Determining the 

Effectiveness, Limitations, 

and Operator Response 

for Very Early Warning 

Fire Detection Systems in 

Nuclear Facilities 

(DELORES-VEWFIRE)

 

Final Report

Office of Nuclear Regulatory Research

NUREG-2180

Determining the 

Effectiveness, Limitations, 

and Operator Response 

for Very Early Warning 

Fire Detection Systems in 

Nuclear Facilities 

(DELORES-VEWFIRE)

Final Report

Manuscript Completed: September 2016

Date Published: December 2016

Prepared by:

G. Taylor1, S. Cooper1, A. D’Agostino1, N. Melly1, and

T. Cleary2 

1U.S. Nuclear Regulatory Commission

2National Institute of Standards and Technology

Office of Nuclear Regulatory Research

NUREG-2180

 

iii

ABSTRACT

Aspirated smoke detection systems have been available on the commercial market for more 

than four decades as an alternative technology to spot-type smoke detection for detecting 

products of combustion. In the United States, several nuclear power plants (NPPs) have 

installed these systems as early as the mid-1990s as an alternative method to conventional fire 

detection systems with the idea to provide advanced warning of potential fire threats. Recently, 

there has been indication that numerous licensees of NPPs transitioning to a performancebased fire protection program have or intend to install these types of systems configured as very

early warning fire detection (VEWFD). In many, but not all cases, the choice to install these 

systems is based on the expectation that these systems may reduce the estimated fire risk in a 

fire probabilistic risk assessment (PRA).

In 2008, the U.S. Nuclear Regulatory Commission (NRC) issued a staff interim position 

documented in a National Fire Protection Association (NFPA) Standard 805 Frequently Asked 

Question (FAQ) 08-0046, “Incipient Fire Detection Systems.” This staff interim position provides 

guidance on the use of these systems and the associated fire PRA quantification for in-cabinet 

applications. At that time, there was limited test data and PRA experience available for those 

applications and as such a confirmatory research program was needed. Research was also 

needed to advance the state of knowledge related to the performance of these systems. This 

report documents the results and findings from the confirmatory research program.

This program provides an evaluation of VEWFD and conventional spot-type smoke detection 

system performance, operating experience, and fire PRA quantification for applications in NPPs 

where these systems are expected to detect fires in their incipient (pre-flaming) stage. The 

results of this report show there is a wide variance in performance for both spot-type and 

VEWFD systems. It has been shown that variables such as ventilation, fuel type, system 

application/design, and operator response play a significant role in the performance of these 

systems to detect low-energy fires. 

Ultimately, this research has shown that (1) the state of knowledge regarding the duration of an 

incipient stage for electrical components found in NPPs, and the associated failure modes with 

regard to fire development of such components is low (uncertain and highly variable), 

(2) in-cabinet smoke detection used to protect electrical enclosures provides the most effective 

and earliest notification of potential fire threats, (3) for area-wide applications the aspirated 

smoke detection systems when configured as VEWFD can potentially notify plant personnel of 

potential fire threats sooner than conventional spot-type smoke detection systems, and (4) plant 

personnel responsible for responding to smoke detection systems must be properly trained, 

follow plant procedures suitable for response to these systems, and ensure that every smoke 

detection system notification has adequate response time and necessary urgency. This report 

concludes with an updated approach to quantify the performance of these systems in Fire PRA 

for in-cabinet and area-wide applications in non-continuously occupied NPP areas.

v

TABLE OF CONTENTS

Section Page

ABSTRACT .................................................................................................................. iii

TABLE OF CONTENTS.................................................................................................. v

LIST OF FIGURES......................................................................................................... ix

LIST OF TABLES ......................................................................................................... xv

EXECUTIVE SUMMARY ............................................................................................ xvii

ACKNOWLEDGMENTS .............................................................................................. xxi

ACRONYMS AND ABBREVIATIONS........................................................................ xxv

1. INTRODUCTION ....................................................................................................1-1

1.1 Overview ..................................................................................................................1-1

1.2 Need for Confirmatory Research .............................................................................1-1

1.3 Purpose and Objectives...........................................................................................1-2

1.4 General Approach and Project History ....................................................................1-3

1.5 Scope of this Report ................................................................................................1-7

1.6 Report Organization and How to Use This Report...................................................1-7

PART I

2. FUNDAMENTALS OF SMOKE GENERATION AND FIRE DETECTION 

TECHNOLOGIES ...................................................................................................2-1

2.1 Background..............................................................................................................2-1

2.2 Dynamics of Fire Stages..........................................................................................2-3

2.3 Aerosol Generation ..................................................................................................2-6

2.4 Smoke Detection Principles .....................................................................................2-7

2.5 Quality Assurance Program ...................................................................................2-14

2.6 System Performance Measures.............................................................................2-17

2.7 Inspection, Testing and Maintenance of Smoke Detection Systems .....................2-21

3. OPERATING EXPERIENCE, STANDARDS, AND LITERATURE REVIEW OF 

VEWFD SYSTEMS.................................................................................................3-1

3.1 Review of VEWFD System Operating Experience ..................................................3-1

3.2 Standards, Listings, Approvals and Codes of Practice ..........................................3-11

3.3 Literature Review ...................................................................................................3-14

4. EXPERIMENTAL APPROACH ..............................................................................4-1

4.1 Detectors..................................................................................................................4-1

4.2 Incipient Fire Sources ..............................................................................................4-3

4.3 Measurement and Control Instrumentation............................................................4-16

4.4 Experimental Configurations..................................................................................4-18
vi

Section Page

4.5 Experimental Procedure ........................................................................................4-49

4.6 Experimental Design..............................................................................................4-52

5. EXPERIMENTAL RESULTS..................................................................................5-1

5.1 Laboratory, Small-Cabinet Experiments ..................................................................5-2

5.2 Laboratory, Large-Cabinet Experiments ..................................................................5-9

5.3 Full-Scale, Small-Room, In-Cabinet Experiments..................................................5-18

5.4 Full Scale, Large Room, Single-Zone, In-Cabinet Experiments ............................5-24

5.5 Full-Scale, Large-Room, Multi-Zone, In-Cabinet Experiments ..............................5-30

5.6 Full-Scale, Small-Room, Area-wide Experiments ..................................................5-36

5.7 Large-Room, Multi-Zone Area-wide Experiments..................................................5-37

5.8 Insulated Electrical Conductor Heat Conduction and Ignition Potential.................5-41

5.9 Evaluation of Test Results .....................................................................................5-46

PART II

6. OVERVIEW OF QUANTIFICATION APPROACH..................................................6-1

6.1 Overview of Fire PRA Model....................................................................................6-1

6.2 Summary of Previous Quantification Efforts ............................................................6-3

6.3 Approaches to Quantifying Smoke Detection Performance.....................................6-8

6.4 Event Trees and Definitions of Event Headings ......................................................6-8

7. PARAMETER ESTIMATION BASED ON PART I..................................................7-1

7.1 Fraction of Fires That Have an Incipient Stage........................................................7-1

7.2 System Performance Measures (Availability, Reliability, Effectiveness)..................7-6

8. TIMING ANALYSIS ................................................................................................8-1

8.1 Detector response time during the incipient stage...................................................8-3

8.2 Estimating the duration of time available for operators to respond..........................8-4

9. HUMAN FACTORS ANALYSIS .............................................................................9-1

9.1 Information Gathering ..............................................................................................9-1

9.2 Human Factors Analysis of VEWFD System Response Operations .......................9-2

9.3 Area-wide Applications ..........................................................................................9-16
vii

Section Page

10.HUMAN RELIABILITY ANALYSIS ......................................................................10-1

10.1 Human Reliability Analysis (HRA) Approach .........................................................10-1

10.2 Define Issue and Scope to Be Addressed .............................................................10-2

10.3 Identification and Definition of Human Failure Events (HFEs)...............................10-5

10.4 Qualitative HRA ...................................................................................................10-10

10.5 Feasibility Assessment ........................................................................................10-28

10.6 HRA Quantification ..............................................................................................10-30

10.7 HRA Dependency and Recovery Analysis...........................................................10-51

10.8 HRA Uncertainty Analysis....................................................................................10-51

11.FIRE SUPPRESSION...........................................................................................11-1

11.1 Enhanced Fire Suppression...................................................................................11-2

11.2 Conventional Fire Suppression..............................................................................11-3

12.QUANTIFICATION OF SMOKE DETECTION PERFORMANCE.........................12-1

12.1 Illustrative Examples ..............................................................................................12-1

12.2 Evaluation of the Event Tree Sensitivity ..............................................................12-14

12.3 Use of Plant Specific or Generic Data .................................................................12-20

13.ASSUMPTIONS AND LIMITATIONS...................................................................13-1

PART III

14.REPORT SUMMARY AND CONCLUSIONS .......................................................14-1

14.1 Conclusions ...........................................................................................................14-3

15.RECOMMENDATIONS FOR FUTURE RESEARCH ...........................................15-1

16.DEFINITIONS .......................................................................................................16-1

17.REFERENCES .....................................................................................................17-1

APPENDIX A VIEWGRAPHS FROM MEETING WITH ASD VENDORS................................ A-1

APPENDIX B SUPPORTING EXPERIMENTAL DATA ........................................................... B-1

APPENDIX C SUPPORTING INFORMATION FOR HUMAN PERFORMANCE 

EVALUATION....................................................................................................C-1

APPENDIX D EVALUATION OF OPERATING EXPERIENCE ...............................................D-1

APPENDIX E LITERATURE REVIEW..................................................................................... E-1

APPENDIX F QUICK REFERENCE FOR PARAMETERS USED IN RISK 

SCOPING STUDY............................................................................................. F-1

APPENDIX G VEWFD SYSTEM DATA COLLECTION ...........................................................G-1

APPENDIX H USER GUIDE FOR VEWFD EVENT TREE NON-SUPPRESSION

PROBABILITY CALCULATION TOOL..............................................................H-1

ix

LIST OF FIGURES

Figure Page

Figure 1-1. Illustration of VEWFD System Confirmatory Research Project..........................1-3

Figure 2-1. Illustration of ASD system in an in-cabinet application.......................................2-2

Figure 2-2. Fire stages..........................................................................................................2-4

Figure 2-3. Illustration of several performance-based t-squared fire growth profiles 

(Note: incipient, steady-state, and decay stages not shown) ............................2-5

Figure 2-4. Image of conventional spot-type detectors.......................................................2-10

Figure 2-5. Illustration of ASD in-cabinet application..........................................................2-11

Figure 2-6. Illustration of an ASD area-wide application.....................................................2-11

Figure 2-7. Illustration of detector signal background noise and drift .................................2-20

Figure 4-1. Schematic of the bus bar....................................................................................4-7

Figure 4-2. XLPO2 wire mounted to the bus bar ..................................................................4-8

Figure 4-3. The bus bar mounted on the stand inside a cabinet...........................................4-9

Figure 4-4. Heating ramp profiles for 15, 60, and 240 minute HRPs, with logarithmic 

and exponential bounding profiles that reach 485 °C in 60 minutes ................4-10

Figure 4-5. Printed circuit board (PCB) mounted to the bus bar.........................................4-11

Figure 4-6. Terminal block (TB) mounted to the bus bar ....................................................4-11

Figure 4-7. The cable bundle sample .................................................................................4-12

Figure 4-8. A single wire test was setup following the British Standard 6266.....................4-13

Figure 4-9. A set of three resistors wired in parallel............................................................4-14

Figure 4-10. A pair of capacitors wired in parallel.................................................................4-14

Figure 4-11. Dome enclosure for the resistor and capacitor tests ........................................4-15

Figure 4-12. Shredded paper test.........................................................................................4-15

Figure 4-13. Thermal image of XLPE wires following a 60.0 minute heating ramp 

and a 5.0 minute soak period ...........................................................................4-17

Figure 4-14. Laboratory instrument cabinet experimental configuration...............................4-18

Figure 4-15. Instrument cabinet ceiling view ........................................................................4-19

Figure 4-16. Instrument cabinet top plate hole pattern .........................................................4-20

Figure 4-17. ASD pipe layout for the instrument cabinet experimental setup.......................4-20

Figure 4-18. Instrument cabinet inside NIST laboratory .......................................................4-21

Figure 4-19. NPP cabinets used in large cabinet experiments .............................................4-22

Figure 4-20. Large cabinet installation and enclosure detail.................................................4-23

Figure 4-21. View of sampling ports and spot detectors installed inside the naturally 

ventilated (left) and force ventilated (right) NPP large cabinets .......................4-23

Figure 4-22. The ventilation configuration of the forced ventilation cabinet..........................4-24

Figure 4-23. Small room full-scale experiment space layout ................................................4-25

Figure 4-24. View of the small room experimental space.....................................................4-26

Figure 4-25. View of the small room full-scale experimental space ceiling...........................4-27

Figure 4-26. Five-cabinet spot detector and ASD sampling port configurations...................4-28

Figure 4-27. In-cabinet ASD pipe layout...............................................................................4-29

Figure 4-28. Smoke source sample locations.......................................................................4-30

Figure 4-29. Area-wide ASD pipe layout ..............................................................................4-31

Figure 4-30. Off angle view of 100 m2 facility .......................................................................4-32

Figure 4-31. Front view of the 100 m2 facility........................................................................4-33
x

Figure Page

Figure 4-32. Large room testing facility layout......................................................................4-34

Figure 4-33. Spot detectors layout inside 100 m2 facility ......................................................4-35

Figure 4-34. Detector and sampling port locations for single-zone experiments ..................4-36

Figure 4-35. Detectors, sampling ports, and vent hole locations for in-cabinet 

experiments......................................................................................................4-37

Figure 4-36. Detector layout and side vent location..............................................................4-38

Figure 4-37. Single zone piping configuration for ASD2 and ASD3......................................4-39

Figure 4-38. Front view of the ASD4 piping setup for the in-cabinet sampling.....................4-40

Figure 4-39. Pressure regulator located above each sampling port for ASD4......................4-40

Figure 4-40. Side view of a cabinet, showing the location of the side vents.........................4-41

Figure 4-41. Side vents allowing flow between three cabinets with ASD sampling..............4-42

Figure 4-42. Sample locations in the experiments performed ..............................................4-43

Figure 4-43. ASD4 piping layout for the in-cabinet and HVAC inlet sampling ......................4-44

Figure 4-44. ASD5 piping layout for the in-cabinet and HVAC inlet sampling ......................4-45

Figure 4-45. ASD5 piping layout, with sampling locations located at the HVAC exhaust.....4-46

Figure 4-46. ASD4 piping layout, with sampling location located at the HVAC exhaust.......4-46

Figure 4-47. Return air grill protected with ASD piping.........................................................4-47

Figure 4-48. Area-wide ASD4, piping layout.........................................................................4-48

Figure 4-49. Area-wide ASD5, piping layout.........................................................................4-49

Figure 5-1. VEWFD pre-alert and conventional alarm times for the 15 minute HRP

instrument cabinet experiments. (Error bars represent ± one standard 

deviation for repeated tests)...............................................................................5-3

Figure 5-2. VEWFD alert and ION alarm times for the 15 minute HRP instrument 

cabinet experiments. (Error bars represent ± one standard deviation for 

repeated tests) ...................................................................................................5-4

Figure 5-3. VEWFD pre-alert and conventional alarm times for the 60 minute HRP

instrument cabinet experiments. (Error bars represent ± one standard deviation 

for repeated tests) ..............................................................................................5-5

Figure 5-4. VEWFD alert and ION alarm times for the 60 minute HRP instrument 

cabinet experiments. (Error bars represent ± one standard deviation 

for repeated tests) ..............................................................................................5-5

Figure 5-5. VEWFD pre-alert and conventional alarm times for the 240 minute HRP

instrument cabinet experiments..........................................................................5-6

Figure 5-6. VEWFD alert and ION alarm times for the 240 minute HRP instrument 

cabinet experiments ...........................................................................................5-7

Figure 5-7. Block temperature at pre-alert, alert or alarm time for various detectors. 

(Error bars represent +/- one standard deviation for repeated tests) .................5-8

Figure 5-8. Time difference between ION alarm time and the ASD alerts and 

SS alert time for large cabinet, natural ventilation, 16.3 minute HRP 

experiments......................................................................................................5-10

Figure 5-9. Time difference between the end of test (EOT) time (1278 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for large cabinet, 

natural ventilation, 16.3 minute HRP experiments ...........................................5-11

Figure 5-10. Time difference between ION alarm time and the ASD alert time for large

cabinet, forced ventilation, 16.3 minute HRP experiments...............................5-11

Figure 5-11. Time difference between the end of test (EOT) time (1278 s) and 

ASD pre-alerts, ASD alerts or ION alarm time for large cabinet, 

forced ventilation, 16.3 minute HRP experiments ............................................5-12
xi

Figure Page

Figure 5-12. Time difference between ION alarm time and the ASD alerts and 

SS alert time for large cabinet, natural ventilation, 65.0 minute HRP 

experiments......................................................................................................5-13

Figure 5-13. Time difference between the end of test (EOT) time (4200 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for large cabinet, 

natural ventilation, 65.0 minute HRP experiments ...........................................5-13

Figure 5-14. Time difference between ION alarm time and the ASD alerts times 

for large cabinet, forced ventilation, 65.0 minute HRP experiments.................5-14

Figure 5-15. Time difference between the end of test (EOT) time (4200 s) and 

ASD pre-alerts, ASD alerts or ION alarm time for large cabinet, 

forced ventilation, 65.0 minute HRP experiments ............................................5-15

Figure 5-16. Time difference between ION alarm time and the ASD alerts and 

SS alert time for large cabinet, natural ventilation, 260.0 minute HRP 

experiments......................................................................................................5-16

Figure 5-17. Time difference between the end of test (EOT) time (15900 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for large cabinet, 

natural ventilation, 260.0 minute HRP experiments .........................................5-17

Figure 5-18. Time difference between ION alarm times and the ASD alerts and 

SS alert for small room, cabinet mock-up, 16.3 minute HRP experiments.......5-19

Figure 5-19. Time difference between the end of test (EOT) time (1278 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for small room, 

cabinet mock-up, 16.3 minute HRP experiments .............................................5-19

Figure 5-20. Time difference between ION alarm time and the ASD alerts and 

SS alert for small room, cabinet mock-up, 65.0 minute HRP experiments.......5-20

Figure 5-21. Time difference between the end of test (EOT) time (4200 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for small room, 

cabinet mock-up, 65.0 minute HRP experiments .............................................5-21

Figure 5-22. Time difference between ION alarm times and the ASD alerts and 

SS alert for small room, cabinet mock-up, 260.0 minute HRP experiments.....5-22

Figure 5-23. Time difference between the end of test (EOT) time (15900 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for small room, 

cabinet mock-up, 260.0 minute HRP experiments ...........................................5-22

Figure 5-24. Mean alert or alarm times for small room, 1, 4, and 5-cabinet mock-up

configurations, 65.0 minute HRP experiments .................................................5-23

Figure 5-25. Mean alert or alarm times for small room, 5-cabinet mock-up 

configurations, 65.0 minute HRP experiments .................................................5-24

Figure 5-26. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, single-zone single-cabinet, 65.0 minute HRP 

experiments......................................................................................................5-25

Figure 5-27. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, single-zone, three-cabinet configuration, 

65.0 minute HRP experiments..........................................................................5-26

Figure 5-28. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, single-zone, no ventilation, in-cabinet, 

65.0 minute HRP experiments..........................................................................5-26

Figure 5-29. Time difference between ION alarm times and the ASD alerts and 

SS alert for Large room, single-zone, 7.4 ACH room ventilation, 

in-cabinet, 65.0 minute HRP experiments........................................................5-27
xii

Figure Page

Figure 5-30. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, single-zone, in-cabinet, 65.0 minute HRP 

experiments......................................................................................................5-28

Figure 5-31. Time difference between the end of test (EOT) time (4200 s) and 

ASD Pre-alerts, ASD and SS alerts or ION alarm time for large room, 

in-cabinet, 65.0 minute HRP experiments........................................................5-28

Figure 5-32. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, single-zone, in-cabinet, 260.0 minute 

HRP experiments .............................................................................................5-29

Figure 5-33. Time difference between the end of test (EOT) time (15900 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for large room, 

in-cabinet, 260.0 minute HRP experiments......................................................5-30

Figure 5-34. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, multi-zone, single-cabinet, 65.0 minute HRP 

experiments......................................................................................................5-32

Figure 5-35. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, multi-zone, three cabinet configuration, 

65.0 minute HRP experiments..........................................................................5-32

Figure 5-36. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, multi-zone, no ventilation, 65.0 minute 

HRP experiments .............................................................................................5-33

Figure 5-37. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, multi-zone, 7.4 ACH room ventilation, 

65.0 minute HRP experiments..........................................................................5-33

Figure 5-38. Time difference between the end of test (EOT) time (4200 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for large room, 

multi-zone in-cabinet, 65 minute HRP experiments .........................................5-34

Figure 5-39. Time difference between ION alarm times and the ASD alerts and 

SS alert for large room, multi-zone, in-cabinet, 260.0 minute HRP 

experiments......................................................................................................5-35

Figure 5-40. Time difference between the end of test (EOT) time (15900 s) and 

ASD pre-alerts, ASD and SS alerts or ION alarm time for large room, 

multi-zone in-cabinet, 260.0 minute HRP experiments ....................................5-35

Figure 5-41. Time difference between the end of test (EOT) time (4200 s) and 

ASD pre-alerts, ASD and SS alerts, PHOTO and ION alarm time 

for small room, area-wide, 65.0 minute HRP experiments ...............................5-37

Figure 5-42. Time difference between the end of test (EOT) time (1278 s) and 

ASD alerts, SS alerts, PHOTO and ION alarm time for large room, 

area-wide, 16.3 minute HRP experiments........................................................5-39

Figure 5-43. Time difference between the end of test (EOT) time (4200 s) and 

ASD alerts, SS alerts, PHOTO and ION alarm time for large room, 

area-wide, 65.0 minute HRP experiments........................................................5-40

Figure 5-44. Heating profiles for 12 AWG XLPE wires at various times during 

the heating process. The temperature of the block can be seen 

in the top left corner in each image. .................................................................5-42

Figure 5-45. Temperature profile for a 12 AWG XLPE wire. The image was taken

at the end of a 60.0+ minute HRP and a 5.0 minute set point hold 

at 450 °C. The bus bar temperature was 446 °C. ............................................5-43
xiii

Figure Page

Figure 5-46. Wire surface temperature as a function of time for a 60.0 minute HRP 

followed by a 5.0 minute hold (± 2 oC). Thermocouples were located 

along the insulated surface of a 12 AWG XLPE sample ..................................5-43

Figure 5-47. Experimental setup...........................................................................................5-45

Figure 5-48. Pilot flame.........................................................................................................5-45

Figure 5-49. Detector response to selected materials (1-hour HRP)....................................5-47

Figure 5-50. Time to detection, by detector..........................................................................5-48

Figure 5-51. Summary plot – ION alarm versus VEWFD alert 

(in-cabinet, natural ventilation) .........................................................................5-50

Figure 5-52. Summary plot – PHOTO versus VEWFD (in-cabinet, natural ventilation)........5-50

Figure 5-53. Detector response versus number of sampling ports in cabinet space............5-52

Figure 5-54. Effect of cabinet ventilation on in-cabinet detector response ...........................5-53

Figure 5-55. Effect of cabinet ventilation on in-cabinet detector response for 

all materials and all HRPs. ...............................................................................5-54

Figure 5-56. System effectiveness by detector and application

(Note: no data for ION area-wide return).........................................................5-55

Figure 5-57. ASD time to detect area-wide ceiling configurations ........................................5-57

Figure 5-58. ASD time to detect area-wide forced ventilation air return and 

ceiling pooled ...................................................................................................5-57

Figure 5-59. ASD time to detect low-energy incipient sources .............................................5-58

Figure 6-1. Electrical fires suppression curve showing 5 minute in-cabinet 

detection reduction .............................................................................................6-4

Figure 6-2. FAQ 08-0046 proposed event tree for assessing fire risk for 

installed VEWFDS in-cabinet .............................................................................6-6

Figure 6-3. FAQ 08-0046 simplified event tree for assessing fire risk for 

installed VEWFDS in-cabinet .............................................................................6-8

Figure 6-4. Basic event tree for in-cabinet smoke detection non-suppression 

probability estimation........................................................................................6-10

Figure 6-5. Basic event tree for area-wide smoke detection non-suppression 

probability estimation........................................................................................6-11

Figure 6-6. Illustration of change to in-cabinet event tree for de-energization strategy ......6-12

Figure 8-1. Generic fire scenario event timeline. Fire scenario progression (top),

VEWFD/operator response (center), conventional smoke detection/

operator response (bottom). Illustration only—event markers may 

not be indicative of actual system response.......................................................8-2

Figure 8-2. Summary of ASD VEWFD in-cabinet test results showing normalized time 

of alert (box and whisker plot shows 10th, 25th, 50th (median), 75th, and 90th

percentiles with outliers shows as dots, mean shown as dashed line)...............8-3

Figure 8-3. Illustration of difference between incipient stage and time available..................8-6

Figure 8-4. Distribution for duration of time available for plant personnel to 

respond to VEWFD system “alert” or conventional “alarm” notification 

of incipient fire conditions, for those fires, which exhibit an incipient 

stage for in-cabinet applications.........................................................................8-7

Figure 9-1. Generic depiction of operations in response to an in-cabinet ASD VEWFD 

alert followed by alarm where a suppression strategy is being used .................9-4
xiv

Figure Page

Figure 9-2. Generic depiction of operations in response to an in-cabinet ASD VEWFD 

alert followed by alarm where a de-energization strategy is being used............9-5

Figure 10-1. Timeline showing difference between time available and entire 

incipient phase. ..............................................................................................10-18

Figure 10-2. Cumulative distribution function of time available for operator response

by detection type ............................................................................................10-20

Figure 10-3. Timelines for MCR operators, field operators, and technician for 

incipient detector response. ...........................................................................10-25

Figure 11-1. Conventional detection suppression event tree................................................11-5

Figure 12-1. Case 1, detection suppression tree for (η1)......................................................12-4

Figure 12-2. Case 1, detection suppression event tree (η2) .................................................12-5

Figure 12-3. Case 4, conventional detection suppression tree for (η1)...............................12-12

Figure 12-4. Case 4, conventional detection suppression tree for (η2)...............................12-13

Figure 12-5. Probability plots for sensitivity of Cloud Chamber ASD VEWFD System

(Case 1)..........................................................................................................12-15

Figure 12-6. Probability plots for sensitivity of Light Scattering (LS1) ASD VEWFD 

System (Case 1).............................................................................................12-16

Figure 12-7. Probability plots for sensitivity of Light Scattering (LS2) ASD 

VEWFD System (Case 1)...............................................................................12-17

Figure 12-8. Probability plots for sensitivity of Sensitive Spot-type (SS) 

VEWFD System (Case 1)..............................................................................12-18

Figure 12-9. Probability plots for sensitivity of Ionization (ION) Spot-Type 

Addressable system (Case 1) .......................................................................12-19
xv

LIST OF TABLES

Table Page

Table 4-1. Smoke Detection Technologies Generic Identification System..........................4-1

Table 4-2. Ignition Temperatures for Plastics Grouped by Polymer Properties (Ref. 33)....4-4

Table 4-3. Materials Used in Experiments...........................................................................4-6

Table 4-4. Laboratory Instrument Cabinet Experiments ....................................................4-53

Table 4-5. Laboratory Large-Cabinet Experiments............................................................4-54

Table 4-6. Large Cabinet Experiments with Reduced HRP...............................................4-55

Table 4-7. Full-Scale, Single-Zone, In-Cabinet XLPE Experiments ..................................4-55

Table 4-8. Full-Scale, Single-Zone, In-Cabinet CSPE Experiments..................................4-56

Table 4-9. Full-Scale, Single-Zone, In-Cabinet PVC2 Experiments ..................................4-56

Table 4-10. Full-Scale, Single-Zone, In-Cabinet PCB Experiments ....................................4-57

Table 4-11. Full-Scale, Single-Zone, In-Cabinet Resistor Experiments ..............................4-57

Table 4-12. Full-Scale, Single-Zone, Area-wide Experiments .............................................4-57

Table 4-13. Full-Scale, Single-Zone, In-Cabinet Experiments.............................................4-58

Table 4-14. Full-Scale, Multi-Zone Experiments.................................................................4-58

Table 5-1. Material Identification Numbers Used in Laboratory—Small-Scale Tests ..........5-2

Table 5-2. Nominal Detector Sensitivities for Laboratory—Small Scale Tests....................5-3

Table 5-3. AMD and MMD Average over the 5.0 Minute Soak Time for 

15.0 Minute HRP Tests ......................................................................................5-7

Table 5-4. Nominal Detector Sensitivities for Laboratory—NPP Large-Cabinet 

Experiments .......................................................................................................5-9

Table 5-5. Nominal Detector Sensitivities for Small Room, Cabinet Mock-Up Tests ........5-18

Table 5-6. Nominal Detector Sensitivities for Large Room, Single-Zone Cabinet Tests ...5-25

Table 5-7. Nominal Detector Sensitivities for Large Room, Multi-Zone Cabinet Tests......5-31

Table 5-8. Nominal Detector Sensitivities for Small Room, Single-Zone, 

Area-wide Experiments ....................................................................................5-36

Table 5-9. Nominal Detector Sensitivities for Large Room, Multi-Zone, 

Area-wide Experiments ....................................................................................5-38

Table 5-10. Persisting Burn Time for Wire Samples after Pilot Flame Removed ................5-46

Table 5-11. Summary of Average Difference in Time to Detection between 

Conventional and VEWFD Systems (Negative Values Represent 

Conventional Spot Responding on Average before VEWFD Systems) ...........5-51

Table 7-1. Summary of Fraction of Electrical Cabinet Fires (Bin 15) That 

Have an Incipient Stage Detectable by a VEWFD System ................................7-4

Table 7-2. Generic ASD Unreliability Estimate per detector per year..................................7-8

Table 7-3. Information Gathered from Site Visits to Inform Availability 

and Reliability Estimates ....................................................................................7-9

Table 7-4. Generic ASD Unavailability Estimate per Detector per Year............................7-10

Table 7-5. ASD VEWFD System In-Effectiveness Estimates Based on Test Data ...........7-11

Table 7-6. Identification of Datasets Used To Estimate the System 

In-Effectivness Estimates (See Table 7-3) .......................................................7-12

Table 10-1. Fraction of Probability Distributions for ASD VEWFD, Cloud Chamber .........10-22

Table 10-2. Fractions of Probability Distribution for Conventional Spot-Type, 

ION Detector ..................................................................................................10-23
xvi

Table Page

Table 10-3. Fractions of Probability Distribution for VEWFD, Light-Scattering 

Detector..........................................................................................................10-23

Table 10-4. Summary of Timing Inputs for Operator Actions after “Alert” Signal ..............10-27

Table 10-5. Feasibility Assessment for ASD VEWFD, Cloud Chamber ............................10-29

Table 10-6. Feasibility Assessment for Conventional Spot-Type, ION Detector ...............10-29

Table 10-7. Feasibility Assessment for VEWFD Light-Scattering Detector 

(ASD & Spot)..................................................................................................10-29

Table 10-8. MCR Operator Response (without recovery) Assessed with EPRI’s 

CBDTM HRA Method .....................................................................................10-32

Table 10-9. HEP Calculations for ASD VEWFD, Cloud Chamber.....................................10-36

Table 10-10. HEP Calculations for Conventional Spot-Type, ION Detector........................10-36

Table 10-11. HEP Calculations for VEWFD, Light Scattering Detector...............................10-37

Table 10-12. Time Required Variation (4 minutes): Revised HEP Calculations 

for Conventional Spot-Type, ION Detector.....................................................10-40

Table 10-13. More Sensitive Detector Setting Variation: Revised HEP Calculation 

for ASD VEWFD, Cloud Chamber..................................................................10-40

Table 10-14. Less sensitive detector setting variation: Revised HEP Calculation 

for ASD VEWFD, Cloud Chamber..................................................................10-41

Table 10-15. HEP Calculations for Cloud Chamber - Local Cabinet De-energization.........10-49

Table 10-16. HEP Calculations for Cloud Chamber - Local Component De-energization ..10-51

Table 11-1. Conventional Detection Suppression Event Tree Outputs ...............................11-5

Table 12-1. Case 1 Input Parameters: Multi-Control Cabinet, In-Cabinet ..........................12-2

Table 12-2. Case 1 Results - Probability of Non-Suppression ............................................12-3

Table 12-3. Case 2, Input Parameters: Single Low Voltage Control Cabinet, In-Cabinet ...12-6

Table 12-4. Case 2 Results - Probability of Non-Suppression ............................................12-7

Table 12-5. Case 3 Input Parameters: Multi-Power Cabinet, In-Cabinet.............................12-8

Table 12-6. Case 3 Results - Probability of Non-Suppression ............................................12-8

Table 12-7. Case 4 Input Parameters: Low-voltage control cabinet Type, Area-wide......12-10

Table 12-8. Case 4 Results for Area-wide Ceiling VEWFD - Probability of 

Non-Suppression............................................................................................12-10

Table 12-9. Case 4 Results for Area-wide Air Return Grill VEWFD - Probability 

of Non-Suppression........................................................................................12-11
xvii

EXECUTIVE SUMMARY

The purpose of this research is to evaluate the relative performance of smoke detection

systems when configured for very early warning fire detection (VEWFD) applications, to 

conventional spot-type detection systems for use in nuclear power plant (NPP) applications. 

There has been recent interest in quantifying potential risk enhancement associated with these 

systems to support fire probabilistic risk assessments (PRAs). The fire PRAs are primarily 

being developed to support NPPs transitioning to performance-based fire protection programs 

per National Fire Protection Association (NFPA) Standard 805, “Performance-Based Standard 

for Fire Protection for Light Water Reactor Electric Generating Plants,” 2001 Edition. The

performance objective for using these systems is to provide earlier warning to plant personnel 

that may allow for additional time for human intervention before fire conditions threaten the 

ability to achieve safe shutdown conditions.

The need for this research is a result of limited test data and understanding of the performance 

of these systems in NPP applications to detect low energy pre-flaming (incipient) fire conditions

typically originating in electrical enclosures. The availability of applicable empirical data is 

scarce and operating experience in NPP applications for detection of electrical enclosures fires 

are limited. Specifically, data on the detection of slowly developing, incipient stage, pre-flaming 

conditions is not available. The focus of the research presented in this report is to better 

understand these systems performance, operating experience, and their potential risk benefits 

via a risk scoping study. Specific needs included evaluation of the effectiveness of using 

VEWFD systems for in-cabinet and area-wide applications, response to representative products 

of combustion, system design aspects, comparison to conventional spot-type smoke detection, 

and operator response.

The focus of this research is related to the use of these systems as potential fire risk reduction 

measures, associated with electrical enclosures fire hazards by providing enhanced warning of 

pre-flaming (incipient) fire conditions to support fire probabilistic risk assessments. A common 

failure mode of electrical enclosures occurs as a result of slow overheating followed by electrical 

component thermal decomposition (pyrolysis) that may eventually lead to flaming fire conditions,

if sufficient heat and ignition conditions exist. 

This research includes a literature review, a review of available operating experience, several 

scales of testing, and an evaluation of human performance. All of these elements taken 

together support a risk scoping study. A literature review was conducted early on in the project 

to understand the availability of information to support the risk analysis and test plan 

development. The literature review concluded that, in general, there has been substantial 

research supporting the use of aspirating smoke detection (ASD) VEWFD systems in special 

applications, such as telecommunication facilities, warehouses, atria, and as a reference tool to 

support evaluation of model prediction of conventional spot-type detector activation. However, 

most of the available information was developed to acquire specific data needed to support 

specific applications. Where applicable to NPP scenarios, these data have been used to 

support the risk scoping study documented in this report. In addition to test data, several 

sources have provided valuable information on the characteristics of smoke, and the 

parameters that affect smoke aging and detector response associated with electrical enclosure 

fires.

Concurrently, operating experience was obtained as related to VEWFD systems, by conducting 

site visits, interviews with plant operating staff, procedure review and assessing the historical 
xviii

fire events in NPP electrical enclosures. The operating experience supported the human 

performance evaluation by providing an understanding of common plant personnel response

and an understanding of where and how operators interface with VEWFD systems and 

associated fire alarm annunciator response. Observations made during the site visits also 

supported test development to ensure testing was representative of their use in NPPs. Though 

not found in direct support of the research objectives, other valuable information was obtained 

during the site visits, and is also documented in this report to allow communication of lessons 

learned from using these systems.

Following the operating experience and literature review, actual VEWFD system testing was 

conducted. The testing evaluated three single-port ASD and two multi-port ASDs from three 

different vendors, all configured to VEWFD. One spot-type VEWFD detector and two types of 

conventional spot-type detectors were also evaluated. Three scales of testing were completed. 

Laboratory scale tests evaluated detector response to a variety of material in a small instrument 

cabinet and in reactor protection system cabinets procured from an unfinished NPP. The next 

scale of testing evaluated both in-cabinet and area-wide detector response in a small room. 

This testing included variations in both cabinet and room ventilation conditions. The final largescale testing again evaluated the in-cabinet and area-wide detector response, but also included 

testing VEWFD system performance in an air return grill application. The test results provide a 

wealth of information regarding the performance of these systems to support a better

understanding of their risk benefit of detecting low energy pre-flaming fire conditions.

Objectives Supported by Testing

Testing and the analysis of the results provided insights that supported several objectives of this 

project. The first objective supported was the evaluation of the effectiveness of in-cabinet and 

area-wide VEWFD system applications. The results confirmed that in-cabinet smoke detection

provides the earliest notification of low-energy incipient fires originating in the cabinet. This is 

because of the close proximity of ASD sampling ports and spot-type smoke detectors to 

protected equipment that could generate products of combustion during an incipient stage. 

Phenomena such as dilution and stratification are minimized as compared to area-wide

applications. The data indicated that cabinet ventilation can have a negative effect on smoke 

detection system performance when high air velocities within the electrical enclosure are 

encountered. This negatively impacts both conventional spot-type and ASD VEWFD systems. 

In an area-wide application, the ASD VEWFD systems are also more effective in detecting lowenergy incipient fire sources than conventional spot-type detectors. ASD VEWFD show 

improved protection in area-wide applications with high-airflow room ventilation conditions, as 

compared to conventional ceiling mounted spot-type detectors. A test result comparison 

between air return grill and ceiling mounted ASD applications showed marginally increased 

effectiveness in the air return grill application for the limited experimental conditions. However, 

competing parameters such as ceiling height and ventilation influence the performance of 

ceiling versus air return ASD VEWFD application effectiveness.

Contrasting the performance of conventional spot-type smoke detection devices to VEWFD 

systems, the following insights were identified. The amount of additional time will vary based on

the failure mechanism of the degrading component and the associated length of its incipient 

stage. In an effort to capture an estimate on the range of additional warning time provided by 

ASD VEWFD systems over conventional spot detectors a wide variety of materials, range of 

heating rates, and space configurations were explored. For area-wide applications, all ASD 

VEWFD systems performed better, on average, than the conventional spot type detectors when 

responding to low-energy smoke sources. In-cabinet application showed mixed results based 

upon detection technology. The experimental tests show for naturally ventilated in-cabinet 
xix

applications, a conventional ionization (ION) spot type detector performed better, on average, 

than three of the five ASD VEWFD systems tested. For fast developing fires the amount of 

additional warning between these two systems is marginal, regardless of application (in-cabinet 

vs. area-wide).

VEWFD system response to common products of combustion encountered in NPP electrical 

enclosures was also evaluated. Laboratory scale tests evaluated the characteristics of the 

products of combustion generated from the selected components expected to be found in NPP 

electrical enclosures, and allowed for a reduction of the materials tested in large-scale testing. 

The mean diameters of smoke particles were measured and shown to vary by a factor of three 

for the materials tested. This particle characteristic information was used to down select the 

number of materials used in subsequent testing to bound the range of particle characteristics.

Literature and testing supported an evaluation of several parameters that affect in-cabinet 

VEWFD system layout and design characteristics with regard to system response. The 

following findings were made with regard to this objective:

• Cabinet design, loading, and ventilation effects can have an influence on the 

performance of ASD VEWFD systems, as well as conventional spot-type detectors 

installed inside electrical cabinets. Mechanical (forced) cabinet ventilation is a primary 

influence factor on detector response, especially with high rates of cabinet air exchange. 

As cabinet ventilation rates increase, so does smoke dilution. High ventilation conditions 

affect both time to detection and the effectiveness of the VEWFD systems to detect 

low-energy incipient stage fires. However, in the empty ventilated cabinet tests in which 

lower rates of cabinet ventilation were used, the ASD VEWFD response marginally 

improved relative to the naturally ventilated cabinet case.

• For in-cabinet applications, the presence of openings, or lack of partitions between 

adjacent cabinet sections having ASD sampling ports, reduces the time to detection. 

This is because of the cumulative effect of drawing samples from multiple sampling 

ports. The full-scale, small room, in-cabinet tests indicated that ASD response to a 

single cabinet with no openings to adjacent cabinets, was slowest, compared to multisection cabinets without cabinet partitions.

• Source location inside the electrical cabinet also has an effect on VEWFD response. In 

the full-scale small room tests where the source was elevated off the cabinet floor

approximately two-thirds of the height of the cabinet, the ASDs responded approximately 

9 percent faster, on average, than when the sources were located on the floor.

• Other parameters not explicitly explored in this program, but covered in the literature,

relate to soot deposition and loss of aerosol thorough ventilation. Soot deposition 

internal to the electrical cabinet will be influenced by the obstructions (impactation), 

thermal gradients (thermophoresis), and electric fields (electrophoresis). Cabinets with a 

large surface area of ventilation, such as louvered vents compounded by 

thermophoresis, could result in a fraction of aerosol being lost through these vents. 

These phenomena would cause less aerosol to transport to the ASD sampling ports or 

spot-type detectors located at the ceiling of the electrical cabinet, resulting in a delay in 

detection, as compared to the data in this report, and a decrease in effectiveness in the 

detection of low-energy fire during the incipient stage.
xx

Objective Supported by Human Performance

The human factors were also evaluated to foster a broader understanding of both types of tasks 

required by plant personnel, and the factors that affect human performance. A tabletop analysis 

was developed to present main control room, field operator, and technician response to VEWFD 

systems. Factors identified as affecting human performance include, the use of special 

equipment, such as portable ASDs, or thermal imaging cameras; human-system interface; 

procedures; training; staffing; communications; complexity; and perceived workload, pressure, 

and stress.

Information obtained from operating experience, literature review, and the tabletop analysis,

supported a human reliability analysis. Based on the expected operational response and timing 

estimate developed from operating experience and test results, and the overall strategy that 

parallels post-initiator operator actions, a human reliability analysis was conducted. The results 

of this HRA analysis indicate that human error probabilities vary with the type of in-cabinet 

smoke detection system used.

Risk Scoping Study Objectives

A model to quantify the non-suppression probability for use in fire PRAs is presented. The 

model uses the best available test data, operating experience, and expected operator 

responses. It has been shown that a dominant contributor to the risk quantification is the 

estimation of the fraction of potentially challenging or greater fires which exhibit an incipient fire 

stage of sufficient duration to allow for successful operator response. Since fire PRAs only 

quantify those fires that initiate and can potentially grow to a damaging state, the majority of 

smoking events are not modeled (i.e., not included as a fire initiator). The previous methods to 

estimate this fraction were mostly subjective, lacked supporting data relevant to the types of 

fires postulated in fire PRAs and could not be confirmed based on the evaluation of the 

operating experience.

The reliability and availability was evaluated for ASD systems. Data from the Electric Power 

Research Institute (EPRI) report EPRI 1016735 “Fire PRA Methods Enhancements: Additions, 

Clarification, and Refinements to EPRI 1019189,” literature on German NPP operating 

experience, information collected during site visits, and reliability estimates as part of smoke 

detector listings were used to estimate unreliability estimates for ASDs. Based on the 

information collected during the site visits, a wide variance of system downtime was observed. 

It was noted that system availability improved for facilities that had these systems installed and 

operating for a substantial period of time. Facilities that were using ASD VEWFD systems for

the first time indicated longer system downtime likely because of the lack of understanding of 

the system start-up and maintenance requirements to ensure proper operation. This early 

downtime was not included in the unavailability estimates. For area-wide air return grill 

applications, the reliability and availability of the ventilation system need to be modeled into the 

risk quantification as the air return grill application requires forced ventilation to perform as 

intended.

The risk benefit for using these systems varies by application with in-cabinet detection being the 

optimal approach for detecting low-energy incipient sources early enough to allow for enhanced 

suppression capabilities and avoidance of damage to targets outside of the electrical cabinet. 

Area-wide applications also provide some risk benefit; however, they are usually slower to 

detect low-energy fires when compared to in-cabinet applications because of a number of 

contributing factors, which are identified above. Overall, the approach and information 

presented in this report provides the best available information on VEWFD system performance 

and PRA application.
xxi

ACKNOWLEDGMENTS

The authors acknowledge and appreciate the many people who contributed to this project. 

First, we acknowledge the contributions of the staff at the National Institutes of Standards and 

Technology, who not only supported the testing program, but evaluated and documented the 

results. In particular, the assistance from Mariousz Zarzecki and Mike Selepak is greatly 

appreciated. The authors would like to thank all of the individuals who supported site visits, 

meetings and phone interviews, including; 

Wayne Aho (Xtralis)

Grant Cherkas

(Canadian Nuclear Safety Commission)

Joelle DeJoseph (Duke Energy)

Keenan Dodson

(NASA Goddard Space Flight Center)

Jeff Ertman (Duke Energy)

Nick Gomez (System Sensor)

Ashley Lindeman

(Electric Power Research Institute)

Jim Lloyd (Safe Fire Detection)

Edward O’Connell

(NASA Goddard Space Flight Center)

Ron Robertson (Safe Fire Detection)

Robert Robertson (Safe Fire Detection)

Scot Robertson (Safe Fire Detection)

Sean Taylor (Exelon)

Chip Wellington

(NASA Goddard Space Flight Center)

The authors would also like to thank Montgomery County Public Service Training Academy for 

use of their facilities. U.S. Nuclear Regulatory Commission (NRC) staff members also 

supporting this research include Tammie Pennywell, David Stroup and Nathan Siu, and their 

support is greatly appreciated. Additionally, Eric Knowles and Woody Machalek must be 

acknowledged for several of the high quality graphics presented in this report. Jake Stefanick 

developed the early versions of the calculation tools included in the back of this report, and his 

support is greatly appreciated.

Acknowledgements must also be made to those individuals who provided feedback and 

constructive comments on the early draft versions of this report, including Christine LaFleur 

(Sandia National Laboratories); Harrold Barrett, Daniel Frumkin, Raymond Gallucci, Brian 

Metzger, of the NRC staff; and individuals from the Southwest Research Center under contract 

to the NRC. A special thanks to Anita Aikins-Afful at the NRC for her technical editing support.

Internal and external stakeholders provide comments and suggestions on the draft of this report 

when it was published in the Federal Register (80 FR 38755) on July 7, 2015. Those 

stakeholders who commented are listed and acknowledged below. Resolution of those 

comments is documented in the NRC Agencywide Documents Access and Management 

System (ADAMS) under Accession No. ML16280A343.

Victoria K. Anderson

(Nuclear Energy Institute)

Jeffery Ertman

(Duke Energy)

David T. Gudger

(Exelon Generation)

Meghan Housewright

(National Fire Protection Association)

Stuart Lewis

(Electric Power Research Institute)

Ron Robertson (Safe Fire Detection)

Dwight Wills (Building Reports)
xxii

A public workshop was held on April 26, 2016 at the NRC Headquarters Offices located in 

Rockville, Maryland, and simultaneously presented via a webinar. During this all-day workshop, 

participants provided valuable insights and feedback. The authors would like to acknowledge 

those who participated, including:

Victoria Anderson

(Nuclear Energy Institute)

Dennis Andrukat, (U.S. NRC)

Harold Barrett, (U.S. NRC)

Paul Bemis, (Pacific Gas and Electric)

Michael Birnkrant, 

(United Technologies Research Center)

Harrison Brown, 

(TVA Brown’s Ferry Nuclear Plant)

Greg Casto, (U.S. NRC)

Stan Chingo, (Duke Energy)

Daniel Davidson, (Southern Nuclear)

Joelle DeJoseph, (Jensen Hughes)

Josh Dinaburg, (Jensen Hughes)

Robert Dukes, (Enercon)

Jeffery Ertman, (Duke Energy)

Stephen Fleger, (U.S. NRC)

Daniel Frumkin, (U.S. NRC)

Nick Gomez, (System Sensor)

James Gregerson, 

(Pacific Gas and Electric)

Scott Groesbeck, (DP Engineering)

Kenneth Hamburger, (U.S. NRC)

Naiv Hughes, (U.S. NRC)

Mark Hulet, (APS)

JS Hyslop, (U.S. NRC)

Boemhee Jeong, (Southern Company)

Brian Krystek, 

(Engineering Planning and Management)

Greg Kvamme, (Xcel Energy)

Sawyer Lance, (Pacific Gas and Electric)

Ashley Lindeman, 

(Electric Power Research Institute)

James Lloyd, (Safe Fire Detection, Inc.)

Christopher Lord, 

(FENOC Beaver Valley Power Station)

Laroy Martin, (Dominion)

Shivani Mehta, (U.S. NRC)

Stephanie Morrow, (U.S. NRC)

Charles Moulton, (U.S. NRC)

Victor Ontiveros, (Jensen Hughes)

JongSeuk Park, (KINS, Korea)

Andrew Ratchford, (Jensen Hughes)

Robert Robertson, (Safe Fire Detection, Inc.)

Scot Robertson, (Safe Fire Detection, Inc.)

Frederick Ross, (TVA)

Mark Henry Salley, (U.S. NRC)

Thomas Shudak, 

(Nebraska Public Power District)

Brenda Simril, (TVA)

Philip Smith, (U.S. NRC)

Joseph Soscia, (Enercon)

Susumu Tsuchino, (JNRA)

Travis Weber

(Engineering Planning and Management)

Stephanie Weimer (Entergy)

Keith Williams (Duke Energy)

Mark Wishart (Jensen Hughes)

Boyee Wong (Pacific Gas and Electric)

Mark Wright, 

(Defense Nuclear Facilities Safety Board)

Joel Yurkosky, (Enercon)

Kiang Zee, (Jensen Hughes)
xxiii

A public meeting was held on September 20, 2016 at the NRC Headquarters Offices located in 

Rockville, Maryland. During this meeting the NRC staff provided feedback on a tabletop pilot 

exercise of NUREG-2180 performed by industry and additional concerns raised on the 

completeness, benefit to plant safety, need for an expert elicitation and removal of single 

component damage end state from the draft report. The authors would like to acknowledge 

those who participated, including:

Victoria Anderson

(Nuclear Energy Institute)

Harold Barrett (U.S. NRC)

Jana Bergman (Curtiss-Wright)

Pamela Burns (Southern Nuclear)

Greg Casto (U.S. NRC)

Thomas Collopy (FENOC)

Richard Correia (U.S. NRC)

Joelle DeJoseph (Jensen Hughes)

Jeffery Ertman (Duke Energy)

Jason Floyd (Jensen Hughes)

Michael Gartman (U.S. NRC)

John Giddens (Southern Nuclear)

Joseph Giitter (U.S. NRC)

Kenneth Hamburger (U.S. NRC)

JS Hyslop (U.S. NRC)

Boemhee Jeong (Southern Company)

Francisco Joglar (Jensen Hughes)

Robert Krsek (U.S. NRC)

Brian Krystek 

(Engineering Planning and Management)

Greg Kvamme (Xcel Energy)

Ashley Lindeman

(Electric Power Research Institute)

Sara Lyons (U.S. NRC)

David Miskiewicz

(Engineering Planning and Management)

Marty Murphy (Xcel Energy)

Joe Pollock (Nuclear Energy Institute)

Andrew Ratchford (Jensen Hughes)

Robert Rishel (Duke Energy)

Joseph Reed, (Nuclear Energy Institute)

Eric Ruesch, (U.S. NRC)

Mark Henry Salley, (U.S. NRC)

Harold Stiles, (Duke Energy)

David Stroup, (U.S. NRC)

Mike Testa, (FENOC)

Keith Vincent, (NextEra Energy)

Travis Weber, 

(Engineering Planning and Management)

Mark Wishart, (Jensen Hughes)

Kiang Zee, (Jensen Hughes)

xxv

ACRONYMS AND ABBREVIATIONS

ACH air changes per hour

ADAMS Agencywide Documents Access and Management System

AHJ authority having jurisdiction

AHU air handling unit

AMD arithmetic mass diameter

APCSB Auxiliary and Power Conversion Systems Branch

ARP alarm response procedure

ASD aspirating smoke detection (or detector)

ASD-CC cloud chamber aspirating smoke detector

ASD-LS light-scattering aspirating smoke detector

ASIC application specific integrated circuit

ASME American Society of Mechanical Engineers

AUO auxiliary unit operator

AW area-wide

AWG American wire gauge

BS British Standard

BSI British Standard Institution

BTP branch technical position

CBDTM cause based decision tree method

cd candela

CDF Core damage frequency

CFR Code of Federal Regulations

CPT control power transformer

CSA Canadian Standards Association

CSPE chlorosulfonated polyethylene

CVPC chlorinated polyvinyl chloride

DCRDR detailed control room design review

DI&C digital instrumentation and controls

EDG emergency diesel generator

ELPI electrical low pressure impactor

EOP emergency operating procedure

EOT end of test

EPRI Electric Power Research Institute

EWFD early warning fire detection

FACP fire alarm control panel

FAQ frequently asked question

FCC Federal Communications Commission

FIA Fire Institute Association

FM Factory Mutual

FO field operator

GL generic letter
xxvi

HCR/ORE human cognitive reliability/operator reliability experiments

HEAF high-energy arc fault

HEP human error probability

HEPA high efficiency particulate air

HF human factors

HFE human failure event

HRA human reliability analysis

HRP heating ramp period

HSI human-system interface

HVAC heating, ventilation, and air conditioning

I&C instrumentation and controls

ID inside diameter

IEEE Institute of Electrical and Electronic Engineers

IN information notice

ION ionization detector

IPEEE individual plant evaluations of external events

IR infrared radiation

IST in-service testing

IT information technology

LCS local control station

LED light emitting diode

LER licensee event report

LQ lower quartile

MCC motor control center

MCR main control room

MMD mass mean diameter

MOU memorandum of understanding

NASA National Aeronautics and Space Administration

NEI Nuclear Energy Institute

NFPA National Fire Protection Association

NIST National Institute of Standards and Technology

NPP nuclear power plant

NRC U.S. Nuclear Regulatory Commission

PCB printed circuit board

PHOTO photoelectric detector

PPE personal protective equipment

PRA probabilistic risk assessment

PVC polyvinyl chloride

QA quality assurance

RA return air

RES Office of Nuclear Regulatory Research

RoHS restriction of hazardous substances
xxvii

SDP significance determination process

SIS synthetic insulated switchboard

SPAR-H standardized plant analysis risk human reliability analysis

SRO senior reactor operator

SS sensitive spot detector

SSC systems, structures, and components

STA shift technician advisor

TB terminal block

THERP technique for human error rate prediction

THT total heating time

UL Underwriters Laboratories

ULC Underwriters Laboratories Canada

UQ upper quartile

VEWFD very early warning fire detection

XLPE cross-linked polyethylene

XLPO cross-linked polyolefin insulated 

1-1

1. INTRODUCTION

1.1 Overview

This report describes an evaluation of the performance of smoke detection systems configured 

as either conventional spot-type or very early warning, including aspirating smoke detection 

(ASD)1 systems, for use in nuclear power plant (NPP) applications. In addition to this 

evaluation, general information applicable to NPP installation is presented. This research is 

confirmatory in nature, such that its primary purpose is to evaluate the technical adequacy of a 

U.S. Nuclear Regulatory Commission (NRC) interim staff position documented in a National Fire 

Protection Association (NFPA) Standard 805 Frequently Asked Question (FAQ) 08-0046, 

“Incipient Fire Detection Systems.” FAQ 08-0046 provides an interim staff position on the use of 

ASD systems configured as very early warning fire detection (VEWFD) to protect electrical 

enclosures2 containing low-voltage control components found in U.S. NPPs. This research was 

funded and managed by the NRC Office of Nuclear Regulatory Research (RES). Testing was 

performed by the National Institute of Standards and Technology (NIST). In addition to the 

testing, staff from the NRC supported this project by conducting site visits, reviewing literature,

and evaluating human performance and smoke detection system performance as part of a fire 

risk scoping study.

The report is broken into three parts. Part I presents information gathered to develop a 

knowledge base for this project. Information contained in Part I includes presentation of 

fundamental smoke detection terminology and theory; a summary of operating experience and 

literature review; the experimental approach, basis and results. Part II evaluates the 

performance of smoke detection technologies in quantitative terms. This includes an overview 

of a fire risk scoping study (including assumptions and limitations), estimation of parameters 

used, timing analysis, human performance assessment, and an evaluation of the results for 

common NPP applications and comparisons to the interim staff position. Part III concludes the 

report and provides a summary, conclusions and future research recommendations, along with 

supporting information such as definitions of key terms and a list of references.

1.2 Need for Confirmatory Research

On March 31, 2008, FAQ 08-0046 “Incipient Fire Detection Systems” was proposed by the 

Nuclear Energy Institute (NEI) NFPA 805 Task Force to describe the treatment of VEWFD

systems in a fire probabilistic risk assessment (PRA), because guidance for the treatment of 

such a system with respect to hardware failure rates and its relationship with the EPRI 1011989 

(NUREG/CR-6850) Appendix P treatment of fire suppression was insufficient (Ref. 1). In an 

addendum on fire risk under its memorandum of understanding (MOU), the NRC-RES and the 

Electric Power Research Institute (EPRI) began developing guidance for determining the effect 

on the probability of non-suppression in fire areas that have these VEWFD systems installed. 

Before the conclusion of this work, EPRI published an interim report 1016735 titled, “Fire PRA 

Methods Enhancements (Additions, Clarifications, and Refinements to EPRI 1011989),” in 

December 2008. The EPRI report presented an interim methodology and guidance for fire 

 1 Definitions are presented in Section 16. 2 “Electrical enclosure,” “electrical cabinet,” and “electrical panel” are used synonymously in this report to 

mean a surrounding case or housing used to protect the contained equipment or prevent personnel from 

accidentally contacting live parts.
1-2

PRA, including re-evaluation of fire ignition frequency, a framework for quantifying incipient-fire 

detection systems in fire PRA, and treatment of large oil fires caused by main feed water 

pumps. Chapter 3 and Appendix C contain information pertaining to incipient detection 

systems. Although, EPRI was working with NRC-RES on many of these issues, the methods 

presented in the interim report were never endorsed by the NRC.

To improve accuracy and realism, and in an effort to close out FAQ 08-0046, the NRC staff took 

the EPRI approach and modified it to address several issues and conditions, and to develop an 

approach to evaluate the performance of ASD VEWFD systems in fire PRA applications. On 

June 24, 2009, the NRC released a draft interim position on FAQ 08-0046, regarding the use of 

VEWFD systems for use in NFPA 805 applications, on which the staff requested comments. 

The NRC staff reviewed all comments received on the draft interim position, and on November 

30, 2009, closed out the FAQ as the final interim staff position, which was later incorporated into 

NUREG/CR-6850, Supplement 1, “Fire Probabilistic Risk Assessment Methods Enhancements,”

dated September 2010.

Given the number of comments received on the draft interim position and the authors’

discussions with knowledgeable individuals from both the industry and the regulatory 

arenas/sides, vastly differing views regarding these systems’ performance, and suitable 

application in fire PRA, were apparent. Notably, both empirical data and operating experience 

in NPP applications are scarce; additionally, terminology is commonly used inconsistently. 

Because of these difficulties, the NRC identified a need to obtain a better understanding of 

these systems’ performance and their operating experience(s). Thus, the NRC began a 

confirmatory research program to address the objectives identified below.

1.3 Purpose and Objectives

The research completed by NRC and NIST staff as documented in this report provides an

assessment on the use of smoke detection systems in NPP applications. This research focuses 

on the use of these systems in risk-informed performance-based applications.

The objectives of this report are as follows:

A. To evaluate the effectiveness of smoke detection systems

o This includes an evaluation of in-cabinet and area-wide applications.

B. To compare the performance of common smoke detection systems currently used in 

NPPs to VEWFD systems

C. To evaluate the response and effectiveness of equipment used to locate a pre-fire 

source(s) through the use of human reliability analysis (HRA)

D. To evaluate ASD availability and reliability

E. To evaluate smoke detection system response to common products of combustion 

applicable to NPPs

F. To evaluate electrical cabinet layout and design effect on smoke detection system 

response
1-3

G. To evaluate the performance of smoke detection technologies in various applications, 

including in-cabinet and area-wide

o The evaluation should support fire PRA applications and provide a technical 

basis and approach for updating the interim approach described in FAQ 08-0046, 

“Incipient Fire Detection Systems.”

1.4 General Approach and Project History

To achieve the stated purpose and objectives, this confirmatory research project was broken 

down into three distinct areas: review of literature, operating experience, and testing. Each area 

has its own subtasks, as shown in Figure 1-1. These three areas support the risk scoping 

study, as well as providing input to the human performance evaluation.

Figure 1-1. Illustration of VEWFD System Confirmatory Research Project

Early in the project, staff from NIST reviewed available literature on ASD VEWFD systems 

to support development of a test plan. Once the literature review and the majority of the site 

visits were complete, NIST developed a draft test plan that focused on providing data to address 

objectives A, B, E, and F. Upon finalizing the test plan, systems and materials were procured 

and testing commenced.

Once the literature, test data, operator response characteristics, and operating experience were 

understood, a risk scoping study was completed to evaluate the performance of these systems. 

NRC VEWFD System Confirmatory 

Research Program

Fire Events Database

Site Visits

Procedure Review

Plant Personnel 

Inteviews

Operating Experience Testing

3 Physical Scales

5 ASDs systems

3 Spot detector types

Aerosol measurements

In-Cabinet & Area Wide 

Applications

Literature Review

Standards, Codes of 

Practice

Test Reports

Journal Articles

Vendor Information Variations in 

- Materials 

- Heating rate

- Ventilation

VEWFD System Risk Benefit Quantification
1-4

Project History

This project took over five years to complete. To help understand the elements, complexity, and 

interactions of this project, a short history is provided.

The need for this work developed out of the initial NFPA 805 pilot plant application and the

desire to characterize the benefit of having VEWFD systems and their effect on the fire PRA 

quantification. In 2008, frequently asked question (FAQ) 08-0046 was proposed by the Nuclear 

Energy Institute (NEI), through its NFPA 805 Task Force to seek additional guidance on 

modeling the use of VEWFD systems in fire PRAs. Later that same year, EPRI 1016735 titled, 

“Fire PRA Methods Enhancements (Additions, Clarifications, and Refinements to 

EPRI 1011989),” was issued. Based on this industry report, discussions between NRC staff 

and industry, and the beliefs at the time of these systems performance, an interim staff position 

was issued in 2009 (see FAQ 08-0046 closure memo). The interim guidance was based on 

vendor expectation and consensus standard performance objectives. A lack of applicable data

was apparent. In 2010, a user need request was transmitted from the Office of Nuclear Reactor 

Regulation requesting confirmatory research in this area. The Office of Nuclear Regulatory

Research (RES) contracted with NIST in 2011 to perform testing to support addressing several 

of the user need request objectives. That same year, NPP sites that have VEWFD systems 

installed were identified and site visits were planned, including the development of a list of 

questions that was reviewed and commented on by the user need office. 

Early in 2012, several USA and Canadian NPP sites and a National Aeronatutic and Space 

Administration (NASA) site were visited to understand how these systems were being used and 

to obtain operating experience to support the objectives of this project. Site visits were 

conducted at the following facilities:

• NASA Goddard Space Flight Center (Maryland)

• Shearon Harris Nuclear Power Plant (North Carolina)

• H.B. Robinson Nuclear Generating Station (South Carolina)

• Bruce Nuclear Generating Station (Ontario, Canada)

• Darlington Nuclear Generating Station (Ontario, Canada)

• Pickering Nuclear Generating Station (Ontario, Canada)

• Three Mile Island Nuclear Station (Pennsylvania)

• Operator response questionnaire (through the Electric Power Research Institute)

By the middle of 2012 a draft test plan was shared with the user need office and comments 

were received and incorporated as appropriate. The small-scale laboratory and small room 

testing were completed by the end of the year. 

In 2013, the preliminary results from the testing indicated that for in-cabinet applications, there 

were little performance differences between spot-type detection (ionization) and ASD VEWFD 

systems. This preliminary information was communicated to the NRC stakeholders during a 

public teleconference held on March 21, 2013 (Agencywide Documents Access and 

Management System (ADAMS) Accession No. ML13080A166). After the exchange of 

preliminary information, the user need office requested larger scale testing and testing of multizone ASD VEWFD systems. It also was requested that RES engage the ASD vendors and 

EPRI to better understand the test program and the results. Two meetings were held with these

external parties. The first meeting occurred on May 16, 2013 between the NRC staff and 

vendors of ASD VEWFD systems equipment being tested. The second meeting occurred on 
1-5

July 24, 2013, between the NRC and EPRI. Both meetings were considered information 

exchanges, and were used to receive additional feedback on the project approach, specifically 

testing. The user need office staff was in attendance during both meetings. The NRC 

presentations are available electronically (ADAMS, Accession No ML14356A581). The test 

plan was shared with the user need office and EPRI for review and comment. Those comments 

were incorporated as appropriate and testing commenced in the fall of 2013, however a delay of

approximately 6 weeks was experienced because of a lack of appropriations (i.e., government 

shutdown). The larger room testing finished in early 2014.

In March 2014, the data from all testing was received, and the NRC staff began data processing 

and analysis tasks to support the risk scoping study documented in Part II of this report. In May, 

a high-level description of the risk scoping study approach was presented during a routine user 

need meeting. A question related to this research was asked during a June Commission 

meeting on NFPA 805 and staff from the user need office responded. A follow-on briefing for 

Commissioner Magwood was requested and provided, with other Commissioner staff in 

attendance. The first draft of the report was transmitted to the user need office on August 1, 

with a two part presentation of the reported provided to the user need office that same month. 

High level comments on the first draft were provided to RES staff by the end of August and a 

revised version of the report was returned to the user need office on November 2. In late 

November 2014 UNR office comments were received on the second draft report and a request 

to have senior staff meet to thoroughly review the data that were used to estimate one of the 

parameters in the risk scoping study approach. A consensus on the disposition of the lowvoltage control operating experience was reached in December and the intent to publish memo 

was routed before the end of the year. 

In early 2015, the user need office requested that the draft report be sent to EPRI for review and 

comment. EPRI comments were received January 21. After a January 29 FAQ call, it was 

clarified that the user need office sought an extended EPRI review; however, RES disagreed 

because publishing this report as a draft document for public comment served the same 

purpose. As a compromise, RES agreed to transmit to EPRI the questions that were included in 

the Federal Register (80 FR 38755) and have EPRI seek for responses from the industry to 

provide additional operating experience to better inform the report prior to issuance for public 

comment. The questions were transmitted to EPRI in February and by the end of March, no 

responses were provided. The draft report was issued for a 60-day public comment on July 7, 

2015. 

The public comment period closed on September 8, 2015 and approximately 200 comments on 

the report were received. In addition to the public comments, additional comments from the 

user need office and its contractor were received. By early November most of the comments 

were resolved and communicated to the user need office. In addition, NEI identified in its cover 

letter that “...industry has some very recent operating experience with these systems that could 

provide useful input to this report. Once a complete report on the operating experience is 

available, the industry will provide this to the NRC for consideration in the final version of this 

NUREG.” In mid-October operating experience from Shearon Harris was provided. Upon 

reviewing the information and after discussions with the user need request office, it was 

determined that the information provided was not complete and a site visit could facilitate a 

better understanding of this recent operating experience for incorporation into the final report. 

The site visit occurred on January 13, 2016 and the draft-final version of this report started the 

publication process in early February 2016.
1-6

Subsequent discussions with the user need office indicated that holding a public workshop to 

communicate the final results from this work, discussing changes to the report between the draft 

and final versions, and presenting the event tree non-suppression probability estimation tool 

would be of value. A public workshop was held on April 26, 2016, with 55 participants in 

attendance. In June of 2016, NEI requested that a tabletop pilot of the risk quantification 

approach be performed prior. Those results were provided to the NRC in July and a public 

meeting was held on September 20, 2016 to communicate the NRC feedback on the tabletop 

pilot along with the staffs’ response to additional comments provided by NEI. The following 

provides a high level milestone timeline of this project.

2010

- User need request (June)

2011

- NIST contract in place (July)

2012

- Site visits conducted (March – May)

- Small scale testing (summer)

- Full scale testing (November – December)

NRR/EPRI observe testing at Montgomery County Police Training Academy

2013

- Preliminary results presented during Fire PRA Methods and FAQ call with NEI.

- Meeting with ASD venders (May)

- Meeting with EPRI (July)

- Requested to test multi-zone ASD

- Government shutdown

- Full scale testing multi-zone (November – December)

NRR/EPRI observe testing at Hughes Associates Inc.

2014

- Data received from NIST (March)

- Commission meeting on NFPA 805 (June)

- First draft report to user need office (August)

- Second draft report to user need office (November)

- All NRC staff comments resolved and draft report finalized for issuance (December)

2015

- User need office requests EPRI be provided opportunity to review and comment 

(January)

- EPRI comments received (January)

- User need office requests additional feedback from EPRI (January)

- Report formally issued draft for public comment (80 FR 38755, July)

- Comment period closed (September)

- NEI provided additional operating experience received (October)

- User need office requests site visit to better understand recent operating experience

- Public comments resolved (November)
1-7

2016

- Site visit conducted (January)

- Data collection standardization sheet developed (January)

- Report finalized for final issuance (February)

- NEI requests tabletop pilot exercise be performed (June)

- Tabletop results provided to NRC (July)

- Public meeting to provide NRC feedback on tabletop results and response to additional 

NEI comments

1.5 Scope of this Report

This report provides information on ASD and spot-type systems configured for VEWFD and 

conventional spot-type smoke detector performance in various NPP applications, with a focus 

on their response to low-energy fires during the early stages (pre-flaming). The potential risk 

benefits from using these systems and associated operator response characteristics are also 

provided. The focus on these two types of systems was directed by the regulatory need and 

does not represent any determination that these are the only fire detection methods suitable for 

NPP applications. Additionally, this report does not explore negative impact from using 

ASD systems such as whether single failure potential is increased, configurations where other 

detections systems perform better, or any significant risks associated with inappropriate 

equipment de-energization.

This report specifically focuses on evaluating ASD VEWFD systems’ ability to detect electrical 

enclosure fires during low-energy incipient stage fire conditions. Electrical enclosures are 

defined as items such as switchgears; motor control centers; direct current (dc) distribution 

panels; relay cabinets; control and switch panels, (excluding panels that are part of machinery);

fire protection panels, etc. Voltages in electrical enclosures vary from low voltage to 6.9kV 

switchgear. Although other types of equipment found in NPPs are likely to have equipment 

failure modes which exhibit an incipient stage of sufficient duration to allow for enhanced 

operator response, this report does not provide an evaluation of ASD VEWFD performance to 

protect those other types of equipment.

1.6 Report Organization and How to Use This Report

This report is broken into three parts. Part I contains a collection of supporting information 

associated with smoke characteristics, detection technologies, operating experience and 

presentation of the experimental program approach and results. Part II presents an approach

for quantifying the performance of smoke detection in fire PRA applications. Part III provides 

report summary, conclusions, definitions and references. Each part is organized as follows:

PART I

• Section 2 provides general background information on fire dynamics; smoke detection 

principles; system performance measures; and the importance of quality assurance,

inspection, testing, and maintenance programs. 

• Section 3 presents a review of operating experience associated with VEWFD systems, 

NPP use of these systems, and information obtained during site visits. An overview of 
1-8

national consensus standards, listing and approval standards and information found in 

codes of practice is also provided. A literature review summary is also presented.

• Section 4 describes the experimental approach taken to address the objectives of this 

project. Included in this section are descriptions of the detectors, incipient fire source, 

instrumentation, test facilities, test protocols and experimental design.

• Section 5 documents the test results obtained and presents them graphically. 

Characteristics of the incipient fire source with regard to heat conduction and ignition 

potential are also presented. The last subsection presents the results in a format to 

support the scoping risk study documented in Part II.

PART II

• Section 6 presents a summary of previous efforts used to quantify the performance of 

ASD VEWFD systems in fire PRA. An overview of the model used in this project is also 

presented.

• Sections 7–11 provides a basis for estimating the parameters of the model presented in 

Section 6.

• Section 12 presents illustrative examples using the model and parameters developed in 

this project to quantify the performance of various smoke detection technologies.

• Section 13 presents assumptions and limitations of the risk scoping study.

PART III

• Section 14 presents a summary from the findings of this project and conclusions.

• Section 15 identifies recommendations for future research

• Section 16 provides definitions for terms commonly used in report

• Section 17 provides a list of references

• Appendices A–H contain supporting information including; view graphs from meetings 

with vendors, experimental data, human performance, operating experience, literature 

reviewed, quick reference for risk scoping study parameters, VEWFD data collection 

sheet, and user guides for the event tree non-suppression probability estimation tool.
PART I

Knowledge Base

2-1

2. FUNDAMENTALS OF SMOKE GENERATION AND FIRE 

DETECTION TECHNOLOGIES 

Success in limiting or even preventing fire damage is dependent on the rate of fire development. 

The earlier a fire is detected the sooner fire suppression activities can be initiated to reduce the 

likelihood of damage to equipment. This section provides an overview of the fundamental fire 

science underlying the performance of fire detection systems and key definitions of the fire 

stages used throughout this report. Included in this discussion are a generalized representation 

of fire growth and fire classification, fire byproduct generation and the principles of smoke 

detection. The fundamental information presented here supports assessing smoke detection 

system performance and quantifying the use of these systems in fire probabilistic risk 

assessments. 

2.1 Background

Very early warning fire detection (VEWFD) is defined in National Fire Protection Association 

(NFPA) 76, “Standard for Fire Protection of Telecommunication Facilities,” as systems that 

detect low-energy fires before the fire conditions threaten telecommunications service. VEWFD

systems are used extensively in the telecommunications industry in area-wide applications to 

meet the intent of NFPA 76, protect high value or mission critical contents and limit interruption 

of services. Their extensive use in mission critical and telecommunications industries is a result 

of smoke damage being the biggest risk to electrical equipment, not fire. Telecommunications

facilities also find VEWFD systems useful because of the high air exchange rates needed to 

cool electronic equipment, whereas conventional spot-type smoke detector performance is 

degraded because of smoke dilution. Most Canadian and some U.S. nuclear power plants 

(NPPs) also use some form of air aspirated VEWFD systems to reduce risk and provide 

advanced warning of fire conditions. In general, VEWFD systems are finding wide applications 

in a variety of other industries, especially in performance-based design.

Air aspirated (sampling-type) smoke detectors are commonly used to meet the NFPA 76 

requirements for VEWFD systems. These ASD VEWFD systems actively sample air from the 

protected space and transport the air samples through a smooth bore piping network back to a 

centralized detector unit where the air samples are monitored for combustion-based products 

(in accordance with either light-scattering or cloud chamber smoke detection principles). An 

illustration of such a system is shown in Figure 2-1. These systems have the potential to 

provide numerous advantages over conventional systems. However, their difference from 

conventional spot-type detectors presents several challenges to successful implementation and 

proper quantification of any risk improvements in fire PRA.
2-2

Figure 2-1. Illustration of ASD system in an in-cabinet application

ASD VEWFD systems have been used at several U.S. NPPs (e.g., Three Mile Island Nuclear 

Station, H.B. Robinson Steam Electric Plant, Clinton Power Station) for over a decade as a 

measure to reduce fire risk contributors identified during the individual plant examinations of 

external events (IPEEEs) or for enhanced fire detection means to support exemptions (Ref. 2 

and 3). However, only recently has there been an interest to use these systems in the 

regulatory context in fire PRAs, to support the application of NFPA Standard 805, “PerformanceBased Standard for Fire Protection for Light Water Reactor Electric Generating Plants,” 2001 

Edition. The performance objective for using these VEWFD systems is to provide earlier 

notification to plant personnel that may allow for additional time for human intervention before

fire conditions threaten reactor safety. However, these initiating devices could also be used to 

initiate heating, ventilation, and air conditioning (HVAC) changes or to automatically initiate a 

suppression system. Hypothetically, these systems could even be used to automatically deenergize the electrical equipment which they are protecting, reducing the likelihood of any 

potential fire threat, without human intervention. Operating experience has also indicated that 

the benefit from using these systems may extend beyond fire risk reductions. In some 

instances, these systems may be capable of detection component failures that if left unattended 

could result in extended system or plant down time.

NFPA-805 Frequently Asked Question (FAQ) 08-0046 (Ref. 4), was later incorporated into

Supplement 1 to NUREG/CR-6850/EPRI 1019259. NFPA-805 FAQ 08-0046 provided an 

interim staff position on questions raised by the pilot plants during their transition to NFPA 805. 

Section 13 of Supplement 1 titled, “Incipient Fire Detection Systems,” provides an interim 

position for determining the non-suppression probability for fire scenarios that have installed 
2-3

incipient fire detection systems.1 Because of the lack of information and test data, the interim 

staff position limited the applicability of VEWFD systems with regard to quantifying these 

systems in fire PRA. 

Following the issuance of FAQ 08-0046, “Incipient Fire Detection Systems,” the U.S. Nuclear 

Regulatory Commission (NRC) initiated a research program along with confirmatory testing at 

NIST to ensure the interim position is technically adequate. This report documents that 

research.

2.1.1 Fire protection defense-in-depth

A fundamental understanding of the concept of defense-in-depth will be important later when 

evaluating the entire fire protection safety performance objectives. Fire protection programs at 

U.S. NPPs must ensure that both the probability of occurrence and consequences of fire and 

explosions are minimized. To achieve the required level of fire safety, licensees use the 

concept of defense-in-depth to provide echelons of protection from fire effects. This concept 

was first introduced in NRC Branch Technical Position, Auxiliary and Power Conversion 

Systems Branch 9.5-1 (BTP APCSB 9.5-1) as a result of Browns Ferry Special Review Group 

recommendations (NUREG-0050). Subsequently, defense-in-depth for fire protection is a 

design concept applicable to deterministic [Sections 50.48(a) and (b) of Title 10 of the Code of 

Federal Regulations (10 CFR)] and performance-based [10 CFR 50.48(c)] fire protection plans. 

The three echelons of defense-in-depth related to fire protection are:

a. Preventing fires from starting.

b. Detecting fires quickly, suppressing those fires that occur, putting them out quickly and 

limiting their damage.

c. Designing plant safety systems such that if a fire does get started in spite of the fire 

prevention program, and burns for a considerable time, in spite of fire protection 

activities, it will not prevent essential plant safety functions from being performed.

VEWFD systems partially support the second echelon by providing a means of quickly detecting 

fires. Because VEWFD systems support defense-in-depth, there have been differing views on 

the role of VEWFD systems in performance-based fire protection programs, leading to 

complexity in the evaluation of these systems’ performance in a fire PRA.

2.2 Dynamics of Fire Stages

A fire development profile is typically discussed in terms of “fire stages.” These are commonly

referred to as the “incipient,” “growth,” “steady-state,” and “decay” stages as illustrated in 

Figure 2-2 (Ref. 5). This idealized representation provides a foundation for understanding the 

various fire stages; however, the shape and, more importantly, the duration and transition point 

of each stage are scenario dependent. Having a clear definition and understanding of the

 1 As a matter of clarification, the term incipient fire detection system will not be used in this report. Instead,

the term very early warning fire detection (VEWFD) systems will be used. The use of the terms VEWFD is 

to reduce any confusion with regard to regulatory applications where licensees have installed conventional 

non-VEWFD SYSTEMS spot-type detectors in cabinets or other areas and classified these detectors as 

incipient detection in licensing documentation. 
2-4

incipient stage and transition point as it relates to performance-based methods, is paramount to 

the research performed under this project.

Figure 2-2. Fire stages

The incipient stage includes the preheating, gasification and smoldering phases, which are all 

stages before flaming combustion. The preheating phase is the process of heating combustible 

materials to a point where gasification begins. As combustible material continues to heat up, it 

is decomposed, or broken down into more simple molecular compounds; this stage is known as 

pyrolysis. Smoldering is a slow, low-temperature, flameless form of combustion, sustained by 

the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel (Ref. 6). 

True self-sustaining smoldering conditions will not occur for many of the materials of interest in 

electrical enclosures (Ref. 7).

A common fire scenario includes an initial pre-heat phase, typically followed by a pyrolysis 

phase, then followed by a transition to either smoldering or flaming conditions. In electrical 

enclosures, the preheating phase could start as a result of circuit, component, or 

inter-connecting electrical conductor failure, or by some other mode. Regardless of how the 

initial degradation begins, a source of energy to cause the preheating is needed to initiate the 

potential fire scenario. Once sufficient concentrations of combustible material are present 

during the gasification phase, electrical energy within the electrical enclosures provides the 

potential ignition source to end the incipient stage.

There are also variations on this prototypical fire scenario. The degradation mechanism may 

not continue to progress in severity. For example, the pyrolysis phase may be reached, but 

sufficient vapor may not be evolved to support combustion, or an ignition source may not be 

present at a physical location where flammable concentrations are present. Alternatively,

component degradation could begin to decrease in severity before the ignition and fire growth 

stage. Although these cases may produce considerable combustion products, the heat output 

could be relatively low, and thus, not all situations involving an incipient stage actually result in a 

fire.

Preheating

Pyrolysis

Established Burning 

(Sustained Ignition)

Flame Spread

Fully Developed

Extinction

Fire Growth

Smoldering (material dependant)

Incipient Steady State Decay
2-5

For the purposes of this report, “ignition” will be defined as the point where “self-sustained

flaming combustion is initiated.” This definition of ignition corresponds to the start of the growth 

phase as depicted in NUREG/CR-6850. The logic for using this definition will become apparent 

later on when the risk scoping study structure is presented, showing the dependency between 

fire-initiating events that are risk-significant and are counted in the fire initiating frequency, and

the fraction of those fires which exhibit an incipient stage of sufficient duration to support 

enhanced suppression capabilities. 

In addition to understanding the stages of a fire, it should also be emphasized that different fire 

growth profiles have been defined. Figure 2-3 illustrates common fire growth profiles across 

various electrical enclosure heat release rate categories, as found in performance-based 

designs, such as the slow, medium, fast and ultrafast growth profiles as presented in fire 

protection literature (Ref. 5 and 8) and NUREG/CR-6850 Appendix G. Notably, the growth 

profiles of actual fires that occur in NPPs will vary, and are functions of the component failure 

mode and configuration of combustibles; simply, just as the fire growth profiles are variable, the

incipient stage duration can vary dramatically as well (Ref. 9).

Figure 2-3. Illustration of several performance-based t-squared fire growth profiles 

(Note: incipient, steady-state, and decay stages not shown)

Alternative definitions

For completeness, it should also be mentioned that there are alternative definitions of an 

incipient stage. For instance, Heskestad and Yao define three fire stages for solid materials 

with an input heat source of relatively low-energy (Ref. 10). The three phases included 

“incipient,” “smoldering,” and “flaming” stages of a fire. Here, Heskestad and Yao identified 

ignition as the point where smoldering combustion starts. Therefore, they did not consider 

smoldering combustion to be included in the incipient stage.

Even within the same reference material, definitions differ; for example, the NFPA Fire 

Protection Handbook commonly cites an incipient fire as one that can be extinguished by 
2-6

portable fire suppression equipment (Ref. 5). If this definition were used to classify fires that 

have occurred in NPP, all but a very small fraction of fires would be classified as incipient fires. 

In the same NFPA reference, in the section discussing smoke detection, the incipient fire is 

defined as a stage when there is smoldering, but insufficient flaming to achieve established 

burning. For this project, the latter definition is used, because it has a stronger physical 

connection to the observed fire phenomena, and is not based solely on the success of human 

intervention to suppress fires.

2.3 Aerosol Generation

Fire signature response

From its inception, fire produces a variety of changes to the ambient conditions within the fire 

environment. These changes are referred to as “fire signatures” and have the potential to be

measured by detection systems. Examples of the fire signatures include aerosol (commonly 

referred to as smoke), energy release, gas, and transport fire signatures. For a specific fire 

signature to be of value, a measurable change in ambient condition is required, and that 

magnitude change (“the signal”) must be greater than the normal background variations (“the 

noise”). Thus, the preferred fire signature for a specific application will be that which generates

the highest signal-to-noise ratio in the earliest period of fire development (Ref. 11). Proper 

application of fire detection technologies requires an understanding of the fire conditions for 

which detection system response is required.

Aerosols are the type of fire signatures that can be detectable by smoke detectors, and will be 

studied exclusively for the purpose of this research. Smoke is defined as, “the airborne solid 

and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, 

together with the quantity of air that is entrained or otherwise mixed into the mass” (Ref. 6). 

Some ASD vendors do sell components for their systems that have the capability of detecting 

gas signatures; the performance of ASDs used in the latter application will not be evaluated in 

this report.

Aerosols are classified as solid and liquid particles ranging in size from 5×10-04 to 10 

micrometers (μm) and suspended in air. The characteristics of the aerosol are a function of the 

source material (source composition), combustion stage (incipient, smoldering or flaming), and 

amount of dilution with air, coagulation from Brownian motion, and surface deposition. These

factors play an important role in determining the chemical composition, refractive index, particle 

size distribution, and concentration. The characteristics of the aerosol play an important role in 

the response of the detector because specific characteristics will affect sensing technologies

differently. For instance, flaming fires tend to produce smokes that have a large fraction of 

sub-micron particles that tend to absorb a greater fraction of incident light than the fraction 

scattered, while smokes from smoldering fires tend to have a larger fraction of particles 

micrometer sized or greater, and they tend to scatter more incident light than the fraction 

absorbed. Based primarily on these factors, an ionization type detector is better suited for 

detecting flaming fires and a photoelectric type detector is better suited for detecting smoldering 

fires. Because of these differences, combination detectors have been developed and are 

available on the commercial market (Ref. 12).
2-7

2.4 Smoke Detection Principles

Reliable fire detection is an essential part of the fire protection program in NPPs. The use of 

smoke detectors is common because they typically detect fires before heat detectors and 

sprinkler activation. The NFPA Glossary of Terms defines the following types of smoke 

detectors (Ref. 13):

Cloud Chamber Smoke Detection:

The principle of using an air sample drawn from the protected area into a high-humidity 

chamber combined with a lowering of chamber pressure to create an environment in which 

the resultant moisture in the air condenses on any smoke particles present, forming a cloud.

The cloud density is then measured by a photoelectric principle. The density signal is 

processed and used to convey an alarm condition when it meets preset criteria.

Ionization Smoke Detection:

The principle of using a small amount of radioactive material to ionize the air between two 

differentially charged electrodes to sense the presence of smoke particles. Smoke particles 

entering the ionization volume decrease the conductance of the air by reducing ion mobility.

The reduced conductance signal is processed and used to convey an alarm condition when 

it meets preset criteria.

Photoelectric Light Obscuration Smoke Detection:

The principle of using a light source and a photosensitive sensor onto which the principal 

portion of the source emissions is focused. When smoke particles enter the light path, some 

of the light is scattered and some is absorbed, thereby reducing the light reaching the 

receiving sensor. The light reduction signal is processed and used to convey an alarm 

condition when it meets preset criteria.

Photoelectric Light-Scattering Smoke Detection:

The principle of using a light source and a photosensitive sensor arranged so that the rays 

from the light source do not fall (as normal) onto the photosensitive sensor. When smoke 

particles enter the light path, some of the light is scattered by reflection and refraction onto 

the sensor. The light signal is processed and used to convey an alarm condition when it 

meets preset criteria.

Video Image Smoke Detection:

The principle of using automatic analysis of real-time video images to detect the presence of 

smoke.

In NPP applications, smoke detectors have traditionally been considered best-suited for fire 

detection in spaces with physical barriers where rapid heat generation and smoke confinement 

can be expected in the event of a fire. The purpose of these systems is to provide early warning 

to building occupants, and rapid notification of the fire brigade. Some detection devices will also 

perform the function of automatically actuating suppression systems, and interfacing with other 

building systems such as HVAC. Advancements in electronics have aided in the improvement 

of the smoke detector signal processing, allowing for algorithms to be developed to reduce 

nuisance alarms from non-fire combustion products.

For room fire detection, smoke detectors have typically been placed in the uppermost space of 

the protected area. This placement assumes a growing or high-energy (steady-state) fire in 

which the energy released causes a strong buoyant plume to force the products of combustion 
2-8

upward and outward along the horizontal ceiling where the detectors are located. For 

low-energy fires, characterized by low temperatures and relatively small amounts of combustion

products, the plume strength may not be sufficient to transport the products of combustion to the 

uppermost level where the detectors are located. In addition, if the room has a vertical 

temperature gradient (on the order of a few degrees Celsius) such a weak plume could result in 

stratification of the smoke (Ref. 14).

2.4.1 Smoke characteristics

Smoke production of a given fuel will vary with the type of fuel; mode of combustion; size of fuel 

package; arrangement; physical configuration; material moisture content; and ignition input 

energy. The earliest indication of a fire occurrence usually involves the heating of materials 

during the pre-ignition (incipient) stages, which initially produces (through pyrolysis) submicron 

particles ranging in size from 5×10-04 to 1×10-03 micrometers2 (Ref. 11). Under ambient 

conditions, particles of this size are normally found in concentrations from several thousand per 

cubic centimeter to several hundred thousand per cubic centimeter. Incipient stage conditions 

can raise the sub-micrometer particle concentration sufficiently above the background levels 

(noise) to be used as a fire detection signal. As a reference point, a match flame can produce 

ten million particles per cubic centimeter (Ref. 15). 

The size of the particle produced by diffusion flame combustion3 also varies with the heating of 

the air and the development of the fire progressing to flaming combustion. Large particles are 

formed by coagulation, with the particle size distribution varying between 0.1 micrometer and 

4.0 micrometers. The smaller particles below 0.1 micrometer tend to disappear as a result of 

the formation of larger particles by coagulation, while the larger particles tend to settle out 

through the process of sedimentation (Ref. 15). Both of these properties contribute to smoke 

aging.

The performance of ASD systems exposed to smoke is dependent on the particular ASD 

technology (i.e., light-scattering vs. cloud chamber), because the ASD technologies respond 

differently to varying particle sizes and particle concentrations. For instance, cloud chamber 

technology is more sensitive to particle concentration and less sensitive to particle size. This is 

because the particles act as a condensation nucleus when the cloud is formed, and the 

response of the system is similar whether 100 large particles or 100 small particles are present. 

Light-scattering ASD technology requires the particles to be of sufficient size to scatter light. In 

this sense, the system response would differ between 100 large particles (more light-scattering

occurs) and 100 small particles (less light-scattering occurs). Smoke detector response 

depends on detector type and accumulation of smoke particulate within the sensing chamber. 

In addition to the differences among detector technology, the performance within a technology 

may differ because of the design and characteristics of the detector. For instance, 

light-scattering based technologies may use forward scattering, back scattering or a 

combination, along with employing different wavelengths of light sources. These design 

variations result in variable performance levels. Consequently, the motivation for choosing one 

ASD technology over another for use in NPP applications is not clear. The test results 

documented in this report confirm this statement (see Section 5).

 2 This range is representative of the typical particle sizes observed during the early stages of pyrolysis. 

Larger particles sizes outside of this range are likely and dependent on materials and/or smoldering 

combustion. 3 Diffusion flame combustion refers to a mode of combustion where fuel and air mix or diffuse together at the 

region of combustion.
2-9

2.4.2 Smoke properties

The Lambert-Beer Law (also known as Bouguer’s law) provides an expression for the light 

intensity reduction caused by smoke. The Lambert-Beer Law is shown mathematically as:

II = II0 ∙ exp(−κκκκκκ)

where: I = intensity of transmitted monochromatic light over pathlength d, (cd)

I0 = initial intensity of monochromatic light (cd)

κ = extinction coefficient, (m2/g)

C = mass concentration of smoke particles, (g/m3) and

d = pathlength of the optical beam passing through smoke, (m).

The use of this law allows for the development of a parameter known as optical density (OD)

per unit length (meter or foot),

OOOO = 1

llllll10 �

II0

II

� = κκκκ

2.303

Obscuration is the effect that smoke has on reducing visibility. Higher smoke concentrations 

result in higher obscuration, which results in lower visibility. Light obscuration in percent is 

defined as:

II0 − II

II0

∙ 100

Obscuration is the standard definition of smoke detector sensitivity in the fire protection industry 

today. Detector sensitivity is reported in units of percent obscuration per unit length (e.g., %/m

obscuration or %/ft obscuration). Percent obscuration per unit of length (meter or foot) is shown 

mathematically as:

�1 − �

II

II0

�

1

dd

� ∙ 100

2.4.3 Spot-type detectors

Most of the devices associated with conventional fire detection are typically located near the 

ceiling surfaces of NPP compartments. In the event of a fire, hot gases in the buoyancy-driven 

fire plume rise directly above the burning fuel, and impinge upon the ceiling. The ceiling surface 

causes the flow to turn and move horizontally beneath the ceiling, to other areas of the room,

located some distance from the fire. The response of detection devices installed below the 

ceiling, submerged in this hot flow of combustible products, provides the basis for the 

construction of active fire detection features.

The response of conventional spot-type (also referred to as “point type”) detectors like those 

shown in Figure 2-4, are influenced by several parameters. Smoke characteristics, smoke 

transport and detector characteristics are the predominant factors that influence detector 

response. The performance of spot-type smoke detectors is also dependent on fire-induced 
2-10

flow velocities near the detector. Typically, spot-type smoke detectors operate on two types of 

detection principle: ionization or photoelectric.

Figure 2-4. Image of conventional spot-type detectors

Conventional photoelectric spots, laser spots and the non-cloud chamber ASDs all sense 

scattered light. In the photoelectric spot the beam is an infrared radiation (IR) diode, in the laser 

spot it’s a diode laser, and in ASDs it could be either. All have a detector located at some fixed 

angle from the beam. PHOTO will be used throughout this report when referring to the 

conventional spot-type photoelectric detector. ION will be used throughout this report when 

referring to the conventional spot-type ionization detector. SS will be used throughout this 

report to when referring to the sensitive spot-type detector used in testing that was configured to 

the VEWFD sensitivities of NFPA 76.

2.4.4 Aspirating smoke detectors

ASDs, also known as air sampling-type detectors, provide a means of smoke detection that

actively draws air samples from the protected space through a network of sampling pipes into a 

centrally located smoke detector unit. Figure 2-5 and Figure 2-6 provide illustrations of an ASD 

in-cabinet and area-wide application, respectively. These figures show the detector unit, 

smooth bore pipe network with two zone and sample ports.
2-11

Figure 2-5. Illustration of ASD in-cabinet application

Figure 2-6. Illustration of an ASD area-wide application

Zones 

(2 shown)

Sampling Ports

(not shown)

Detector Unit
2-12

Two types of detector unit technology are commonly used in ASDs; one is based on the lightscattering principle, while the other uses cloud chamber technology. The light scattering-type 

ASDs detect smoke particles using the same principles as photoelectric spot detectors, but 

typically have an improved detection unit that is more immune to external light sources that can 

affect conventional photoelectric spot detectors. The cloud chamber detector places the air 

sample in a humidifier where distilled water is used to bring the relative humidity to nearly 100 

percent. Then, a vacuum pump reduces the chamber pressure to cause supersaturated 

conditions (i.e., relative humidity above 100 percent). When this occurs, any smoke particles 

present will act as condensation nuclei for water droplets to form on, resulting in the formation of 

a cloud in the sensing chamber. The particle concentration (cloud density) is measured by lightscattering detector principles, which provide an output that is proportional to the number of 

droplets. Either of the detector technologies will respond when a pre-programmed threshold is 

exceeded (Ref. 5). 

The ability to mechanically transport air to the detector allows for the use of filters to remove 

dust. Physical filters remove large dust particles from the air sample, before it being analyzed. 

The filters can minimize unwanted alarms and contamination of the detector. Filter placement 

and designs vary by manufacturer, but typically are built or fused into the ASD detector unit or 

installed in the pipe network.

ASD systems typically have multiple alarm thresholds that are determined by the performance 

objectives of the application and the pipe network design. Per NFPA 76, for an ASD to be 

classified as a VEWFD system, the following minimum sensitivity setting above ambient air 

borne levels must be achieved:

Alert condition: 0.2 percent obscuration per foot (effective sensitivity at each sampling 

port)

Alarm condition: 1.0 percent obscuration per foot (effective sensitivity at each sampling 

port)

Alert and alarm threshold settings more sensitive than these may be achievable in an

application and will provide for an enhancement over these minimum requirements. However, 

the ability to use more sensitive settings will be dependent on a number of variables, including 

operational transients on background aerosol noise levels. Thus, most vendors recommend at 

least a two week burn-in period, (sometimes referred to an auto learn cycle), during which the 

detector monitors background noise variations, such that an optimum alert and alarm threshold 

can be chosen, which enables a sensitive system with few unwanted nuisance alarms. 

Additionally, per NFPA 76 to be classified as VEWFD a maximum transport time from the most 

remote port to the detection unit of an air-sampling system shall not exceed 60 seconds.

It is important to point out that the sensitivity of a detector unit is not equivalent to the sensitivity 

at the sampling port. The sensitivity of each sampling port is a function of “detector unit”

sensitivity and the number of sampling holes in a sampling zone. Most ASD VEWFD systems 

require multiple sampling ports per zone. The smoke entering the air sampling port network will 

be diluted by air entering the network from other ports that does not contain smoke. Thus, for 

an ASD system to be able to detect a specific smoke concentration, the detector must have 

greater sensitivity than at the air sampling port for which the system is designed. For example,

assume a 2,000 square feet (ft2) room is protected by an area-wide ASD system consisting of a 

single zone piping network located at the ceiling, having 10 sampling ports, spaced per 
2-13

NFPA 76 requirements, and all sampling ports have been calculated to have an equivalent 

sensitivity (i.e., balanced). If the design requires each sampling port to have a sensitivity of 1.0

percent per foot obscuration, the detector sensitivity would be required to be set at 0.1 %/ft

obscuration. This estimation method is applicable to a balanced system; however, it is typically 

not sufficient to ensure the performance of the ASD VEWFD system. Such assurance can only 

be given through product testing and approvals. Vendors of ASD systems have software tools 

to assist in designing an ASD system pipe network and calculating detector unit sensitivity 

threshold setting to achieve the required sampling port sensitivities. As discussed in the 

literature and confirmed in this project’s test results (Section 5), ASD VEWFD systems may 

have varying response times if they are from different vendors; use identical piping networks;

are set to the same sampling port sensitivities; and are exposed to the same smoke sources. 

Given the different technologies and processing algorithms, it is reasonable to expect 

differences in response.

It is important to understand the difference because much of the literature reports the detector

unit sensitivity, and not the various sensitivities at the sampling ports, which are not equivalent,

and will differ by design. The use of a single piping zone sampling from multiple ports also has 

an effect on the performance of the system. For instance, in area-wide applications, the 

phenomenon known as cumulative air sampling may improve the performance of ASD systems,

and allow for earlier detection by permitting smoke sampled from multiple ports to contribute to 

the total smoke particulate being sampled at the detector from a protected space. Sample port 

spacing has a direct correlation to the cumulative air sampling effect. As the spacing of 

sampling ports is reduced, there is a greater possibility for smoke particles from a single source 

to enter into more than one sampling port, thus improving detector response. Theoretically, the 

concept of cumulative air sampling can be understood; however, there are many variables that 

influence the effect of this phenomenon, and unless specific validation testing is performed for 

the scenario under evaluation, it is a difficult phenomenon to quantify. Thus, standards such as 

NFPA 76 specify minimum port sensitivity (above background) for each sampling port.

Spot-type air sampling detectors are also available on the market. These detectors combine the 

spot-type light-scattering smoke detector with filtered aspirating features. Typical applications 

are heated stables, paper plants, cotton and textile mills, commercial laundries, food processing 

areas, and other applications where very dusty conditions exist. Performance of these types of 

detectors is not evaluated in this research.

2.4.5 Ambient conditions affecting detector response

Ambient environmental conditions influence the performance of smoke detection technologies. 

The improper selection of detector type and location can lead to problems ranging from false or

delayed alarms, to, in some cases, no alarms when fire conditions exist. Issues to consider

when selecting and locating detectors/sampling ports include, but are not limited to: 

• Background noise

– Detectors responding to invisible aerosol fire signatures are prone to detecting 

signals from cigarette smoke and automobile exhaust fumes. Thus, placement of 

a smoke detection system which responds to invisible aerosol fire signatures in 

proximity to an emergency diesel generator (EDG) should be evaluated to ensure 

that the frequent operation of the EDG does not result in numerous nuisance 

alarms.
2-14

– Routine maintenance of plant structures, systems and components may increase 

the background noise level above the alarm threshold for detection systems set 

up to signal very early warnings. 

• HVAC effects

– Ventilation conditions within rooms and electrical enclosures are important to 

understand. Detector/sampling port location without considering the air 

movement and thermal effects within the room, especially for low-energy 

incipient fires, may slow the detection systems’ response(s), and could result in 

the detection system missing the fire signal completely. Areas of low air flow or 

stagnation should be considered when designing the detection system layout. 

Depending on the detector location and type, these conditions may result in 

delayed detection for very early warning applications. Areas of high air flow can 

have an effect on smoke dilution as well. The detection technology best suited 

for such applications should be carefully considered.

– ASD systems may be prone to pressure change within a room because of

periodic changes in HVAC operational state.

• Humidity

– High relative humidity of the air space affects the smoke transport to detectors or 

sampling ports located at the ceiling such that a high relative humidity will 

enhance the agglomeration of smoke particles. Depending on room conditions, 

the moist smoke laden air may not be transported to the elevations where 

detectors and sampling ports are located.

– Humidity and ambient conditions can also result in condensation being trapped 

within the smooth bore air sampling piping, if not properly designed and installed 

to prevent water accumulation in low points. The use of moisture traps and 

drains in the piping system may be necessary is some applications.

• Radiation

– Ion type detectors are not suitable for use in applications in which high 

radioactivity levels are to be expected; the radiation causes a reduced sensitivity 

of ion type detectors.

2.5 Quality Assurance Program

Each licensed NPP is required to have a quality assurance (QA) program that provides 

reasonable assurance that the requirements for design, procurement, installation, testing and 

administrative controls for the fire protection program are satisfied. Licensees typically meet the 

fire protection QA program criteria by (1) implementing those fire protection QA criteria as part

of their QA program under 10 CFR Part 50 Appendix B, or by (2) providing for NRC review a 

description of the fire protection QA program and the measures for implementing the program. 

Commitments made by licensees regarding fire protection quality assurance are applicable to 

both deterministic [10 CFR 50.48 (b)], and performance-based [10 CFR 50.48(c)] fire protection 

plans. 

In 1977, a letter sent to each licensee titled, “Nuclear Power Fire Protection Functional 

Responsibilities, Administrative Controls, and Quality Assurance,” provided NRC supplemental 

guidance on the quality assurance necessary to assure an effective fire protection program, 

which was reiterated in Generic Letter (GL) 82-21, “Technical Specifications for Fire Protection 
2-15

Audits.” These documents provided supplemental guidance on the 10 fire protection 

QA program criteria, which included: 

1. design control and procurement document control

2. installations, procedures, and drawings

3. control of purchased material, equipment, and services

4. inspection

5. test and test control

6. inspection, test and operating status

7. non-conforming items

8. corrective action

9. records

10. audits

All 10 of these criteria have application to the QA of VEWFD systems. However, because of the 

fact that ASD VEWFD systems are engineered systems, several of these QA criteria can have a 

high impact on assuring the adequate performance of VEWFD systems. Some of these criteria 

are discussed in detail below.

Design Control and Procurement Document Control

a. Control of design and procurement documents changes—including field changes and 

design deviations are subject to the same level of controls, reviews, and approvals that 

were acceptable to the original document—is controlled.

The sensitivity of the ASD at the sampling port is dependent on the size and number of 

sampling port holes on an individual zone. Any deviations or field changes (sometimes referred 

to as “as-built”) from the original engineered design of the system (as-designed) will have a 

direct impact on the performance of the system. Depending on the differences between the 

as-built and the as-designed system this may improve or degrade the performance of the 

system.

b. Quality standards are specified in the design documents such as appropriate fire 

protection codes and standards, and deviations and changes from these quality 

standards are controlled.

c. New designs and plant modifications, including fire protection systems, are reviewed by 

qualified personnel to assure inclusion of appropriate fire protection requirements.

Installations, Procedures, and Drawings

a. Configuration control activities such as design, installation, inspection, test, 

maintenance, and modification of fire protection systems are prescribed and 

accomplished in accordance with documented instructions, procedures, and drawings.

b. Instructions and procedures for design, installation, inspection, test, maintenance, 

modification and administrative controls are reviewed to assure that proper inclusion of 

fire protection requirements.

Clear and coherent procedures ensure that installation, inspection, testing, maintenance, and 

modifications are completed with a high certainty of success. Procedures also support 
2-16

maintaining the system with a high reliability of proper operation under conditions requiring their 

response.

Inspection

A program for independent inspection of activities affecting fire protection should be established 

and executed by, or for the organization performing the activity to verify conformance to 

documented installation drawings and test procedures for accomplishing activities. The 

independent inspectors should be knowledgeable in the design and installation requirements of 

the structures, systems and components (SSCs) being inspected and follow appropriate 

procedures, instructions and checklists to perform a comprehensive inspection. For ASD 

VEWFD systems, these inspections and, the applicable code requirements should be followed 

and could include, but are not limited to, the following:

• Verify that sampling ports or points are not obstructed.

– If drop-down flexible capillary tubing is used, this should include a verification that 

no blockage has occurred at the sampling end of the capillary or at the junction of 

where the capillary connects to the rigid smooth bore piping.

• Verify that filters are clean and have been changed per manufacturer’s 

recommendations, plant procedure, or code requirements.

• Visually verify that sampling piping has been permanently installed per design 

requirements, or as-built if calculations based on as-built configurations, fittings appear 

air tight, and piping is clearly identified.

• Verify that the system sensitivity settings are consistent with any code requirements or 

regulatory commitments.

• Verify that the system sensitivity calculations are adequate and correct.

• Verify that system testing has been completed per code, plant, or regulatory 

requirements.

Test and Test Control

A test program should be established and implemented to ensure that testing is performed and 

verified by inspection and audit to demonstrate conformance with design and system 

requirements. Following construction, modification, repair or replacement, sufficient testing 

(referred to as “installation testing” or “start-up testing”) is performed to demonstrate that the 

detection system will perform satisfactorily when it is placed in service and that all design criteria 

are met. Written test procedures for installation tests incorporate the requirements and 

acceptance limits contained in applicable design documents. Periodic testing (referred to as 

“in-service testing” or IST) should be conducted on a pre-defined schedule to assure that the 

system will properly function and continue to meet the design criteria. For example, testing 

should be conducted with smoke or other acceptable product per manufacturers’

recommendations and instructions. Testing should be conducted at the farthest end sampling 

port or test port in each piping run (zone). Airflow through all ports on each piping run should 
2-17

also be verified. Additionally, sensitivity testing should be conducted to ensure detector 

operability per design requirements.

Records

Records should be prepared and maintained to furnish evidence that the criteria enumerated 

above are being met for activities affecting fire protection systems. Records should include 

results of inspections, tests, reviews, and audits; non-conformance and corrective action 

reports; and construction, maintenance and modification records and certified manufacturers’

data. Records can also be an important part of documenting the trending system performance 

or aging management; such records could support advancement to PRA modeling of any such 

system.

Audits

Audits should be conducted and demonstrated to verify compliance with design and 

procurement documents, instructions, procedures, and drawings and inspection and test 

activities. Audits should follow written procedures, and be conducted by knowledgeable 

personnel not directly responsible for the area being audited. Audit results should be 

documented, and follow-up actions should be taken by responsible management to correct any 

deficiencies identified.

2.6 System Performance Measures

As with any system, there are attributes that affect the systems performance. Performance of a 

specific detector will be dependent on the as-built system configuration4, manufacturing 

procedures, quality and reliability control procedures, and the training and supervision of the 

persons who install, use, and maintain the system. Quality assurance programs for fire 

protection are maintained at each U.S. NPP and provide a level of assurance that fire protection 

systems are designed, fabricated, erected, tested, maintained, and operated so that they will 

function as intended (Ref. 16). Although no QA program will be able to identify all deficiencies, 

the application of the fire protection defense-in-depth concepts provide added assurance that if 

system deficiencies are not identified other echelons of protection are available to ensure 

safety. Additionally, NFPA 805 requires procedures be established for inspection, testing, and 

maintenance for fire protection features credited by the fire protection program. 

This subsection provides a high-level overview of some of the system performance measure 

associated with smoke detection systems. Several of these system performance measures will 

be used to quantify the performance of smoke detection systems in Section 7.2. In addition, the 

NFPA 805 standard requires monitoring programs to be established with methods to monitor 

system effectiveness measures such as availability, reliability, and performance5.

 4 ASD VEWFD systems are engineered systems, and any deviations between as-built and as-designed 

configurations will have an effect on system performance.

5 See Paragraph 2.6 of the 2001 edition of NFPA Std. 805, “Performance-Based Standard for Fire Protection 

of Light Water Reactor Electric Generating Plants.”
2-18

2.6.1 Reliability

Reliability is an important aspect to consider when quantifying the usefulness of a detection 

system. Reliability relates to the ability of the system and each individual component to be in 

proper working condition at all times ready to perform its intended function (Ref. 17). The

complement to reliability is unreliability, which is commonly used in PRA and is defined in the 

ASME/ANS PRA Standard as the probability that a system or component will not perform its 

specified function under given conditions upon demand or for a prescribed time.

2.6.2 Availability

Availability is defined by NFPA Std. 805 as the probability that a system, structure or component 

of interest is functional at a given point in time (Ref. 18). In PRA terms, the complement to 

availability is unavailability, which is an attribute that may affect a plants response to an initiating 

event. Unavailability is defined in the ASME/ANS PRA Standard as the probability that a 

system or component is not capable of supporting its function including, but not limited to, the 

time it is disabled for test or maintenance.

2.6.3 Effectiveness

System effectiveness is a measure of how well a design solution will perform or operate given 

anticipated operational scenarios (Ref. 19). Effectiveness estimates for smoke detection 

systems are influenced by the several parameters including detector technology; fire 

combustion type; smoke generation material; smoke characteristics (particle size, concentration, 

and transport length); ventilation configurations; and stratification effects. For ASD systems, the 

as-built system configuration with regard to the application, layout, and sensitivities is also 

important with regard to system effectiveness. Thus, the design of the smoke detection system 

should be suited for the systems and components being protected. Effectiveness provides a 

measure of how well a particular design solution will perform in meeting its design objectives.

2.6.4 Maintainability

The maintainability of detection units varies directly according to the complexity of the design

(Ref. 17). Smoke detectors are typically designed for a life expectancy of 10 years or more. 

Extended use beyond 10 years should be evaluated, and, for systems expected to perform in 

excess of 10 years, detectors should be replaced or sent out for re-calibration to ensure proper 

functionality. In addition, in-field calibrations such as re-baselining the alert and alarm 

thresholds, drift compensation, or any other methods that could reduce the sensitivity of the 

system to slowly developing fires, should provide evidence that such calibrations do not 

compromise the early detection function of the system. Any reductions in sensitivity will affect 

system performance, and the fire PRA quantification should be modified as a reduction in the 

system’s effectiveness (Ref. 20). Vendor recommendations should be followed regarding 

calibration requirements.

Filters can be used for different purposes depending on the design of the system, but are 

commonly used to remove dust particles from the air sample. High-efficiency particulate air 

(HEPA) filters may be used on some ASDs as part of a dual air filtering design where the HEPA 

filtered air is used to protect and isolate the detector optics from the actual air sample (nonHEPA filtered) being analyzed. Protecting the detector optics can extend the life of the detector 

and reduce the likelihood of detector soiling. Soiling of detector optics over time will reduce the 

sensitivity of the system. The non-HEPA type filters are used to remove dust particles from the 
2-19

sampled source but allow smaller particles of combustion to pass through into the sampling 

chamber of the detector unit. These filters may be found inside the detector unit or in the 

sampling manifold external to the detector unit. Depending on the environment where air 

samples are taken, the rate of filter loading will vary, and periodic maintenance based on vendor 

recommendations or field operating experience should be factored into determining the filter 

replacement frequency. 

The ASD smooth bore piping network must be maintained to ensure sampling points are not 

blocked because of accumulation of dust or other foreign materials. Although the detector units 

are capable of annunciating a trouble alarm because of low- or high-flow conditions, most are 

not sensitive enough to alert when only one air sampling point is blocked. Gottuk and McKenna 

reported that ASD system supervisory trouble alarms did not provide a low air flow warning until 

76 percent of the total air flow was blocked on one sampling line for the system they use in 

testing (Ref. 21). U.S. NPP operating experience has identified at least one instance in which 

systems were commissioned, but not all sampling points were verified to be open and able to 

sample from the protected space. Thus, for an extended period of time, the system was 

nonfunctional. Operating experience from Canadian NPPs has also identified that collection of 

dust balls within the ASD piping having degraded system performance. This has resulted in the 

development of internal cleaning methods using compressed air and condenser balls to pass 

through the ASD piping network to push out and clear any obstructions. 

Ensuring sampling point functionality (ability to sample at each sampling point within a zone) is 

important when the ASD VEWFD systems are installed for in-cabinet applications. Because 

cleanliness of the air being sampled will vary among applications and environmental conditions, 

an increased surveillance beyond the vendors’ recommended surveillance period may be 

warranted to ensure proper system function. In addition to using compressed air or vacuum 

cleaning methods (depending upon vendor recommendations), a verification of openness of 

ASD sampling points should be conducted, especially for in-cabinet applications. Although a 

blocked sampling point in an area-wide ceiling-mounted ASD type application could degrade the 

systems performance, the blockage in sampling point(s) will result in increased flow in the other 

sampling points. In addition to ensuring the ASD piping is clear and clean of any foreign 

materials and accumulation of dust particles, the ASD piping network must also be periodically 

inspected to ensure that no portions of the pipe have become dislodged or broken from other 

plant activities. Any openings in the pipe will reduce the VEWFD system effectiveness, because 

the system is no longer balanced as designed and the volume of air from the sampling points is 

both reduced and diluted by the air flowing through any unintentional pipe openings.

2.6.5 Stability

The stability of a detector relates to its ability to sense fires over extended periods of time with 

no change of sensitivity (Ref. 17). Stability is sometimes also referred to as detector sensitivity 

drift. For ionization spot-type detectors, the accumulation of dust within the sensing chamber 

over time can interfere with the detectors’ sensitivity rendering them more sensitive, and hence 

more prone to spurious nuisance alarms. The same effect occurs in photoelectric detectors 

where by the accumulation of dust in the sensing chamber results in increased internal 

reflectance and detector sensitivity, and makes the detector more prone to spurious nuisance 

alarms (Ref. 5). However, light-scattering detectors become less sensitive as light intensity is 

decreased because of the accumulation of dust and film (Ref. 15).

Detector sensitivity drift is a uni-directional gradual shift in the range of combustion products that 

will activate the smoke detector. Background noise is the short time variation of the detector 
2-20

signal. Most smoke detector use some form of drift compensation algorithm to counter this 

effect.

Figure 2-7 provides an illustration of noise and drift. Noise is considered to be the high 

frequency peak-to-peak fluctuations. Background noise levels may fluctuate throughout the 

day, or as a result of changing environmental conditions (e.g., HVAC changes, maintenance, 

housekeeping, etc.). During commissioning of ASD VEWFD systems, background noise levels

are typically monitored for an extended period of time (up to 90 days) before making the system 

operational, such that the relative sensitivities for the alert and alarm set points have a 

background reference point that will minimize nuisance alarms.

Figure 2-7. Illustration of detector signal background noise and drift

2.6.6 Serviceability

The ease of which a system can be repaired is referred to as serviceability, and can be a 

desirable aspect from a cost perspective (Ref. 21). Serviceability is a characteristic of the 

system’s design and is usually evaluated in the design stage. It is difficult to measure on a 

numerical scale, and typically is evaluated by comparing various alternatives and assigning, a 

ranking to each system in terms of its ease of serviceability.

For smoke detection systems in NPPs, numerous system design aspects should be considered 

when evaluating serviceability. For instance, detection systems require periodic inspection, 

testing and maintenance. The purpose of periodic inspection, testing and maintenance is to 

ensure operational integrity of the system. Equipment performance can be affected by building 

modifications; environmental changes; physical obstructions; physical damage; configuration 

issues related to as-built and as-designed differences; improper installation; improper system 

startup testing; degree of cleanliness; and other problems that may not be readily apparent. 

The accessibility of the detection systems to support visual inspections and functionality and 

sensitivity testing should be evaluated. In many areas of an NPP, obstructions in the upper 

Time

Signal

Noise

Drift = θ
2-21

volume of the compartment can negatively impact the ease of performing periodic inspections 

and testing. In some instances, the amount of obstructions (cable trays in particular), can make 

it difficult or impossible to view the detector from floor level. On some occasions, scaffolding 

would need to be constructed to support inspection and testing of detection systems. 

Depending on the number of detectors or sampling ports in a room requiring scaffolding to be 

erected, there could be a negative impact on the cost of performing such inspections and 

testing, along with potentially increasing the risks to plant worker safety and NPP operational 

continuity.

2.6.7 Nuisance alarms

Nuisance alarms (also known as unwanted alarms) are associated with non-fire conditions that 

produce ambient conditions which mimic fire signatures and can cause a smoke detector to go 

into an alarm condition. The detector sensitivity is frequently tailored to the particular 

application to achieve the performance capability desired without being susceptible to nuisance

alarms.

2.7 Inspection, Testing and Maintenance of Smoke Detection Systems

The use of performance-based approaches places greater dependence on fire detection system 

success as a foundation of the fire protection strategy. The use of ASD VEWFD systems in fire 

PRA as a risk reduction method requires the ASD systems to achieve a high level of 

performance. With the increased reliance on ASD VEWFD systems to improve plant fire safety, 

an increased importance on maintaining the predictability of these types of systems is required. 

The required high level of performance necessitates the use and implementation of an effective 

inspection, testing and maintenance program. The use of an ASD VEWFD system results in the 

use of a single detector unit to cover a large area (up to 20,000 square feet) where traditionally 

a large number of single spot-type detectors would have been needed. Simply, an ASD 

VEWFD system detector unit failure (loss of protection in an entire area), causes further 

consequences than spot-type detector failures in which the additional number of detectors 

provides redundancy. Thus, use of ASD systems necessitates a high level of performance be 

maintained to achieve the performance-based objectives and assumptions used in the 

quantification of these systems. This section provides an overview of the types of inspections, 

testing, and maintenance that support ensuring a high level of performance for ASD VEWFD 

systems.

The performance of any fire detection system is dependent on system elements such as design, 

installation, equipment, and maintenance (Ref. 5). An adequate inspection, testing, and 

maintenance program allows for a method to reduce deficiencies in these elements. 

For instance, room ventilation conditions may not allow for smoke to be transported to the 

location where smoke detector sampling points are located. Initial inspection and testing of the 

system should identify any problems that are designed into the system so that they can be fixed 

before the system being accepted and placed into operation. Unlike spot-type detectors, 

ASD systems are engineered systems. That is, these systems can be designed to meet a wide 

variety of performance goals and objectives. As such, any deviations in the system from 

as-designed to as-built, will affect the ASD’s performance. Depending on the deviation, the 

“as-built” system could be performing better or worse than expected. Thus, a thorough initial 

inspection and testing of the system should be conducted to identify any deviations and then it 

should be evaluated to ensure adequate system performance. Periodic maintenance and 

inspection programs also provide assurance that statistical failures of the electronic components 
2-22

and mechanical blockage or breakages of the smooth bore piping network are promptly 

identified and properly corrected.

The inspection, testing and maintenance methods of smoke detection systems are designed to 

ensure that smoke can enter the sensing chamber of the detector; the detection system 

achieves an alarm state at the smoke concentrations for which the detection system was 

designed; and the detector alarm signal is received and processed by the fire alarm control 

panel. Inspection, testing, and maintenance methods are typically specified by the vendor for 

ASD systems. The test methods must verify that all of the air-sampling ports operate at their 

designed flow rates and that the detector unit, including the sampling fan, operates within the 

parameters established by the listing. This implies a sensitivity measurement similar to that 

required by all of the other detector types (Ref. 5).

The telecommunications industry consensus standard NFPA 76 on the use of these systems in 

prescriptive-based applications requires installation, testing and maintenance in accordance 

with NFPA 72 (Ref. 22). Some licensees using these systems have committed to the 

requirements of NFPA 72 associated with inspection, testing, and maintenance. NFPA 72

provides methods and schedules for inspection, testing, and maintenance of fire detection 

systems. With regard to air sampling-type initiating devices, the NFPA 72 standard 

recommends following vendor guidance for testing detector alarm response (functionality) and 

verifying air flow through all sampling ports (Ref. 8). It also provides the frequency for 

conducting the recommended inspections, testing and maintenance.

Based on the available information and the need to ensure a highly reliable system through 

proper inspection, testing and maintenance, it is recommended that, for NPP applications, the 

requirements of NFPA 72 and vendor-recommended practices be followed. The fire risk 

scoping study described in Part II is based on the assumption that these practices are followed 

at a minimum.
3-1

3. OPERATING EXPERIENCE, STANDARDS, AND LITERATURE 

REVIEW OF VEWFD SYSTEMS

3.1 Review of VEWFD System Operating Experience

Early on, the project team determined that testing alone would not provide all of the information 

needed to quantify the performance of very early warning fire detection (VEWFD) systems to 

support fire probabilistic risk assessment (PRA) applications. Project objective Item C, “human 

reliability analysis”; Item D “system availability and reliability”; and in part, Item E “system 

response to common products of combustion applicable to NPP,” could not be adequately 

addressed by testing alone. 

Subsequently, the project team coordinated several site visits and expanded the team with 

human reliability and human factor experts. The site visits fostered a general understanding of 

the installation and response to the VEWFD systems. This supported development of a test 

plan that represented NPP applications to the extent possible given certain laboratory 

limitations. Teleconferences were held with licensees who have VEWFD systems installed to 

allow the HRA and human factors experts to understand how the plant operators and staff 

respond to these systems and any actions taken. This information allowed the team to identify 

assumptions and place bounds on the analysis developed on Part II of the report. This more 

detailed operational information is discussed in greater detail in Sections 9, 10, and Appendix C.

This section summarizes the information received through site visits to gain a better 

understanding of these systems applications, support development of a testing approach, 

estimation of system unreliability and unavailability, along with understanding general operating

experience. Operating experience documented in the EPRI fire events database was also 

reviewed and used to support Part II of this report. However, that operating experience 

information is described in Sections 7, 8, and Appendix D along with how it is used to support 

the objectives of this report.

3.1.1 Site visits

The National Institute of Standards and Technology (NIST) and U.S. Nuclear Regulatory 

Commission’s (NRC’s) Office of Nuclear Regulatory Research (RES) staff conducted several 

site visits to nuclear and non-nuclear facilities to gather information and operating experience for 

aspirating smoke detection (ASD) applications. These site visits provided two benefits. First, 

observing how the systems were used in the field allowed the planning of the testing to be 

conducted in a manner that reflected actual use. Second, the team gathered information on the 

operating experience (e.g., reliability and availability) of these systems and plant operator

/technician response to system notifications to address project objectives. Site visits were 

conducted at the following facilities:

• NASA Goddard Space Flight Center (Maryland)

• Shearon Harris Nuclear Power Plant (North Carolina)

• H.B. Robinson Nuclear Generating Station (South Carolina)

• Bruce Nuclear Generating Station (Ontario, Canada)

• Darlington Nuclear Generating Station (Ontario, Canada)

• Pickering Nuclear Generating Station (Ontario, Canada)
3-2

• Three Mile Island Nuclear Station (Pennsylvania)

• Operator response questionnaire (through the Electric Power Research Institute)

To facilitate a structured, consistent, and thorough exchange of information during these site 

visits, a checklist of questions was developed and used. Topics included system performance 

objectives, system design, installation and maintenance, operator interface, and actual system 

performance. The Shearon Harris site was visited a second time in early 2016. The purpose of 

this visit was to better understand recent operating experience that was provided during the 

public comment on the draft version of this report. A summary of the insights obtained during 

these visits is provided herein.

3.1.1.1 U.S. nuclear facilities

For the U.S. nuclear facilities visited, licensees indicated that VEWFD systems were installed as 

risk reduction measures to support risk quantification studies, or to support an exemption from 

deterministic fire protection requirements. The VEWFD systems used were of the ASD type.

Deterministic Applications

The use of ASD systems has been in support of exemptions from NRC requirements. At Three 

Mile Island, ASD systems are used to enhance the second echelon of fire protection,

defense-in-depth, by providing early warning to complement the passive fire protection features 

and other fire protection measures, such as transient combustible control zones. In this 

instance, an ASD system was used in area-wide configurations. The system has been in use 

since 1998 and consists of two multi-zone detectors protecting battery rooms, battery/inverter 

rooms and a switchgear room. For this application, the licensee chose to install the system to 

provide early detection as an alternative to upgrading other passive fire protection features 

within the protected rooms. Room sizes varied from 600 feet squared to 1,200 feet squared,

with an average room height of 18 feet. Typically, the sampling ports were within 2 feet of the 

ceiling, and each room had two sampling ports on a single zone. One additional application 

was protecting an office type space adjacent to a switchgear room, which was previously 

classified as the Technical Support Center. Ceiling height in this space was approximately 8

feet (Ref. 2).

Operating experience obtained during interviews (with fire protection staff regarding the 

successes from these systems), included a case in which a potential fire from an air-handling 

unit (AHU) fan motor failure was detected by this ASD system and operators responding to the 

alarm were able to identify the failing component and isolate it before any fire conditions. It was 

indicated that the early detection and response to this event possibly prevented a plant trip. 

Other events detected by these systems, but not directly related to reactor safety, include

detecting hot work in a battery room and burned popcorn from a microwave oven. 

Documentation of success cases can play an important role in understanding and quantifying 

the reliability of operator response to ASD system notifications. Unfortunately, documentation of 

these events was not readily retrievable.

Risk Applications

The use of VEWFD ASD systems and the associated risk reductions were either prompted by 

risk insights obtained from the results of Generic Letter 88-20, “Individual Plant Examination of 

External Events (IPEEE) for Severe Accident Vulnerabilities;” or to meet a risk metric from 

Regulatory Guide 1.174, “An Approach for Using Probabilistic Risk Assessment in RiskInformed Decisions on Plant-Specific Changes to the Licensing Basis.”
3-3

In an IPEEE case, the licensee postulated a fire scenario in a control room reactor turbine 

generator board contributing to a core damage frequency (CDF) of 4.5×10-05 per year. Installing 

the in-cabinet ASD VEWFD system resulted in an increased time period between fire detection 

and mitigation, which the IPEEE states a drop in the overall CDF for the postulated fire scenario 

to 6.9×10-06 per reactor year (Ref. 23). 

The use of ASD VEWFD systems in fire PRA for plants transitioning to NFPA 805 typically 

followed the interim staff position method described in Supplement 1 to NUREG/CR-6850, 

Frequently Asked Question (FAQ) 08-0046 titled, “Incipient Fire Detection Systems.” In one 

scenario, several installations of the VEWFD systems were used as defense-in-depth measures 

only and were not quantified in fire PRA. 

For risk reduction applications, ASD VEWFD systems were exclusively used for in-cabinet 

(within electrical enclosures) applications in non-continuously occupied areas and in most 

cases, are complemented by conventional NFPA 72 standard area-wide spot detectors within 

the same room. The installation of ASD VEWFD in-cabinet detection either replaced 

conventional in-cabinet spot detectors, or provided new detection within the electrical enclosure. 

The electrical enclosures protected by ASD VEWFD systems contain safe shutdown system 

function and components. Within these enclosures vital electronic circuitry, instrumentation and 

control devices are found along with electrical cable and wiring used by these systems. 

Ventilation flow rates were not known, but all electrical enclosures that used VEWFD systems 

were of a vented design. There were no applications identified at the sites visited where 

VEWFD systems were used to detect smoke entering the return air ducts of the compartment 

ventilation system.

NFPA 72, “National Fire Alarm and Signaling Code,” was used1 as the code of record for these 

systems and in one case the response time of 60 seconds found in NFPA 76, “Standard for the 

Fire Protection of Telecommunications Facilities” was adopted over the 120-second maximum 

response time specified in NFPA 72. Section 8.5 of NFPA 76 states several requirements for 

VEWFD and early warning fire detection (EWFD) systems, such as the maximum coverage 

area per port; minimum sensitivity settings for alert and alarm conditions; and the maximum 

transport time from the most remote sensing port to the detector (Ref. 22). Section 3.2 of this 

report provides more detail on smoke detection standards.

Two types of ASD technologies were found to be in use at the sites visited. One site used a 

laser-based detection technology that had been in use for over 14 years. The other site used a 

cloud chamber detector technology that had been in service since 2010 (about 20 months of 

operation at the time of the site visit). Both systems were ASD VEWFD, Underwriters 

Laboratories (UL) listed, and installed by certified installers. In all cases, the detector unit was 

located within the same room as the electrical enclosures that were being protected. The 

distance the detector is located away from the cabinets was dependent on meeting the NFPA 

response time criteria. One unit consisted of a single zone with five sampling ports, whereas 

the other system was capable of sequentially sampling multiple zones (up to four) connected to 

the detector. In the latter case, the cycle time to complete sampling of all four zones was one 

minute.

The systems were installed by plant craft (instrumentation and control, electrical maintenance, 

or electrical craft personnel). Once installed, plant operations, engineering, system certifying 

 1 One licensee used the 1996 edition of NFPA 72, while the other used the 2007 edition as the code of 

reference.
3-4

officials, and the VEWFD system vendor technical representative performed the initial system 

configuration and testing. The VEWFD systems were initially configured with sensitivity settings

consistent with the vendors experience in similar monitoring locations and environments. For 

one site using a cloud chamber detector, the sensitivity settings were set to a gain of 7 or 8 out 

of 10, with 10 being the most sensitive range. The specific sensitivity settings within this gain 

setting were set to vendor predetermined values (30 percent, 50 percent, 70 percent, and 

90 percent). This site used the 30 percent set point to represent the “alert” response, while the 

50 percent set point represented the “alarm” response.2 Acceptance testing followed, which,

depending on the VEWFD systems included the following:

• validation of detector response to smoke source 

– (small element heat gun) provided by the vendor for generation of particles of 

combustion

– NFPA 76 Annex B/British Standard (BS) 6266 annex tests

– test gas/aerosol

• validation of transport times (detector response times)

• placing the detector in fault by restricting air flow

• placing the backup batteries in fault condition

• placing the air separator in a fault condition

• putting the alternating current (ac) power in a fault condition

• confirming proper alarms on detector panel, local interface panel, and fire alarm console 

in control room.

Acceptance testing is an important process to ensure that ASD systems are properly installed 

and functional. Feedback from one of the sites visited, indicated that the installation instruction 

did not contain a critical step from the design requirements to include opening all sampling 

ports. Bacause of the cabinet configuration, acceptance testing did not verify the ASD systems 

response to a smoke source at each sampling port. Thus, the system was non-functional from 

commissioning until the installation error was identified by plant personnel performing period 

testing on the system.

Following this initial setup, the systems were allowed to operate with this configuration over a 

period of several weeks to validate set points and lack of nuisance alarms. Upon completion of 

these activities, the systems were commissioned and placed into operation. Site operators test 

the systems per their plant procedures, either quarterly or semi-annually and annually. Revisit 

to the plant in early 2016 determined that set points were changed after commissioning for one 

of the 10 systems installed. In this instance, the detector was experiencing significantly more 

nuisance alarms (alerts) than the other VEWFD systems installed in similar applications. As 

such an engineering evaluation was performed and the detector unit was decreased in 

sensitivity. Instrumentation and control technicians maintain and repair the systems with vendor 

 2 These detector gain and % of gain settings are vendor specific. Sampling point (i.e., port) sensitivity is 

dependent on the number of sampling points per zone along with the detector sensitivity setting. The 

identification of these gain settings DO NOT imply an equivalency to the NFPA 76 minimum sensitivity 

requirements for VEWFD systems.
3-5

support when needed. As required by the VEWFD system vendor, plant technicians who install 

and service the detector systems received initial classroom training on the detectors and 

associated software. Plant operators were also provided classroom and hands-on training for 

operation of the detector system software and were required to show proficiency with the 

software to gain qualification. Additionally, teaching aids were provided to the site’s training 

department for continued training.

Availability of these systems varied among the two sites visited. In one instance, the systems 

were down for semi-annual and annual testing, trouble alarms and for any cases in which the 

systems were intentionally turned off because of hot work, or other activities that would cause 

unwanted alarms. Any time the system is off, fire watches are in place. Testing alone could 

account for up to 8 hours per device per year of downtime. Trouble alerts/alarms received 

included lower water level; vacuum fault (cloud chamber failure); airflow exceed set point (out of 

set point tolerance, but within system range); and transport time faults. Most of these 

troubleshooting alerts/alarms required vendor input to resolve. The other site experienced 

roughly the same down time for testing, but also experienced several weeks of outage time 

because of hardware failure. At this site, the system was out for 27 days in 2002 for a panel 

replacement. The lengthy interruption resulted from not having a replacement panel on site, nor

the lead time to acquire a replacement from non-domestic sources. The second outage 

occurred in 2007 and lasted for 20 days. In this instance, staff initially thought that another 

panel failure had occurred. This time there was a replacement panel on site, but upon 

installation, it was determined that the replacement panel experienced an infant mortality failure. 

The site ultimately determined that the root cause of the initial panel failure was the need for a 

battery replacement. Upon battery replacement in the initial panel, the system was returned to 

operation. In all cases, trouble alarms were received from the system and the site 

instrumentation and controls technicians promptly began their investigation. The sites 

corrective measure to avoid future excessive downtime was to have spare parts available 

on site.

Appendix D.2 of this report describes a limited number of events where VEWFD systems 

detected pre-flaming conditions (incipient stage). Several other cases were observed in which 

the systems generated nuisance alarms because of hot work, grinding, and operation of a floor 

buffer. None of these instances resulted in fires, but this information does provide some 

quantitative information on the sensitivity of these systems. In response to future unwanted 

alarms, the utility intends to turn off the VEWFD systems and station fire watches while the 

activity/work is completed (that may cause these systems to generate unwanted alarms), and 

then return the systems to service. From the end users’ point of view, the VEWFD systems 

reacted as designed because of the presence of pre-combustion products being present, and 

both sites found that the installed VEWFD systems met performance expectations. 

As identified by the users, a drawback to these systems was that the required quarterly and 

annual surveillances were time-consuming and expensive. The sites identified that the systems 

were complicated and required significant vender interface to ensure proper operation. Lastly, 

one of the sites ran into an issue with receiving replacement parts in a timely manner, which 

resulted in an extended outage of the system. Their recommendation was for end users to 

determine the timeliness of acquiring replacement parts, and to, wherever possible, have spare 

units on site to reduce extended outage times.

Operator actions in response to system alert/alarm conditions are important to understand in

characterizing the failure probability of operators trying to complete specific actions. During the 

site visits, several questions were asked focusing on how operators would respond to 
3-6

notification from the installed VEWFD systems. The operator responses were very similar, with 

some slightly different between facilities. 

For several sites, when an “alert” (pre-alarm) notification is received in the main control room 

(MCR), an auxiliary unit operator (AUO) and technician are dispatched to the detector zone 

electrical enclosure(s) to look for source of the notification. The technician is responsible for 

bringing along a portable ASD, that he/she is trained and qualified to operate. The AUO uses 

his/her human senses (sight and smell) to try to identify the source of the notification, while the 

technician uses the portable device. Meanwhile, in the control room, the VEWFD systems 

software is monitored from the shift technical advisor (STA) desk. If the ASD goes into an alarm 

state, the fire brigade is activated and sent to the area where the detector is located with 

necessary equipment for fighting any potential fires.

At one site an ASD VEWFD system is installed in the reactor turbine generator board located in 

the MCR. The RTBG is located in the front of the MCR, visible to the operators. Both the 

VEWFD system detector and fire alarm panel are located within the MCR. Upon detector alarm, 

the response procedure prescribes than an operator is to investigate the five sections of the 

reactor turbine generator board for smoke, charring, or overheating components. In addition to 

a visual inspection, thermography equipment can be used to further investigate the source of 

the alarm. Upon identification of the component causing the detector alarm condition, shutdown 

and repair of the affected component are initiated. If necessary, the fire brigade is to be 

activated. If no source component can be located, then the operator is to attempt to reset the 

detector. If detector resetting does not clear the alarm, a fire watch is to be started, and 

preparations should be made to activate the fire brigade. If an additional alarm on an opposite 

zone is received, then the fire brigade is to be activated and the ventilation fans are to be 

secured.

In all U.S. sites visited, any changes to the ASD VEWFD system sensitivity set points would 

require an engineering change evaluation. If such changes were deemed appropriate, 

personnel trained and qualified in the use of VEWFD system software for the installed detectors 

were to make the approved changes.

3.1.1.2 Non-U.S. nuclear facilities

The non-U.S. facilities visited use ASD VEWFD systems because at the time of installation 

(mid-1990s to mid-2000s), they were thought to be the best available detection technology to 

protect critical areas of the plant that house critical control equipment. In addition, these 

systems were installed in areas where spot detectors were difficult to install, inspect, test and 

maintain. Currently, a regulatory design basis standard CSA N293-07, “Fire Protection for 

Nuclear Power Plants” requires the installation of ASD VEWFD systems that meet the 

requirements of NFPA 76 where redundant safe shutdown systems are located within the same 

fire compartment. This standard also has a “control room complex” requirement to detect fires 

at their incipient stages. In addition, some sites identified the use of standard Underwriters 

Laboratories Canada (ULC) 536, “Fire Alarm Verification.” Some respondents identified using a 

0.1 factor in their fire PRAs for detection using ASD VEWFD system in non-continuously 

occupied cases. No basis for this estimate was provided.

The VEWFD systems were installed in area-wide configurations protecting critical electrical 

control and instrumentation equipment. VEWFD systems were used to protect control 

equipment and instrumentation rooms that contained critical reactor control and shutdown 

equipment. These rooms included the main control room (four unit), control equipment rooms, 
3-7

digital control computer rooms, and cable spreading rooms. The ASD VEWFD systems were 

typically employed in areas where highly congested overhead components existed, making 

routine maintenance, and testing difficult for conventional spot detection heads, or multi-sensor 

spot detectors. The ASD VEWFD systems complemented, replaced, or provided new detection 

to the areas being protected.

Laser-based ASD VEWFD systems, which were used exclusively, operate on a light-scattering 

principle and use dual photoelectric sensors. The multi-zone detector was the most commonly 

used design, but there were a few instances in which single-zone detectors were used to protect 

small rooms. As a pre-requisite for procurement, all detectors had to be ULC listed. 

Additionally, the systems used also contained listings from UL and Factory Mutual (FM). 

The detector units are typically located within the protected zone to minimize any pressure 

differential effects. The ASD air sample is exhausted into the protected space to ensure that 

smoke or airborne radioactive particles are not inadvertently propagated to other areas. The 

detector sampling port coverage is typically 50-75 ft2, which is less than that required by NFPA 

76 (200 ft2). However, there are several areas near but still below the 200 ft2 coverage area 

required by NFPA 76. Ventilation conditions in rooms using ASD ranged from 2.5 to 4.5 air 

changes per hour.3 A third party installed the piping network and detectors, while an 

independent certified engineering firm verified the installation. Commissioning of the system 

was completed by a trained and qualified entity. Acceptance testing included verifying 

parameter setup; determining alarm set points; verifying relay/wiring configurations; taking flow;

pressure and transport time measurements from each sample hole, conducting heated wire 

testing, and third party verification.

The systems are maintained by the station control maintenance technicians who received 

week-long training on the ASD system, the operation, detection panel, and software. Annual 

preventive maintenance accounts for 2-3 hours per detector, per year; monthly maintenance 

accounted for approximately 12 hours of system down time per detector per year. With the 

large population of ASD in use at the sites, the typical down time from system failure or trouble 

is approximately 2–3 hours. As time progressed from initial installation to current conditions, the 

maintenance staff understanding of the systems operation and failure modes has greatly 

reduced the down time from random failures or system trouble alarms. Types of trouble alarms 

received from the devices include: laser problems, filter fault, major/minor flow trouble, and 

detector failure. One of the sites identified that the typical frequency of trouble alarms from the 

two-dozen detectors on site was approximately six times per year. When the ASD system is out 

of service, the site implements continuous fire watches and restricts work activities in the 

associated protected area.

Operator Interface

Before commissioning the systems, they are placed into an “auto-learn” operating state in which 

background conditions are monitored to determine the baseline ambient conditions. Following 

this typically 2-week auto-learn state, the detector sensitivities are set to meet NFPA 76 

sensitivity requirements. Temporary changes may be made following the plants temporary 

configuration change process for cases in which heavy smog or wildfires are found to cause 

nuisance alarms. Permanent changes to detector sensitivity settings cannot be changed 

without a design change, as well as the approval of the detection system’s responsible design 

engineer and system engineer.

 3 These values are representative of air changes in rooms other than the control room.
3-8

When an “alert” (pre-alarm) notification is received, operators attempt to clear the pre-alarm. 

The purpose of this action is to help support a determination of a continuous, intermittent, or 

nuisance system response. If unsuccessful, an operator initiates an alert and requests 

emergency response technician support. When detector alarm conditions are met, operators 

consult the fireworks computer and data gathering panels for fire alarm message information. In 

an alarm condition, Operations staff sounds the emergency response tones for fire alarm 

response, which results in the response of the fire brigade. Upon entry into the protected area, 

staff uses thermal imaging cameras to detect incipient fires.

System Performance

Operating experience identified several system notifications from potential fire sources, 

including smoke from grinding and welding activities within the protected room; however no 

opportunities presented themselves for detection of incipient fires that progressed to a flaming 

state. Air handling unit belt and cork isolator fires were detected. In one instance, a series of 

alarms was received from an ASD system during commissioning of the reactor’s safety system 

during restart. It was concluded that overheating/charring wire insulation was the cause of the 

alarms.

Early on during the use of these systems, several nuisance alarms were received. Over the 

years of use, some of these systems’ set points have been offset to account for the 

environmental conditions, which as a result, have reduced the number of nuisance alarms. The 

causes of these nuisance alarms include airborne charcoal dust from charcoal filters, fumes 

from floor stripping, nitrogen purging, dust accumulation within the air sampling ports, but, in 

some cases, are unknown. The most common cause of nuisance alarms was related to work 

activities in the protected area when the system was not bypassed for such activities. Also, 

during the summer immediately following the installation, heavy smog and wildfires in the 

adjacent province resulted in higher than normal background conditions and resulted in 

nuisance alarms.

Some final qualitative insights from the non-U.S. nuclear facilities visited included issues with 

maintenance and sensitivity settings. For maintaining the systems, the users found that it is 

very difficult to perform any maintenance outside of what is identified by the manufacturer. For 

instance, if issues with flow, pressure, or transport time measurements are observed at the test 

point, it may be required to test the individual sample holes. This can be difficult, time 

consuming, and may potentially have a negative impact on safety because of the complexity of 

accessing the sampling pipes and ports in very sensitive areas. In addition, foreign particles or 

dust may accumulate in the air piping, and in several cases it has been found that the 

manufacturer’s recommended practice of using compressed air is not effective. Alternative

actions were required to restore system operations. This specific problem occurred in a 3⁄4-inch 

diameter piping network. Regarding sensitivity, both operations and control maintenance have 

indicated that the system is overly sensitive to dust and smog. It was also emphasized that 

plant personnel are pleased with the ASD VEWFD systems and they work well for small rooms, 

but in several cases in-duct, non-ASD type detectors were preferred for protecting larger rooms.

3.1.1.3 Other U.S. facilities (non-nuclear)

Non-nuclear U.S. facilities used VEWFD systems because they were specified in their safety 

standards for fire protection in mission-critical areas.

NASA employed VEWFD systems in mission-critical applications for asset protection. They 

have over 100 ASD systems designed as VEWFD systems at the site visited, and have a similar 
3-9

number of systems at other major installations. All systems in use when the site visits occurred 

were of the laser-based technology. Cloud chamber type systems were initially used for a short 

period of time, but none remain. The decision to discontinue the use of the cloud chamber 

systems were partially due to the large number of systems planed and maintenance of the 

systems water supply. The vast majority of ASD systems used were from a single 

manufacturer. They are also considering or have plans to install video smoke detection and 

wide-area, beam-type smoke detection in high bay applications. 

The basis for using VEWFD systems is specified in NASA-STD-8719.11 Revision A, Safety 

Standard for Fire Protection (Ref. 24). The standard refers to very early smoke detection, or 

automatic smoke detection equipment capable of early warning, and references NFPA 72 and 

NFPA 75. There is no reference to VEWFD systems specifically, nor is NFPA 76 referenced.

All new constructions are required to have VEWFD systems installed. In retrofits, spot detectors 

remain and are maintained. Applications include clean rooms (tents), high bays, and computer 

rooms, and all are general area systems; no in-cabinet monitoring using ASD is employed. 

VEWFD systems are used for duct detection in protected spaces (installed on returns). The 

layout is designed to meet the time requirements for detection, stated as 60 second–plus 

delays. There are pipe length restrictions, which can be relaxed if the system is engineered,

(i.e., pipe flow software).

Systems are installed by factory-certified contractors, and are serviced in-house by factory 

trained technicians, who are certified (Level II) by the National Institute for Certification in 

Engineering Technologies. There is a 30-day burn-in period after installation, and 

commissioning includes formal acceptance testing. The performance test procedure was not 

specified, but is most likely handled by the contractor to industry specifications; the assumption 

is that they follow the test procedures outlined in NFPA 76 Appendix B. 

The NASA experience with VEWFD systems includes the following observations:

1. Fewer nuisance alarms occurred than when spot detectors were installed.

2. Issues arose from flow imbalances tripping airflow trouble alerts. The fix usually involves 

piping the exhaust back to the protected space.

3. One system sensitivity adjustment was made (lowered) because of picking up a circuit 

board “fry,” which is not unusual in areas with instrumentation assembly.

4. Nuisance alarms occurred because of vacuum dust.

5. Fire was recently picked up in a lab space.

The technicians check all pre-alert and trouble signals; (response is not time sensitive) Fire I 

level does not evacuate the building; however, Fire II results in fire department response. 

Sensitivity set to an equivalent 3 %/ft obscuration at sampling locations.
3-10

3.1.2 Other sources of operating experience information

Conference Proceedings

Forell and Einarsson identified one weakness in the fire detection systems evaluated in German 

NPP units associated with the battery system (Ref. 25). Fire detection systems are connected 

to the emergency power supply and equipped with an additional battery, making the fire 

detection systems’ power supply both reliable and redundant. However, in one reportable case 

it was identified that the emergency power supply failed, the fire detection system properly 

functioned on the backup battery supply until its energy was depleted. It wasn’t until sometime 

later (time of discover not reported) that operators identified the failure, leaving portions of the 

plant unprotected for an extended period of time. Although the fire detection system backup 

power supply performed as designed, human error, in not recognizing that the primary power 

supply had failed, resulted in unavailability of the system. 

Forell and Einarsson also reported that the primary cause of failures in ASD systems can be 

attributed to flow changes caused by clogging of inlets and leaks in the pipework. The use of 

ASD systems in German NPP units is a result of changes to Insurance Europe (formally the 

Comite Europeen des Assurances) and the European Standard EN 54 Part 20. Because of the 

clogging and pipe leakage issue, the new specifications require an increased tolerance of 

changes in ASD air volume flow from ±50 percent to ±20 percent (Ref. 25).

3.1.3 Experience within other industries

Telecommunications

The Network Reliability Council of the Federal Communications Commission (FCC) identified 

fire protection challenges related to the network reliability of public telecommunications. Their

focus on a need to improve reliability in protecting the network from fire effects was largely a 

result of a main switching room fire that occurred in the Hinsdale Central Office of the Illinois 

Bell Telephone Company on May 8, 1988. 

At the time, the Hinsdale Central Office was one of the largest switching systems in the state of 

Illinois; the facility processed more than 3.5 million calls per day, including calls from numerous 

hospitals, as well as the communications between Chicago’s O’Hare and Midway Airports. 

As the Hinsdale Central Office was not continually occupied, after receiving fire and power 

failure alarms at the Alarm Reporting Center, it took nearly an hour before a technician arrived 

at the facility. Upon arrival it was determined that the fire had become large enough to require 

fire department response, and had knocked out much of the region’s telephone service,

requiring the responding technician to drive to the fire department to initiate their response. 

Battling the fire involved additional complications in that the uninterruptible electrical power 

supply associated with the fire area was still live, and the lack of telephone services meant that 

the local power company could not be contacted to remove power. Two hours were lost while 

firefighters manually removed all fuses from the power feeds to the building. In addition, toxic 

fumes from fiber optic equipment required the response of hazardous materials experts and 

evaluation of civilians living within five blocks of the Central Office. All told, the fire lasted for 

more than six hours, and it took nearly two weeks to completely restore service (Ref. 26). 

Although the Hinsdale event is described in detail, there have been numerous other 

catastrophic fires in telecommunications facilities, including one that occurred only one month 

before the Browns Ferry Fire of 1975, in a New York City Telephone Exchange.
3-11

As a result of the recurrence of severe fire events in telecommunication facilities, and the need 

to maintain high reliability and business continuity for these types of installations, the 

telecommunications industry developed NFPA 76, “Standard for Fire Protection of 

Telecommunications Facilities.” This standard provides the minimum level of fire protection in 

telecommunications facilities to protect equipment and service continuity where services such 

as telephone (landline, wireless), data, internet, voice-over Internet protocol, and video 

transmission are rendered to the public. Details on this standard are provided in Section 3.2. 

As a result of this standard, the telecommunications industry has extensively used ASD 

technology in certain applications to provide advanced warning. ASD technologies are 

employed in telecommunication facilities, in part, as a result of difficulties in implementing

conventional spot-type smoke detection in environments with high air ventilation rates and 

numerous complex physical configurations. Common telecommunication challenges to 

detecting fires in information technology (IT) server room environments include the following 

(Ref. 27):

(1) varying fuel loads and ignition sources 

– large number and concentration of electronic devices generate excessive 

amounts of heat

(2) obstructions that interfere with movements of smoke toward detection points

– server enclosures

(3) high airflow

– smoke dilution

U.S. Navy

Engineers at the Naval Sea Systems Command indicated that they were unaware of any U.S. 

Navy ships that employ these types of systems. Spot-type smoke, heat and flame detectors are 

commonly used on surface ships. Because of the large size and number of compartments 

aboard, the increased cost and installation difficulties make using ASD systems challenging. 

However, future ship designs are expected to use ASD systems (not configured as VEWFD) in 

a limited number of special applications, because of unique overhead structures (Ref. 28).

3.2 Standards, Listings, Approvals and Codes of Practice

Standards, listings, and approvals provide a means to demonstrate that a product meets a 

minimum level of performance. Applicable standards for air sampling smoke detection systems 

include NFPA 72, “National Fire Alarm and Signaling Code,” and NFPA 76, “Standard for the 

Fire Protection of Telecommunications Facilities.” BS 6266, “Fire Protection of Electronic 

Equipment Installations – Code of Practice,” also provides useful information on the use of ASD 

systems. Listings and approvals provide a structured and inspectable process to ensure that 

equipment, materials, and services meet identified standards, or have been tested and found 

suitable for a specific purpose. Listings and approvals are provided by organizations that are 

acceptable to the authority having jurisdiction (AHJ), such as UL or FM.

The remainder of this subsection provides a brief overview of the associated standards and 

listings. For specifics on these standards, the reader may reference the applicable standard(s).
3-12

NFPA Standards

NFPA 72 establishes the minimum required level of performance for the application, installation, 

location, performance, inspection, testing and maintenance of fire alarm systems. It provides 

installation guidance for air sampling-type smoke detectors, but does not identify any 

requirements for VEWFD or EWFD systems. The guidance provided includes maximum 

transport time, each sampling port to be treated as a spot-type detector, air flow trouble signal, 

pipe labeling and a requirement the sampling pipe networks be designed and supported by fluid 

dynamic principles.

NFPA 76 provides installation requirements for VEWFD systems used in telecommunication 

facilities. The requirements of NFPA 76 detail the maximum coverage area; requirements for 

monitoring air return ventilation from protected space; minimum sensitivity settings for alert and 

alarm conditions; and the maximum transport time of the system. Although NFPA 76 provides 

requirements for protection of telecommunications facilities, there is no other U.S. consensus 

standard that is available for these systems, and as such, most U.S. NPP utilities reference 

NFPA 76 as the standard applicable to their VEWFD system.

In addition to specifying the performance requirements of VEWFD systems, Annex B of

NFPA 76 also provides performance test procedures. Two types of tests are presented in the 

annex. One uses a heated wire, while the other uses chemicals. Both tests are designed to 

simulate small amounts of visible smoke that would be present in the early stages of a fire in a 

telecommunications equipment area. The intent of the tests is to provide a quick, easy, and 

repeatable functionality test (quantity, temperature, and color of smoke), while minimizing the 

potential hazard to the facility and health of personnel in the test area. These test methods are 

not intended to serve as a calibration of the ASD detection unit sensitivity.

Listings and Approvals

UL provides a URXG product category listing that covers smoke-automatic fire detectors, 

including air sampling types, employing a special construction different from conventional 

detectors and designed to detect products of combustion in a specific location. Detectors with 

this listing are installed in accordance with manufacturer installation instructions, in a manner 

acceptable to the AHJ, and in accordance with NFPA 72. The basic standard used to 

investigate products in this category is UL 268, “Smoke Detectors for Fire Alarm Signaling 

System.”

UL 268 covers smoke detectors defined as “an assembly of electrical components arranged to 

detect one or more products of combustion.” It provides a standard set of requirements for 

smoke detectors employed in ordinary indoor locations, in accordance with NFPA 72. These 

include assembly and component requirements, evaluation of detector performance under 

numerous conditions, manufacturing and production requirements, along with required markings 

and installation instructions. The standard evaluates detector performance by conducting 

numerous types of tests, including detector sensitivity, fire, temperature, humidity, and 

endurance, among others. Because ASD VEWFD sensitivities can be outside the sensitivity 

test range of 0.5 to 4.0 %/ft obscuration, specified under Section 30, the UL URXG product 

category uses the sensitivities recorded during the fire test for the detector listing. The standard 

also requires a means for measuring or indicating the nominal sensitivity or sensitivity range of 

the detector after it has been installed as intended. This is to verify that the sensitivity of the 

detector is within its marked range (UL 268, Section 6.2 & 30). The sensitivity testing can be 

conducted using the typical United States sensitivity smoke test chamber as described in 
3-13

Annex B of UL 268, and a smoldering cotton lamp wick, or an aerosol generator, either of which

will produce gray smoke.

FM provides independent testing and approval of smoke actuated detectors, including any 

aspirating-type detectors for indoor locations, per its Class Number 3230 “Approval Standard for 

Smoke Actuated Detectors for Automatic Alarm Signaling.” FM approval criteria include

performance and marking requirements, manufacturing facility examinations, quality assurance

procedure audits, and a follow-up program. Performance requirements include air flow, 

transport time, sensitivity, and fire tests, per UL 268 Section 39 guidance.

Codes of Practice

British Standards Institution (BSI) publication BS 6266, “Fire Protection of Electronic Equipment 

Installations—Code of Practice,” provides recommendations for the protection of electronic 

equipment from fire. It identifies electronic equipment such as computer servers, 

communications systems, design, manufacturing and distribution equipment. Because the 

scope of this standard is for the protection of electronic equipment, it covers a variety of fire 

protection topics, including separation, construction, building services, detection, suppression, 

smoke control, and management. BSI 6266 indicates that all ASD systems should conform to 

BS EN 54-20, “Fire Detection and Fire Alarm Systems - Aspirating Smoke Detectors,” and be 

used in accordance with the Fire Industry Association (FIA) Code of Practice. For the interests 

of this report, the information on detection, along with the material presented in Annex A on 

spacing and location, are useful.

Annex A of BSI 6266 provides recommendations for the spacing and location of aspirating 

sampling holes. For return air vent applications, it is recommended that each sampling port 

have a maximum 0.4 square meter (m2) area of coverage, be a Class A system per EN 54-20, 

and that manufacturers’ recommendations should be followed. For ceiling applications 

(including floor or ceiling voids), a nominal effective sampling hole coverage area of 25 m2 is 

specified. The Annex then identifies that the coverage area can be increased or reduced 

depending on various room attributes (e.g., air flow velocity, air conditioning state, detector 

class, or layered detection configuration). For in-cabinet applications, the Annex recommends 

that the sampling hole be placed where the ventilation exits the electrical enclosure, or within 

the top 10 percent of a cabinet with no ventilation (sealed). There are also provisions for using 

multiple sampling points within the same cabinet that has large vents. Annexes B through H 

describe the various performance tests, including smoke pellet; paper; overheated enamel wire;

polyvinyl chloride/low smoke and fume (PVC/LSF) wire; PVC coated wire; resistor; polyurethane 

mat; and potassium chlorate/lactose chemical test.

The FIA (England) has also developed a code of practice document titled, “Design, Installation, 

Commissioning and Maintenance of Aspirating Smoke Detector (ASD) Systems” (Ref. 29). 

This document defines three sensitivity classes (Class A, B, C),with Class A being described as 

“Very High Sensitivity” applicable for in-cabinet application with high risk, as well as five ASD 

sampling methods, including in-cabinet applications. It also explains that when operating as a 

Class A or Class B system, the source of the alarm may not be readily visible and special 

training should be provided to acquaint responders with the performance of these systems. 

With regard to in-cabinet detection, the code of practice recommends that a sample point in 

each cabinet be installed, and specifies preferred locations of sampling within the cabinet for 

cabinet ventilation configuration (sealed, natural or forced). The in-cabinet application section 

also provides recommended limits on the number of cabinets protected by various classes of 

systems. In general, the FIA document provides fundamental design guidance of ASD systems 
3-14

for various applications and supports the use of vendor experts to ensure that the design will 

meet the intended design goals.

3.3 Literature Review

A literature review was conducted to better understand the information available on the use of 

ASD VEWFD systems. Literature was collected from publically available sources, academia, 

Internet, vendor Web sites, and journal articles. The collected literature was reviewed and 

evaluated for its applicability in developing the test plan and to better advice regarding the 

capabilities of this technology. Appendix E provides a summary of relevant literature reviewed.

In general, there has been substantial work in supporting the use of ASD VEWFD systems in 

special applications, such as telecommunications facilities, warehouse, atria, and as a reference 

tool to support evaluation of model prediction of conventional spot-type detector activation. 

Unfortunately, most of the available test programs were developed to acquire data needed to 

support a specific application.
4-1

4. EXPERIMENTAL APPROACH

The purpose of the experiments conducted in this program was to provide a quality data set to 

allow for the evaluation of the responsiveness and effectiveness of aspirating smoke detection 

(ASD) very early warning fire detection (VEWFD) systems and make comparisons to 

conventional spot-type detectors. The experiments were focused on evaluating the 

responsiveness of detectors to aerosols generated from the degradation of polymer 

components commonly found in NPP electrical enclosures. Experiments were conducted at 

three different facilities, a laboratory space at NIST; a 38 m2 floor area, 90 m3 volume fire test 

room (small room) located at the Montgomery County Public Service Training Academy in 

Rockville, MD; and a 93 m2 floor area, 283 m3 fire test room (large room) at Hughes Associates 

Inc., located in Arbutus, MD. 

Experimental configurations were selected to represent a limited range of possible in-cabinet 

and area-wide arrangements and low-energy (incipient stage) fire scenarios, and as such, the 

results alone do not represent a complete performance assessment. The experimental designs 

for the different size scales were developed to assess performance of specific smoke sources;

and variations in the location of sources; detectors; sampling ports; and ventilation (in-cabinet 

and area-wide). Each set of experiments added to the overall performance assessment of 

ASDs in VEWFD applications.

4.1 Detectors

There are currently several vendors that offer air-aspirated VEWFD systems and many vendors 

that offer conventional spot detectors. It was not the intent of this research to perform a product 

comparison or evaluation of specific vendor products, but rather, to provide information on the 

performance of VEWFD systems with regard to the objectives listed in Section 1.3. As such, 

several different air-aspirated VEWFD systems were procured, and are generically identified in 

Table 4-1 as ASD1-ASD5. The sensing technologies of these ASDs included both 

light-scattering and cloud chamber based. Single-zone ASDs have a single sampling pipe 

directed to the detector, thus all sampling locations are incorporated into the air flow being 

monitored by the detector. Multi-zone ASDs have more than one sampling pipe directed to the 

detector and a selection valve cycles between different incoming pipes, thus more than one 

zone can be monitored by an individual multi-zone detector. Conventional spot detectors were 

procured to provide representative spot-type technology comparisons. The following three 

types of spot detectors were included: PHOTO, ION and SS that can be used in a VEWFD 

system.

Table 4-1. Smoke Detection Technologies Generic Identification System

Detector ID Technology Application

ASD1 Single-zone, air-aspirated, light-scattering VEWFD

ASD2 Single-zone, air-aspirated, cloud chamber VEWFD

ASD3 Single-zone, air-aspirated, light-scattering VEWFD

ASD4 Multi-zone, air-aspirated, cloud chamber VEWFD

ASD5 Multi-zone, air-aspirated, light-scattering VEWFD

SS Spot detector head, sensitive photoelectric VEWFD*

PHOTO Spot detector head, photoelectric Conventional

ION Spot detector head, ionization Conventional

* For area-wide installations, VEWFDS sensitivity settings were used, but coverage was the same as 

the conventional spot detectors. 
4-2

The aspirated detector systems ASD1-ASD5 are stand alone, while the spot detectors are 

interfaced to a fire alarm control panel (FACP). Because the spot detectors interfaced to the 

FACP were addressable, the individual detector response times were identifiable.

The intent of this research was to examine the gross differences between VEWFD systems and 

conventional detectors, and the expected performance from a range of VEWFD system 

implementations. Because National Fire Protection Association (NFPA) 76 specifies the 

minimum sensitivity settings above ambient background levels for VEWFD systems as an Alert

level of 0.2 %/ft obscuration at each port (or sensitive spot detector), and an Alarm level of 1.0

%/ft obscuration, these were the target sensitivities for ASDs and the sensitive spot detector. 

These are the minimum allowable levels to meet the definition of VEWFD system. If set to 

these levels (at each hole location), then it may be considered to meet the minimum sensitivity 

requirements of NFPA 76 for VEWFD. For area-wide detection, the added benefit of the active 

air sample would likely improve ASD performance, but in cabinets with very little airflow there 

may be little to no difference. The primary benefit of ASD is that you can set them to alert at 

much more sensitive levels, whereas the 0.2 %/ft obscuration is about the limit for most high 

sensitive spot detectors. In addition to Alert and Alarm settings, ASDs typically have additional 

adjustable settings. Thus, a Pre-alert setting, more sensitive than the Alert setting, may be 

considered to instigate a non-emergency investigation. Conventional photoelectric and 

ionization detector sensitivities were set to the factory default settings. The specific 

photoelectric and ionization detectors were individually addressable and had the feature of two 

sensitivity settings, a Pre-alarm and an Alarm threshold. Per NFPA 72, 2013 Edition, an alarm 

condition poses immediate threat to life, property or mission, while a pre-alarm condition poses 

a potential threat but time is available for investigation. For the testing completed, the pre-alarm 

setting is more sensitive than an alarm setting, but still within the listing of the detectors used.

There was an issue identified concerning sensitivity settings for the procured cloud chamber 

ASDs, which was not fully resolved. These ASDs do not report detector sensitivity in terms of 

ANSI/UL 268 standard engineering units of percentage of obscuration per foot, but, in terms of 

numeric (dimensionless) settings. That being said, for the testing performed to support this 

research, the dimensionless units were converted to a nominal particle number concentration 

obtained from the cloud chamber ASD software. An assumption was made that the number 

concentration is a linear function of the obscuration, as would be the case with low 

concentrations of the same smoke particles. The cloud chamber ASD sensitivity settings were 

selected to represent Alert and Alarm settings that covered a range. These settings were not 

fixed, but may have changed for different experimental configurations. The exception was the 

last series of experiments with the multi-zone cloud chamber ASD, in which the vendor 

commissioned the system, and provided the sensitivity settings. The vendor’s process involved 

initially setting the detector to a gain setting commonly used for the application. Then the 

background signal is observed of a period of time to ensure remains below a specific reading to 

reduce the likelihood of nuisance alarms. If it does (as it did for our testing) that set point is 

maintained. Since this process differs from how the light scattering based detectors were 

configured, direct comparison between the two technologies is not strait forward.

The terms Alert and Pre-alarm are synonymous. Both intended to present a condition where a 

potential threat is posed and sometime of unknown duration is available for investigation. 

However, this report will use the terms consistent with the associated NFPA standards. That is, 

Alert will be used exclusively for VEWFD systems per NFPA 76 and Pre-alarm will be used 

exclusively for the conventional spot-type detectors per NFPA 72. 
4-3

4.2 Incipient Fire Sources

A key to the assessment of any detection system is identifying challenging scenarios that need 

to be detected, then evaluating detection systems against surrogate test conditions that 

represent those scenarios. For example, the performance requirements for residential smoke 

alarms were developed from relevant household fire scenarios, while the performance 

requirements for VEWFD systems in telecommunications facilities were developed from 

scenarios deemed to be appropriate for that application. For NPP in-cabinet and area-wide

applications, there is no consensus opinion characterizing the duration and failure mechanisms 

of a challenging incipient fire scenarios that could have formed the basis for surrogate test 

conditions. Thus, a major task in this research was to develop surrogate test conditions that 

were plausible, and challenging for both in-cabinet and area-wide applications.

It is assumed that the most probable, slowly developing incipient fire sources of the type that a 

VEWFD system would be used to detect, would be electrically initiated. That is, electrical power 

is the energy source used to produce the heat needed for the incipient source. Electrically

initiated fires are often preceded by some form of arcing or joule heating of electrical 

components. The literature lists various ways electrical fires may be initiated including arcing, 

overloads, poor connections and corrosion (Ref. 30, 31, 32). Regardless of the failure 

mechanism, heat is typically conducted from a metallic electrical conductor to an insulating 

polymeric material. Upon heating, insulating polymeric materials degrade, and pyrolysis 

products can condense into smoke particles, which in sufficient concentration, can be detected.

Therefore, to assess the performance of VEWFD systems, smoke sources were developed that 

mimic slow overheat conditions which degrade polymeric insulating materials and produce 

smoke before flaming combustion.

The smoke sources were designed to mimic smoke evolution from a particular scenario likely to 

be experienced in various electrical fires. A current overload, a high-resistance connection, or 

combination of both produces joule heating that conducts heat to polymeric insulating material, 

here, wire insulation, a printed circuit board, or terminal block insulation. As the material heats 

up, it starts to thermally degrade, gases are released, and a fraction forms pyrolysis smoke 

particles. The temperature at which particles are formed, the amount, and particle size depends 

in part on the specific material, including the base polymer and additives. 

The goal was to produce sufficient smoke to initiate alert and alarm conditions in some or all 

detectors during a test. No attempt was made to achieve a flaming combustion transition as a 

test end point. It is not necessary to have flaming ignition to provide a relative comparison 

between detector technologies. It is, however, useful to specify a condition indicative of 

imminent hazard for performance analysis purposes. In the data analysis, the end of test is 

specified as an imminent hazard based on the heating source end temperature, and the extent 

of thermal damage to the materials being heated. The rationale for choosing the final heat 

source temperature achieved at the end of the test as an imminent hazard given the ignition 

potential of the materials studied is detailed below.

Transition to flaming requires an ignition event which would be scenario specific and stochastic 

event. The ignition event could be piloted or non-piloted. A piloted ignition would occur when a 

flammable mixture of the pyrolysis gases and air encounter an electrical arc or spark whereas it 

ignites and a flame is established. A non-piloted ignition would occur when the temperature of 

the degrading material is such that the reaction of the pyrolysis gases and air spontaneously 

ignites and establishes a flame. For solids, piloted ignition is often characterized by a piloted 

ignition temperature specific to a material, while non-piloted ignition is characterized by a non-
4-4

piloted ignition or auto-ignition temperature specific to a material. Piloted ignition temperatures 

are lower than auto-ignition temperatures. Babrauskas tabulated a range of ignition 

temperatures of plastics obtained by various literature sources based on broad polymer property 

classes for both piloted ignition and auto-ignition experiments (Ref. 33). The tabulated values 

are shown in Table 4-2 below. The classes in the table represent a range of electrical insulation 

materials. 

Table 4-2. Ignition Temperatures for Plastics Grouped by Polymer Properties (Ref. 33)

Polymer Property Class Piloted Ignition 

Temperature

Auto-ignition Temperature

°

C

Thermoplastic 369 ± 73 °C 457 ± 63 °C

Thermoset 441 ± 100 °C 514 ± 92 °C

Elastomer 318 ± 42 °C 353 ± 56 °C

Halogenated 382 ± 70 °C 469 ± 79 °C

One can imagine three scenarios influencing the probability of ignition. In the first scenario, an 

ignition source of sufficient strength is available at the onset of material heating. The material 

must be heated to a point where the necessary gaseous pyrolysis fuel and air form a flammable 

mixture. At that time, the gases ignite and establish a flame. This would be piloted ignition at 

the lowest possible material temperature. An example would be continuous arcing that provides 

sufficient heating to form a flammable mixture, with the arc itself acting as the ignition source. 

In the second scenario, no piloted ignition source is available, and the material must be heated 

to its auto-ignition temperature, whereas the material ignites. Whereas the auto-ignition 

temperature is higher than piloted ignition temperature, these two temperatures bound the 

minimum temperatures for ignition, below the minimum piloted ignition temperature the 

necessary conditions do not exist for ignition, and at or above the auto-ignition temperature both 

necessary and sufficient conditions exist and ignition will occur. 

The third scenario involves an intermediate temperature range above the minimum piloted 

ignition temperature and below the auto-ignition temperature. This scenario would involve the 

introduction of an ignition source at some time when the material is in the intermediate 

temperature range. The introduction of an ignition source would be a stochastic ignition event 

involving a shorting spark, a wire breaking spark, a tracking arc, or a glowing hot surface either 

of which at the right location and of sufficient energy to ignite the material. For a current 

overload condition, or a high resistance connection, it is plausible to assume that the probability 

of any stochastic ignition event described above would increase as the heating time increased 

because of the level of local damage to the insulating material.

Given the smoke sources developed for the VEWFD experiments, the ignition scenario above is 

an appropriate assumption to make, where some stochastic ignition event occurs before autoignition of the material being heated. The end of test heating source temperature was initially 

selected as 450 °C, for scoping experiments conducted in the laboratory at NIST, and 

subsequently raised to 485 °C to produce more smoke at the end of the test for full-scale 

experiments. Given the tabulated values of the piloted and auto-ignition temperature ranges 

above, the end of test was specified as a condition of imminent hazard because the end of test 

heat source temperature is in the range of auto-ignition temperatures and the materials being 

heated appear to be above piloted ignition temperatures based on temperature measurements 
4-5

detailed in Section 5.8. Also, ignitability experiments in Section 5.8 show that piloted ignition is 

achievable at heating source temperatures well below the end of test value for three downselected materials. A more detailed analysis of ignition scenarios or ignition probability is not 

within the scope of this research. 

A range of polymeric materials was initially selected for research, including electrically insulated

materials, representing polymers found in nuclear power plants (Ref 31), and other electrical 

insulating materials. It is important to note here that such materials are not virgin polymers, but 

a mixture of polymeric material and additives necessary for processing, electrical 

characteristics, flame retardancy, etc. The first 11 materials listed in Table 4-3 were used in the 

incipient smoke source experiments. Other materials included in this research, which were not 

considered slowly developing incipient sources, were overheated resistors and capacitors, an 

insulated wire used in standard tests (British Standard BS 6266), and smoldering, shredded 

copy paper. These materials were used to provide challenging, but short-lived smoke sources 

for in-cabinet experiments with resistors and capacitors; a non-polymeric smoldering source for 

area-wide experiments; and a standardized source to assess area-wide system design.

Experiments with these sources were included as reference sources and were not considered in 

any analysis of VEWFDS effectiveness in this report. 

The materials used in the in-cabinet and area-wide experiments are described in Table 4-3. 

The material names and ID numbers are short-hand descriptions used in the report; wire 

descriptions refer to conductor size using the American Wire Gauge (AWG) nomenclature.

The RoHS descriptor refers to materials that pass the European Union “Restriction of 

Hazardous Substances Directive”.
4-6

Table 4-3. Materials Used in Experiments

ID # Name Description of Material*

1 PVC1 Polyvinyl chloride insulated, 18 AWG wire, RoHS lead-free

2 PVC2 Polyvinyl chloride insulated, 14 AWG wire, RoHS lead-free

3 Silicone Silicone insulated , 18 AWG wire, RoHS lead-free

4 PTFE wire Polytetrafluoroethylene insulated, 14 AWG wire, lead free 

5 XLPO1 Cross-linked polyolefin insulated, 12 AWG wire, RoHS lead free 

6 XLPO2 Cross-linked polyolefin insulated, 12 AWG wire, lead free 

7 XLPE Cross-linked polyethylene insulated, 12 AWG wire, lead free 

(Synthetic Insulated Switchboard, SIS wire) 

8 CSPE Chlorosulfonated polyethylene insulated, 10 AWG wire , lead free

9 PCB FR4, glass-reinforced epoxy laminate circuit board

10 TB Phenolic barrier terminal block

11 Cable Bundle NPP cable XLPE jacket, XLPO insulation 7 wire, 12 AWG wire

12 Resistor 12 ohm, 1⁄4 W, carbon film resistor

13 Capacitor Small electrolytic can type

14 BS 6266 Wire PVC, BS 6266 test wire

15 Shredded Paper Copy paper run through paper shredder, ignited with a smouldering wick

* Wires 4, 6, 7, and 11 were classified as qualified per the flame propagation- test of IEEE 383-1974, “IEEE 

Standard for Type Test of Class 1E Electric Cables, Field Splices and Connections for Nuclear Power 

Generating Stations.”

4.2.1 Bus bar heat source (insulated conductors, terminal block and printed circuit 

board)

The first 10 materials were degraded by conduction of heat from a copper block bus bar whose 

temperature was ramped from ambient to 450 oC or 485 oC. The bus bar was 9.84 cm long and 

3.2 cm square with a 9.5 mm hole drilled out along the long axis to accommodate a 500 watt 

electric cartridge heater. Figure 4-1 is a schematic of the bus bar. 
4-7

Figure 4-1. Schematic of the bus bar

Wire samples were cut to 10.0 cm lengths, with 3.0 cm of insulation stripped from one end.

Up to five samples of the same wire type were attached to the bus bar by wrapping the stripped 

wire end around a machine-head screw that passed through the bus bar and was held in place 

with a nut. The screw was tightened to the bus bar with a torque wrench to 110 ± 5 N-m. Wires 

were mounted such that a 5-mm length of stripped wire separated the bus bar from the 

insulated wire and 7 cm of insulated wire extended horizontally from the bus bar. An example of 

a prepared assembly before heating is shown in Figure 4-2. (Smoke production from wire 

sources was adjustable based on the number of wires attached to the bus bar. Early scoping 

experiments were conducted with fewer than five wires, but it was observed that the best 

chances for conventional alarm response were realized with the five-wire arrangement.) 
4-8

Figure 4-2. XLPO2 wire mounted to the bus bar

The bus bar was mounted on a stand such that the wires were located 13 cm above the ground.

The stand had four posts that fit inside the bus bar through holes. The stand provided a stable 

platform, and thermally insulates the copper block from the floor to some extent. The bus bar 

with wire samples attached and placed on the holder, can be seen in Figure 4-3. The assembly

was then placed inside a cabinet or on the floor for area-wide experiments. A cartridge heater 

was inserted into the bus bar, a ground wire was attached to the stand, and the thermocouple 

was connected to the temperature controller.

5 mm stripped wire

7 cm insulated wire

Bus bar

Thermocouple
4-9

Figure 4-3. The bus bar mounted on the stand inside a cabinet

Six heating profiles were specified using three set point heating ramp periods (HRPs) and two 

final set point ending temperatures. The first set of preliminary experiments were conducted 

with three heating profiles: a 15 minute set point ramp from ambient to 450oC with a 5 minute

soak period where the set point remained at 450oC, a 60 minute ramp to 450oC with a 5 minute

soak, and a 240 minute ramp to 450oC with a 5 minute soak. Subsequent experiments were 

conducted using three set point ramps to 485oC, keeping the same slope as the first set, which

extended the HRPs to 16.3, 65, and 260 minutes, and maintaining the 5 minute soak period for 

total heating times (THTs) of 21.3, 70, and 265 minutes, respectively. Set points read back from 

the controller and bus bar temperatures were recorded in a heater log file for all experiments.

The three heating profiles of nominally 15, 60, and 240 minute heating duration represent a 

factor of four increase from the first to the second, and the second to the third heating duration, 

and heating rates of 28.33oC/min, 7.08oC/min, and 1.77oC/min, respectively. 

Actual failure mechanisms that cause heating of polymeric insulating materials could present 

other than linear heating profiles in the incipient phase that could be increasingly faster or 

slower than a linear heating ramp. Linear heating ramps were chosen since actual heating 

profiles are unknown, and the three linear heating ramps cover a wide range of heating rates.

Figure 4-4 shows set point ramps for 15, 60, and 240 minute HRPs along with two bounding 

heating profiles, a logarithmic and exponential increase in the heating profiles ending at 485 °C 

after 1 hour. The three linear ramp slopes are bound by the logarithmic profile from 265 °C to 

412 °C, and the exponential profile from 41 °C to 446 °C which covers a temperature range 

where degradation is detectable by the ASDs.

Copper bus bar

Electric cartridge heater

Wire sample

Thermocouple

Stand
4-10

0

100

200

300

400

500

0 500 1000 1500 2000 2500 3000 3500

15 min HRP

60 min HRP

240 min HRP

Logarithmic

Exponential

Bus Bar Temperature (

oC)

Time (s)

Figure 4-4. Heating ramp profiles for 15, 60, and 240 minute HRPs, with logarithmic 

and exponential bounding profiles that reach 485 °C in 60 minutes

A load cell was used to weigh the wire samples before and after each test. The expanded 

combined uncertainty of the mass measurement was 5 mg. Other test samples were not

weighed. 

In addition to the wire samples, printed circuit board (PCB, 10.1 cm by 10.2 cm by 0.1 cm thick) 

and phenolic resin terminal block (TB, 5.5 cm by 1.8 cm by 0.6 cm thick) samples were also 

used in experiments. The PCB was clamped between two 25 mm by 10 mm by 6 mm thick 

copper plates, with the bolts tightened using a torque screwdriver to 110 ± 5 N-m. Because the 

degradation was localized to the side being heated, a PCB could be reused for an additional 

experiment. The terminal block was mounted to a copper plate (2.5 cm by 10 cm by 0.3 cm 

thick) with two screws, with the plate then bolted to the bus bar. A mounted PCB and terminal 

block can be seen in Figure 4-5 and Figure 4-6, respectively. 
4-11

Figure 4-5. Printed circuit board (PCB) mounted to the bus bar

Figure 4-6. Terminal block (TB) mounted to the bus bar

Copper plate

Printed circuit board

Thermocouple

Copper plate

Phenolic resin terminal block

Sample mounting

bolt

Thermocouple
4-12

4.2.2 Cable bundle sample preparation

The cable bundle source was a 7-wire bundle of 12 AWG cross-linked polyolefin insulated 

(XLPO) insulated wires with an outside jacket of cross-linked polyethylene (XLPE). The cable 

diameter was 13 mm. Six 12 cm long pieces were attached to a 15.2 mm long, 12.7 mm OD, 

11.1 mm ID copper tube. The cable sections were held firmly to the tube by three nickelchromium wires wrapped tightly around the cables. A thermocouple was clamped to one end of 

the copper tube (away from the cable sections) for temperature control, and was attached to the 

same temperature controller used for the bus bar block. A 15 cm long 9.5 mm diameter 400 W 

heater cartridge was inserted inside the cooper tube as seen in Figure 4-7. The 400W heater 

replaced the 500W heater because the extra power was not needed to elevate the surface of 

the cable material to the 485 °C and the 400W heater was longer, allowing for more contact with 

the cable samples. Three HRPs were specified for these experiments: 16.3, 65.0, and 260.0

minutes, and THTs to 21.3, 70.0, and 265.0 minutes, respectively. The cable bundle rested on 

ceramic insulating paper on top of an aluminum foil-covered piece of gypsum wallboard. 

Because the cartridge heater did not fit tightly inside the copper tube, but would tend to rest on 

the bottom of the tube, the inner tube may have experienced temperature non-uniformities.

However, the relatively high thermal conductivity of the cooper tube will tend to reduce any 

exterior temperature non-uniformity. Because of the limited number of experiments with the 

cable bundle, repeatability of the source was not evaluated.

Figure 4-7. The cable bundle sample

Copper tube

Wire bundle

Thermocouple

Electric cartridge heater

Insulation
4-13

4.2.3 BS 6266 PVC wire test

A series of heated wire tests were performed, following the performance test procedures in 

Annex B of NFPA 76, 2012 Ed., and based on British Standard BS 6266. A 1 m long polyvinyl 

chloride (PVC) insulated wire was heated for 60 seconds by passing a current through it from a 

power supply capable of generating current up to 30 amps at 6 VAC. Additional tests were 

based on a modified BS 6266 test which involves replacing the single wire with two 1 m wires in 

parallel. Wire samples were placed on electrically and thermally insulated ceramic paper on top 

of gypsum wallboard. An example of a single wire configuration can be seen in Figure 4-8. The 

current causes significant resistive heating that subsequently cooks off the PVC insulation. For 

these tests, any alarm verification or time delay features were disabled. A pass/fail criterion of 

detection system response (any sensitivity) was used within 120 seconds after the end of the 

electrical power application for the single wire source or Alert for two parallel wire tests for 

VEWFD systems. 

Figure 4-8. A single wire test was setup following the British Standard 6266

Experiments were conducted by exposing resistors and capacitors to excessive voltage. The 

methodology closely followed the Fire Industry Association’s “Code of Practice for Design, 

Installation, Commissioning, and Maintenance of Aspirating Smoke Detection (ASD) Systems”

using resistors. A set of three 12 Ohm, 0.5 Watt resistors were wired in parallel. The BS 6266 

6 VAC power supply was used as the power source. The power supply timer was set to 90

seconds and turned on. After 90 seconds the power supply automatically turns off. Detectors 

were monitored for an additional 300 seconds. Similarly, two capacitors were mounted in 

parallel. The power supply timer was set to 30 s. After the power was shut off, the alarms were 

monitored for an additional 300 s. The setup for the resistor and capacitor tests can be seen in

Figure 4-9, and Figure 4-10, respectively. A 15 cm diameter plastic dome with holes drilled 

through it (Figure 4-11) was placed on top of the setup during experiments to contain debris, in 

case of any material expulsion, but allow for smoke to escape.

Power supply

Wire sample Insulation
4-14

Figure 4-9. A set of three resistors wired in parallel.

Figure 4-10. A pair of capacitors wired in parallel

Resistors

Ceramic terminal 

blocks

Capacitors

Ceramic terminal 

blocks
4-15

Figure 4-11. Dome enclosure for the resistor and capacitor tests

4.2.4 Shredded paper smoldering source

Two experiments were conducted with a smoldering paper source. The purpose of introducing 

this source was to evaluate detector response to smoldering conditions. Two cotton wicks

(same used in UL smoke box) were inserted into a clean one gallon can, filled with shredded 

paper. Approximately two handfuls of shredded copy paper were used. The wicks were ignited 

before they were placed inside the can. The test began when the sample was placed in the 

center of the test room, and concluded when the fire transitioned to flaming combustion. The 

test generated a substantial amount of visible smoke during the smoldering process before it 

transitioned to flaming combustion. An example of the smoldering paper can be seen in Figure 

4-12.

Figure 4-12. Shredded paper test

Dome enclosure

Can

Shredded paper

Vent holes
4-16

4.3 Measurement and Control Instrumentation

Data acquisition and control of the bus bar or cable bundle heater were accomplished by a 

program running on a PC. Details of the programs are given in the Appendix.

A humidity probe with a built-in thermistor was used to record the relative humidity and air 

temperature during the experiments. For the small room, area-wide setup the center room 

vertical temperature profile was recorded. A thermocouple tree was placed in the center of the 

room, with seven Type-K thermocouples respectively placed 2.54 cm, 5.08 cm, 7.62 cm, 

0.31 m, 0.61 m, 0.914 m, and 2.13 m below the ceiling. A set of four more thermocouples was 

placed on the ceiling in the center of each quadrant of the room. 

4.3.1 Temperature controls

The front panel of the main program had a button to open and start the temperature controller 

program. The ramp slope was fixed by the final set point temperature and the heating rate 

period. About every 30 seconds, the program sends the temperature controller a new set point.

During the update, the power to the heater is disabled for about 5 seconds. This latency 

produced little lag in the rate of block temperature rise. The soak period was specified by a 5.0

minute heating period at the final set point temperature. During the soak period, the program 

stops updating the controller, thus the power is not disabled during the 5 minute soak period. 

Preliminary tests identified a problem of potential contamination of the bus bar or cartridge 

heater during handling, leading to particle generation upon heating of a bus bar with no wires 

attached. Thus, after every experiment, the bus bar previously used was heated by itself in a 

separate location for a cleaning cycle. This would remove residue from the previous experiment 

or handling. The reheating took about 20 minutes, which includes a 15 minute heating ramp

period and a 5 minute soak period. A total of three bus bars were used during the experiments 

and were regularly rotated during testing; after an experiment the bus bar was put through its 

cleaning cycle, while a prepared test sample bus bar was readied for the next experiment and a 

clean bus bar cooled, waiting for sample preparation.

4.3.2 Smoke detector monitoring

The ASD VEWFD response times were recorded by monitoring the state of the ASD relay 

switches on digital inputs of the data acquisition card. The FACP detector response times were 

obtained by monitoring the data stream that typically is sent to a printer or other output device.

The ASCII text data was parsed by the program automatically to light program indicator buttons 

and log the pre-alarm and alarm times of all detectors; up to 16 detectors were monitored during 

these experiments.

4.3.3 Aerosol instrumentation

Selected aerosol measurement instrumentation was used to gather particle size and aerosol 

concentration data at the ASD sampling locations for laboratory in-cabinet experiments. An 

electrical low pressure impactor (ELPI) measured the aerosol size distribution during most small 

cabinet experiments to help characterize the smoke sources. Additional experiments were 

conducted with three instruments that recorded the particle number concentration (zero-th 

moment of the size distribution), mass concentration (third moment of the size distribution), and 
4-17

the total aerosol length, (a measure of the first moment of the size distribution). These 

measurements complement the ELPI data. The aerosol data is detailed in Appendix B.

4.3.4 Ambient environment measurements

Measurements were taken at all locations to monitor the ambient environment before, during,

and after each test. For small-scale tests, ambient temperature was monitored near the top and 

bottom of the cabinet. In most laboratory experiments, the particle concentration was also being 

recorded. The large-scale tests in the 38 m2 room had its room temperature monitored at 

various heights in the center of the room. A portable particle counter was used to monitor the 

ambient particle concentrations. Humidity was monitored at all the test locations for the duration 

of all tests. Background information was taken for up to 120 seconds.

4.3.5 Thermal imaging

Some small-scale, in-cabinet experiment setup used an infrared camera to monitor the 

temperature of the heat source and degrading samples. The camera was placed 1 meter above 

the sample, on the outside of the test cabinet. An IR window was installed between the camera 

and the sample. A series of top view, thermal images were taken using an FLIR E30 infrared 

camera. Twenty images were taken for each heating rate. The time intervals between the 

images were 1, 3 and 10 minutes for 15, 60, and 240 minute ramps, respectively. 

The emissivity of the bus bar was taken to be 0.78, that of oxidized copper, and was kept 

constant for all the tests. The transmissivity of the IR window was taken to be 0.5. The camera 

was controlled remotely from the computer via a USB cable. The settings on the camera, such 

as emissivity and transmissivity, could be changed after the test by using the accompanying 

software. The camera was limited to measuring temperatures up to 370oC. An example of a 

temperature profile measured using the thermal camera can be seen in Figure 4-13. All images 

for the monitored experiments are given in the Appendix.

Figure 4-13. Thermal image of XLPE wires following a 60.0 minute heating ramp and a 

5.0 minute soak period
4-18

4.4 Experimental Configurations

4.4.1 Laboratory, instrument cabinet configuration

A laboratory space was used for the small cabinet experiments. The cabinet was an empty 

instrumentation cabinet with dimensions of 0.56 m by 0.61 m by 1.32 m tall. While the height of 

this cabinet is shorter than those commonly encountered in NPP facilities, it is a reasonable

surrogate to represent an upper portion of an NPP cabinet. The cabinet was placed inside a 

ventilated enclosure. The laboratory instrument cabinet experimental configuration can be seen 

in Figure 4-14.

Figure 4-14. Laboratory instrument cabinet experimental configuration

The instrument cabinet had spot detectors and air-sampling detector ports installed on its 

ceiling. Figure 4-15 shows how they were mounted inside the cabinet, and a schematic of the 

top plate hole pattern is shown in Figure 4-16. The ASD detector units were located outside the 

cabinet and individual but identical piping networks were used to transport air samples from 

A

B

C

D

E

F

A: ASD piping D: Fire alarm control panel

B: Ventilated enclosure E: ASD systems

C: Instrument cabinet F: Blower motor
4-19

within the cabinet to the detector units. The pipe used was a 1.91 cm inside diameter (ID)

chlorinated polyvinyl chloride pipe (CPVC). Flexible tubing, 1.24 cm ID, was used to connect 

the sampling port to the CPVC piping. The ASD piping layout can be seen in Figure 4-17. Each 

detector had four sampling ports, one routed to the inside of the cabinet and three sampling 

laboratory-space ambient air. The sampling port diameters were 3.2 mm. 

Figure 4-15. Instrument cabinet ceiling view

A: Spot detectors E: Top vent holes 

B: ASD sampling ports F: IR camera view port

C: Aerosol sampling port G: Side vent holes

D: Thermocouple

A

B

B

E

A

A

B

E

E E

C D

G

G

F
4-20

Figure 4-16. Instrument cabinet top plate hole pattern

Figure 4-17. ASD pipe layout for the instrument cabinet experimental setup

The cabinet had two sets of vent holes, four through the ceiling and three each on the left and 

right sides. The locations of the top vent holes and some of the side vent holes can be seen in

Figure 4-16. The top vent hole configuration consisted of four 5.08 cm holes, while the side 

vent configuration had six. The side vent holes, consisting of three holes separated by 0.18 m, 

were placed on left and the right sides 0.2 m from the top of the cabinet. Each test had either 

the top or side vents opened, but in no test were both open. 
4-21

The cabinet was located inside a ventilated enclosure to contain and evacuate the smoke. The 

ventilated enclosure was 0.91 m by 0.91 m and 1.75 m tall. A blower motor installed on top of 

the enclosure provided the ventilation to exhaust smoke fumes through the top of the cabinet,

and into the laboratory fume hood. A 3.8 cm gap between the bottom of the enclosure and the 

floor, allowed fresh air to enter the enclosure. Inside the test cabinet the air was quiescent, 

except for the thermal plume generated by the heat source. A close-up of the cabinet inside the 

ventilated enclosure can be seen in Figure 4-18.

Figure 4-18. Instrument cabinet inside NIST laboratory

4.4.2 Laboratory, large NPP cabinet configurations

These experiments used two surplus NPP electrical cabinets, which can be seen in Figure 4-19.

The cabinets were 0.61 m by 0.61 m by 2.13 m tall. The cabinet on the left (Cabinet 1) had 

cable bundles hanging from the back wall and was naturally ventilated. The cabinet on the right 

(Cabinet 2) was a compartment with multiple shelves with circuit card slots in them; no circuit 

cards were in place. A piece of sheet metal covered the front opening simulating a 

compartment with all the circuit cards in place. Each cabinet was placed inside the ventilated 

enclosure for experimentation, which can be seen in Figure 4-20. Spot smoke detectors and 

ASD sampling ports were installed inside at the top of each cabinet, as shown in Figure 4-21.

Only ASD2 and ASD3 were installed during these experiments. SS and ION spot detectors 

D

A: IR camera D: Source location

B: Sampling tubes E: Humidity probe

C: Instrument cabinet F: Cabinet door

A

B

C

E

F
4-22

were installed inside the naturally ventilated cabinet, while the force-ventilated cabinet only had

space for the ION spot detector.

Figure 4-19. NPP cabinets used in large cabinet experiments

A: Source location D: Naturally ventilated cabinet 

B: Air vents E: Force ventilated cabinet

C: Cabinet enclosure

D

A

B

C

A

B

C

E
4-23

Figure 4-20. Large cabinet installation and enclosure detail

Figure 4-21. View of sampling ports and spot detectors installed inside the naturally 

ventilated (left) and force ventilated (right) NPP large cabinets

A: Air purifier E: Force ventilated cabinet

B: Cabinet cooling fan F: Naturally ventilated cabinet

C: Exhaust fan G: Particle counter

D: Cabinet enclosure H: ASD piping

C

A

B

D E

H

C

D

F

G

H

A: Spot detectors C: Air vent 

B: ASD sampling ports D: Aerosol sampling port

A

B

C

A

B

B

D
4-24

Cabinet 2 had a blower installed on top that was used for cooling of the circuit cards. The 

blower forced air down through a series of vents and across the circuit card slots, then back up 

to the top of the cabinet. To limit the effects of air recirculation in the cabinet enclosure, air to 

the blower was sampled from the bottom of the enclosure into a plastic shroud containing a 

HEPA-filter air cleaner. A close-up of the top of the cabinet with and without the blower can be 

seen in Figure 4-22. This configuration was meant to simulate a large compartment with 

abundance of clean air, such that the air being sucked into the cabinet would be clean air.

ASD1 was used to monitor the air entering the cabinet.

Figure 4-22. The ventilation configuration of the forced ventilation cabinet

4.4.3 In-cabinet, small room, cabinet mock-up configurations

The single-zone experiments were conducted at the Montgomery County Public Safety Training 

Academy, in the Burn Prop building. A space on the lower floor was used to configure an 8.2 m

by 4.6 m room containing electrical cabinet mock-ups. The ceiling height was 2.4 m. A forcedair ventilation scheme could be implemented in the room using a variable-speed blower and 

ducting to direct air flow to wall registers, which exhaust air grills vented to the outside of the 

building. The cabinet mock-ups were constructed to simulate naturally ventilated, or forced-air 

ventilated electrical equipment cabinets. Individual cabinets were 0.61 m wide by 0.61 m deep 

and 1.78 m tall. Figure 4-23 shows the small room experimental space layout. Photographs of 

the setup can be seen in Figure 4-24 and Figure 4-25.

A: Cabinet Air inlet D: Force ventilated cabinet

B: Blower E: Cabinet enclosure

C: Blower enclosure 

A

D

E

B

C

D

E
4-25

Figure 4-23. Small room full-scale experiment space layout
4-26

Figure 4-24. View of the small room experimental space

E

A A

B

C

D

D

D

A: Cabinet mock-ups D: Cabinet ventilation fans

B: Sampling tubes E: Room ventilation air inlet

C: ASD pipes
4-27

Figure 4-25. View of the small room full-scale experimental space ceiling

Experiments were conducted with ASD1, ASD2, and ASD3 running simultaneously. The 

in-cabinet configurations had spot detectors installed in a similar fashion to the small-scale 

setup. The aspirated smoke detectors had ports installed into the top of selected cabinets, 

which directed the sampled air to the piping connected to each ASD VEWFD system.

Single-cabinet and multiple open-side cabinet designs were constructed in a row with top 

ventilation. The following (three) cabinet configurations were tested: single cabinet with ASD 

ports and spot detectors; a 4-cabinet arrangement where two cabinets had ASD ports and spot 

detectors; and a 5-cabinet arrangement where three cabinets had ASD ports and spot 

detectors. Figure 4-26 shows the alarm layout inside the 5-cabinet configuration. The cabinets 

were raised 5.08 cm above the ground to allow air to enter.

A: Cabinet mock-ups C: ASD pipes

B: Sampling tubes D: Room ventilation air inlet

A

A

C

B

D
4-28

Figure 4-26. Five-cabinet spot detector and ASD sampling port configurations

Figure 4-27 shows the ASD piping layout for in-cabinet sampling configuration. The four- and 

five-cabinet arrangements had ceiling ventilation holes with ASD ports and spot detectors, and 

no internal side wall partitions.

A

C

C

C

C

B

A

A A

B

B

B

C C

C

C

A: Spot detectors C: Vent holes

B: ASD sampling ports
4-29

Figure 4-27. In-cabinet ASD pipe layout

Three small muffin fans provided forced ventilation during some experiments. They were 

installed in the first, third, and fifth cabinets (in each) of the 5-cabinet configurations, on the back 

wall along the central axis, and 30.5 cm above the cabinet floor. The fans were installed in the 

cabinets that had the spot alarms and the ASD sampling ports. Unlike the previous tests, the 

openings on the bottoms of the cabinets were covered to prevent air entrainment. The fans can 

be seen in Figure 4-24. 

Figure 4-28 shows in-cabinet and area-wide source locations. The samples were placed in 

cabinets 1, 2, and 7 for the single cabinet, 4-cabinet and 5-cabinet experiments, respectively.

The spot alarms and ASD sampling ports were located in cabinets 1, 3, 5, 6, 8, and 10. The 

in-cabinet layout for the spot detectors, the ASD sampling ports and the vent holes can be seen 

in Figure 4-27. 

For area-wide ASD experiments, the samples were placed in either cabinet 7, the center of the 

room or one of the room quadrants.
4-30

Figure 4-28. Smoke source sample locations

4.4.4 Area-wide, small room configuration

The area-wide ASD configuration had spot detectors on the ceiling of the room. The piping 

system for the aspirated smoke detectors was modified for this configuration. Area-wide piping 

and detector locations can be seen in Figure 4-29. The area-wide piping had 3.2 mm holes 

(DIA) drilled into it to serve as sampling ports. There were four sampling holes in total for each 

detector.
4-31

Figure 4-29. Area-wide ASD pipe layout

4.4.5 In-cabinet, large room, electrical cabinet configurations

The experimental space for the large-room, full-scale experiments was 10 m by 10 m by 3 m 

high ceiling with a variable speed ventilation fan. The facility can be seen in Figure 4-30 and 

Figure 4-31, and the complete layout can be seen in Figure 4-32. With access doors closed, air 

was pulled through two openings located on the ceiling in the rear of the room, and exhausted 

at the front through a 76.2 cm high by 61.0 cm wide louver with a 49.5 cm by 54.6 cm opening 

behind it with the center of the louver located in the center of the wall.
4-32

Figure 4-30. Off angle view of 100 m2 facility

A: Source location D: In-Cabinet ASD Sampling Lines

B: Test Cabinets, Instrumented E: Areawide ASD piping

C: Test Cabinets, Not instrumented F: Areawide sampling port

A

C

D

B

E

F
4-33

Figure 4-31. Front view of the 100 m2 facility

A

B C

D

A: Source location, Wire bundle C: Test Cabinets, Not Instrumented

B: Test Cabinets, Instrumented D: In-Cabinet ASD Sampling Lines

 

 
4-34

Figure 4-32. Large room testing facility layout

Experiments were conducted with single-port ASD VEWFD systems for in-cabinet coverage and 

multi-port ASD VEWFD systems for in-cabinet and area-wide coverage. Figure 4-33 shows the 

ceiling-mounted and in-cabinet spot detector locations. Ceiling mounted detectors included 

photoelectric, ionization, and sensitive spot detectors, while in-cabinet detectors included 

ionization and sensitive spot detectors.

Figure 4-34 shows the detector layout inside the cabinet and the ventilation hole pattern, and 

Figure 4-35 gives the locations on the ceiling plate. Figure 4-36 shows the detector layout, 

ASD sampling pipe vertical entrance, and ventilation configuration between cabinets in the 

three-cabinet bank.
4-35

Figure 4-33. Spot detectors layout inside 100 m2 facility
4-36

Figure 4-34. Detector and sampling port locations for single-zone experiments

A: Spot detectors C: Vent holes

B: ASD sampling ports D: Cabinet door

A

B

D

A

B C

C

C

C
4-37

Figure 4-35. Detectors, sampling ports, and vent hole locations for in-cabinet 

experiments
4-38

Figure 4-36. Detector layout and side vent location

Figure 4-37 shows the piping diagram for the single-zone, in-cabinet experiments. Separate 

piping with a single port in four cabinets was directed to either ASD2 or ASD3. One cabinet was 

isolated from adjacent cabinets by sealing the side wall opening. Figure 4-37 and Figure 4-38 

show details of ASD4 (cloud chamber type) configuration where individual pressure regulators 

were installed in piping. These allowed for flow adjustments. 

Figure 4-40 and Figure 4-41 show the details of the openings between the three-cabinet 

configurations. Figure 4-42 shows the source locations.

D

E

D

C

C

C C

B

A

B

A

A: Spot detectors D: Side vents 

B: ASD sampling ports E: ASD piping

C: Top vents
4-39

Figure 4-37. Single zone piping configuration for ASD2 and ASD3
4-40

Figure 4-38. Front view of the ASD4 piping setup for the in-cabinet sampling

Figure 4-39. Pressure regulator located above each sampling port for ASD4
4-41

Figure 4-40. Side view of a cabinet, showing the location of the side vents
4-42

Figure 4-41. Side vents allowing flow between three cabinets with ASD sampling

Side vents
4-43

Figure 4-42. Sample locations in the experiments performed

4.4.6 Area-wide, large room configurations

In the multi-zone ASD VEWFD experiments, two area-wide smoke detection zones were 

covered, (i.e., the return air grill, and the ceiling). A separate zone monitored the four-cabinet 

that were monitored during the single-zone experiments. Figure 4-43 and Figure 4-44 show a 

plan view of the piping network layout for the return air grill, and the cabinets for ASD4 and 

ASD5. Figure 4-45 and Figure 4-46 show elevation views of the return air grill piping network 

layouts. Figure 4-47 shows a picture of the return air grill and the piping for the two ASDs. 

Figure 4-48 and Figure 4-49 show the plan view of the area-wide ceiling ASD piping networks. 

The piping network design was supplied by the system vendor, and also met the intent of 
4-44

NFPA 76. For conventional detection, per NFPA 72, no return air monitoring is required. Duct 

detectors were not able to be installed at the facility where the testing was being performed.

Figure 4-43. ASD4 piping layout for the in-cabinet and HVAC inlet sampling
4-45

Figure 4-44. ASD5 piping layout for the in-cabinet and HVAC inlet sampling
4-46

Figure 4-45. ASD5 piping layout, with sampling locations located at the HVAC exhaust

Figure 4-46. ASD4 piping layout, with sampling location located at the HVAC exhaust
4-47

Figure 4-47. Return air grill protected with ASD piping

A: ASD 5 B: ASD 4

A

A

A

A

A

A

B

B
4-48

Figure 4-48. Area-wide ASD4, piping layout
4-49

Figure 4-49. Area-wide ASD5, piping layout

4.5 Experimental Procedure

The experimental procedure for each set of experiments is detailed below. The protocol 

included verifying detector configuration and design, including ASD smoke transport times and 

individual port suction pressures, and identifying individual spot detector locations. Either 

heated wire test per NPFA 76 Annex B.2 or vendor recommended performance tests were 

conducted before each series of tests.

4.5.1 Laboratory instrument cabinet experiments

The initial setup of the instrument cabinet detectors and ASDs verified that the spot detectors 

responded to challenging smoke; the pre-alarm and alarm times were recorded properly and the 

ASD system setups were performed, and faults cleared. The suction pressure at each ASD port 

was measured to verify that it was nominally the same as the others, and thus, presumably, 
4-50

produces equal flows for the ports leading to each ASD. A pressure gauge with a range of 

0.20 kPa was used. Flexible tubing was attached to the low pressure port of the gauge and the 

ASD port. The port pressures were 0.20 kPa, 0.14 kPa, and 0.15 kPa for ASD1, ASD2, and 

ASD3, respectively. Smoke from a smoldering punk was used to verify the smoke transport 

times. Each ASD responded to smoke presented at the furthest sampling port within 10 s. 

At the beginning of each day that experiments were to be conducted, the ASDs, FACP, and 

electrical low pressure impactor were turned on and allowed to run for at least 30 minute before

the start of an experiment. The data acquisition computer started logging typically 1 or 2 minute

before starting the heating ramp. The end of the experiment was set at the end of the heating 

period. The bus bar was allowed to cool, and any residual smoke was exhausted from the 

enclosure during sample cool-down. 

4.5.2 Laboratory large-cabinet experiments

The initial setup of the large-cabinet experiments followed the instrument cabinet setup. Only 

two ASDs were used to monitor the cabinets, ASD2 and ASD3. The measured port suction 

pressures were the same for each ASD piping arrangement, 0.11 kPa for ASD2, and 0.12 kPa 

for ASD3.

One large cabinet, used for ventilated cabinet experiments, had a cooling fan mounted on top.

Experiments were conducted to estimate the fan flow given the specific internal arrangement of 

the cabinet as tested. Carbon dioxide was injected at a fixed rate into the fan inlet. The carbon 

dioxide mixed with the incoming air, and was diluted to a lesser value based on the ventilation 

air. Injection of ~ 10 l/min of carbon dioxide yielded 0.20 percent (by volume), once mixed with 

the ventilation air. The mean of four measurements was 4,800 l/min ± 100 l/min. Assuming a 

cabinet volume of 0.8 m3, an air exchange rate of about 360 air changes per hour (ACH) would 

be expected.

4.5.3 Small room in-cabinet experiments

The small-room experiments posed some challenges, since the offsite test location had no 

central heating, and the experiments were conducted in the month of December. Electric space 

heaters were used to heat the small room and the attached data acquisition room. The heaters 

were turned on in the morning and allowed to heat the test room to a temperature above 12oC. 

A vertical array of thermocouples stretching from 30 cm above the floor to the ceiling was

monitored to determine the temperature gradient. The maximum temperature difference was 

typically about 1oC. 

The suction pressures for the six in-cabinet sampling ports for the three ASDs were 0.12 kPa, 

0.07 kPa, and 0.08 kPa for ASD1, AD2 and AD3, respectively. 

The smoke transport time was measured at cabinets 1 and 6. Those cabinets represented the 

closest and furthest points in the setup. The average smoke response times for three 

measurements at the first cabinet were 19.4 ± 2.4, 33.2 ± 6.7, and 13.7 ± 0.2 seconds for ASD1, 

ASD2, and ASD3, respectively. The measurements at the last cabinet were 20.3 ± 1.8, 32.0 ± 

1.7, and 15.3 ± 1.2 seconds for ASD1, ASD2, and ASD3, respectively.

Experiments were conducted with forced ventilation of the row of five interconnected cabinets.

The air exchange rate between the cabinet and the room air was estimated by monitoring 

carbon dioxide concentration inside the cabinets, following a discharge of a carbon dioxide fire 
4-51

extinguisher. The decay in the concentration was fitted to an exponential equation to yield a 

time constant for the decay, giving the air changes per hour (ACH). A detailed description for 

calculating the exchange rate can be found in the ASTM E741 standard. The combined relative 

uncertainty in this technique is ± 10 percent. For the row of five cabinets, two measurements 

were made following two separate extinguisher discharges yielding 8.4 ACH and 7.5 ACH, and 

a mean of 8.0 ACH.

Additional experiments were conducted with room ventilation air flow. The air exchange rate of 

the room with air ventilation was estimated by monitoring carbon dioxide concentration in the 

center of the room, following a discharge of a carbon dioxide fire extinguisher, and fitting the 

decay curve as described above. For the room with mechanical ventilation running, three 

measurements were made yielding a mean value of 9.2 ACH ± 1.4 ACH.

4.5.4 Small room area-wide experiments

The small-room, area-wide experiments were conducted with three ASD VEWFD systems 

monitoring ceiling-mounted pipe network. Four sampling holes were drilled into each piping 

network to cover each quadrant of the room. The suction pressures for the four sampling holes 

for the three ASDs were 0.14 kPa, 0.15 kPa, and 0.20 kPa for ASD1, AD2, and AD3, 

respectively. Smoke response times were less than 30 seconds for all ASDs and sampling 

holes.

Experiments were conducted with and without room ventilation air flow. The fan setting was the 

same one used during the in-cabinet room ventilation experiments (i.e. 9.2 ACH ± 1.4 ACH).

The room was heated with electric space heaters before the start of an experiment. For no 

room ventilation experiments, the heaters were turned off just before the start of an experiment.

With no ventilation, the room temperature was nominally constant for the duration of an 

experiment. For experiments with room ventilation, after heating the room before the start of an 

experiment, the heaters were moved to the room where the blower was located, so the 

incoming air during an experiment was heated to maintain the room air temperature throughout 

an experiment. 

4.5.5 Large room single-zone in-cabinet experiments

ASD2 (cloud chamber type) and ASD3 (light-scattering type ) were used for the large-room,

single-zone in-cabinet experiments, where a single cabinet (second in line) isolated from 

adjacent cabinets was one sample location configuration, and the fourth cabinet in line that had 

vent holes to the third and fifth cabinets was the other sample location. Sample port openings 

were 4.75 mm in diameter for both piping networks. The sample port suction pressures for 

ASD2 were 0.07 kPa for the first two sampling ports (closest to the detector), and 0.06 kPa for 

the last two sampling ports. For ASD3, the suction pressures were 0.09 kPa for the first two 

sampling ports, and 0.08 kPa for the last two sampling ports. 

The air exchange rate of the room with the air exhaust fan fixed at a speed of 32 Hz was 

estimated by monitoring carbon dioxide concentration in the center of the room following a 

discharge of a carbon dioxide fire extinguisher, and fitting the decay curve to an exponential 

function. Three measurements were made yielding a mean value of 7.4 ACH ± 0.6 ACH.
4-52

4.5.6 Large room multi-zone in-cabinet experiments

ASD4 (cloud chamber type) and ASD5 (light-scattering type) were used for the large room multizone in-cabinet experiments. The configurations examined were the same as the single-zone 

experiments where a single cabinet (second in line) isolated from adjacent cabinets was one 

sample location configuration, and the fourth cabinet in line that had vent holes to the third and 

fifth cabinets was the other sample location. The multi-port ASDs were set up to monitor three 

separate zones, in-cabinet, area-wide ceiling, and the exhaust grill representing a return air grill.

Sample port openings were 4.75 mm in diameter for the ASD5 piping network. ASD4 used a 

vendor-provided design and verification (commissioning) testing. ASD4 used different sampling 

ports for in-cabinet applications. The flow through each port was adjusted to approximately 1.5 

L/min from an adjustable inline pressure regulator. The smoke transport times for ASD5 incabinet piping was 25 ± 1 seconds for the last port (furthest from the detector) and 16 ± 2

seconds for the closest port. The transport time for ASD4 was 52 seconds for the last in-cabinet 

port. 

The air exchange rates of the room with the room air exhaust fan fixed at speeds of 32 Hz and 

60 Hz were estimated by monitoring carbon dioxide concentrations in the center of the room 

following a discharge of a carbon dioxide fire extinguisher, and fitting the decay curve to an 

exponential function. Repeated measurements were made yielding mean values of 6.5 ACH ± 

0.6 ACH, and 14.0 ACH ± 1.0 ACH for 32 Hz and 60 Hz fan settings respectively. Note, there 

was a difference between single-zone and multi-zone the 32 Hz fan setting ACHs, which was 

because of a ducting configuration change between the two test series.

4.5.7 Large room multi-zone area-wide experiments

ASD4 (cloud chamber type) and ASD5 (light-scattering type) were used for the large room multizone area-wide experiments. The smoke transport times for ASD4 area-wide and return air grill 

sampling ports furthest from the detector were 33 and 26 seconds, respectively. The smoke 

transport times for ASD5 area-wide and return air grill sampling ports furthest from the detector 

were 45 ± 1 seconds and 25 ± 0.5 seconds, respectively.

4.6 Experimental Design

The following tables detail the experimental conditions for each experiment conducted. The 

experimental design evolved throughout the project schedule. Information gained from 

experiments, and feedback from observers was used to tailor successive experimental designs.

As part of the experimental design, select experimental conditions were replicated to allow an 

assessment of repeatability of the experimental results.

4.6.1 Laboratory, instrument cabinet experiments

The laboratory instrument cabinet experiments were conducted at 15-, 60- and 240 minute

HRPs each with a 5 minute fixed ending set point soak period. Samples were located with the 

bus bar block centered on the floor of the cabinet. The cabinet was naturally ventilated with 

vents located on the top of the cabinet or on the cabinet sides, 0.2 m below the top of the 

cabinet. Table 4-4 presents the test matrix for the Laboratory Instrument Cabinet experiments 

using single-zone detectors.
4-53

Table 4-4. Laboratory Instrument Cabinet Experiments

Configuration 

Number

Experimental Configuration Experiments 

Material Conducted Heating Ramp 

Period (min) Vent locations

1 XLPO2 15.0 Top 3

2 XLPO2 15.0 Side 1

3 XLPO2 60.0 Top 1

4 XLPO2 60.0 Side 3

5 XLPO2 240.0 Side 1

6 PFTE 15.0 Top 3

7 PFTE 15.0 Side 1

8 PFTE 60.0 Top 1

9 PFTE 60.0 Side 3

10 PFTE 240.0 Top 1

11 XLPE 15.0 Top 4

12 XLPE 15.0 Side 2

13 XLPE 60.0 Top 1

14 XLPE 60.0 Side 4

15 XLPE 240.0 Side 3

16 Silicone 15.0 Top 3

17 Silicone 15.0 Side 1

18 Silicone 60.0 Top 1

19 Silicone 60.0 Side 3

20 XLPO1 15.0 Top 3

21 XLPO1 15.0 Side 1

22 XLPO1 60.0 Top 1

23 XLPO1 60.0 Side 3

24 PVC1 15.0 Top 3

25 PVC1 15.0 Side 1

26 PVC1 60.0 Top 1

27 PVC1 60.0 Side 3

28 PVC2 15.0 Top 3

29 PVC2 15.0 Side 1

30 PVC2 60.0 Top 1

31 PVC2 60.0 Side 3

32 PVC2 240.0 Side 1

33 CSPE 15.0 Top 3

34 CSPE 15.0 Top 1

35 CSPE 60.0 Top 1

36 CSPE 60.0 Top 3

37 CSPE 240.0 Top 1

38 TB 15.0 Top 3

39 TB 15.0 Side 1

40 TB 60.0 Top 1

41 TB 60.0 Side 3

42 TB 240.0 Side 1

43 PCB 15.0 Top 3

44 PCB 15.0 Side 1

45 PCB 60.0 Top 1

46 PCB 60.0 Side 3
4-54

Table 4-4. Laboratory Instrument Cabinet Experiments (Continued)

Configuration 

Number

Experimental Configuration Experiments 

Material Conducted Heating Ramp 

Period (min) Vent locations

47 PCB 240.0 Top 1

48 Resistors 90 s Top 3

49 Resistors 90 s Side 3

50 Capacitors 30 s Top 3

51 Capacitors 30 s Side 3

Configurations below used different ASD2 and ASD3 detector sensitivities

52 XLPE 15.0 Top 4

53 XLPE 60.0 Top 4

54 PVC2 15.0 Top 4

55 PVC2 60.0 Top 4

56 CSPE 15.0 Top 4

57 CSPE 60.0 Top 4

58 One Resistor 70 s Top 3

59 Two Resistors 80 s Top 3

4.6.2 Laboratory, large-cabinet experiments

Table 4-5 presents the test matrix for the Laboratory Large Cabinet experiments using singlezone detectors. Table 4-6 presents the test matrix for the limited number of Laboratory Large 

Cabinet experiments using a reduced HRP.

Table 4-5. Laboratory Large-Cabinet Experiments

Configuration

Number

Experimental Configuration

Experiments

Conducted Material Heating 

Ramp Period 

(min)

Cabinet

Ventilation

1 XLPE 16.3 Natural 3

2 XLPE 16.3 Forced 3

3 XLPE 65.0 Natural 3

4 XLPE 65.0 Forced 3

5 XLPE 260.0 Natural 1

6 XLPE 260.0 Forced 1

7 CSPE 16.3 Natural 3

8 CSPE 16.3 Forced 3

9 CSPE 65.0 Natural 3

10 CSPE 65.0 Forced 3

11 CSPE 260.0 Natural 1

12 CSPE 260.0 Forced 1

13 PVC2 16.3 Natural 3

14 PVC2 16.3 Forced 3

15 PVC2 65.0 Natural 3

16 PVC2 65.0 Forced 3

17 PVC2 260.0 Natural 1

19 PVC2 260.0 Forced 1

20 PCB 16.3 Natural 1

21 PCB 16.3 Forced 1
4-55

Table 4-5. Laboratory Large-Cabinet Experiments (Continued)

Configuration

Number

Experimental Configuration

Experiments

Conducted Material Heating 

Ramp Period 

(min)

Cabinet

Ventilation

22 PCB 65.0 Natural 1

23 PCB 65.0 Forced 1

24 BS 6266 - Natural 6

Table 4-6. Large Cabinet Experiments with Reduced HRP

Configuration

Number

Experimental Configuration

Material Maximum Set 

point (oC)

HRP

(min)

Hold Time

(min)

1 XLPE 275 8.8 12.2 

2 XLPE 300 9.7 60.3 

3 XLPE 325 10.6 10.4 

4 CSPE 200 6.2 14.8 

5 CSPE 225 7.1 62.9 

6 CSPE 250 8.0 13.0 

4.6.3 Full-scale, small room tests

A series of mock cabinets was built in a 37.7 m2 facility to simulate an electrical room. The 

cabinets were connected forming two rows of 5 cabinets each. One of the rows had a partition 

placed in between two cabinets, creating a set of four connected cabinets alongside a single 

one. The five-cabinet configuration had a set of fans installed, allowing for a comparison 

between naturally and forced-ventilated conditions. The sample was located either on the 

ground or about 1.2 m above the floor. The tests were performed with and without room 

ventilation. Table 4-7 through Table 4-11 list all the tests where the detectors and the aerosol 

sources were located inside the cabinet. For the tests in Table 4-12, the spot detectors from the 

front row of the cabinets were placed on the ceiling of the room. The source location varied 

from either inside the cabinet or outside.

Table 4-7. Full-Scale, Single-Zone, In-Cabinet XLPE Experiments

Configuration 

Number

Experimental Configuration

Experiments 

Conducted HRP

(min)

Source 

Cabinet

Source 

Elevation

(m)

Cabinet 

Ventilation

(ACH)

Room 

Ventilation

(ACH)

1 16.3 1 0 0 0 1

2 16.3 4 0 0 0 1

3 16.3 5 0 0 0 2

4 16.3 5 1.2 0 0 1

5 65.0 1 0 0 0 1

6 65.0 4 0 0 0 3
4-56

Table 4-7. Full-Scale, Single-Zone, In-Cabinet XLPE Experiments (Continued)

Configuration 

Number

Experimental Configuration

Experiments 

Conducted HRP

(min)

Source 

Cabinet

Source 

Elevation

(m)

Cabinet 

Ventilation

(ACH)

Room 

Ventilation

(ACH)

7 65.0 5 0 0 0 1

8 65.0 5 1.2 0 0 1

9 65.0 5 0 7.9 0 1

10 65.0 5 0 0 9.1 3

11 260.0 5 0 0 0 1

Table 4-8. Full-Scale, Single-Zone, In-Cabinet CSPE Experiments

Configuration 

Number

Experimental Configuration

Experiments 

Conducted HRP

(min)

Source 

Cabinet

Source 

Elevation

(m)

Cabinet 

Ventilation

(ACH)

Room 

Ventilation

(ACH)

1 16.3 1 0 0 0 1

2 16.3 4 0 0 0 3

3 16.3 5 0 0 0 1

4 16.3 5 1.2 0 0 1

5 65.0 1 0 0 0 1

6 65.0 4 0 0 0 1

7 65.0 5 0 0 0 1

8 65.0 5 1.2 0 0 1

9 65.0 5 0 7.9 0 1

10 65.0 5 0 0 9.1 1

11 260.0 5 0 0 0 1

Table 4-9. Full-Scale, Single-Zone, In-Cabinet PVC2 Experiments

Configuration

Number

Test Configuration

Experiments

Conducted HRP

(min)

Source 

Cabinet

Source 

Elevation

(m)

Cabinet 

Ventilation

(ACH)

Room 

Ventilation

(ACH)

1 16.3 1 0 0 0 1

2 16.3 4 0 0 0 1

3 16.3 5 0 0 0 1

4 16.3 5 1.2 0 0 3

5 65.0 1 0 0 0 1

6 65.0 4 0 0 0 1

7 65.0 5 0 0 0 3

8 65.0 5 1.2 0 0 1

9 65.0 5 0 7.9 0 1

10 65.0 5 0 0 9.1 1

11 260.0 5 0 0 0 1
4-57

Table 4-10. Full-Scale, Single-Zone, In-Cabinet PCB Experiments

Configuration

Number

Experimental Configuration

Experiments

Conducted HRP

(min)

Source 

Cabinet

Source 

Elevation

(m)

Cabinet 

Ventilation

(ACH)

Room 

Ventilation

(ACH)

1 16.3 1 0 0 0 1

2 16.3 5 0 0 0 1

3 65.0 5 0 0 0 1

4 65.0 5 0 7.9 0 1

5 65.0 5 0 0 9.1 1

Table 4-11. Full-Scale, Single-Zone, In-Cabinet Resistor Experiments

Configuration

Number

Experimental Configuration

Experiments

Conducted PS

(s)

Source 

Location

Source 

Elevation

Cabinet 

Ventilation

(ACH)

Room 

Ventilation

(ACH)

1 90 1 0 0 0 3

2 90 4 0 0 0 3

3 90 5 0 0 0 3

4 90 5 1.2 0 0 3

Table 4-12. Full-Scale, Single-Zone, Area-wide Experiments

Configuration

Number

Experimental Configuration

Experiments

Material Conducted HRP 

(min)

Source

Location

Room 

Ventilation

(ACH)

1 Cable bundle 65.0 Center 0 1

2 Cable bundle 65.0 Corner 0 1

3 Cable bundle 65.0 Corner 9.1 1

4 Cable bundle 260.0 Corner 0 1

5 CSPE 16.3 5 0 3

6 CSPE 16.3 5 9.1 1

7 CSPE 16.3 Corner 9.1 5

8 CSPE 16.3 Center 0 1

9 CSPE 65.0 5 0 1

10 CSPE 65.0 5 9.1 1

11 CSPE 65.0 Corner 0 1

12 XLPE 16.3 5 0 3

13 XLPE 16.3 Corner 0 3

14 XLPE 65.0 5 0 3

15 XLPE 65.0 Corner 0 1

16 Shredded Paper Corner 0 1

17 Shredded Paper Center 9.1 1

19 BS Wire Corner 0 3

20 BS Wire Center 0 3

21 BS Wire Corner 9.1 3

22 Mod BS Wire Corner 0 3

23 Mod BS Wire Center 0 3

24 Mod BS Wire Corner 9.1 3
4-58

4.6.4 Full-scale, large room tests

Table 4-13 and Table 4-14 present the test matrix for the full-scale, large room experiments 

using single-zone and multi-zone detectors, respectively.

Table 4-13. Full-Scale, Single-Zone, In-Cabinet Experiments

Configuration 

Number

Experimental Configuration

Experiments 

Material HRP (min) Conducted Source 

Cabinet

Room 

Ventilation 

(ACH)

1 CSPE 65.0 1 0 1

2 CSPE 65.0 1 7.4 1

3 CSPE 65.0 3 0 1

4 CSPE 65.0 3 0 1

5 CSPE 260.0 3 0 1

6 CSPE 260.0 3 7.4 1

7 XLPE 65.0 1 0 1

8 XLPE 65.0 1 7.4 1

9 XLPE 65.0 3 0 1

10 XLPE 65.0 3 7.4 1

11 XLPE 260.0 3 0 1

12 XLPE 260.0 3 7.4 1

Table 4-14. Full-Scale, Multi-Zone Experiments

Configuration 

Number

Test Configuration

Experiments 

Material Conducted HRP 

(min)

Source 

Location1

Source 

Elevation

Room 

Ventilation

(ACH)

1 CSPE 65.0 2 0 0 1

2 CSPE 65.0 2 0 6.5 1

3 CSPE 65.0 4 0 0 1

4 CSPE 65.0 4 0 6.5 1

5 CSPE 65.0 4 0 14 1

6 CSPE 260.0 4 0 0 1

7 CSPE 260.0 4 0 6.5 1

8 CSPE 65.0 Center 0 0 1

9 CSPE 65.0 Center 0 6.5 1

10 CSPE 65.0 Second row 0 0 1

11 CSPE 65.0 Second row 0 6.5 1

12 CSPE 65.0 Second row 0 14 1

13 CSPE 65.0 Left rear 0 6.5 1

14 CSPE 65.0 Right front 0 6.5 1

15 XLPE 65.0 2 0 0 2

16 XLPE 65.0 2 0 6.5 1

 1 Refer to Figure 4-42 for a graphical representation of the various source locations. Numbers represent the 

cabinet that the source was located in the bank of instrumented cabinets.
4-59

Table 4-14. Full-Scale, Multi-Zone Experiments (Continued)

Configuration 

Number

Test Configuration

Experiments 

Material Conducted HRP 

(min)

Source 

Location

Source 

Elevation

Room 

Ventilation

(ACH)

17 XLPE 65.0 2 0 7.4 1

18 XLPE 65.0 4 0 0 2

19 XLPE 65.0 4 0 6.5 1

20 XLPE 65.0 4 0 7.4 1

21 XLPE 65.0 4 0 14 1

22 XLPE 260.0 4 0 0 2

23 XLPE 260.0 4 0 6.5 1

24 XLPE 260.0 4 0 7.4 1

25 XLPE 65.0 Center 0 0 1

26 XLPE 65.0 Center 0 6.5 1

27 Cable 

Bundle 16.3 Center 0 0 1

28 Cable 

Bundle 16.3 Center 0 6.5 2

29 Cable 

Bundle 16.3 Center 0 14 1

30 Cable 

Bundle 16.3 Center 1.8 0 1

31 Cable 

Bundle 16.3 Center 1.8 6.5 1

32 Cable 

Bundle 16.3 Cabinets not instrumented 2.11 14 2

33 Cable 

Bundle 65.0 Center 0 0 1

34 Cable 

Bundle 65.0 Center 0 6.5 1

35 Cable 

Bundle 65.0 Cabinets not instrumented 0 0 1

36 Cable 

Bundle 65.0 Cabinets not instrumented 0 6.5 1

37 Cable 

Bundle 65.0 Cabinets not instrumented 0 14 1

38 Cable 

Bundle 65.0 Left rear corner

0 6.5 1

39 Cable 

Bundle 65.0 Left rear corner

0 14 1

40 Cable 

Bundle 65.0 Front rightcorner

0 6.5 1

41 Cable 

Bundle 65.0 Front right corner

0 14 1

42 Cable 

Bundle 260.0 Center 0 0 1

43 Mod BS 

Wire

Front right 

corner

0 0 1

44 Mod BS 

Wire

Front right 

corner

0 6.5 1

45 Mod BS 

Wire

Front right 

corner

0 14 2
4-60

Table 4-14. Full-Scale, Multi-Zone Experiments (Continued)

Configuration 

Number

Test Configuration

Experiments 

Material Conducted HRP 

(min)

Source 

Location

Source 

Elevation

Room 

Ventilation

(ACH)

46 Mod BS 

Wire Center 0 0 2

47 BS Wire Center 0 0 1

48 Mod BS 

Wire Center 0 6.5 1

49 BS Wire Center 0 6.5 1

50 Mod BS 

Wire Center 0 14 1

51 BS Wire Center 0 14 1

52 Mod BS 

Wire

Left rear 

corner

0 0 1

53 Mod BS 

Wire

Left rear 

corner

0 6.5 2

54 BS Wire Left rear 

corner

0 6.5 2

55 ModBS 

Wire

Left rear 

corner

0 14 1

56 Cable 

Bundle 16.3 Center 0 14 1
5-1

5. EXPERIMENTAL RESULTS

The test plan included a set of experimental configurations to assess performance over a range 

of conditions. The objective was to capture conceivable particle evolution scenarios that would 

be sensed by the various detectors, given the experimental conditions. It is impossible to 

ensure these experiments capture the most likely, along with the worst-case scenarios that 

would be experienced by systems deployed in nuclear power plants (NPPs). However, it is 

reasonable to assume the relative performance of aspirating smoke detection (ASD) very early 

warning fire detection (VEWFD) systems or sensitive spot detectors to conventional detectors 

observed in these experiments would apply in real-world scenarios.

Individual detector pre-alerts, alerts, pre-alarms and alarms, where applicable, were recorded 

during all experiments. The relative performance of the ASD and sensitive spot VEWFD 

systems were evaluated by comparing their activations to the ionization detector alarm for 

in-cabinet experiments and to the ionization and photoelectric detector for area-wide

experiments. For in-cabinet configurations, ionization detectors were considered to be the 

conventional detector for comparison to the ASDs or sensitive spots VEWFD systems for two 

reasons: (1) ionization alarms were observed in-service inside electrical cabinets in NPP visits, 

and (2) ionization alarms are typically more sensitive to the early pyrolysis smokes generated by 

the chosen sources.

 

An absolute performance measure of the time interval between activation and the end of a test 

was used to represent time available for response before a potential ignition event. The 

assumption made here is that the wire temperature at the end of the test is nominally at or 

above the piloted ignition temperature and within the range of the materials auto ignition 

temperature. Given the measured wire temperature profile and thermal imaging camera 

images, this appears to be a reasonable assumption (Ref. 34). Piloted ignition tests conducted 

after the experimental series was concluded and as detailed in Section 5.8 support this 

assumption. It may also be worth noting that the final block temperatures are above the generic 

critical temperature thresholds to estimate functionality failures of cables, namely 205 and 

330°C.

For the experiments in which two light-scattering ASDs were monitored, only one of those ASDs 

was used in the analysis, since similar results were obtained. In addition, the analysis 

considered ASD VEWFD system Pre-alert and Alert settings the sensitive spot (SS) Alert 

setting, and the conventional ionization and photoelectric detector Alarm settings; the 

assumption here is that the ASD or SS Alert would initiate a defined VEWFD system response. 

The ASD Pre-alert and Alert settings were not held constant throughout the different 

experimental setups. In the case of light-scattering ASDs, the Pre-alert setting ranged from 4 to 

10 times the sensitivity of the VEWFD Alert setting (0.2 %/ft obscuration). In the case of cloud 

chamber ASDs, the Pre-alert, Alert and Alarm settings were specified to cover a range of 

sensitivity settings in all experimental setups except the large-room, multi-zone experiments in 

which the vendor specified the sensitivity settings.

It is important to note that this experimental research was not designed to assess the 

performance of VEWFD models or types against one another, but rather, was designed to 

assess the potential VEWFD performance against conventional detectors. The VEWFD system 

sensitivity settings and system designs may not be optimal for the configuration being studied

and will vary based on the environmental conditions of the application. The guidance on in-
5-2

cabinet applications is less developed than the area-wide design specifications, therefore incabinet designs rely on input from manufacturers and system integrators.

5.1 Laboratory, Small-Cabinet Experiments

These experiments were the first set conducted and were designed to gain an understanding of 

the heated wire/component source, typical ASD, and conventional detector response to the 

various materials selected. In addition, it was a goal of these experiments to find a rational 

basis for down-selecting the number of materials to be used in subsequent experiments; the 

materials tested in this series are identified in Table 5-1.

Table 5-1. Material Identification Numbers Used in Laboratory—Small-Scale Tests

ID # Name Description of Material

1 PVC1 Polyvinyl chloride insulated, 18 AWG wire

2 PVC2 Polyvinyl chloride insulated, 14 AWG wire

3 Silicone Silicone insulated , 18 AWG wire

4 PTFE Polytetrafluoroethylene insulated, 14 AWG wire 

5 XLPO1 Cross-linked polyolefin insulated, 12 AWG wire 

6 XLPO2 Cross-linked polyolefin insulated, 12 AWG wire 

7 XLPE Cross-linked polyethylene insulated, 12 AWG wire. 

8 CSPE Chlorosulfonated polyethylene insulated, 10 AWG wire

9 PCB FR4, glass-reinforced epoxy laminate circuit board

10 TB Phenolic barrier terminal block

Two sets of experiments were conducted with different heating rate and experimental time 

conditions. One condition was a heating ramp period (HRP) of 15.0 minutes to a final set point 

of 450 °C, followed by a 5.0 minute period heating period at the final set point, for a total heating 

time (THT) of 20.0 minutes. The other condition was a HRP of 60.0 minutes to a final set point 

of 450 °C, followed by a 5.0 minute period heating period at the final set point, for a THT of 65.0

minutes. The results aggregate those experiments with top and side ventilation conditions,

because there was no apparent response time difference between these configurations. 

The ASD and spot detector sensitivities are listed in Table 5-2. ASD piping configuration was 

nominally 3.7 m long with four equally spaced tee’d sampling ports, three drawing ambient 

laboratory room air and one drawing cabinet air. In the case of the ASD3, the pre-alarm and 

alarm sensitivities represent the port sensitivities equal to 0.2 %/ft obscuration and 

1.0 %/ft obscuration. The sensitivities for ASD2, the cloud chamber device, were factory default 

settings, and not necessarily what would be used for in-cabinet VEWFD system applications, 

but most likely less sensitive settings.
5-3

Table 5-2. Nominal Detector Sensitivities for Laboratory—Small Scale Tests

Sensitivity

Setting

ASD1

Detector /

Port

%/ft Obsc

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFDS Pre-alert

0.013 / 

0.05

5.1×105 /

2.0×106

0.025 / 

0.10 - - -

VEWFDS Alert

0.05 /

0.20

1.2×106 /

4.8×106

0.05 /

0.20

0.20 - -

VEWFDS Alarm

0.25 /

1.00

1.5×106 /

6.0×106

0.25 /

1.00

1.00 - -

Conventional 

Pre-Alarm - - - - 0.5 1.3

Conventional Alarm - - - - 1.0 2.1

Figure 5-1 shows the pre-alert or alarm times for experiments conducted in the small cabinet 

with the 15.0 minute HRP. The materials are ordered in terms of increasing average ION spot 

alarm times. In most cases, ASD2 pre-alerted first, typically before 600 seconds of heating. 

With silicone and PTFE, ASD3 did not reach a pre-alert threshold before the end of 

experiments. The ION did not alarm with PTFE wire, while the PHOTO did not alarm with 6 out 

of 10 materials. 

0

200

400

600

800

1000

1200

8 7 1 3 2 10 5 6 9 4

ASD2 - Pre-alert

ASD3 - Pre-alert

PHOTO - Alarm

ION - Alarm

Pre-alert or Alarm Time (s)

Material ID Number

Figure 5-1. VEWFD pre-alert and conventional alarm times for the 15 minute HRP 

instrument cabinet experiments. (Error bars represent ± one standard 

deviation for repeated tests)
5-4

Figure 5-2 shows VEWFD system alert and ION alarm times for experiments conducted in the 

small instrument cabinet with the 15.0 minute HRP. Again, the materials are ordered in terms of 

increasing average ION spot alarm times. ASD2 alerted first for all materials except CSPE

(chlorosulfonated polyethylene). ASD3 alerted before ION alarm with two materials.

ASD3 typically alerted before SS, while neither alerted with two materials. 

0

200

400

600

800

1000

1200

8 7 1 3 2 10 5 6 9 4

ASD2 - Alert

ASD3 - Alert

SS - Alert

ION - Alarm

Alert or Alarm Time (s)

Material ID Number

Figure 5-2. VEWFD alert and ION alarm times for the 15 minute HRP instrument cabinet 

experiments. (Error bars represent ± one standard deviation for repeated 

tests)

Figure 5-3 shows the pre-alert or alarm times for experiments conducted in the small cabinet 

with the 60.0 minute HRP, and Figure 5-4 shows alert and ION alarm times. In both figures, the 

materials are ordered in terms of increasing average ION spot alarm times. The trends are 

similar to the 15.0 minute HRP results. ASD3 pre-alerted and alerted before other detectors 

with CSPE, like the 15.0 minute HRP experiments. Neither ASD3 nor SS alerted before the end 

of test with four materials. 
5-5

0

500

1000

1500

2000

2500

3000

3500

8 2 7 3 1 10 5 9 6 4

ASD2 - Pre-alert

ASD3 - Pre-alert

PHOTO - Alarm

ION - Alarm

Pre-alert or Alarm Time (s)

Material ID Number

Figure 5-3. VEWFD pre-alert and conventional alarm times for the 60 minute HRP 

instrument cabinet experiments. (Error bars represent ± one standard 

deviation for repeated tests)

0

500

1000

1500

2000

2500

3000

3500

8 2 7 3 1 10 5 9 6 4

ASD2 - Alert

ASD3 - Alert

SS - Alert

ION Alarm

Alert or Alarm Time (s)

Material ID Number

Figure 5-4. VEWFD alert and ION alarm times for the 60 minute HRP instrument cabinet 

experiments. (Error bars represent ± one standard deviation for repeated 

tests)
5-6

Figure 5-5 shows the pre-alert or alarm times for experiments conducted in the small cabinet 

with 240.0 minute HRP, while Figure 5-6 shows the alert and ION alarm times The materials 

are ordered in terms of increasing average ION spot alarm times. 

The overall trend between the three heating rates was the same. Essentially the order and 

relative time that the detectors alarmed did not depend on the heating rate. Therefore, the 

concept of “end of test” (EOT,) is used later in this report to merge test results from similar tests 

that only varied in heating rate and test length. ASD2 typically pre-alerted or alerted before 

ASD3 and the ION detector. The ION typically alarmed before ASD3 alerted, and ASD3 

typically alerted before the SS detector alert. The one exception to this pattern is Material 8, in 

which case the ASD3 alerted first, followed by the ASD2, ION, and PHOTO.

0

5000

10000

15000

2 8 7 9 5 10

ASD2 - Pre-alert

ASD3 - Pre-alert

PHOTO - Alarm

ION - Alarm

Pre-alert or Alarm Time (s)

Material ID Number

Figure 5-5. VEWFD pre-alert and conventional alarm times for the 240 minute HRP 

instrument cabinet experiments

Table 5-3 shows typical smoke particle arithmetic mean diameter (AMD) and mass mean 

diameter (MMD) results averaged over the 5 minute soak period for the 15.0 minute HRP

experiments for each wire sample. Both the AMD and MMD vary by a factor of three from PFTE 

to CSPE insulation. These results, plus the alert and alarm activation results, were used to 

down-select the wire samples for full-scale experiments. PVC wire (2), XLPE wire, and CSPE 

wire materials were selected to (1) cover the observed (relatively) small, medium, and large 

mean particle sizes, and (2) to have the ability to produce sufficient smoke to activate the 

detectors being studied in the various experimental configurations. Therefore, these selected 

materials (PVC, XLPE, and CSPE) are intended to represent the aerosol characteristics 

generated from a large variety of materials commonly found in NPP electrical enclosures. 

The selection is not intended to bound all materials and these materials (PVC, XLPE, and 

CSPE) likely do not represent the most difficult aerosols to detect.
5-7

0

5000

10000

15000

2 8 7 9 5 10

ASD2 - Alert

ASD3 - Alert

SS - Alert

ION - Alarm

Alert or Alarm Time (s)

Material ID Number

Figure 5-6. VEWFD alert and ION alarm times for the 240 minute HRP instrument 

cabinet experiments

Table 5-3. AMD and MMD Average over the 5.0 Minute Soak Time for 15.0 Minute HRP 

Tests

ID # Name AMD (μm) (± 20 %) MMD (μm) (± 20 %)

1 PVC1 0.12 0.27

2 PVC2 0.11 0.26

3 Silicone 0.14 0.23

4 PTFE 0.10 0.21

5 XLPO1 0.13 0.21

6 XLPO2 0.23 0.45

7 XLPE 0.20 0.33

8 CSPE 0.33 0.64

To examine trends by extending the HRP from 15.0 to 60.0 minutes, the block temperature1 at 

alarm was plotted for the various detector activations in Figure 5-7. The trend between the 

15.0- and 60.0 minute HRP for ASD2 and ASD3 was consistent for all three materials, but in 

opposite directions. ASD2 responded at higher block temperatures when the HRP was 

 1 The “block temperature” is the surface temperature measured by the thermocouple attached to the surface 

of the copper bus bar block. Refer to Section 4.2.1 for details.
5-8

increased, while ASD3 responded at lower block temperatures. The magnitude of the 

temperature difference was not large, thus the practical implications may be minor.

100

200

300

400

500

ASD2 Pre-alertASD2 Pre-alert

ION AlarmION Alarm

ASD3 Pre-alertASD3 Pre-alert

SS AlertSS Alert

XLPE 15 min

XLPE 1 h

PVC(2) 15 min

PVC(2) 1 h

CSPE 15 min

CSPE 1 h

Block Temperature at Detector Activation

 (oC)

Figure 5-7. Block temperature at pre-alert, alert or alarm time for various detectors. 

(Error bars represent +/- one standard deviation for repeated tests)

For the small-cabinet laboratory results the following observations are made:

1. ASD2 pre-alerted and alerted before ASD3 and SS for all materials with the exception of 

CSPE.

2. At both HRPs, ASD2 was the only detector to respond to PTFE samples before the end 

of test.

3. The overall trend between the three heating rates was the same, ASD2 typically 

pre-alert or alerted before ASD3 and the ION detector. The ION typically alarmed before 

ASD3 alerted, and ASD3 typically alerted before the SS detector.

4. By the end of the test, both the arithmetic mean diameter and the mass mean diameter 

of the smoke varied by a factor of three from PFTE wire (smallest particles) to CSPE 

wire (largest particles), but this made no difference except with ASD3 and SS’s better 

performance in detection CSPE.

5. The heating ramp period affected the observed block temperature when ASDs 

responded. ASD2 responded at higher block temperatures when the HRP was 

increased, while ASD3 responded at lower block temperatures. 
5-9

5.2 Laboratory, Large-Cabinet Experiments

Experiments were conducted in two surplus NPP cabinets, one with natural ventilation, and one 

with forced ventilation provided by a fan and ducting. The cabinet sizes were dimensionally the 

same, but the internal configurations were different. These tests primarily show the effects of 

natural ventilation versus high-flow forced ventilation cabinet conditions. These experiments 

were conducted in the laboratory with an ASD2, an ASD3, an ionization spot detector (ION) and 

a sensitive spot detector (SS). The detector sensitivities are listed in Table 5-4. The ION spot 

was installed in both cabinets and the sensitive spot was only installed in the naturally ventilated 

cabinet, as there wasn’t sufficient room in the forced ventilation cabinet to install both. The ASD 

piping configuration was four equally spaced ports, three sampling laboratory room air and one 

sampling cabinet air. The materials used in these experiments were polyvinyl chloride insulated 

(PVC2), XLPE, CSPE, and PCB.

For these experiments, the heating ramp period and final set point were extended. Based on 

the results of the small-cabinet experiments, it was decided to increase the final block 

temperature set point to 485 °C. The slope of the set point ramp remained the same as the 

450 °C final set point experiments, but the duration of the ramp period was increased to 16.3, 

65, and 460 minutes for the new HRPs. The THTs were 1,278, 4,200 and 15,900 seconds for 

the three HRPs, respectively.

Table 5-4. Nominal Detector Sensitivities for Laboratory—NPP Large-Cabinet

Experiments

Sensitivity

Setting

ASD2

Detector/Port

Particles/cm3

ASD3

Detector/Port

%/ft Obsc

SS

%/ft Obsc

ION

%/ft Obsc

VEWFD Pre-alert 3.8×104/1.5×105 0.0063/0.025 - -

VEWFD Alert 1.4×105/5.5×105 0.05/0.20 0.20 -

VEWFD Alarm 6.4×105/2.6×106 0.25/1.00 1.00 -

Conventional Pre-Alarm - - - 0.5

Conventional Alarm - - - 1.0

Results are presented in box plots where the box’s vertical limits represent the range in which

the middle 50 percent of the data lay, the vertical line inside the box indicates the median, and 

bars above and below the box represent the upper quartile (UQ) and lower quartile (LQ), with 

individual outlier values represented by an open circle symbol. Outliers are defined as values 

greater than (UQ+1.5*(UQ-LQ)) or less than (LQ-1.5(UQ-LQ)), in other words, 1.5 times the 

inter-quartile range above the upper quartile or below the lower quartile. A filled circle symbol 

represents the mean of all values including outliers.

The first data set examined consists of the naturally ventilated cabinet, 16.3 minute HRP 

experiments. Three experiments with each of the four materials were conducted for a total of 

12 experiments. Figure 5-8 shows the difference between the ION alarm and the ASD and SS 

alert times. Only ASD2 had mean and median values greater than 0, and in all cases, ASD2 

alerted before the ION alarmed. ASD3 and SS have about the same average response.
5-10

Figure 5-9 shows the differences between the end of the test (1,278 s) and the ASD pre-alerts, 

ASD and SS alerts, and ION alarm times. All detectors alerted or alarmed before the end of 

test.

-500

0

500

1000

ION - ASD2ION - ASD3

ION - SS

Mean

ION Alarm - Alert (s)

Figure 5-8. Time difference between ION alarm time and the ASD alerts and SS alert 

time for large cabinet, natural ventilation, 16.3 minute HRP experiments
5-11

0

200

400

600

800

1000

1200

ASD2 Pre-alertASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-9. Time difference between the end of test (EOT) time (1278 s) and ASD prealerts, ASD and SS alerts or ION alarm time for large cabinet, natural 

ventilation, 16.3 minute HRP experiments

The second data set examined consists of the forced ventilation cabinet, 16.3 minute HRP 

experiments. Three experiments with each of the four materials, plus an additional experiment 

with PCB were conducted for a total of 13 experiments. Figure 5-10 shows the difference 

between the ION alarm and the ASD alert times. Both ASD VEWFD systems alert before the 

ION alarmed on average.

-500

0

500

1000

ION - ASD2ION - ASD3

Mean

ION Alarm - Alert (s)

Figure 5-10. Time difference between ION alarm time and the ASD alert time for large 

cabinet, forced ventilation, 16.3 minute HRP experiments
5-12

Figure 5-11 shows difference between the end of the test (1,278 s) and the ASD VEWFD 

system pre-alerts, ASD VEWFD system alerts, and ION alarm times.

0

200

400

600

800

1000

ASD2 Pre-alertASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-11. Time difference between the end of test (EOT) time (1278 s) and ASD prealerts, ASD alerts or ION alarm time for large cabinet, forced ventilation, 

16.3 minute HRP experiments

The third data set examined consists of the naturally ventilated cabinet, 65.0 minute HRP 

experiments. Three experiments with each of the four materials were conducted for a total of 

12 experiments. Figure 5-12 shows the difference between the ION alarm and the ASD and SS 

alert times. Only ASD2 had mean and median differences greater than 0, and it responded 

before the ION in all experiments. 

Figure 5-13 shows the difference between the end of the test (4,200 s) and the ASD pre-alerts, 

ASD and SS alerts, and ION alarm times. The average detector response was about 

1,500 seconds or greater before the end of test. 
5-13

-1000

-500

0

500

1000

ASD2 Alert

ASD 3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-12. Time difference between ION alarm time and the ASD alerts and SS alert 

time for large cabinet, natural ventilation, 65.0 minute HRP experiments

0

500

1000

1500

2000

2500

3000

ASD2 Pre-alertASD3 Pre-alart

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-13. Time difference between the end of test (EOT) time (4200 s) and ASD prealerts, ASD and SS alerts or ION alarm time for large cabinet, natural 

ventilation, 65.0 minute HRP experiments
5-14

The fourth data set examined consists of the forced ventilation cabinet, 65.0 minute HRP 

experiments. Three experiments with each of the four materials were conducted for a total of 

12 experiments. Figure 5-14 shows the difference between the ION alarm and the ASD alert 

times. Both ASDs alerted before the ION alarm on average.

-500

0

500

1000

1500

2000

2500

ASD3 AlertASD2 Alert

Mean

ION Alarm - Alert (s)

Figure 5-14. Time difference between ION alarm time and the ASD alerts times for large 

cabinet, forced ventilation, 65.0 minute HRP experiments

Figure 5-15 shows difference between the end of the test (4,200 s) and the ASD pre-alerts, ASD 

alerts, and ION alarm times.
5-15

0

500

1000

1500

2000

2500

ASD2 Pre-alert

ASD3 Pre-alert

ASD2 Alert

ASD3 Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-15. Time difference between the end of test (EOT) time (4200 s) and ASD prealerts, ASD alerts or ION alarm time for large cabinet, forced ventilation, 

65.0 minute HRP experiments

The fifth data set examined consists of the naturally ventilated cabinet, 260.0 minute heating 

period experiments. Three experiments each with XLPE and CSPE materials were conducted 

for a total of six experiments. Figure 5-16 shows the difference between the ION alarm and the 

ASD and SS alert times. Both the median and mean are greater than 900 seconds for the 

ASDs and SS detector.
5-16

-1000

0

1000

2000

3000

4000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-16. Time difference between ION alarm time and the ASD alerts and SS alert 

time for large cabinet, natural ventilation, 260.0 minute HRP experiments

Figure 5-17 shows the difference between the end of the test (15,900 s) and the ASD pre-alerts, 

ASD and SS alerts, and ION alarm times. The ASDs pre-alerted more than 8,000 seconds 

before the end of the test on average. ASD3 typically pre alerted before ASD2, a distinct 

difference in the trend compared to 65 and 16.3 minute HRP experiments.
5-17

0

2000

4000

6000

8000

1 104

1.2 104

ASD2 Pre-alert

ASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-17. Time difference between the end of test (EOT) time (15900 s) and ASD prealerts, ASD and SS alerts or ION alarm time for large cabinet, natural

ventilation, 260.0 minute HRP experiments

For the large-cabinet laboratory results the following observations are made:

1. For the naturally ventilated cabinet, the ASD VEWFD systems tended to pre-alert before 

the conventional ionization alarm for all heating period experiments.

2. For the forced ventilation cabinet experiments, the ASD VEWFD systems significantly 

outperformed the ionization detector, which in many cases did not alarm during both 

16.3 and 65.0 minute heating period experiments.

3. For the naturally ventilated cabinet, some of the difference between the 16.3 or 65.0

minute heating period experiment trend and the 260.0 minute heating period experiment 

trend is attributed to different sets of test materials.
5-18

5.3 Full-Scale, Small-Room, In-Cabinet Experiments

Experiments were conducted using cabinet mock-ups in the small room. As presented in 

Section 4.4.3, each ASD piping network covered a total of 10 cabinets divided into three 

separate spaces; a single, isolated cabinet monitored by one sampling port, a set of four 

cabinets monitored by two sampling ports, and a set of five cabinets monitored by three 

sampling ports. The cabinet sizes were the same, and the set of four and five cabinets were 

without internal side walls, effectively creating one large space. Experiments were conducted in 

each of the three separate spaces with no forced ventilation flow in the cabinet or in the room. 

Additional tests were conducted with the five-cabinet configuration with forced ventilation in the 

cabinet or in the room. Several tests included the smoke source elevated for the typical cabinet 

floor location to 2/3 of the cabinet height with the source located within the cabinet. The 

detector sensitivities are listed in Table 5-5. The materials used in these experiments were 

PVC2, XLPE, CSPE, and PCB.

Table 5-5. Nominal Detector Sensitivities for Small Room, Cabinet Mock-Up Tests

Sensitivity

Setting

ASD1

Detector /

Port

%/ft Obsc

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFD

Pre-alert

0.013 / 

0.08

5.1×105 /

3.1×106

0.0083 / 

0.05 - - -

VEWFD

Alert

0.05 /

0.30

1.2×106 /

7.2×106

0.033 /

0.20

0.20 - -

VEWFD

Alarm

0.25 /

1.50

1.5×106 /

9.0×106

0.167 /

1.00

1.00 - -

Conventional

Pre-alarm - - - - 0.5 1.3

Conventional

Alarm - - - - 1.0 2.1

The first data set examined consists of 16.3 minute HRP experiments in the three cabinet 

configurations: single cabinet with XLPE, CSPE, PVC and PCB materials, the set of four 

cabinets with XLPE, CSPE and PVC materials, the set of five cabinets with XLPE, CSPE, PVC 

and PCB materials, and the set of five cabinets with XLPE, CSPE and PVC sources elevated to 

2/3 of the cabinet height.

Figure 5-18 shows the difference between the ION alarm and the ASD and SS alert times. Both 

the median and mean are greater than 0 for the ASDs, and less than 0 for the SS.

Figure 5-19 shows difference between the end of the test (1,278 s) and the ASD pre-alerts, ASD 

and SS alerts, and ION alarm times.
5-19

-500

-250

0

250

500

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-18. Time difference between ION alarm times and the ASD alerts and SS alert 

for small room, cabinet mock-up, 16.3 minute HRP experiments

0

200

400

600

800

1000

ASD2 Pre-alertASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-19. Time difference between the end of test (EOT) time (1278 s) and ASD 

pre-alerts, ASD and SS alerts or ION alarm time for small room, cabinet 

mock-up, 16.3 minute HRP experiments
5-20

The second data set examined consists of 65.0 minute heating period experiments in the three 

cabinet configurations: single cabinet with XLPE, CSPE, and PVC materials, the set of four 

cabinets with XLPE, CSPE and PVC materials, the set of five cabinets with XLPE, CSPE, PVC 

and PCB materials with no ventilation flows, cabinet ventilation and room ventilation, and the set 

of five cabinets with XLPE, CSPE and PVC sources elevated to 2/3 of the cabinet height. The 

single, four, and five cabinet configurations are presented in Section 4.4.3.

Figure 5-20 shows the difference between the ION alarm and the ASD and SS alert times. The 

ASD mean time differences were greater than 0 and less than 0 for the SS.

Figure 5-21 shows the difference between the end of the test (4,200 seconds) and the ASD prealerts, ASD and SS alerts, and ION alarm times. Most detectors responded before 500 seconds 

to the end of test.

-1000

-500

0

500

1000

1500

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-20. Time difference between ION alarm time and the ASD alerts and SS alert for 

small room, cabinet mock-up, 65.0 minute HRP experiments
5-21

0

500

1000

1500

2000

2500

3000

ASD2 Pre-alertASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-21. Time difference between the end of test (EOT) time (4200 s) and ASD prealerts, ASD and SS alerts or ION alarm time for small room, cabinet mockup, 65.0 minute HRP experiments

The third data set examined consists of 260.0 minute HRP experiments in the set of five 

cabinets. Three experiments with XLPE, CSPE, or PVC wire sources were conducted. 

Because only three data points are plotted, the box limits represent the minimum and maximum 

values.

Figure 5-22 shows the difference between the ION alarm and the ASD and SS alert times. 

Figure 5-23 shows difference between the end of the test (15,900 s) and the ASD pre-alerts, 

ASD and SS alerts, and ION alarm times.
5-22

-2000

-1000

0

1000

2000

3000

4000

5000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-22. Time difference between ION alarm times and the ASD alerts and SS alert 

for small room, cabinet mock-up, 260.0 minute HRP experiments

0

2000

4000

6000

8000

10000

12000

ASD2 Pre-alertASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-23. Time difference between the end of test (EOT) time (15900 s) and ASD 

pre-alerts, ASD and SS alerts or ION alarm time for small room, cabinet 

mock-up, 260.0 minute HRP experiments
5-23

The XLPE, CSPE, and PVC2 sources were each tested at 65.0 minute HRP in each of the six 

experimental configurations to examine the effect of the various configurations on time to alert 

or alarm. The six conditions were as follows: an isolated single cabinet with the source at the 

bottom (single cabinet—1C); a group of four cabinets and two sampling port locations with the 

source at the bottom (four cabinets—4C); a group of five cabinets with three sampling port 

locations with the source at the bottom of the cabinet (five cabinets—5C); configuration 5C with 

the source elevated two-thirds from the bottom of the cabinet (5 ES); configuration 5C with room 

ventilation (5 RV); and configuration 5C with cabinet ventilation (5 CV). 

Figure 5-24 shows ASD alert, SS alert and ION alarm time averaged over the three materials 

experiments for the isolated cabinet (1C), group of four cabinets (4C), and group of five cabinets 

(5C). Pre-alarm or alarm times for XLPE, PVC2 and CSPE wire samples subject to 65.0 minute 

heating periods.

Figure 5-25 shows ASD alert, SS alert and ION alarm time averaged over the three materials 

experiments for the four group of five cabinets (5C) experimental configurations. Increasing and 

decreasing alert or alarm time trends were observed, moving from 1C to 4C to 5C 

configurations. Elevated-sample experiments tended to yield shorter alarm times than the base 

case (sample at bottom, no ventilation). 

2000

2500

3000

3500

1C4C5C

ASD2 Alert

ASD3 Alert

SS Alert

ION Alarm

Alert or Alarm Time (s)

Figure 5-24. Mean alert or alarm times for small room, 1, 4, and 5-cabinet mock-up 

configurations, 65.0 minute HRP experiments
5-24

2000

2500

3000

3500

5C

 ES

 RV CV

ASD2 Alert

ASD3 Alert

SS Alert

ION Alarm

Alert or Alarm Time (s)

Figure 5-25. Mean alert or alarm times for small room, 5-cabinet mock-up 

configurations, 65.0 minute HRP experiments

The small-room cabinet mock-up results gave rise to the following observations:

1. The ASDs tended to pre-alert before the ION alarm for both heating period experiments.

2. All ASDs alerted before the end of test time, 

3. On average, the SS detector pre-alarmed after the ION alarm for both heating period 

experiments

4. Cabinet-ventilated experiments and elevated-sample experiments tended to yield shorter 

alarm times than the base case (sample at bottom, no ventilation).

5.4 Full Scale, Large Room, Single-Zone, In-Cabinet Experiments

Eight 65 minute HRP in-cabinet experiments were conducted in the large room, four of which 

with XLPE wire sources and four with CSPE wire sources. For each wire source, experiments 

were conducted in the isolated cabinet with and without room air ventilation flow, and in the

three-cabinet arrangement (where the cabinet with the source had openings to its two 

neighbors), with and without room air ventilation flow. The detector sensitivity settings are 

presented in Table 5-6. For ASD2, the highest sensitivity setting was often below background 

room concentrations before the start of an experiment. Therefore, the pre-alert and alert 

settings were shifted to the next highest sensitivity. 
5-25

Table 5-6. Nominal Detector Sensitivities for Large Room, Single-Zone Cabinet Tests

Sensitivity

Setting

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft Obsc

ION

%/ft Obsc

VEWFD

Pre-Alert

3.8×104 /

1.5×105

0.0063 / 

0.025 - -

VEWFD

Alert

1.4×105 /

5.5×105

0.05 /

0.20

0.20 -

VEWFD

Alarm

6.4×105 /

2.6×106

0.25 /

1.00

1.00 -

Conventional

Pre-alarm - - - 0.5

Conventional

Alarm - - - 1.0

Results for the single cabinet and the three-cabinet arrangement are shown in Figure 5-26 and

Figure 5-27. On average, the ASDs responded sooner than the ION alarm for the three-cabinet 

arrangement suggesting some cooperative ASD sampling from adjacent cabinets. 

-1000

-500

0

500

1000

1500

2000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-26. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, single-zone single-cabinet, 65.0 minute HRP experiments
5-26

-1000

-500

0

500

1000

1500

2000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-27. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, single-zone, three-cabinet configuration, 65.0 minute HRP 

experiments

Results for the ventilated and non-ventilated room experiments are shown in Figure 5-28 and

Figure 5-29.

-1000

-500

0

500

1000

1500

2000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-28. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, single-zone, no ventilation, in-cabinet, 65.0 minute HRP 

experiments
5-27

-1000

-500

0

500

1000

1500

2000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-29. Time difference between ION alarm times and the ASD alerts and SS alert 

for Large room, single-zone, 7.4 ACH room ventilation, in-cabinet, 65.0

minute HRP experiments

Figure 5-30 shows the results of the difference between the ION alarm, and the ASD alerts or 

SS pre-alarm time for all eight experiments. 

Figure 5-31 shows the results of the time difference between the end of the test (4,200 s) and 

the ION alarm, the ASD alerts, and SS pre-alarm time. Both ASD pre-alert mean and median 

time differences were greater than 1,900 seconds. The decreasing mean time difference trend 

was ASD2 alert, ASD3 alert, SS alert, and ION alarm.
5-28

-1000

-500

0

500

1000

1500

2000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-30. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, single-zone, in-cabinet, 65.0 minute HRP experiments

0

500

1000

1500

2000

2500

3000

3500

ASD2 Pre-alert

ASD3 Pre-alert

ASD2 Alert

ASD3 alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-31. Time difference between the end of test (EOT) time (4200 s) and ASD 

Pre-alerts, ASD and SS alerts or ION alarm time for large room, in-cabinet, 

65.0 minute HRP experiments
5-29

Four 260.0 minute HRP in-cabinet experiments were conducted in the large room, two 

experiments with XLPE wire sources and two with CSPE wire sources. For each wire source, 

experiments were conducted in the three-cabinet arrangement with and without room air 

ventilation flow.

Figure 5-32 shows the results of the difference between the ION alarm, and the ASD alerts or 

SS pre-alarm time for all eight experiments. 

Figure 5-33 shows the results of the time difference between the end of the test (15,900 s) and 

the ION alarm, the ASD alerts, and SS pre-alarm time. The decreasing mean time difference 

trend was ASD2 alert, SS alert, ASD3 alert and ION alarm. 

-1000

0

1000

2000

3000

4000

ASD2 AlertASD3 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-32. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, single-zone, in-cabinet, 260.0 minute HRP experiments
5-30

0

2000

4000

6000

8000

1 104

1.2 104

ASD2 Pre-alertASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-33. Time difference between the end of test (EOT) time (15900 s) and ASD prealerts, ASD and SS alerts or ION alarm time for large room, in-cabinet, 260.0

minute HRP experiments

The large room single-zone cabinet results gave rise to the following observations:

1. The time difference between the ION alarm and the ASD alerts was greater for the 

three-cabinet arrangement than the single cabinet arrangement.

2. Room ventilation tended to reduce the time difference between the ION alarm and the 

ASD alerts.

3. All detectors alerted before the end of test time for both heating periods.

5.5 Full-Scale, Large-Room, Multi-Zone, In-Cabinet Experiments

Ten in-cabinet 65 minute heating period experiments were conducted in the large room, five 

with XLPE wire sources and five with CSPE wire sources. For each wire source, experiments 

were conducted in the isolated cabinet with and without room air ventilation flow (7.5 ACH), and 

in a three cabinet arrangement (where the cabinet with the source had openings to its two 

neighbors), with and without room air ventilation flow (7.5 ACH and 15 ACH). Table 5-7

presents the detector sensitivity settings used in these tests. ASD4 (cloud chamber) sensitivity 

setting were vendor-specified.
5-31

Table 5-7. Nominal Detector Sensitivities for Large Room, Multi-Zone Cabinet Tests

Sensitivity

Setting

ASD4

Detector /

Port

Particles/cm3

ASD5

Detector /

Port

%/ft Obsc

SS

%/ft Obsc

ION

%/ft Obsc

VEWFD

Pre-Alert

1.5×105 /

6.0×105

0.0159 / 

0.064 - -

VEWFD

Alert

2.5×105 /

1.0×106

0.0334 /

0.13

0.20 -

VEWFD

Alarm

4.5×105 /

1.8×106

0.1665 /

0.67

1.00 -

Conventional

Pre-alarm - - - 0.5

Conventional

Alarm - - - 1.0

Results for the single cabinet are shown in Figure 5-34. ASD4 (cloud chamber) responded 

before the ION alarm in all experiments, while ASD5 (light-scattering) responded after the ION

alarm on average. Results for the three-cabinet arrangement are shown in Figure 5-35. 

Results for the non-ventilated and ventilated room experiments are shown in Figure 5-36 and

Figure 5-37. The results are mixed: however, ASD4 responded before the ION alarm in all 

experiments.

Results for all 10 experiments are shown in Figure 5-38. ASD4 responded 750 seconds sooner 

than the ION alarm on average, while ASD5 and SS responded about 150 seconds sooner than 

the ionization alarm, on average. The decreasing mean time difference trend was ASD4 alert, 

ASD5 alert and SS alert (tie), and ION alarm.
5-32

-1000

-500

0

500

1000

1500

ASD4 AlertASD5 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-34. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, multi-zone, single-cabinet, 65.0 minute HRP experiments.

-1000

-500

0

500

1000

1500

ASD4 AlertASD5 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-35. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, multi-zone, three cabinet configuration, 65.0 minute HRP 

experiments
5-33

-1000

-500

0

500

1000

1500

ASD4 AlertASD5 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-36. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, multi-zone, no ventilation, 65.0 minute HRP experiments

-1000

-500

0

500

1000

1500

ASD4 AlertASD5 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-37. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, multi-zone, 7.4 ACH room ventilation, 65.0 minute HRP 

experiments
5-34

0

500

1000

1500

2000

2500

3000

ASD4 Pre-alert

ASD5 Pre-alert

ASD4 AlertASD5 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-38. Time difference between the end of test (EOT) time (4200 s) and ASD prealerts, ASD and SS alerts or ION alarm time for large room, multi-zone incabinet, 65 minute HRP experiments

Four 260 minute heating period in-cabinet experiments were conducted in the large room, two 

experiments with XLPE wire sources and two with CSPE wire sources. For each wire source, 

experiments were conducted with the three cabinet arrangements with and without room 

ventilation (7.5 ACH).

Results for both the ventilated and non-ventilated room experiments are shown in Figure 5-39. 

Figure 5-40 shows the results of the time difference between the end of test (15,900 seconds)

and the detector response.
5-35

-2000

0

2000

4000

6000

ASD4 AlertASD5 Alert

SS Alert

Mean

ION Alarm - Alert (s)

Figure 5-39. Time difference between ION alarm times and the ASD alerts and SS alert 

for large room, multi-zone, in-cabinet, 260.0 minute HRP experiments

0

2000

4000

6000

8000

ASD4 Pre-alertASD5 Pre-alert

ASD4 AlertASD5 Alert

SS Alert

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-40. Time difference between the end of test (EOT) time (15900 s) and ASD prealerts, ASD and SS alerts or ION alarm time for large room, multi-zone 

in-cabinet, 260.0 minute HRP experiments
5-36

The large-room multi-zone in-cabinet results gave rise to the following observations:

1. ASD4 responded before the conventional ION alarm for both heating period 

experiments.

2. The time difference between the ION alarm and the ASD alerts was greater for the 

three-cabinet arrangement than the single-cabinet arrangement.

3. Room ventilation tended to reduce the time difference between the ionization alarm and 

the ASD alerts.

4. All detectors responded before end of test for all 60 minute HRP experiments. 

5. ASD5 and SS did not alarm with XLPE samples before end of test for 260 minute HRP 

experiments. 

5.6 Full-Scale, Small-Room, Area-wide Experiments

The data set examined consists of 65.0 minute heating period experiments for XLPE, CSPE and 

three cable bundle sources located on the floor. There was room air ventilation during one of 

the cable bundle experiments. The detector sensitivity settings used in these tests are shown in 

Table 5-8.

Table 5-8. Nominal Detector Sensitivities for Small Room, Single-Zone, Area-wide

Experiments

Sensitivity

Setting

ASD1

Detector /

Port

%/ft Obsc

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFD

Pre-alert

0.013 / 

0.052

5.1×105 /

2.0×106

0.025 / 

0.10 - - -

VEWFD

Alert

0.05 /

0.20

8.5×105 /

3.4×106

0.05 /

0.20

0.20 - -

VEWFD 

Alarm

0.25 /

1.00

1.5×106 /

6.0×106

0.25 /

1.00

1.00 - -

Conventional

Pre-alarm - - - - 0.5 1.3

Conventional 

Alarm - - - - 1.0 2.1

Figure 5-41 shows the results for the time difference between the end of the test (4,200 s) and 

the ASD pre-alerts and alerts, SS alert, ION alarm, and the PHOTO alarm. The SS, photo and 

ION alarms did not respond before the end of the test in two of the five experiments.
5-37

0

500

1000

1500

2000

ASD2 Pre-alert

ASD3 Pre-alert

ASD2 AlertASD3 Alert

SS Alert

PHOTO Alarm

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-41. Time difference between the end of test (EOT) time (4200 s) and ASD prealerts, ASD and SS alerts, PHOTO and ION alarm time for small room, areawide, 65.0 minute HRP experiments

From these experimental results the following observations are made:

1. The trend in mean alert/conventional alarm time from earlier to later was ASD3, SS, 

ASD2, PHOTO, and lastly ION.

2. Each detector failed to alert or alarm before the end of test in at least one experiment.

5.7 Large-Room, Multi-Zone Area-wide Experiments

The large-room, multi-zone, area-wide experiments included in-cabinet, return air grill, and areawide ceiling ASD coverage and conventional alarm coverage on the ceiling and inside cabinets. 

Table 5-9 documents the detector sensitivity settings used for these tests. ASD4 (cloud 

chamber) sensitivity settings were vendor-specified. Area-wide and return air grill locations are 

referred to as AW and RA, respectively.
5-38

Table 5-9. Nominal Detector Sensitivities for Large Room, Multi-Zone, Area-wide

Experiments

Sensitivity

Setting

ASD4

Detector /

AW Port / 

RA Port

Particles/cm3

ASD5

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFD

Pre-Alert

1.5×105 /

9.0×105 / 

7.5×105

0.0159 / 

0.10 - - -

VEWFD

Alert

2.5×105 /

1.5×106 /

1.3×106

0.0334 /

0.20

0.20 - -

VEWFD 

Alarm

4.5×105 /

2.7×106 /

2.3×106

0.167 /

1.0

1.00 - -

Conventional

Pre-alarm - - - 0.5 1.3

Conventional

Alarm - - - 1.0 2.1

Figure 5-42 and Figure 5-43 show the results for 16.3 and 65.0 minute HRP area-wide

experiments with ventilation air flow. 
5-39

0

100

200

300

400

500

ASD4 Alert AW

ASD4 Alert RA

ASD5 Alert AW

ASD5 Alert RA

SS Alert

PHOTO Alarm

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-42. Time difference between the end of test (EOT) time (1278 s) and ASD alerts, 

SS alerts, PHOTO and ION alarm time for large room, area-wide, 16.3

minute HRP experiments
5-40

0

500

1000

1500

ASD4 Alert AW

ASD4 Alert RA

ASD5 Alert AW

ASD5 Alert RA

SS Alert

PHOTO Alarm

ION Alarm

Mean

EOT - {Pre-alert, Alert, or Alarm} (s)

Figure 5-43. Time difference between the end of test (EOT) time (4200 s) and ASD alerts, 

SS alerts, PHOTO and ION alarm time for large room, area-wide, 65.0

minute HRP experiments

From these experimental results the observations are made:

1. For the 16.3 minute HRP experiments ASD5 and SS detectors were the only systems 

that alerted before the end of test

2. For the 16.3 minute HRP experiments SS area-wide outperformed the ASD5 return air 

zone and the ASD5 area-wide. 

3. For the 65.0 minute HRP experiments the ASD and SS detectors were the only systems 

that on average alerted before the end of test.

4. For the 65.0 minute HRP experiments the ASD5 area-wide and return air zone provided 

about the same average alert times, while ASD4 return air zone outperformed all 

detectors on average.
5-41

5.8 Insulated Electrical Conductor Heat Conduction and Ignition Potential

Measurements were made to characterize the heat conduction from the heated bus bar block to 

wire samples, and the ignition potential of such heated wires to a small pilot flame. The wire 

temperature governs when and how much smoke is generated from the sample to some extent.

The wire insulation temperature profile also determines the ease of ignition from a small pilot 

flame. The ease of ignition from such a pilot flame is an indication of the potential hazard of the 

heated wire insulation. 

Heat conduction from the block to the wire, down its length and through the insulation is a 

transient heat transfer process. The controlled variable is the block temperature via the 

temperature controller’s power-cycling of the cartridge heater. The wire insulation closest to the 

block heats up first and produces the smoke particles sensed by detectors. Subsequently, more 

wire insulation is heated to produce more smoke. The IR camera recorded images at different 

times during HRPs. A series of images is shown in Figure 5-44. The temperature profile 

measured at the end of the nominal 60 minute test can be seen in Figure 5-45. The locations of 

the thermocouples were represented by the cross symbol, with the corresponding IR camera 

temperature measurements. The emissivity on the IR camera in was set for plastic (0.93). 
Figure 5-44. Heating profiles for 12 AWG XLPE wires at various times during the heating process. The 

block can be seen in the top left corner in each image.

5-42
5-43

Figure 5-45. Temperature profile for a 12 AWG XLPE wire. The image was taken at the 

end of a 60.0+ minute HRP and a 5.0 minute set point hold at 450 °C. The 

bus bar temperature was 446 °C.

A series of tests was performed where the thermocouples measured the temperature along a 

single 12 AWG XLPE wire. Thermocouples were attached to the wire at 10, 30, 45, and 68 mm 

from the bus bar. Three tests were performed, one for each heating ramp. The thermocouple 

measurements along the wire and the bus bar can be seen in Figure 5-46. The figure shows 

the temperature profile at different times during the test, and for different locations along the 

wire. Both the thermocouples and the IR camera show about a 100 oC gradient across the wire.

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60 70

5

15

25

35

45

55

65

Surface Temperature (oC)

Thermocouple Location (mm)

Heating Time (min)

Start of Wire 

Insulation

Block

Figure 5-46. Wire surface temperature as a function of time for a 60.0 minute HRP 

followed by a 5.0 minute hold (± 2 oC). Thermocouples were located along 

the insulated surface of a 12 AWG XLPE sample
5-44

The temperature measurements suggest that the wire insulation closest to the bus bar 

approaches temperatures close to the piloted ignition temperature. During the experimental 

design, it was surmised that, given the final block temperature chosen, wire insulation would be 

easily ignitable by the end of the test period and thus, poses an imminent fire hazard. 

To support the assumption, ignitability experiments were conducted on the wire samples at 

different times during the HRP, and at the end of test time. XPLE, PVC2, and CPSE wire 

samples were attached to bus bars, and heated using the 16.3 minute HRP with the final set 

point of 485 °C. A small flame was positioned under a wire sample for 5 seconds then moved 

away. The time of persistent flame attachment after the pilot was moved was recorded. The 

flame was from a horizontal 0.3 mm ID tube with a flow rate of 25 L/min of propane. The end of 

the tube was 9.5 mm from the bus bar and the center of the tube was located 12.7 mm below 

the wire sample. The tube was attached to a slide rail so it could be positioned under heated 

wires rapidly. Figure 5-47 is a picture showing the bus bar mounted on its stand, two wire 

samples, the ignition tube, and an a enclosure located in a chemical hood. Each experiment

used two wire samples and was videotaped for subsequent timing analysis. Figure 5-48 is a 

picture of the pilot flame before it was positioned under a wire sample. 

Table 5-10 shows the results for the persistent burn time after the pilot flame was removed for 

the four different pre-heat times. The nominal block temperature for the pre-heat times is also 

indicated. After a heating period of 1,200 seconds and a block temperature of about 480 °C, the 

average persistent burn times were 5, 26, and 50 seconds for PVC2, XLPE and CSPE, 

respectively. After pre-heating for 900 seconds and a block temperature of about 435 °C, XLPE 

and CSPE wires continued to burn for greater than 20 seconds on average. After a heating 

period of 600 seconds and a nominal block temperature of about 300 °C, only the XLPE wire 

sustained flaming for longer than 1 second on average, with an average persistent burn time of 

14 seconds. After a heating period of 500 seconds and a nominal block temperature of about 

250 °C, none of the wire samples sustained flaming for longer than 1 s. XLPE appears to be 

the easiest of the three wires to ignite, followed by CPSE, then PVC2. The trend appears to be 

counter-intuitive, whereas thermoset insulation materials, such as XLPE and CSPE, burned 

longer after pilot flame removal, compared to the thermoplastic PVC. Possible factors affecting 

the ignitability here are possible flame retardant additives, and differences in the wire 

conduction, which could lead to different temperature profiles.
5-45

Figure 5-47. Experimental setup

Figure 5-48. Pilot flame
5-46

Table 5-10. Persisting Burn Time for Wire Samples after Pilot Flame Removed

Heating Period (s)

(15 minute HRP) Block Temperature (oC) Persistent Burn Time (s)

XLPE CSPE PVC2

1,200 480 28 45 7

1,200 480 20 53 5

1,200 480 31 52 5

1,200 480 4

900 435 17 38 3

900 435 36 26 2

900 435 18 32 3

900 435 5

600 300 17 1 1

600 300 11 0 1

600 300 13 0 0

500 250 1 0

500 250 1 0

500 250 0 0

Although these experiments do show that a small pilot flame can ignite pre-heated wires, it is 

not unreasonable to expect that such pre-heated wire could ignite from a brief electric arc, or 

glowing conductor, following an electrical failure; without sufficient pre-heating, ignition would 

not occur. Refer to Section 4.2 for more detail on the design and intended purpose of the 

incipient fire source.

5.9 Evaluation of Test Results

The results from testing provide information that supports several project objectives. This 

section presents the test results specific to those objectives, and supporting the risk scoping 

study presented in Part II.

5.9.1 System response to common products of combustion (Objective E)

Detector response to smoke signature from materials included in the test program is presented 

generically. The in-cabinet, naturally ventilated test data is used. This data set limits variability 

of cabinet ventilation rate, and area-wide detector-to-source location influences on detector 

response. Figure 5-49 presents data from all one-hour HRP tests. The figure shows the 

detection time for each detector type and material. The “alert” response is reported for the 

VEWFD systems (SS, ASD CC, ASD LS1, ASD LS2) and the “alarm” response for the 

conventional spots (PHOTO, ION). Note that the “alert” response is the 0.2 %/ft obscuration of 

the laser based systems (ASD LS1, ASD LS2, and SS)2, while the “alert” response for the ASD 

CC is the vendor recommended sensitivity. Since the latter does not report in percent 

obscuration per foot, comparisons between the ASD CC and other VEWFD systems should not 

be inferred to be tested at equivalent sensitivity settings. Instances in which the detector did not 

 2 LS1 and LS2 represent light-scattering ASDs from different vendors. LS1 represents the results from ASD1. 

LS2 represents the results from ASD3 and ASD5 which were from the same manufacture but differed in the 

model and number of sampling zones per detector. CC represents the results from ASD2 and ASD4 which 

were from the same manufacture but differed in the model and number of sampling zones per detector. 
5-47

respond before the end of the test are not shown. Plots for the 15 minute and four-hour HRP 

show similar responses and are presented in Appendix B.

Figure 5-49. Detector response to selected materials (1-hour HRP)

Figure 5-50 presents the mean time to detection results for in-cabinet experiments. The results 

for all three HRP normalized to the respective HRP are presented by detector and material. 

 

 

Time (Minutes)

0 10 20 30 40 50 60 70

CSPE

PCB

PTB

PVC

XLPE

XLPO

PHOTO

ION

SS

ASD CC

ASD LS2

ASD LS1
5-48

Figure 5-50. Time to detection, by detector
5-49

The insights from the data presented in Section 5.9.1 indicate:

1. Based on the mean detection time normalized to the test duration, there is no apparent 

trend for the mean time to detection and the HRPs used in testing. In general, the mean 

time to detect increases with increasing HRP in a non-linear manner. This change 

between the 15 minute and 1 hour HRP is more pronounced than the change between 

the 1 hour and 4 hour HRP. This likely indicates the aerosol characteristics are 

dependent on the rate of material heat up and aerosol generation. The normalization of 

the test durations in Figure 5-50 also indicates that the mean detection range between 

the three HRP is narrow on the order of 1/10th of the test duration. This observation is 

used to support the timing analysis discussed in Section 8.

2. The PHOTO spot-type detector only responded to the CSPE, PCB, PVC, and XLPE 

materials. CSPE was the only material that the PHOTO consistently detected.

3. The CSPE material appears to be the easiest to detect. All detectors were effective at 

detecting the CSPE. For VEWFD systems, including the sensitive spot detector, all 

responded to the CSPE aerosol at approximately the same time, with ASD LS2 

responding earliest (on average).

4. PTB, SR, and TEF are typically detected latest in the low-energy (incipient stage) fire

and are the most difficult to detect of the materials tested. The Silicone and Teflon 

conductor insulation materials were only effectively detected by ASD CC and ION.

5. The printed circuit board data shows an opposite trend compared to the other materials,

in that the time to detect decreases with increased HRP from the 15-mintue to 1-hr 

HRPs.

6. With the exception of CSPE, the ASD CC and ION spot-type responded at an “alert” and 

“alarm” setting, respectively, before either ASD LS1 or ASD LS2 “alert” response.

5.9.2 Comparison between common detection systems and VEWFD systems 

(Objective B)

A comparison between the performance of conventional spot-type detectors (ION, PHOTO) and 

VEWFD systems (ASDs and sensitive spot) is presented in summary plots. Each point on the 

summary plots represents the time to detection within a single test. These plots present all 

three HRP data on a single plot. The diagonal line represents equal performance. In Figure 

5-51 and Figure 5-52, points below the diagonal line represent a test in which the VEWFD 

system responded to an “alert” before the conventional detector “alarm.” Points above the 

diagonal line indicate the conventional spot-type detector responded with an “alarm” before the 

VEWFD system “alert” response. The dashed diagonal lines represent two standard deviations 

from the mean conventional detector response used for evaluations.

Table 5-11 provides a summary of this data shown as the percent difference in time to detection 

between conventional and VEWFD systems. Negative values represent conventional detection 

responding before VEWFD systems. Based on these test results, the ION spot-type detector

responded 2.9 percent slower on average than the VEWFD systems, whereas the PHOTO spottype responded 19.3 percent slower, on average. Also, based on study observations, it is highly 

likely that the ASD CC will respond before the ION detector and the ASD LS2 will respond 

before the PHOTO. This is consistent with the technologies involved, in that both the ION and 
5-50

ASD CC perform well at detecting small particles, while the PHOTO and ASD LS perform well at 

detecting larger particles. It is also notable, that the results for the two ASD VEWFD LS 

detectors show performance differences, even though both were set to the same port sensitivity 

of 0.2 %/ft. obscuration.

Figure 5-51. Summary plot – ION alarm versus VEWFD alert 

(in-cabinet, natural ventilation)

Figure 5-52. Summary plot – PHOTO versus VEWFD (in-cabinet, natural ventilation)
5-51

Table 5-11. Summary of Average Difference in Time to Detection between Conventional 

and VEWFD Systems (Negative Values Represent Conventional Spot 

Responding on Average before VEWFD Systems)

ION PHOTO

Mean 5th/95th

Percentile Mean 5th/95thPercentile

ALL VEWFD 2.9% -39.8/38.0% 19.3% -10.7/50.4%

ALL ASD 6.7% -38.8/40.0% 20.6% -10.5/50.6%

ASD LS1 - 15.7% -54.4/29.5% 12.1% -18.7/45.7%

ASD LS2 -0.1% -27.6/35.3% 23.6% 4.0/53.9%

ASD CC 23.5% 3.4/47.7% 25.9% -7.6/50.8%

SS -7.9% 41.1/30.0% 15.2% -13.4/52.6%

5.9.3 Evaluation of in-cabinet VEWFD system layout and design versus system 

response (Objective F)

In-cabinet installations of VEWFD systems are consistent among vendors and with the 

community of practice in that it is recommended to locate the air sampling port in the upper 

10 percent of the cabinet volume being protected. Since the testing followed vendor 

recommended practices, the evaluation presented here will focus on the detection system 

response to different cabinet configurations and operating conditions. The variations evaluated 

include cabinet ventilation conditions, such as forced versus natural ventilated cabinets; and

cabinet bank arrangement, as single partitioned cabinets versus multiple connected cabinets,

with a common air space.

Figure 5-53 illustrates the responses of several detectors to two different material aerosols for 

different cabinet configurations. The “1/C” case represents a cabinet where a single sampling 

port is located. The “M/C” case represents a cabinet space where multiple sampling ports 

protect a section of cabinets that have a shared air space without any partitions. The intent of 

presenting the data in this form is to evaluate the potential cumulative effect of ASDs. These

results show that, in general, having more sampling ports within a cabinet will shorten the time 

to detection. However, for the CSPE case, the ASD LS2 shows an average increase in the time 

to detection between single and multiple sampling ports. These results also show that, in 

general, the spot-type detectors respond more slowly when used to protect multiple cabinets. 

A similar trend is shown in Section 5.5, Figure 5-24. On average, the spot-type detectors were 

approximately 2.9 percent slower and the ASDs were 2.6 percent faster in response when 

multiple detection points were used in a bank of cabinets compared to a single detection point in 

a single cabinet.
5-52

Figure 5-53. Detector response versus number of sampling ports in cabinet space

Figure 5-54 present the effect cabinet ventilation configurations have on detector response. 

Three states of cabinet ventilation are shown. The “Natural” state represents tests where no 

mechanical ventilation was used. The “Forced – Low” state represents cases in the full scale 

testing where an 8.0 ACH cabinet ventilation rate was used. The “Forced – High” state 

represents the NPP cabinet tests where the ventilation rate was estimated at 300 ACH. 

In general, as cabinet ventilation rate is increased, the time to detection also increases. This 

general trend does not hold for the CSPE material where the ASD LS2 and ION spot-type 

detector responded sooner at the “Forced – Low” state than at the naturally ventilated condition.

Figure 5-55 presents the results from in-cabinet detectors mean response time for all materials 

by cabinet ventilation conditions. From this graphic, it becomes apparent that the mean time for 

detector response increases with increasing cabinet ventilation conditions. It should also be 

noted that LS1 was only included in the naturally ventilated case and the PHOTO was no 

included in the high forced ventilation experiments.
5-53

Figure 5-54. Effect of cabinet ventilation on in-cabinet detector response
5-54

Figure 5-55. Effect of cabinet ventilation on in-cabinet detector response for all 

materials and all HRPs.

5.9.4 Effectiveness of in-cabinet and area-wide VEWFD system applications, including 

an evaluation of system applicability for various NPP applications (Objective A)

System effectiveness is a measure of how well a design solution will perform or operate, given 

anticipated operational scenarios (Ref. 19). Effectiveness estimates for ASD VEWFD systems 

expected to operate during the pre-flaming (incipient) stage are based on the test data collected 

from the experiments conducted in this program. As indicated above, the source materials are 

heated to a temperature that can support flaming conditions. The effectiveness differs between 

in-cabinet and the two area-wide configurations, primarily because of smoke dilution and 

stratification effects. Section 7.2 quantifies the effectiveness of the detection systems tested for 

use in the risk scoping study presented in Part II of this report. Figure 5-56 present the mean

effectiveness for the systems and applications tested. As shown in Figure 5-55 and Figure 

5-56, in-cabinet detection without forced ventilation provides the earliest and most effective 

application for detecting pre-flaming fire conditions.

For the in-cabinet applications, the test data was initially analyzed for system effectiveness for 

two cases (i.e., naturally ventilated and forced ventilation conditions). The data showed a large 

difference between the two cases, with regard to system effectiveness. Further evaluation of 

the forced ventilation data indicated that the reactor protection system cabinet, taken out of a 

U.S. NPP had a much lower VEWFD system effectiveness compared to the mock-up cabinet 

used in the MCPTA tests, and the naturally ventilated cases. Further evaluation of the 

differences between cabinet design and testing conditions identified several parameters that are 

likely influencing these results. Photographs and descriptions of these cabinet configurations 

are shown in Sections 4.4.2 and 4.4.3.

Time (Normalized to test duration)

0.4 0.5 0.6 0.7 0.8 0.9

Natural

Low Forced

High Forced

ASD LS1

ASD LS2

ASD CC

SS

ION

PHOTO
5-55

Figure 5-56. System effectiveness by detector and application 

(Note: no data for ION area-wide return)

Cabinet Design Influence

Air streams within the cabinet caused by internal structural members used to hold circuit cards, 

power supply, electronic modules, etc., and cabinet vent locations in relation to the location of 

the ASD sampling point may reduce effectiveness. If the products of combustion cannot be 

sampled by the VEWFD system, the system should be expected to either not respond or to 

experience a delayed response. Electrical cabinets within NPPs have a variety of designs with 

regard to ventilation. Common examples include louvered vents on the full length of the front 

and back panels/doors; half height vents on the front and/or back cabinet panel height; vents on 

the top and bottom portion of the front and/or back panels/doors; vents on the bottom (floor) and 

top; combinations of these configurations also exist. Thus, the cabinet design, (with regard to 

ventilation), and physical component layout, (with regard to the location of the VEWFD system 

sampling point), have an effect on the ability of any combustion products reaching the VEWFD 

system sampling points. Because only a limited number of vent/cabinet layout designs were 

evaluated in the NIST testing, those results should be used with caution, and do not bound all 

cases found in NPPs.

Ventilation rate 

Ventilation rate has a direct effect on aerosol dilution. As the ventilation rate increases, so does 

dilution, which results in lower concentration of combustion products. Dilution affects both 

particle concentration and light obscuration.

0.0

0.2

0.4

0.6

0.8

1.0

PHOTO

ASD LS1

SS

ASD LS2

ION

ASD CC

Area-Wide Ceiling

Area-Wide Return

In-Cabinet Forced

In-Cabinet Natural Effectiveness
5-56

For in-cabinet applications, two parameters with an influence on system response were 

identified: cabinet ventilation rates and cabinet internal obstructions, the latter being a 

secondary or tertiary order effect. Three ventilation rates were available from the testing, 

namely, naturally ventilated, 7.9 and approximately 360 cabinet air changes per hour. The 

results indicated that, in general, as the ventilation rate increased, the effectiveness of any 

smoke detector technology in detecting the incipient source decreased and the time to detection 

increase. For light scattering ASDs VEWFD systems and SS detectors, the low flow rate data 

indicates a slight improvement in detector effectiveness compared to the naturally ventilated 

case. Whereas the influence on system response from cabinet internal obstructions is less 

clear. In the naturally ventilated cases, there isn’t sufficient data to differentiate between the 

influences/impacts made from the various obstructions. In the force-ventilated cabinet cases, 

the test series which had differences in cabinet internal obstructions also had different 

ventilation rates. Because there is no constant for comparison, the results do not provide clear 

insights on the performance. 

Additionally, cabinet configurations such as fully louvered doors with internal heat loading and 

room ventilation effects were not evaluated during this program; yet they could potentially have 

an effect on the system performance, especially if sufficient thermal gradients are present within 

the cabinet.

Cabinet internal component layout and ventilation configurations vary, and are likely to be an 

important influencing parameter on the effectiveness of the system. Because of the limited 

number of cabinets available for testing, the test data may not be representative of other 

configurations. Several different estimates of system effectiveness are developed as shown in 

Table 7-5. These estimates support the risk scoping study presented in Part II. The system 

effectiveness estimates are all based on test data from this program.

5.9.5 Area-wide ASD comparison

The individual data sets were evaluated for the ability to be pooled into larger datasets using the 

Kolmogorv-Smirnov (K-S) tests. For ceiling-mounted ASDs, VEWFD systems the data was able 

to be pooled into the following three data sets: all tests using forced ventilation; naturally

ventilated cases using block smoke source; naturally ventilated case using bundle smoke 

source. Figure 5-57 shows these data sets according to their time to detect the low energy 

sources. A similar evaluation was conducted for the tests using forced ventilation with a 

comparison between air return grill, and ceiling-mounted ASD performance. These results are 

shown in Figure 5-58 and indicate that the air return grill and ceiling-mounted performance 

results are similar enough to be pooled. From a practicality standpoint, given the fact that there 

are a variety of source materials found in electrical enclosures and testing materials that 

produce a range of aerosol characteristics representative of slow overheating conditions, 

pooling of these data sets seems reasonable.
5-57

Figure 5-57. ASD time to detect area-wide ceiling configurations

Figure 5-58. ASD time to detect area-wide forced ventilation air return and ceiling pooled

The last comparison of the area-wide data is to evaluate the effectiveness of the ASD system 

tests. The effectiveness estimates are shown in Figure 5-58. These results indicate that for the 

room air change rates tested, the air return grill application response sooner on average than 
5-58

ceiling-mounted ASD systems. With the bundle tests producing more smoke than the block 

tests, these results consistently show that either area-wide application is better at detecting the 

bundle source.

5.9.6 ASD comparison: In-cabinet versus area-wide

The ASD time to detection results for a variety of configurations are presented in Figure 5-59, 

including the effect of forced ventilation on the time for detection. As shown in Figure 5-59, 

forced-ventilation also impacts the effectiveness of smoke detection; however, the term 

‘effectiveness’ is not incorporated into this assessment. As expected the closer a smoke 

detector is located to a potential hazard, the sooner the response.

Figure 5-59. ASD time to detect low-energy incipient sources
PART II

Smoke Detection Risk Scoping Study

6-1

6. OVERVIEW OF QUANTIFICATION APPROACH

This section provides a high-level overview of the current fire probabilistic risk assessment 

(PRA) quantification method as described in NUREG/CR-6850; a summary of previous efforts; a 

discussion on possible approaches for quantifying smoke detector performance in fire PRA; and 

an overview of the approach pursued under this research project.

6.1 Overview of Fire PRA Model

The fundamental concept of a fire PRA is to estimate the total core damage frequency (CDF) 

arising from fire initiators and develop risk insights. The total CDF is the sum of the CDF 

contributions from individual fire-initiated scenarios. The CDF contribution from an individual fire 

scenario can be divided into three principal components (Ref. 35): 

1. frequency of the fire scenario

2. conditional probability of fire-induced damage to critical equipment given the fire

3. conditional probability of core damage given the specific equipment damage 

Mathematically, the total CDF is characterized as follows:

CC = � CCii

ii

= �λλii ��ppeeee,jj|ii ��ppCCCC,kk|ii,jj

kk

�

jj

�

ii

Where λi Frequency of fire scenario i

Ped,j|i Conditional probability of damage to critical equipment set (“target set”)

j given the occurrence of fire scenario i

PCD,k|i,j Conditional probability of core damage caused by plant response 

scenario k given fire scenario i and damage target set j

Fire ignition frequency is defined as the occurrence rate of a potentially challenging or greater 

fire involving a specific component or specific compartment of the plant (Ref. 36).

The probability of equipment damage is decomposed into two parts:

PPeeee,jj|ii = SS jj|ii × PPnnnn,jj|ii

Where SFj|i severity factor for damage to target set j given fire source i

Pns,j|i probability of non-suppression before damage to target set j given fire 

scenario i

The severity factor reflects the fraction of fires that have the potential to damage the critical 

equipment in the fire scenario. The non-suppression probability represents the probabilistic 

outcome of the fire damage versus fire suppression race given a fire that has the potential to 

damage critical equipment. 
6-2

6.1.1 Application of VEWFD System Probability of Non-Suppression: Pns

Specific care must be taken when applying the probability of non-suppression (Pns ) for very 

early warning fire detection (VEWFD) systems. For the simplest application of this 

methodology, the probability of non-suppression is only applicable to target damage sets 

located outside of the electrical cabinet. That is, the VEWFD system ONLY affects the adverse 

consequences related to cabinet fires resulting in fire growth outside the initiating electrical 

cabinet which could damage secondary combustibles and targets. The fire ignition source is 

assumed to be damaged given any fire involving itself as the source. The probability of 

non-suppression represents the probabilistic outcome that the fire will damage critical 

secondary equipment before some means of fire suppression can be achieved.

However, the methodology can be extended beyond the application discussed above, and 

consider target sets within the electrical enclosure. When a fire scenario contains several target 

sets, the scenario CDF equation can be written as follows, where the quantities in the square 

brackets represent the target set (j) contributions:

CCii = λλii × ��SS jj|ii × PPnnnn,jj|ii

jj

��ppCCCC,kk|ii,jj

kk

��

Where λi Frequency of fire scenario i

SFj|i severity factor for fire scenario i

Pns,j|i probability of non-suppression before damage to target j given fire 

scenario i

pCD,k|i,j, Conditional probability of core damage caused by plant response 

scenario k given fire scenario i and damage target set j 

The scenario targets sets (j) can be within the electrical enclosure, as well as external to the 

electrical enclosure. The analysis of target sets within the electrical enclosure and external to 

the electrical enclosure is briefly described below.

Target sets within initiating electrical enclosure:

Under a suppression strategy Pns,j|i is assumed to be 1. This is done due to the difficulty in 

assessing the likelihood of successful prevention prior to damage to targets within the 

enclosure. This approach is intended to take credit for the fire suppression capability and 

assumes that the cabinet will be damaged. For the suppression strategy to be effective, the 

licensee needs to establish procedures that require appropriately trained personnel to be in 

place until the problem has been successfully resolved. Success in this approach is ultimately 

judged based on the ability to suppress the fire rather than prevent it. 

Under a de-energization strategy, the suppression event trees presented in Section 6.4 could be 

modified as illustrated in Figure 6-6 to estimate Pns,j|i. A de-energization strategy is considered 

one in which the end state is achieved by removing power from the affected cabinet (or part of 

the cabinet) and repairing the degraded component (i.e., this is a prevention strategy/approach). 

Section 9.2 provides a generic description of a tabletop analysis performed for a de-energization 

strategy. Section 10.6.4 provides human failure event quantification items for a de-energization 
6-3

strategy that should be considered when performing the detailed human reliability analysis

(Section 10) to support a scenario-specific de-energization approach and quantification of

scenario-specific human error probabilities. Fire scenario and human response information, 

including timing and feasibility analysis, would be needed to support estimation of the human 

error probabilities via a detailed Human Reliability Analysis. Pre-planning the steps to deenergize the cabinets would be needed to ensure power is removed in a timely manner thus 

insuring the prevention of flaming conditions. The estimates developed in Section 10 and 11 

are for suppression capability only and as such do not apply to a de-energization strategy, 

which requires detailed scenario-specific information. The process outlined in Section 10, for 

suppression capability, could be modified with alternative timing assumptions to quantify the 

parameter ξde-ss. Using the parameter ξde-ss, it is possible to quantify the modified in-cabinet 

event tree for a de-energizations strategy, as shown in Figure 6-6. In the de-energization 

strategy the initiating enclosure term Pns,j|i is calculated as the summation of all “cabinet 

damage” and “fire damage outside cabinet” end states as illustrated in Figure 6-6.

Target sets external to the initiating electrical enclosure:

Under a suppression strategy, Pns,j|i is calculated as the summation of all “fire damage outside 

cabinet” end states as illustrated in the in-cabinet event tree (Figure 6-4) or the area-wide event 

tree (Figure 6-5) depending on the scenario application. This approach is intended to take

credit for the fire suppression capability and assumes that the cabinet will be damaged. To be 

effective, the licensee needs to establish procedures that require appropriately trained 

personnel to be in place until the problem has been successfully resolved. Success in this 

approach is ultimately judged based on the ability to suppress the fire rather than prevent it.

Under a de-energization strategy, Pns,j|i is calculated as the summation of all “fire damage 

outside cabinet” end states as illustrated in Figure 6-6.

6.2 Summary of Previous Quantification Efforts

This section presents a summary of the previous approaches used to quantify the performance 

of VEWFD system fire PRAs. This summary does not present any new information nor does it 

evaluate the adequacy of previous approaches. It is suggested that the reader reference the 

associated documents for a full understanding of the methods summarized herein.

NUREG/CR-6850—EPRI 1011989, Appendix P, Detection and Suppression Analysis

NUREG/CR-6850 (EPRI 1011989), Appendix P, “Appendix for Chapter 11, Detection and 

Suppression Analysis, (2005)” provides an approach for solving the Detection-Suppression 

Event Tree to estimate a non-suppression probability. That event tree evaluates the prompt, 

automatic, and manual detection and suppression capabilities. For in-cabinet smoke detection 

devices, Appendix P provides the following information:

If in-cabinet smoke detection devices are installed in the electrical cabinet 

postulated as the ignition source, the analyst should assume that the fire will be 

detected in its incipient stage. The incipient stage is assumed to have a duration 

of 5 minutes. To account for these 5 minutes, the analysts should add them to 

the time to target damage (or, equivalently, add them to the time available for 

suppression).
6-4

The non-suppression probability is calculated by the following equations:

Pr(TT > tt) = ee−λλλλ

Where λ is the rate at which a fire is suppressed and t is the time available for suppression 

before target damage calculated as follows:

tt = tt dddd − ttffff − tt

where tdam is the time to target damage, tfb is the response time of the fire brigade, and tdet is the 

time to detection. Given the information in NUREG/CR-6850 (EPRI 1011989), tdet can be set to 

-5. As shown in Figure 6-1, adding 5 minutes to the time available for suppression results in a 

maximum reduction of 0.39 from the non-suppression probability, excluding the timing 

contributions from the tdam and tdet.

 

Figure 6-1. Electrical fires suppression curve showing 5 minute in-cabinet detection 

reduction

The NUREG/CR-6850 (EPRI 1011989) approach does not specify the type of in-cabinet smoke 

detection. Additionally, the NUREG/CR-6850 references “high sensitivity detectors” as a form of 

prompt detection, but does not define this term. The approach also specifies that prompt 

suppression is for hot work fires only.

Supplement 1 to NUREG/CR-6850, Section 14, titled “Manual Nonsuppression Probability (FAQ 

08-0050),” presents clarifications on the use and updated to the non-suppression probabilities. 

The Supplement differs from NUREG/CR-6850 method in that the fire brigade response time is 

not directly used in the calculation of time available for suppression. Instead, an industry 

average is included in the non-suppression curves presented in the supplement. Thus 

Supplement 1 presents two approaches. The first approach assumes an industry-average fire 

brigade response. The time available for suppression is simply the difference between the time 

of detection and the time of damage. The second approach uses a correction factor for cases 

where it is judged that the scenario specific fire brigade response time distribution is significantly 

different from the underlying events reported in the EPRI Fire Events Database (the source of 

information used to develop the revised non-suppression probability curves). Under the 

scenario specific adjustment approach the non-suppression probability is calculated as:

Time (Minutes)

0 10 20 30 40 50 60 70

Non-Supp Probability

0.0

0.2

0.4

0.6

0.8

1.0

0.39
6-5

PPnnnn(tt) = exp[−λλ(tt ∙ )]

where Cs is a scenario-specific adjustment factor calculated as:

= �

〈TTffff− 〉 − 〈TTffff− 〉

〈TTffff− 〉 + 〈TTffff− 〉

�

where Tfb-s and Tfb-t are the mean scenario specific and typical fire brigade response times, 

respectively.

EPRI 1016735 Fire PRA Method Enhancements

In 2008, EPRI published EPRI 1016735, “Fire PRA Methods Enhancements: Additions, 

Clarifications and Refinements to EPRI 1019189.” In section 3 of that report, a method for 

quantifying the performance of aspirating smoke detection (ASD) is provided. Appendix C of 

the EPRI report also provides supporting information. The quantification approach is in the form 

of an event tree with three events. The method adjusts the fire ignition frequency by multiplying 

the location-weighted ignition frequency (λω) by:

1. μ : the fraction of ignition source J components in location L that are effectively covered 

by the very early warning fire detection (VEWFD) system, represented as (μ)

2. R : availability and reliability of VEWFD system in location L, and 

3. P : the pre-emptive suppression probability

The ignition frequency adjusted to account for VEWFD systems, would then be:

λλωωVVVVVVVV = λλ (1 − μμμμ ).

The report limits the applicability of this method of using VEWFD systems to components 250V 

or less such as Bin 1 (batteries); 4 (main control board); 9 (air compressors); 10 (battery 

chargers); and 15 (electrical cabinet) components; and 480V or less components such as Bin 

11 (welding and cutting); 14 (electric motors); 18 (junction box); 21 (pumps); and 22 (RPS MG 

set) components.

The method also makes the following assumptions:

• VEWFD alarms will indicate incipient conditions approximately an hour or more before 

ignition occurs (based on manufacturers’ claims, NFPA 76 objectives).

• Technicians respond within 15 minutes.

• Incipient condition will be identified and prevented from achieving ignition for 

approximately 99.9 percent or more of true incipient conditions. Based on control room 

suppression curve and 15 minutes or more suppression time.

• Prompt detection (plus alarm delay time) for system excluded from guidance, but within 

coverage area of VEWFD system.

The report section concludes with two examples. In the first example the system is assumed to 

be highly capable of detecting incipient conditions and estimates a reduction in fire ignition 

frequency by 0.994. The second example assumes a more limited capability of the VEWFD 

system and estimates a reduction in the fire ignition frequency by 0.503.
6-6

NFPA 805 FAQ 08-0046, Incipient Fire Detection Systems

FAQ 08-0046, “Incipient Fire Detection Systems, (2009)” documents the NRC staff’s interim 

position with regard to crediting VEWFD systems in fire PRAs to support NFPA 805. This 

position provides a method for determining the probability of non-suppression in fire areas 

where incipient fire detection systems are installed. The FAQ approach follows the EPRI 

approach using an event tree, but provides refinements intended to improve accuracy and 

realism. The FAQ event tree is shown in Figure 6-2. The event tree has five events with three 

possible outcomes.

Figure 6-2. FAQ 08-0046 proposed event tree for assessing fire risk for installed 

VEWFDS in-cabinet

The first event represents the percentage of components that do not exhibit an incipient 

degradation phase: this is detectable by the ASD VEWFDS in cabinets containing components 

with voltages less than or equal to 250 Volts (represented by α). Components that do not 

exhibit this phase include fast-acting components defined in the FAQ as electrical/electronic 

circuit boards that contain the following:

• electrolytic capacitors

• chart recording devices

• cooling fans

• mechanical timers driven by electric motors

• other components that may fail abruptly without a degradation phase detectable by an 

ASD VEWFDS

(1-δ)

(1-γ) (1-ε1)

δ

(1-β) ε1

(1-ε2)

γ

(1-α) ε2

(1-ε2)

β

λ

ε2

(1-ε2)

α

ε2

Fire Damage

No Fire Damage to Targets

Outside Cabinet

Fire Damage

No Fire

No Fire Damage to Targets

Outside Cabinet

Fire Damage

No Fire Damage to Targets

Outside Cabinet

Fire Damage

No Fire Damage to Targets

Outside Cabinet

Fire Initiating Event End Point

Fration that have an 

Incipient Phase 

Detectable by 

System

Detector System 

Availability and 

Reliability

Successful Operator 

Response to Alert

Technician 

Successful in 

Preventing Fire in 

Incipient Stage

Fire Suppressed
6-7

The next event represents the failure probability for the ASD VEWFDS to issue an alert,

provided that conditions exist within the protected cabinet that would cause a properly 

functioning system to go into alert (represented by β). The NRC staff’s interim position suggests 

that this value can be set to 1×10-2.

The next event represents the likelihood that plant personnel may/will fail to adequately respond 

to an alert signal in a timely manner (before flaming). Gamma (γ) can be determined based on 

a human reliability analysis (HRA) or conservatively set to 1×10-2 for applications in which an 

ASD VEWFDS zone is dedicated to multiple cabinets and γ can be set to 5×10-3 when the ASD 

VEWFDS is addressable to an individual protected cabinet. The recommended values assume

that the ASD VEWFDS provides at least one hour of warning before the actual outbreak of an 

open flaming fire. 

The next event represents the probability of failure to remove power from the device once it has 

been located. The NRC staff determined that because of the complexity of identifying and 

removing all power from the affected component, this parameter’s (δ) value should be set to 1 

representing a zero probability of successfully removing power.

The last event “Fire Suppressed” has two parameters associated with it. ε1 is the “enhanced”

non-suppression parameter, and represents the probability that, given the operator has 

successfully responded to the alert (1-λ), the personnel staged at the cabinet associated with 

the ASD VEWFD system alert fails to promptly suppress the fire. Here, failure represents a 

scenario in which the suppression activity is not performed quickly enough to prevent damage 

outside of the protected cabinet, once the affected components’ fire growth enters the flaming 

stage. The NRC staff’s interim position indicated that a value of 1×10-3 should be used for ε1.

The second suppression parameter, ε2 the probability of “normal” non-suppression, addresses 

cases in which the operator fails to properly respond to an alert, or cases in which the detector 

fails to issue an alert (availability/reliability). The values used for ε2 in this event tree should be 

taken from the Detection Suppression Event Tree in NUREG/CR-6850, Appendix P, using the 

electrical suppression curve for manual suppression, as appropriate. Additionally, FAQ 08-0050 

in Supplement 1 to NUREG/CR-6850 provides updated information to Appendix P of 

NUREG/CR-6850. Credit should be given as described in Appendix P for automatic detection 

and suppression (normal spot detectors and automatic suppression in the area), as well as 

delayed manual detection, manual actuation of fixed suppression, and manual suppression via 

the fire brigade. Using the updated numerical results presented in Table 14-1 of Supplement 1 

to NUREG/CR-6850 for Electrical Fires, making the assumption that the fire brigade has 5 

minutes to suppress the fire, the non-suppression probability value of 0.602 should be used for 

ε2.

Given the discussion provided in the FAQ, and summarized above, a simplified event tree can 

be developed based on the fact that these systems can only be credited for protecting 

equipment that exhibits an incipient stage detectable by the system (α set to 0) and no credit is 

given for removing power from the component that caused the system alert (δ set to 1); this 

simplified event tree is presented in Figure 6-3. Given this treatment, the simplified 

FAQ 08-0046 event tree represents a reduction of 0.979 to 0.984 in the non-suppression 

probability, dependent on addressability of the system and ignoring normal non-suppression 

analysis.
6-8

Figure 6-3. FAQ 08-0046 simplified event tree for assessing fire risk for installed 

VEWFDS in-cabinet

6.3 Approaches to Quantifying Smoke Detection Performance

There are several methods that could be applied to evaluate the non-suppression conditional 

probability estimate associated with smoke detection systems. Examples include simulation, 

decision trees, and expert judgment. Some past quantification efforts1 have used event trees to 

quantify risk reductions. This report also uses an event tree approach to estimate the 

probability of non-suppression for different smoke detector technologies. Event trees are 

models that group a broad range of possible scenarios into a small number of categories. For 

example, there is an infinite variety of pre-flaming scenarios; however, the event tree groups this 

infinite variety into two categories: slowly developing (where VEWFD might give substantial 

improvement over conventional), and rapidly developing (where VEWFD and conventional are 

likely to perform about the same). Typically, event trees are used to evaluate different end 

states (e.g., fire causes damage to 1. initiating component, 2. secondary targets, 3. room, etc.) 

and the risk reduction of any detection or suppression system.

The method described here builds on the previous event trees’ quantification efforts. However, 

this method attempts to provide improvements over past efforts by using test data, operating 

experience, and expected operational response to smoke detection alarms. 

6.4 Event Trees and Definitions of Event Headings

Two event trees were developed. The first event tree, shown in Figure 6-4, estimates the nonsuppression probability for in-cabinet smoke detection applications, while the second, shown in

Figure 6-5, represents the non-suppression probability for area-wide type applications. The 

event trees presented in Figure 6-4 and Figure 6-5 provide a structure to estimate the non-

 1 See EPRI 1016735 and NRC FAQ 08-0046

Fire Initiating 

Event

OK

ξ1

NS

γ (1-ξ2)

OK

ξ2

NS

β (1-ξ2)

OK

ξ2

NS

(1-ξ1)

Fire Suppressed End Point

Fire Damage

No Fire Damage to Targets 

Outside Cabinet

Fire Damage

No Fire Damage to Targets 

Outside Cabinet

Fire Damage

No Fire Damge to Targets 

Outside Cabinet

Detector System 

Availability and 

Reliability

Successful Operator 

Response to Alert

λ (1-β) (1-γ)
6-9

suppression probability for fire scenarios where smoke detection systems are used to protect 

electrical cabinet ignition sources. Figure 6-6 provides an illustration of how the event tree 

could be modified to credit a de-energization strategy. However, use of the de-energization 

event tree requires scenario-specific information and as such, cannot be performed under a 

generic approach. As such the HEP probabilities developed in Section 10 of this report do not 

apply to the de-energization event tree shown in Figure 6-6.

The non-suppression probability event trees (Figures 6-4 and 6-5) have two end states (1) 

cabinet damage and (2) fire damage outside cabinet. These end states are defined as follows:

Cabinet damage end state assumes that damage is not limited to the initiating component and 

that other components within the cabinet are damaged. Suppression activities, regardless of 

form are also assumed to damage the ability of components located within a cabinet to perform 

their intended design function.

Fire damage outside cabinet assumes damage could not be limited to the initiating cabinet,

and target sets outside of the initiating cabinet may be damaged.

The event tree headings include estimation of fire phenomena, detector performance, human 

performance measures, and fire suppression. The basis for the development of these estimates 

is provided in the subsequent subsections. 

The first event, “Detector System Availability, Reliability” quantifies the systems operational 

performance (Section 7.2). The failure branch (down) represents the probability that a detection 

system will be unable to perform its function because of system outage or hardware failure. The 

next event, “Fractions that have an incipient phase” separates events that exhibit rapidly 

developing fires from those that exhibit longer incipient stages (Section 7.1). The next branch 

“Effectiveness,” evaluates the system’s ability to detect low-energy (pre-flaming) fires for a 

specific installed application (Section 7.2). The success branch represents a detection system’s 

probability of effectively detecting a low energy fire in its incipient stage. Success of the “MCR 

Response” event represents that the main control room (MCR) operating crew has 

acknowledged a smoke detector alert or alarm and has directed first level field response to the 

alerting/alarming fire location (Section 10.6). Success in the “first Level Field Response 

(Technician/Field Operator) Fire Watch Posted” represents the probability that the field 

response plant personnel have arrived at the smoke detector alert/alarm location (Section 10.6). 

Success in the enhanced suppression event represents the probability that any potential fire is 

suppressed before fire damage to targets of concern (Section 11.1). The last event 

“Conventional Detection/Suppression” estimates the probability of successfully suppressing a 

fire given a failure of one of the earlier events (Section 11.2). To estimate the success of these 

branches, the suppression/detection event tree from NUREG/CR-6850 (EPRI 1011989) can be 

solved.

A Microsoft Excel® based tool has been developed and included with this report to support a 

consistent and automated process for solving the event trees presented in Figures 6-4 and 6-5. 

Appendix H to this report provides a user guide for these VEWFD event tree non-suppression 

probability calculation tools.
Figure 6-4. Basic event tree for in-cabinet smoke detection non-suppression probability estimation

Fire

μ

1-β

NS

 η2

1-η2

1-η1

1-η3

 η3 

 η1 NS 

 η2 NS 

β 1-η1 OK 

 η2 NS 

α 1-η2 OK 

NS 

τ 1-η2 OK 

OK 

λi x SF 1-α 1-τ 1-μ 1-ξ 1-π1 OK

ξ OK 

NS 

π1 OK 

 η1

Detector System 

Avaliability, 

Reliability

Fraction of Fires that 

have an Incipient Stage

System Effective 

Detecting Incipient 

Stage

Successful MCR 

Response

Successful Field Operator 

Response 

(Fire Watch Posted)

Enhanced Suppression

Conventional 

Detection / 

Suppression

6-10
Figure 6-5. Basic event tree for area-wide smoke detection non-suppression probability estimation

Fire

Conventional 

Detection / 

Suppression

λi x SF 1-τ

Detector System 

Availability, 

Reliability

Fraction of fires that 

have an incipient 

stage

System Effective 

Detecting Incipient 

Stage

Successful MCR 

Response

Successful Field 

Operator Response 

(Fire Watch Posted)

Enhanced 

Suppression

1-ξ 1-μ 1-α 1-β 1-π2

1-η3

OK 

OK 

 π2 OK 

 η1 NS 

 τ 1-η2

OK 

 η3

NS 

 μ 1-η1

OK 

 ξ

NS 

 β 1-η1

OK 

 η2

NS 

 α 1-η2

OK 

 η1

NS 

 η2

NS 

1-η2

 η2

6-11
Figure 6-6. Illustration of change to in-cabinet event tree for de-energization strategy

ξde-ss is only applicable to the de-energization event tree fo

applications. ξde-ss represents the probability that given su

response to the VEWFD ‘alert,’ the field operator and techn

energize the equipment prior to the onset of flaming combu

This parameter is estimated by performing a detailed scena

reliability analysis on the de-energization strategy. The hu

probabilities presented in this report do not apply.

6-12
7-1

7. PARAMETER ESTIMATION BASED ON PART I

7.1 Fraction of Fires That Have an Incipient Stage

The first event represents the fraction of potentially challenging or greater fires that have an 

incipient stage of greater than or equal to 30 minutes.1 The failure branch of this event is 

represented “α.” This event does not include the effectiveness measures of the performance of 

a smoke detection system. To estimate this parameter, the types of fires that are determined to 

be a potentially challenging or greater fire as represented by the fire ignition frequency “λ,” were 

reviewed. Thus “α” is dependent on “λ”. As such, the fire events used to estimate λ were 

reviewed to ensure that this dependency is understood and conserved.

Operating experience has shown that many, but not all, electrical cabinet fires2 that occur in a

nuclear power plant (NPP) and are considered potentially challenging or greater fires have an 

incipient stage (Ref. 37). Potentially challenging or greater fires are classified as challenging, 

potentially challenging, or undetermined with regard to the fire severity classification 

documented in Electric Power Research Institute (EPRI) 1025284, “The Updated Fire Events 

Database: Description of Content and Fire Event Classification Guidance.” Fires not considered 

important to risk are classified as “non-challenging.” The fire ignition frequency (λ) is the 

occurrence rate on a generic plant wide basis of a potentially challenging or greater fire

involving a specific type of component (Ref. 36). These generic plant fire frequencies are 

adjusted based on weighting factors for plant areas and the number of components in the plant. 

Non-challenging fires do not contribute to the development of the generic fire ignition frequency 

estimates. EPRI 1025284 defines non-challenging fires as, follows:

Fires that did not cause or would not have caused adjacent objects or 

components to become damaged or ignite regardless of location for essentially 

any amount of time. These fires could be detected automatically by an incipient 

fire detection system and could be related to component failures involving ignition 

of the component followed by self- extinguishment without any required 

intervention. Fires that remained in a smoldering state with no apparent potential 

for open flaming might also be classified as non-challenging using the criteria 

provided in Appendix B. Another typical example of non-challenging would 

include component overheating incidents with light or moderate smoking but 

without any flaming. The non-challenging classification is also applied to fires of 

a type or in a location that would not be considered relevant to a fire PRA (e.g., 

an automobile fire in an on-site parking lot or an off-site grass fire). Fires that 

occurred during plant construction would also be classified as non-challenging. 

(See additional discussion and criteria in Appendix B, Section 2.5).

Additionally, the EPRI report states:

The event classification criteria for non-challenging or undetermined categories 

are consistent with NUREG/CR-6850. The definitions for key terms used to 

determine whether a fire could be non-challenging, including ‘incipient’, ‘flaming 

combustion’, ‘smoldering’, and ‘ignition’ are now defined in accordance with 

NFPA 901, NFPA 921, and other NFPA standards and publications. These 

 1 The basis for the 30 minutes criterion is presented later in this section. 2 Electrical cabinet fires are classified as Bin 15 in NUREG/CR-6850
7-2

definitions are provided in Appendix A [of EPRI 1025284] and were used as a 

consistent means of classifying fire event information.

Thus, to understand the fraction of electrical panel fires that exhibit an incipient stage, the fire 

events database was reviewed. Bin 15 “Electrical Cabinet” of the EPRI fire events database 

was evaluated to inform the fraction of events, which exhibit an incipient stage of sufficient 

duration, (greater than or equal to 30 minutes) such that detection and operator response could 

enhance the suppression activities. All Bin 15 Electrical Cabinet events that contribute to fire 

ignition frequency (i.e., Challenging, Potentially Challenging, and Undetermined) were reviewed. 

Events classified as “non-challenging” were not evaluated because they do not contribute to the 

fire frequency and are not considered important to plant risk. However, review of these “nonchallenging” events does indicate that the majority of these events exhibit failure mechanisms 

for which an in-cabinet smoke detection system could provide enhanced warning. Nonetheless, 

as they do not contribute to fire risk (as presented in Section 6.1), they are not examined further.

The results of this evaluation are documented in Appendix D. The results are based on the 

independent review of events by two NRC staff members. Determination of whether or not an 

event has an incipient stage is subjective in nature, and highly dependent upon the amount and 

quality of information available. As such, there were ‘rules’ developed to assist in making this 

classification exercise practical, and as consistent as possible. The process involved 

developing ground rules and definitions to support the classification (discussed below). Next 

the reviewers independently reviewed and classified the events per the definitions. Following 

the initial review, the staff members compared their classifications and discussed events where 

their classifications differed. Based on this discussion the reviewers may or may not have 

changed their classification. There was not attempt to force a consensus. In addition, the 

classification made by these two staff members, the events classified as low-voltage control 

cases were studied by NRR staff and thoroughly discussed until agreement was achieved and 

prior to the issuance of the draft version of this report in July 2015. 

First, an attempt was made to make no assumptions regarding the event. If the necessary 

information was not available, the reviewer ventured no guesses. In many events with limited 

information, this rule likely directed the classification as being “Undetermined.” For example, 

many events identify a breaker fault, but do not identify the component of the breaker which 

failed. Since breakers have numerous failure modes that could result in a breaker fault, and 

because the various failure modes may or may not exhibit an incipient stage, there was no 

assumption made regarding any one particular failure mode; more-information was needed to 

make such a determination possible.

Next qualitative definitions for “Yes,” “No,” and “Undetermined” were developed to support a 

consistent classification of the events. The definitions used were as follows:

Yes, the description of the event provides sufficient detail to determine that slow component 

degradation occurred. Additionally, if the description of the event does not provide a direct 

indication of slow component degradation, but can be inferred from the component which 

failed, then it is still a ‘yes’.

An example of the latter circumstance would be a control power transformer (CPT) 

within a motor control center (MCC) which fails due to internal winding failure, and not 

from an over voltage condition on the primary side of the CPT.
7-3

No, description of event identifies rapid failure, failure during work activities (maintenance, 

inspection, testing, cleaning, etc.), failure on demand, or the description of the event does 

not provide direct information regarding the time frame for component degradation but can 

be inferred from other information presented.

Undetermined, event does not provide sufficient details to determine that an incipient stage 

occurred or did not occur.

A nominal minimum threshold of 30 minutes should be used to classify as being of “sufficient 

duration.” The 30 minute threshold is based on an assumed maximum 15 minute response time 

for plant personnel to arrive at the scene and the test data that shows detector response 

typically occurring half-way through the test duration (assuming test duration represents 

incipient stage duration). A 30 minute incipient stage duration represents a detector response 

at approximately 15 minutes (1/2 of incipient stage (test) duration) and a maximum assumed 

operator response of 15 minutes.

The 30 minute rule is not the sole determiner of whether or not an event is classified as having 

an incipient stage. The reviewer should understand the failure mechanisms described in the 

event and make an informed decision based on the information and the objective of using a 

VEWFD system to provide sufficient time for operators to respond and be capable of providing 

suppression. Typically, the event descriptions do not have sufficient information to quantify the 

incipient stage in terms of seconds, minutes, and/or hours of pre-flaming degradation. 

Alternatively, if an event identifies a root cause as being (a result of) age or in-service use over 

many years or decades, this should not be used as justification that the incipient stage began 

when the component was installed. This 30 minute threshold is not intended to indicate that 

fires only exhibit incipient stages that are greater than 30 minutes, but to model the fraction of 

fires that do have a shorter incipient stage and would be more difficult for an operator to 

respond to the ignition source prior to flaming conditions. Lastly, this 30 minute quantity is used 

for screening events into categories to support quantification of the “fraction of fires that have an 

incipient stage,” and does not, nor should it be used to quantify the actual incipient stage 

duration as discussed in Section 11 of this report.

The results of this event review process are presented in Appendix D. Table 7-1 summarizes 

the results of this review for Bin 15 fires. Two of the events resulted in a differing classification 

among the two reviewers. In both cases one reviewer assigned an “undetermined”

classification, while the other classified the event as having an incipient stage (“yes”

classification). For these two events, a 0.75 weight was applied. The last column identifies the 

mean point estimate for “α” shown in bold font, with the 5th and 95th percentiles shown in 

brackets. These estimates are calculated excluding the “Undetermined” category and using a 

Jeffery’s non-informed method to estimate the uncertainty.

The two bins presented in Table 7-1 represent a demarcation in the fundamental assumption 

that voltage is an influencing parameter in the duration of the incipient. Stated differently, an 

assumption is being made that higher voltage equipment such as medium voltage switchgear 

are more likely to experience a short incipient stage than a low voltage control cabinet. The 

results presented in Table 7-1 tend to confirm this belief. For instance, the power cabinet class 

of equipment tend to have a 50% chance of experience an incipient stage of sufficient duration 

to support early intervention. While the low voltage control cabinets have a higher likelihood of 

experiencing an incipient stage of sufficient duration. The footnote to Table 7-1 provides 

examples of the types of equipment that is intended to be classified as “Power Cabinets.” The 

“low voltage control cabinets” classification is intended to cover those cabinets that only have 
7-4

low voltage control equipment of voltages less than 250V. For equipment that has both power 

and low voltage control components, such as a motor control center, that equipment should be 

classified as power cabinets for this evaluation. 

Table 7-1. Summary of Fraction of Electrical Cabinet Fires (Bin 15) That Have an 

Incipient Stage Detectable by a VEWFD System

Category

Incipient stage Detectable by 

VEWFD System Total # 

Events

Fraction (alpha)

Mean [lower/upper] Yes No Undetermined

Power 

Cabinets# 16.5 16 22.5 55 0.50[0.36 / 0.64]

Low Voltage 

Control 

Cabinets

6 2 5 13 0.28

[0.08 / 0.54]

# Power cabinets include electrical distribution electrical enclosures such as motor control centers, load 

centers, distribution panels, and switchgear

This current methodology differs from that provided in FAQ 08-0046. No visible inspection of 

“low voltage” cabinets needs to be performed to take credit for the provided alpha factor. The 

presence of fast-acting components such as electrical/electronic circuit boards that contain 

electrolytic capacitors, chart recorder drives, cooling fan motors, mechanical timers driven by 

electric motors, etc. was incorporated into the underlying analysis and creation of the alpha 

factor. The lack of these types of components was not seen to influence the tendency for fast 

acting fires to occur. 

7.1.1 Discussion of difference between this method and FAQ 08-0046

This sub-section provides background information to support the approach described above as 

well as comparison to the interim staff position presented in Frequently Asked Questions 

(FAQ) 08-0046.

The estimates developed above differ from the deterministic criteria presented in the U.S. 

Nuclear Regulatory Commission (NRC) interim staff position FAQ 08-0046. In that FAQ, the 

alpha term was estimated using a criteria of “fast acting components,” and a limitation on the 

maximum voltage of electrical cabinets (<250V). 

The FAQ 08-0046 defines “fast-acting components” as:

• Electrical/electronic circuit boards that contain electrolytic capacitors

• Chart recorder drives

• Cooling fan motors

• Mechanical timers driven by electric motors, etc.

Given these criteria are met, the FAQ presents a simplified model that assumes all fires have an 

incipient stage of sufficient duration that an ASD VEWFD system installed in the electrical 

enclosure containing the fire initiating component will provide at least one hour of advanced 

warning.
7-5

Per the FAQ approach, for electrical cabinets that contain “fast acting” components, those 

elements/parts are to be proportioned, and a fraction should be estimated as the (number of 

fast acting components)/(total number of components within the cabinet). The FAQ does not 

provide information on counting the number of components.

Although the intent of the FAQ approach was to eliminate those types of components that do 

not have long incipient stages; review the FEDB, inspection reports and licensee event reports 

(LERs); feedback received during site visits; and information from vendors (Ref. 20) it could not 

be confirmed that this approach adequately dispositions equipment that does not exhibit an 

incipient stage of sufficient duration to support enhanced suppression quantification. 

Before the review of the EPRI Fire Events Database, the authors of this report were aware of at 

least two events that conflicted with the FAQ 08-0046 interim staff position with regard to the 

definition of fast acting components. The first event is documented in Inspection Report 

05000348/364-11-012 and final significance determination process (SDP) (Refs. 38 and 39). 

In this fire event, the mis-wiring of the 1A reactor coolant pump oil lift pump pressure switch 

(cross-connection of 125 Vdc and 130 Vac circuit leads) caused a fire when the 1A RCP hand 

switch was taken to start. Within seconds of operating the hand switch a fire was observed. 

Given that the initiating component does not meet the definition of a fast-action component per 

the FAQ and was determined to be potentially challenging based on the SDP, an extended 

review of applicable operating experience was reviewed. The second event is documented in 

licensee event report (LER) 96-005-00, dated May 20, 1996. In this event, a turbine lockout 

relay failed when the turbine tripped on high vibration and resulted in a fire 3 minutes after the 

relay failure (Ref. 40).

Other examples that conflict with the FAQ position include FEDB FID #30276, #30338. In the 

first event, a malfunctioning charging circuit board caused a fire in a power transformer. The 

location of the fire was on the power transformer and not the charging board. In the second 

event, a panel blower (fan) failed because of accumulation of dust and dirt. The accumulation 

occurred over a long period of time and the panel blower did not fail abruptly, but over some 

period of time. During site visits (Section 3.1.1) an event was identified where VEWFD systems 

were installed where failure of an AHU fan motor was detected by the VEWFD system; operator 

actions were successful in responding to prevent a plant trip because of the unavailability of 

required equipment.

Review of the applicable operating experience indicates that it does not support the use of 

deterministic go/no-go criteria as presented in FAQ 08-0046. Although some of failure modes of 

the “fast-acting components” do not involve an incipient stage (i.e., locked rotor), using these 

deterministic criteria do not adequately represent the failure modes and associated duration of 

the component degradation phase. In addition, review of the operating experience has 

demonstrated that use of VEWFD system to protect power cabinets is applicable for Bin 15 type 

components as well as other types of components. However, the focus of this project is 

exclusively Bin 15 type equipment.

There may be cases where the use of VEWFD systems combined with appropriate operator 

actions may enhance the likelihood of preventing high-energy arc fault (HEAF) type events 

(Ref. 41). There are also cases where use of these systems has not prevented HEAF events 

(Ref. 42). Although the use of VEWFD systems to protect power distribution type equipment in 

combination with adequate and prompt operator response could help reduce the likelihood of 

HEAF (Bin 16) type event fires; it should not be considered a HEAF prevention system. 
7-6

Evaluating the risk benefit from the use of these systems with regard to HEAF events is outside 

the scope of this project.

7.2 System Performance Measures (Availability, Reliability, Effectiveness)

This section quantifies the smoke detection system performance measures of availability, 

reliability, and effectiveness; system effectiveness (τ) is represented separately. The parameter 

“β” represents the combined unavailability and unreliability of a smoke detection system to 

perform its intended design function. The success branch (1-β) represents the smoke detector 

being both available and reliable, calculated as the arithmetic sum of these two estimates. The 

parameter “τ” represents the smoke detection system’s effectiveness in detecting pre-flaming 

(incipient) phase conditions. The success in this branch represents that the smoke detection 

system has responded with an alert for the VEWFD system, or an alarm for the conventional 

system during the incipient stage. Other system effectiveness measures such as serviceability, 

maintainability, repairability, and operational readiness, although important to consider during 

system selection, are not useful measures when quantifying the system’s ability to perform its

intended function. 

The availability and reliability of VEWFD systems are estimated generically below using 

information collected during site visits (Section 3.1.1), vendor supplied information and 

literature. The system effectiveness measure is also estimated below using available test data. 

In addition to the operating experience reported, vendor literature has identified that the 

following U.S. NPPs used ASD systems; Palo Verde, Calvert Cliffs, Nine Mile Point, Clinton 

Generation Station, Seabrook, and Ft. Calhoun. Availability estimates with higher certainty 

could be achieved if data from these sites could be collected and integrated into the assessment 

presented here. For air return applications, the unavailability and unreliability of the HVAC 

system should also be explained or justified and included in the unreliability/unavailability 

estimate.

7.2.1 Estimate of unreliability

System reliability is the probability that an item (system) will perform its intended function for a 

specified interval under stated conditions (Ref. 43). Its complement is referred to as 

unreliability. The unreliability can be estimated by using the standby failure rate probability 

model. This model assumes that the detector transitions to the failed state while the system is 

in standby (i.e., period when no fire condition are present to detect). The transition to the failed 

state is also assumed to occur at a random time with a constant transition rate. The latent failed 

condition ensures that the detector will fail when it is next demanded, but the condition is not 

discovered until the next inspection, test, or actual demand. This model will be used to 

calculate smoke detector unreliability as:

= EE(λλ)TT

2

Where E(λ) : expected failure rate (per hour)

Ts : the surveillance test interval
7-7

The expected failure rate is

EE(λλ) = kk + 0.5

TT

Where k : observed failures in T

T : total operating period (hours)

Several sources of information associated with ASD system reliability were identified. Each 

source is explored individually and a generic unreliability estimate is provided based on the 

average of the individual unreliability estimates. The surveillance test interval is assumed to be 

semi-annual (4380 hours).

Germany NPPs

Forell and Einarsson reported that three of the six German NPP units (2 PWR and 4 BWR) 

investigated for fire protection systems and component reliability data included data on the 

performance of ASD systems. Approximately 250 ASDs were included in the research, which 

identified 5 failures in 12.19 million operating hours (Ref. 25). This operating experience results 

in an expected failure rate of 4.5×10-7/hr and an unreliability of 9.86×10-4 based on a semiannual surveillance.

U.S. NPPs

U.S. NPP unreliability estimates are based on information collected during the site visits (see

Sections 3.1, Table 7-3, and Appendix C.1) and information obtained from the EPRI report 

1016735. Information on 181 system years shows four system failures. This results in a mean

failure rate of 2.84×10-6/hr, with 5th and 95th percentiles of 1.05×10-6/hr and 5.34×10-6/hr and an 

unreliability of 6.22×10-3.

ASD Vendors

Two of the ASD vendors provided or have available on their Web sites information on system 

reliability; one of whose unreliability estimates are based on the minimum acceptable reliability 

per the listing standard UL 268, namely:

Section 4.1 of UL 268 references a method for detector reliability prediction as follows (Ref. 44):

The maximum failure rate for a detector unit shall be 4.0 failures per million hours as 

calculated by a full part stress analysis prediction as described in MIL-HDBK 217 or 3.5 

failures per million hours as calculated by a simplified parts count reliability prediction as 

described in MIL-HDBK 217, or equivalent, see Annex D. A ′′Ground Fixed′′ (GF) 

environment is to be used for all calculations. When actual equivalent data is available 

from the manufacturer, it is permissible that it be used in lieu of the projected data for the 

purpose of determining reliability.

In addition, a component failure rate of not greater than 2.5 failures per million is referenced for 

light emitting diode (LED) type smoke detectors using a photocell-light assembly. The same 

component failure rate suggested for application specific integrated circuit (ASIC) employed in a 

smoke detector unit.
7-8

Vendor 1: a. Per UL268 simplified parts count reliability analysis3

3.5 failures per million hours

4×10-6 mean 

Unreliability of 8.76×10-3

b. Per UL268 full parts stress analysis

4 failures per million hours

4.5×10-6 mean 

Unreliability of 9.86×10-3

Another vendor provided reliability estimates based on a Military Handbook method:

Vendor 2: Per MIL-HDBK-217 simplified parts count method

Overall MTBF = 6.09×106 hours

Overall FITS = 164.149

2.46×10-7 mean 

Unreliability of 5.39×10-4

The average vendor unreliability estimate is 6.4×10-3

Based on the information collected from Germany and U.S. NPP operating experience along 

with the reliability estimates provide by the vendors (UL 268 or MIL-HDBK-217), the generic 

unreliability is shown in Table 7-2,of 1.6×10-3 is estimated based on a semi-annual surveillance 

period (4,380 hours) and equal weighting of these three sources of data. The 5th and 95th

percentiles are 9.7×10-4, 2.3×10-3, respectively.4 This generic estimate was developed using a 

multi-stage Bayesian update using all three data sets. Note that this is a generic estimate. Site 

(or fleet) specific data could be used to develop and justify site specific unreliability estimates.

For the air return application the unreliability of the ventilation system should also be included 

since the function of the ASD system in this application relies upon the operation of the 

ventilation system.

Table 7-2. Generic ASD Unreliability Estimate per detector per year

Parameter Mean 5th percentile 95th percentile

ASD Unreliability 1.6×10-3 9.7×10-4 2.3×10-3

 3 UL 268 simplified part count estimates are not used in the generic unreliability estimates developed below 

as the full part stress analysis provides a bounding estimate. A sensitivity showed that the difference was 

on the order of 5x10-5, almost two orders of magnitude below the generic estimate. 4 Mean expected failure rate of 7.2x10-7, modeled as a gamma distribution with alpha = 15, beta = 20.9x10+6

hours
7-9

Table 7-3. Information Gathered from Site Visits to Inform Availability and Reliability 

Estimates

Source Technology 

(# Detectors)

Fire 

Events

System 

Failures

Total Operating 

Time (Years)

Down Time 

(hours)

TMI 1 Cloud Chamber 

(2) 2

PMT failure 

caused by 

improper 

maintenance

14.66 years per 

detector 

Total : 30 years

Maintenance

2-4hrs/qtr/device

Trouble Alarms 

4hrs/yr/device

Hot work 

20hrs/yr/device

Total

6-8hrs/year/device

Robinson Laser 

(1) None

Power Supply 

2002 

(20day outage)

Battery and 

Processor 

Replaced 2006 

(27day outage)

Total : 17 years

Maintenance

24hrs/yr/device

Trouble Alarms

2hrs/yr/device

Total

26hrs/year

Harris Cloud Chamber 

(10) 2 None identified

4 years per 

detector

Total : 40 years

Maintenance

24hrs/yr/device

Trouble Alarms

2hrs/yr/device

Total

26hrs/year/device

Darlington Laser

(# not provided)

Machine 

belt smoke None identified 8-9 years per detector

Maintenance

Annual 1-2hrs/device

Pickering Laser 

(# not provided)

Cork 

isolator 

overheating 

in air 

handling 

unit

None identified

Nuisance Alarms

Nitrogen Purging,

Maintenance 

Activities

12 years per 

detector

No information

provided

Bruce Laser 

(24)

Incipient, 

source not 

identified 

before 

flaming fire 

conditions

None identified

19 years per 

detector

Total : 456 years

Maintenance

Annual:2-3hrs/device

Quarterly:3 

hrs/device

Trouble Alarms

1-2hrs/yr/device

Total

15-17hrs/yr/device

NASA Laser 

(>100)

Circuit 

board 

failure,

ShopVac, 

fire in lab 

space

Nuisance Alarms

Ventilation 

Changes

>300 years

Maintenance

1-3 hrs/yr/device
7-10

7.2.2 Estimate of generic system unavailability

Availability is defined as the probability that a system is operating satisfactorily at any point in 

time and considers only operating time and downtime (Ref. 21). Availability is a measure of the 

ratio of the operating time of the system to the operating time plus downtime; its complement is 

referred to as unavailability. Unavailability can be estimated by:

UU = xx̅= 1

nn�xxii

nn

ii=1

xxii = tt

TT

Where td : system downtime for testing, preventative maintenance, and corrective 

maintenance

Ts : the surveillance test interval

Operating experience data collected from site visits shows that there is a large variation in 

system down time. With a limited set of data, a frequentist summary approach was taken to 

estimate the unavailability. This approach is described in NUREG/CR-6823, “Handbook of 

Parameter Estimation for Probabilistic Risk Assessment,” in Section 6.7.2.2. Following this 

approach, a generic unavailability is shown in Table 7-4. A plant-specific (or fleet-specific) 

unavailability estimate could be used instead of this generic estimate, if sufficient data is 

available to support/justify such an estimate.

Table 7-4. Generic ASD Unavailability Estimate per Detector per Year

Parameter Mean 5th percentile 95th percentile

ASD Unavailability 2.0×10-3 3.0×10-4 3.7×10-3

For the air return application, the unavailability of the ventilation system should also be included 

since the function of the ASD system in this application relies on the operation of the ventilation 

system.

7.2.3 Estimate of generic system ineffectiveness

System effectiveness is a measure of how well a design solution will perform or operate given 

anticipated operational scenarios (Ref. 19). Effectiveness estimates for ASD VEWFD systems 

expected to operate during the pre-flaming (incipient) phase are based on the test data 

collected from the experiments conducted in this program. As indicated above, the source 

materials are heated to a temperature that can support flaming conditions. The effectiveness 

differs between in-cabinet, and the two area-wide configurations, primarily because of smoke 

dilution and stratification effects. The effectiveness of these systems is determined generically 

using data collected during the NIST experiments, which applied a “one-stage” bayes approach

(Jeffery’s non-informed) modeled as a binomial. Only data from the 15, 60 and 240 minute HRP 

tests were used. Short tests such as resistor, capacitor, and shredded paper were not used. 
7-11

Several different estimates of system in-effectiveness are developed to support quantification of 

the event tree parameter τ, as shown in Table 7-5. The specific datasets used to develop these 

estimates are identified in Table 7-6. As an example, for the ION Spot type detector within a 

naturally ventilated cabinet, the data indicates 145 alarms out of 162 trials. The mean 

effectiveness is calculated as a beta distribution, with:

αα = 145 + 0.5 = 145.5

ββ = 162 − 145 + 0.5 = 17.5

ττ = 1 − (αα⁄(αα + ββ)) = 1 − (145.5⁄(145.5 + 17.5)) = 0.1

Table 7-5. ASD VEWFD System In-Effectiveness Estimates Based on Test Data

Application Cabinet/Room

Ventilation Detector Type Mean (τ)

[5th/95th percentile]

In-Cabinet

Natural 

and

Forced <100

cabinet ACH

ION Spot 1.0×10-1

[6.8×10-2 / 1.4×10-1]

PHOTO Spot 7.7×10-1

[7.1×10-1 / 8.3×10-1]

SS 2.6×10-1

[2.1×10-1 / 3.2×10-1]

ASD CC 2.7×10-3

[1.1×10-5 / 1.0×10-2]

ASD LS1 5.3×10-1

[4.5×10-1 / 6.5×10-1]

ASD LS2 1.9×10-1

[1.4×10-1 / 2.4×10-1]

Forced ≥100

cabinet ACH

ION Spot 7.9×10-1

[6.5×10-1 / 9.0×10-1]

PHOTO Spot No Data

SS No Data

ASD CC 1.9×10-2

[7.8×10-5 / 7.3×10-2]

ASD LS1 No Data

ASD LS2 3.7×10-1

[2.2×10-1 / 5.2×10-1]
7-12

Table 7-5. ASD VEWFD System In-Effectiveness Estimates Based on Test Data

(Continued)

Application Cabinet/Room

Ventilation Detector Type Mean (τ)

[5th/95th percentile]

Area-wide, Air Return 

Grill HVAC in room

ASD CC 3.0×10-1

[1.7×10-1 / 4.5×10-1]

ASD LS2 5.2×10-1

[3.6×10-1 / 6.7×10-1]

Area-wide, Ceiling Any

ION Spot 8.1×10-1

[7.3×10-1 / 8.9×10-1]

PHOTO Spot 8.7×10-1

[8.0×10-1 / 9.3×10-1]

SS 5.7×10-1

[4.7×10-1 / 6.7×10-1]

ASD CC 3.2×10-1

[2.3×10-1 / 4.2×10-1]

ASD LS2 1.1×10-1

[4.6×10-2 / 1.8×10-1]

Table 7-6. Identification of Datasets Used To Estimate the System In-Effectivness 

Estimates (See Table 7-3)

In-Cabinet Area-wide

Natural and 

Forced <100 

ACH

Forced >100 

ACH

Air Return 

Grille Ceiling

Laboratory Small

 ASD1 (LS1) X

 ASD2 (CC) X

 ASD3 (LS2) X

 SS X

 ION X

 PHOTO X

Laboratory Large

 ASD1 (LS1)

 ASD2 (CC) X X

 ASD3 (LS2) X X

 SS X

 ION X X

 PHOTO

Full Scale, 

Small Room

 ASD1 (LS1) X

 ASD2 (CC) X X

 ASD3 (LS2) X X

 SS X X

 ION X X

 PHOTO X X
7-13

Table 7-6. Identification of Datasets Used To Estimate the System In-Effectivness 

Estimates (See Table 7-3) (Continued)

In-Cabinet Area-wide

Natural and 

Forced <100 

ACH

Forced >100 

ACH

Air Return 

Grille Ceiling

Full Scale, 

Large Room

 ASD1 (LS1) X

 ASD2 (CC) X

 ASD3 (LS2) X

 ASD4 (CC) X X X

 ASD5 (LS2) X X X

 SS X X

 ION X X

 PHOTO X

7.2.3.1 Estimating “in-effectiveness” when system sensitivity differs from that reported here

All of the detectors evaluated in this test program have the capability to be configured to a range 

of sensitivity settings. Most ASD VEWFD systems are capable of being configured with multiple 

alarm settings (three to five for the systems tested). The data analyzed above to develop the 

“in-effectiveness” estimates (Table 7-5) were based on either the ‘alert’ sensitivity of National 

Fire Protection Association (NPFA) 76 (i.e, 0.20 percent obscuration/ft. at the sampling port 

above background) or from a range of particle count per unit volume sensitivities for the cloud 

chamber technology.

In actual applications there may be a desire to configure the VEWFD system either more or less 

sensitive than reported here. If actual system sensitivity differs from those used here, the “ineffectiveness” parameter should be estimated using data applicable to that sensitivity. Since 

data for multiple sensitivity settings were recorded as part of this testing program, the data 

included here could potentially be used to support such an analysis. However, if the data 

collected as part of this research do not support the sensitivities used in actual plant application, 

then additional data may be required to support parameter estimation. In addition, because the 

focus of this research was to evaluate VEWFD systems that met the minimum sensitivity 

requirements of NFPA 76, less data is available at other sensitivities. As such, limited data will 

broaden the uncertainty of the parameter estimate. Good engineering practice would suggest

conducting a sensitivity analysis with the development of any new parameter. Appendix B 

describes the process to follow to estimate a detector in-effectiveness (τ) parameter for VEWFD 

detection systems configured to “alert” sampling port sensitivities other those reported in 

Section 5.

8-1

8. TIMING ANALYSIS

To determine the likelihood of success in suppressing a fire, timing information needs to be 

provided such that operator failure rates and non-suppression probabilities can be estimated 

and used in the event tree. The timing information that is directly applicable includes the time 

frame that plant personnel have to respond to a smoke detection system alert, or alarm 

notification before commencement of flaming conditions. Figure 2-2 provides a conceptual 

illustration of the fire stages, showing the incipient, fire growth, steady state, and decay stages. 

The flaming conditions’ end point represents the demarcation between the event tree end 

states, “No Damage Beyond Initiating Component” and “Cabinet Damage.” Cabinet damage in 

PRA terms refers to the point at which components within the electrical enclosure other than the 

initiating component become damaged, or when suppression activities are initiated. This 

approach assumes that once flaming combustion commences, adjacent components to the 

initiating component are damaged. A more detailed fire modeling approach could potentially 

provide for an additional amount of time before other components within a cabinet being 

damaged, if initiating component growth profiles, peak heat release rates, and component 

physical layout information were known, along with the thermal failure threshold for the adjacent 

components. However, to model all of the ignition sources within a cabinet and evaluate the 

time delay in damage to adjacent components may provide too little additional benefit for the 

level of effort involved to be worthwhile. 

In addition to estimating the duration of an incipient stage for an electrical panel fire scenario, 

the timing information of when the smoke detection system will respond with an alert or alarm 

must also be known, such that the time available for operator response can be estimated. 

Figure 8-1 presents timelines for the fire event for a generic fire scenario (top timeline), along 

with timelines for very early warning fire detection (VEWFD) and conventional spot-type smoke 

detection and operator response. Given the variability of the incipient stage duration, the start 

of flaming combustion could begin before or after the operator response event. The analysis 

assumes that since the fire probabilistic risk assessment (PRA) is quantifying the risk from 

potentially challenging or greater fires that ignition can be expected to occur at some point if not 

prevented/hampered by plant personnel, per the definition of fires characterized by the fire 

ignition frequency.

To estimate the time available for operators to respond (i.e., time between VEWFD alert or 

conventional alarm and flaming fire conditions), two approximations were actually needed:

• Timing of smoke detection systems response during an incipient fire phase 

– Results are presented in Section 8.1

• Estimate of incipient stage duration for electrical enclosure equipment

– Results are presented in Section 8.2
8-2

Figure 8-1. Generic fire scenario event timeline. Fire scenario progression (top), 

VEWFD/operator response (center), conventional smoke detection/operator 

response (bottom). Illustration only—event markers may not be indicative 

of actual system response

At some point following flaming combustion the cabinet is damaged because of fire growth and 

if not successfully suppressed, the fire may develop such that targets external to the cabinet 

become damaged. Details on how to model the fire growth stages are not discussed here,

however, the generic concepts will be highlighted as they relate to estimating the nonsuppression probabilities. For information on modeling electrical enclosure fire hazards can be 

found in NUREG-2178, “Refining and Characterizing Heat Release Rates from Electrical 

Enclosures During Fire (RACHELLE-FIRE).”

The testing conducted as part of this program provides insights with regard to the amount of 

time available from the time a VEWFD system alert is received to the point where flaming 

combustion is assumed to occur (end of test). However, because the testing involved only three 

different incipient fire durations, the data must be normalized to some baseline that represents 

typical incipient event duration for the types of components found in the plants which these 

systems are intended to protect. An analysis of available test data and operating experience 

are used to develop these estimates as presented below and in Appendix D.

Begin Degradation

Begin gassification/ 

 

pyrolysis

Begin Smoldering 

 

(if applicable)

Begin burning / 

 

flames

Damage to target 

component

Tincipient phase

Tdamage

VEWFD alert,

 

start MCR response

Begin Enhanced

 

Suppression

Fire Suppressed

Tdet,VEWFD

Time Available for

Operator Response Tsupp,VEWFD

Tdet,conventional Tfb,conventional Tsupp,conventional

Conventional Alarm

Begin Suppression

Fire Suppressed
8-3

8.1 Detector response time during the incipient stage

This subsection evaluates the performance of VEWFD system response with regard to the time 

of “alert” during an incipient stage. As discussed in Section 4, the copper block end point was 

485  ̊C. This temperature is in the range of auto-ignition and above the point in which piloted 

ignition of many polymer materials occurs (Ref. 34). Thus, the end of the test (including hold 

time) will be used as the point where the incipient stage ends. Figure 8-2 presents the VEWFD 

system test results showing an example of the normalized time for in-cabinet VEWFD system 

response with an “alert.” Statistical K-S tests were run between the groups (test durations) and 

found to be poolable. The pooled dataset is shown as “All” in Figure 8-2 and has a median of 

0.54 and mean of 0.56. The results presented in Figure 8-2, provide a generalized 

representation of the response of ASD VEWFD systems used for in-cabinet applications. The

mean for individual detectors are used to support development of an estimate of time availability 

for operator response as discussed in the next section. A similar approach is used for the 

conventional system evaluation. If detector sensitivities other than those discussed in Section 

7.2.3.1 are used in actual plant applications, additional data and analysis will be required to 

develop a normalized mean time to detection, which is used to estimate the time available 

curves (Section 8.1.2). A description of the process to develop time available curves for 

systems using different sensitivities is presented in Appendix B and D. These curves (see 

Figure 8-4) are used in Section 10 to estimate a human error probability (HEP) for the field 

operator response. The process used here is summarized in Appendix D.

Figure 8-2. Summary of ASD VEWFD in-cabinet test results showing 

normalized time of alert (box and whisker plot shows 10th, 25th, 50th

(median), 75th, and 90th percentiles with outliers shows as dots, mean 

shown as dashed line)
8-4

8.2 Estimating the duration of time available for operators to respond

The incipient stage was described previously in Section 2.1 as the preheating, gasification, and 

smoldering phases. Thus, the incipient stage includes everything from the start of component 

degradation up to ignition, which is the point of self-sustained flaming combustion.

The duration of an electrical component incipient stage may vary from less than a second to 

hours or days, and possibly even extended out to years; if age-related degradation mechanisms 

are considered as the fire initiator. With this variability it becomes difficult to quantify, with any 

certainty, the phenomena that affect the duration of the incipient stage; this is primarily because 

of the sheer number of parameters affecting this data for many types of components, and the 

many failure modes that prevent development of a precise characterization of this phenomenon. 

This variability can be attributed to numerous factors, one being that, from a fire perspective, 

component degradation does not equate to a functional failure of a component or system; it is 

only (a breakdown) when the degradation progresses to a point where the thermal heat 

dissipation (development of a fire or flaming conditions) and combustion byproduct cause 

prompt or delayed component; or system functional failures. With no obvious failure, no action 

can be taken; no one attempts to fix things that he/she does not know are broken. Another 

related contributor is the lack of a detailed understanding of the failure mechanisms of electrical 

components, with regard to the duration of the incipient stage; further, and more importantly, 

there must be knowledge or understanding of the point during the incipient stage at which 

sufficient concentrations of products of combustion are available at the smoke detector to

exceed a detector set point. The last point is particularly significant. In testing, a linear heating 

ramp was used as an approximation of the incipient stage; however, in reality, the smoke 

generation from degrading electrical equipment might not follow this approximation, but could 

potentially follow a logarithmic or, exponential growth profile, or anything in between. Therefore, 

there is presently no agreed upon method to predict the time duration from when an electrical 

component begins to degrade, and degrading components generates smoke, to when flaming 

combustion commences.

Although it can be said that most fires have an incipient stage, for a VEWFD system to be 

modeled in the event tree above, the duration of an incipient stage must be sufficient to allow 

the VEWFD system to respond to the products of combustion. It must allow plant personnel 

sufficient time to locate and suppress the fire before damage to external targets occur. From a 

safety standpoint, use of smoke detection systems becomes increasingly beneficial, as 

additional time is available to respond to the particular event. Thus, the risk scoping study for 

ASD VEWFD systems hinges on being able to determine the time available for operators to 

respond to degrading conditions that have the potential to pose a potentially challenging or 

greater fire threat. For fires that have a short incipient stage, there may not be sufficient time for 

operators, technicians, or fire brigade to respond before fire damage occurring. For every 

scenario, the personnel response time will be different, as will the duration of the incipient stage. 

Thus, the variability of the response time and the incipient stage duration makes the 

quantification of a successful response both uncertain and difficult to estimate. In addition, 

although the incipient stage may last “x” minutes, the ASD VEWFD system (or spot-type 

detector) will not provide advanced warning of “x” minutes but rather some “x-y” minutes, with 

“y” not necessarily being less than “x”. That is, the VEWFD alert (or spot-type alarm) threshold 

may not occur before the development of a flame or flaming conditions, in which case the only 

“benefit” may be additional time for suppression and a reduction in the non-suppression 

probability.
8-5

With the lack of available information and the importance of understanding the duration of an 

incipient stage, one approach could be to conduct a formal expert elicitation-type effort to 

develop a consensus or community opinion. Unfortunately, the efficacy for such an effort was 

not realized until late in this project, and as such, the needed resources, including budget and 

time, were not available. Consequently, in an attempt to estimate the necessary information, a 

detailed evaluation of the fire events database was conducted; the details of this research are 

documented in Appendix D, identifying a limited number of cases to inform a duration estimate. 

The events reviewed included those events contained in the recently updated fire events 

database (Ref. 45, 46), along with more recent operating experience obtained from LERs and 

presentations made at NEI fire protection information forums.

For the fire PRA quantification to be of use, what is being quantified as an incipient stage must 

first be understood; then, using operational experience, test data, and judgment to develop an 

estimate of the amount of time available for operators to respond. Therefore, in developing a 

scenario that can quantify any potential risk enhancements from using these systems, several 

simplifying assumptions are required, and they are noted herein. 

Assumption #1: Incipient duration information is collected only for fires that exhibit incipient 

stages of sufficient duration to allow for operator response before ignition.

Because each specific plant scenario will differ in the determination of the time to damage 

and the specific target set, it would be unrealistic to use target damage as the end state of 

the incipient duration. This assumption is also consistent with the definition of the incipient 

stage provided in Section 2.1.

Assumption #2: Incipient duration information is collected only for electrical/electronic 

component failures which are contained within an electrical enclosure (cabinet). 

Interest in using ASD VEWFD systems in U.S. NPP applications has focused on electrical 

enclosure fires. In the EPRI/NRC-RES Fire PRA Methodology, 37 different generic fire 

frequency bins are identified. The fire frequency bin of most interest here is Bin 15, 

“Electrical Cabinet.”

Although other types of components found in an NPP can, and will exhibit an incipient stage 

of sufficient duration to allow enhanced suppression in applications in which VEWFD 

systems are able to detect the products of combustion early on, the need for such 

applications has not yet presented itself. However, a similar process could be followed to 

develop such estimates.

Assumption #3: Use the experimental test results based on a linear heating ramp to estimate 

the time available for operator response in instances in which the operating experience provides 

information of the incipient stage duration. 

In testing, a linear heating ramp was used as an approximation of the incipient stage. In 

reality, the smoke generation from degrading electrical equipment might not follow this 

approximation, but rather, could follow a logarithmic, exponential, or other growth profile. 

Therefore, there is presently no agreed upon method to predict the time duration from when 

an electrical component begins to degrade, degrading component show smoke 

characteristics, and when flaming combustion commences. Thus, the use of the test data 

seems reasonable, but has inherent uncertainty associated with its ability to represent actual 

electrical equipment failure modes.
8-6

Appendix D provides a detailed description of the research undertaken to identify operating 

experience to support informing the duration of the available time for operator response. In 

addition, recent operating experience with VEWFD systems in NPPs has provided several data 

points on VEWFD system timing information. It should be noted that the incipient stage duration 

and time available for operators response differ as shown in Figure 8-3. In using the operating 

experience to inform a time available curve, the mean time to detection for the systems tested is 

used. This information, along with the VEWFD system performance as presented above, allows 

for the development of distributions representing the time available for operators to respond as 

shown in Figure 8-4. This time available duration begins at VEWFD system “alert” or 

conventional system “alarm” notification, and ends at the fire flaming stage. However, it should 

be understood that the process followed contributes its own uncertainties, which may not be 

adequately quantified to represent an informed technical community’s beliefs. The authors of 

this report suggest that a formal process (such as an expert elicitation) be followed if better 

resolution of this quantification is needed. It is also suggested that any such effort employ 

experts knowledgeable in electrical component design and failure characteristics, such as 

individuals from vendors of the equipment being protected (i.e., relays, transformers, power 

distribution equipment, etc.) or from industries with extensive operating experience and 

components similar to those found in NPPs.

Figure 8-3. Illustration of difference between incipient stage and time available.
8-7

Figure 8-4. Distribution for duration of time available for plant personnel to respond to 

VEWFD system “alert” or conventional “alarm” notification of incipient fire 

conditions, for those fires, which exhibit an incipient stage for in-cabinet 

applications

Figure 8-4 illustrates the exponential distribution representing the duration of time available for 

operator response before the onset of flaming conditions. These distributions were developed 

using available operating experience and modified based on system response test data. Where 

operating experience provided the duration of the incipient stage, that time was adjusted by 

using the normalized mean response time for the smoke detection system in-cabinet response 

from the test results. That is, if the incipient duration lasted 60 minutes and the normalized 

mean detection time for detector X was 0.5, then the time available estimate for that event and 

detector is 30 minutes. Three events having incipient stage durations of 0.5, 0.9, and 7 hours, 

respectively, were identified from the fire events database. Each of these values involved cases 

in which timing information regarding system start or change of stage was documented, and 

ended when the potentially challenging or greater fire event was identified. Thus, it is assumed

that the incipient stage started when the equipment was turned on, which may or may not be 

true, and therefore, the actual incipient duration could be shorter in duration than used here. 

Two of the events identified were from recent operating experience in which ASD VEWFD 

systems were present. In both cases, the ASD VEWFD systems were located in equipment 

other than the initiating equipment; thus these two values could be longer in duration. Since the 

VEWFD detection type was known (Cloud Chamber) the timing information from that event 

could be used directly for the cloud chamber estimate. However, those two events timing 

information was adjusted for the other detectors by using the normalized mean difference in 

performance. For instance, if a cloud chamber event provided for 60 minutes of advanced 

warning from the VEWFD “alert,” and detector X provided half of the advanced warning as the 

cloud chamber (based on normalized mean time to detection), then detector X would have 

allowed for 30 minutes of advanced warning for that event. In the draft report, the interim staff 

position estimate of “at least one hour of warning” was used, since this value was presented in 

both industry and NRC documents, it could be viewed as somewhat of a consensus opinion. 
8-8

Because of public comments, the authors reconsidered the applicability of using this estimate. 

Neither the industry nor the NRC document provided any technical basis to support such an 

estimate. Review of operating experience has also demonstrated that these assumptions are 

not bounding. Since the interim staff position does not differentiate between detection 

technologies, the use of this estimate does not support a consistent treatment of smoke detector 

performance for low-energy fires. As such, the 1-hour estimate has been removed and the 

distributions recreated in this final report. With its removal, the lambda parameter for the 

exponential distributions changed less than 5 percent for all distributions. However, an 

additional event occurring at Shearon Harris in August of 2015 was also provided as part of the 

set of comment received on the draft report. The authors reviewed this event and ended up 

including it to update the time available distributions. This event is described in detail in 

Appendix D. Accordingly, the information in Appendix D and Section 10 have been updated in 

this final report to reflect this change.

These distributions are developed to support human error probability quantification (see 

Section 10.4) and to evaluate the varying performance of different smoke detectors, 

distributions for time available for ASD CC, ASD LS, SS, PHOTO and ION detectors. 

Appendix D provides additional information on the events identified and the numerical 

estimates.
9-1

9. HUMAN FACTORS ANALYSIS

The objective of the human factors (HF) analysis was to identify personnel tasks involved in the 

planned response to aspirating smoke detection (ASD) very early warning fire detection

(VEWFD) alerts and alarms; and factors that may adversely affect personnel task performance 

(e.g., insufficient training, poor alarm design) during response operations.

The way in which an ASD VEWFD system is implemented (e.g., planned response, system

design, application) determines how humans will interact with the system and, ultimately, may 

affect whether prevention or prompt suppression is achieved. As such, variations in 

implementation of an ASD VEWFD system can impact the system’s effectiveness in the amount 

of advanced warning provided. Thus, the results of the human factors analysis are used 

primarily as input to the detailed human reliability analysis and, consequently, the risk scoping 

study presented in this report (Section 12). However, the HF analysis also serves to inform 

designers, reviewers and users about potential factors related to the way in which ASD VEWFD 

systems are implemented, that may adversely affect human performance.

Few licensees have implemented ASD VEWFD systems within the context of quantifying 

system performance in a fire probabilistic risk assessment (PRA). Therefore, information is 

limited regarding system implementation practices and effectiveness; thus, this HF analysis 

should be considered an early analysis. To supplement our knowledge of ASD VEWFD 

systems, in addition to the U.S. nuclear industry, we studied non-U.S. NPPs and non-nuclear 

facilities using these systems. It is recommended that this analysis be updated as more 

information becomes available regarding the usage of ASD VEWFD systems in the U.S. nuclear 

industry.

9.1 Information Gathering

To support the HF analysis, information about ASD VEWFD systems was gathered using 

various methods. Information gathering activities were strategic or resourceful in nature and 

yielded qualitative data. The following activities were conducted:

1. Document Review: 

o trip reports1 from various facilities (nuclear and non-nuclear) currently using ASD 

VEWFD systems (trips are summarized in Section 3.1.1; See appendix C for 

detailed information) including:

• 3 Canadian NPPs

• NASA’s Goddard Space Flight Center

• 3 U.S. NPPs

o ASD VEWFD alert and alarm response procedures at two U.S. NPPs

o vendor documentation for special equipment (i.e., thermal imaging cameras and 

portable ASDs)

2. Expert consultation

o developed and administered a set of questions regarding ASD VEWFD response 

operations to: 1) personnel at a licensee intending to use ASD VEWFD systems 

 1 The trips were conducted by the NRC team lead, Gabriel Taylor, and Tom Cleary from NIST. The HF and 

HRA NRC personnel (Amy D’Agostino and Susan Cooper) were not part of the team at this time and, thus, 

were not present during these trips.
9-2

to support its transition to NFPA 805 and 2) personnel at a licensee currently 

using ASD VEWFD to support an approved NFPA 805 transition (See Appendix 

C for questions and answers). One licensee provided written responses2 and the 

other provided responses via teleconference. The following departments were 

represented on the teleconference:3

• fire protection 

• licensing 

• instrumentation and controls (I&C) maintenance

• systems engineering

• corporate

• operations (a senior reactor operator (SRO))

3. Site Visit:

o trip to a licensee currently using an ASD VEWFD system as a surrogate to a 

conventional spot-type smoke detector

• plant tour focused on the ASD VEWFD system (e.g., ASD VEWFD alarm 

display in MCR, local fire alarm control panel)

• discussions with personnel regarding system implementation and 

performance

4. Site Visit

o trip to a licensee currently using ASD VEWFD system to follow-up on recent 

operational experience

• discussions and tour focused on the event and the subsequent response

9.2 Human Factors Analysis of VEWFD System Response Operations

This section describes a two-step HF analysis of ASD VEWFD alert/alarm response operations. 

The first step consisted of a tabletop analysis to identify the personnel tasks involved in the 

response operations, captured in Section 9.2.1. The second step was an evaluation to identify

factors that may adversely affect task performance, captured in Section 9.2.2.

The HF analysis conducted was a generic analysis (i.e. not plant specific), which was an 

intentional choice by the project team. As there is no standardized way in which licensees must 

implement these systems, a “generic analysis” allowed for exploration of various possible 

implementations, while concurrently developing an understanding of the fundamental tasks

involved in response operations. The analysis identified the general structure of the humansystem and human-human interactions that are, likely, common to all licensees during ASD 

VEWFD response operations. The analysis also highlighted variations in implementation 

(e.g., alarm location) that are a product of licensee-preferred practices. Variations are of 

interest because they can impact the efficacy of the human-machine system (i.e., interaction of 

detection system and personnel response), such that they can either facilitate or deter fire 

prevention or prompt suppression. 

 2 Positions and titles of the personnel that contributed to the written responses were not provided.

3 The staff members present for the teleconference were chosen by the licensee. The project team had no 

involvement in the selection process.
9-3

9.2.1 Step 1: Tabletop analysis

A tabletop task analysis is a technique that involves consulting with a group of experts who have 

an understanding of a system to define/assess particular aspects of that system. The 

discussions are typically directed around some basic framework (e.g., procedures). This 

technique can be used to “deepen task knowledge of a system ... [It] can create (on-line) 

detailed task information and/or can analyze that information in a problem-solving and 

explanatory way.” (Ref. 47)

A tabletop analysis was conducted on ASD VEWFD response operations, which rely solely on 

human response. Before consulting experts, the project team gathered information via 

document review (described in 9.1) to gain a basic understanding of ASD VEWFD response 

operations (e.g., necessary tasks/equipment/personnel). Experts were then asked targeted 

questions aimed at validating our understanding of response operations, gathering missing 

information, identifying gaps, and gaining a deeper understanding of specific aspects of 

response operations.

The scope4 of the tabletop analysis was the personnel response to an ASD VEWFD alert 

followed by an alarm for an in-cabinet application. An alert occurs at 0.2 %/ft obscuration

(effective sensitivity at each sampling port) and an alarm occurs at 1.0 %/ft obscuration

(effective sensitivity at each sampling port). Plant personnel can take one of two strategies to 

respond to incipient fires, either a fire suppression strategy or a de-energizing strategy. A fire 

suppression strategy is one in which the end state is a posted fire watch at the location of the 

VEWFD in “alert” and positioned in close proximity to the specific affected cabinet, thus, 

ensuring personnel are in position for prompt suppression. A de-energization strategy is one in 

which the end state is removing power from the affected cabinet (or part of the cabinet) and 

repairing the degraded component; this is a prevention strategy/approach.

The results of the tabletop analysis are depicted in Figure 9-1 and Figure 9-2. The figures are 

“generic” in the sense that they do not represent plant-specific response operations, but, rather, 

depict an illustrative case. Figure 9-1 is a depiction of a fire suppression strategy and Figure 

9-2 is a depiction of a de-energization strategy. Response operations primarily involve four 

types of personnel: 1) MCR operators 2) field operators 3) digital instrumentation and controls 

(DI&C) technicians and 4) the fire brigade. MCR operators are responsible for detecting an 

alert, using the correct alarm response procedure (ARP), dispatching personnel to the alert 

location, monitoring the situation from the MCR during the field investigation and, on alarm, 

activating the fire brigade. The field operator is responsible for serving as the initial fire watch 

(with suppression capabilities) and maintaining communications with the control room. The 

technician is responsible for gathering necessary equipment, traveling to the fire location,

unlocking and opening cabinets, and using the equipment to find the incipient fire source. The 

fire brigade is responsible for suppression duties once they’ve arrived on the scene. Variations

in system implementation and response operations that were observed during the informationgathering stage are noted by superscript letters in the figures and addressed further in Section 

9.2.1.1.

 4 The scope of the analysis was determined by the information required to support the HRA (discussed in 

Section 10).
9-4

Figure 9-1. Generic depiction of operations in response to an in-cabinet ASD VEWFD 

alert followed by alarm where a suppression strategy is being used5

 5 If at any time during response operations flaming conditions are observed, the main control room would be 

alerted and the fire brigade dispatched.

Event MCR Response Field Operator 

Response

Technician

Response

Fire Brigade 

Response

Component Continues to Degrade

Alarm 

sounds in 

MCR

Flaming

fire

Detects Alerta

Begins using 

Alarm 

Response 

Procedure

Consults MCR 

computerb to 

determine fire 

location (i.e., 

bank of 

cabinets)

Dispatches FO

to fire location

Dispatches tech 

to fire location

Dispatches 

fire brigade 

to fire 

location

Travels to fire 

location

Begins serving 

as posted fire 

watchc at 

identified bank 

of cabinets

Retrieves 

equipment to 

locate incipient 

fire source

Travels to fire 

location

Begins using 

equipmente to 

locate affected 

cabinet

FB travels to 

fire location

Suppress fire

Initial fire 

suppression if 

FB is not 

on-site

VEWFD Response Operations

Uses 

equipment to 

locate 

degrading 

component

Operational 

Goal

Posted fire 

watch at the 

correct location 

of VEWFD 

system “alert,” 

positioned in 

close proximity 

to the affected 

cabinet to 

initiate 

suppression in 

the case of a 

flaming fire.

Fire Brigade 

on-site for 

prompt 

suppression

Alert

sounds in

MCR

Component 

begins 

degrading

Alert Response

Operations

Alarm Response

Operations

Opens affected 

cabinetd

Continues 

monitoring the 

MCR computer 

screen during 

the field 

investigation

Maintains 

communication 

with FO 

throughout 

investigation

Maintains 

communication 

with MCR 

throughout 

investigation
9-5

Figure 9-2. Generic depiction of operations in response to an in-cabinet ASD VEWFD 

alert followed by alarm where a de-energization strategy is being used

Event MCR Response Field Operator 

Response

Technician

Response

Fire Brigade 

Response

Detects Alert

Begins using 

Alarm 

Response 

Procedure

Consults MCR 

computer to 

determine fire 

location (i.e., 

bank of 

cabinets)

Dispatches FO

to fire location

Dispatches tech 

to fire location

Makes decision 

to de-energize

Travels to fire 

location

Begins serving 

as posted fire 

watch at 

identified bank 

of cabinets

Retrieves 

equipment to 

locate incipient 

fire source

Travels to fire 

location

Begins using 

equipment to 

locate affected 

cabinet

Opens affected 

cabinet

Communicates 

information 

about degraded 

component to 

MCR

Instructs FO to 

de-energizeg

De-energizes 

Component

VEWFD Response Operations

Uses 

equipment to 

locate 

degrading 

component

Communicates 

information 

about 

degrading 

component to 

FO

Operational 

Goal

Locate 

degraded 

component and 

de-energize 

before flaming 

fire occurs

Component Continues to Degrade

Alert

sounds in

MCR

Component 

begins 

degrading

Continues 

monitoring the 

MCR computer 

screen during 

the field 

investigation

Maintains 

communication 

with FO 

throughout 

investigation

Maintains 

communication 

with MCR 

throughout 

investigation
9-6

9.2.1.1 Variations in ASD VEWFD system implementation and response operations

Variations that were observed or discussed during the information-gathering stage and that 

were noted in Figure 9-1 and Figure 9-2 are addressed further here:

a. Alarm location varies. There are licensees that located the ASD VEWFD alarm display 

on the front panel and others located it on a back panel in the MCR. The implications of 

this variation are discussed in Section 9.2.2.2.

b. Licensees differ with regard to how personnel retrieve information regarding fire 

location. At one licensee site, this information is provided via a MCR computer that 

indicates the fire location/bank of cabinets in an alarm (represented in Figures 9-1 and

9-2). Others require operators to consult a main fire alarm control panel in or near the 

MCR, which will indicate the room or area that is alarming; then, the field operator will 

check a local fire alarm control panel and/or a local ASD VEWFD detector to get more 

detailed location information (e.g., bank of cabinets). The more steps involved in 

determining the fire location, the longer the overall response will be delayed, which may 

decrease the probability of successful prevention or prompt suppression.

c. The level and quality of training that fire-watch personnel receive varies. One licensee 

reported that fire-watch personnel are trained in basic fire suppression using a fire 

extinguisher and another reported that 95 percent of field operators are fire brigade 

trained. The implications of this variation are discussed in Section 9.2.2.4.

d. Cabinets at some sites require keys to be opened. The implications of this variation are 

discussed further in Section 9.2.2.1.1. In addition, the personnel responsible (i.e., FO or 

I&C tech) for retrieving cabinet keys and unlocking cabinets varies between sites. This 

variation can affect the response timeline, and thus, affect the probability of successful 

response operations.

e. The type of equipment used to locate the degrading component varies. Some licensees 

use portable ASDs, and others use thermal imaging cameras. The impact of equipment 

type on response operations is discussed further in Sections 9.2.2.1.3 and 9.2.2.1.4.

f. The personnel responsible (e.g., FO, MCR operator, another technician) for deenergizing equipment varies based on the type of components/equipment being deenergized. This variation may impact the necessary communications (e.g., the 

operators may have to dispatch personnel who are not currently at the fire location). 

This variation can affect the response timeline, and thus, impact the probability of 

successful response operations. For example, if the equipment can be de-energized 

from the MCR, this may shorten the timeline; however if other personnel have to be 

dispatched to the fire location, this can extend the timeline.
9-7

9.2.2 Step 2: Factors that Affect Human Performance

In this section, factors that may adversely affect human performance during ASD VEWFD 

response operations are identified and described. For each factor, there is a general discussion 

of the factor itself, the unique concerns regarding the factor’s influence on ASD VEWFD 

response operations, and guidance and/or operational experience if relevant. 

The factors include:

• special equipment

• human-system interface

• procedures

• training 

• staffing

• communications

• complexity

• workload, pressure and stress

9.2.2.1 Special equipment

Special equipment is the unique equipment or tools needed to successfully carry-out human 

actions in a specified scenario or under certain conditions (e.g., fire, flooding) (Ref. 48). For 

VEWFD response operations, special equipment may include portable ASDs and/or thermal 

imaging cameras, keys and PPE. As noted in NUREG-1921, “EPRI/NRC-RES Fire Human 

Reliability Analysis Guidelines,” special equipment must be readily available and functional, 

located in a known and designated area, and able to be located and accessed by plant 

personnel (Ref. 49). In addition, it is important that the equipment is used and maintained 

properly, so that staff may have confidence in the information it provides.

9.2.2.1.1 Keys

Keys may be required to access certain cabinets once the degrading component has been 

located. If response personnel do not routinely carry the keys to access locked cabinets, 

retrieving the keys will add time to the response operations timeline, and may decrease the 

probability of successful prevention or rapid suppression. 

9.2.2.1.2 Personal protective equipment

Depending on the type of cabinet, personal protective equipment (PPE) may be required to 

open it. PPE may include protective clothing such as gloves, safety glasses, or other special 

purpose gear. The appropriate PPE will vary based on the cabinet’s contents. PPE can have a 

significant effect on performance. For example, gloves may make manipulating portable 

ASDs/thermal imaging cameras more difficult, increasing the likelihood of errors or increasing 

the time required to complete the task.

9.2.2.1.3 Portable ASDs

Vendors of ASD systems market portable ASD equipment that can be used to help locate low 

energy pre-flaming fires. These systems are intended to help in cases in which the fire aerosol 
9-8

signature is not producing visible smoke to aid the responding personnel in locating the fire 

source. These devices have the potential to locate the incipient fire before damage occurring to 

other equipment. However, there are certain aspects related to the use of these systems that 

affect their utility.

Differences in detection technologies (i.e., handheld and locally mounted ASD), should not be 

mixed. For instance, if an electrical cabinet is protected by a cloud chamber based ASD, a 

laser-based portable ASD is not suitable for use and vice-versa. This is because of the 

differences in these two technologies’ ability to detect different fire aerosol signatures (i.e., a 

large number of small particles versus a small number of large particles). Therefore, there can 

be instances where a handheld light-scattering based detector may not be effective at locating a 

low energy fire source when a permanently mounted cloud chamber device is being used, and 

vice versa.

Maintenance and testing are important to ensure the operability of these handheld systems. 

The portable ASD systems are battery operated. Battery life typically varies from 2-3 hours. 

As such, the battery should be properly maintained to vendor recommendations to ensure that 

power is available when needed. Units based on cloud chamber technology require a suitable 

water source to function. Thus, the supply should be routinely inspected and maintained to 

ensure operability. Lastly, testing the units with the vendor-recommended frequency will ensure 

that they are functioning properly, or that malfunctions are discovered and repaired in a timely 

manner.

9.2.2.1.4 Thermal imaging cameras

A thermal imaging camera is a non-contact instrument which is able to quickly scan temperature 

distribution of entire surfaces of machinery and electrical equipment and detect infrared 

radiation (IR) emitted by objects. The amount of radiation emitted is dependent on the object’s 

temperature and emissivity properties. Emissivity is the efficiency with which an object emits 

radiation and can range from 0 (not-emitting) to 1 (completely emitting). This is highly 

dependent on material properties and also varies with temperature. In order for the thermal 

imaging camera to read correct temperatures, emissivity must be taken into account. Most 

thermal imaging cameras allow for emissivity as an input and use an algorithm to calculate the 

temperature of a viewed object to most closely match the actual temperature. When using 

thermal imaging cameras to survey the internals of electrical cabinets, it is important to select an 

emissivity within an acceptable range.

The quality and effectiveness of thermal imaging cameras can vary with camera resolution, 

thermal sensitivity and camera accuracy. Equipment should be chosen that is appropriate for 

application in NPPs and is proven to be appropriately sensitive. Similar to portable ASDs, 

thermal imaging cameras require periodic calibration, maintenance, and testing to ensure that 

they are functioning properly.

When responding to a VEWFD “alert” or “alarm,” the ability to use thermal imaging cameras to 

locate the incipient source will vary. If thermal imaging cameras are used in conjunction with 

portable ASDs, and a portable ASD is used to locate the affected cabinet, the thermal imaging 

camera may support locating degrading components. In this scenario, the cabinet configuration 

and thermal operating characteristics will affect the ability to locate the degrading 

component. For instance, if the camera’s view is clear and unobstructed, the ability to locate 

degrading components should be enhanced. Conversely, if the cabinet has substantial 

partitions and obstructions which do not allow for a direct line of sight for the camera to sense 
9-9

an objects emitted IR, the thermal imaging camera is less effective; and consequently, locating 

the component may take substantially more time and the potential exists that the component 

may not be located before flaming conditions.

The use of thermal imaging cameras requires the user to process the viewed image. Thus, it 

may be beneficial to have accurate baseline images available and accessible. Baseline images 

are images taken when equipment is at normal operating temperatures. Baseline images serve 

as comparison data to assess whether or not acceptable temperatures have been surpassed,

and are commonly used as a periodic surveillance tool. Depending on how thermal images 

must be processed, (e.g., operator compares current image to baseline, through the use of 

software that processes images, etc.), the timeline for incipient fire identification should be 

adjusted accordingly.

9.2.2.2 Human-system interface

A human-system interface (HSI) is the part of the system through which personnel interact to 

perform their functions and tasks. The availability, functionality, and usability of human-system 

interfaces can impact personnel performance. Guidance for the evaluation of HSIs is provided 

in NUREG-0700 Revision 2, “Human-System Interface Design Review Guidelines” (Ref. 50). 

HSIs that are poorly designed (e.g., poor labeling, subpar computer interfaces), have been 

damaged, or are difficult to use, can negatively impact performance. Also, if the HSI does not 

display required information, or if the information is inaccurate, performance can be adversely 

affected. HSIs involved in ASD VEWFD alarm response operations include MCR HSIs, portable 

equipment HSIs, and may include HSIs of local fire alarm control panels and local VEWFD 

detectors. 

9.2.2.2.1 MCR HSI

The value of the ASD VEWFD system is in creating the opportunity for fire prevention or prompt 

suppression. As stated earlier, an effective response to a VEWFD signal relies on the actions of 

NPP personnel. Thus, ASD VEWFD MCR alarm displays are aspects of the MCR HSI that 

deserve consideration, as there can only be an effective operator response if the operator is 

aware there is a problem. Broadly defined, alarms are signals/warnings that inform personnel 

that a plant parameter, component, system, or function is currently in a state requiring the 

attention of plant personnel. Both ASD VEWFD alerts and alarms that require personnel to 

respond would fall under the broad definition of “alarm.” This is pertinent because the 

subsequent information in this section will discuss “alarm” characteristics, which, in the current 

context, applies to both ASD VEWFD alerts and alarms that require personnel to respond.

As stated in NUREG-0700, Revision 2, “To be effective, an alarm system should attract 

attention and help the operator focus attention on more-important rather than less-important 

alarms.” An alarm should be designed such that operators can reliably discern it. Alarms can be 

made discernible through aspects such as signal level, visual coding, visual intensity, and 

frequency of tonal signals. NUREG-0700, Revision 2, Section 4 provides detailed information

for alarm system design.

With regard to ASD VEWFD, there are several aspects of alarm design that should be 

specifically noted: 

• Signal level of alarms - NUREG-0700, Revision 2 states that the signal “should be such 

that users can reliably discern the signal above the ambient control room noise.” 
9-10

Specifically, it advises that a signal approximately 10 decibels (dB) above average 

ambient noise is adequate and that the sound intensity should be limited to a maximum 

of 95 decibels in most circumstances. NFPA 72, states that the combination of ambient 

noise and alarms “shall not exceed 110 dB at the minimum hearing distance” (Ref. 8). In 

NPPs, there are a multitude of alarms. Although NFPA 72 states that fire alarms shall 

not exceed 110 dB, caution should be taken to ensure that the ASD VEWFD alarms are 

not “drowning out” reactor alarms (Ref. 8). This is possible if the reactor alarms are set 

at 95 dB or less and the ASD VEWFD alarm at 110 dB. 

• Alarm set-points - Nuisance alarms are alarms that have no operational significance to 

current plant conditions. One type of nuisance alarm occurs when alarm set-points are 

established so close to the normal operating value that many “false” alarms occur. If 

nuisance alarms occur frequently, operators may become less likely to respond. 

Operating experience indicates that VEWFD systems tend to alarm in response to 

maintenance activities (e.g., welding/grinding activities), “dirty” environmental conditions 

(e.g., dust, charcoal in the air) and even momentary increases in background signal 

“spikes.” Licensees have used several mitigating strategies to reduce the amount of 

false alarms including disabling the system and posting a fire watch during maintenance 

activities; and adjusting sensitivity settings that account for environmental conditions. 

Another variety of nuisance alarms is status indications or “messages that indicate the 

status of plant systems but are not intended to alert the user to the need to take action.” 

Status indications may increase the processing demand on operators and result in the 

operator being unsure when a response is required and/or delay operator response. 

ASD VEWFD systems offer users the capability to have multiple alert and alarm setpoints. If used, those alerts and alarms with no associated operator actions are

nuisance alarms. As suggested in Section 4.1.2-5 of NUREG-0700, Revision 2, status 

indications should be segregated from alarms and should be presented to operators via 

a non-alarm display unless there are unique aspects of the design that justify presenting 

the information within the alarm display. 

• Alarm Location - According to NUREG-0700, Revision 2, Section 4.6-5, alarm displays 

and controls should be positioned such that responsible personnel can access the alarm 

information with adequate time to respond. For example, if the VEWFD alarm is located 

on the back panel in the MCR and only monitored periodically, it may take the MCR 

operators longer to detect the alarm than if it were on the front panel, thus reducing the 

time available to respond, and potentially decreasing the probability of fire prevention or 

prompt suppression. Through site visits and expert consultation, it was determined that 

there are licensee MCRs where the alarm is located on a front panel and others where it 

is located on a back panel. 

9.2.2.2.2 Local fire alarm control panels, local VEWFD detectors, and special equipment

ASD VEWFD response operations require technicians to interact with HSIs of special 

equipment (portable ASDs/thermal imaging cameras) and may require field operators to interact 

with the HSIs of local fire alarm control panels and/or ASD VEWFD systems. As explained in 

NUREG-1921, MCR HSIs (including main fire alarm control panels) are subject to detailed 

control room design reviews (DCRDRs) which has led to modification or elimination of many 

problematic HSIs. However, local panels and special equipment have not received the same 

level of regulatory review. HSIs of local fire alarm control panels, local detectors and special 
9-11

equipment deserve consideration with regard to their potential impact on VEWFD response 

operations. 

Local fire alarm control panels and local detectors are akin to what NUREG/CR-6146, “Local 

Control Stations: Human Engineering Issues and Insights,” refers to as local control stations 

(LCSs) (Ref. 51). LCSs are defined as “an operator interface related to process control that is 

not located in the main control room. This includes multifunction panels, as well as singlefunction LCSs, such as controls (e.g., valves, switches, and breakers) and displays (e.g., 

meters) that are operated or consulted during normal, abnormal, or emergency operations” (Ref. 

50). Local fire alarm control panels and local VEWFD detectors are displays that are consulted 

during abnormal operations, thus, meeting the aforementioned definition. 

In NUREG/CR-6146, the results of a study to evaluate human engineering of LCSs in the U.S. 

nuclear industry are captured. Approximately 3,000 LERs involving “poor ergonomics or human 

environment” were reviewed and in-plant assessments of LCSs were conducted. Several items 

of note resulted from this study: 1) many events were identified as having occurred as a result 

of a specific human interface deficiency, 2) human engineering deficiencies at LCSs are quite 

common across the industry and 3) human engineering deficiencies at LCSs can negatively 

affect plant operation. Interestingly, while reviewing the LERs, it was noted that nearly half of 

them involved equipment used for testing/calibration. Although testing/calibration equipment 

events were not of interest in the study, the information is relevant for VEWFD response 

operations. It suggests that special equipment HSIs may be less than ideal. Another 

complicating factor for both LCSs and special equipment may be that, as opposed to MCR HSIs 

that operators interact with frequently, personnel may rarely interact with LCSs and special 

equipment. They must take action in much less familiar surroundings, using equipment with 

less than ideal HSIs, potentially resulting in adverse effects on performance. 

Project activities included one observation of a local fire alarm control panel and local VEWFD 

system. Both appeared to be located in a readily accessible area and had readable and 

understandable displays. Vendor documentation and videos were reviewed for thermal imaging 

cameras and portable ASDs; however, the images of the equipment were not clear enough to 

properly evaluate the quality of the HSIs. More research should be done to evaluate the quality 

of the thermal imaging camera/portable ASD HSIs, and to explore personnel usage difficulties.

As stated in NUREG/CR-6146, “Like the workstations in the control room, LCSs are interfaces 

between the operators and the plant, and the approach to their design should reflect the same 

human engineering considerations given to the main control room...” (Ref. 51). High-level 

guidance regarding design and evaluation of local control stations and portable diagnostic tools 

(e.g., portable ASDs) is provided in Sections 12.2 and 13.8.3.2 of NUREG-0700 Revision 2. 

9.2.2.3 Procedures

Procedures or instructions for performing actions, can impact human performance negatively or 

positively depending on their availability, accessibility and quality. To create high-quality 

procedures, they should be developed using accepted human factors engineering principles. 

As stated in Section 9.2 of NUREG-0711, Revision 3, “Human Factors Engineering Program 

Review Model,” procedures should be “technically accurate, comprehensive, explicit, easy to 

use, and validated” (Ref. 52). 

Regarding ASD VEWFD response operations, MCR Alarm Response Procedures (ARPs) must 

be available and accessible to guide the human response to a MCR alert or alarm. Personnel 
9-12

should have immediate access to ARPs from the alarm location (Ref. 50). If procedures are not 

readily available or accessible, it may negatively impact performance by increasing the time to 

respond, thus decreasing the probability of fire prevention or prompt suppression. 

Local actions (e.g., locating degrading components) are required in ASD VEWFD response 

operations. Typically, there are procedures available for local actions; however, some tasks 

may be considered “skill-of-the-craft”6 and, thus, are not proceduralized. Project research did 

not yield information regarding the usage of procedures for local actions. If procedures are not 

used, a strong case must be made for labeling a task “skill-of-the-craft” to provide reasonable 

assurance of a safe operator response. Even when procedures do exist, it may not be practical 

to page through a procedure while performing a task. For example, if a technician is using a 

portable ASD to locate the degraded component, one hand would be used to hold the body of 

the device and the other would hold the probe, thus making paging through a procedure 

impractical. Thus, personnel must either be trained to perform the steps from memory or there 

must be a contingency plan for providing the procedural steps (e.g., via portable radio).

9.2.2.4 Training

Nuclear power plant personnel must receive training in accordance with current regulations. 

With regard to ASD VEWFD response operations, 10 CFR 50.120 applies to the non-licensed 

operators (FOs) and I &C technicians and10 CFR 55 applies to licensed operators. Both 

regulations identify a systems approach as acceptable methodology for training nuclear power 

plant personnel. A systems approach to training consists of the following five elements (10 CFR 

55.4):

• systematic analysis of the jobs to be performed

• learning objectives derived from the analysis which describe desired performance after 

training

• training design and implementation based on the learning objectives

• evaluation of trainee mastery of the objectives during training

• evaluation and revision of the training based on the performance of trained personnel in 

the job setting

Ultimately, the training program must provide the instruction necessary to produce qualified 

personnel to operate and maintain the facility in a safe manner. In general, training should 

establish familiarity with procedures and operation of any special equipment; prepare personnel 

to handle departures from the expected sequence of events; and provide opportunities to 

practice the skills required to accomplish the task (Ref. 48). 

Some of the training that may be necessary for ASD VEWFD response operations includes:

 6 “Skill of the craft” is a term describing those tasks in which it is assumed that the workers know certain 

aspects of the job and need no written instructions (NUREG/CR-1278) (Ref. U).
9-13

• training for MCR operators on ASD VEWFD ARPs

• training for FOs and technicians on any applicable procedures

• training for technicians on the operation of thermal imaging cameras and/or portable 

ASDs

• training for personnel who will serve as fire watch

• training for fire brigade personnel

• human performance training related to electrical safety for personnel involved in opening 

energized electrical cabinets

Special equipment operation training may be especially important, as it is the key to locating the 

degrading component. Portable ASDs have, more often, been used in other domains 

(e.g., telecommunications); licensees should be cognizant that vendor training may have been 

developed with domains other than nuclear in mind. Nuclear power plants may introduce 

unique elements that warrant domain-specific training. Project research yielded limited 

information with respect to the U.S. nuclear industry’s current approach to special equipment 

training. Personnel at one site receive initial training followed by retraining every 2 years for 

portable ASDs. The training has classroom and practical aspects and lasts approximately

4 hours. The practical section consists of simulating a situation in which technician has to find a 

degrading component by placing a “smoking” wire in a room beforehand that trainees must 

locate. Another site reported that, although training has not yet been implemented, they expect

that all operators will have specific training in the use of portable ASDs, with a qualification 

sign-off, as part of their basic operator training. In addition, one site reported that they receive 

several nuisance alarms per month, which allows personnel to gain experience in using the 

equipment and helps them maintain their level of proficiency. Regarding thermographic 

cameras, one site reported that personnel are required to complete a qualification card to use 

the thermographic camera which includes one week of offsite training and over 100 hours of 

working with the camera.

Training is also an important consideration with respect to fire-watch and fire-suppression 

activities. In some cases, the personnel may be trained in basic fire suppression using a fire 

extinguisher or may have fire brigade level training.7 One licensee reported that approximately 

95 percent of its field operators are fire brigade qualified, they receive specific incipient fire 

training and are qualified as incipient fire watch, and personnel receive refresher training yearly. 

Another licensee reported that all operators are trained in the proper use of fire extinguishers. It 

follows that more confidence can be placed in those with more extensive training to successfully 

suppress a fire.

 7 Fire brigade training acceptance criteria are laid out in the SRP section 13.2.2. Professional standards are 

further defined by NFPA 1081, “Standard for Industrial Fire Brigade Member Professional Qualifications.”
9-14

9.2.2.5 Staffing

According to NUREG-0711, Revision 3, staffing levels are an important consideration when 

plant modifications are undertaken. Plant modifications can impact important human actions, 

thus, applicants should assess staffing needs to assure that required actions can be 

successfully accomplished. Applicants should determine the following needs: 1) the type of 

staff (i.e., qualifications); 2) the number of staff and 3) the (required) availability of the staff. 

Information about regulations and guidance regarding staffing is provided in NUREG-0800, 

“Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: 

LWR Edition,” Section 18 and NUREG-1764, “Guidance for the Review of Changes to Human 

Actions” (Refs. 53 and 54).

The installation of an ASD VEWFD system is a plant modification; thus, staffing needs should 

be assessed. ASD VEWFD response operations require both MCR and non-MCR staff. 

According to NUREG-1792, “Good Practices for Implementing Human Reliability Analysis 

(HRA),” “for control room actions, the availability of [MCR] staff is not a concern because plants 

are required to maintain a minimum crew with qualified staff in or near the control room”

(Ref. 55). As a note of caution, applicants should ensure that MCR operators responsible for 

control room actions do not have collateral duties that would threaten their availability in the 

MCR. An operator who could be called upon to complete MCR actions should not, for example, 

also be a member of the Fire Brigade for the same fire.

Non-MCR staff including an FO for initial fire watch duties, a DI&C technician to locate the 

degraded component, and potentially, additional personnel for a long-term fire-watch, are also 

needed for ASD VEWFD response operations. According to the distribution presented in 

Section 8.2, Figure 8-4, the incipient stage duration distribution is not narrowly defined (i.e., 

ranges from several minutes to several hours). With a large amount of variability in the duration 

of the incipient stage, it is a reasonable conclusion that personnel involved in response 

operations need to be available on site to ensure a timely response. If a sufficient number of 

qualified personnel are not available, the planned response should not be considered feasible.

8 

The information gained from trip reports and expert consultation indicated that FOs and 

technicians are available on site continuously. However, level of staffing may be of concern 

with regard to a long-term fire-watch. The time between an incipient alarm and flaming 

conditions may be an extended period of time (refer to Section 8 for timing estimates), hence

necessitating that licensees have personnel available for an extended fire-watch. Depending on 

staffing levels, this may affect whether or not the fire watch is roaming or constant. A roaming 

watch is not desirable as it could extend the timeline for detecting and suppressing a fire.

9.2.2.6 Communications

Much of the communication during ASD VEWFD response operations takes place between the 

MCR and personnel outside the control room. The MCR must dispatch FOs, technicians and 

the fire brigade. The MCR must also be in contact with the FO during the field investigation to 

provide pertinent information gained from monitoring the MCR computer. This communication 

is critical as the success of the alarm response operations rests with the FO and technician 

arriving at the fire location and completing their tasks in a timely manner. Communications 

equipment must be available, accessible and functional to ensure communication can occur. 

 8 Feasibility is defined as the ability to accomplish a task in the context within which it will be performed and 

there is adequate time available to perform the action, considering any adverse contextual or personnel 

factors that may delay or degrade performance. (Ref. 45, 46)
9-15

According to NUREG-0700, Revision 2, “Where communications are critical, users should not 

be precluded from communicating with other plant personnel by the loss of one method” (Ref. 

50). A complement of communications equipment for this context might include phone lines, the 

intercom system, sound-powered phones and portable radios. Sound-powered telephone 

systems do not require a separate electrical power supply to transmit signals; the force of the 

user’s speech on the mouthpiece generates small electrical impulses, which are transmitted as 

a signal. They are beneficial for situations in which electricity is not available; however, it should 

be noted that training is required to operate them properly. Portable radio transceivers include 

battery-powered communication devices that transmit messages through the airways rather 

than through wires. However, there are places in the plant where radio usage is not permitted, 

thus, strengthening the case for having several diverse methods of communication. This 

complement of communications equipment provides a variety of equipment that uses various 

media for communication, and is a good example of establishing diverse communication 

methods. Guidance for speech-based communications regarding topics such as sound quality 

and area coverage is provided in NUREG-0700, “Human-System Interface Design Review 

Guidelines,” Revision 2, Section 10.2.

9.2.2.7 Complexity

Complexity refers to the ambiguity and mental effort associated with the situation to be 

diagnosed, the decision to be made, or the action to be performed (Ref. 55). High levels of 

complexity, particularly in the absence of training and practice, can negatively impact human 

performance.

Sources of complexity that may affect task performance in VEWFD response operations 

include:

• ambiguity from difficult-to-interpret cues and indications

• the need to consider multiple variables simultaneously 

• the need to perform many unfamiliar steps in rapid succession

The indications from portable ASDs and thermal images may be ambiguous or difficult to 

interpret because of the fact that, as stated previously, HSIs of special equipment often do not 

receive the same level of review as MCR HSIs. The equipment HSIs may be less than ideal, 

thus increasing the complexity of identifying the exact component that is degrading. Also, being

unfamiliar with the necessary equipment and procedures/task steps can make the task more 

complex. If personnel use the portable equipment and perform the steps in VEWFD response 

operations rarely, this may result in a situation in which personnel are performing many 

unfamiliar steps in rapid succession. High-quality initial training and periodic refresher training 

along with well-designed procedures and equipment can help to mitigate complexity. 

The decision to de-energize the affected cabinet may also be a complex task. There are 

multiple variables that personnel must take into consideration. First, personnel must have an 

understanding of the contents of a particular cabinet to determine the effect that de-energizing 

will have on the safe operation of the NPP. They must also consider the effort involved in the 

task of de-energizing. According to experts (see section 9.1), de-energizing equipment can be 

a very simple or a very involved and complex operation. For example, if an entire cabinet is 

being de-energized, there are instances in which it can be de-energized from the MCR within 

seconds. At one site, the logic for the cabinet power is laid out on the MCR control board,

making the de-energization process simple. Alternatively, if only one breaker is being de-
9-16

energized (partial cabinet), local action may be required, and the task would likely require 

multiple steps. This is an important consideration because if the process takes an extended 

amount of time, it may not be possible to complete it before the transition to a flaming 

fire. Personnel must weigh the benefits of de-energizing to preserve equipment with the 

ancillary effects on the rest of the plant. Some of the complexity associated with this task can 

be mitigated by pre-planning the steps needed to de-energize the cabinets. One strategy would 

be to “pre-locate” the isolation devices for all ignition sources within each cabinet in an effort to 

speed up the process. This would include predetermining the isolation devices, conveniently 

displaying that information for use in response to VEWFD alerts, training responders to rapidly 

locate and operate the isolation device(s), and conducting drills to periodically demonstrate this 

ability.

9.2.2.8 Workload, pressure and stress

Workload, pressure and stress, collectively, refer to the extent to which personnel experience 

(time) pressure to perform an action, along with their overall sense of being threatened in some 

way with respect to accomplishing their task. The emphasis for this factor is on the amount of 

work that must be accomplished in the available time. If workload, pressure, and stress are too 

high, they may adversely impact personnel performance. 

For ASD VEWFD response operations, it appears on the surface, that there would be little time 

pressure as this is very early warning fire detection. However, based on project research, 

personnel are trained to handle incipient alerts and alarms with urgency, such that they are to 

drop everything and respond. The value of these systems lies with the human reacting promptly

to prevent a fire, or quickly suppress it. Thus, creating some time pressure is an important 

component of providing reasonable assurance of the feasibility and reliability of VEWFD 

response operations. However, while some time pressure may help provide a feasible reliable 

response, too much time pressure may result in degraded task performance (Ref. 56). 

The fire itself can create time pressure because of the fact that the length of the incipient stage 

of a fire can vary widely (see Figure 8-4); thus, the time available for personnel to respond will 

also vary. It may be that being aware of the variability in the time available to respond, may 

alone, create pressure and stress for personnel.

Another source of stress during VEWFD response operations may be a concern for one’s 

personal safety. If a VEWFD detector is being used for power distribution equipment, there is 

the potential for significant safety hazards (e.g., explosion upon opening a cabinet).

9.3 Area-wide Applications

Because of the considerable amount of unknown variables (e.g., room size, content of room, 

etc.) area-wide applications were not specifically addressed in the HF analysis as there is no 

“prototypical scenario” to be assessed. However, based on discussions with plant personnel, it 

is expected that the personnel response to an area-wide incipient alert and alarm will be 

fundamentally the same as for in-cabinet applications. The only difference is that the FO and 

technician will be sent to a room rather than a bank of cabinets, after receipt of an “alert.” The 

technician will need to locate the incipient fire source within that room. The larger area that 

must be surveyed should be accounted for in the timing analysis.
10-1

10. HUMAN RELIABILITY ANALYSIS

10.1 Human Reliability Analysis (HRA) Approach

The objective of this section is to develop an improved, detailed HRA quantification to support a 

fire PRA quantification of VEWFD system performance. To accomplish this objective, existing 

HRA approaches will be used to the extent applicable to this particular context.

First, the HRA process (i.e., steps needed to perform HRA) used in this report is based on 

existing HRA processes. The Joint EPRI/NRC-RES Fire Human Reliability Analysis Guidelines 

(Ref. 49) and “A Technique for Human Event Analysis,” (ATHEANA) (Refs. 57 and 58) describe

two similar HRA processes. The HRA process given in NUREG-1921 is specific to a fire 

context. However, since the analysis in this report must address operator1 actions taken before 

or without a reactor trip, the NUREG-1921 process is not general enough to adequately address 

this context. Therefore, the HRA process used in this analysis is a combination of those in 

NUREG-1921 and ATHEANA. Namely, the first two steps in ATHEANA are added to the 

NUREG-1921 HRA process. As a result, the HRA process used in this report consists of the 

following steps:

1. Define and interpret the issue (Section 10.2.1).

2. Define the scope of analysis (Section 10.2.2).

3. Identify and define human failure events (HFEs) (Section 10.3).

4. Perform qualitative analysis (including feasibility assessment) (Sections 10.4 and 10.5).

5. Perform quantitative analysis to develop the human error probability (HEP) for each HFE

(Section 10.6).

6. Perform dependency analysis (Section 10.7).

7. Perform recovery analysis (Section 10.7).

8. Perform uncertainty analysis (Section 10.8).

9. Complete documentation.

Second, as will be discussed further in Section 10.6, HRA quantification is based on existing 

HRA methods and their associated HEPs.

Each of the HRA process steps is addressed in the sections below. There is no explicit 

discussion for documentation as this section serves as the documentation portion of the 

process. Also, like NUREG-1921, HFE identification and definition is discussed before 

qualitative analysis (even though qualitative analysis must be performed to support this task). 

In addition, documentation of feasibility assessment, including the development of necessary 

timing inputs, is provided in a section separate from qualitative analysis.

 1 For the application of very early warning fire detectors discussed in this report, human actions and activities 

are required of: 1) MCR crew of operators and associated supervisor(s) 2) a field operator, 3) a technician 

(assumed to be an Instrument and Control technician), 4) the fire brigade, and 5) other plant personnel who 

may be needed for de-energization (e.g., electrical engineers, electricians). However, for simplicity, the 

authors describe the activities of all human activities (except the fire brigade) as "operator actions." Also, 

the failure probabilities associated with the fire brigade are quantified, as is described in NUREG/CR-6850 

and elsewhere, using non-suppression curves that are supported by statistical data. The next section 

(i.e., Section 11) provides the quantification for suppression activities.
10-2

10.2 Define Issue and Scope to Be Addressed

This section lays out the requirements of the HRA needed to support the overall quantification 

approach outlined in Section 10.6 and the quantification illustrative examples in Section 12.

10.2.1 Define issue

The issue to be addressed in this study, as described in Section 1.3, is to provide HRA input 

(both qualitative and quantitative) to support fire PRA for VEWFD applications. In particular, 

this analysis addresses VEWFD applications that:

• are focused on in-cabinet installations

• are intended to support quantification of fire non-suppression probabilities

• achieve such reductions by enabling quicker fire suppression response than is typically 

modeled in fire PRAs

From the discussions in Section 9 regarding human factors analysis, it can be inferred that the 

HRA task for supporting fire PRA for VEWFD applications will be different from that in traditional 

HRA/PRA contexts.

Some of the key differences are as follows:

• All operator actions are taken before (or without) a reactor trip, but after a signal 

in the main control room (MCR). HRA/PRA traditionally only addresses the time 

phase before a reactor trip with modeled operator actions that could result in system or

component unavailability.2 Consequently, the type of operator actions associated with 

VEWFD installations has not been addressed in the development or application of 

previous PRA studies.3 Some low power and shutdown (LP&S) PRAs include operator 

actions that cause reactor trips (i.e., human-induced initiators), but these actions also 

differ from the exact context of VEWFD applications in that they occur before or 

simultaneous with reactor trip (rather than after); and they do not have as their goal to 

prevent a reactor trip or other worsening condition.

• For VEWFD applications discussed in this report, human actions and activities 

involve the following personnel: 1) MCR crew of operators and associated 

supervisor(s), 2) a field operator, 3) a technician (such as an Instrument and Control 

[I&C] technician), 4) the fire brigade, and 5) other plant personnel who may be needed 

for de-energization (e.g., electrical engineers, electricians). However, for simplicity, all of 

these human activities (except the fire brigade) are called “operator actions” in the 

context of the HRA, including definition of HFEs.

 2 Even those actions modeled in some fire PRAs that are preventative in nature, differ from those associated 

with VEWFD applications because, again, they occur following a reactor trip. 3 Initiating events for support system failures (e.g., loss of instrument air, loss of component cooling water) 

may involve control room alarms before reactor trip. However, as discussed later in this section, the time 

available for operator response to VEWFD system “alerts” is limited, so operators must respond with some 

urgency. This is not typically the case for support system initiating events.
10-3

• There are no standard requirements for traditional job aids (e.g., procedures, 

training, human-machine interface) supporting the operator actions of interest. 

While there are currently a few applications of VEWFD that are consistent with the 

objectives of this report, the operator response and supporting job aids have not been 

standardized either by regulation or common practice.

10.2.2 Define Scope

The scope of this HRA is limited and shaped by previous quantification efforts (described in 

Section 6.2) and current, existing VEWFD applications in NPPs and available supporting data.

These limiting and/or shaping factors include:

• As discussed in Section 1, the typical objective of VEWFD system applications 

is reduction of the overall fire non-suppression probability for the fire PRA. 

Consequently, this HFE defines success of operator actions as an end state that allows 

“enhanced” (i.e., quicker than usual) suppression of a fire that would otherwise 

eventually result in a reactor trip. Conversely, operator failure is defined as conventional 

fire suppression (i.e., normally used fire non-suppression probability). Section 11

discusses the use of non-suppression curves from NUREG/CR-6850 for the 

development of these failure probabilities.

• Following the event tree discussed in Section 6.4, HFEs must represent: (1) main 

control room operator response, and (2) the combined response from a field 

operator and technician. In the main control room (MCR), operators must 

acknowledge the incipient fire detector alert or alarm, then initiate and direct the 

field response. The field response is focused on the field operator being trained and 

positioned appropriately to provide fire suppression before flaming conditions occur.

• The input data for this HRA are consistent with current VEWFD applications 

at NPPs, but do not represent any specific NPP. To the extent possible, real 

operational experience for VEWFD applications is represented in this HRA. However, 

as illustrated by the plant interviews summarized in Appendix C, there was not always 

alignment among the NPPs that provided input. In general, a composite input was 

developed for the analysis. Examples of such inputs are how MCR operators are 

expected to respond to incipient fire detectors and the time required for various operator 

responses (including I&C technicians). Consequently, any plant-specific HRA would 

need to develop their own information for certain HRA inputs.

• Inputs from NPPs with existing VEWFD installations have defined “enhanced fire 

suppression.” “Enhanced fire suppression” requires earlier than typical arrival of fire 

suppression capability at the location where an incipient fire detectors has alarmed.4 

Using the HF tabletop analysis described in Section 9 (an shown in Figure 9-1), this 

HRA is based on: (1) fire brigade training for all field operators (i.e., fire suppression 

capability), (2) special incipient fire and/or incipient fire detector training for all field 

operators, (3) field operator arrival at the relevant location before there is any visible 

evidence of a fire, (4) stationing of a field operator at the location until there is visible 

 4 As discussed in Section 9.2.2.3, both VEWFD alerts and alarms require operator response fall under the 

broad definition of “alarm.” Furthermore, whatever the signal in the MCR from the VEWFD system, it is 

expected that the operator response will be guided by Alarm Response Procedures (ARPs).
10-4

evidence of a fire, and (5) fire suppression equipment is available at the relevant 

location. The success or failure of fire suppression is addressed in a different event 

tree heading, as discussed in Section 11.

• As discussed further in Section 10.4.1, the relevant success criteria, especially the 

time available after which operator actions are not helpful, have been difficult to 

establish. Timing inputs are crucial to HRA. As discussed in NUREG-1921 (Ref. 49), 

for successful performance, there must be more time available for the action than the 

action requires; otherwise, the operator action is not feasible (i.e., guaranteed failure). In

traditional HRA/PRA, success criteria are developed using plant-specific information and 

analyses. In the case of time available for HRA for VEWFD system applications, use of 

generic information, such as the timing analysis in Section 8, is likely to be the only 

option.

• In this analysis, a distribution for the time available for operator action has been 

developed, rather than a single data point. To address this distribution, the HRA 

has evaluated feasibility and quantified human failure probabilities for multiple 

times available, each of which can have a different associated success criterion. 

As shown in the event tree for fire suppression strategy for in-cabinet installations (see 

Figure 6-4), there are two event tree headings following the two operator responses: 

(1) enhanced suppression (π1) and (2) conventional detection/suppression (η). As 

shown in Section 11, these factors are developed using non-suppression curves, with 

the associated timing inputs linked to the success (or failure) of field operator/technician 

response. Consequently, failure of the field operator/technician response essentially 

represents a failure to effectively use the “extra time” provided by the VEWFD system 

for fire suppression such that credit can be taken in the fire PRA.

These limitations indicate that a much more thorough qualitative HRA must be performed. 

In particular, the human factors analysis described in Section 9 is an important input to 

understanding the operational context (and potential variations with associated changes in 

human performance). For example, the tabletop analysis5 (and associated Figure 9-1)

described in Section 9.2.1 is an essential input to understanding what is required of operators 

and technicians to successfully implement VEWFD for fire PRA. In modern day, at-power,

internal event Level 1 HRA/PRA studies, a formal task analysis is seldom required for any or all 

of the following reasons:

• Formal task analyses have already been performed in the development of the control 

room design and procedures (Ref. 59).

• Decades of simulator training and operating experience are available as inputs to the 

HRA analyst to understand the role and responsibilities of operators that are relevant 

to PRA.

• It is likely that any previously performed HRA/PRA study was supported by something 

resembling a “cognitive task analysis.”

 5 The “tabletop analysis” described in Section 9 is similar to a "task analysis" performed by HRA. However, 

“task analysis” is considered a more formal analysis by HF experts.
10-5

Overall, unlike the VEWFD application considered in this report, the HRA methods and 

approaches for addressing at-power, internal events Level 1 HRA/PRA are mature and 

supported with a wealth of relevant, realistic information.

Consequently, the development of an appropriate HRA approach for this study on VEWFD 

applications must recognize the following:

• There is no “standard” or requirement for how VEWFD is implemented (and, therefore, it

resembles the equivalent of an “unconstrained, mathematical problem”).

• There is very limited information on existing VEWFD applications in NPPs.

• HRA results must support VEWFD applications, in general, rather than a plant-specific 

HRA of a VEWFD implementation (as expected in HRA/PRAs that follow ANS/ASME 

PRA Standard (Ref. 60)).

In addition, this analysis, like the human factors discussion in Section 9, focuses on in-cabinet 

installations of VEWFD. Section 10.3 “Identification and Definition of HFEs” discusses the 

different types of installations and associated strategies for success.

10.3 Identification and Definition of Human Failure Events (HFEs)

In traditional PRAs, operator actions that mitigate an accident or that worsen plant conditions 

should be considered for representation in PRA models as HFEs. This analysis is similar, but 

not exactly the same as traditional HRA/PRAs. In particular, the objective of this analysis is to 

model operator actions whose success would result in early fire suppression. Consequently, an 

operator “failure” in this analysis is “no early fire suppression” (i.e., normal fire suppression).

NUREG/CR-6850, Supplement 1 (Ref. 61) identified two event tree headings associated with 

human actions for determining fire PRA. While this earlier work was an important input to this 

report, HFEs are identified herein by using the more thorough analysis of human actions and 

associated performance provided by the human factors analysis discussed in Section 9. In 

particular, the overall tabletop task analysis discussion and Figure 9-1 and Figure 9-2 has been 

the basis for the identification and definition of HFEs. In addition, this project included extensive 

coordination between the HRA team and the event tree developers on the structure, headings 

and success criteria associated with the event trees shown in Section 6.4. This event tree 

structure also requires close coordination with the task for assigning fire non-suppression 

probabilities (see Section 11).

In particular, Section 6.4 provides two event trees to address two different VEWFD applications:

(1) As shown in Figure 6-4, in-cabinet installations of VEWFD where the cabinet(s) are not 

targets of concern for the fire PRA (i.e., “success” for the operator action is achieved 

when a field operator (or other appropriately trained plant personnel) arrives at the fire 

location, providing fire suppression capability upon arrival, and the field operator begins 

fire suppression as soon as flaming conditions are visible. Figure 6-4 shows (and 

Sections 11.1 and 12.1 provide further discussion) that two different cases for operator 

success are considered for earlier fire suppression than normal fire suppression.
10-6

(2) As shown in Figure 6-5, area-wide installations of VEWFD are similar to the other 

installation types, but require additional time needed for the I&C technician to survey 

the entire room or fire area to identify the affected bank of cabinets or other equipment, 

especially since in-cabinet installations can be addressable (i.e., the specific alarming 

cabinet can be identified immediately if each cabinet is linked to its own detector).

Each of these installations will have different success criteria and, again, there will also be 

differences in the operator actions required. The first type of installation is analyzed in 

detail in the remainder of the HRA discussion. At the end of this section (i.e., Sections 

10.6.4 and 10.6.5), some HRA-related notes are provided regarding de-energization strategies 

and area-wide applications, respectively.

10.3.1 Success criteria for in-cabinet, fire suppression strategy

Success criteria must be defined for each event tree heading, including identified HFEs. These 

criteria are:

• MCR response—After the detector “alert” signals in the MCR, MCR operators must 

dispatch both the field operator and technician to the correct location for the VEWFD 

system in “alert.” This dispatching must be performed in a timely manner.

• Field operator response—Once the detector “alert” signals, this must result in the field 

operator 1) arriving at the correct location for the VEWFD system in “alert,” 2) having the 

ability to initiate suppression in the absence of the fire brigade, and 3) positioning himself 

or herself, in a timely manner, in close proximity to the specific cabinets with the “alert”

condition. In addition, the field operator must provide continuous fire suppression 

capability while degrading conditions occur, up through transition to flaming conditions or 

until the degraded component is de-energized, repaired, or replaced.6

• Technician response—The technician must 1) arrive at the correct location for the 

VEWFD system in “alert” with the required equipment, 2) correctly and timely identify the 

specific cabinet where the degraded component is located (especially for cases in which 

there is more than one cabinet associated with a detector, and 3) correctly identify the 

degraded components.

 6 The phrase ‘continuous fire suppression capability’ is used here rather than the more familiar phrase of 

‘continuous fire watch’ because not all NPPs define ‘continuous fire watch’ in the same way.
10-7

There are a few important things to note:

(1) In Figure 6-4, the event tree heading for successful MCR response is “1-μ.” Conversely, 

Figure 6-4 uses “μ” to represent MCR operator failure.

(2) For the purposes of quantification (as discussed in both Section 6 and Section 12), the 

field operator and technician response represented together as a combined outcome.

(3) Successful, combined response of the field operator and technician is represented as 

“1-ξ” in Figure 6-4. Conversely, the combined failure of the field operator and technician 

is represented as “ξ.”

(4) According to Figure 9-1 (and the event tree shown in Figure 6-4), the overall objective 

of this VEWFD application is for the field operator to arrive at the “alert” location and 

provide fire suppression capability (earlier than would be provided by a fire brigade 

dispatch on “alarm”).7

Timing inputs are almost always an important part of the success criteria defined for each 

operator action. First, the operator actions must be feasible (i.e., sufficient time available as 

defined in NUREG-1921). Second, shortness of time to complete actions also can influence 

the reliability (therefore, the failure probability) of operator actions. If time is limited for the PRA 

scenario modeled, operator actions must be completed before a worsened state occurs 

(e.g., equipment or plant damage). Discussion of timing and timing inputs is provided, first, 

in the qualitative analysis (i.e., Section 10.4), then in the feasibility assessment given in 

Section 10.5. Section 10.6 discusses all factors (including timing) that are considered relevant 

to HRA quantification. Overall, the critical timing inputs for this analysis are:

(1) the total time available for operator actions measured as the time starting from the 

VEWFD “alert” to when the incipient stage ends and a fire begins (as discussed in 

Section 8)

(2) the total time required to complete all operator actions, ending with the field operator 

positioned and ready to perform fire suppression activities

10.3.2 HFEs for “fire suppression” strategy, in-cabinet installations

The tabletop analysis provided in Section 9 describes in detail the operator and technician 

actions that could be performed to provide earlier than traditional fire suppression capability (per 

definition by the fire PRA). These actions are, in turn, aggregated or collected logically, then 

represented as HFEs and, for this analysis, shown as top events in event trees.

Specifically, the operator activities for the in-cabinet, fire suppression VEWFD strategy that are 

represented in event tree headings (see Section 6.4) are:

1. MCR operator response to VEWFD system signals (“μ” defines failure)

2. field operator and I&C technician response (“ξ” defines failure)

 7 A different event tree would be required if a VEWFD application was not focused on fire suppression and 

preventing fire damage to targets outside the affected cabinet.
10-8

3. fire suppression

a. “Enhanced” fire suppression (for cases in which fire suppression capability is 

provided by the field operator (or other appropriately trained personnel) who is 

already at the fire location when the transition from low-energy 

(incipient) fire to a flaming fire condition occurs) (“π1” defines failure)

b. Conventional fire suppression (e.g., MCR operator response to VEWFD system 

signal fails, field operator/technician response fails) (“η” defines failure)

It is important to note that the suppression activities in the list above are not addressed 

explicitly by HRA. Instead, the failure probabilities for these activities are quantified using 

non-suppression curves as described in Section 11. Further details on the definitions of these 

two HFEs are given below.

10.3.2.1 HFE definition: MCR operator response (in-cabinet, fire suppression strategy)

Based on the tabletop analysis in Section 9, interviews with plant personnel from two NPPs with 

VEWFD installations, and the discussion of the qualitative analysis (in Section 10.4 below), the 

following are the key events and responsibilities for the MCR operator once the VEWFD system 

signals “alert” in MCR:

(1) VEWFD system signals “alert” in MCR.

(2) MCR operators immediately note “alert” and switch focus on this signal.8

(3) MCR operators find the appropriate alarm response procedure (ARP) for the VEWFD 

system “alert.”

(4) MCR operators consult main fire alarm control panel in MCR to identify which detector

is signaling and the associated location, including specific cabinet or cabinet bank.

(5) MCR operators dispatch field operator9 closest to the alerting detector, providing location 

information.

(6) MCR operators dispatch technician to alerting detector.

(7) MCR operators continue to monitor degrading conditions from the notification panel in 

the MCR and phone with the field operator with changing conditions.

(8) When VEWFD system signals “alarm,” MCR operators activate the fire brigade.

Alternative conditions (e.g., detector “alert” is not provided on the main fire alarm control 

panel, degrading conditions are displayed on a source other than a notification panel) can be 

addressed by HRA. However, the above combination of events, responsibilities, and conditions 

have been assessed in the HRA provided in this report to represent a realistic but fast and 

reliable MCR operator response.

 8 Recall that no PRA credit for VEWFD system installations if a reactor trip or other similar higher priority 

event has occurred.

9 Field operator is expected to have fire brigade training.
10-9

10.3.2.2 HFE definition: Field operator and technician response (in-cabinet, fire suppression 

strategy)

Based on the tabletop analysis in Section 9, interviews with plant personnel from two NPPs with 

VEWFD installations, and the discussion of the qualitative analysis (in Section 10.4 below), the

following are the key events and responsibilities for the field operator and technician response in 

the fire suppression strategy for in-cabinet VEWFD installations:

(1) Upon dispatch, the field operator expeditiously travels to the fire location provided by the 

MCR operator.

(2) Upon dispatch, the technician expeditiously collects required equipment and travels to 

the fire location provided by the MCR operator.

(3) Upon arrival at the fire location, the field operator takes up the responsibility of fire watch 

(with suppression capability). If there is only one cabinet served by the VEWFD system, 

the field operator can have his/her attention immediately focused on changing conditions 

with and within this cabinet (even while the technician performs his survey - see the next 

step). If there are multiple cabinets served by the detector, the field operator draws 

upon his/her incipient fire detector training to monitor potentially changing conditions 

(e.g., smelling off-gases, seeing smoke, monitoring any local incipient detector 

notification panels in the room, taking phone calls from MCR operators with updates 

on changing conditions).

(4) Upon arrival at the fire location, the technician begins use of special equipment to 

identify the affected cabinet. In addition, the technician also is alert to the smell of 

off-gases, seeing smoke, and so forth that would provide more immediate evidence of a 

degrading component. The technician will change his/her survey plan to address 

cabinets that have these indications. With the “alert” signal locked in, the field operator 

remains at the fire location after the technician arrives and during the technician’s 

survey.

(5) The technician identifies the affected cabinet, using hand-held “sniffer.”

(6) The technician identified the degraded component, using hand-held “sniffer.” (It 

is assumed that the cabinet door will need to be opened for the “sniffer” to find the 

degraded component, and that the door will be closed after the technician has finished 

his/her survey.)

(7) The field operator provides continuous fire suppression capability, remaining alert to 

changing conditions in the cabinet where the degraded component has been identified. 

Changing conditions are monitored in multiple ways. The indications from the incipient 

detector will continue to be monitored. The MCR operators will be monitoring the 

VEWFD system display panel and communicating with the field operator by phone with 

changing conditions, especially as the setpoint for “alarm” is approached. In addition, 

there may be a display at the “alert” location that provides useful information. Finally, 

the field operator will be visually monitoring the cabinet, using his/her sense of smell, 

and generally monitoring for pre-flaming fire effects.
10-10

In this analysis, the field operator is trained to perform the simultaneous visual monitoring and 

detector monitoring10 (as well as fire brigade trained), the multiple cabinet case can be treated 

similarly to the single cabinet case. In other words, the success of the field operator being 

prepared to provide fire suppression as soon as the incipient phase ends (and flaming 

conditions begin) does not depend on the identification of the affected cabinet by the technician 

(as long as there are a finite number of cabinets involved in the VEWFD installation).

Although the technician response is not necessary for success of the field operator, the 

identification of the affected cabinet and a degraded component is necessary in a practical 

sense. First, given the sensitivity of the incipient detectors, it is expected to be important to 

verify that the degraded component is within the cabinet (or bank of cabinets) served by the 

incipient detector, as opposed to somewhere else in the room, or even in a different room. 

From the standpoint of operational resources, the field operator is likely to be posted at the 

“alert” location to provide fire suppression capability only IF he is needed for this role. For 

example, if the technician’s survey reveals that the source of degradation is elsewhere, then the 

field operator can return to his/her other duties. Second, although not addressed in this HRA 

and associated event tree, NPP personnel are expected to try de-energizing and repairing the 

degraded component, after it is identified and if flaming conditions do not occur first.

Section 10.4, HRA quantification, discusses these end states further and Section 11

continues this discussion with respect to which non-suppression curves are appropriate.

10.4 Qualitative HRA

As is described in NUREG-1921 (Ref. 49), HRA qualitative analysis is a vital step in HRA that 

provides the foundation for all other HRA products (i.e., identified and defined HFEs, human 

error probabilities developed through application of HRA quantification tools). Also, qualitative 

HRA is performed throughout the analysis, ending only when final outcomes/output have been 

produced or claimed. Consequently, for this report, the qualitative analysis step is listed ahead 

of all other purely technical tasks.

As noted above, one of the ways that this analysis is different from other HRAs is that it has 

not been performed for a specific NPP installation of VEWFD. Instead, this analysis is for a 

“representative plant” that:

• to the extent possible, uses the “real-world” information that was collected as part of this 

project, coupled with a general understanding of NPP operations and operators

• is consistent with the operational practices that support a “good” or “best case” with 

respect to successful and reliable operator performance for a VEWFD application

• is based on the human factors analysis provided in Section 9 (especially, the “tabletop 

analysis” of the steps required for VEWFD system response and the descriptions of 

contextual factors that either are licensing requirements or are common to the at-power 

internal events Level 1 PRA context)

• is based on the typical needs of HRA quantification tools so far as required inputs and 

underlying assumptions

 10 If the field operator is NOT trained for such simultaneous monitoring, then “enhanced suppression” can be 

credited only AFTER the technician has identified the affected cabinet and degrading component. This 

situation is not addressed in this report.
10-11

Instead of collecting and interpreting plant-specific information through plant site visits and other 

interactions, this analysis has used, for example:

• a composite of information collected from three plants (e.g., a plant visit, NPP-supplied 

procedures, two plant surveys) for VEWFD applications

• general NPP information, such as conduct of operations (including protocols for 

communications), alert/alarm designs, alarm response procedures, control interfaces, 

fire brigade training 

• a limited number of assumptions to take the place of information that is not 

available generically and was not supplied by the NPPs either visited or surveyed

For the “representative plant” analyzed, the subsections below provide descriptions of the MCR 

operator response and the responses of the field operator and technician at the plant location 

where a VEWFD system has detected a degrading component. Then, timing information for 

each operator action (both time required and time available) is discussed. The two most 

essential HRA timing inputs are: (1) the time available for operator action and (2) the time 

required for operator action. A plant-specific HRA would require the development of similar 

information, but based on that NPP’s VEWFD application.

It should be noted that qualitative HRA described below for the “representative plant” is intended 

to represent a realistic, yet fast response, from MCR operators, as well as from field operators 

and technicians. As discussed in Section 8, the currently available timing information indicates 

that the time available for operator response (see Figure 8-4) can be quite short 

(i.e., 10 minutes or less), especially when compared to timing information regarding the time 

required for combined operator response, both in control room and ex-control room (see 

Section 10.4.3). Consequently, the qualitative analysis below also attempts to capture those 

factors that support a relatively fast operator response.

10.4.1 Qualitative analysis for all operator actions

General information on all operator responses is discussed below.

10.4.1.1 Qualitative analysis for MCR operator response

For the MCR operator response to in-cabinet installations of VEWFD, the keys to a fast 

response that can be addressed with existing HRA quantification tools are: (1) an overall plant 

philosophy that VEWFD response is “priority number one,” supporting a quick, consistent, 

reliable, and predictable response, (2) procedures and training that support quick and 

immediate response to incipient fire detector alerts/alarms, and (3) any factors that support 

a response closely resembling that for post-reactor trip MCR operator response in at-power, 

internal events Level 1 PRA.

The following information provided by NPPs with existing VEWFD installations is used to define 

the “representative plant” for the MCR operator response:

• “MCR operator response” represents the collective effort of the MCR operating crew.

• VEWFD system alert/alarm annunciator panel(s) is/are located in the MCR on the front 

panel (i.e., where operators are accustomed to seeing critical alarms). There is a main 
10-12

fire alarm annunciator panel11 in the MCR, providing MCR operators with quick and easy 

access to information on both the specific fire area and location of the VEWFD system 

zone in “alert” or “alarm” condition and the specific cabinet or bank of cabinets where the 

detector is installed.

• Operators respond to VEWFD system alerts and alarms with urgency (that may be 

inconsistent with other alarms that occur during normal operating conditions). This 

sense of urgency is reinforced by training and procedures, and might be aided by 

distinguishing tape or other markings for VEWFD system alarm panels. In particular, 

MCR operators are trained to “drop everything” when a VEWFD system signals.12

• Alarm response procedures (ARPs), which are part of the emergency operating 

procedures set, guide MCR response, including consultation of the main fire alarm 

control panel and calls to the field operators, technicians, and fire brigade.

• MCR operators expeditiously dispatch the field operator closest to the detector in “alert,”

providing essential location information.13

• MCR operators expeditiously dispatch a technician trained in portable ASD use, 

providing essential location information.

• MCR operators expeditiously dispatch the fire brigade when the detector is in “alarm”

state, providing essential location information.

• Nuisance alarms for the VEWFD system are minimal (e.g., through appropriate initial 

testing and appropriate set points), such that MCR operator response is not slowed or 

questioned to be correct. In addition, the results of investigation into the causes of 

nuisance alarms (by technician surveys or other means) should increase confidence 

in the sensitivity of the plant-installed and hand-held incipient detectors.

• Accessibility is not a concern (as the VEWFD system in “alert” or “alarm” is outside 

the MCR).

• MCR operators are trained and directed by procedural guidance (i.e., ARPs) to 

continually monitor detector indications and to communicate these conditions (especially 

changing conditions and conditions approaching the “alarm” setpoint) to the field 

operator at the “alert” location.

This analysis also uses the following assumptions that: (1) are typical of an at-power, internal 

events Level 1 HRA, and (2) are based on generic information and a general understanding of 

NPP operations and operators (e.g., policies and practices given in “conduct of operations”

 11 VEWFD system alarm control panels can be located in the MCR or in the plant. For one of the NPPs 

providing information for this project, a dedicated computer on the STA's desk provided all information 

collected by the VEWFD system. The authors have assumed an equivalent to this situation because: a) no 

additional time is required for a field operator to travel to a local fire alarm control panel and report back to 

the MCR, and b) the reliability of the MCR crew interpreting information from the main fire control alarm 

panel should be higher than that of a single field operator reading a local panel. 12 This specific response was provided by one of the licensees that provided input to this project.

13 Again, this is consistent with information provided by one of the NPPs consulted in this project. 
10-13

procedures, NRC requirements for MCR design and emergency operating procedures (EOPs), 

NRC requirements for operator licensing and training):

• VEWFD system alert/alarm signals are audible, according to other MCR alarm standards 

(see Section 9.2.2.2).

• The number of VEWFD system alert/alarms that require urgent response should be few 

as compared to other fire alarms, and measures are taken to avoid confusion of such 

alerts/alarms with other alarms that do not require such urgency.

• The instructions for VEWFD system response in the ARP are formatted and worded 

consistently with other instructions given in the ARPs.

• There is normal staffing, such that there is no shortage of manpower.

– MCR operators use formal, three-way communication to describe the fire 

location to both the field operator and technician to minimize the likelihood of 

miscommunication. Three-way communication also can facilitate recoveries of 

miscommunication (e.g., repeat back from field operator while looking at the main 

fire alarm control panel can serve as a check). Or, other operators in the MCR, 

who also are looking at the main fire alarm control panel, can serve as an 

independent check.

• Peer checking and other activities that support reliable response by the MCR operators 

are used.

Changes to any of the above inputs are likely to change the timing of operator responses (and, 

therefore, feasibility) and/or the reliability of operator responses. For example, if the main fire 

alarm control panel in the MCR, does not provide the specific fire area and location of the 

VEWFD system zone in “alert” or “alarm” condition, and the specific cabinet or bank of cabinets 

where the detector is installed, then additional time will be needed for the field operator to travel 

to the panel where this information is provided.

10.4.1.2 Qualitative analysis for field operator response

For the field operator response to in-cabinet installations of VEWFD, the keys to a fast response 

that can be addressed with existing HRA quantification tools are: (1) an overall plant philosophy 

that VEWFD response is “priority number one,” supporting a quick, consistent, reliable, and 

predictable, response, (2) fire brigade training that results in the fastest possible arrival of fire 

suppression capability, and (3) any factors that support a response closely resembling that for 

post-reactor trip, operator response for field or local actions in at-power, internal events Level 1 

PRA.

The following information provided by NPPs with existing VEWFD installations is used to define 

the “representative plant” for the field operator response:

• The field operator closest to the fire location is dispatched by the MCR operator.

• VEWFD system response is the highest priority job for the field operator (upon receiving 

dispatch from the MCR), such that no new activities will be started, and current, 
10-14

non-critical activities will be suspended.

• The field operator is trained to travel expeditiously to the low-energy (incipient) 

fire location.

• The field operator is trained to suppress fires (e.g., fire brigade training).

• The field operator is trained regarding incipient fire detectors and associated response. 

This training includes initiation of fire suppression activities only AFTER there are visible 

effects of a fire (e.g., a flame).

• Required equipment (e.g., portable fire extinguisher) is available and accessible.

• Accessibility is not a concern (because the field operator is fire brigade-trained).

This analysis also uses the following assumptions that: (1) are typical of an at-power, internal 

events Level 1 HRA, and (2) are based on generic information and a general understanding of 

NPP operations and operators (e.g., policies and practices given in “conduct of operations”

procedures, NRC requirements for MCR design and emergency operating procedures (EOPs), 

NRC requirements for operator licensing and training):

• Formal three-way communication is used between the MCR operator and field operator 

so that the likelihood of miscommunication is very small.

• There is normal staffing, such that there is no shortage of manpower.

• The time needed for the field operator to travel to the VEWFD installation in “alert” can 

be based on similar timing information that has been demonstrated for other analyses.

Three additional assumptions are important to this analysis:

(1) Based on his/her incipient fire detector training, the field operator actively assumes the 

role of on-site fire suppression capability immediately arrival at the “alert” location, and 

provides this fire suppression capability continuously until it is no longer needed. For 

example, the field operator will be alert to changes in degrading component conditions, 

both from changes in detector indications and from fire effects that he/she may be able 

to sense (e.g., see, smell). These responsibilities continue even while the technician is 

performing surveys with the hand-held “sniffer.”

(2) The door to the affected cabinet does not need to be open (or unlocked) for the field 

operator to identify flaming conditions that require fire suppression.

(3) Although the plant-installed incipient detector is expected to produce nuisance alarms14

(because of its sensitivity), the field operator is trained and procedurally driven to 

respond to any “alert” signal that is “locked in” as if the (potentially not-yet identified) 

degrading component can transition to flaming conditions within minutes from the start of 

the “alert” signal.

 14 One NPP’s experience is approximately one nuisance alarm per month.
10-15

These last three assumptions and the provision of incipient fire detectors and fire brigade 

training for all field operators15 are especially important. In particular, as discussed in the 

section on definition of HFEs and associated success criteria, the definition of operator 

“success” with respect to “enhanced” fire suppression is based on when plant personnel 

having fire suppression capability are in place for the VEWFD system in an “alert” or “alarm.”

10.4.1.3 Qualitative analysis for technician response

From the discussion in of the HFE definition for the field operator and technician response

(Section 10.3.2.2), it was identified that, for this HRA and conditions described in this report, 

the success of the field operator is not dependent on the success (or failure) of the technician.

However, analysis of the technician response is carried forward in this report because:

(1) Other analyses (with different conditions and assumptions) may need to include the 

technician’s response as an integral part of the HFE defined for the field operator and 

technician response.

(2) Different analyses that consider, for example, de-energization (or repair) would require 

analysis of the technician response (as the identification of the affected cabinet and 

degraded component are essential to such strategies).

(3) Identification of the affected cabinet and degraded component are important 

operationally (either to minimize the field operator’s time spent providing fire suppression 

capability when it is not needed or to support efforts to de-energize or repair the 

degraded component).

For the technician response to in-cabinet installations of VEWFD, the keys to a fast response 

that can be addressed with existing HRA quantification tools are: (1) an overall plant philosophy 

that VEWFD response is “priority number one,” supporting a quick, consistent, reliable, and 

predictable, response, and (2) any factors that support a response closely resembling that for 

post-reactor trip, operator response for field or local actions in at-power, internal events Level 1 

PRA. 

The following information provided by NPPs with existing VEWFD installations is used to define 

the “representative plant” for the technician response:

• VEWFD system response is the highest priority job for the technician (upon receiving 

dispatch from the MCR); such that no new activities will be started, and current, 

non-critical activities will be suspended. In addition, work activities that are interrupted 

by dispatch from the MCR will be secured quickly.

• The technician is trained to expeditiously collect necessary equipment and travel to the 

low-energy (incipient) fire location.

 15 Section 9.2.2.4 discusses variations between plants on what, if any, fire training is provided to field 

operators.
10-16

• If cabinets that require survey are locked, the technician will travel to the MCR to obtain 

keys for these cabinets (allowing the field operator to travel directly to the “alert” location.

• If cabinets that require survey are locked, the technician will unlock the cabinet and open 

the cabinet doors for the survey with the hand-held “sniffer.” When the technician 

completes his/her survey, cabinet doors will be closed and re-locked. Also, the 

technician will return the cabinet keys to the MCR.

• Required equipment is available and accessible.

• The technician is trained in using special equipment for identifying low-energy (incipient) 

fire conditions, including detection of both the relevant cabinet (if the VEWFD installation 

is for a bank of cabinets) and the degrading component in the cabinet.

• Although the technician’s main function is to identify the affected cabinet and degrading 

component with the hand-held “sniffer,” he/she also will be alert to any fire effects that 

can be detected by observation or smell.

• Accessibility is not a concern (because the field operator, who is fire brigade-trained, 

is responsible for operator response if the degraded conditions transition to a fire).

• Although nuisance alarms can be expected because of the sensitivity of the 

plant-installed equipment16, technician response to these alarms, and subsequent 

discovery of the root causes of such alarms (e.g., adjacent overheating equipment), 

provides an on-the-job mechanism for understanding the sensitivity of plant-installed and 

hand-held incipient detectors and general confidence in their effectiveness.

This analysis also uses the following assumptions that: (1) are typical of an at-power, internal 

events Level 1 HRA, and (2) are based on generic information and a general understanding of 

NPP operations:

• Formal three-way communication is used between the MCR operator and technician so 

that the likelihood of miscommunication is very small.

• There is normal staffing, such that there is no shortage of manpower.

• Overall, technician activities for VEWFD are similar to that for instrument calibration 

(which are often represented in traditional HRA/PRAs) with respect to “operator”

reliability (e.g., the technician is adequately trained).

Interactions with NPPs for this project indicated that there is limited experience and 

understanding of the technician activities for VEWFD applications in NPPs. Consequently, 

the final assumption above is likely to be the most crucial to this analysis.

10.4.2 Time available for operator actions

 16 As noted in the qualitative analysis for the field operator, one NPP’s experience is approximately one 

nuisance alarm per month.
10-17

HRA typically relies on inputs developed by and for other PRA tasks. While this research does 

not involve a formal PRA study, it follows the traditional path in that several inputs to the HRA 

are developed by other analysts. In particular, HRA/PRA requires determination of the “system 

time window,” or the time from the start of “event” to the operator action is no longer beneficial.17 

Another term used in HRA for “system time window” is “time available for operator response.” 

Figure 8-1 shows several timelines that are used in this report, including the time available for 

the combined operator response.

In typical PRAs, the “event” is a reactor trip and the system time window is determined with 

thermal-hydraulic calculations for when, for example, core damage occurs. The context for this

analysis is very different. In particular, the “plant damage” of significance is flaming conditions 

for a component(s) in the electrical cabinet(s) being monitored by VEWFD. Therefore, the 

component degradation and associated VEWFD response provide the basis for determining the 

system time window. Section 8 discusses the limited, appropriate data on the duration of the 

incipient phase (i.e., component degradation before flaming), while the first part of this report 

provides results of how various VEWFD respond to degrading components. Consequently, 

unlike traditional PRAs where plant-specific calculations are used to determine the system time 

window, this analysis, as well as any other similar analysis, must use the timing information 

developed in Section 8.18

10.4.2.1 How time available determination is different for VEWFD applications

In this analysis, system time window, or time available for operator action, is defined as the time 

from when an “alert” signal from the VEWFD system is displayed in the MCR (i.e., when the cue 

for action is received) until the incipient stage ends (e.g., flaming conditions begin). Section 8

discusses how the timing of the “alert” signal is dependent on the type of VEWFD system, as 

reported in Section 5 “Experimental Results.” Consequently, the time available for operator 

actions will be different, depending on which detector is being used. Figure 10-1 shows

three different timelines (fire, VEWFD, and conventional response) and how the time available 

is determined.

 17 NUREG-1921 defines the system time window in this way, drawing upon long-time HRA/PRA conventions. 18 The timing information in Section 8 might be updated by future data collection efforts on VEWFD response 

and the duration of the incipient phase for degrading components of various types. Refer to Section 12.3 for 

further discussion.
10-18

Figure 10-1. Timeline showing difference between time available and entire incipient 

phase.

Begin Degradation

Begin gassification/ 

 pyrolysis

Begin Smoldering 

 (if applicable)

Begin burning / 

 flames

Damage to target 

component

Tincipient phase

Tdamage

VEWFD alert,

 start MCR response

Begin Enhanced

 Suppression

Fire Suppressed

Tdet,VEWFD Time Available for Operator Response

Tsupp,VEWFD

Tdet,conventional

Tfb,conventional

Tsupp,conventional

Conventional Alarm

Begin Suppression

Fire Suppressed
10-19

Figure 8-4 (and the associated raw data given in Appendix D) shows how the time available for 

operator actions varies between the four types of fire detection technology tested and shown in 

the form of complementary cumulative probability distribution function (i.e., survivor function). 

These four distributions represent five fire detection technologies, include three VEWFD 

systems and two conventional fire detectors; respectively:

VEWFD systems

(1) air-aspirated VEWFD (cloud chamber)

(2) air-aspirated VEWFD (light-scattering)19

(3) sensitive spot-type VEWFD (light-scattering)

Conventional spot-type detectors

(4) spot detector (ionization)

(5) spot detector (photoelectric)

Typically, a single value for the system time window, or time available, is developed for defining 

the success criteria in HRA/PRA. Also, if timing ranges or distributions are developed, the 

shape of the distribution resembles either a normal distribution with small error factors (or 

another distribution with small “tails” at the low and high probability ends), justifying the use of a 

single value.

For this analysis, Figure 8-4 illustrates why a single value is not an appropriate way to represent 

the distribution of operating experience for component failures having an incipient phase. In 

particular, the existing data is sparse and this data is equally distributed in time between 

relatively short times available (e.g., less than 1 hour) and longer times available (i.e., longer 

than 1 hour).

In summary, the figures cited above provide several important insights for the HRA analyst:

(1) The input for the system time window is a probability distribution, rather than a single 

data point, providing an indication of the sparseness of data for the incipient phase and 

how it is distributed.

(2) The input for the system time window is different for different detectors.

In turn, there are several implications for this HRA:

(1) The HRA will need to address the system time window as a distribution, rather a single 

data point.

(2) Separate analyses will be required for each detector.

The probability distributions for the amount of time available for operator action (i.e., the system 

time window) shown time available is not necessarily very long (e.g., less than 1 hour), making 

all operator actions time critical.

 19 Note that the performance of the ASD VEWFD LS and the non-ASD VEWFD LS sensitive spot (SS) were 

very similar. As such, there response time characteristics were combined and represented as a single 

distribution for VEWFD light scattering type detectors.
10-20

10.4.2.2 Determining time available for operator response in VEWFD applications

To treat time available as a distribution, one approach occasionally used in PRA is to “sample”

the distribution, then to assign split fractions associated with the sample points. In 

quantification, human failure probabilities will be developed for the time available associated 

with each sample point and split fraction.

In this analysis, cumulative probability distributions are used to determine sample points. Figure 

10-2 shows such distributions for each of the four detectors investigated. These cumulative 

distributions are based on the same data and curves fit as the complimentary cumulative 

density functions are shown in Figure 8-4 and discussed further in Appendix D. In particular, 

both the probability of the incipient phase ending AND the sensitivity of the detector to gases 

emitted during the incipient phase are represented. The differences in the curves are related to 

the sensitivity in the detectors. The fact that a probability distribution is needed to show these 

differences is related to the uncertain nature of the data on the incipient phase. Two points to 

note that are illustrated by Figure 10-2 are that:

(1) Based on the test data presented in Section 5 and the smoke source materials tested, 

the PHOTO spot detector was generally not capable of detecting an incipient phase prior 

to transition to a flaming fire state, while the other detectors provide more advanced 

warning (see Figure 5-56).

(2) For even the earliest responding detector (i.e., ASD CC), the probability of the 

incipient stage transitioning to a flaming significant through and up to an hour.

Figure 10-2. Cumulative distribution function of time available for operator response by 

detection type
10-21

For this analysis, the selection of time available points was driven by the need for the following:

• representation of the “most likely” situation with respect to duration of the time available 

for operator response following an VEWFD system “alert” signal

• representation of the situations in which all variations of in-cabinet, VEWFD installations 

are feasible (i.e., the time available is equal to or less than the total time required for all 

operator responses)

• representation of the mean of the probability distribution and/or the approximately 1 hour 

time available duration (which represents more time available than time required for all 

cases of operator response)

Using both the time required information (developed in Section 10.4.3 below) and the 

cumulative probability distributions, the following cases were chosen with respect to the duration 

of the time available (from “alert”):

• the time from “alert” that corresponds with a 90 percent likelihood that the transition to a 

fire has NOT occurred

• 30 minutes after “alert” (from Section 10.4.3 below) is approximately the maximum 

amount of time for the field operator to be positioned for immediate fire suppression

• 1 hour

The cumulative probability distributions for each of the four detectors investigated is shown in 

Figure 10-2 (These cumulative distributions are based on the same data and curves fit as the 

complimentary cumulative density functions are shown in Figure 8-4.) In addition, Appendix D 

provides minute-by-minute cumulative probabilities for each of the detectors. This information 

from Appendix D was used to develop Table 10-1, Table 10-2, and
10-22

Table 10-3 that display the fraction of the cumulative probability represented by each of these 

time points for the ASD VEWFD cloud chamber, ionization and VEWFD light-scattering (both 

ASD and SS) detectors, respectively. These fractions, in turn, can be are used as split fractions 

for the outcome of the HFE for “First Level Field Response.” In Sections 10.6.1 and 10.6.2, 

human error probabilities are developed to be paired with each of these split fractions.

The four sample points selected for this analysis are as follows:

• Sample 1: Incipient stage ends in only 10 percent of cases

• Sample 2: Incipient stage ends by 30 minutes after “alert” for VEWFD, “alarm” for 

conventional ION

• Sample 3: Incipient stage ends after 30 minutes but before approximately1 hour

• Sample 4: Incipient stage ends after approximately 1 hour

Note that, with these selections, the cumulative probability is fixed for one sample point 

(i.e., time available for operator response ends in 10 percent of cases) but the time from alert is 

not. All other sample points are associated with certain times and, therefore, the cumulative 

probabilities will be different for different detectors.

Feasibility assessments for each detector type and associated sample points are shown in 

Section 10.5. For those human error probabilities for each of the four distribution sample points 

and their associated-split fractions are developed in Section 10.6.

Table 10-1. Fraction of Probability Distributions for ASD VEWFD, Cloud Chamber

Sample

Cumulative 

probability for 

incipient stage

ended

Time from alert Fraction of probability 

distribution (split fraction)

1 0.10 0-12 minutes* 0.10

2 0.23 >12 minutes AND 

< 30 minutes**

0.13

3 0.40 > 30 minutes AND 

< ~1 hour**

0.17

4 1.0 > ~ 1 hour** 0.60

* From the text above, this is the result from the cumulative distribution function associated with the definition 

of this sample point 

** From the text above, this is the definition of this sample point
10-23

Table 10-2. Fractions of Probability Distribution for Conventional Spot-Type, 

ION Detector

Sample

Cumulative 

probability for 

Incipient stage

ended

Time from alert Fraction of probability 

distribution (split fraction)

1 0.10 0-8 minutes* 0.10

2 0.33 > 8 minutes AND 

< 30 minutes**

0.23

3 0.55 > 30 minutes AND 

< ~1 hour**

0.22

4 1.0 > ~ 1 hour** 0.45

* From the text above, this is the result from the cumulative distribution function associated with the definition 

of this sample point 

** From the text above, this is the definition of this sample point

Table 10-3. Fractions of Probability Distribution for VEWFD, Light-Scattering Detector

Branch

Cumulative 

probability for 

Incipient stage

ended

Time from alert Fraction of probability 

distribution (split fraction)

1 0.10 0-6 minutes* 0.10

2 0.41 > 6 minutes AND 

< 30 minutes**

0.31

3 0.65 > 30 minutes AND 

< ~1 hour**

0.24

4 1.0 > ~ 1 hour** 0.35

* From the text above, this is the result from the cumulative distribution function associated with the definition 

of this sample point 

** From the text above, this is the definition of this sample point

10.4.3 Time required for all operator actions

Figure 10-3 expands upon the operator response timeline included in Figure 8-1, 

showing separate timelines for MCR operators, the field operator, and the technician.

For this analysis, time required for operator actions is defined to be the time from when the 

“alert” is received in the MCR until all operator actions, both those in the MCR and those taken 

by the field operator and technician locally, are expected to be completed.

HRA-specific timing inputs for a “representative plant” were developed from information 

collected through interviews of operations and fire protection personnel at NPPs that have 

installed VEWFD. To the extent possible, the NUREG-1921 (Ref. 49) guidance was used to 

develop of these inputs. For example, as recommended in Section 4.6.2 of NUREG-1921, 

inputs from the two NPPs have been used to develop a range of times, rather than a point 

estimate. (See Appendix C for details of the interviews and the development of these ranges 
10-24

which recognize variabilities in timing.) However, timing information from both NPPs for and 

technician response was not based on walkdowns or demonstrations.

HRA analysts who perform a similar analysis would need to collect their own plant-specific 

information for the time required for completing operator actions. 

For this analysis, time required for operator actions is defined to be the time from when the 

“alert” is received in the MCR until all operator actions, both those in the MCR and those taken 

by the field operator and technician locally, are expected to be completed. Estimates for time 

required for all of those operator actions were developed using information provided by NPPs 

with existing VEWFD installations. (See Appendix C for the details of the interviews and 

surveys used to developing this timing information.) In particular, a range of times is used to 

define the “representative plant” for each type of action. A plant-specific analysis should be 

based on similar plant-specific demonstrations and/or estimates of these required times.
10-25

Figure 10-3. Timelines for MCR operators, field operators, and technician for incipient 

detector response.

VEWFD Alert in MCR

Call to Field Operator

Call to Technician

Begin burning / 

 flames

Arrive at VEWFD

 Alert Location

Component Identified

Begin burning/flames

 and Enhanced 

 Suppression

Travel Time

Travel and 

get equipment

Time to Identify 

Cabinet

Component Identified

Call from MCR

Call from MCR

Arrive at VEWFD

 Alert Location

Cabinet Identified

Begin burning /

 flames

MCR Operators

Field OperatorTechnician

Time to Identify 

Component

 (ready to provide enhanced fire suppression)
10-26

Figure 10-3 shows individual timelines for the MCR operators, field operator, and technician. 

Important features of this figure are:

• Each of the three timelines represent the total time available for operator actions, 

starting with the “alert” signal at the far left and ending with “begin burning/flames” when 

fire suppression starts (and also the endpoint of the operator actions).

• t=0 for the MCR operator response starts with the “alert” signal, as shown by the leftmost 

start of all three timelines.

• Start times for field operator and technician responses start with a call from the MCR 

operators, which occurs at different times.

• For the field operator, another key time on the timeline is arrival at the “alert” location, at 

which time suppression capability is provided.

• The technician is always expected to arrive at the “alert” location after the field 

operator because:

– The MCR operators call the technician after the field operator

– In addition to travel time, the technician also must collect the hand-held detector 

equipment from its storage location (and may need to collect keys from the MCR)

• For the technician, other key features on the timeline are:

– time to identify the affected cabinet

– time to identify the degraded component

Table 10-4 below summarizes the HRA timing inputs developed. The ranges of times are either 

directly provided by plant personnel or are interpretations of more informal estimates (e.g., a 

“few” minutes can be translated into 3 or 4 minutes). Also, the durations of the field operator 

response and technician response shown in Table 10-4 represent the range of time estimates 

provided by both NPPs. For example, the duration of the field operator response (i.e., average 

time between when the field operator receives dispatch call to arrival at the appropriate location) 

was reported as 2 to 8 minutes for Plant X and 3 to 4 minutes for Plant Y, while Table 10-4 uses 

the broader range of times provided by Plant X for this analysis. Although the information 

sources for this timing data are limited, it was the only such information available when this 

analysis was performed and is adequate for this particular analysis.20

 20 For a plant-specific analysis, it is recommended that timing estimates be developed for each detector 

location. Such plant-specific and location-specific timing estimates may have a narrower range. However, it 

is still recommended that a range of timing estimates be developed, rather than a point estimate.
10-27

Table 10-4. Summary of Timing Inputs for Operator Actions after “Alert” Signal

Start of 

response

Who and Where? Action(s) required for 

success Time required (minutes)

Alert signal MCR operator; MCR

Detect signal, use alarm 

response procedures, 

identify location of 

detector, and call to 

dispatch field operator

1-2

Alert signal MCR operator; MCR Dispatch technician to 

detector location 1

Call from MCR Field operator in plant

Travel to location of 

VEWFD system in “alert”: 

standby as fire watch by 

cabinet(s)

2-8

Call from MCR Technician

Obtain necessary 

equipment and travel to 

location of VEWFD 

system in “alert”

5-11

Arrival at location Technician

Uses equipment to 

identify cabinet

1 cabinet: 0

3 cabinets: 5

6 cabinets: 10

10 cabinets: 15

Cabinet identified Technician

Uses equipment to 

identify degraded 

component in cabinet 3-4

Feasibility assessments (Section 10.5) and HRA quantification (Section 10.6) need to consider 

the appropriate combination of required times for the MCR operators, field operator, and 

technician. For this analysis, the total time required is for only the MCR operators and field 

operator actions. In particular, the total time required for these two actions starts from the “alert”

signal and ends when the field operator arrives at the “alert” location and provides fire

suppression capability.

The total time required can be calculated with the following:

ttotal time required = tMCR to field op + ttravel for field op

where:

tMCR to field op = total time from “alert” to ending the call to field operator (i.e., 1 to 2 minutes)

ttravel for field op = 2 to 8 minutes (per Table 10-4 above)

Consequently, 

ttotal time required = (1 to 2 minutes) + (2 to 8 minutes)

or,

ttotal time required = 3 to 10 minutes

Note that from the discussion above and that regarding success criteria in Section 10.3, this 

total time required applies to all cabinet configurations considered in this analysis (e.g., single 

cabinets or 3-, 6-, or 10-cabinet installations).
10-28

10.5 Feasibility Assessment

Section 4.3.4 of NUREG-1921 (Ref. 49), identifies the following criteria for determining the 

feasibility of an operator action:

• sufficient time

• sufficient manpower

• primary cues available and sufficient

• proceduralized and trained actions

• accessible location

• equipment and tools available and accessible

NUREG-1921 further states that if an operator action is not feasible, it should be assigned a 

failure probability of 1.0.

As a result of the assumptions made in Section 10.3, all of the above criteria are met for 

MCR operator response and field operator/technician response, except sufficient time.

To determine the feasibility of overall operator response to in-cabinet, VEWFD installations 

for the fire suppression strategy, the total time required for operator actions calculated in the 

previous section, Section 10.4.2 (i.e., 3 to 10 minutes) must be compared with the time available 

shown in Table 10-1,Table 10-2, Table 10-3 for the different detectors, using the four sample 

points selected for this analysis, i.e.,

• Sample 1: Incipient stage ends in only 10 percent of cases

• Sample 2: Incipient stage ends by 30 minutes after “alert”

• Sample 3: Incipient stage ends after 30 minutes but before approximately1 hour

• Sample 4: Incipient stage ends after approximately 1 hour

Tables Table 10-5, Table 10-6, and Table 10-7 show the feasibility assessment results for the 

cloud chamber, ION, and light-scattering detectors, respectively, which apply to all cabinet 

configurations considered in this analysis. These tables show that:

• All sample points are feasible for the cloud chamber.

• Sample points #1 and #2 are only partially feasible for the ION and light scattering 

detectors because the upper bound on the total time required (i.e., 10 minutes) is greater 

than the ranges for time available, i.e., 

o For the ION detector:

 Sample point #1: Upper bound of time available (i.e., 8 minutes) is less 

than 10 minutes

 Sample point #2: Lower bound of time available (i.e., 8 minutes) is less 

than 10 minutes
10-29

o For the light-scattering detector:

 Sample point #1: Upper bound of time available (i.e., 6 minutes) is less 

than 10 minutes

 Sample point #2: Lower bound of time available (i.e., 6 minutes) is less 

than 10 minutes

Table 10-5. Feasibility Assessment for ASD VEWFD, Cloud Chamber

Time 

required Sample Time available from 

alert Feasible?

3-10 minutes

1 0-12 minutes* Yes

2 >12 minutes AND 

< 30 minutes**

Yes

3 > 30 minutes AND 

< ~1 hour**

Yes

4 > ~ 1 hour** Yes

* From Section 10.4.2.1, this is the definition of this sample point.

** From Section 10.4.2.1, this is the result from the cumulative distribution function 

associated with the definition of this sample point

Table 10-6. Feasibility Assessment for Conventional Spot-Type, ION Detector

Time 

required Sample Time available from 

alert Feasible?

3-10 minutes

1 0-8 minutes* Partial

2 > 8 minutes AND 

< 30 minutes**

Partial

3 > 30 minutes AND 

< ~1 hour**

Yes

4 > ~ 1 hour** Yes

* From Section 10.4.2.1, this is the result from the cumulative distribution function 

associated with the definition of this sample point 

** From Section 10.4.2.1, this is the definition of this sample point

Table 10-7. Feasibility Assessment for VEWFD Light-Scattering Detector (ASD & Spot)

Time 

required Sample Time available from 

alert Feasible?

3-10 minutes

1 0-6 minutes* Partial

2 > 6 minutes AND 

< 30 minutes**

Partial

3 > 30 minutes AND 

< ~1 hour**

Yes

4 > ~ 1 hour** Yes

* From Section 10.4.2.1, this is the result from the cumulative distribution function 

associated with the definition of this sample point 

** From Section 10.4.2.1, this is the definition of this sample point
10-30

10.6 HRA Quantification

Because HRA quantification methods and associated human error probabilities (HEPs) were 

principally developed by and for post-initiator operator response, the overall strategy for 

quantification of the operator actions in this report is to either describe or prescribe operational 

conditions for the low-energy (incipient) fire response actions that are similar or parallel to the 

more familiar post-initiator operator actions. Section 10.4 (HRA Qualitative Analysis) is the 

starting point for the assumptions that will be used in HRA quantification to follow this strategy.

10.6.1 Basis for human error probabilities

Since the objective of the VEWFD installations for the scope of this analysis is to provide earlier 

than traditional fire suppression, by definition of a fire PRA, the context for operator response is 

very different than that typically modeled by HRA. Discussions given in Section 9 and earlier in 

this section have been aimed at describing such important differences.

However, since most HRA quantification methods, and their associated HEPs, are intended for 

post-reactor trip response, the strategy for identifying appropriate HEPs for this fire suppression 

analysis using VEWFD installation is to: (1) identify or define similarities in the characteristics of 

the two contexts, and (2) use identified similarities in context to justify similar HEPs. In addition, 

the authors have chosen to analyze a set of conditions, realistic yet representing fast operator 

responses, for HRA quantification, as illustrated below.

The HRA quantification approaches used in this comparison are not exhaustive. Methods 

included are:

1. technique for human error rate prediction (THERP) (Ref. 62) 

2. EPRI’s HRA Approach (Ref. 63), consisting of two methods addressing cognition:

a. cause-based decision tree method (CBDTM)

b. human cognitive reliability and operator reliability experiments (HCR/ORE)

method

3. standardized plant analysis risk human reliability analysis (SPAR-H) (Ref. 64)

4. ATHEANA (Ref. 58)

10.6.1.1 HEP basis for MCR operator response (in-cabinet, fire suppression)

Combining and building on the descriptions of MCR operator response given in the success 

criteria and the HF analysis, respectively, the principal activities required for MCR operator 

response for the in-cabinet VEWFD installation and the suppression strategy are:

1. detection of the VEWFD system alert (and, later, alarm), and

2. use of alarm response procedures (ARPs) to:

a. Identify the fire location and cabinet bank for VEWFD system in “alert,” and
10-31

b. Make phone calls to dispatch to fire location:

i. first (on alert), a field operator

ii. next (on alert), to an I&C technician

iii. monitor detector output for changing conditions

iv. notify field operator at “alert” location with important changes in detector 

output (e.g., detector output that is approaching the “alarm” set point)

v. finally (on alarm), to the fire brigade response

In the context of post-reactor trip for at-power, internal events PRA, alarm detection and use of 

procedures to guide response are typical activities for MCR operator response. In general, 

these MCR operator activities comprise the cognitive (or diagnostic) portion of overall operator 

response. Communication between operators in the crew is most typical, but occasionally, 

emergency operating procedures (EOPs) will require MCR operators to make a phone call to a 

dispatch field operator. Explicit concerns for such communications to personnel outside the 

MCR have been more thoroughly considered in the context of fire PRA (e.g., NUREG-1921).

In the context of post-reactor trip MCR operator actions, there are many design requirements 

and associated guidance for supporting such MCR operator actions (as discussed in Section 9). 

This analysis assumes that these same requirements are used to support the pre-trip, “fire 

suppression” MCR operator responses. As a result, failure modes associated with diagnosis 

or cognition are expected to be the dominant contributor to these MCR operator activities.

THERP is not appropriate to use directly for this analysis (and, often, generally, for current NPP 

designs) because it does not explicitly address cognition or diagnosis as most modern HRA 

methods do; and it was developed before the Three Mile Island 2 accident (Ref. 59) and the 

ensuing upgrades to NPP control room designs, operating procedures, operator licensing and 

training programs. In addition, THERP’s “annunciator response model” does not appropriately 

take into account the pattern-matching of annunciator tiles that modern NPP operators do when 

responding to an event. HEPs for THERP’s “annunciator response model” (shown in 

Table 20-23 of NUREG/CR-1278) range from 1×10-04 to 2.5×10-01.

The EPRI HRA approach includes two quantification methods that address cognitive failures. 

EPRI HCR/ORE method is a time reliability correlation and typically used when available time 

is relatively short. However, the HCR/ORE method should not be used for operator actions that 

are extremely well-practiced or skill-based, such as manual reactor trip after trip signals and 

alarms are received. The qualitative analysis in Section 10.4 is intended to describe a similar 

operator response. Consequently, for this reason (and because of the HCR/ORE method 

provides little insight on the potential causes of operator failure), EPRI’s CBDTM method is 

used to calculate the cognitive contribution to operator failure.

EPRI’s CBDTM model consists of eight decision trees, four of which address failures in the 

plant information-operator interface and another four that address failures in operator-procedure 

interface. Both sets of these decision trees match well with the MCR operator actions described 

above. The contextual factors assumed in Section 10.4 (as they relate to the VEWFD system 
10-32

alarm in the MCR, the ARP used, the interface used to identify the fire and cabinet bank 

location, and the phone calls made to the FO, I&C technician, and brigade) are used to apply 

the CBDTM decision trees.

Table 10-8 summarizes these assessments and shows the resulting HEP assignments. For 

instance, EPRI’s CBDTM assigns as “negligible” the contribution from all of the “data” decision 

trees, and all but one of the “procedure” decision trees. The results in Table 10-8 also show

that the contribution from “procedures” for this HFE is the lowest possible HEP (i.e., 1×10-03 in 

decision tree pc-e). If CBDTM recovery factors (e.g., self-recovery and other, or extra crew) are 

applied, this HEP can be reduced to an HEP in the 1×10-4 range.

A similar analysis using SPAR-H for at-power contexts and the qualitative analysis in 

Section 10.4 would result in the following assessments:

• Available time is nominal.

• Stress from stressors is nominal.

• With respect to complexity, it is an “obvious diagnosis” (i.e., better than nominal).

• Experience and training are nominal.

• Procedures are diagnostic/symptom-oriented (i.e., better than nominal).

• Ergonomics/HMI is good (i.e., better than nominal).

• Fitness for duty is nominal.

• Work processes are good (i.e., better than nominal).

Using the worksheets in Appendix A of NUREG/CR-6883 with the information above, an HEP of 

2×10-04 is obtained.

Table 10-8. MCR Operator Response (without recovery) Assessed with 

EPRI’s CBDTM HRA Method

Decision Tree 

Type

Decision Tree 

Branch

Decision Tree Branch 

Assessment

Associated 

HEP 

Assignment 

Plant Information 

(regarding 

VEWFD system

“alert” and 

“alarm”

Pc-a

(Data not 

available)

• is available in the MCR

• is accurate (e.g., no spurious 

signals or nuisance alarms)

• is trained upon

negligible

Pc-b

(Data not attended 

to)

• occurs during low workload 

conditions

• is a “check” (by virtue of this cue 

being an “alarm”)

• is on the MCR front panel

negligible

Pc-c

(Data misread or 

miscommunicated)

• is easy to locate

• is a “good” indicator with respect 

to HSI concerns

• is communicated to the crew 

using formal, 3-way 

communications

negligible

Pc-d

(Information 

missing)

• cue is “as stated” negligible
10-33

Table 10-8. MCR Operator Response (without recovery) Assessed with 

EPRI’s CBDTM HRA Method (Continued)

Decision Tree 

Type

Decision Tree 

Branch

Decision Tree Branch 

Assessment

Associated 

HEP 

Assignment 

Procedure Pc-e 

(Relevant step in 

procedure missed)

• is obvious (i.e., stand-alone step)

• is the only guidance being used at 

this time (i.e., no other procedures 

or activities are in play)

• is graphically distinct (e.g., typical 

HSI concerns with respect to 

procedure formatting have been 

addressed)

is followed using place keeping aids

1×10-3

Pc-f

(Information 

misleading)

• uses standard, unambiguous 

wording

contains all required information

negligible

Pc-g

(Error in 

interpreting logic)

• does not contain “NOT”

statements

• does not contain “and” or “or”

statements

does not contain both “and” and “or”

statements

negligible

Pc-h

(Deliberate 

violation)

• is relevant for a practiced scenario

is appropriate to the situation 

negligible

10.6.1.2 HEP basis for field operator and technician response (in-cabinet, fire suppression)

As has already been discussed, the principal activities for the field operator are as follows:

• Take call from MCR operator

• Travel to low-energy (incipient) fire location

• Actively monitor conditions at the “alert” location, including:

o monitoring output from incipient detector, by local panels present at the “alert”

location and/or by information provided by phone calls from MCR operators who 

are monitoring incipient detector signals in the MCR

o monitoring cabinets for any fire effects, such as visible signs of smoke or burning 

smells

• Stand at-ready to perform fire suppression21 at first sign of flaming conditions inside any 

of the cabinets covered by the detector in “alert.”

These activities are similar to those that might be modeled for an ex-control room action 

modeled in an at-power, post-reactor trip Level 1 PRA, including some time urgency. However, 

 21 This analysis is based on the expectation that fire suppression capability will need to be continuously 

present until either the incipient stage ends or the degraded component and/or cabinet is de-energized.
10-34

typically, such field operator activities also would involve some sort of equipment manipulation. 

In this case, however, the only “execution” modeled for the field operator is the success or 

failure of fire suppression (which is addressed in Section 11 with the assignment of appropriate 

non-suppression curves). Therefore, the field operator activities modeled in this HRA represent 

only part of what is typically modeled in HRA/PRA. Consequently, only the general HEP 

guidance given in THERP or ATHEANA associated with “unlikely” or “very unlikely” failure 

probabilities (i.e., 1×10-2 or 1×10-3, respectively) is appropriate.

Following the discussion given in Section 10.3.2.2 on the definition of this HFE for this analysis, 

the technician does not need to identify the affected cabinet or degrading component for the 

overall success of the combined field operator/technician response, provided the field operator 

is appropriately trained to perform the activities described above. However, for completeness 

(and in case a different analysis scope is addressed), the technician response is discussed 

here. So, for the technician, the principal activities are:

• Take call from MCR operator.

• Gather necessary equipment (including keys for locked cabinets, if necessary).

• Travel to low-energy (incipient) fire location.

• Use portable ASD equipment to identify affected cabinet (only needed for installations 

involving more than three cabinets).

These activities are very different than those usually modeled in at-power, post-reactor trip 

Level 1 HRA/PRA. Instead, these activities are similar to those modeled with pre-initiator HFEs 

(e.g., test or maintenance activities), except these actions may be performed with some 

urgency, and there is no opportunity to verify the actions. Consequently, an HEP (more) typical 

of THERP or ASEP (Ref. 65) is the best fit (e.g., 1×10-3).

10.6.2 Results: HFE quantification for in-cabinet, fire suppression strategy

This section presents the HRA quantification results for operator actions associated with the incabinet, VEWFD system installations (ASD VEWFD cloud chamber, conventional spot-type 

ionization, and VEWFD light-scattering detectors), using the fire suppression strategy. These 

results only apply to the analysis scope and operational factors that have been described in 

this report.

10.6.2.1 MCR operator response, μ

The HEP for the MCR operator response (μ), in all cases, is assigned as 1×10-04. The basis for 

this assignment is described below.

The state-of-practice in HRA currently is to treat each HFE, and its associated operator actions, 

holistically, rather than analyzing a breakdown of the various subtasks and individual influencing 

factors then “adding them up.” In particular, operator response in an existing U.S. NPP, in the 

MCR, following a reactor trip, and using the existing emergency operating procedures (EOPs), 

is understood to be supported by, for example (Note: this is not a complete or comprehensive 

list):
10-35

• a well-designed control room (including alarm panel placement and layout, 

clear indication of different trains, appropriate lighting and ventilation)

• effective training and certifications

• job aids (such as procedures that follow human factors guidance with respect to content, 

presentation and format)

• communication protocols

• a co-located crew and adequate and qualified staffing

• a strong safety culture

The combination of factors such as those given above may result in a certain expectation and 

reliability of operator performance. Because of differences between NPPs and even operator 

crews, there is no set combination of factors or features, or right or wrong implementation of job 

aids or designs except that they must be demonstrated (via operating experience, namely);

each NPP’s combination of job aids, influential factors and EOPs achieves the level of 

performance expected. If a plant design (e.g., size of steam generators) is such that faster 

response is required, then certain steps, training and procedures may be altered to compensate 

for or address this need.

Consequently, the “right” set of design features, job aids, and so forth cannot be definitively 

identified as it may come in a variety of forms. However, certain factors are expected to be 

important to successful operator response (since they underlie operator response for postreactor trip events and using EOPs). For the purposes of this analysis, the qualitative analysis 

described in Section 10.4 are taken to be adequate to describe a context in which the MCR 

operator response is similar to that for post-reactor trip response. Thus, under those conditions 

described and assumed above, MCR operators are expected to act quickly, and with a reliability 

consistent with alarm response and procedural use traditionally addressed by PRAs for postinitiator actions.

Overall, given the qualitative analysis results, and the implication that MCR operator response 

for the fire suppression strategy of in-cabinet VEWFD installations, should mirror that for alarm 

response in HRA:

• does not consider any failure modes associated with detection

• considers only potential failure modes for situation assessment or diagnosis (which 

would result either in delayed MCR operator response or difficulty in identifying the 

specific cabinet(s) associated with the VEWFD system signal, which is expected to

result in delayed MCR operator response)

• does not consider any failure modes associated with response planning

• considers only delays as potential modes associated with execution

Based on this discussion, an HEP for this HFE is justified in being similar to those HEPs 

developed in Section 10.6.1.1; namely, an HEP of 1×10-04 is assigned (which is independent 
10-36

of the number of cabinets per bank in the installation and type of VEWFD system) for failure of 

MCR response.

10.6.2.2 Field operator and technician response, ξ

As discussed earlier, the key to the success of this HFE for the field operator to arrive quickly at 

the “alert” location and immediately provide fire suppression capability. Consequently, the only 

contribution to the failure probability for this HFE is from the assessment of the field operator 

response.

The HRA quantification results for the field operator response, ε, are shown in Tables 10-9, 10-

10, and 10-11 for the cloud chamber, ION, and light-scattering detectors, respectively. The 

results for “total HEP” (along with the results of Section 11) are used to quantify the event trees 

shown in Section 6.4. (Note that these results, per this particular analysis, are for all cabinet 

configurations — single, 3-, 6-, or 10-cabinet installations.)

Table 10-9. HEP Calculations for ASD VEWFD, Cloud Chamber

Sample Time available from 

alert

Split Fraction 

from Table 10-1 Base HEP Base HEP x Split Fraction

1 0-12 minutes* 0.1 1×10-03 1.0×10-04

2 >12 minutes AND 

< 30 minutes**

0.13 1×10-03 1.3×10-04

3 > 30 minutes AND 

< ~1 hour**

0.17 1×10-03 1.7×10-04

4 > ~ 1 hour** 0.60 1×10-04 6.0×10-05

TOTAL HEP (ξ) 4.6×10-04

* From Section 10.4.2.1, this is the definition of this sample point

** From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point

Table 10-10. HEP Calculations for Conventional Spot-Type, ION Detector

Sample Time available from 

alert

Split Fraction 

from Table 10-2 Base HEP

Base HEP x 

Split Fraction

1 0-8 minutes* 0.10 0.10*** 1×10-02

2 > 8 minutes AND 

< 30 minutes**

0.23 3×10-02*** 6.9×10-03

3 > 30 minutes AND 

< ~1 hour**

0.22 1×10-03 2.2×10-04

4 > ~ 1 hour** 0.45 1×10-04 4.5×10-05

TOTAL HEP (ξ) 1.7×10-02

* From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point 

** From Section 10.4.2.1, this is the definition of this sample point

*** Partially feasible.
10-37

Table 10-11. HEP Calculations for VEWFD, Light Scattering Detector

Sample Time available from 

alert

Split Fraction 

from Table 10-3 Base HEP

Base HEP x 

Split Fraction

1 0-6 minutes* 0.10 0.30*** 3.0x10-02

2 > 6 minutes AND 

< 30 minutes**

0.31 5x10-02*** 1.6 x10-02

3 > 30 minutes AND 

< ~1 hour**

0.24 1x10-03 2.4x10-04

4 > ~ 1 hour** 0.35 1x10-04 3.5x10-05

TOTAL HEP (ξ) 4.6x10-02

* From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point 

** From Section 10.4.2.1, this is the definition of this sample point

*** Partially feasible.

As summarized in the timing input tables and feasibility assessments (Sections 10.4 and 10.5,

respectively), the assignment of HEPs for the field operator is more complicated than that for 

the MCR operator response because of variations in HRA inputs, namely:

• differences in the sensitivity of detectors, which results in different times available

• different “branches” that represent different times available (i.e., durations of the time 

available for operator response measured from “alert”), as a function of probability 

distributions

• feasibility assessments for each combination of timing inputs

Using the base HEP of 1×10-03 developed in Section 10.6.1, the following considerations were 

used to develop the “Base HEP” values shown in Tables 10-9, 10-10, and 10-11:

• If the overall operator response is not feasible, then an HEP of 1.0 is assigned. (Per 

Section 10.5, there are no cases where complete unfeasibility has been assessed.)

• If partial feasibility has been assessed (i.e., one of the endpoints in the range of times 

available for a particular sample point is less than the time required),the following HEP 

assignments are made:

• For the ION detector:

– The range of times available for Sample Point #1 (i.e., 0 to 8 minutes) is less 

than the range of times required (3 to 10 minutes) with a “gap” of 2 minutes. An 

HEP of 0.1 is assigned for this particular Sample Point.

– The range of times available for Sample Point #2 (i.e., 8 - 30 minutes) is less 

than the range of time required (3 -10 minutes). Although this “gap” (i.e., 2 

minutes) is the same as for Sample Point #1, the ratio of this time gap with the 

range of times available is smaller than that for Sample Point #1. Consequently, 

an HEP of 3.0E-2 is assigned for this case.
10-38

• For the light scattering detectors (i.e., ASD VEWFD and SS light scattering):

– The range of times available for sample point #1 (i.e., 0 - 6 minutes) is less than 

the range of time required (3 -10 minutes) with a “gap” of four minutes. An HEP 

of 0.3 is assigned for this particular Sample Point.

– The range of times available for Sample Point #2 (i.e., 6 - 30 minutes) is less 

than the range of time required (3 -10 minutes). Although this “gap” (i.e., 4 

minutes) is the same as for Sample Point #1, the ratio of this time gap with the 

range of times available is smaller than that for Sample Point #1. Consequently, 

an HEP of 5.0E-2 is assigned for this case.

• If the time available is more than the time required, then an HEP of 1×10-03 is assigned

(which is the recommended base HEP from Section 10.6.1).

• If the time available is significantly more than the time required (e.g., time available is 

greater than 1 hour), an HEP of 1×10-4 is assigned.

• An HEP floor of 1×10-4 is used.

In each table (for each detector type), the “Total HEP” is calculated by summing the results of all 

four sample points (i.e., the product of “Base HEP x Split Fraction” for each sample point).

10.6.3 Exploration of a variation in HRA quantification

From the discussion above, it is apparent that the HRA quantification results for the HFE 

representing the field operator response are extremely sensitive to the comparison between the 

time required for operator actions, and the time available from VEWFD system “alert” to when 

the incipient stage ends. For this reason, this section explores how a variation in HRA timing 

inputs would influence HRA quantification results. 

Three variations are explored:

1. Variations in the time required for field operator response,

2. Variations in the time available data for the cloud chamber:

a. under the most sensitive setting tested (i.e., 150,000 particles/cm3)

b. under the least sensitive setting tested (i.e., 9,000,000 particles/cm3) 

10.6.3.1 Variation in the time required

As discussed in Section10.4.3, the time required for the field operator to arrive at the location 

where the VEWFD system is in “alert” varies from 2 to 8 minutes. In addition, the answers to 

Question #14 in Section C.1.1 were:

• Plant X: varies but typically, 2 to 8 minutes

• Plant Y: 3 to 4 minutes
10-39

These variations in reported times could be explained by a variety of factors (principally related 

to the field operator travel time), including:

• plant-to-plant differences in layout of fire areas

• for a specific plant, differences in field operator travel times, depending on the specific 

location of the VEWFD system and where the field operator is located when the MCR 

operator calls

• lack of operational experience with this field operator action

The qualitative analysis chapter in NUREG-1921 (Ref. 49) provides guidance on the 

development of timing inputs for fire HRA, especially the discussions of sufficient time as 

a feasibility assessment criterion (see Section 4.3.4.1 in NUREG-1921) and of timing as 

a performance shaping factor (see Section 4.6.2 in NUREG-1921). This guidance, along with 

the possible explanations for variations given above, points to a corresponding selection of 

ways to explore variations in this timing input, such as:

1. The “Plant Y” data could be used as a plant-specific input (rather than a generic 

representation of this input), or

2. Uncertainty in the estimate of this timing input could have been reduced by either: 

a. collecting a more robust set of operational experience that reduces such 

variations, or 

b. establishing a job performance measure for this field operator action (or, even 

better, for the integrated response of the MCR operators and the field operator) 

that requires operator performance to meet certain targets (usually time required 

for performance.)

Despite uncertainties regarding the time required for field operator actions, this analysis 

explores the change in HRA quantification results if the time required for the field operator to 

arrive on location is assumed to be four minutes , plus an additional one minute for the MCR 

operator dispatch of the field operator. In other words, the total time required in this variation is 

5 minutes (rather than the 3 to 10 minutes used in the quantification shown in Section 10.6.2).

Table 10-12 shows the results of this variation (in which all sample points are assessed to be 

feasible). In particular, the total HEP for this variation case is almost a factor of 20 smaller than 

that shown in Table 10-10.
10-40

Table 10-12. Time Required Variation (4 minutes): Revised HEP Calculations for 

Conventional Spot-Type, ION Detector

Sample Time available from 

alert

Split Fraction 

from Table 10-2 Base HEP

Base HEP x 

Split Fraction

1 0-8 minutes* 0.10 1×10-03 1×10-04

2 > 8 minutes AND 

< 30 minutes** 0.23 1×10-03 2.3×10-04

3 > 30 minutes AND 

< ~1 hour** 0.22 1×10-03 2.2×10-04

4 > ~ 1 hour** 0.45 1×10-04 4.5×10-05

TOTAL HEP (ξ) 1×10-03

* From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point 

** From Section 10.4.2.1, this is the definition of this sample point

10.6.3.2 Variation in cloud chamber sensitivity – more sensitive

Section 5 discusses the various test cases for the cloud chamber and their results. Specifically, 

the most sensitive cloud chamber detector setting tested was 150,000 particles/cc.

The most important change from the HRA perspective is how the time available distribution 

changes with the detector on the most sensitive setting. Table 10-13 provides the results for 

this variation, showing, as expected, that there is more time available for the first sample point 

than was calculated in Section 10.6.2 (i.e., 0 to 15 minutes in the revised case versus 

0 to 12 minutes in Section 10.6.2). Also, the split fractions for each sample are changed, 

shifting an even larger bulk of the distribution to longer times.

The base HEPs multiplied for each split fraction/sample point are unchanged.

Although the time available ranges are significantly changed, the revised total HEP is not much 

changed from that calculated in Section 10.6.2 (i.e., 4.2×10-04 in the revised cases versus 

4.6×10-04 in Section 10.6.2).

Table 10-13. More Sensitive Detector Setting Variation: Revised HEP Calculation for 

ASD VEWFD, Cloud Chamber

Sample Time available from 

alert

Split Fraction 

from Section

B.4

Base HEP

Base HEP x Split 

Fraction

1 0-15 minutes* 0.10 1×10-03 1×10-04

2 >15 minutes AND 

< 30 minutes** 0.09 1×10-03 9×10-05

3 > 30 minutes AND 

< ~1 hour** 0.16 1×10-03 1.6×10-04

4 > ~ 1 hour** 0.65 1×10-043 6.5×10-05

TOTAL HEP (ξ) 4.2×10-04

* From Section 10.4.2.1, this is the definition of this sample point

** From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point
10-41

10.6.3.3 Variation in cloud chamber sensitivity – less sensitive

Section 5 also discusses the least sensitive cloud chamber detector setting tested (i.e., 

9,000,000 particles/cc.)

As in the variation with test results using the most sensitive setting, the most important change 

from the HRA perspective for the least sensitive test setting is changes to the time available 

range for the first sample point, and changes to the split fractions for all sample points.

Table 10-14 shows:

• the revised time available range for Sample Point #1

• the revised split fractions for all split fractions

• the same “Base HEP” assignments as in Section 10.6.1

• the revised HEP x split fraction calculations

• the revised total HEP

As for the more sensitive detector setting test results, the revised total HEP for this variation is 

not much changed from that calculated in Section 10.6.2 (i.e., 5.4×10-04 in the revised cases 

versus 4.6×10-04 in Section 10.6.2).

Table 10-14. Less sensitive detector setting variation: Revised HEP Calculation for ASD 

VEWFD, Cloud Chamber

Sample Time available from 

alert

Split Fraction 

from Section 

B.4

Base HEP

Base HEP x Split 

Fraction

1 0-9 minutes* 0.1 1×10-03 1×10-04

2 >9 minutes AND 

< 30 minutes** 0.19 1×10-03 1.9×10-04

3 > 30 minutes AND 

< ~1 hour** 0.20 1×10-03 2×10-04

4 > ~ 1 hour** 0.51 1×10-04 5.1×10-05

TOTAL HEP (ξ) 5.4×10-04

* From Section 10.4.2.1, this is the definition of this sample point

** From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point

10.6.4 HFE quantification notes for de-energization strategy

As stated earlier Section 10.1, the discussion given in this report is intended to be a guide for 

HRA analysts and reviewers. It is not intended to be a complete illustration of the detailed HRA 

that would be required for this application. The notes given in this section are even more 

simplified so far as HRA and its documentation, principally because de-energization strategies 

will be very plant-specific and no such details support this report. Consequently, the reader 

should view the discussion below as a general guide, but follow other guidance (e.g., NUREG1921 [Ref. 49], the ASME/ANS PRA Standard [Ref. 60]) for the level of detail in their plantspecific analysis.
10-42

10.6.4.1 General HRA Notes on De-energization Strategies

As described in the HF tabletop analysis (Section 9.2.1), in-cabinet installations of VEWFD

systems that use a de-energization strategy will involve additional operator actions and 

associated times, all of which must be performed before the incipient stage ends to credit fire 

prevention. Addressing both cabinet and component de-energization strategies, these 

additional actions include:

1. The technician must identify the degraded component first.

2. The field operator must communicate to the MCR operators what component is 

degraded.

3. MCR operators must: 

a. Determine if the degraded component can be de-energized.

b. Determine how (e.g., with what steps and by whom) the degraded component 

can be safely de-energized (and without reactor trip).

4. The degraded component must be de-energized by plant personnel, such as:

a. electrical engineers or electricians (who may assist in developing the 

de-energization strategy and in actually performing de-energization steps).

5. The field operator may assist in the de-energization.

Based on NPP interviews conducted for this project (see Appendix C), the complexity and 

associated amount of time needed to de-energize can vary widely (e.g., from minutes to a 

“research project” that takes days or more). 

Similarly, the determination of whether or not a degraded component can be de-energized is 

not always straightforward.

10.6.4.2 Recommended Approaches for De-Energization Strategies

Based on the general notes immediately above (especially regarding the amount of time 

required versus the time available), it is apparent that de-energization strategies would be most 

successful in terms of HRA/PRA risk reduction, if:

1. The de-energization strategy is pre-planned, including the decision to de-energize 

(whether cabinet or component) and specific instructions for what operator actions must 

be taken.

2. The amount of time needed to identify the affected cabinet, and degraded components 

within a cabinet is minimized.
10-43

3. The amount of time needed to de-energize the affected cabinet, and degraded 

components within a cabinet is minimized.

4. All times required for operator actions are demonstrated.

The sub-sections that immediately follow provide further discussion on how to address deenergization strategies with detailed HRA, using these recommendations in the following way:

1. The alarm response procedure (ARP) used to response to a VEWFD "alert" also 

contains the necessary instructions for de-energization. (Also, if de-energization is 

performed locally, instructions are available at that location.)

2. A single-cabinet installation (e.g., addressable cabinet) is assumed in this 

discussion.

3. A cloud chamber VEWFD installation is assumed, providing the most time available 

(see Figure 10-2).

4. Only those de-energization strategies that are very simple to carry out are 

considered (e.g., only 1 to 3 simple and quick steps).

5. Only MCR operators, the field operator, and technician are required to perform all 

actions; no additional personnel are needed.

6. All actions are proceduralized and trained such that both speed and high reliability 

can be expected in task performance.

10.6.4.3 Issue / Scope

The issue and scope described in Section 10.2 are adjusted to represent the three different deenergization strategies considered, namely:

• Focused on in-cabinet VEWFD installations that are addressable devices22 (i.e., unique 

“alert” signals) for each cabinet

• Intended to support quantification of cabinet or component de-energization 

• Allows credit fire suppression if de-energization fails

• Assumes that the MCR operators, field operator, and technician (rather than other plant 

personnel) can perform all actions needed to support the de-energization strategy and 

that these actions can be performed either at the “alert” location or in the MCR

 22 “addressable device” is defined in Section 16, “Definitions”
10-44

10.6.4.4 Success Criteria

Success criteria must be defined for each event tree heading (such as those shown in Figure 

6-6), including identified HFEs. These criteria, which are similar but not the same as those 

stated in Section 10.3.1, are:

• MCR response:23 After the detector “alert” signals in the MCR, MCR operators must 

dispatch both the field operator and technician to the correct location, including the 

specific cabinet, for the VEWFD system in “alert.” This dispatching must be performed 

in a timely manner.

o Local de-energization: In addition to the actions above, the MCR operators 

must direct the field operator to perform actions for de-energization immediately 

upon arriving at the correct location (i.e., cabinet).

o In-MCR de-energization: A board operator is assigned the task of performing 

de-energization steps expeditiously from the MCR.

• Field operator/Technician response: 

o Cabinet de-energization: Once the detector “alert” signals, field operator must:

1) arrive at the correct location, including the specific cabinet, for the 

VEWFD system in “alert,” 

2) confirm that the VEWFD "alert" is valid locally, 

3) perform necessary actions for de-energizing the cabinet in a timely 

manner, and 

4) provide continuous fire suppression capability while degrading conditions 

occur, in case there is a transition to flaming conditions before deenergization actions are complete.

o Component de-energization: 

Once the detector “alert” signals, the field operator must:

1) arrive at the correct location, including the specific cabinet, for the 

VEWFD system in “alert,” and

2) confirm that the VEWFD "alert" is valid locally.

 23 The decision to de-energize is not addressed here because it is assumed that pre-planning addresses this 

decision, per the recommendations in Section 6.4.2 which are the basis of these illustrative examples. If the 

decision to de-energize is NOT pre-planned, then procedures, training, and cues must be provided to 

support decision-making. Also, the time required for MCR operator response must be increased to account 

for the additional time required to make the decision.
10-45

The technician must:

3) arrive at the correct location,24 and

4) locate the degraded component (using, for example, a “sniffer” tool, visual

inspection, or thermography).

Then the field operator must:

5) perform necessary actions for de-energizing the component in a timely 

manner, and 

6) provide continuous fire suppression capability while degrading conditions 

occur, in case there is a transition to flaming conditions before deenergization actions are complete.

10.6.4.5 Identify and define HFEs

Two HFEs can be used to represent the required operator actions for de-energization 

strategies, as has been done throughout Section 10. The HFE definitions for this deenergization strategy are somewhat different than those given in Section 10.3.2. The revised 

and new aspects of both HFEs are:

1. HFE definition: MCR operator response (in-cabinet, addressable cabinet deenergization strategies)

a. MCR operators find the appropriate alarm response procedure (ARP) for the 

VEWFD system “alert,” and the specific cabinet. The ARP provides clear 

indication that this cabinet (or any degraded component in the cabinet) is to be 

de-energized when the VEWFDS is in “alert.”

b. MCR operators consult main fire alarm control panel in MCR to verify which 

detector is signaling and the associated location, including specific cabinet.

c. MCR operators:

• Local de-energization: i) dispatch field operator closest to the alerting 

detector, providing both location and cabinet information, ii) direct the field 

operator to de-energize the identified cabinet or degraded component, iii) 

dispatch the technician to the location of the cabinet in "alert."

• In-MCR de-energization: i) direct a board operator to de-energize the 

identified cabinet, ii) in parallel, dispatch field operator2 closest to the 

alerting detector, providing both location and cabinet information, iii) 

dispatch the technician to the location of the cabinet in "alert."

 24 The time required for the technician to arrive at the location must include the time required to retrieve the 

required equipment to identify the degraded component in the component de-energization case (e.g. the 

“sniffer” tool, thermography equipment, etc.).
10-46

d. Local de-energization: Depending on the field operator’s success/failure, MCR 

operators:

• receive verification from the field operator that the cabinet is de-energized, 

• receive verification from the field operator that a fire was suppressed, or 

• activate the fire brigade when VEWFD system signals “alarm,”

2. HFE definition: Field operator and technician response (in-cabinet, addressable cabinet 

de-energization strategies)

a. Upon arrival at the fire location:

• Local cabinet de-energization- The field operator: i) locates the cabinet in 

“alert” and any necessary procedure guidance for de-energizing the 

cabinet, and ii) then performs de-energization steps expeditiously.

• Local component de-energization - i) The field operator locates the cabinet 

in “alert” and any necessary procedure guidance for de-energizing 

degraded components in the cabinet, ii) the technician performs necessary 

actions to identify the degraded component within the affected cabinet (for 

example, using “sniffer” tool, visual inspection, thermography), iii) then the 

field operator performs de-energization steps expeditiously.

b. The field operator takes the responsibility of fire watch (with suppression 

capability. [both in-MCR and local de-energization]

c. The field operator calls the MCR when the cabinet or component is de-energized 

(or fire is put out, if transition to flaming occur before de-energization steps have 

been completed.) [local de-energization only]

10.6.4.6 Qualitative HRA

All of the information and assumptions presented in Section 10.4 still applies for this 

de-energization strategy. However, there are some key, new assumptions for the MCR and 

field operators, including:

• MCR operator is trained on ARPs, especially if different responses are required for 

VEWFD system in “alert” (i.e., some, but not all, cabinets have instructions for deenergization)

• De-energization steps have been pre-planned and documented in appropriate 

procedures

• field operator is trained on both de-energization steps and fire suppression such that 

“high reliability” is expected for these actions
10-47

• the cabinet being de-energized is not locked (or there are other provisions made so that 

there is no additional time required to obtain keys) 

• there are few (e.g., 1-3) steps required for de-energization and these steps are either 

memorized, skill-of-the-craft, or posted at the “alert” location (such that no additional time 

is required to find procedure binders and locate necessary procedure)

• de-energization steps require only 1-2 minutes to complete

In addition, while the time available has not changed for the VEWFD system – cloud chamber, 

there is a minor adjustment to time required for operator actions (as shown in Table 10-4), 

namely:

• MCR operators:

o All de-energization strategies (cabinet / component): Additional time is needed to 

review ARP instructions for de-energizing cabinet or component and 

communicating these instructions to the field operator. For illustrative purposes, 

a time required of two minutes (i.e., the maximum time required shown in Table 

10-4) is used in the examples below.

o In-MCR de-energization strategy: For the examples below, the time required for 

de-energizing is assumed to be 1-2 minutes in addition to the two minutes 

needed to review the ARP and dispatch the field operator/technician. 

• Field operator/technician:

o Local cabinet de-energization: The time required for the field operator is adjusted 

to account for additional time needed to perform de-energization (i.e., 1-2 

minutes for de-energizing added to 2-8 minutes required for the field operator's 

travel time), resulting in 3-10 minutes time required.

o Component de-energization: The time required is adjusted to account for 

additional time associated with locating the degraded component AND 

performing de-energization, in addition to the technician's travel time to the 

location. From the notes above and Table 10-4, the technician requires 5-11 

minutes to arrive at the location and 3-4 minutes to identify the degraded 

component in an affected cabinet, resulting 8-15 minutes required for the 

technician plus 1-2 minutes for the field operator to de-energize the degraded 

component, resulting in 9-17 minutes of time required for both the field operator 

and technician.

Consequently, the total time required (i.e., sum of time required for MCR operators and field 

operator/technician) for each de-energization strategy is:

• Local cabinet de-energization: 5-12 minutes 

• In-MCR cabinet de-energization: 3-4 minutes

• Local component de-energization: 11-19 minutes
10-48

This example is not intended to be a complete illustration of the detailed HRA that would be 

required for this application. The timing assumptions given above are for a simple-optimized 

scenario, principally because de-energization strategies will be very plant-specific and no such 

details support this illustrative example.

10.6.4.7 Feasibility Assessment

All of the same assumptions made in Section 10.5 are assumed to be valid for these illustrative 

de-energization examples.25 In addition, the feasibility assessments for these illustrative 

examples (using the cloud chamber VEWFDS to determine the time available) will be similar to 

that shown in Table 10.5. However, the total time required for MCR operator and field 

operator/technician actions has changed for the de-energization strategies, resulting in the 

following feasibility assessments:

• Local cabinet de-energization: All cases remain feasible (however, only barely so for the 

first sample point of the time available probability distribution).

• In-MCR cabinet de-energization: All cases remain feasible.

• Local component de-energization: 

o Sample point #1: Not feasible

o Sample point #2: Partially feasible

o Sample point #3: Feasible

o Sample point #4: Feasible

10.6.4.8 HRA Quantification

The discussion provided in Section 10.6.1 and 10.6.2 still applies to these illustrative examples. 

However, adjustments to the analysis and associated HEPs need to be made in order to 

address the specifics of each de-energization strategy (all for an addressable cabinet). Each 

de-energization strategy is considered separately in the sections below.

10.6.4.8.1 HRA Quantification for Local, Cabinet De-energization

The following adjustments are needed for the local, cabinet de-energization strategy:

• MCR operator response – It is assumed that the same assessments and assumptions 

made previously still apply. However, to account for the additional ARP instructions and 

communication with the field operator, an HEP of 2E-4 (instead of 1E-4) is used.

• Field operator response – Again, the same assessments and assumptions made 

previous still apply. In this case, the HFE for the field operator represents actual 

equipment manipulation, i.e., an execution task. There are variety of unknown potential 

 25 For these illustrative de-energization examples, the same partitioning of the cumulative distribution function 

curve for time available was used. However, if the time required for a de-energization strategy was 1.5 

hours (e.g., a time required greater than the lower limit of the fourth bin used in previous example) then it 

would be appropriate to establish additional partitions (and associated sample points) in the time available 

distribution (e.g., two additional sample points corresponding with time frames of: 1) greater than1 hour to 

less than 2 hours, and 2) greater than 2 hours.
10-49

influencing factors (that would be known for the HRA analyst in a plant-specific analysis), 

such as environmental factors (e.g., lighting), ergonomics (e.g., layout of items to be 

manipulated), and potential offsetting factors for the short time available for operator 

actions, that would be important inputs to determining the appropriate HEP. As noted 

above, for the first sample point of the time available probability distribution, there is only 

just enough time available to perform required actions. The scoping HRA quantification 

approach in NUREG-1921 and SPAR-H address concerns about adequate time for 

performing operator actions. EPRI's HCR/ORE HRA method, a time-reliability 

correlation, is not likely to be appropriate in this case since its estimates can be too high 

for high-practiced and memorized actions taken in short time frames. For these 

reasons, no attempt is made here to formally assign an HEP to this modified HFE. 

However, for illustrative purposes, the inputs used in Table 10-9 are modified to produce

Table 10-15. The base HEP of 5E-2 shown in Table 10-15 represents a THERP-like 

HEP for multiple, possible failure modes (e.g., errors of omission and commission), plus 

an adjustment for barely enough time. As a result, the overall HEP for the field operator 

de-energization (ξde-ss) shown in Table 10-15 (i.e., 6.4x10-03) is larger than that shown in 

Table 10-9 (i.e., 4.6x10-04) by a factor of 10. These HEPs can be used in an event tree, 

such as that shown in Figure 6-6, for failure of the MCR operator actions and of 

"prevention," respectively.

Table 10-15. HEP Calculations for Cloud Chamber - Local Cabinet De-energization

Sample Time available from 

alert

Split Fraction 

from Table 10-1 Base HEP Base HEP x Split Fraction

1 0-12 minutes* 0.1 5x10-02 5.0x10-03

2 >12 minutes AND <30 

minutes**

0.13 5x10-03 6.5x10-04

3 > 30 minutes AND < ~1 

hour**

0.17 1x10-03 1.7x10-04

4 > ~ 1 hour** 0.60 1x10-03 6.0x10-04

TOTAL HEP (ξde-ss) 6.4 x10-03

* From Section 10.4.2.1, this is the definition of this sample point

** From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point

10.6.4.8.2 HRA Quantification for In-MCR, Cabinet De-Energization

This case is similar to that discussed immediately above except:

• MCR operators perform de-energization steps in the MCR

• Specifically, it is assumed that, for example, a board operator would be directed to 

perform the de-energization steps while either the shift supervisor or shift technical 

advisor would dispatch the field operator to "alert" location to provide fire suppression 

capability.

• It is assumed that operator performance influencing factors (e.g., ergonomic design with 

respect to possible errors of commission) in the MCR are more favorable than they 

would be at the cabinet with the "alert" condition.
10-50

In this case, the total time required for de-energization involves only MCR operator actions, i.e., 

1-2 minutes for MCR operator response to the "alert" plus 1-2 minutes to perform deenergization steps. Consequently, not only would this set of be feasible, but a more than 

adequate amount of time is available for operator actions. 

Based on the discussion immediately above and that for the context of de-energization locally, a 

base HEP of 1.0x10-03 is assigned for de-energization steps only in the MCR.

In order to use the event tree in Figure 6-6, assigned HEPs could be used as follows:

• the failure probability for MCR operator actions developed in Section 10.6.4.2 should be 

used in the same way for this case

• the failure probability for de-energization should include both the failure probability of 

MCR de-energization (i.e., HEP of 1.0x10-03) and field operator positioning for fire 

suppression (i.e., HEP of 4.6x10-04) 

Consequently, the overall HEP for de-energization (ξde-ss) in this case is 1.4x10-03.

10.6.4.8.3 HRA Quantification for Local, Component De-energization

This case is similar the strategy for local cabinet de-energization, but more complicated due to 

the timeframes in which this strategy is not feasible. 

Adjustments include:

• MRC operator response: The HEP assigned for the MCR operator response is the 

same as for the local cabinet de-energization (i.e., 2.0x10-04). 

• Field operator/technician response:

o Field operator response: The considerations for this strategy are identical to that 

for the local cabinet de-energization strategy, except that there are sample points 

in the time available probability distribution where de-energization is either not 

feasible or partially feasible. An HEP of 1.0 is assigned for the infeasible case. 

The HEP assignment used in Table 10-10 was used as the basis for the partially 

feasible case. Table 10-15 was used as a guide to assign the remaining HEPs.

o Technician response: Although considerable effort (including reviews and 

walkdowns of operational events) has been devoted to understanding the 

process involved in the technician identifying a degraded component within an 

affected cabinet, the failure modes (and reasons for success) for this activity are 

still not well understood. In principle, the identification of a degraded component 

should be successful, given adequate time available. This assumption is used in 

this illustrative example, but is recommended to be further supported in an actual 

plant-specific application.
10-51

Table 10-16. HEP Calculations for Cloud Chamber - Local Component De-energization

Sample Time available from 

alert

Split Fraction 

from Table 10-1 Base HEP Base HEP x Split Fraction

1 0-12 minutes* 0.1 1.0*** 0.1

2 >12 minutes AND <30 

minutes**

0.13 0.1  1.3E-2

3 > 30 minutes AND < ~1 

hour** 

0.17 5x10-03 8.5x10-04

4 > ~ 1 hour** 0.60 1x10-03 6.0x10-04

TOTAL HEP (ξde-ss) 0.14

* From Section 10.4.2.1, this is the definition of this sample point

** From Section 10.4.2.1, this is the result from the cumulative distribution function associated 

with the definition of this sample point

*** Not feasible.

**** Partially feasible.

10.6.5 HFE quantification notes for area-wide applications

HRA for area-wide applications of VEWFD would be similar to that for the in-cabinet, 

de-energization strategy, except that even more time will be needed for field operator and 

technician actions. A value of 1.0 was used to represent the HEP failure based on area-wide

applications. That is, the timing information available for detecting fires before flaming 

conditions will vary widely on a room by room basis. To inform HEP’s for area-wide application 

analysts should perform timing feasibility studies which take into account room factors such as; 

area (i.e. size, ceiling height), contents of room (i.e. number of cabinets protected, alternative 

fire sources), accessibility of room (i.e. ease of movement within the room, security, locked 

doors, radiation areas etc.).

10.7 HRA Dependency and Recovery Analysis

Dependencies between HFEs addressed in this analysis have been treated directly through the 

event tree representation.

Self- or within-crew recoveries have been addressed in the development of base HEPs (see 

Section 10.6.1). Recoveries, as traditionally defined by PRA, are only addressed with respect to 

fire suppression capability (i.e., “enhanced” versus “conventional”).

10.8 HRA Uncertainty Analysis

The uncertainty analysis guidance provided in Section 6.3 of NUREG-1921 (Ref. 49) is 

appropriate for this study. However, because of the limitations in the information inputs for this 

project, new uncertainty sources could not be identified (and uncertainty sources identified in 

NUREG-1921 are assumed to be applicable). In particular, uncertainty in timing estimates are 

expected to be especially important to this analysis.

11-1

11. FIRE SUPPRESSION

The objective of this section is to describe how the suppression events are modeled. Enhanced 

and conventional detection/suppression modeling is presented. The modeling of both events 

are adopted and modified as needed from the current suppression and detection analysis 

approach described in NUREG/CR-6850 and supplemental documents such as Supplement 1

to NUREG/CR-6850 and NUREG-2169. Reliance on current models allows for any future 

updates or advancements to be incorporated into this analysis. In other words, the suppression 

modeling presented in this section relies on current state-of-the-art methods and this 

dependence allows for the suppression portion of the event tree presented in Section 6.4 to be 

changed, as the state-of-the-art methods for suppression and detection analysis is advanced.

The current approach takes credit for the fire suppression capability. This approach assumes 

appropriately trained personnel remain in place until the problem has been resolved. Success 

in this approach is ultimately judged based on the ability to suppress the fire rather than prevent 

it. So long as the fire is prevented from growing significantly, the adverse consequences related 

to a large cabinet fire, and the associated fire growth because of secondary combustibles, are 

prevented. Estimation of both the enhanced and conventional non-suppression probabilities 

necessitates the estimation of the manual suppression time. This quantity is estimated as 

described in Supplement 1 to NUREG/CR-6850 (see Section 14 Manual Non-suppression 

probability). Mathematically, the manual suppression time is estimated as:

tt = tt dddddddd − tt

where: tms : time for manual suppression

tdamage : time to target damage

tdet : time to detection

The time to damage (tdamage) is based on scenarios specific information and fire modeling used 

to estimate the time of target damage. For quantification of VEWFD system performance, the 

time to detection (tdet) should be assumed to be zero minutes. No time should be added to the 

time to target damage (tdamage) or subtracted from the time to detection (tdet) to represent the 

advanced warning provided by the VEWFD system (detection during the incipient stage). The 

event tree parameters “α” and “ξ” along with the use of different manual non-suppression 

probability curves, as explained below, support quantification of the performance of the VEWFD 

system. Adjusting the manual suppression time to account for an incipient stage time would be 

non-conservative and inconsistent with the method described in this report. Thus, as stated in 

NUREG/CR-6850, no incipient stage duration is included in the time to damage estimate due to 

its uncertainty in duration and that it is not expected to generate thermal conditions that threaten 

the integrity of other targets in the room (emphasis added).

If a licensee desires to obtain more credit in this process, a more detailed evaluation of deenergization strategies, including adequate and appropriate justification in the form of a detailed 

human reliability analysis must be performed. One way a licensee could achieve additional 

benefit would be to “pre-locate” the isolation devices for all ignition sources within each cabinet 

in an effort to speed up the process. This would need to include predetermining the isolation 

devices, conveniently displaying that information for use in response to VEWFD system alerts, 

training responders so that they could rapidly locate and operate the isolation device(s), and 

conducting drills to periodically demonstrate this ability. Given the scenario specific details that 
11-2

would need to be known to conduct such an analysis, the de-energization approach is beyond 

the scope of this study.

11.1 Enhanced Fire Suppression

Enhanced Fire Suppression is credited following the success path of the following 

branch points:

(1) very early warning fire detection (VEWFD) system availability, reliability, and

effectiveness (β)

(2) fraction of fires that have an incipient stage of sufficient duration (α)

(3) system effective at detecting incipient stage (τ)

(4) main control room (MCR) response (μ)

(5) field operator and technician response (ξ)

The “π” factor in the event tree represents the enhanced suppression probability. The “π” factor 

differs between the two event trees (in-cabinet – π1; area-wide – π2).

The “π1” factor is applicable for the in-cabinet event tree (see Figure 6-4) and represents the 

probability that, given success of the technician/field operator to respond to the VEWFD “alert,”

suppression has failed to limit the fire damage to the enclosure of origin. The field operator in 

the area of the cabinet responsible for the VEWFD system alert fails to promptly suppress the 

fire quickly enough to prevent damage to PRA targets outside the cabinet. The MCR curve 

(λ=0.324) should be used for this case. This is considered to be reasonable representation 

given that the field operator, a trained responder, will be near the bank of cabinets where the 

VEWFD system alert was initiated, actively searching for the source location of the alert.

The “π2” factor is applicable for the area-wide event tree and represents the probability that, 

given success of the technician/field operator in the room responsible for the VEWFD system

alert, suppression activities fail to prevent damage to PRA targets outside the cabinet.1 This 

branch path takes into account that the field operator has arrived at the room causing the 

VEWFD system alert, but was unable to locate the source of the condition causing the VEWFD 

system “alert” before the low-energy (incipient) fire progresses to a flaming condition. A newly 

developed non-suppression probability curve should be used with λ = 0.194. This value is 

based upon an analysis of fire events from the Updated Fire Events Database (Ref. 63). All 

fires in electrical cabinets were sampled for occurrences in which personnel were present in the 

room of origin when a flaming condition began. The approximation for the non-suppression 

value to be used was then evaluated against the MCR suppression curve. Differences between 

the MCR and newly developed curve were sufficient to warrant a new suppression curve for this 

application. This is considered to be reasonable representation given that the field operator, a 

 1 Due to the large variation among area-wide applications, including room size, ignition sources, ventilation 

conditions, and limited operator experience in locating ignition sources for area-wide application, the HEP for 

the area-wide application is assumed to fail (HEP = 1.0). The generic approach presented in this report 

assumes that the field operator fail to 1) arrive at the correct location, 2) initiate suppression, and 3) position 

him/herself in close proximity to the specific cabinet responsible for the ‘alert’ condition. Plant specific 

information and supporting justification and documentation may support an HEP estimate other than 1.0.
11-3

trained responder, will be in the room where the VEWFDS alert was initiated, actively searching 

for the source location of the alert.

The probability that the fire brigade or other first responders will fail to suppress the fire 

[Pr(Tsupp ≥ t)] is estimated with suppression probability curves developed using the 

suppression time data reported in FEDB. In EPRI 3002002936 (NUREG-2169), new 

non-suppression probability estimates are provided. These new estimates are used in 

Section 12 “Illustrative examples.” These curves were developed using U.S. Fire Event 

Experience through 2009 where manual suppression was involved and suppression time 

information was available. Suppression time was defined as the time the fire was extinguished 

or the time the fire was reported to have been under control by the fire brigade on scene

(Ref. 45). Events including self-extinguished fires, supervised burnouts, and fires extinguished 

with automatic fire suppression systems were excluded from the curves. If the time from 

detection to suppression was not known, but the duration of the fire event from start to 

suppression was known, then the reported fire duration was used instead.

The mathematical model to derive Pr(Tsupp ≥ t) is described in NUREG/CR-6850 as follows: 

The data for analysis consists of reported fire durations in commercial 

U.S. NPPs. These times are treated as being generated by an underlying 

probabilistic model. The final output of interest is the suppression curve, which 

gives the probability that a fire lasts longer than a specified time. If T is the 

random variable describing when the fire is suppressed, and λ(t) is the rate 

at which the fire is suppressed (possibly time-dependent), this probability of nonsuppression is given by: 

Pr(TT > tt) = ee�− ∫ λλ( ) tt

0 �

In this equation, λ(t) is a function of the parameters of the probabilistic model 

chosen for T. The simplest model for T is the exponential distribution, whose 

probability density function is:

ff(tt) = λλee−λλλλ

In this model, λ is estimated directly and is not a function of time, giving

Pr(TT > tt) = ee−λλλλ

The non-suppression probability is calculated using the above equation, usually 

selecting t as the time to target damage.

The same mathematical model was used to derive the non-suppression probability curve 

applicable to “π2.”

11.2 Conventional Fire Suppression

Conventional detection/suppression is credited following the failure path of the following branch 

points:
11-4

1. VEWFD system availability and reliability (β)

2. fraction that have an incipient stage of sufficient duration (α)

3. detector effectiveness (τ)

4. MCR response (μ)

5. field operator and technician response (ξ)

The “η” factor in the event tree represents the conventional suppression probability. There are 

two cases for this factor which are dependent upon the detection strategies used within the fire 

area. “η1” represents the failure probability of redundant detection and/or automatic 

suppression systems, given that the VEWFD system has failed. “η2” represents the failure 

probability of redundant detection and/or automatic suppression systems, given that the 

VEWFD system was not able to provide enhanced detection.

As used here, its estimated using the conventional Detection Suppression Event Tree in

NUREG/CR-6850, Appendix P, using the electrical fire suppression curve for manual 

suppression as appropriate. The conventional detection suppression event tree is shown in 

Figure 11-1. The end points for conventional suppression are shown below in Table 11-1. 

When crediting automatic suppression systems, the analyst must first determine that the 

automatic suppression will actuate prior to the predicted time to fire damage or else auto 

suppression fails.
11-5

Fire

Automatic Manual

Sequence

End State

Detection Suppression Detection Fixed Fire Brigade

FI AD AS MD MF FB

1.0 F OK

G OK

H OK

I NS

J OK

K OK

L OK

M NS

N NS

Figure 11-1. Conventional detection suppression event tree

Table 11-1. Conventional Detection Suppression Event Tree Outputs

Sequence Detection Suppression

F Automatic detection by

• Heat detectors

• Smoke detectors

Fire suppression by an automatically actuated fixed 

system 

G Fire suppression by a manually actuated fixed system

H Fire suppression by the fire brigade

I Fire damage to target items 

J Delayed detection by

• Roving fire watch

• Control room verification

Fire suppression by an automatically actuated fixed 

system 

K Fire suppression by a manually actuated fixed system

L Fire suppression by the fire brigade

M Fire damage to target items 

N Fire damage to target items 
11-6

For In-Cabinet or Area-wide applications, in which redundant detection and/or automatic 

suppression systems are available in the area, “η1” represents sequences F – N from Figure 

11-1. That is, given a failure of the VEWFD system or MCR to respond, the redundant detection

and/or automatic suppression capability still exists. From NUREG-6850, the probability of 

random failure of a conventional spot-type smoke detector system is assumed to be no larger 

than 0.05. 

For In-Cabinet or Area-wide applications, in which redundant detection and/or automatic 

suppression systems are available in the area, “η2” represents sequences F – I from Figure 

11-1. That is, given a failure of the VEWFD system to provide sufficient advance warning, the 

VEWFD system will still provide prompt detection functions. Time to detection is assumed to be 

at ignition.

For In-Cabinet or Area-wide applications, “η3” represents the failure of an independent 

automatic fire suppression system (including automatic detection system if the automatic 

suppression system is dependent on the automatic detection system) to suppress the fire prior 

to fire damage when the enhanced suppression capabilities fail. If an independent automatic 

suppression system is not present in the fire scenario, then “η3” is assumed 1.0. For all other 

cases, the reliability of the independent automatic suppression system (and automatic detection 

system, if applicable) is modeled consistent with NUREG/CR-6850, including an evaluation of 

any timing considerations.
12-1

12. QUANTIFICATION OF SMOKE DETECTION PERFORMANCE

The purpose of this section is to provide a description and evaluation of a method to estimate 

the non-suppression probability associated with using different types of smoke detectors in 

several different fire scenarios.

12.1 Illustrative Examples

This subsection provides illustrative examples of how the information developed in the report 

could be used in the detection/suppression analysis to estimate a probability of non-suppression

for several smoke detection systems. For these examples, the same room will be used to 

evaluate different fire scenarios, smoke detection applications, and ventilation conditions. The 

room contains both control and power-type electrical enclosures. The examples evaluated 

include the following five cases:

Case 1 Control cabinet ignition source, in-cabinet smoke detection, bank of 10 cabinets that 

are naturally ventilated

Case 2 Control cabinet ignition source, in-cabinet smoke detection, single cabinet that is forced

ventilated

Case 3 Power cabinet ignition source, in-cabinet smoke detection, bank of 10 cabinets that are 

naturally ventilated

Case 4 Mix of control and power cabinets, area-wide air return grill mounted ASD VEWFD

Case 5 Mix of control and power cabinets, area-wide air return grill mounted ASD VEWFD with 

room suppression system

All cases consist of an electrical cabinet fire (ignition source) affecting a cable tray containing a 

fire PRA target cable. In typical fire PRA analyses, the time to damage will vary depending on 

the cabinet configuration (associated heat release rate value from Table G-1 of 

NUREG/CR-6850 or from NUREG-2178), level of fire modeling (empirical, zone, CFD, THIEF, 

etc.), and the location of the cable tray. For these illustrative examples, it will be assumed that 

the time to damage this target cable is 10 minutes, regardless of the electrical cabinet fire 

hazard characteristics. This approach was chosen to reduce the number of variables, allowing 

for a more direct comparison of the benefit from the use of ASD VEWFD among applications. 

For each example, the results using the approach presented in this report will be shown, along 

with the results from FAQ 08-0046 simplified model approach on page 13-8 of Supplement 1 to

NUREG/CR-6850, EPRI 1019259. 

12.1.1 Case 1: In-cabinet smoke detection, bank of 10 control cabinets, 

natural ventilation

In this example, various in-cabinet smoke detection systems are evaluated for their ability to 

enhance the suppression capability for a potential fire hazard located in a bank of 10 low 

voltage control cabinets. The cabinets are naturally ventilated. Each in-cabinet smoke 

detection system will be evaluated separately. It is assumed that a single zone of the ASD 

protects the entire cabinet bank and the spot-type detectors are addressable to a single cabinet. 
12-2

The in-cabinet detection systems will be evaluated with and without crediting other detection 

and suppression systems in the room. The following information is necessary before 

proceeding with the analysis:

• Based on fire modeling analysis, the estimated time to target damage is 12 minutes.

• From fire drills records, the reasonable fire brigade response time is 10 minutes.

• Procedures to de-energize entire cabinets or specific components within the cabinet are 

not available.

• When a redundant ceiling mounted conventional area-wide smoke detection system is 

credited, fire modeling analysis has shown that the estimated time for conventional areawide smoke detection is 2 minutes. 

Based on the preceding information, the values presented in Table 12-1 are to be used in 

applying the in-cabinet ASD VEWFD event tree approach; each parameter estimate is provided 

with source information as to where the particular estimate value can be located. The results are 

presented in Table 12-2 as non-suppression probability estimates.

Table 12-1. Case 1 Input Parameters: Multi-Control Cabinet, In-Cabinet

Parameter Estimate Source

β

3.6×10-03 (ASD)

5×10-02 (Spot)

System unavailability (Table 7-4) and 

unreliability (Table 7-2) in Section 7.2 of this 

report for ASD VEWFD systems

NUREG/CR-6850 – Appendix P

α 2.8×10-01 Fraction of fires that have an incipient phase

Table 7-1, Low Voltage Control Cabinets

τ

2.7×10-03 : ASD CC

5.3×10-01 : ASD LS1

1.9×10-01 : ASD LS2

2.6×10-01 : SS

1.0×10-01 : ION

System ineffectiveness

Table 7-5, In-cabinet, Natural Ventilation

μ 1×10-04 MCR operator response Section 10.6.2.1

ξ

4.6×10-04 : ASD CC

4.6×10-02 : ASD LS

4.6×10-02 : SS

1.7×10-02 : ION

Field operator response Section 10.6.2.2

Table 10-9 – ASD VEWFD CC

Table 10-10 – Ionization

Table 10-11 – ASD VEWFD LS and SS

π1 2.0×10-02 See discussion below

η1

η2 Varies with redundant systems credited in scenario.

η3
12-3

Table 12-2. Case 1 Results - Probability of Non-Suppression

In-Cabinet 

Detection Type

In-Cabinet 

Detection Only

In-Cabinet Detection with

Redundant 

Auto Detection

Auto 

Suppression

[Halon]

Auto 

Suppression

[Wet Pipe]

LS1 2.2×10-02

[4.6]

2.2×10-01

[4.6]

1.1×10-02

[92]

4.3×10-03

[231]

LS2 1.5×10-01

[6.6]

1.5×10-01

[6.7]

7.5×10-03

[134]

3.0×10-03

[335]

CC 1.1×10-01

[9.4]

1.0×10-01

[9.6]

5.2×10-03

[193]

2.1×10-03

[482]

SS 2.0×10-01

[4.9]

1.7×10-01

[5.7]

8.7×10-03

[115]

3.5×10-03

[287]

ION 1.7×10-01

[5.9]

1.4×10-01

[7.2]

7.0×10-03

[144]

5.6×10-03

[180]

None 1.0

Without In-Cabinet Detection

3.6×10-01

[2.8]

1.8×10-02

[56]

8.1×10-03

[123]

Note: non-suppression probability estimates presented in Table 12-2 are scenario dependent and 

not generic. Values in “[ ]” represent the inverse of the probability of non-suppression, sometime 

referred to as the “reduction factor.”

The enhanced suppression parameter (π) is the probability that, given success of event ξ, the 

field operator in the area of the electrical cabinet responsible for the VEWFD system alert fails 

to promptly suppress the fire quickly enough to prevent damage to PRA targets outside the 

cabinet. This is calculated using the MCR non-suppression curve. For this case, both the time 

to detection of the VEWFD system and time to fire brigade response are set to 0 minutes, 

considering that the system has already gone into alarm and the field operator is present in the 

room. The field operator acts as the fire brigade as he/she has been trained to suppress fires 

(e.g., fire brigade training). The failure probability of the automatic detection, in this case, the 

VEWFD system, is set to 0 as well. At this point in the event tree the system is assumed to 

have already operated successfully and the appropriate response is underway. 

The probability of failure to extinguish the fire (π1), once ignition has occurred, is calculated 

using the MCR curve (λ = 0.324) using a manual suppression time of 12 minutes as:

ee(−λλMMMMMM∗ ) = ee(−0.324∗12) = 2.0 × 10−02

As identified in Table 12-1, the conventional detection/suppression estimates vary with the types 

of redundant detection and suppression systems credited in the fire scenario. Figure 12-1 and 

Figure 12-2, illustrate the solution of the detection suppression event tree from NUREG/CR6850 for the scenario where only redundant fire detection is available.
12-4

Figure 12-1 represents η1, the presence of redundant conventional ceiling mounted spot-type 

detection system is assumed for this example calculation. η1 is calculated through sequences F 

to N of the conventional suppression/detection event tree. Automatic conventional spot-type 

detection has a failure probability of 0.05 (Ref. 36). The probability of failure for the fire brigade 

is calculated using the electrical suppression curve using a manual suppression time of 12 - 2 = 

10 minutes (tdam - tdet = tms) as:

ee(−λλeeeeeeee eeeeeeeeee∗ ) = ee(−0.098∗10) = 0.375

If the automatic detection fails, delayed detection is credited. Sequences J to N refer to this 

situation. Assuming a time to delayed detection of 15 minutes, the fire brigade has no time to 

suppress the fire before target damage. 

Accordingly, the non-suppression probability is the sum of sequences I, M and N, which is 0.36

+ 0.00 + 0.05 = 0.41.

Fire

Automatic Manual

Sequence

End 

State

Pr (nonsuppression)

Detection Suppression Detection Fixed Fire Brigade

FI AD AS MD MF FB

1.0 0.95 0.0 F OK

1.0 0.0 G OK

1.0 0.625 H OK

0.375 I NS 3.6E-01

0.05 0.0 J OK

1.0 0.0 0.0 K OK

1.0 0.0 L OK

1.0 M NS 0.0

1.0 N NS 5.0E-02

Total 4.1E-01

Figure 12-1. Case 1, detection suppression tree for (η1)

Figure 12-2 represents (η2), the presence of in cabinet VEWFD system providing prompt 

detection for event sequences where a fast developing fire occurs or the VEWFD system is not 

effective in detecting the low energy fire in its incipient stage. That is, given a failure of the incabinet ASD VEWFD system to provide sufficient advance warning, the VEWFD system will still 

perform prompt automatic detection functions. That is, for in-cabinet applications, prompt 

detection assumes the ASD VEWFD system time to detect is 0 minutes. (η2) is calculated

through sequences F to I. The probability of failure for the fire brigade is calculated using the

electrical suppression curve using a manual suppression time of 12 minutes as:
12-5

ee(−λλeeeeeeee eeeeeeeeee∗10) = ee(−0.098∗12) = 0.309

Accordingly, the non-suppression probability for (η2) sequence I or 0.309.

Fire

Prompt Automatic Manual

Sequence

End 

State

Pr(nonsuppression)

Detection Detection Suppression Detection Fixed Fire Brigade

FI PD AD AS MD MF FB

1.0 1.0 1.0 0.0 F OK

1.0 0.0 G OK

1.0 0.691 H OK

0.309 I NS 3.1E-01

0.0 0.0 J OK

1.0 0.0 0.0 K OK

1.0 0.691 L OK

0.309 M NS 0.0

1.0 N NS 0.0

Total 3.1E-01

Figure 12-2. Case 1, detection suppression event tree (η2)

“η3” represents the failure of an independent automatic fire suppression system (including 

automatic detection system failure, if the automatic suppression system is dependent on the 

automatic detection system.) For scenarios where no independent automatic fire suppression 

systems are available or credited, “η3” is set to 1.0. Where automatic suppression systems are 

available and credited and not dependent on an automatic detection system (such as wet pipe 

sprinklers), then the failure probability of that system should be represented by “η3.” Where 

automatic suppression systems are available and credited and dependent on an automatic 

detection system the combined failure probability of the suppression and detection system 

should be represented by “η3.” For example, if an automatic suppression system with a failure 

probability of 0.04 is dependent on an automatic detection system with a failure probability of 

0.05, then the combined failure probability to use for η3 should be (0.05) + (1-0.05) ×

(0.04) = 0.088.
12-6

12.1.2 Case 2: In-cabinet, single control cabinet, force ventilation high ACH

In this example, various in-cabinet smoke detection systems are evaluated for their ability to 

enhance the suppression capability for a potential fire hazard located in a single control cabinet 

with a high rate of forced ventilation (approximately 300 cabinet ACH). Each in-cabinet smoke 

detection system will be evaluated separately. It is assumed that a single zone of the ASD 

protects the cabinet and the spot-type detectors are addressable to a single cabinet. The incabinet detection systems will be evaluated with and without crediting other detection and 

suppression systems in the room. The following information is necessary before proceeding 

with the analysis:

• Based on fire modeling analysis, the estimated time to target damage is 12 minutes.

• From fire drills records, the brigade response time is 10 minutes.

• Procedures to de-energize entire cabinets or specific components within the cabinet are 

not available.

• When a redundant ceiling mounted conventional area-wide smoke detection system is 

credited, fire modeling analysis has shown that the estimated time for conventional areawide smoke detection is 2 minute. 

Based on the above information the values presented in Table 12-3 are to be used in applying 

the in-cabinet ASD VEWFD event tree approach; each parameter estimate is provided with 

source information as to where the particular estimate value can be located. The results are 

presented in as non-suppression probability estimates.

Table 12-3. Case 2, Input Parameters: Single Low Voltage Control Cabinet, In-Cabinet

Parameter Estimate Source

β

3.6×10-03 (ASD)

5×10-02 (Spot)

System unavailability (Table 7-4) and 

unreliability (Table 7-2) in Section 7.2 of this 

report for ASD VEWFD systems

NUREG/CR-6850 – Appendix P

α 2.8×10-01 Fraction of fires that have an incipient phase

Table 7-1, Low Voltage Control cabinet

τ

1.9×10-02 : ASD CC

3.7×10-01 : ASD LS2

7.9×10-01 : ION

System ineffectiveness

Table 7-5, In-cabinet, Forced Ventilation

μ 1×10-04 MCR operator response Section 10.6.2.1

ξ

4.6×10-04: ASD CC

4.6×10-02 : ASD LS

1.7×10-02 : ION

Field operator response Section 10.6.2.2

Table 10-9 – ASD VEWFD CC

Table 10-10 – Ionization

Table 10-11 – ASD VEWFD LS and SS

π1 2.0×10-02 Same as Case 1

η1

η2 Varies with redundant systems credited in scenario

η3
12-7

Table 12-4. Case 2 Results - Probability of Non-Suppression

In-Cabinet 

Detection Type

In-Cabinet 

Detection Only

In-Cabinet Detection with

Redundant 

Auto Detection

Auto 

Suppression

[Halon]

Auto 

Suppression

[Wet Pipe]

LS2 1.9×10-01

[5.4]

1.8×10-01

[5.4]

9.2×10-03

[108]

2.0×10-02

[271]

CC 1.1×10-01

[9.2]

1.1×10-01

[9.3]

5.4×10-03

[187]

2.1×10-03

[467]

ION 3.0×10-01

[3.3]

2.7×10-01

[3.7]

1.4×10-02

[73]

5.5×10-03

[183]

None 1.0

Without In-Cabinet Detection

3.6×10-01

[2.8]

1.8×10-02

[56]

8.1×10-03

[123]

Note: non-suppression probability estimates presented in Table 12-4 are scenario dependent and 

not generic. Values in “[ ]” represent the inverse of the probability of non-suppression, sometime 

referred to as the “reduction factor.”

For this case the solution of the conventional detection suppression event tree from 

NUREG/CR-6850 for the terms η1, η2 and η3, are identical to Case 1 and will not be repeated 

here.

12.1.3 Case 3: In-cabinet ASD VEWFD, power cabinet with 10 vertical sections, natural 

ventilation

In this example, various in-cabinet smoke detection systems are evaluated for their ability to 

enhance the suppression capability for a potential fire hazard located in power cabinets (motor 

control center with ten vertical sections). The cabinets are naturally ventilated (not well sealed). 

Each in-cabinet smoke detection system will be evaluated separately. It is assumed that a 

single zone of the ASD protects the entire cabinet bank and the spot-type detectors are 

addressable to a single cabinet. The following information is necessary before proceeding with 

the analysis.

• Based on fire modeling analysis, the estimated time to target damage is 12 minutes.

• From fire drills records, the brigade response time is 10 minutes.

• Procedures to de-energize entire cabinets or specific components within the cabinet are 

not available.

• When a redundant ceiling mounted conventional area-wide smoke detection system is 

credited, fire modeling analysis has shown that the estimated time for conventional areawide smoke detection is 2 minutes.
12-8

Based on the above information the values presented in Table 12-5 are used in applying the incabinet ASD VEWFD event tree approach; each parameter estimate is provided with source 

information as to where the particular estimate value can be located. The results are presented 

in

Table 12-6 as non-suppression probability estimates.

Table 12-5. Case 3 Input Parameters: Multi-Power Cabinet, In-Cabinet

Parameter Estimate Source

β

3.6×10-03 (ASD)

5×10-02 (Spot)

System unavailability (Table 7-4) and 

unreliability (Table 7-2) in Section 7.2 of this 

report for ASD VEWFD systems

NUREG/CR-6850 – Appendix P

α 5.0×10-01 Fraction of fires that have an incipient phase 

Table 7-1, Power Cabinets

τ

2.7×10-03 : ASD CC

5.3×10-01 : ASD LS1

1.9×10-01 : ASD LS2

2.6×10-01 : SS

1.0×10-01 : ION

System ineffectiveness

Table 7-5, In-cabinet, Natural Ventilation

μ 1×10-04 MCR operator response Section 10.6.2.1

ξ

4.6×10-04 : ASD CC

4.6×10-02 : ASD LS

4.6×10-02 : SS

1.7×10-02 : ION

Field operator response Section 10.6.2.2

Table 10-9 – ASD VEWFD CC

Table 10-10 – Ionization

Table 10-11 – ASD VEWFD LS and SS

π1 2.0 ×10-02 Same as Case 1

η1

η2 Varies with redundant systems credited in scenario.

η3

Table 12-6. Case 3 Results - Probability of Non-Suppression

In-Cabinet 

Detection Type

In-Cabinet 

Detection Only

In-Cabinet Detection with

Redundant 

Auto Detection

Auto 

Suppression

[Halon]

Auto 

Suppression

[Wet Pipe]

LS1 2.5×10-01

[4.0]

2.4×10-01

[4.1]

1.2×10-02

[82]

4.9×10-03

[205]

LS2 2.0×10-01

[5.0]

2.0×10-01

[5.0]

9.9×10-03

[101]

4.0×10-03

[253]

CC 1.7×10-01

[5.9]

1.7×10-01

[6.0]

8.3×10-03

[120]

3.3×10-03

[300]

SS 2.5×10-01

[4.1]

2.2×10-01

[4.6]

1.1×10-02

[92]

4.3×10-03

[231]
12-9

Table 12-6. Case 3 Results – Probability of Non-Suppression (Continued)

In-Cabinet 

Detection Type

In-Cabinet 

Detection Only

Without In-Cabinet Detection

Redundant 

Auto Detection

Auto 

Suppression

[Halon]

Auto 

Suppression

[Wet Pipe]

ION 2.2x10-01

[4.5]

1.9x10-01

[5.2]

9.6x10-03

[104]

3.8x10-03

[260]

None 1.0 3.6x10-01

[2.8]

1.8x10-02

[56]

8.1x10-03

[123]

Note: non-suppression probability estimates presented in Table 12-6 are scenario dependent and 

not generic. Values in “[ ]” represent the inverse of the probability of non-suppression, sometime 

referred to as the “reduction factor.”

For this case the solution of the detection suppression event tree from NUREG/CR-6850 for the 

terms η1, η2, and η3, are identical to Case 1 and 2, and will not be repeated here.

12.1.4 Case 4: Area-wide ASD VEWFD system protecting low-voltage control cabinets

In this example, various area-wide smoke detection systems are evaluated for their ability to 

enhance the suppression capability for a potential fire hazard located in control cabinet. The 

room has an HVAC system. The following information is necessary before proceeding with 

the analysis.

• Based on fire modeling analysis, the estimated time to target damage is 10 minutes.

• From fire drills records, the brigade response time is 10 minutes.

• There are no redundant smoke detection systems in the fire area.

• Procedures to de-energize entire cabinets or specific components within the cabinet are 

not available.

• When a redundant ceiling mounted conventional area-wide smoke detection system 

is credited, fire modeling analysis has shown that the estimated time for conventional 

area-wide smoke detection is 2 minutes.

Based on the above information the values presented in Table 12-7 are to be used in applying 

the incipient event tree approach:
12-10

Table 12-7. Case 4 Input Parameters: Low-voltage control cabinet Type, Area-wide

Parameter Estimate Source

β

3.6×10-03

(ASD ceiling)

4.0×10-03

(ASD air return grill)

5×10-02 (Spot)

System unavailability (Table 7-4) and unreliability

(Table 7-2) in Section 7.2 of this report for ASD 

VEWFD systems

Note that a 4.x10-04 value for HVAC unreliability has been 

added to the ASD when used in an air return application. 

There is no basis for this value and is used for illustrative 

purposes only. Plant specific HVAC unreliability estimates 

should be used.

NUREG/CR-6850 – Appendix P

α 0.28×10-01 Fraction of fires that have an incipient phase 

Table 7-1, Bin 15 – All Cabinet Types

τ

Ceiling

3.2×10-01 : ASD CC

1.1×10-01 : ASD LS

5.7×10-01 : SS

System ineffectiveness

Table 7-5, Area-wide, Ceiling

Air Return

3.0×10-01 : ASD CC

5.2×10-01 : ASD LS

System ineffectiveness

Table 7-5, Area-wide, Air Return Grill

μ 1×10-04 MCR operator response Section 10.6.2.1

ξ 1.0 Field operator & technician response

Section 10.6.5

π2 9.7×10-02 See discussion below

η1

η2 Varies by redundant systems credited in scenario.

η3

Table 12-8. Case 4 Results for Area-wide Ceiling VEWFD - Probability of NonSuppression

In-Cabinet 

Detection Type VEWFD Only

Area-wide VEW Detection with

Redundant 

Auto Detection

Auto 

Suppression

[Halon]

Auto 

Suppression

[Wet Pipe]

LS2 3.1×10-01

[3.2]

3.1×10-01

[3.2]

1.5×10-02

[65]

6.2×10-03

[162]

CC 3.1×10-01

[3.2]

3.1×10-01

[3.2]

1.5×10-02

[65]

6.2×10-03

[162]

SS 3.4×10-01

[2.9]

3.1×10-01

[3.2]

1.6×10-02

[63.8]

6.3×10-03

[160]

None 1.0

Without Area-wide VEW Detection

3.6×10-01

[2.8]

1.8×10-02

[56]

8.1×10-03

[123]
12-11

Table 12-9. Case 4 Results for Area-wide Air Return Grill VEWFD - Probability of NonSuppression

In-Cabinet 

Detection Type VEWFD Only

Area-wide VEW Detection with

Redundant 

Auto Detection

Auto 

Suppression

[Halon]

Auto 

Suppression

[Wet Pipe]

LS2 3.1×10-01

[3.2]

3.1×10-01

[3.2]

1.5×10-02

[65]

6.2×10-03

[162]

CC 3.1×10-01

[3.2]

3.1×10-01

[3.2]

1.5×10-02

[65]

6.2×10-03

[162]

None 1.0

Without In-Cabinet Detection

3.6×10-01

[2.8]

1.8×10-02

[56]

8.1×10-03

[123]

Note: non-suppression probability estimates presented in Table 12-9 are scenario dependent 

and not generic. Values in “[ ]” represent the inverse of the probability of non-suppression, 

sometime referred to as the “reduction factor.”

Note that as a default, the field operator/technician response is set to failure (ξ = 1.0). As 

discussed in Section 10, unless a detailed analysis following the process outlined is followed to 

develop scenario specific human error probabilities for the area-wide applications is conducted, 

the field operator/technician response is assumed to fail (ξ = 1.0). If a value other than 1.0 can 

be justified, then the enhanced suppression parameter (π2) is the probability that, given 

success of event ξ, the field operator in the area of the electrical cabinet responsible for the 

VEWFD system alert fails to promptly suppress the fire quickly enough to prevent damage to 

PRA targets outside the cabinet. This is calculated using the field operator non-suppression 

curve. For this case, both the time to detection of the VEWFD system and time to fire brigade 

response are set to 0 minutes, considering that the system has already gone into alarm and the 

field operator is present in the room. The field operator acts as the fire brigade as he/she has 

been trained to suppress fires (e.g., fire brigade training). The failure probability of the 

automatic detection, in this case, the VEWFD system, is set to 0 as well. At this point in the 

event tree the system is assumed to have already operated successfully and the appropriate 

response is underway.

The probability of failure to extinguish the fire (π2), once ignition has occurred, is calculated 

using the field operator detection curve (λ = 0.194) using a manual suppression time of 

12 minutes as:

ee(−λλMMMMMM∗ ) = ee(−0.194∗12) = 9.7 × 10−02

Figure 12-3 estimates (η1), conventional non-suppression probability for a redundant 

detection systems when the area-wide VEWFD system or main control room response to an 

ASD VEWFD response fails. Delayed detection is assumed 1.0 for cases where no prompt or 

automatic detection is credited (or fails) if the estimated time for manual detection is less than 

the time to target damage. “η1” is calculated through sequences F – N. Assuming a time to 

delayed detection of 15 minutes, the fire brigade has no time to suppress the fire before target 
12-12

damage. For this case, there is no redundant detection systems credited. Accordingly, the nonsuppression probability is the sum of Sequences I, M and N, which is 0.36 + 0.0+ 0.05 = 0.41.

Fire

Automatic Manual

Sequence

End State Pr(nonsuppression)

Detection Suppression Detection Fixed Fire Brigade

FI AD AS MD MF FB

1.0 0.95 0.0 F OK

1.0 0.0 G OK

1.0 0.625 H OK

0.375 I NS 0.36

0.05 0.0 J OK

1.0 0.0 0.0 K OK

1.0 0.0 L OK

1.0 M NS 0.0

1.0 N NS 0.05

Total 0.41

Figure 12-3. Case 4, conventional detection suppression tree for (η1)

Figure 12-4 represents η2, the probability that the VEWFDs system providing prompt detection 

for event sequences where a fast developing fire occurs or the VEWFD system is not effective 

in detecting the low energy fire in its incipient stage. That is, given a failure of the VEWFD 

system to provide sufficient advance warning, the VEWFD system will still provide prompt 

detection. η2 is calculated through sequences F to I with VEWFD system represented with a 

failure probability of 0.0 given that detector system availability, reliability, and effectiveness is 

accounted for with β. The probability of failure for the fire brigade is calculated using the 

“electrical” suppression curve at time 12 - 0 = 12 minutes as:

ee(−0.098∗12) = 0.309

According to Section P.1.5, the non-suppression probability is the sum of Sequences I, M and 

N, which is 3.1×10-1 + 0.0 + 0.0 = 3.1×10-01.
12-13

Fire

Automatic Manual

Sequence

End State Pr(nonsuppression)

Detection Suppression Detection Fixed Fire Brigade

FI AD AS MD MF FB

1.0 1.0 0.0 F OK

1.0 0.0 G OK

1.0 0.691 H OK

0.309 I NS 3.1E-01

0.0 0.0 J OK

1.0 0.0 0.0 K OK

1.0 0.0 L OK

1.0 M NS 0.0

1.0 N NS 0.0

Total 3.1E-01

Figure 12-4. Case 4, conventional detection suppression tree for (η2)

“η3” represents the failure of an independent automatic fire suppression system (including 

automatic detection system failure, if the automatic suppression system is dependent on the 

automatic detection system.) See case 1 for further details.
12-14

12.2 Evaluation of the Event Tree Sensitivity

The sensitivity of the event trees are evaluated in this section. This evaluation is conducted for 

Case 1. The evaluation is performed by adjusting a single parameter to its bounding estimate 

(5th/95th or max/min) and plotting the results on a time based probability plot. The event trees

are evaluated for the “Fire Damage Outside Cabinet” damage state. For each ASD VEWFD

system case, the parameters evaluated include α, β, τ, and ξ. The HEP for MCR response “μ,”

enhanced suppression “π1-2,” and conventional detection/suppression “η1-3,” are not evaluated 

here as they will have a minor effect on the end result compared to these parameters. 

Figure 12-5 to Figure 12-9 present the sensitivities for case 1. As shown in these results, the 

“α” (fraction of fires that do not have an incipient stage) results in the largest change in the 

estimated probability of non-suppression. The next sensitive parameter is “τ” (system 

effectiveness), followed by “ξ” (human error probability), and lately “β” (system 

reliability/availability).
12-15

Figure 12-5. Probability plots for sensitivity of Cloud Chamber ASD VEWFD System

(Case 1)

Case 1 – Cloud Chamber ASD VEWFD System

Parameter Mean Lower (5th) Upper (95th)

α 0.28 0.08 0.54

β 3.6×10-3 8.6×10-04 6.3×10-03

τ 2.7×10-3 1.1×10-05 1.0×10-02

ξ 4.6×10-04 1.7×10-05 1.7×10-03
12-16

Figure 12-6. Probability plots for sensitivity of Light Scattering (LS1) ASD VEWFD 

System (Case 1)

Case 1 – Light Scattering 1 ASD VEWFD System

Parameter Mean Lower (5th) Upper (95th)

α 0.28 0.08 0.54

β 3.6×10-03 8.6×10-04 6.3×10-03

τ 0.53 0.45 0.65

ξ 4.6×10-02 5.7×10-03 1.4×10-01
12-17

Figure 12-7. Probability plots for sensitivity of Light Scattering (LS2) ASD VEWFD 

System (Case 1)

Case 1 – Light Scattering 2 ASD VEWFD System

Parameter Mean Lower (5th) Upper (95th)

α 0.28 0.08 0.54

β 3.6×10-03 8.6×10-04 6.3×10-03

τ 0.19 0.14 0.24

ξ 4.6×10-02 5.7×10-03 1.4×10-01
12-18

Figure 12-8. Probability plots for sensitivity of Sensitive Spot-type (SS) VEWFD System

(Case 1)

Case 1 – Sensitive Spot-type (SS) VEWFD System

Parameter Mean Lower (5th) Upper (95th)

α 0.28 0.08 0.54

τ 0.26 0.21 0.32

ξ 4.6×10-02 5.7×10-03 1.4×10-01
12-19

Figure 12-9. Probability plots for sensitivity of Ionization (ION) Spot-Type Addressable 

system (Case 1)

Case 1 – Ionization (ION) Spot-Type 

Parameter Mean Lower (5th) Upper (95th)

α 0.28 0.08 0.54

τ 0.10 6.8×10-02 0.14

ξ 1.7×10-02 2.1×10-03 5.3×10-02
12-20

12.3 Use of Plant Specific or Generic Data

The analyses provided in this report use a variety of data and other information sources, as well 

as calculated inputs. Although this analysis is not intended to represent a specific NPP, PRA 

conventions and the ASME/ANS PRA Standard (Ref. 60) have been followed to the extent 

possible. Specific NPPs that perform their own analyses will need to make different choices 

to satisfy, for example, the PRA Standard.

12.3.1 Use of Plant-Specific Data

Parameters used in this analysis that should be informed using plant-specific information 

include:

(1) Detector system availability and reliability (β) (Section 7.2)

(2) Time required for operator response (Section 10.4)

(3) MCR operator response (μ) (Section 10.6)

(4) Field operator/technician response (ξ) (Section 10.6)

In addition, the quantification tool (discussed in Section 6.4, Appendix H and provided in the 

companion CD to this report), fire scenario specific parameters should be included:

1. time to target damage

2. redundant automatic fire detection system parameters, if applicable

o time to detection

o type of system

o failure probability

3. independent automatic fire suppression system1 parameters, if applicable

o type of system

o failure probability

o dependency on redundant automatic fire detection system

4. manual fixed fire suppression failure probability, if applicable.

12.3.2 Use of Generic Data

The PRA Standard (e.g., initiating events and data analysis Supporting Requirements in Part 2, 

fire ignition frequency in Part 4) identifies certain situations in which generic data is the 

appropriate data choice. In particular, the Standard states that generic data is appropriate 

when very limited data is available on an industry-wide basis, as is the case for components 

within electrical cabinets having an incipient phase. Also, the Standard states that the collection 

of plant-specific data and calculation of plant-specific parameters should be, for example:

• accomplished in a consistent manner with respect to design, operational practices, 

and experience

• based on a clear definition for the parameter

 1 To credit an automatic fire suppression system, an analysis should show than the suppression system is 

capable of suppressing the fire prior to target damage.
12-21

• have a clear basis for identifying failures (versus degraded states)

Although this report used ground rules for consistent review of operational experience, it is 

recommended that further work on establishing appropriate practices for identifying and using 

operating experience for evaluating certain parameters used in this analysis.

For the reasons outlined above, generic data should be used for the following parameters used 

in this analysis:

(1) Fraction of components that have an incipient phase (α) (Section 7.1)

(2) Effectiveness (τ) (Section 7.2)

(3) Time available (Sections 8 and 10.4)

(4) Non-suppression probabilities (enhanced – π, conventional – η)

Until or unless future analyses of generic data are performed, it is recommended that the 

generic values provided in this report be used as the generic data inputs.

13-1

13. ASSUMPTIONS AND LIMITATIONS

The risk scoping study described in Part II of this report is based on numerous assumptions and 

limitations. For the approach presented to provide meaningful results, the following 

assumptions and limitations apply:

1. System is designed and installed by trained and qualified technicians following 

appropriate vendor guidance. System should pass the vendor’s acceptance test(s), 

including any extended period of commissioning prior to being placed in service. Any 

deviations between as-built and as-designed are evaluated for effects on system 

performance.

2. Systems are inspected, tested and maintained per applicable national consensus 

standards (e.g., NFPA 72 and 76) and vendor requirements.

3. Functionality testing via detector alarm response to smoke stimulus should be conducted 

following guidance in Annex B “Performance Test Procedures for Very Early Warning 

and Early Warning Fire Detection Systems,” of NFPA 76 or vendor equivalent methods. 

Functionality testing supports confirmation that transport times are met and verifies air 

flow through each sampling port credited for protection. 

Note that the performance test method presented in Annex B of NFPA 76 or vendor 

equivalent methods (such as overheated polymeric material) used to test system 

response and transport times are not equivalent to a calibration (sensitivity) test, 

such as the “sensitivity test” outlined in UL 268, “Smoke Detectors for Fire Alarm 

Systems.”

4. For in-cabinet application, the cabinet characteristics must allow for the application of 

aspirated VEWFD systems, such that the cabinet is not tightly sealed. For area-wide 

applications, the cabinet(s) being protected must have openings (vents, grates, etc.) to 

allow products of combustion to exit the cabinet and migrate to the VEWFD system 

sampling ports.

5. The parameter estimates used to support the risk scoping study assume the sensitivity 

settings of the VEWFD system is setup to meet or exceed (be more sensitive than) the 

following

Light-scattering [NFPA 76 requirements]

a. An alert is set to 0.2 %/ft obscuration at the sampling point above background.

and 

b. An alarm is set to 1.0 %/ft obscuration at the sampling point above background.

Cloud Chamber

An alert is set to 1.0×106 particles/cm3 at the sampling point. [Note NFPA 76 

does not specify a particles/cm3, this set point is based on the set points

commonly used in the testing conducted as part of this report. This 
13-2

recommended value does not imply that it is equivalent to the NFPA 76 

requirements.]

Because the sensitivity directly affects the detector performance, using sensitivities other 

than those specified here requires justification and adjustments to several risk scoping 

study parameter estimates (i.e., τ, and ξ). A description of the general process to 

estimate these parameters, consistent with this report, is presented in Appendix B.4.

6. The sensitivity setting for the conventional system used for in-cabinet applications meet 

or exceed the sensitivities tested, namely;

ION Spot-type

a. An alarm is set to 1.0 %/ft obscuration

PHOTO Spot-type

b. An alarm is set to 2.1 %/ft obscuration

7. The VEWFD system alert corresponds to the main control room operator, field operator,

and technician response; and a VEWFD alarm corresponds to an expeditious fire 

brigade response as described in Section 10. Requirements described in Section 10 for 

timely response of all relevant personnel include:

a. MCR operator response requirements (regarding procedures, training, 

alert/alarm design, monitoring, etc).

b. Field operator response requirements (regarding procedures, fire watch

responsibilities, available equipment, acceptable training, etc.)

c. Technician response requirements (especially regarding the responsibility to 

obtain keys for locked cabinets, allowing a quicker arrival time for the field 

operator; also training, equipment availability, etc. to allow identification of 

affected cabinet and degraded component)

d. When using a detection strategy with no ability to differentiate between the alert 

and alarm set points (i.e., in-cabinet ionization detectors with one setpoint), the 

alarm response should correspond to an expeditious fire brigade response as 

described in Section 10.

e. Time available for all responses is discussed in Sections 8 and 10 for the 

detector types addressed in this report. Time required for responses should be 

developed using plant specific information.

8. The approach presented is not applicable to main control room applications or other 

spaces that are continuously occupied. Human subject testing was not performed to 

assess the performance of human senses to the response of smoke detection systems

tested in this project.

9. Compensatory measures are put in place whenever the VEWFD or conventional 

spot-type smoke detection system is unavailable.

10. Effective methods should be established for locating the source of the 

systems response.
13-3

a. Plant personnel responding to VEWFD or conventional spot-type systems are 

properly trained to respond to incipient conditions, identify the faulted cabinet, 

and suppress potential fires.

b. Personnel using portable equipment to locate incipient degradation must be 

trained in its use, including on-the-job training such that they are familiar with the 

equipment, procedures for its use and any limitations and/or precautions 

required.

c. Any portable devices used to locate the degrading component should be 

dedicated for use, maintained in an operable condition, available on site at 

all times and appropriately staged to be rapidly accessed when need.

11. The use of ASD VEWFD or conventional spot-type systems as a risk reduction measure 

does not replace the requirement to demonstrate the ability to meet the nuclear safety 

performance criteria for a fire scenario and its impact(s) on safe shutdown equipment

and does not eliminate the requirement to maintain defense-in-depth.

12. Area-wide HEP’s are set to fail (‘ξ’ = 1.0). The risk scoping study does not include timing 

feasibility considerations for area-wide application, as discussed in Section 10.5.6. An 

area-wide HEP other than 1.0 could be used if a feasibility and timing study were 

conducted for the specific fire scenario, following the same process described in Section 

10.

13. The generic unreliability and unavailability estimates presented in Section 7.2 are based 

on several data sets that include cloud chamber and laser based aspirated systems. If 

other technologies are used, the applicability of these generic estimates should be 

evaluated. If site (or fleet) specific unavailability or unreliability estimates are used, a

basis for their use should be justified.

14. Application of the risk scoping study for ‘low voltage electrical cabinets’ (“alpha” term

point estimate of 0.72) requires that the maximum voltage level within the cabinet be 

250 volts or less. Electrical cabinets that contain any electrical components above this 

250 volt threshold should be considered “power cabinets.”

15. The probability of non-suppression estimates developed from the approach presented in

Part II of this report is only applicable to target damage sets located outside of the 

electrical enclosure. That is, the VEWFD system ONLY impacts the adverse 

consequences related to cabinet fires resulting in fire growth outside the initiating 

electrical enclosure which could damage secondary combustibles and targets. For 

implementation guidance see Section 6.1.1.

PART III

Conclusions and Perspectives

Definitions and References

14-1

14. REPORT SUMMARY AND CONCLUSIONS

This section summarizes the specific findings based on the analysis of the test data and review 

of the operating experience and literature related to the performance of very early warning fire 

detection systems, specifically aspirating smoke detection (ASD) configured as very early 

warning fire detection (VEWFD) systems. These findings are specific to the objectives outlined 

in Section 1.3

Operational experience and the tests performed under this program show that aspirated 

VEWFD systems, when designed, installed, and maintained are effective in detecting lowenergy pre-flaming fire conditions. However, the testing has also shown that other forms of 

smoke detection such as conventional ION spot-type detectors perform equally well in naturally 

ventilated in-cabinet applications. The test results show that in-cabinet applications are the 

most effective use of smoke detection technologies in detecting low-energy incipient stage

electrical cabinet fires as compared to area-wide ceiling level detection. Area-wide applications

using an air return grill and those using ceiling-mounted air sampling port locations perform 

similarly. However, as ceiling height increases, ceiling area-wide aspirated VEWFD 

applications will become less effective at detecting low-energy incipient fire sources, unless 

system sensitivities are increased or sampling port spacing is decreased.

VEWFD systems don’t always provide enhanced warning over conventional spot-type detection 

during the low-energy incipient stage. The performance of either type of system (VEWFD or 

conventional spot-type) is dependent upon the material thermal decomposition rate, and aerosol 

characteristics. For in-cabinet applications, the cloud chamber ASD typically outperformed the 

other systems tested, followed by the ION spot-type detector which typically outperformed the 

light-scattering ASD and sensitive spot-type detectors. The PHOTO spot-type detector typically 

responded the slowest during the in-cabinet tests. The cloud chamber and ION in-cabinet 

performance is largely because of the aerosol characteristics at the early stage, which are 

typically spherical in nature; also the effects of aerosol aggregation and agglomeration have not 

developed to a point where light-scattering type detectors can be effective. However, the 

exception is CSPE material (or materials with similar aerosol characteristics), which had the 

largest particle size for the materials tests.

In area-wide applications, the ASD and sensitive spot-type VEWFD systems typically performed 

better than the ION and PHOTO spot-type detectors. The ASD’s ability to use cumulative 

sampling is largely the cause of the effect. ASD systems also have the added benefit of using 

filters to reduce nuisance alarm rates and can be designed to allow for more efficient inspection, 

maintenance, and testing. For fast-developing fires, the amount of additional warning between 

VEWFD and conventional systems is marginal, for the typical NPP configuration.

With the exception of ASD systems designed to protect a single electrical cabinet, human 

interaction with equipment such as handheld thermal imaging cameras or portable ASDs will be 

needed to pin point the incipient fire. As part of the human factors analysis, a table top analysis 

of a generic plant personnel response to ASD VEWFD system ‘alert’ was conducted. The task 

analysis supports a human reliability analysis. Based on the expected operational response 

and timing estimate developed from operating experience and test results, a human reliability 

analysis was conducted based on the overall strategy that parallels post-initiator operator 

actions. The results of this HRA indicate that HEPs vary but combined with the suppression 

analysis, conclude that the trained human response is likely to succeed.
14-2

Review of operating experience, vendor supplied information, and literature has been used to 

estimate generic unreliability estimates of 1.6×10-3 per detector unit per year. This is lower than 

the generic estimate provided in EPRI 1016735, “Fire PRA Methods Enhancements: Additions, 

Clarification, and Refinements to EPRI 1019189.” However, ASD system unavailability differs 

from that report in the EPRI document, and is estimated at 2.0×10-3 per detector unit per year,

based on an average annual system down-time from plants where information was available. A 

wide variance of system downtime was observed from site visits and literature. It was noted 

that system unavailability improved for facilities that had these systems installed and operating 

for a number of years. Facilities which were using ASD systems for the first time indicated 

longer system downtime, likely because of the lack of understanding of the system maintenance 

requirements to ensure proper operation. A plant specific (or fleet specific) unavailability 

estimate could be used instead of this generic estimate, if sufficient data is available to support 

such an estimate. For area-wide air return grill applications, the unreliability/unavailability of the 

ventilation system should also be modeled, since the air return grill application requires forced 

ventilation to perform as designed.

The experimental testing program has confirmed that cabinet design, fill/obstructions and 

ventilation effects can influence the performance of VEWFD systems. Forced cabinet 

ventilation is a primary influence factor on detector response, especially with high rates of 

cabinet air exchange. As cabinet ventilation rates increase, so does smoke dilution. For the 

forced-ventilation rates used in this project’s tests, the ASDs were slower to respond in forceventilated (high air exchange rate) cabinets than naturally ventilated cabinets. In addition the 

ASDs were less effective in reaching an “alert” threshold in force-ventilated cabinet (high air 

exchange rate) tests.

For in-cabinet applications, the presence of openings or lack of partitions between adjacent 

cabinet sections having ASD sampling ports enhances the time to detection; this is because of

the cumulative effect of drawing samples from multiple sampling ports. For this effect to be 

beneficial openings between cabinets would have to be sufficient in size to allow for the air 

space communication between cabinet vertical sections.

Smoke source location also has an effect on VEWFD response. The closer the source is to the 

detector or sampling point, the more rapid the response. In the full-scale small room tests 

where the source was elevated approximately two-thirds of the height of the cabinet off the 

cabinet base; the ASDs responded approximately 9 percent faster than when the sources were 

located on the base.

Other parameters not explicitly explored in this program’s tests, but covered in the literature 

relate to soot deposition and loss of aerosol thorough ventilation. Soot deposition internal to the 

electrical cabinet will be influenced by the obstructions/fill (impaction), thermal gradients 

(thermophoresis), and electric fields (electrophoresis). Cabinets with a large surface area of 

ventilation, such as louvered vents compounded by thermophoresis, could result in a fraction of 

aerosol being lost through these vents. These phenomena would cause less aerosol to be 

transported to the ASD sampling ports located at the ceiling of the electrical cabinet resulting in 

a delay in detection (compared to the data in this report), and a decrease in the effectiveness in 

detection of low-energy fires during the incipient stage.

An evaluation of the non-suppression probability shows that the use of these systems can 

reduce plant risk from the consequences of electrical cabinet fires. It has been shown that a 

dominant contributor to the risk model is the estimation of the fraction of potentially challenging
14-3

or greater fires which exhibit an incipient fire stage of sufficient duration to allow for operator 

response. Since fire PRAs only quantify those fires that initiate and can potentially grow to a 

damaging state, the majority of smoking events are not modeled (included as a fire initiator). 

The previous methods (EPRI 1011989 and FAQ 08-0046) used to estimate this characteristic 

could not be confirmed based on the evaluation of the operating experience.

The risk benefit for using these systems varies by application with in-cabinet detection being the 

most effective approach for detecting low-energy incipient sources early enough to allow for 

suppressing before target damage to multiple components within or outside the electrical 

cabinet. Area-wide applications also provide some risk benefit; however, they are usually 

slower to detect low energy fires because of a number of previously discussed contributing 

factors.

14.1 Conclusions

This confirmatory research program has shown that the performance of smoke detection to 

detect low-energy pre-flaming conditions varies by detection technology, application, and 

aerosol characteristics (dependent on material degradation characteristics).

For in-cabinet applications, the ASD cloud chamber VEWFD and ION spot-type detection 

systems performed better than all light-scattering based technologies (three of the five ASD 

VEWFD systems, sensitive spot VEWFD and PHOTO spot-type detector). This conclusion is 

based on the systems response (ability to detect and mean time to detection) to the materials 

and methods used in testing. 

In area-wide applications, the ASD systems outperformed the conventional spot-type detectors 

(ION, PHOTO) for detecting low energy fire sources. This program has also confirmed that the 

earliest and most effective method of detecting low energy fires is when the detector or 

sampling port is located within the NPP electrical enclosure being protected.

This research has also provided a refined approach to quantify the performance of smoke 

detection systems that could be used in fire PRA applications to estimate the non-suppression 

probability. This refined approach uses operating experience, literature, test results, human 

reliability methods, and the exponential suppression model to characterize the systems 

performance. The approach relies more heavily on timing based information to characterize the 

performance of the systems tested. Because of the uncertainty associated with characterizing 

the duration and aerosol generation of the incipient stage for equipment commonly found in 

nuclear power plant electrical enclosures, there are several parameters and assumptions that 

could enhance the overall risk characterization. Most notably, the refined approach is sensitive 

to the characterization of Potentially Challenging or Greater Fires1 which exhibit an incipient 

stage with a short duration incipient stage.

All methods currently available use some form of assumptions and limitations to bound the 

evaluation. Validation of these assumptions and limitations could be better understood by 

industry support to facilitate collecting operating experience directly related to the performance 

of these ASD VEWFD systems in NPP applications or within other industries with similar 

components and equipment. Information such as nuisance alarm rate, scheduled and 

 1 Potentially challenging or greater fires are classified as challenging, potentially challenging, or undetermined 

with regard to the fire severity classification documented in EPRI 1025284, “The Updated Fire Events 

Database: Description of Content and Fire Event Classification Guidance.” 
14-4

unscheduled system down time, total number of operating detectors and years of operation for 

each system would be useful in evaluating ASD performance. In addition, consistent and 

detailed reporting of instances where potential fires were caught in an incipient stage, how long 

it took operators from time of VEWFD system alert to de-energize the equipment, instances 

where flaming fires occurred and the associated time to suppress those fires that did occur

would help support any future risk quantification effort. The number and frequency of nuisance 

alarms and the total number of detectors and ignition sources protected would also be useful. 

Complete and consistent reporting could be coordinated by a nuclear industry users group. 

General information on lessons learned from the use of these systems in NPPs could be 

communicated via industry forums to benefit the use of these systems such that an 

understanding of the performance of these systems could be achieved.
15-1

15. RECOMMENDATIONS FOR FUTURE RESEARCH

This report has provided a consolidation of the best information to date related to aspirating 

smoke detection (ASD) system response to smoke sources typically found in U.S. nuclear 

power plant (NPP) electrical enclosures, along with a method for quantifying the performance of 

these systems in fire probabilistic risk assessment (PRA). In the process of quantifying these 

systems several assumptions had to be made. The most important quantity to define is the time

duration distribution of the incipient stage. However, design fires typically do not model the 

incipient growth stage of fire. NUREG/CR-6850 (EPRI 1011989) models fires with a power law

growth profile starting at time zero. Thus, following this model provides little to no incipient 

stage, depending on how you define the transition point of incipient to growth. Thus, for a more 

consistent application of this technology’s potential advantages in performance-based 

applications, it would be desirable to develop scenario-specific (electrical enclosures, pump 

fires, transient fires, etc.) design fires that account for the incipient stage of burning. This is not 

to say that the incipient stage would exist in all scenarios, but development of a consensus 

definition of the incipient stage for various scenarios, and a consensus on how and when to 

model the incipient stage, could allow for greater certainty on the quantification of these systems 

in fire PRA.

The human error probability (HEP) estimates were based on an estimate of time available for 

operator response developed from a limited number of fire events in which sufficient information 

was available to quantify this duration; as such, this estimate carries some uncertain. An 

alternative method to quantify this duration would be to conduct a formal elicitation process

whereby a group of qualified experts with diverse backgrounds and knowledge provide 

professional judgment for use in quantifying the incipient stage duration. A panel constituted of 

equipment manufacturing experts, fire PRA experts, and experts from other industries with fire 

response field experience, could develop a comprehensive view point to represent the scientific 

communities view. Although this process may provide additional insights and knowledge to 

support an alternative approach to quantifying the incipient stage duration, the outcome of such 

effort cannot be predicted at this time, nor would it be simple to complete. In addition, the 

sensitivity study presented in section 12.2 of this report indicates that the overall quantification is 

insensitive to this parameter based on the HRA assumptions that model current practices with 

regard to human response to these types of events. While the performance of such an effort 

could potentially provide additional insights, the impact to the quantification approach presented 

in this report would be minimal.

A better measure of ASD very early warning fire detection VEWFD system performance may 

occur when sufficient operating experience in nuclear facilities is obtained and compared to 

similar applications lacking the use of this technology. Depending on use of this technology and 

quality of records, operating experience gained over a period of 5 or 10 years, may provide a 

sufficient database to support such an evaluation. However, to make for a useful measure of 

system performance, it is recommended that a user group associated with the nuclear industry

support a comprehensive and consistent operating experience reporting program. Such a 

program would provide useful information such as the number of detectors in use; nuisance 

alarm rate; availability estimates; reporting of operator response; including time to locate 

incipient source; number of cabinets being protected per detector zone; flaming fires that do 

occur in equipment protected by ASD VEWFD systems; and associated time to suppress such 

fires. Appendix G of this report provides a list of questions that may be helpful in collecting 

operating experience associated with VEWFD systems performance.
15-2

The testing program was limited by the availability of electrical enclosures to test. As such, the 

applicability of the test results for cabinets with louvered doors and/or back panels has not been 

determined. Additional data on cabinet ventilation configurations and a more rigorous 

evaluation of the effects of varying mechanical ventilation rates, would help support an 

evaluation of the performance of these systems.

As mentioned in the experimental approach section, an issue was identified concerning 

sensitivity settings for the cloud chamber ASDs, which was not fully resolved. These ASDs do 

not report detector sensitivity in terms of U.S. detection industry standard engineering units of 

percentage of obscuration per foot, but in terms of numeric (dimensionless) settings. Although 

the authors are not implying that the cloud chamber technology is deficient, guidance to support 

the selection of set points to achieve the NFPA 76 sensitivity settings would be beneficial.

This report focused exclusively on the fire hazards associated with electrical enclosure fires. 

Other types of equipment found in NPPs such as pumps, motors, air handling units, transient 

combustibles, among others can exhibit an incipient stage and ASD VEWFD systems may

provide enhanced warning and a risk reduction. An evaluation similar to what was done in this 

report would be beneficial. Follow-on work could catalog the types of smoke sources and 

materials found in NPPs that contribute to the potentially challenging or greater fires

characterized by fire PRA.
16-1

16. DEFINITIONS

Acceptance test – The process wherein every sampling port is provided an appropriate 

stimulus that simulates the existence of the design fire, and the design sequence of 

operations of each system component in the entire system is verified and recorded in 

written form.[1]

Addressable Device – A fire alarm system component with discrete identification that can have 

its status individually identified or that is used to individually control other functions. [2]

Alarm condition – An abnormal condition that poses an immediate threat to life, property, or 

mission.[2]

Aspirating smoke detector (ASD) – A detector that consists of a piping or tubing distribution 

network that runs from the detector to the area(s) to be protected. An aspiration fan in 

the detector housing draws air from the protected area back to the detector through air 

sampling ports, piping, or tubing. At the detector, air is analyzed for fire products. This 

type of detector is also known as an Air Sampling-Type Detector.

[2]

Diffusion flame – A flame in which fuel and air mix or diffuse together at the region of 

combustion.[1]

Early warning fire detection systems (EWFDS) – Systems that use smoke, heat, or flame 

detectors to detect fires before high heat conditions threaten human life or cause 

significant damage to telecommunications service.[3]

Enhanced fire suppression – as used in this report, refers to providing fire suppression 

capability earlier than typical conventional systems allow. With respect to operator 

response to very early warning fire detection systems, this implies arriving at the location 

of a potential fire threat with suppression capability before that threat transitioning to a 

flaming condition. This differs from prompt detection, as used in fire PRA suppressiondetection analysis. 

Pre-alarm condition – An abnormal condition that poses a potential threat to life, property, or 

mission, and time is available for investigation.[2]

Pyrolysis – A process in which material is decomposed or broken down, into simpler molecular 

compounds by the effects of heat alone; pyrolysis often precedes combustion.

[1]

Response time – The time between the generation of combustion aerosols at their source and 

the indication of their presences at the ASD. [4]

Sampling port – An orifice, through which air is drawn to an air sampling-type detector.

[3]

Sensitivity – Relative degree of response of a detector measured in percent per meter

obscuration (%/ft obscuration). A higher sensitivity denotes response to a lower 

concentration of smoke than a low sensitivity, under identical smoke build-up 

conditions.[5]
16-2

Sensitivity measurement – A quantitative measurement and recording of the stimulus 

necessary to achieve an alarm signal from an initiating device. A sensitivity 

measurement determines how large a stimulus is necessary to cause an alarm 

response. This measurement is to be compared to the value for the unit as shipped [as 

designed] to quantify any change in the performance one can anticipate from the unit. 

Thus, the sensitivity measurement is intended to assess the ability of the detector to 

perform its intended function when the design fire occurs. A sensitivity measurement 

differs from a test in that a test does not imply that the stimulus is of a similar magnitude 

to that obtained from the design fire.[1]

Smoke – The airborne solid and liquid particulates and gases evolved when a material 

undergoes pyrolysis or combustion, together with the quantity of air that is entrained or 

otherwise missed into the mass.

[1]

Smoke dilution – A reduction in the quantity of smoke per unit of air volume of smoke reaching 

the detector.[2]

Smoldering combustion – A slow, low-temperature, flameless form of combustion, sustained 

by the heat evolved when oxygen directly attacks the surface of a condensed-phase 

fuel.[6]

Spot-type smoke detector – A device whose detecting element is concentrated at a particular 

location.[5]

Standard fire detection systems (SFDS) – Systems that use fire detection-initiating devices to 

achieve certain life-safety and property protection in accordance with applicable 

standards.

[3]

Stratification – The phenomena whereby the upward movement of smoke and gases ceases 

because of loss of buoyancy.

[2]

Very early warning fire detection systems (VEWFD systems) – Systems that detect 

low-energy fires before the fire conditions threaten telecommunications service.[3]

Definition references:

1. National Fire Protection Association (NFPA) Fire Protection Handbook, 20th Edition, 

National Fire Protection Association, Quincy, MA, 2008.

2. NFPA Standard 72, National Fire Alarm and Signaling Code, National Fire Protection 

Association, Quincy, MA, 2010.

3. NFPA Standard 76, Standard for the Fire Protection of Telecommunications Facilities, 

National Fire Protection Association, Quincy, MA, 2012.

4. Fire Industry Association, Code of Practice; Design, Installation, Commissioning & 

Maintenance of Aspirating Smoke Detector (ASD) Systems (Vol. Issue 3). Middlesex, 

Hampton, England, 2012.

5. Underwriters Laboratories Standard 268, Smoke Detectors for Fire Alarm 

Systems, 2009.

6. Society of Fire Protection Engineers (SFPE) Handbook of Fire Protection Engineering, 

Third Edition, National Fire Protection Association, Quincy, MA, 2008.
17-1

17. REFERENCES

1. Electric Power Research Institute (EPRI), “Submittal NFPA 805 Transition Pilot Plant FAQ 

08-0046, ‘Incipient Fire Detection Systems’,” 2008. Agencywide Documents Access and 

Management System (ADAMS) Accession No. ML081200120.

2. U.S. Nuclear Regulatory Commission (NRC), Safety Evaluation, “Exemption from Apx. R 

Section III.G.2 of 10 CFR 50,” 1997. ADAMS Accession No. 9707170067.

3. Krich, R. M., “H.B. Robinson Unit 2, Response to GL-88-20, Supplement 4, IPEEE for 

Severe Accident Vulnerabilities,” 1995. ADAMS Accession No. 080100059.

4. NRC, “Closure of National Fire Protection Association 805 Frequently Asked Question 

08-0046 Incipient Fire Detection System,” 2009. Accession No. ML0932220426.

5. National Fire Protection Association (NFPA), “NFPA Fire Protection Handbook, 

20th Edition,” Quincy, MA, 2008.

6. NFPA, “Society of Fire Protection Engineering Handbook of Fire Protection Engineering,”

Quincy, MA, 2002.

7. Gottuk, D., et al., “Validation of Modeling Tools for Detection Design in High Air Flow 

Environments,” The Fire Protection Research Foundation, NFPA Quincy, MA, 2012.

8. NFPA, “NFPA Standard 72, National Fire Alarm and Signaling Code,” NFPA,

Quincy, MA, 2010.

9. NFPA, “NFPA Standard 921, “Guide for Fire and Explosion Investigations,” NFPA, 

Quincy, MA, 2014.

10. Heskestad, G., and C. Yao, “A New Approach to Development of Installation Standards 

for Fire Detectors,” FMRC Technical Proposal 19574, 1971.

11. Bukowski, R.W., R.J. O’Laughlin, and C.E. Zimmerman, “Fire Alarm Signaling Systems 

Handbook,” National Fire Protection Association, Quincy, MA, and The Society of Fire 

Protection Engineers, Boston, MA, 1987.

12. Drysdale, D., “An Introduction to Fire Dynamics,” West Sussex, Wiley, 2011.

13. NFPA Glossary of Terms, Quincy, MA, 2013.

14. Meacham, B.J., “Factors Affecting the Early Detection of Fire in Electronic Equipment and 

Cable Installation,” International Telecommunications Fire Protection Symposium, 

pp. 34-59, 1992.

15. Custer, R.L., and R.G. Bright, “Fire Detection: The State-of-the-Art,” U.S. Department of 

Commerce, National Bureau of Standards, Washington, DC, 1974.

16. NRC, “Fire Protection for Nuclear Power Plants,” Regulatory Guide 1.189, 

Revision 2, 2009.
17-2

17. Bryan, J., “Fire Suppression and Detection Systems,” Englewood Cliffs, NJ, Prentice-Hall, 

Inc., 1993.

18. NFPA, “NFPA Standard 805, Performance-Based Standard for Fire Protection for Light 

Water Reactor Electric Generating Plants,” NFPA, Quincy, MA, 2001.

19. Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard 100, The 

Authoritative Dictionary of IEEE Standards Terms, 2000.

20. Xtralis, “Letter to Barrett, H. and Moulton, C. Regarding FAQ 08-0046 - Crediting Incipient 

Fire Detection Systems (IFDS) for Fire PRA,” 2009. ADAMS Accession 

No. ML091970034.

21. Kapur, K. L., “Reliability in Engineering Design,” John Wiley & Sons, Detroit, MI., 1977.

22. NFPA, “NFPA Standard 76, Standard for the Fire Protection of Telecommunications 

Facilities,” 2012.

23. NRC, “IPEEE H.B. Robinson Review,” 2000.

24. NASA-STD-8719.11 Revision A, Safety Standard for Fire Protection

25. Forell, B., and S. Einarsson, “Extension of the Plant Specific Database of Fire Protection 

Systems and Components,” International Conference on Fire PRA, OECD/NEA 

Committee on the Safety of Nuclear Installations, Garching, Germany, 2014.

26. Groves, “Illinois Bell Telephone Fire, 1988,” 2006. Retrieved from IDEALS: 

https://www.ideals.illinois.edu/bitstream/handle/2142/95/Illinois%20Bell%20Telephone%20

Fire,%201988.pdf?sequence=2.

27. Xtralis, “IT/Server Room Fire Test Demonstrations – VESDA & photoelectric conventional 

point detectors, Technical Report,” October 2008.

28. Peters, A., “E-mail to Salley, M.H., Re: Does the Navy use Incipient Fire Detection 

systems?” 2014. ADAMS Accession No. ML14211A409.

29. Fire Industry Association, “Code of Practice; Design, Installation, Commissioning and

Maintenance of Aspirating Smoke Detector (ASD) Systems,” Issue 3, Middlesex, 

Hampton, England, 2012.

30. Meese, W., and R. Beausoliel, “Exploratory Study of Glowing Electrical Connections,”

NBS Building Science Series 103, October 1977.

31. Babrauskas, V., “Mechanisms and Modes for Ignition of Low-Voltage PVC Wires, Cables, 

and Cords,” pp. 291-309 in Fire & Materials 2005, Interscience Communications Ltd., 

London (2005).

32. Keski-Rahkonen, O., J. Mangs, and A. Turtola, A., “Ignition of and fire spread on cables 

and electronic components,” VTT Building Technology, VTT Automation, Technical 

Research Centre of Finland, Espoo, 1999.
17-3

33. Babrauskas, V., “Ignition Handbook, Fire Science Publishers/Society of Fire Protection 

Engineers, Issaquah, WA, 2003.

34. Fernandez-Pello, A., et al., “A study of the Fire Performance of Electrical Cables,” pp.

237-247, Fire Safety Science, Proceedings of the third International Symposium (IAFSS).

35. Siu, N.O., et al., “Fire Risk Analysis for Nuclear Power Plants,” 2014. ADAMS Accession 

No. ML14084A314.

36. NRC and EPRI, “NUREG/CR-6850, EPRI 1011989, EPRI/NRC-RES Fire PRA 

Methodology for Nuclear Power Facilities,” 2005.

37. EPRI, “Fire PRA Methods Enhancements, Additions, Clarifications, and Refinements to 

EPRI 1019189,” Electric Power Research Institute, 2008.

38. Joseph M. Farley Nuclear Plant, NRC Inspection Report 05000348/2011-012 and 

05000364/2011-012 and Preliminary White Finding, June 2011. ADAMS Accession 

No. ML111590912.

39. Joseph M. Farley Nuclear Plant, Final Significance Determination of a Green Finding 

(NRC Inspection Report 05000348/2011013 and 05000364/2011013), August 2011. 

ADAMS Accession No. ML112270513.

40. Licensee Event Report 1996-005-00, “Automatic Scram due to Turbine Vibration During

Planned Power Reduction,” ADAMS Accession No. 9605280291.

41. Licensee Event Report 2014-001-00, “Manual Reactor Trip due to Indications of a Fire,”

2014. ADAMS Accession No. ML14078A320.

42. Event Notification 49249, “Alert Declared due to a switchgear explosion that supplies safe 

shutdown equipment,” 2013.

43. Roush, M., and W. Webb, “Applied Reliability Engineering,” Volume 2, 5th Edition, The 

Center for Reliability Engineering, University of Maryland, College Park, MD, 2006.

44. Underwriters Laboratories Inc., “Standard for Safety, Smoke Detectors for Fire Alarm 

Systems,” Sixth Edition, 2009.

45. NUREG-2169, EPRI 3002002936, “Fire Ignition Frequencies and Non-Suppression 

Probability Estimation Using the Updated Fire Events Database,” 2014.

46. EPRI, “The Updated Fire Events Database: Description of Content and Fire Event 

Classification Guidance,” EPRI 1025284, Palo Alto, 2013.

47. Kirwan, B., and L.K. Ainsworth, “A guide to task analysis,” London, UK, p. 156, 1992.

48. NRC, “NUREG-1852, Demonstrating the Feasibility and Reliability of Operator Manual 

Actions in Response to Fire,” 2007.
17-4

49. NRC and EPRI, “NUREG-1921, EPRI 1023001, EPRI/NRC-RES Fire Human Reliability 

Analysis Guidelines,” 2012.

50. NRC, “NUREG-700, Human-System Interface Design Review Guidelines,” Revision 2, 

2002.

51. NRC, “NUREG/CR-6146, Local Control Stations: Human Engineering Issues and 

Insights,” 1994.

52. NRC, “NUREG-0711, Human Factors Engineering Program Review Model,” 2012.

53. NRC, “NUREG-800, Standard Review Plan for the Review of Safety Analysis Reports for 

Nuclear Power Plants: LWR Edition.”

54. NRC, “NUREG-1764, Guidance for the Review of Changes to Human Actions,” 2007.

55. NRC, “NUREG-1792, Good Practices for Implementing Human Reliability Analysis,” 2005.

56. Yerkes, R.M., Dodson, J.D., “The relation of strength of stimulus to rapidity of 

habit-formation,” Journal of Comparative Neurology and Psychology 18: pp. 459–482,

1908.

57. NRC, “NUREG-1624, Technical Basis and Implementation Guidelines for a Technique for 

Human Event Analysis (ATHEANA),” 2000.

58. NRC, “NUREG-1880, ATHEANA User’s Guide,” 2007.

59. NRC, “NUREG-1355, The Status of Recommendations of the President’s Commission on 

the Accident at Three Mile Island, A ten year review.”

60. ASME/ANS RA-Sa-2009, “Standard for Level 1 / Large Early Release Frequency 

Probabilistic Risk Assessment for Nuclear Power Plant Applications,” The American 

Society of Mechanical Engineers, 2009.

61. NRC and EPRI, “NUREG/CR-6850 Supplement 1, EPRI 1019259, Fire Probabilistic Risk 

Assessment Methods Enhancements,” 2010.

62. NRC, “NUREG/CR-1278, Handbook of Human Reliability Analysis with Emphasis on 

Nuclear Power Plant Applications,” 1983.

63. EPRI, “An Approach to the Analysis of Operator Actions in Probabilistic Risk Assessment,”

EPRI 100259, Palo Alto, CA, 1992.

64. NRC, “NUREG/CR-6883, The SPAR-H Reliability Analysis Method,” August 2005.

65. NRC, “NUREG/CR-4772, Accident Sequence Evaluation Program Human Reliability 

Analysis Procedure,” 1987.
A-1

APPENDIX A

VIEWGRAPHS FROM MEETING WITH ASD VENDORS

A.1 Summary of Meeting

On May 16, 2013, the U.S. Nuclear Regulatory Commission (NRC) Office of Nuclear Regulatory 

Research conducted a meeting between the three aspirating smoke detection (ASD) vendors 

whose equipment was tested. Staff from NRC, National Institute of Standards and Technology 

(NIST), and at least one technical representative from each ASD vendor were present. The 

purpose of this meeting was to inform the vendors of how their systems were being setup and 

tested, present how their systems may be used in fire probabilistic risk assessment applications, 

and to elicit feedback from the vendors on specific engineering design guidance regarding ASD 

systems for nuclear power plant applications. Additionally, discussion on equipment listings and 

approvals occurred. The morning portion of the meeting consisted of presentations given by 

NRC and NIST staff; these presentations are located in the NRC Agencywide Document Access 

and Management System (ADAMS), under Accession No. ML14356A581. The afternoon 

portion of the meeting consisted of open discussions among all participants.

B-1

APPENDIX B

SUPPORTING EXPERIMENTAL DATA

B.1 Alarm Times and Experimental Conditions and Sample Images

A file naming convention was followed to distinguish the various experiments, data files, and 

image files. Table B-1 describes the naming convention.

Table B-1. File name convention

A_XLPE_5_E_2.1

Alarm 

configuration

Material Cabinet 

configuration

Test

conditions

Heating 

Rate

Test repeat #

C: In-Cabinet 

A: Area-wide 

M: Multi-zone 

PVC1: PVC 

wire (1)

PVC2: PVC 

wire (2)

Teflon: PTFE 

wire

Silicone: 

Silicone wire

XLPO1: XLPO 

wire (1)

XLPO2: XLPO 

wire (2)

XLPE: XLPE 

wire

CSPE: CSPE 

wire

PCB: Epoxy 

Printed Circuit 

Board

PTB: Phenolic 

Terminal block

1XLPE : Single 

XLPE wire

1wire: Single 

wire test

2wire: Dual 

wire test

#Resistor: # of 

resistors

Capacitor: 2 

Capacitors

L: Laboratory

Instrument 

cabinet 

1: Single 

cabinet 

3:Three 

cabinets 

configuration

4: Four 

cabinets 

configuration

5:Five cabinets 

configuration

B: Laboratory

Bellefonte 

cabinet 

O: Center of 

the Room 

C: Corner 

(Small Room)

RL: Rear Left 

corner (Large 

Room)

FR: Front right 

corner (Large 

Room)

A:Top vents 

B:Side vents 

C: Room 

Ventilation 

D: Elevated 

Sample 

E: Cabinet

Ventilation 

F: 7.4 ACH 

Room 

Ventilation 

G: 6.5 ACH

Room 

Ventilation 

H: 14 ACH 

Room 

Ventilation 

0 : Single Wire 

test: 60 

second charge

Dual Wire 

test: 90 

seconds 

charge

1: 15 min 

nominal 

heating ramp

2: 1 hour 

nominal 

heating ramp

3: 4 hours 

nominal 

heating ramp

4: 15 min 

nominal ramp 

with extended 

hold period.
B-2

B.1.1 Raw Data File Header Descriptions

The following text describes the column header for the (raw) experimental data files for each set 

of experiments conducted: 

Alarm Time Files—Laboratory Instrument and Large Cabinet Experiments

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

ASD1 C Alarm - ASD 1 system C Alarm relay (0 : OFF, 1 : ON)

ASD1 Alarm - ASD 1 system Alarm relay (0 : OFF, 1 : ON)

ASD1 I2 - ASD 1 system I2 alarm relay (0 : OFF, 1 : ON)

ASD1 Alert - ASD 1 system Alert alarm relay (0 : OFF, 1 : ON)

ASD1 Pre-alert - ASD 1 system Pre-alert relay (0 : OFF, 1 : ON)

ASD1 Fault - ASD1 system Fault relay (0 : OFF, 1 : ON)

ASD2 Alarm- ASD 2 system Alarm relay (0 : OFF, 1 : ON)

ASD2 Alert- ASD 2 system Alert relay (0 : OFF, 1 : ON)

ASD2 I1- ASD 2 system I1 alarm relay (0 : OFF, 1 : ON)

ASD2 Pre-alert - ASD 2 system Pre-alert relay (0 : OFF, 1 : ON)

ASD2 Fault- ASD 2 system Fault relay (0 : OFF, 1 : ON)

ASD3 Alarm- ASD 3 system Alarm relay (0 : OFF, 1 : ON)

ASD3 Alert - ASD 3 system Alert relay (0 : OFF, 1 : ON)

ASD3 Fault/Pre-alert - ASD 3 system Fault/Pre-alert relay (0 : OFF, 1 : ON)

FACP Alarm - Fire alarm control panel Alarm relay (0 : OFF, 1 : ON)

FACP Trouble - Fire alarm control panel Trouble relay (0 : OFF, 1 : ON)

PHOTO (1) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located outside the 

instrument cabinet .

PHOTO (1) C Alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located outside the 

instrument cabinet.

PHOTO (2) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

instrument cabinet. 

PHOTO (2) C Alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

instrument cabinet.

SS (3) Pre-alert -SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the instrument 

cabinet.

SS (3) Pre-alert-SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the instrument 

cabinet.

ION (4) Alarm -ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the instrument 

cabinet.

ION (4) C Alarm -ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the instrument 

cabinet.

Temperature at Humidity Probe (oC) – The temperature (oC) at the location of the humidity 

probe inside the instrument cabinet.

Relative humidity (%)– Relative humidity (%) inside the instrument cabinet. 

Temperature, Top cabinet (oC) – Temperature (oC) measurement, at the base of the Instrument 

cabinet.

Temperature, Bottom cabinet (oC) - Temperature (oC) measurement , at the ceiling of the 

Instrument cabinet.

Alarm Time Files – Small Room In-cabinet Experiments 
B-3

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

ASD1 C Alarm - ASD 1 system C Alarm relay (0 : OFF, 1 : ON)

ASD1 Alarm - ASD 1 system Alarm relay (0 : OFF, 1 : ON)

ASD1 I2 - ASD 1 system I2 alarm relay (0 : OFF, 1 : ON)

ASD1 Alert - ASD 1 system Alert alarm relay (0 : OFF, 1 : ON)

ASD1 Pre-alert - ASD 1 system Pre-alert relay (0 : OFF, 1 : ON)

ASD1 Fault - ASD1 system Fault relay (0 : OFF, 1 : ON)

ASD2 Alarm- ASD 2 system Alarm relay (0 : OFF, 1 : ON)

ASD2 Alert- ASD 2 system Alert relay (0 : OFF, 1 : ON)

ASD2 I1- ASD 2 system I1 alarm relay (0 : OFF, 1 : ON)

ASD2 Pre-alert - ASD 2 system Pre-alert relay (0 : OFF, 1 : ON)

ASD2 Fault- ASD 2 system Fault relay (0 : OFF, 1 : ON)

ASD3 Alarm- ASD 3 system Alarm relay (0 : OFF, 1 : ON)

ASD3 Alert - ASD 3 system Alert relay (0 : OFF, 1 : ON)

ASD3 Fault/Pre-alert - ASD 3 system Fault/Pre-alert relay (0 : OFF, 1 : ON)

FACP Alarm - Fire alarm control panel Alarm relay (0 : OFF, 1 : ON)

FACP Trouble - Fire alarm control panel Trouble relay (0 : OFF, 1 : ON)

PHOTO (1) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

single cabinet.

PHOTO (1) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the single 

cabinet.

ION (2) Pre-alarm - ION spot detector pre-alarm (0 : OFF, 1 : ON) ) Located inside the single 

cabinet.

ION (2) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) ) Located inside the single cabinet.

SS (3) Pre-alert - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the single 

cabinet.

SS (3) Alarm - SS spot detector alarm (0 : OFF, 1 : ON) Located inside the single cabinet.

PHOTO (4) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

four- cabinet configuration.

PHOTO (4) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) ) Located inside the fourcabinet configuration.

ION (5) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) ) Located inside the fourcabinet configuration.

ION (5) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) ) Located inside the four- cabinet 

configuration.

SS (6) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) ) Located inside the four- cabinet 

configuration.

SS (6) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) ) Located inside the four- cabinet 

configuration.

ION (7) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) ) Located inside the fourcabinet configuration.

ION (7) C Alarm - ION spot detector alarm (0 : OFF, 1 : ON) ) Located inside the four- cabinet 

configuration.

PHOTO (8) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) ) Located inside the 

four- cabinet configuration.

PHOTO (8) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) ) Located inside the fourcabinet configuration.

SS (9) Pre-alert - SS spot detector pre-alarm (0 : OFF, 1 : ON) ) Located inside the four- cabinet 

configuration.
B-4

SS (9) Alarm - SS spot detector alarm (0 : OFF, 1 : ON) ) Located inside the four- cabinet 

configuration.

ION (10) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (10) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (11) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (11) Alarm - SS spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

PHOTO (12) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

five - cabinet configuration.

PHOTO (12) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (13) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (13) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

PHOTO (14) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

five - cabinet configuration.

PHOTO (14) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

SS (15) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (15) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

PHOTO (16) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

five - cabinet configuration.

PHOTO (16) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (17) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (17) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (18) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (18) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

Temperature at Humidity Probe (oC)– The temperature (oC) at the location of the humidity probe 

inside the instrument cabinet.

Relative humidity (%) – Relative humidity (%) inside the test room.
B-5

Alarm Time Files – Small Room Area-wide Experiments 

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

ASD1 C Alarm - ASD 1 system C Alarm relay (0 : OFF, 1 : ON)

ASD1 Alarm - ASD 1 system Alarm relay (0 : OFF, 1 : ON)

ASD1 I2 - ASD 1 system I2 alarm relay (0 : OFF, 1 : ON)

ASD1 Alert - ASD 1 system Alert alarm relay (0 : OFF, 1 : ON)

ASD1 Pre-alert - ASD 1 system Pre-alert relay (0 : OFF, 1 : ON)

ASD1 Fault - ASD1 system Fault relay (0 : OFF, 1 : ON)

ASD2 Alarm- ASD 2 system Alarm relay (0 : OFF, 1 : ON)

ASD2 Alert- ASD 2 system Alert relay (0 : OFF, 1 : ON)

ASD2 I1- ASD 2 system I1 alarm relay (0 : OFF, 1 : ON)

ASD2 Pre-alert - ASD 2 system Pre-alert relay (0 : OFF, 1 : ON)

ASD2 Fault- ASD 2 system Fault relay (0 : OFF, 1 : ON)

ASD3 Alarm- ASD 3 system Alarm relay (0 : OFF, 1 : ON)

ASD3 Alert - ASD 3 system Alert relay (0 : OFF, 1 : ON)

ASD3 Fault/Pre-alert - ASD 3 system Fault/Pre-alert relay (0 : OFF, 1 : ON)

FACP Alarm - Fire alarm control panel Alarm relay (0 : OFF, 1 : ON)

FACP Trouble - Fire alarm control panel Trouble relay (0 : OFF, 1 : ON)

PHOTO (1) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling.

PHOTO (1) C Alarm - PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (2) Alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (2) C Alarm - ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

SS (3) Pre-alert - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling.

SS (3) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

PHOTO (4) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling.

PHOTO (4) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (5) Pre-alarm - ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (5) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON Located on the ceiling.

SS (6) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON Located on the ceiling.

SS (6) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (7) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (7) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

PHOTO (8) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling.

PHOTO (8) C Alarm - PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

SS (9) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling.

SS (9) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling.

ION (10) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (10) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (11) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (11) Alarm - SS spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

PHOTO (12) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

five - cabinet configuration.
B-6

PHOTO (12) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (13) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (13) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

PHOTO (14) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

five - cabinet configuration.

PHOTO (14) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

SS (15) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (15) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

PHOTO (16) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the 

five - cabinet configuration.

PHOTO (16) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (17) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five -

cabinet configuration.

ION (17) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (18) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

SS (18) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located inside the five - cabinet 

configuration.

Temperature at Humidity Probe (oC)– The temperature (oC) at the location of the humidity probe 

inside the instrument cabinet.

Relative humidity (%)– Relative humidity (%) inside the test room.
B-7

Alarm Time Files – Large Room Single-zone Experiments 

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

ASD1 C Alarm - ASD 1 system C Alarm relay (0 : OFF, 1 : ON)

ASD1 Alarm - ASD 1 system Alarm relay (0 : OFF, 1 : ON)

ASD1 I2 - ASD 1 system I2 alarm relay (0 : OFF, 1 : ON)

ASD1 Alert - ASD 1 system Alert alarm relay (0 : OFF, 1 : ON)

ASD1 Pre-alert - ASD 1 system Pre-alert relay (0 : OFF, 1 : ON)

ASD1 Fault - ASD1 system Fault relay (0 : OFF, 1 : ON)

ASD2 Alarm- ASD 2 system Alarm relay (0 : OFF, 1 : ON)

ASD2 Alert- ASD 2 system Alert relay (0 : OFF, 1 : ON)

ASD2 I1- ASD 2 system I1 alarm relay (0 : OFF, 1 : ON)

ASD2 Pre-alert - ASD 2 system Pre-alert relay (0 : OFF, 1 : ON)

ASD2 Fault- ASD 2 system Fault relay (0 : OFF, 1 : ON)

ASD3 Alarm- ASD 3 system Alarm relay (0 : OFF, 1 : ON)

ASD3 Alert - ASD 3 system Alert relay (0 : OFF, 1 : ON)

ASD3 Fault/Pre-alert - ASD 3 system Fault/Pre-alert relay (0 : OFF, 1 : ON)

FACP Alarm - Fire alarm control panel Alarm relay (0 : OFF, 1 : ON)

FACP Trouble - Fire alarm control panel Trouble relay (0 : OFF, 1 : ON)

SS (1) Pre-alert - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front right corner of the room.

SS (1) Alarm - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the front 

right corner of the room.

ION (2) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front right corner of the room.

ION (2) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the front 

right corner of the room.

PHOTO (3) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling 

in the front right corner of the room.

PHOTO (3) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front right corner of the room.

ION (4) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear right corner of the room.

ION (4) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

right corner of the room.

SS (5) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

right corner of the room.

SS (5) Alarm - SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear right 

corner of the room.

PHOTO (6) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling in the rear right corner of the room.

PHOTO (6) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear right corner of the room.

PHOTO (7) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling in the rear right corner of the room.

PHOTO (7) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear left corner of the room.

ION (8) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear left corner of the room.
B-8

ION (8) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

left corner of the room.

SS (9) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

left corner of the room.

SS (9) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear left 

corner of the room.

ION (10) Pre-alarm - ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front left corner of the room.

ION (10) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the front 

left corner of the room.

SS (11) Pre-alert- SS spot detector pre- alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front left corner of the room.

SS (11) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the front left 

corner of the room.

PHOTO (12) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling in the front left corner of the room.

PHOTO (12) C Alarm - PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in 

the front left corner of the room.

ION (13) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

ION (13) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

SS (14) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

SS (14) Alarm- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

ION (15) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located in the single-cabinet 

configuration.

ION (15) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located in the single-cabinet 

configuration.

SS (16) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

SS (16) Alarm- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

Temperature at Humidity Probe (oC) – The temperature (oC) at the location of the humidity 

probe inside the instrument cabinet.

Relative humidity (%) – Relative humidity (%) inside the test room.

TC1 – Not used
B-9

Alarm Time Files – Large Room Multi-zone Experiments 

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

ASD4_Zone1_Alarm- ASD 4 system Alarm relay (0 : OFF, 1 : ON), located in zone 1.

ASD4_Zone1_Alert- ASD 4 system Alert relay (0 : OFF, 1 : ON) , located in zone 1.

ASD4_Zone1_I1- ASD 4 system I1 relay (0 : OFF, 1 : ON) , located in zone 1.

ASD4_Zone1_Pre-alert- ASD 4 system Pre-alert relay (0 : OFF, 1 : ON), located in zone 1.

ASD4_Zone2_Alarm- ASD 4 system Alarm relay (0 : OFF, 1 : ON), located in zone 2.

ASD4_Zone2_Alert- ASD 4 system Alarm relay (0 : OFF, 1 : ON) , located in zone 2.

ASD4_Zone2_I1- ASD 4 system I1 relay (0 : OFF, 1 : ON) , located in zone 2.

ASD4_Zone2_Pre-alert- ASD 4 system Pre-alert relay (0 : OFF, 1 : ON) , located in zone 2.

ASD4_Zone3_Alarm- ASD 4 system Fire 3 alarm relay (0 : OFF, 1 : ON), located in zone 3.

ASD4_Zone3_Alert- ASD 4 system Fire 2 alarm relay (0 : OFF, 1 : ON) , located in zone 3.

ASD4_Zone3_I1- ASD 4 system Fire 1 alarm relay (0 : OFF, 1 : ON) , located in zone 3.

ASD4_Zone3_Pre-alert- ASD 4 system Pre-alarm relay (0 : OFF, 1 : ON) , located in zone 3.

ASD4_Zone4_Alarm- Not used

ASD4_Zone4_Alert- Not used

ASD4_Zone4_I1- Not used

ASD4_Zone4_Pre-alert - Not used

ASD4_Fault - ASD 4 system Fault relay (0 : OFF, 1 : ON)

ASD5_Global_Pre-alert- ASD 5 system Alert relay (0 : OFF, 1 : ON), for all 3 zones (global.)

ASD5_Global_Alarm- ASD 5 system Fire 1 alarm relay (0 : OFF, 1 : ON), for all 3 zones 

(global.)

ASD5_Global_C Alarm- ASD 5 system Fire 2 alarm relay (0 : OFF, 1 : ON), for all 3 zones 

(global.)

ASD5_Zone2_Pre-alert- ASD 5 system Alert relay (0 : OFF, 1 : ON), located in zone 2.

ASD5_Zone2_Alert- ASD 5 system Action alarm relay (0 : OFF, 1 : ON), located in zone 2.

ASD5_Zone3_Pre-alert- ASD 5 system Alert relay (0 : OFF, 1 : ON), located in zone 3.

ASD5_Zone3_Alert- ASD 5 system Action alarm relay (0 : OFF, 1 : ON), located in zone 2.

ASD5_Zone1_Pre-alert- ASD 5 system Alert relay (0 : OFF, 1 : ON), located in zone 1.

ASD5_Zone1_Alert- ASD 5 system Action alarm relay (0 : OFF, 1 : ON), located in zone 2.

ASD5_Fault- ASD 5 system Fault relay (0 : OFF, 1 : ON)

FACP Alarm - Fire alarm control panel alarm relay (0 : OFF, 1 : ON)

FACP Trouble - Fire alarm control panel trouble relay (0 : OFF, 1 : ON)

SS (1) Pre-alert - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front right corner of the room.

SS (1) Alarm - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the front 

right corner of the room.

ION (2) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front right corner of the room.

ION (2) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the front 

right corner of the room.
B-10

PHOTO (3) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling 

in the front right corner of the room.

PHOTO (3) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front right corner of the room.

ION (4) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear right corner of the room.

ION (4) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

right corner of the room.

SS (5) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

right corner of the room.

SS (5) Alarm - SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear right 

corner of the room.

PHOTO (6) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling in the rear right corner of the room.

PHOTO (6) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear right corner of the room.

PHOTO (7) Pre-alarm - PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling in the rear right corner of the room.

PHOTO (7) C Alarm- PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear left corner of the room.

ION (8) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

rear left corner of the room.

ION (8) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear 

left corner of the room.

SS (9) Pre-alert - SS spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in 

the rear left corner of the room.

SS (9) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the rear left 

corner of the room.

ION (10) Pre-alarm - ION spot detector pre-alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front left corner of the room.

ION (10) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the front 

left corner of the room.

SS (11) Pre-alert- SS spot detector pre- alarm (0 : OFF, 1 : ON) Located on the ceiling in the 

front left corner of the room.

SS (11) Alarm- SS spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in the front left 

corner of the room.

PHOTO (12) Pre-alarm- PHOTO spot detector pre-alarm (0 : OFF, 1 : ON) Located on the 

ceiling in the front left corner of the room.

PHOTO (12) C Alarm - PHOTO spot detector alarm (0 : OFF, 1 : ON) Located on the ceiling in 

the front left corner of the room.

ION (13) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

ION (13) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

SS (14) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

SS (14) Alarm- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

ION (15) Pre-alarm- ION spot detector pre-alarm (0 : OFF, 1 : ON) Located in the single-cabinet 

configuration.
B-11

ION (15) C Alarm- ION spot detector alarm (0 : OFF, 1 : ON) Located in the single-cabinet 

configuration.

SS (16) Pre-alert- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

SS (16) Alarm- SS spot detector pre-alarm (0 : OFF, 1 : ON) Located in the 3-cabinet 

configuration.

Temperature at Humidity Probe (oC)– The temperature (oC) at the location of the humidity probe 

inside the instrument cabinet.

Relative humidity (%) – Relative humidity (%) inside the test room.

Bus Bar Heater File (Files ending with _T)

Column 1 - CPU time (Hour : Minute : Seconds)

Column 2 - Counter (s)

Column 3 – Set point temperature (oC)

Column 4 – Actual Bus Bar temperature (oC)

Room Temperature File (Files ending with _RT)

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

TC 1 - Temperature (oC) in the center of the room, 2.54 cm below the ceiling.

TC 2 - Temperature (oC) in the center of the room, 5.08 cm below the ceiling.

TC 3 - Temperature (oC) in the center of the room, 7.62 cm below the ceiling.

TC 4 - Temperature (oC) in the center of the room, 0.31 m below the ceiling.

TC 5 - Temperature (oC) in the center of the room, 0.61 m below the ceiling.

TC 6 - Temperature (oC) in the center of the room, 0.914 m below the ceiling.

TC 7 - Temperature (oC) in the center of the room, 2.13 m below the ceiling.

TC 8 - Ceiling temperature (oC) in the corner of the room.

TC 9 - Ceiling temperature (oC) in the corner of the room.

TC 10 - Ceiling temperature (oC) in the corner of the room.

TC 11 - Ceiling temperature (oC) in the corner of the room.

TC 12 - Room ventilation inlet temperature (oC).

TC 13 - Room ventilation outlet temperature (oC).

Wire Thermocouples (Files ending with _WT)

Time - CPU time (Hour : Minute : Seconds)

Count - Loop time increment (s)

TC 1 (X mm)- Thermocouple #1 located X mm form the bus bar. 

TC 2 (X mm)- Thermocouple #2 located X mm form the bus bar. 

TC 3(X mm)- Thermocouple #3 located X mm form the bus bar. 

TC 4(X mm)- Thermocouple #4 located X mm form the bus bar. 
B-12

B.1.2 Sample before and after experiment images, and thermal camera image 

sequences

Before and after sample images use the file naming convention appended with “Before” or 

“After”. Table B-2 described exemplar image file names for before and after pictures of 

materials tested. Sequences of thermal images at fixed time intervals are given in folders 

labeled using the file naming convention appended with “TI”. The name of each thermal image 

corresponds to the time in minutes at which it was taken. The image analysis software that 

ships with the thermal imaging camera applies an imaging softening filter to reduce pixilation. 

These images do not include that filtering process.

Table B-2. Exemplar image file names for materials tested.

Material Heating ramp Before and after images Thermal images 

folder

PVC wire (1) 15 min N/A C_PVC1_L_B_1.1_TI

1 hour C_PVC1_L_B_2.3_Before

C_PVC1_L_B_2.3_After

C_PVC1_L_A_2.1_TI

PVC wire (2) 15 min C_PVC2_L_A_1.2_Before

C_PVC2_L_A_1.2_After

C_PVC2_L_B_1.1_TI

1 hour C_PVC2_L_B_2.2_Before

C_PVC2_L_B_2.2_After

C_PVC2_L_A_2.1_TI

4 hours N/A C_PVC2_L_B_3.1_TI

Silicone wire 15 min C_Silicone_L_A_1.2_Before

C_ Silicone_L_A_1.2_After

C_Silicone_L_B_1.1_TI

1 hour C_Silicone_L_B_2.3_Before

C_ Silicone_L_B_2.3_After

C_Silicone_L_A_2.1_TI

PTFE wire 15 min C_Teflon_L_A_1.2_Before

C_Teflon_L_A_1.2_After

C_Teflon_L_B_1.1_TI

1 hour C_Teflon_L_B_2.2_Before

C_Teflon_L_B_2.2_After

C_Teflon_L_A_2.1_TI

XLPO wire (1) 15 min C_XLPO1_L_A_1.2_Before

C_XLPO1_L_A_1.2_After

C_XLPO1_L_B_1.1_TI

1 hour C_XLPO1_L_B_2.2_Before

C_XLPO1_L_B_2.2_After

C_XLPO1_L_A_2.1_TI

XLPO wire (2) 15 min C_XLPO2_L_A_1.2_Before

C_XLPO2_L_A_1.2_After

C_XLPO2_L_B_1.1_TI

1 hour C_XLPO2_L_B_2.2_Before

C_XLPO2_L_B_2.2_After

C_XLPO2_L_A_2.1_TI

4 hours C_XLPO2_L_A_3.1_Before

C_XLPO2_L_A_3.1_After

C_XLPO2_L_A_3.1_TI

XLPE wire 15 min C_XLPE_L_A_1.3_After C_XLPE_L_B_1.1_TI

1 hour C_XLPE_L_B_2.2_Before

C_XLPE_L_B_2.2_After

C_XLPE_L_A_2.1_TI

4 hours N/A C_XLPE1_L_B_3.1_TI

CSPE wire 15 min C_CSPE_L_A_1.2_Before

C_ CSPE_L_A_1.2_After

C_CSPE_L_B_1.1_TI

1 hour C_CSPE_L_B_2.2_Before

C_ CSPE_L_B_2.2_After

C_CSPE_L_A_2.1_TI

Epoxy Printed Circuit 

Board

15 min C_PCB_L_A_1.2_Before

C_PCB_L_A_1.2_After

C_PCB_L_B_1.1_TI

1 hour C_PCB_L_A_2.2_Before

C_PCB_L_A_2.2_After

C_PCB_L_A_2.1_TI

4 hours C_PCB_L_B_3.1_Before

C_PCB_L_B_3.1_After

C_PCB_L_B_3.1_TI

Phenolic Terminal block 15 min C_PTB_L_A_1.2_Before

C_PTB_L_A_1.2_After

C_PTB_L_B_1.1_TI

1 hour C_PTB_L_B_2.2_Before C_PTB_L_A_2.1_TI
B-13

C_PTB_L_B_2.2_After

4 hours N/A C_PTB_L_B_3.1_TI

Single XLPE wire 1 hour N/A C_1XLPE_L_A_2.1_TI

B.1.3 Tabulated Alarm Times

Alarm times, the block temperatures at alarm, and the beginning and end of test air 

temperatures and in some cases humidity were extracted from the raw data files for each 

experiment and tabulated in Alarm Time files. The files include individual test file names, test 

configuration and sample mass loss. Table B-4 lists the experimental series, the report section 

describing the series, and the spreadsheet file name. 

Table B-4. Spreadsheet file names for tabulated alarm times

Experimental Series Spreadsheet file name

Aerosol exposure experiments - Section B.2.1 Instrument cabinet CMAG and Gemini exps

Instrument Cabinet aerosol measurement 

experiments - Section B.2.1 

Instrument Cabinet exps - Aerosol moments

Large Cabinet experiments - Variable hold times Large Cabinet Experiments - Variable hold 

times

Laboratory small cabinet experiments - Section 5.1 Instrument Cabinet Experiments

Laboratory large cabinet experiments - Section 5.2 Large Cabinet Experiments - Naturally 

Ventilated

Large Cabinet Experiments - Force Ventilated

Small room, in-cabinet experiments - Section 5.3 Small Room In-Cabinet Experiments

Small room, area-wide experiments - Section 5.6 Small Room Area-wide Experiments

Large room, single zone, in-cabinet experiments -

Section 5.4

Large Room Single-zone In-cabinet 

Experiments

Large room, multi-zone, in-cabinet experiments -

Section 5.5

Large Room Multi-zone In-cabinet Experiments

Large room, multi-zone, area-wide experiments -

Section 5.7

Large Room Multi-zone Area-wide Experiments

The ASD VEWFD systems tested allow for three to five setpoints (depending on vendor). The 

raw data files included with this report contains the data for the setpoints used and recorded in 

this program. The following information identifies the setpoints used for the different systems 

and test series. ASD 1, 3, and 5 are light scattering based technologies and the sensitivities are 

reported in % obscuration per foot. ASD 2 and 4 are cloud chamber based technologies and 

the sensitivities are reported in particles per cubic centimeter. Sensitivities for the other 

detectors such as the PHOTO, ION, and SS are reported in the main body of the report. The 

sensitivities for each of the testing configurations are presented in Tables B-5 to B-10.
B-14

Table B-5. Laboratory Scale Small Cabinet Experiments

Setpoint ASD 1 ASD 2 ASD 3

Detector Port Detector Port Detector Port

Pre-Alert 0.012 0.048 5.1×10+5 2.0×10+6 0.025 0.100

I1 Alarm N/A 8.5×10+5 3.4×10+6 N/A

Alert 0.050 0.200 1.2×10+6 4.8×10+6 0.050 0.200

I2 Alarm 0.100 0.400 N/A N/A

Alarm 0.250 1.000 1.5×10+6 6.0×10+6 0.250 1.000

C Alarm 0.500 2.000 N/A N/A

# of ports / 

zone

4 4 4

Table B-6. Laboratory Scale Large Cabinet Experiments

Setpoint ASD 2 ASD 3

Detector Port Detector Port

Pre-Alert 3.8×10+4 1.5×10+5 0.025 0.100

I1 Alarm 1.4×10+5 5.5×10+5 N/A

Alert 1.4×10+5 5.5×10+5 0.050 0.200

I2 Alarm N/A N/A

Alarm 6.4×10+5 2.6×10+6 0.250 1.000

C Alarm N/A N/A

# of ports / 

zone

4 4

Table B-7. Full Scale Small Room In-Cabinet Experiments

Setpoint ASD 1 ASD 2 ASD 3

Detector Port Detector Port Detector Port

Pre-Alert 0.012 0.072 5.1×10+5 3.1×10+6 0.008 0.050

I1 Alarm N/A 8.5×10+5 5.1×10+6 N/A

Alert 0.050 0.300 1.2×10+6 7.2×10+6 0.033 0.200

I2 Alarm 0.100 0.600 N/A N/A

Alarm 0.250 1.500 1.5×10+6 9.0×10+6 0.167 1.000

C Alarm 0.500 3.000 N/A N/A

# of ports / 

zone

6 6 6
B-15

Table B-8. Full Scale Large Room In-Cabinet Experiments

Setpoint ASD 2 ASD 3 ASD 4 ASD 5

Detector Port Detector Port Detector Port Detector Port

Pre-Alert 3.8×10+4 1.5×10+5 0.006 0.02

5 1.5×10+5 6.0×10+

5 0.016 0.06

4

I1 Alarm 1.4×10+5 5.5×10+5 N/A 2.5×10+5 1.0×10+

6 N/A

Alert 1.4×10+5 5.5×10+5 0.050 0.20

0 3.5×10+5 1.4×10+

6 0.033 0.13

4

Alarm 6.4×10+5 2.6×10+5 0.250 1.00

0 4.5×10+5 1.8×10+

6 0.166 0.66

6

# of ports 

/ zone 4 4 4 4

Note that ASD 4 and ASD 5 are multi-zone detectors. For ASD 4 Zone 1 provided the in-cabinet 

sampling. For ASD 5 Zone 3 provided the in-cabinet sampling.

Table B-9. Full Scale Small Room Area-wide Experiments

Setpoint ASD 1 ASD 2 ASD 3

Detector Port Detector Port Detector Port

Pre-Alert 0.012 0.048 5.1×10+5 2.0×10+6 0.025 0.100

I1 Alarm N/A 8.5×10+5 3.4×10+6 N/A

Alert 0.050 0.200 1.2×10+6 4.8×10+6 0.050 0.200

I2 Alarm 0.100 0.400 N/A N/A

Alarm 0.250 1.000 1.5×10+6 6.0×10+6 0.250 1.000

C Alarm 0.500 2.000 N/A N/A

# of ports / 

zone

4 4 4

Table B-10. Full Scale Large Room Area-wide Experiments

Setpoint ASD 4 ASD 5

Detector Port Detector Port

Pre-Alert 1.5×10+5 9.0×10+5 0.016 0.095

I1 Alarm 2.5×10+5 1.5×10+6 N/A

Alert 3.5×10+5 2.1×10+6 0.033 0.200

Alarm 4.5×10+5 2.7×10+6 0.166 0.999

# of ports / 

zone

6 6

Note that ASD 4 and ASD 5 are multi-zone detectors. For ASD 4 Zone 2 provided the air return 

sampling, while Zone 3 provided the ceiling air sampling. For ASD 5 Zone 2 provided the air return 

sampling, while Zone 1 provide the ceiling air sampling.
B-16

B.2 Smoke Aerosol Measurements

Instrumentation descriptions

Aerosol characterization equipment included an electrical low pressure impactor (Dekati, ELPI) 

to record aerosol concentration and size distribution, a condensation particle counter (CPC, TSI 

Model 3775) to provide the aerosol number concentration, an electrical aerosol detector (EAD, 

TSI Model 3007) to provide a measure of the sum of particle diameters, and a tapered element 

oscillating microbalance (R&P Inc. TEOM Model 1105) to record aerosol mass concentration. 

The ELPI consists of a 13-stage multi-orifice, low-pressure impactor that classifies particles 

according to their aerodynamic size (equivalent diameter unit density sphere). A schematic 

diagram of the instrument is shown in Figure B1. Beginning at the first stage, particles of a 

narrow size range (defined by a cut-off size, 50%, where 50 percent of particles of a given size 

are collected) impact on that stage’s collection plate, while smaller particles move on to the next 

stage. The process repeats itself until the last stage is reached. The particle concentration 

below 0.03 μm is not measured, thus the size distribution is truncated. This may affect 

concentration and average size measurements, and the instrument results are subject to this 

bias. 

The particles are separated according to their inertial properties, thus sizes are reported in 

terms of the diameter of unit density spheres with the same inertial properties, termed the 

aerodynamic diameter. The flow through the instrument is 10 l/min. Typically, cascade 

impactors rely on a gravimetric determination of the amount of particles collected on any stage, 

thus the sampling time must be sufficient to gather a weighable amount of material on each 

stage. This impactor is unique in that it detects particles that impact on the different stages by 

measuring the charge transferred to the stage from the elemental charges carried by the 

particles. Aerosol particles will achieve a statistically average charge level based on particle 

diameter, initial charge state, and exposure to charging mechanisms. 

The ELPI conditions the aerosol to such a state by a two-step process. The initial charge state 

is forced to an equilibrium, Boltzmann charge distribution by passing the aerosol through a 

charge neutralizer (external to the ELPI). Then, a high-voltage corona wire unipolar charger 

puts known excess charge on the aerosol particles based on their size and the residence time 

the aerosol remains in the charging section. Excess ions and very small, charged particles are 

removed by an ion trap just past the charger. Each impactor stage (excluding the first which 

removes particles with aerodynamic diameters larger than 10 μm) is electrically isolated and 

connected to an electrometer. As aerosol particles impact on the various stages, they transfer 

their charges and a current is measured. From the current measurement, impactor stage cut-off 

sizes, flow through the instrument, and the relationship between the particle size and average 

charge, the number of particles that impact each stage is computed and the number size 

distribution is characterized. The number distribution can be converted into diameter, surface 

area, or mass distribution, etc., and the total number, or mass (assuming spherical unit density 

particles) can be computed.
B-17

+

-

KR85 Aerosol Neutralizer

Corona Charger

Electrometers

A/D Converter

Microcontroller

Isolated

Impactor 

Stages

To Vacuum

Pump

Corona Wire

Trap

Aerosol Flow

Figure B-1. Schematic diagram of the electrical low pressure impactor

Table B-11 shows the impactor stage cut-off sizes and the geometric mean of the size range of 

particles collected on a given stage, ii, for standard impaction plates covered with aluminum 

foil. 

Table B-11. ELPI Imactor Plate Cutoff and Median Diameters

Impactor Stage

Standard Impactor Plates

Aerodynamic 50%

(μm)

Aerodynamic 

ii

(μm)

1 0.0280 0.0395

2 0.0557 0.0727

3 0.0948 0.122

4 0.157 0.203

5 0.263 0.317

6 0.383 0.485

7 0.615 0.764

8 0.950 1.23

9 1.60 1.96

10 2.40 3.10

11 4.00 5.18

12 6.70 8.16

13 9.93 -
B-18

If density is known, then the aerodynamic diameter can be replaced with the Stokes diameter, 

defined as the diameter of a sphere with the same density and settling velocity of the particle.

The analysis of the ELPI data takes into account known aerosol particle densities. For the 

unknown aerosol densities of the smokes generated from material degradation, a density of 

1.00 g/cm3 is assumed, but this adds to the uncertainty in the results for the aerosol 

concentrations and size distribution. 

The CPC can detect and count particles 4 nm in diameter and larger up to number 

concentrations of 1×107 particles/cm3. The CPC draws aerosols into a heated saturator where 

alcohol vapor and the aerosol mix, this particle/vapor mixture then flows to a condensing section 

where the vapor becomes superheated and condenses on the aerosol particles greater than 4 

nm. The particles grow rapidly to large individual particles that are counted optically. In that 

respect, the detection principle of the CPC is similar to a cloud chamber ASD. The CPC 

combined relative uncertainty is better than ±10 percent of the reading.

The EAD measures an aerosol concentration referred to as the total aerosol length, equal to the 

sum of the diameter of all particles in a unit volume. This quantity is also referred to as the first 

moment of the size distribution (the number concentration is the zeroth moment). In the EAD,

particles flow into a charging section where positive ions accumulate on particles to a net 

charge state proportional to the particle diameter. An aerosol electrometer measures the net 

charge, which is proportional to the total aerosol length. The combined relative uncertainty in 

the EAD measurement is typically better than ±10 percent of the reading.

The TEOM is a direct measure of the mass concentration of an aerosol. The sampled aerosol 

with a fixed volumetric flow is passed through a vibrating filter that accumulates the aerosol 

particles. The vibration frequency is proportional to the mass of the filter. The frequency is 

measured as a function of time, and with the volumetric flow rate, the mass concentration is 

computed. The TEOM combined standard uncertainty is ± 0.5 mg/m3. 

In the various laboratory test configurations, the transport time from the in-cabinet sample inlet 

to the various instruments was on the order of 2 seconds or less. The response time (95

percent of input change) of each instrument varied, with values for the TEOM, ELPI, CPC and 

EAD on the order of 5, 5, 2, and 2 seconds, respectively.

Properties of the size distribution of interest include the first moment of the number distribution 

with the mean size defined as the arithmetic mean diameter (AMD), given by

AAAA = ∑(nnii ∙ ii)

NN

B-1

where nnii is the number of particles of size group ii, and NN is the total number of particles. 

The first moment correlates with the response of the ionization chamber in smoke detectors. 

Another property of interest is the mass (or volume) distribution, which is a better predictor of 

the response of light-scattering, photoelectric alarms than the diameter distribution. The third 

moment of the size distribution can be represented by the total mass, M (or volume) with a 

mass mean diameter ( ), given by

 

= ∑(mmii ∙ ii) B-2
B-19

where mmii is the number of particles of size group ii, and is the mass of all particles. 

Comparing these two mean diameters gives a sense of the width of the size distribution. If both 

AMD and are the same the particles are of a single size (monodisperse) while an 

increasing difference between the two diameters indicates ever broadening distribution. For 

log-normally distributed aerosols with an AMD of 0.100 μm and geometric standard deviations 

(σσ ) of 1.30 and 1.70, the is 0.123 μm and 0.233 μm respectively. 

The diameter of average mass can be computed if the mass and number concentration are 

known using equation 3.

= �

6

ρρρρ �

1

3�

B-3

B.2.1 Instrument Cabinet Experiments

Experiments were conducted using a condensation mono-disperse aerosol generator (CMAG, 

TSI Model 3475), an instrument that can produce high concentrations of narrow size distribution 

aerosols from 0.1 μm up to 8 μm in diameter, and a Gemini smoke detector tester aerosol 

generator, which produces aerosol designed to mimic smolder smokes. The CMAG produces 

particles from Di-Ethyl-Hexyl-Sebacate (DEHS, density of 0.912 g/cm3), while the smoke 

detector tester produces particles from mineral oil (density of 0.85 g/cm3).

Aerosols from the CMAG or smoke detector tester were introduced into the cabinet from a tube

located at the cabinet floor and pointing up. The aerosol was sampled from the ceiling of the 

instrument cabinet and directed to the ELPI. Experiments were conducted with 9 different 

settings on the CMAG, and one base setting on the smoke detector tester. Data for the 

experiments are located in the spreadsheet files labeled {Instrument cabinet CMAG and Gemini 

exps} and {Test_aerosol_ELPI}. The first file contains the ASD alarm times and the end of test 

number and mass concentrations, and arithmetic mean diameter and diameter of average 

mass, while the second file contains the number, total aerosol length, and mass concentrations, 

along with the arithmetic mean diameter and diameter of average mass values as functions of 

time. The device settings for these experiments are given in Table B-2.
B-20

Table B-12. Nominal Detector Sensitivities for Instrument Cabinet Experiments

Sensitivity

Setting

ASD1

Detector /

Port

%/ft Obsc

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFDS Pre-alert 0.013 / 

0.05

5.1×105 /

2.0×106

0.025 / 

0.10 - - -

VEWFDS Intermediate 

(I1) - 8.5×105 / 3.4×106 - - - -

VEWFDS Alert 0.05 / 0.20 1.2×106 /

4.8×106 0.05 / 0.20 0.20 - -

VEWFDS Intermediate 

(I2) 0.10 / 0.40 - - - - -

VEWFDS Alarm 0.25 / 

1.00

1.5×106 /

6.0×106 0.25 / 1.00 1.00 - -

Pre-Alarm - - - - 0.5 1.3

Conventional Alarm 0.50 / 2.00 - - - 1.0 2.1

The arithmetic mean particle size at the end of the test for the CMAG experiments ranged from 

0.12 μm to 0.82 μm, depending on the CMAG settings, and 0.33 μm for the smoke detector 

tester aerosol. In only one CMAG setting did ASD2 (cloud chamber type) respond at alert or 

alarm settings, and in fact, only pre-alerted in one other CMAG setting. The light-scattering 

ASDs tended to respond at all CMAG settings. The smoke detector tester aerosol triggered all 

light-scattering response levels, while the cloud chamber ASD responded at the level between 

pre-alert and alert consistently. These results show the response to various concentrations of 

given particle size distributions, but by themselves do not indicate effectiveness, nor 

appropriateness for a given application. 

An example of the ELPI data is shown below in Figures B-2 and B-3. The aerosol concentration 

curves are proportional to each other since the size distribution of the CMAG aerosol does not 

change much with time. The mean size fluctuates prior to the arrival of the CMAG aerosol at 

the sampling port due to the low concentration background (room) aerosol. The mean 

diameters show a slight growth trend in time which may be due to the aerosol generator, or 

aging of the aerosol at the ceiling of the cabinet. 
B-21

0

1 106

2 106

3 106

4 106

5 106

0

5

10

15

20

25

0 200 400 600 800 1000

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B-2. ELPI aerosol concentrations for CMAG experiment 40.

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 200 400 600 800 1000

Arithmetic mean diameter

Diameter of average mass

Diameter (

μm)

Time (s)

Figure B-3. ELPI aerosol diameters for CMAG experiment 40.
B-22

B.2.1.2 Degraded materials 

Almost all experiments conducted in the instrument cabinet with the degrading materials were 

monitored with the ELPI. The spreadsheet labeled {Instrument_cabinet_ELPI} contains the 

ELPI results number, total aerosol length, and mass concentrations, along with the arithmetic 

mean diameter and diameter of average mass values as functions of time. The spreadsheet 

labeled {Instrument Cabinet Experiments} contains the configuration and alarm times. The 

device setting for these experiments are given in Table B-12. Figures B-4 – B-9 show ELPI 

60.0 minute HRP results for XLPE, PVC(2), and CSPE samples. 

Additional experiments were conducted in which the smoke sampled from the aerosol sampling 

port was directed to the CPC, EAD and TEOM as alternate direct measurements of number 

concentration, total aerosol length, and mass concentration. The materials examined were 

limited to XPLE, PVC(2) and CSPE at HRPs of 15.0 minute and 60.0 minute, and overheated 

resistors. The spreadsheet labeled {Instrument_cabinet_moments} contains the measurement 

results and the bus bar heating measurements for those experiments.

0

5 105

1 106

1.5 106

2 106

2.5 106

0

2

4

6

8

10

0 1000 2000 3000 4000

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B-4. ELPI aerosol concentration for XLPE and 60.0 minute HRP.
B-23

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1000 2000 3000 4000

AMD 

DAM 

Mean Diameter (

μm)

Time (s)

Figure B-5. ELPI mean particle diameters for XLPE and 60.0 minute HRP.

0

2 106

4 106

6 106

8 106

1 107

0

2

4

6

8

10

0 1000 2000 3000 4000

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B-6. ELPI aerosol concentration for PVC(2) and 60.0 minute HRP.
B-24

0.05

0.1

0.15

0.2

0 1000 2000 3000 4000

AMD 

DAM 

Mean Diameter (

μm)

Time (s)

Figure B-7. ELPI mean particle diameters for PVC(2) and 60.0 minute HRP

0

2 106

4 106

6 106

8 106

1 107

0

5

10

15

20

25

0 1000 2000 3000 4000

Number 

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B-8. ELPI aerosol concentration for CSPE and 60.0 minute HRP.
B-25

0.05

0.1

0.15

0.2

0.25

0.3

0 1000 2000 3000 4000

AMD 

DAM 

Mean Diameter (

μm)

Time (s)

Figure B-9. ELPI mean particle diameters for PVC(2) and 60.0 minute HRP

Additional experiments were conducted where the smoke sampled from the aerosol sampling 

port was directed to the CPC, EAD and TEOM as alternate direct measurements of number 

concentration, total aerosol length, and mass concentration. The materials examined were 

limited to XPLE, PVC(2) and CSPE at HRPs of 15.0 minute and 60.0 minute, and overheated 

resistors. The spreadsheet labeled {Instrument Cabinet exps - Aerosol moments} contains the 

configuration and alarm times. The spreadsheet labeled {Instrument_cabinet_moments} 

contains the measurement results and the bus bar heating measurements for those 

experiments. The device settings for these experiments are given in Table B-13 for the first 15

minute and 1 hr heating ramp period experiments and Table B-14 for the next three 15 minute 

and 1 hr heating ramp period experiments. 
B-26

Table B-13. Nominal Detector Sensitivities for Laboratory—Aerosol Moments 

Experiments

Sensitivity

Setting

ASD1

Detector /

Port

%/ft Obsc

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFDS Pre-alert 0.013 / 

0.05

1.9×105 /

7.7×106

0.0063 / 

0.025 - - -

VEWFDS 

Intermediate (I1) - 2.5×105 / 1.0×106 - - - -

VEWFDS Alert 0.05 / 0.20 3.7×105 /

1.5×106

0.0125 / 

0.050 0.20 - -

VEWFDS 

Intermediate (I2) 0.10 / 0.40 - - - - -

VEWFDS Alarm 0.25 / 

1.00

8.5×105 /

3.4×106

0.025 / 

0.10 1.00 - -

Pre-Alarm - - - - 0.5 1.3

Conventional Alarm 0.50 / 2.00 - - - 1.0 2.1

Table B-14. Nominal Detector Sensitivities for Laboratory—Aerosol Moments 

Experiments

Sensitivity

Setting

ASD1

Detector /

Port

%/ft Obsc

ASD2

Detector /

Port

Particles/cm3

ASD3

Detector /

Port

%/ft Obsc

SS

%/ft 

Obsc

ION

%/ft 

Obsc

PHOTO

%/ft 

Obsc

VEWFDS Pre-alert 0.013 / 

0.05

8.4×104 /

3.4×105

0.0063 / 

0.025 - - -

VEWFDS Intermediate 

(I1) - 1.3×105 / 5.2×105 - - - -

VEWFDS Alert 0.05 / 0.20 1.9×105 /

7.7×105

0.0125 / 

0.050 0.20 - -

VEWFDS Intermediate 

(I2) 0.10 / 0.40 - - - - -

VEWFDS Alarm 0.25 / 

1.00

8.5×105 /

3.4×106

0.025 / 

0.10 1.00 - -

Pre-Alarm - - - - 0.5 1.3

Conventional Alarm 0.50 / 2.00 - - - 1.0 2.1
B-27

B.2.2 Large Cabinet 

During the large cabinet experiments, smoke was sampled from the top of the cabinets and 

directed to the ELPI. Data was collected for the 16.3 and 65.0 minute HRP experiments for 

XLPE, PVC(2), CSPE, and PCB materials, and for 260 minute HRP experiments for XLPE and 

CSPE materials. Experiments were conducted with XLPE and CSPE samples in which the 

HRP was reduced to achieve a lower bus bar temperature, and the hold time was increased so 

the EOT times, 21.3 and 70 minutes, were the same. In addition, BS 6266 1 meter wire tests 

were conducted. The spreadsheet labeled {Large_cabinet_ELPI} contains the ELPI results 

number, total aerosol length, and mass concentrations, along with the arithmetic mean diameter 

and diameter of average mass values as functions of time.

Figures B10 – B13 show the aerosol concentration results for naturally ventilated cabinet and 

XLPE and CSPE experiments, with the normal 16.3 minute HRP and shorter HRPs to set points 

of 325 °C and 225 °C, respectively.

0

2 105

4 105

6 105

8 105

1 106

0

2

4

6

8

10

0 250 500 750 1000 1250 1500

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B10. Large, naturally ventilated cabinet ELPI aerosol concentration for XLPE and 

16.3 minute HRP.
B-28

0

2 104

4 104

6 104

8 104

1 105

0

0.2

0.4

0.6

0.8

1

0 250 500 750 1000 1250 1500

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B11. Large, naturally ventilated cabinet ELPI aerosol concentration for XLPE and 

10.5 minute HRP to a set point of 325 °

C and held until 21.3 min.

0

5 105

1 106

1.5 106

2 106

0

2.5

5

7.5

10

0 250 500 750 1000 1250 1500

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B12. Large, naturally ventilated cabinet ELPI aerosol concentration for CSPE 

and 16.3 minute HRP.
B-29

0

1 105

2 105

3 105

4 105

5 105

0

0.5

1

1.5

2

0 250 500 750 1000 1250 1500

Number

TAL

Mass

Number Concentration (#/cm3) and 

TAL (

μm/cm3 X10)

Mass Concentration (mg/m3

)

Time (s)

Figure B13. Large, naturally ventilated cabinet ELPI aerosol concentration for CSPE 

and 7.9 minute HRP to a set point of 250 °

C and held until 21.3 min.

B.3 VEWFD System Performance versus Cloud Chamber ASD Sensitivity 

Setting

Experiments using the single-zone cloud chamber (ASD2) did not consistently use a fixed alert 

sensitivity setting, nor was a fixed estimated alert sensitivity setting used in the analysis 

reported in Part II of this report. The analysis in Part II used specific recorded alert times, or 

averaged time from two ASD2 settings, and were estimated values for sensitivity levels 

considered representative of what would meet the minimum sensitivity requirements of NFPA 

76. It was noted that the cloud chamber sensitivity range appeared to cover a smaller sensitivity 

range compared to the smoke obscuration range for the overheated wire and material 

experiments performed, and moderate changes in sensitivity did not necessarily translate into 

significant alert time differences. The reason for this appears to be due to the observed 

exponential increase in particle number concentration throughout the cloud chamber sensitivity 

range during the linear heating ramp experimental test scenarios.

To assess the effects sensitivity setting on the analysis in Part II for a fixed cloud chamber ASD 

alert sensitivity over all experiments, estimates were made of the time to alert for a sensitivity 

specified by the manufacturer of the cloud chamber ASD in the multi-zone cloud chamber 

experiments for all experiments conducted. Note, there was no modification of the cloud 

chamber device sensitivity for in-cabinet nor area-wide and return air duct zones, thus effective 

port sensitivities will depend on the number of ports and the particle concentration sampled at 

each port. This followed the manufacturer’s specifications for the different multi-zone ASD 

experiments.
B-30

Table B-15 gives conversion factors to convert between the gain setting and particle 

concentration. These factors were estimated from the values obtained from the instrument 

software. The method of calibration in unknown, so comparisons to specific particle number 

concentration measurements (in this report or elsewhere) should be made with caution. The 

conversion factors were then used to convert the specific experimental single-zone and multizone device and port settings to particle concentrations.

Table B-15. Estimated ASD2 and ASD4 Gain Factors

Gain Gain Factor

(particles/ cm3)

5 17000

6 7300

7 5000

8 3850

9 2560

10 1670

The particle concentration is obtained by multiplying the gain factor for a specific gain setting by 

the percentage of the gain setting (i.e., the particle concentration for a setting of 50 percent of a 

gain setting of 5 (5/50) would be 17,000 times 50 = 8.5×105 particles/ cm3. The manufacturerspecified alert sensitivity setting for the multi-zone experiments was a Gain of 7 at 30 percent

(7/30), or an estimated particle concentration device sensitivity of 1.50×105 particles/ cm3. It is 

not clear how this sensitivity setting is related to the NFPA 76 alert sensitivity of 0.2 %/ft 

obscuration at the sampling port or spot device location.

Table B-16 shows the specific settings for the single zone ASD2 experiments, the alert setting 

used for the analysis of each set of experiments and the corresponding light scattering ASD3 

settings. Based on the particle concentration conversion, the large cabinet and large room incabinet experiment device alert sensitivity values were very close to the manufacturer-specified 

sensitivity, while for the other experiments the closest sensitivity setting was greater than 5 

times the manufacturer-specified particle concentration sensitivity. Thus, either interpolation or 

extrapolation of time to reach the manufacturer-specified device setting of 7/30 will align all 

experiments to a single fixed alert setting. 

Table B-16. Single Zone ASD2 and ASD3 Device Settings and Alert Levels

Experimental 

Configuration 

Device 

Setting

ASD2 

Device 

Sensitivity

(part./cm3)

ASD2 Port 

Sensitivity

(part./cm3)

Light Scattering 

ASD3 Device 

Setting

(%/ft Obscuration)

Light Scattering 

ASD3 Port Setting

(%/ft Obscuration)

Instrument 

Cabinet

Experiments

5/30 5.1×105 2.0×106 0.025 0.10

5/50 8.5×105 3.4×106 0.050 0.20

5/70 1.2×106 4.8×106 0.25 1.00

5/90 1.5×106 6.0×106

Alert 8.5×105 3.4×106 0.050 0.20

Large 

Cabinet 

Experiments

9/15 3.8×104 1.5×105 0.0063 0.025

9/53 1.4×105 5.5×105 0.05 0.20

6/19 1.4×105 5.5×105 0.25 1.00

6/88 6.4×105 2.6×106

Alert 1.4×105 5.5×105 0.05 0.20
B-31

Table B-16. Single Zone ASD2 and ASD3 Device Settings and Alert Levels (Continued)

Experimental 

Configuration 

Device 

Setting

ASD2 

Device 

Sensitivity

(part./cm3)

ASD2 Port 

Sensitivity

(part./cm3)

Light Scattering 

ASD3 Device 

Setting

(%/ft Obscuration)

Light Scattering 

ASD3 Port Setting

(%/ft Obscuration)

Small Room

In-cabinet 

Experiments

5/30 5.1×105 3.1×106 0.0083 0.050

5/50 8.5×105 5.1×106 0.033 0.20

5/70 1.2×106 7.2×106 0.167 1.00

5/90 1.5×106 9.0×106

Alert 8.5×105 3.4×106 0.033 0.20

Small Room 

Area-wide

Experiments

5/30 5.1×105 2.0×106 0.025 0.10

5/50 8.5×105 3.4×106 0.050 0.20

5/70 1.2×106 4.8×106 0.25 1.00

5/90 1.5×106 6.0×106

Alert 8.5×105 3.4×106 0.050 0.20

Large Room 

In-cabinet 

Experiments

9/15 3.8×104 1.5×105 0.0063 0.025

9/53 1.4×105 5.5×105 0.050 0.20

6/19 1.4×105 5.5×105 0.25 1.00

6/88 6.4×105 2.6×106

Alert 1.4×105 5.5×105 0.050 0.20

Limited experiments were conducted to ascertain the effect of a wide range of sensitivity 

settings on the time to alarm for ASD2. Specifically, experiments in the instrument cabinet 

arrangement with XLPE, CSPE and PVC(2) were conducted with the ASD2 device at settings of 

8/50, 7/50, 6/50 and 5/50, and at 10/50, 9/50/ 8/50 and 5/50 with continuous air sampling 

directed to a condensation particle counter. The results from these experiments were used to 

develop a strategy for interpolation or extrapolation of ASD2 alert times given a different 

sensitivity setting. 

The observed exponential increase in particle concentration begins soon after the time (block 

temperature) the particle concentration begins to rise above the ambient background level 

(typically less than 5×103 particles/cm3 in the NIST laboratory space) during the heating ramp 

phase. This observation is specific to the particle evolution of the linear heating ramp 

exposures of the experiments conducted in this study. Thus, a plot of the activation time for 

each of the four device settings versus the device (or port) sensitivity settings (in particles/cm3) 

would should show a logarithmic increase in activation time over a range of sensitivity settings. 

Examples are shown in Figures B-14, 15, and 16 where the recorded alarm times are plotted 

against the port sensitivity for instrument cabinet experiments. Additional alarm time settings 

were estimated by using a linear correlation between the recorded particle concentration and 

the port sensitivity at the recorded alarm times for each experiment, then interpolating or 

extrapolating estimated alarm times for different port sensitivities. Finally, a curve was fitted 

with a logarithmic function (base 10) through just the recorded alarm times.
B-32

400

450

500

550

600

0 2 106 4 106 6 106

Recorded Alarm

Estimated Alarm

ASD2 Alarm (s)

y = 7.4842 + 78.633log(x) R= 0.99549 

Port sensitivity (particles/cm 3

)

Figure B-14. Instrument cabinet experiment (filename C_CSPE_L_A_1.4) with CSPE wire 

and a 15 minute HRP, ASD2 device settings at 8/50, 7/50, 6/50, and 5/50, and 

estimated settings of 7/30, 5/30, 5/70, and 5/90

400

450

500

550

600

0 2 106 4 106 6 106

Recorded Alarm

Estimated Alarm

y = -177.75 + 109.55log(x) R= 0.99706 

ASD2 Alarm (s)

Port sensitivity (particles/cm 3)

Figure B-15. Instrument cabinet experiment (filename C_XLPE_L_A_1.4) with XLPE wire 

and a 15 minute HRP, ASD2 device settings at 8/50, 7/50, 6/50, and 5/50, and 

estimated settings of 7/30, 5/30, 5/70, and 5/90
B-33

1200

1300

1400

1500

1600

1700

1800

0 2 106 4 106 6 106

Recorded Alarm

Estimated Alarm

y = 525.99 + 163.47log(x) R= 0.98034 

ASD2 Alarm (s)

Port sensitivity (particles/cm 3

)

Figure B-16. Instrument cabinet experiment (filename C_PVC2_L_A_2.4) with PVC(2) 

wire and a 60 minute HRP, ASD2 device settings at 10/50, 9/50, 8/50, and 

5/50 and estimated settings of 7/30, 7/70, 6/50, 5/30, 5/70, and 5/90

This analysis confirmed the strategy to interpolate or extrapolate activation times at other 

sensitivity settings by fitting the recorded alarm times (not all four, but typically two or three 

points closest the 7/30 setting) to a logarithmic function and estimating an alert time at a device 

setting of 7/30. This was done for all experiments using ASD2 and considered in the analysis of 

Phase II.

B.4 Supplemental Data Supporting Objective E

Figures B-17 and B-18 present the in-cabinet, naturally ventilated data for the 15 minute and 

4-hour HRP, respectively. These figures show the detectors’ time of response (“alert” for 

VEWFD systems and “alarm” for conventional spots). This data shows trends similar to the 

data presented in Figure 5-49.
B-34

Figure B-17. Detector response to selected materials (15 minute HRP)

in-cabinet, natural cabinet ventilation

 

 

Time (Minutes)

0 5 10 15 20

CSPE

PCB

PTB

PVC

XLPE

XLPO

PHOTO

ION

SS

ASD CC

ASD LS2

ASD LS1
B-35

Figure B-18. Detector response to selected materials (4-hour HRP)

in-cabinet, natural cabinet ventilation

The supplemental data above is based on the sensitivities reported in the body of this report. 

However, as discussed elsewhere, the ASD systems tested were configurable to several 

different sensitivities and data was logged at sensitivities other than those focused on in body of 

this report. Figure B-19 and B-20 present the system response at different alert set points for 

in-cabinet test configurations with natural or low cabinet ventilation rates. All data recorded is 

available on the enclosed CD. In general, as the detector port sensitivity increases (becomes 

more sensitive) the time to detection decreases (earlier detection). However, the data is 

quite disperse.

 

 

Time (Minutes)

0 53 106 159 212 265

CSPE

PCB

PTB

PVC

XLPE

XLPO

PHOTO

ION

SS

ASD CC

ASD LS2

ASD LS1
B-36

Figure B-19. Light Scattering (LS2) Alert system response at different port sensitivities. 

(Top – scatter plot; Bottom – Box plot with mean bolded)
B-37

Figure B-20. Cloud Chamber (CC) Alert system response at different port sensitivities. 

(Top – scatter plot; Bottom – Box plot with mean bolded)

B.5 Process Used to Estimate Risk Scoping Study Parameters

Several parameters used in the risk scoping study presented in Part II of this report were 

estimated based on a specific set of empirical data presented in Part I. The subset of data used 

represented the minimum sensitivity requirements for a smoke detection system to be classified 

as VEWFD per NFPA 76 for detectors that reported in percent obscuration per foot or from 

vendor recommendations and information obtained from site visits for the cloud chamber 

technology. Since most ASD VEWFD systems are configurable to a range of sensitivity settings 

and number of sampling ports per zone, actual applications may use sensitivities other that 
B-38

those used to estimate the parameters in the body of this report. Some of the VEWFD systems 

tested are configurable to have up to 5 different setpoints. As such the testing configured all 

available setpoints to cover a range of sensitivities. This data is included with this report and 

could be used to support parameter estimates for systems that are configured differently than 

the minimum sensitivity settings used in the main body of this report. The sensitivity settings of 

VEWFD systems will affect the system effectiveness (τ) in detecting pre-flaming (incipient) 

stage conditions. System sensitivity also affects the time when a system goes into ‘alert’ which 

if more sensitive should provide more advanced warning. This will have an effect on the time 

available curves used to support development of HEPs (ξ) for the field operator and technician 

response.

The purpose of the discussion presented here is to describe the process that could be followed 

to develop parameter estimates consistent with the approach taken in this report, but for

systems that use sensitivity settings different from those use in the main body of this report.

B.5.1 Effectiveness (τ) of detecting pre-flaming (incipient) phase conditions

This term is developed based on the smoke detection system response to the incipient fire 

sources. As discussed in Section 7.2.3, this parameter is estimated using a “one-stage” bayes 

approach (Jeffery’s non-informed) assuming a binomial data set (the detector either responds in 

the incipient stage “a success” or it does not “a failure”). The posterior distribution is a beta with 

parameters “α” and “β” calculated as shown below. The effectiveness estimates (τ) presented 

in the body of the report are based on detectors configured to setpoints as presented in 

Table B-17.

αα = (# ssssssss ssss) + 0.5

ββ = (# tttttttt ) − (# ssssssss ssss) + 0.5

ττ = 1 − αα⁄(αα + ββ)

Systems configured to be more sensitive than reported in Table B-17, should result in a higher 

effectiveness estimate. To estimate an effectiveness parameter, applicable data should be 

collected from the applicable available data included with this report or through testing. To be 

consistent with the risk scoping study presented in this report, a similar calculation method as 

described above is recommended.
B-39

Table B-17. Sensitivity settings used estimate effectiveness parameter as presented in 

Table 7-5. 

Application Cabinet/Room

Ventilation Detector Type

Sensitivity Settings at 

sampling port

% obsc/ft. particles / cm3

In-Cabinet

Natural 

and

Forced <100

cabinet ACH

ION Spot 1.0 - - -

PHOTO Spot 2.1 - - -

SS 0.2 - - -

ASD CC - - - 1 x 10+6

ASD LS1 0.2 - - -

ASD LS2 0.2 - - -

Forced ≥100

cabinet ACH

ION Spot 1.0 - - -

PHOTO Spot 2.1 - - -

SS 0.2 - - -

ASD CC - - - 1 x 10+6

ASD LS1 0.2 - - -

ASD LS2 0.2 - - -

Area-wide, Air 

Return Grill HVAC in room ASD CC - - - 1 x 10+6

ASD LS2 0.2 - - -

Area-wide, 

Ceiling Any

ION Spot 1.0 - - -

PHOTO Spot 2.1 - - -

SS 0.2 - - -

ASD CC - - - 1 x 10+6

ASD LS2 0.2 - - -

B.4.2 1st level field response (ξ) Technician / Field Operator

The human error probabilities (HEPs) developed in Section 10 are based in part from the time 

available curves presented in Section 8.2 and Appendix D. The time available curve are

developed from operating experience that provides either an “incipient stage” duration or a “time 

available” duration for a specific technology starting at the time when a VEWFD system goes 

into ‘alert.’ Because the testing illustrated differences in response among the VEWFD systems 

tested, individual time available curves are developed, based on this empirical data and 

operating experience. The following provides a description of the process followed to develop 

these curves. Detector sensitivities other than those reported above (see Table B-17) will alter 

the performance of these systems and thus change the time available curves. To be consistent 

with the risk scoping study presented in this report, a similar calculation method as described 

below is recommended. Figure B-21 provides an illustration of the difference between the 

incipient stage period (duration) and the time available for operator response prior to flaming 

conditions, following a smoke detection system response.
B-40

Figure B-21. Illustration of difference between incipient stage and time available1

.

Operating experience that reports an incipient stage duration, should be multiplied by the 

complement of the mean normalized detector ‘alert’ time. This results in an estimate of the time 

available. The mean estimate should be based on the data for the applicable sensitivities for 

the VEWFD system under evaluation. Table B-18 shows the mean estimates used in the main 

body of this report.

Table B-18. Mean Normalized Time to “Alert” for Sensitivities Reported in Table B-17

OpE 

Detector

Normalized 

Mean time 

to 

detection

Relative difference between detection technologies

CC LS-SS ION PHOTO

CC 0.48 1.00 0.50 0.65 0.15

LS-SS 0.74 2.00 1.00 1.31 0.31

ION 0.66 1.53 0.76 1.00 0.24

PHOTO 0.92 6.50 3.25 4.25 1.00

For example, if an event indicates a 4 hour incipient phase duration, the time available is 

estimated as follows:

Cloud chamber : (4 hours) x (1 - 0.48) = 2.08 hours

Light scattering / sensitive spot : (4 hours) x (1 - 0.74) = 1.04 hours

Ionization spot type : (4 hours) x (1 - 0.66) = 1.36 hours

Photoelectric spot type : (4 hours) x (1 – 0.92) = 0.32 hours

 1 For illustration purposes only. In some configurations detector response may occur after ignition.

Incipient Stage Duration

Start of event (component begins to 

degrade)

Ignition (Start of Flaming Combustion)

Detector Response Time

Time

Time Available

Mean time to 

detection
B-41

As presented in Section 5.9, the detectors response to the incipient smoke source varied over a 

range and did not necessarily overlap, even for detectors employing the same technology and 

configured to the same sensitivities. As such, the use of the mean relative difference estimates 

presented in Table B-18 allows for operating experience from on VEWFD detector technology 

type to be interpolated to another technology based on the data collected from this research 

project. That is, for operating experience events that are based on a specific detection 

technology, the relative difference estimate can be used to modify the time available from one 

technology to represent the time available for another technology. 

For example, if an event indicates that a cloud chamber ASD VEWFD system provided 4 hours 

of advanced warning (time available to respond before flaming conditions), then estimates for 

the other technologies based on this event can be estimated as follows:

Light scattering / sensitive spot : (4 hours for CC) x (0.50) = 2.00 hours for LS-SS

Ionization spot type : (4 hours for CC) x (0.65) = 2.60 hours for ION

Photoelectric spot type : (4 hours for CC) x (0.15) = 0.60 hours for PHOTO

The relative difference estimates are calculated based on the difference between the 

normalized mean time to detection, such as:

% rr = 1 − (1 − OO mm ) − (1 − mm )

(1 − OO mmee )

For the sensitivities used in the main body of this report, the relative difference estimate to 

convert operating experience of a cloud chamber to a light scattering data point is:

1 − (1 − ) − (1 − LLSS. SSSS )

(1 − ) = 1 − LLSS. SSSS nn −

1 − = 1 − 0.74 − 0.48

1 − 0.48 = 0.5

Once available operating experience is converted to a time available for a specific technology, 

the data can be fit to a distribution. This is discussed in Appendix D. The developed distribution 

and plant specific operator response timing can then be used to conduct the human reliability 

analysis to develop plant specific HEPs following the process discussed in Section 10.

Example cases for cloud chamber based ASD VEWFD system

(most vs. least sensitive setpoint tested)

To support the variations in the human error probabilities presented in Section 10.6.3, the ineffectiveness and HEP split fractions estimation process will be illustrated. The least sensitive 

data for the cloud chamber was collected during the laboratory large-cabinet and full-scale 

large-room single-zone in-cabinet experiments. In both of these cases the least sensitive of the 

four setpoints was configured to approximately 9,000,000 particles per cm3 at the sampling port. 

The most sensitive data for the cloud chamber was collected during the full-scale small-room incabinet experiments. In this series of tests, the most sensitive of the four setpoints was 

configured to 150,000 particles per cm3 at the sampling port. Based on these data sets, the 

estimation of the in-effectiveness, mean time to detection, and exponential distribution 

parameters are shown in Table B-19.
B-42

Table B-19. Estimation of Parameters for Cloud Chamber Variation Case.

Least Sensitive (9,000,000 particles / cm3) Most Sensitive (150,000 particles / cm3)

# of tests : 44

# of responses : 44

αα = (44) + 0.5

ββ = (44) − (44) + 0.5

ττ = 1 − 44.5⁄(44.5 + 0.5) = 1.1xx10−02

The normalized mean time to detection at the 

specified sensitivity: 0.60

Exponential Distribution Parameter

λλ = �1

nn �xx

nn

=1

�

−1

= 0.674

# of tests : 35

# responses : 35

αα = (35) + 0.5

ββ = (35) − (35) + 0.5

ττ = 1 − 35.5⁄(35.5 + 0.5) = 1.4xx10−02

The normalized mean time to detection at the 

specified sensitivity: 0.

Exponential Distribution Parameter

λλ = �1

nn �xx

nn

=1

�

−1

= 0.428
B-43

Following the process outlined in Appendix D.2, the sampling point split fractions for the CC at 

these sensitivities are estimated as;

For least sensitive case:

Time (Minutes) Split Fraction Start End

0 60 ∙ �

−1

λλ � ∙ ln(1 − 0.1) = 9 0.10

9 30 ee−λλ∙ 10tth − ee−λλ∙0.5 = 0.19

30 60 ee−λλ∙0.5 − ee−λλ = 0.20

60 ∞ ee−λλ = 0.51

For the most sensitive case:

Time (Minutes) Split Fraction Start End

0 60 ∙ �

−1

λλ � ∙ ln(1 − 0.1) = 15 0.10

15 30 ee−λλ∙ 10tth − ee−λλ∙0.5 = 0.09

30 60 ee−λλ∙0.5 − ee−λλ = 0.16

60 ∞ ee−λλ = 0.65

With these sample times, new HEP estimates are developed in Sections 10.6.3.2 and 10.6.3.3, 

following the process outlined in Section 10 of this report.

Example case for light scattering based ASD VEWFD system

Let’s assume that an ASD VEWFD system using a light scattering based detection technology 

is configured with an “alert” port sensitivity of 0.02 % obscuration / ft. for use in an in-cabinet 

application. From the testing performed, one series of testing has a light scattering system 

configured to 0.0252 % obscuration / ft. at the sampling port. Using this tested configuration, 

the data is collected and summarized below to support estimating both the in-effectiveness and 

the mean time to detection that can be used to support the HRA.

Total number of tests : 30

Total number of system responses at specified sensitivity : 30

αα = (30) + 0.5

ββ = (30) − (30) + 0.5

ττ = 1 − 30.5⁄(30.5 + 0.5) = 1.6xx10−02

The normalized mean time to detection at the specified sensitivity : 0.39

Following the process outlined in Appendix D.2, the exponential distribution parameter and 

sampling point split fractions for the CC at this sensitivity are estimated as;

λλ = �1

nn �xx

nn

=1

�

−1

= 0.442
B-44

Time (Minutes) Split Fraction Start End

0 60 ∙ �

−1

λλ � ∙ ln(1 − 0.1) = 14 0.10

14 30 ee−λλ∙ 10tth − ee−λλ∙0.5 = 0.10

30 60 ee−λλ∙0.5 − ee−λλ = 0.16

60 ∞ ee−λλ = 0.64

With these sample times, new HEP estimates can be developed, following the process outlined 

in Section 10 of this report.
C-1

APPENDIX C

SUPPORTING INFORMATION FOR HUMAN 

PERFORMANCE EVALUATION

C.1 Human Factor/Human Reliability Analysis Evidence Database

This section documents questions and answers received to a written survey, with follow-up 

phone calls, that were collected during the project. After each entry, an application designator 

will appear, which are defined as follows:

IC – in-cabinet application

MCR – main control room application

AW – area-wide application

C.1.1 Response Operations: Questions and Answers1

 1 This set of questions was used to obtain information from 2 NPPs. One NPP provided written responses, 

the other through a teleconference.

Events prior to 

human 

response

Component 

Degrades

Alarm sounds in 

Main Control 

Room

Phase 1

MCR Operator 

Detects Alarm

MCR Operator 

Obtains Alarm 

Response 

Procedure

Uses cues and 

indications on 

Fire Notification 

Panel to 

determine 

general location 

of fire (i.e.

area/room)

Dispatches 

appropriate 

personnel to fire 

location

Phase 2

Field operator 

retrieves 

appropriate 

equipment to 

locate source of 

the incipient fire 

and proceed to 

location of fire

Checks local 

detector for

indication of 

exact fire zone 

(i.e. bank of 

cabinets )

Uses 

appropriate 

equipment to 

locate incipient 

degradation in 

specified zone

Posts Fire 

Watch

Phase 3

Technician 

retrieves 

appropriate 

drawings to 

determine how 

to remove 

power from the

degrading 

component

Removes 

power from 

degraded 

component 

Repairs 

component
C-2

The previous chart (in C.1.1.) depicts an in-cabinet very early warning fire detection system 

(VEWFDS) alarm response operation. It is not specific to any one plant and represents a 

generic case. The alarm response operations are represented here in three phases. The 

authors categorized the responses into three different phases because each phase is 

completed by different groups of personnel. 

The authors recognize that there will be variations in alarm response operations. The aim is to 

capture examples in which operations may vary from those depicted herein. In addition, some 

specific questions regarding individual processes are noted below:

Operations Questions

(1) Is there immediacy associated with VEWFDS alarm response? In other words, is the 

alarm treated like any other fire alarm? If not, how is it different?

Plant X: Yes, if an alert comes in, an operator and instrumentation and control (I&C)

tech are sent. If an alarm, the operator and tech are sent along with the fire brigade. If a 

system trouble annunciator comes in, an operator is sent out. Alarm does not 

necessarily indicate a flaming fire. The response associated with an incipient alert/alarm 

is the same as for spot detectors. First, the MCR dispatches an operator, not the fire 

brigade, as in a VEWFDS alert. If additional signs/signals of fire are detected, then the 

fire brigade is sent, similar to when a VEWFDS alarm comes in. Regarding immediacy, 

fire alarms of any kind trump other activities, so a response from operators comes within 

seconds. Only a reactor trip would trump a fire alarm. (IC)

Plant Y: Field operator dispatched immediately. (IC)

a. Are the VEWFDS systems set to have alerts and alarms? If so, how many of 

each? 

Plant X: Set to have one alert and one alarm (also real-time graphic read-out of 

sampler) (IC)

Plant Y: Yes there is an alert and an alarm. Alerts are at 20 percent above 

background and alarms at 50 percent above. (IC)

b. Are alerts, alarms and system trouble differentiated on the annunciator panel?

Plant X: Yes. (IC)

Plant Y: Yes, three annunciators on front panel. (IC)

c. If both alerts and alarms are used, how does the response differ? 

Plant X: If an alert comes in an operator and I&C tech are sent. If an alarm, the 

operator and tech are sent along with the fire brigade. 

Plant Y: No difference in response, field operator will be immediately dispatched 

to investigate either
C-3

(2) Where is the alarm located in the control room (e.g. front panel/back panel)? 

Plant X: All three annunciators are on the front panel.

Plant Y: Front panel.

(3) Where is the fire notification panel located? 

Plant X: A computer screen located on shift technical advisor (STA) desk. The screen 

is separate from other screens and is dedicated to fire. A reactor operator (RO), senior 

reactor operator (SRO), or STA can look at this. Either SRO will look or he or she will 

assign someone to look (as STA will not be in MCR when this annunciator goes off).

Plant Y: Close to shift managers desk

(4) What personnel are dispatched by MCR operators (e.g. just field operator, field operator 

and digital instrumentation and controls (DI&C) technician)?

Plant X: Field operator and DI&C technician

Plant Y: Field operator only

a. Are the necessary staff available 24/7? If not, what are the contingency actions 

(e.g., DI&C tech called in)? How do contingency actions affect the timing of the 

response? 

Plant X: Yes, including I&C tech. One is required at the plant at all times. 

Probably more than one during the day shift.

Plant Y: Field operator available 24/7.

(5) What equipment is used to locate source of incipient fire (e.g. sniffer, thermal imaging 

camera)? 

Plant X: Portable sniffer—tested weekly.

Plant Y: Portable sniffer.

a. If a thermal imaging camera is used, are baseline heat mappings of each cabinet 

available to compare with current conditions? If so, is comparison done at the 

scene of incipient fire or is additional analysis involved (e.g., software package 

needed to compare conditions)? 

Plant X: Thermal cameras are not used.

Plant Y: Not applicable.

b. What is the reliability and sensitivity of the sniffer technology? 

Plant X: Tested weekly.
C-4

Plant Y: Not specifically known, but probably similar to fixed installation as it is 

the same type of equipment.

(6) How many cabinets are typically part of the same zone? 

Plant X: It varies, from one to nine cabinets. There are typically sampling ports in each 

cabinet. The cabinets are partitioned and sealed.

Plant Y: Between 5 and 15 cabinets

(7) Is the posted fire watch a permanent or roaming fire watch? If it is roaming, how 

frequently is the site observed? 

Plant Y: Continuous fire watch until event is concluded. Fire watch personnel are 

prevented from leaving the post; they cannot go through keycard door. 

Plant X: Permanent fire watch following alert or alarm until situation resolved.

a. Is the dispatched field operator also considered the initial fire watch? If yes, what 

training is required to qualify as a fire watch (e.g., ability to use fire extinguisher, 

fire brigade training)?

Plant X: Yes. All (i.e., 100 percent) AO’s have “fire watch incipient” training. 

About 95 percent of FOs are fire brigade qualified. All FOs are qualified as 

incipient fire watch; (this was added training, includes fire extinguisher use).

Plant Y: Yes. All operators trained in use of fire extinguishers, fire watch, and 

most are fire brigade members.

(8) What type of training is given to operators for using sniffers and thermal imaging 

cameras?

Plant X: Training—initial, continuing training cycle every 2 years, sit down instructional 

and practical at end, about 4-hour training. Practical training includes finding a 

“smoking” wire; no timing data collected on this aspect of training.

Plant Y: Training not implemented as of yet, but expect that all operators will have 

specific training (with qualification sign-off) in use of sniffers as part of basic operator 

training.

(9) Are cabinets opened when using the portable sensing equipment, or is it used with the 

doors closed? 

Plant X: It depends on the zone to which personnel are responding. If it is a 

multi-cabinet zone, personnel start by sniffing the outside of cabinets to determine which 

specific cabinet it is. The identified cabinet is then opened to find the component. If it is

a one-cabinet zone, the cabinet is opened. These actions are training-based.

Plant Y: Yes.
C-5

(10) Is the stopping point for receiving credit at the point of posting a fire watch, at the point of 

de-energizing the panel, or at time of repair? 

Plant X: Assumption is made that 90 percent of incipient fires will be prevented and, 

specifically, the fire watch will prevent the incipient fire from having an affect beyond the 

ignition site.

Plant Y: National Fire Protection Association (NFPA) 805 Fire PRA credits incipient 

detection as a means of detecting fire before propagation from cabinet. No credit is 

taken for de-energizing or repairing affected component, or preventing loss of cabinet 

function.

(11) How often are VEWFDS down for service? How often are false alarms experienced?

Plant X: Annual maintenance requires system to be out-of-service for half a day. 

Overall, Plant X is not seeing many unplanned shutdowns. 

False alarms caused by ongoing work. Personnel usually know about this work and can 

take detector out-of-service and post a fire watch. One detector alarms more often than 

others; it is not necessarily false-alarm, but rather, a bit too sensitive. The alarm often 

does not “lock in.” They are correcting this by changing the sensitivity settings. (IC)

Plant Y: No operating experience as yet.

Timing Questions

(12) Do you have any information regarding timing from the VEWFDS alarm sounding to a 

flaming fire? 

Plant X: Yes, 2 hours and 42 minutes from alert to flaming fire on August 8, 2013.

Plant Y: No plant specific experience as yet.

(13) What is the average time from the alarm sounding in the MCR to the dispatch of field 

personnel? 

Plant X: Immediately. The operator closest to location is dispatched.

Plant Y: Based on normal fire alarm response—Immediate

(14) What is the average timing between the field operator receiving the dispatch call and

arriving at the fire location?

Plant X: Varies, but typically, 2 to 8 minutes. I&C tech are typically a few minutes 

behind auxiliary operator (AO).

Plant Y: Usually 3 to 4 minutes for field operator; 15 minutes or less for brigade 

response.
C-6

(15) What is the average time for the field tech to locate the degraded component using the 

portable equipment? 

Plant X: No empirical information available. A single cabinet zone would require a few 

minutes to narrow down which component.

a. How does the zone size affect this timing (three cabinet zone vs. 10 cabinet 

zone)?

Plant Y: No experience as yet but would estimate 1 minute to sniff one cabinet. 

Therefore time to identify specific cabinet could be 5 to 15 minutes.

b. If thermal imaging is used, how long does comparison between baseline and 

current conditions take? 

Plant X: Do not currently use thermal imaging.

Plant Y: Not applicable.

(16) What is the average time to de-energize a degraded component? 

Plant X: Quickly once the component is identified. If whole thing is on fire, they will 

de-energize the entire cabinet, which takes seconds. It would take minutes for specific 

breaker. One breaker requires a local action whereas the whole cabinet can be done 

from the control room. The logic for the cabinet power is already laid out in MCR on 

control board. No procedures for specific equipment de-energization; this activity is 

directed by CR supervisor.

Plant Y: Do not anticipate de-energizing component as an immediate response unless 

absolutely warranted because of the impact on plant operations 

(17) What is the average time to repair a component? 

Plant X: Varies widely.

Plant Y: Long term.

C.1.2 Information from Trip Reports

(1) How do operators respond to the various notifications, (e.g. pre-alarm and alarm)?

Response 1: They respond in accordance with APP-044-B39. If detector indicates 

alarm, investigate for any indication of smoke, charring or overheating components. In 

addition to visual inspection, other methods such as thermography can be used to locate

any overheating components. If an additional alarm on opposite train is received, 

activate the fire brigade. (MCR)

Response 2: In a pre-alarm condition, operation attempts to clear the pre-alarm, if no 

success, operation will initiate an operator alert and request ERT personnel 

assistance. (AW)
C-7

Response 3: Pre-alarm: control room call responding organizations, shift emergency

response manager; someone checks the panel or visual size-up. Alarm: shift 

manager. (AW)

(2) Has the system false alarmed? 

Response 1: Yes. During the initial installation of the VESDA system, multiple spurious 

alarms were observed. Over the years, some alarm set points have been offset to 

account for the environmental conditions, as a result, numbers of spurious alarms have 

reduced. The cause of the spurious alarm ranges from unknown reasons, to airborne 

charcoal dust from filters, to fumes from floor stripping, to dust bunnies. (AW)

Response 2: Yes. The most common cause is/was other work in protected areas (units 

not bypassed). During the summers immediately following installation, there were some 

issues caused by heavy smog or wildfires, causing high ambient background 

conditions. (AW)

Response 3: Yes, from nitrogen purging, maintenance activities. Protocol to inform 

CSNC, insurance company, (follow procedure for notification, SERM, duration time, 

same notifications if it is a real fire. (AW)

Response 4: No. (MCR)

Response 5: Yes, from burning popcorn, hot work in the battery room, and a fan 

failure. (AW)

(3) How would an incipient fire be located and verified (equipment used, sequence of actions 

taken)?

Response 1: Compensatory measures, thermal imaging (IR), only certified people can 

use sniffers. (AW)

Response 2: Thermal imaging cameras are used to detect incipient fires. (AW)

Response 3: Cabinet doors would be opened, visual and smell senses would be used. 

If necessary, a thermal imager would be used. (MCR)

Response 4: Human senses. (AW)

(4) What kind of training has been provided to the operators, technicians and fire fighters on the 

system?

Response 1: Control maintenance technicians were provided with a one week 

comprehensive training on the EST 3 and the VESDA system at Edwards. They were 

trained on the operation of the software and detection panel. (AW)

Response 2: Operations were provided with classroom and hands-on training for 

operation of the Fireworks and Cirrus-Pro software. Operators were required to show 

proficiency in Fireworks and Cirrus-Pro to gain qualification. Technicians were provided 

initial classroom training on the Cirrus-Pro detectors and the FireWorks and Cirrus Pro 

software which satisfied vendor qualification requirements for plant techs to work on the 
C-8

Cirrus Pro detectors. A training aid that simulated the detector/fire panel/software 

interconnection was provided to the training department for further instruction. (IC)

Response 3: No specific training implemented, the unit is straightforward. (MCR)

(5) Are the operators trained on calibrating the system using proprietary software, if applicable?

Response 1: The control maintenance technicians are trained on the manipulation of 

the system and the use of the applicable software. Yes. Control Maintenance 

technicians have been trained by the product vendor in the use of system configuration 

software. This is required to enable the technicians to conduct proper maintenance, 

testing and troubleshooting of devices. (AW)

Response 2: Operators are trained and qualified to use Fireworks and Cirrus-Pro 

packages. (IC)

Response 3: No. (MCR)

(6) Where is the detector located relative to the detection zone and why?

Response 1: The detector is located outside the RGTB cabinets, but approximately 

5 feet from the nearest RGTB cabinet, because of space limitations. Tech manual 

recommends locating the detector within the protected space. (MCR)

Response 2: Aspirating detector is located remote to the cabinets being monitored and 

air sample is piped from the cabinets being monitored to the detectors. Each detector 

has four zones. (IC)

Response 4: The detectors are typically located in the protected zone. This approach 

acts to minimize any adverse effects caused by pressure differentials within different 

areas of the station. It also serves to ensure that sampled air is discharged to the same 

area from which it originated, eliminating opportunities for the spread of smoke or 

airborne radioactive particles from one area to another (AW).

Response 5: In a different room. Both detectors are on the same wall next to each 

other. (AW)

(7) Are there any drawbacks specific to the VEWFDS?

Response 1: Maintenance requires quarterly and annual surveillance which is time 

consuming and expensive. System is somewhat complicated, requiring a significant 

vendor interface.

Response 2: A spare unit is recommended. MCR)

(8) What compensatory measures do you use when the system is out-of-service?

Response 1: Continuous fire watch. (AW)
C-9

Response 2: None. This detector is just one in a detection circuit made up of other spot 

detectors. The rest of the detector circuit remained operable. The detection circuit is 

located in the constantly manned control room. (MCR)

Response 3: Continuous fire watch. Also used when the fire source can’t be located 

and alarm will not clear. (IC)

Response 4: Continuous fire watch, restricted work activities. (AW)

C.2 Shearon Harris Nuclear Power Plant Site Visit of January 13, 2016

NRC staff visited Shearon Harris Nuclear Power Plant (Harris) on January, 13, 2016 with the 

purpose of better understanding an operational event that occurred on August 2015, including 

information to support this reports’ event tree analysis (Section 6), timing analysis (Section 8), 

the human factors analysis (Section 9), the human reliability analysis (Section 10), and nonsuppression probabilities assignments (Section 11).

Pre-visit information (e.g., purpose, site visit plan, list of questions), and preliminary plant 

responses, are provided in Section C.2.1. The plant site visit itself consisted of three (e) major 

activities:

1. general, roundtable discussions between NRC and utility staff

2. plant walkdowns (including main control room and other plant areas)

3. a follow-up call on January 28, 2016 with utility staff

Notes taken during the site visit (i.e., activities #1 and #2), plus plant visit participants, are 

captured in Sections C.2.2, C.2.3, C.2.4, and C.2.5. Notes from the follow-up call are provided 

in Section C.2.6.

C.2.1 Pre-Visit Information and Preliminary Answers to Questions

This section documents preliminary information exchanged between the NRC and the utility.

The NRC provided a list of questions before the plant visit. At the NRC’s request, the utility 

provided written answers in advance of the visit.

The list of the NRC’s questions and the utility’s responses are the following:

Purpose: Provide RES team with additional information on event, system performance, 

and plant personnel performance.

Desired Outcome: Use information to support refinements to model assumptions and 

evaluate characterization of event and system/human performance.

Site Visit Plan: First, talk through what happened (including discussion of the timeline, 

operator & technician actions, etc.). Then, walkdown the relevant areas (e.g., MCR, 

cabinet area) with plant staff. And finally, have a follow-up discussion to clear up any 

remaining questions. In all cases, having the plant staff who were directly involved in the 

event would be the most helpful.

List of Questions:
C-10

1. Detailed timeline of event.

a. Including timing and actions taken between MCR receipt of first VEWFD 

system “alert”; removal of power from control switch circuit, including 

resetting VEWFD system “alert”; and declaring the VEWFD system 

inoperable.

Response:

1. 08/01/2015 19:52 - Received ALB 30/4-1 IFD System Alert for the ACP area. 

Dispatched Operator and I&C, they reported no unusual conditions. WR# 20002776 

CR#1938438.

2. 08/01/2015 20:05 - ALB 30/4-1 IFD System Alert annunciator is clear.

3. 08/01/2015 23:08 - Received ALB 30/4-1 IFD System Alert for the ACP area. 

Dispatched Operator and I&C, they reported no unusual conditions.

4. 08/01/2015 23:30 - ALB 30/4-1 IFD System Alert annunciator is clear.

5. 08/01/2015 23:34 - The Incipient detection was declared inoperable and a continuous 

Incipient Detection fire watch established. (Fire OOSL –15-5338)

6. 08/03/2015 – Work order 20008305 given to FIN Team

7. 08/03/2015 – following initial troubleshooting, with no results FIN Team requests 

Thermography.

8. 08/03/2015 14:10 PM – Thermography performed (time stamp on picture)– see 

Thermography picture.

9. 08/04/15 13:14 Opened PP-1A312-SA-8, MS LINE ‘C’ PORV (1MS-62) SERVO AMPALTERNATE

10. 08/04/15 13:51 - Hung PSC-1-15-3020-CSGPORVALT-0092 for C SG PORV alt 

power.

11. 08/04/15 14:24 - ALB-030/4-1, IFD SYSTEM ALERT, is clear. 1SFD-E087 for the 

ACP has been reset.

12. 08/04/15 14:58 - Completed FPT-3220A INCIPIENT FIRE DETECTION ALARM 

NOTIFICATION FUNCTIONAL TEST - MONTHLY INTERVAL, Satisfactory

13. 08/05/2015 – 07:30 The detector declared functional and returned to service following 

removal of power from CS-1256.2 (C SG PORV) and the Alarm clearing. (Fire Watch 

secured)

14. 08/05/2015 – 14:45 Work on repair begins

15. 08/06/2015 – 15:00 Work complete
C-11

2. Operator/technician response 

a. Description of actions taken. 

Response:

Operator and I&C dispatched Pre APP-ALB-030. See below

b. Procedural requirements for operator, technician and fire brigade.

Response:

See APP-ALB-030 and/or APP-ALB-030 incipient detection info only 

(attached)

c. Handheld ASDs

i. How much time was used with handhelds and what/how survey was 

conducted? 

Response:

1. The Shift I&C technician sniffed with the portable incipient fire detector for at 

least 30 minutes during the first alarm.

2. The second alarm no abnormal indications reported from field.

3. 1SFD-E087 declared Non-Functional and established a

continuous fire watch (within one hour).

ii. Were doors opened, vents surveyed, etc. 

[May be useful to conduct a walkthrough of procedure.]

Response (on shift SRO):

Yes, I spoke with the I&C technician about sniffing the ACP panel and

sniffing/surveying behind door. An AO was present while I&C tech

monitored behind all the door panels. I&C techs get special IFD training and 

are qualified to respond to IFD alarms (ICC0016H-N).

d. Description of fire watch (actions, responsibilities, procedure).

Response:

This is a continuous Fire Watch performed by personnel qualified as a fire 

watch that are also trained to use a fire extinguisher.
C-12

From AR 405845:

The plan for incipient fire watch is as follows:

1. A new qualification group FH32 will be created called Fire 

Watch/Incipient Detection

2. The qualification will require qual group FPQ0001G which will require 

a fire extinguisher practical.

3. NLOs will be scheduled to undergo training for FPQ0001G.

4. The procedure for requiring a fire watch for incipient fire detection 

equipment failure,

FPP-005 will require any individual performing a fire watch for out of service or 

failed incipient fire detection equipment, to hold the qual FH32.

3. Thermal imaging camera 

a. What process were you in when you used the Thermal Imaging Camera (part 

of fire response, troubleshooting nuisance VEWFD system alarms, part of 

repair/planning, etc.)

Response by Engineer that used the Thermal imaging Camera:

Part of repair/planning

b. What did you do and were procedures followed/available for this?

Response by Engineer that used the Thermal imaging Camera:

There are no procedures for thermography

c. Were baseline images available? 

Response by Engineer that used the Thermal imaging Camera:

No. This component is does not have a thermography route and had not ever

been scanned prior to my knowledge

d. Did investigation stop after control switch was located?

Response by Engineer that used the Thermal imaging Camera: 
C-13

No. The entire Auxiliary Control Panel was scanned.

4. Switch Repair

a. Were actions taken to de-power component (cabinet) prior to replacement? 

Response (FIN Team supervisor) :

Yes, see plant status control tag # PSC-1-15-3020-CSGPORVALT-0092 for

timeline of deenergizing CS-1256.2SA from PP-1A312-SA-8.

b. When was power removed from the switch?

Response:

08/04/15 13:14

c. Component switch cut sheet/bill of material information

i. thermal rating 

Response:

WO 20008305 was used to replace CS-1256.2SA with part number

9220143121 PQL1.

Thermal rating 120 hours at 80 degrees C (176 degrees F), 50 degrees Rise.

ii. qualification, if applicable

Response:

See ESC-STD-1000 Rev 6. Which is EC 62343

Attachment C. Electroswitch Series 20, Part Number 38050M-132/Type M1S1L 

Rev.A (Drawing No. G51, Sheet 1 dated 03-24-06), shall be Nuclear Safety 

Related in accordance with the requirements of ESC-STD-1000 Rev.6 and 

Technical Publication MIN-1 Dated April1, 1991. See ESC-STD-1000, rev 6 

attached.

5. VEWFD system reports 

a. List of “alerts,” “alarms,” nuisance alarms

Response:

Not available
C-14

b. Outcomes of system annunciation (nuisance, fire, smoke, etc.)

Response:

All items to date with the exception of four alarms were no fire products 

found.

Those four items were:

1. Hot resistor found in Main Termination Cabinet. 

This was around 180°F, and was due to inadequate spacing 

around the resistors. The proper gap was established by 

bending the resistor

2. Ground in Transformer 1E2 (ground alarm received). 

This was outside the cabinets being sampled, but the sniffer 

started at the cabinet and was able to follow the combustion 

particles across the room to the ground resistor which was at 

about 250 °F.

3. Ground in Transformer 1D2 (ground alarm received)

This was outside the cabinets being sampled, but the sniffer 

started at the cabinet and was able to follow the combustion 

particles across the room to the ground resistor.

4. The 8/1/15 ACP event

c. Time spent investigating

Response:

About 30 minutes

6. Data to update Availability, Reliability estimates.

No response

7. System set points.

a. What are the system set points for each of the VEWFD systems installed at 

Harris (Gain, % of gain, # ports per zone)? 

Response:

Sensitivity of 7. Alert is 30% of range and Alarm is 50% of range, There is one

port serving Three Zones for 1SFD-E087 (for ACP.

b. Any change to set points following event?

Response:

No – would require an Engineering Change.
C-15

C.2.2 Shearon Harris Plant Site Visit - Participants

Harris plant staff who supported the site visit included:

• Fleet Fire Protection Program Manager

• Site Appendix R / safe shutdown engineer

• Site Fire Protection engineer

• Site Licensing

• Site I&C Technician

• Site Thermal Imaging engineer

• Site Detection System engineer

• Site Operations

NRC staff who participated in this plant site visit were: 

• Office of Nuclear Reactor Regulation (NRR) - Harold Barrett (fire protection)

• Office of Nuclear Regulatory Research (RES):

o Gabe Taylor (fire protection)

o Susan E. Cooper (HRA)

o Nick Melly (fire PRA, fire protection)

o Amy D’Agostino (HF)

C.2.3 Human Factors/Human Reliability Analysis Notes

Plant site visit notes taken by NRC HF and HRA analysts are combined in the text immediately 

below.

C.2.3.1 Expected Sequence

Sequence of activities for incipient detector “alert” response was discussed.

- Alert comes into main control room (MCR)

- Board operator retrieves and follows alarm response procedure (ARP)

- Exact field location of alert is determined via “fireworks” computer program on 

Shift Technical Advisor (STA) desk 

- Board operator directs STA to handle communications, communications flow 

through the STA (Harris STAs are operators and some are Senior Reactor 

Operators [SROs])

- AO is called (can be called via PA, cell phone, radio or to the AO corral)

- AO is dispatched to the alert location to investigate

-the estimated travel time to the field location is 3 minutes

- STA dispatches the Instrumentation and Control (I&C) tech 

NOTE: If the alert clears before the I&C tech arrives, the AO can leave

- I&C tech secures his current work first, then:

- goes to shop area behind the MCR to retrieve portable sniffer

- Stops at MCR to verify location and get keys for any locked cabinets

- Goes to location:

- Puts detector on floor to get background reading (assess initial 

condition outside of cabinets) (takes a few minutes)

- I&C opens each cabinet
C-16

- Inserts sniffer hose (a part constructed by utility; not provided by 

vendor) inside cabinet & partially closes door to sample air inside 

cabinet

- Uses sniffer:

-Starts at most sensitive range

inserts the sniffer hose (the hose it a part constructed by the utility, not provided 

by the vendor) and partially closes the cabinet to sample the air

- I&C says this can take 5-8 minutes (or 2-10 minutes?)2 per cabinet

C.2.3.2 August 2015 Event Notes:

General timeline for event that occurred in August 2015 was discussed.

-An alert was received twice and cleared both times on its own

-In both cases the AO reported “no unusual conditions” meaning no flaming fire 

or signs of fire (smoke, etc.)

-After the second alert and clear, the system was declared inoperable

-This declaration was made to allow a continuous fire watch to be posted, 

however, the incipient system was still in operation

C.2.3.3 Thermography

Plant personnel trained to use thermography equipment discussed use of this equipment.

-Personnel are required to complete a qual card to use the thermographic camera

-includes 1 week of offsite training and over 100 hours of working with the 

camera

-Personnel claim that the hotspot is typically immediately apparent

-Occasionally they need to look at comparison data or look at trends of the temperature 

of a component over time

- In this case, it took thermography about 1 hour to find the hotspot

-Once the hotspot was identified, the cabinet was de-energized in order to fix the 

component

-A thermographer is not always on shift but can be called in

C.2.3.4 Equipment Notes

General discussion on incipient detector equipment included some specifics on plant-specific 

detector installations.

-Portable sniffer has 10 sensitivity settings

-Incipient detectors are set to a setting of 7 or 8, except one that was causing problems 

and is now set to 5

-Computer/notification “panel” on STAs desk in MCR

C.2.3.5 Personnel/Training Notes

-There is one I&C tech on every shift

-Personnel receive IFD response training that is run in-house

 2 Conflicting information in notes; likely from different sources (e.g., group meeting versus walkdown group).
C-17

-More detailed training is given to maintenance staff that will be working on the system

-STA - mostly licensed RO-SRO (plus engineer or tech ed)

C.2.3.6 Opinion/Insights/Operating Experience from plant personnel

-I&C tech is confident that the system will pick something up if there is something to be 

found

-Example instances showing how sensitive detector is:

a) vacuum used to clean out cabinets set off detector in same room

b) microwave cooking kitchen adjacent to MCR set off MCR detectors

c) in transformer event, overhead ion detector went off ~4 hours after incipient 

detector (with incipient detector 20 feet away)

-Thus far there have been no “flaming fires” in rooms with incipient detection

-Personnel estimate that there are 1 to 2 events per month that require the I&C tech to 

go out with the portable sniffer

-Personnel noted that cabinets are always opened in order to inspect using the portable 

sniffer

-Personnel noted that the often see “spikes” on the system. In other words, the particles 

being counted quickly go up and right back down. This indicator was down for 8/2015 

event.

C.2.4 Probabilistic Risk Assessment Notes

During the site visit the question arose as to what the risk drivers were for many of the installed 

systems. This line of questioning led to specific changes to section 6.1.1.1 “Application of 

VEWFDS Probability of Non-Suppression; Pns”. This section describes the applicability of 

suppression system use and highlights the inherent use of these systems to mitigate damage to 

targets outside the enclosure of origin. 

For suppression credit to be applied to targets within an enclosure a more detailed evaluation of 

de-energization strategies, including adequate and appropriate justification in the form of a 

detailed human reliability analysis must be performed. One way a licensee could achieve 

significant fire prevention credit would be to “pre-locate” the isolation devices for all ignition 

sources within each cabinet in an effort to speed up the process. If such an effort was taken, 

additional credit for preventing fires could be allowed. This would need to include 

predetermining the isolation devices, conveniently displaying that information for use in 

response to VEWFD system alerts, training responders so that they could rapidly locate and 

operate the isolation device(s), and conducting drills to periodically demonstrate this ability.

C.2.5 Notes from follow-up call with utility

The NRC team that made the site visit had some follow-up questions that were addressed in a 

conference call.

Those who attended the call included:

• NRC:

o NRR – Harry Barrett

o RES – Gabe Taylor, Susan E. Cooper, Nick Melly
C-18

• Utility:

o Fire protection

o I&C technician

o Operations

Questions:

1. A key was needed for the remote shutdown panel cabinet when we did the 

walkdown. Regarding keys and locked cabinets:

a. Do all cabinets with incipient detectors have keys?

b. Is it typical for the technician to get the keys from the MCR?

c. Is there somewhere else (or another way) to get a key to technician at “alert”

location?

d. Is there a scenario where the field operator would get the keys?

e. Once the technician confirms the location of the degraded component in a cabinet, 

does he close & lock the cabinet?

f. Does the technician return the key to the MCR? Or, does he give it to field operator?

g. If technician returns key to MCR, what is process for field operator if fire suppression 

is needed, e.g.,

i. Does field operator have a key with him?

ii. Is there a key in the room with the affected cabinet?

iii. If there is no key available, what does field operator do if fire suppression is 

needed?

2. If possible, I’d like to have a general talkthrough with a field operator to what he/she does 

when called out for an incipient detector alert

3. Specific questions for field operator, e.g.,

a. When does the field operator stay (versus leave room with “alert”)?

b. Especially what he/she is trained to do per incipient fire detector training IF 

technician does confirm with hand-held detector that there is a degrading 

component, e.g.,

i. An overall talkthrough would be great - for cases where the “alert” signal 

DOES NOT CLEAR

ii. With the “alert” signal confirmed, what is the status of the detector panel (i.e., 

is the signal cleared or does it stay “locked in”)?

iii. Where is he/she positioned with respect to the degraded component & 

affected cabinet (after its identified)?

1. How near affected cabinet?

2. Can he/she see the affected cabinet at all times?

3. If not, what is he/she monitoring, etc.?

4. Would the field operator open the cabinet if the incipient detector 

signals changed (e.g., local panel shows elevated readings and/or 

incipient detector changes from “alert” to “alarm”)?

5. What role, if any, do the local panels for the incipient fire detector 

play (e.g., are they monitored?)

iv. When or under what conditions would the field operator move (per 

training, etc.)?

Answers:

• Operations:

o Need key for some local panels, etc.

o All keys are kept in MCR
C-19

o There’s a key log (for signing in & signing out keys)

o Typically, the I&C tech logs out the key

o MCR operators in touch with stationed fire watch with updates for STA desk 

computer

o ARP- -30 has note on “alert” locked in3

o All ROs, SROs, STAs have qualification cards on use of detector on STA 

desk, including continuous monitoring of detector

o Usually, STA is in the MCR

o MCR operators will put high on “radar screen” to validate “alert” conditions 

• Other:

o Key not issued to continuous fire watch

o Various and diverse indications of fire effects, e.g., 

 Plant-installed incipient detector

 Trend information on computer

 Hand-held “sniffer”

 Fire brigade thermal-imaging

 Noses & eyes

 Etc.

• Fire protection:

o Extinguisher would be used

o Cabinets are vented

o Do not need key (to open cabinet) in order to extinguish fire

o Are situations where detector goes straight to “alarm” (with no “alert”)

o When detector reaches “alarm,” MCR operators dispatch: a) field operator, b) 

I&C tech w/ keys to open cabinet door (if needed), c) fire brigade (w/ thermal 

imaging camera) 

o Will de-power, if possible

 3 Later review of plant procedures identified these steps in the relevant procedure:

• DISPATCH an operator to the Incipient Detector Zone causing the alarm to investigate the source of the 

initiating signal.

• DISPATCH the on-shift I&C technician to the Incipient Detector Zone causing the alarm with the 

portable incipient detector to investigate the source of the initiating signal.

• MONITOR the initiating detector response on the CirrusPro screen during the field investigation.

D-1

APPENDIX D

EVALUATION OF OPERATING EXPERIENCE DATA

D.1 Evaluation of Fraction of Fire That Have Detectable Incipient Stages

This appendix documents the review of potentially challenging or greater fires from the fire 

events database. The results are summarized in Table D-1, which contains several fields 

described below.

Fire ID Record number from EPRI Fire Events database

Fire Cause Identifies apparent cause of event

Detected by Identifies how the event was detected 

Cabinet Type Identifies the type of cabinet where the event occurred

Ignition Component Identifies the component which ignited

Description Provides summary of event

Incipient stage Identifies if the event involved an incipient failure mode. Possible 

classifications are Yes, No, and Undetermined. Definitions are provided 

below.
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

29 Stab 

misalignment

Control 

Room 

instrumentation / 

annunciator

MCC MCCB

Failure on demand. Following start of 

the main turbine turning gear motor a 

fire occurred in the 480V Engineered 

Safety Features (ESF) Motor Control 

Center (MCC) 2B64. Cause is 

attributed to design of breaker cubicle 

allowed misalignment when installing 

the breaker without providing a 

method of verifying proper breaker 

position.

PC 

38

Run Contactor 

Short 

damaged CPT

Control 

Room 

instrumentation / 

annunciator

MG set 

Breaker CPT

Failure During Test. During a bus 

undervoltage and ECSS integrated 

functional test for Units 2/3 Diesel 

Generator, a short in the run 

contactor coil to the 3A RPS MG Set 

Drive Motor Breaker caused 

excessive current flow through the 

control power transformer, which 

caught fire. This resulted in a loss of 

power on 3B RPS bus (because of

the Reserve RPS Power Supply being 

out of service for a modification), a 

half scram and an unplanned 

Engineered Safety Feature (ESF) 

actuation.

CH 

D-2
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

41

Stab 

misalignment Fire Alarm MCC Breaker

Failure on demand. Immediately 

following start of the ‘D’ River Water 

Supply (RSW) pump, a fire alarm was 

received. Investigation identified fit 

between breaker primary disconnects 

and the associated breaker cubicle 

stabs was inadequate. Poor fit 

resulted in arcing in the breaker 

cubicle and subsequent fire. Breaker 

had been recently replaced as part of 

a design modification package and 

insufficient inhouse review of the 

breaker interface design specification 

is the apparent root cause.

PC 

45 Undetermined

Control 

Room 

Instrumentation / 

Annunciator

MCC MCC

Electrical fire in intake structure 

affecting 2 MCCs. Unite 1 Circ. 

Water MOVs and Lube Oil Cooling 

Water Pumps affected, and Unit 2 

Circ. Water Pump Motor Bearings 

affected. Insufficient information to 

determine cause of failure or 

component that failed.

PC 

69 Overheating 

wire

Control 

Room 

Instrumentation / 

Annunciator

Control 

Cabinet

Electrical 

cable 

insulation

Breaker self-closing caused by 

breakdown of insulation in breaker 

control cabinet. Breakdown caused 

by insulation contact with protruding 

tap of a wire wound power resistor, 

associated heat from resistor and 

deterioration caused by water 

intrusion (cabinet located in 

switchyard). Failure is a result of 

accumulated effects of 25 years of 

deterioration.

U (NCPC) 

D-3
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

83.1 Ground Fault Plant 

Personnel Power

Essential 

Lighting UPS / 

Distribution 

Panel

Smoke was discovered in the back 

boards area of the control room by a 

security officer performing an hourly 

fire watch tour. Smoke was 

emanating from the Emergency 

Lighting Uninterruptible Power Supply 

and the Essential Lighting Distribution 

Panel. Cause was short circuit 

current in plant ground system 

because of inadequate grounding 

procedures. Fire was selfextinguished by removal of power by 

opening AC breaker in ELDP.

PC 

83.2 Ground Fault Plant 

Personnel Power

Essential 

Lighting 

Isolation 

Transformer

Following event 83.1, AO was 

surveying duty area and found smoke 

and fire in Train B DC equipment 

room (different room and elevation 

from event 83.1). Fire was contained 

to essential lighting isolation 

transformer. Fire required removal of 

480V power from ELIT by manually 

opening circuit breaker, and 

application of carbon dioxide 

extinguisher by AO and Fire Brigade.

PC 

89 CPT & Relay 

Failure Unknown MCC CPT & HGA Relay

Internal short in the control power 

transformer, which caused the failure 

of the HGA control relay. Root cause 

of CPT failure not reported. Failure of 

these two components prohibited the 

proper operation and automatic 

transfer of the EDG room ventilation 

system from Unit 3 busses to Unit 2.

PC 

D-4
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

98 Undetermined

Plant 

Personnel & 

Fire Alarm

Control 

Cabinet Undetermined

During a 24-hr post-maintenance run 

of emergency diesel generator an 

operator noticed heavy smoke coming 

from the EDG control panel. Initiation 

component and cause of event was 

not identified.

PC 

131 Undetermined Plant 

Personnel

Undeter

mined Undetermined Non available, fire was in turbine building CH 

144 Stab 

Misalignment / 

Ground Fault

Control 

Room / 

Plant 

Personnel

MCC Breaker stabs

Failure on Demand. Concurrent with 

attempted start of containment 

cooling fan (closing of breaker), 

supply circuit breakers for 480VAC 

MCC tripped as a result of a bus to 

ground electrical fault. Responding 

operators discovered a small fire in 

the MCC. Root cause identified 

inadequate design resulted in 

improper placement of circuit breaker 

in MCC. One stab did not make up to 

its associated bus bar correctly, 

resulting in a high resistance 

connection.

PC 

146

Breaker to 

Bus Stab High 

Resistance

Control 

Room / 

Plant 

Personnel

Load 

Center Breaker stabs

Failure on demand. Breaker failure 

following placing breaker in-service 

after restoration steps from a test of 

the automatic start feature of an 

isophase bus cooling fan. Failure 

because of high resistance 

connection between bus bar stabs 

and breaker assembly.

PC 

D-5
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

152

Breaker to 

Bus Stab High 

Resistance

Fire Alarm MCC Breaker stabs

Failure on demand following 

maintenance MCC failure concurrent 

with charging pump starting. Root 

cause identified high resistance 

connection at the stab/bus interface 

likely because of less than adequate 

preventative maintenance and original 

design inadequacy.

PC 

161

Undetermined Plant

Personnel MCC Undetermined

‘D’ control rod drive mechanism 

(CRDM) fan (1-HV-F-37D) tripped. 

Approximately 30 minutes later 

operations locally opened the breaker 

after identifying a strong odor and that 

the breaker associated with CRDM 

fan was smoldering. Upon opening 

the cabinet a “6-inch flame” was 

observed.

PC 

175 Undetermined Undetermined

7.2kV 

Switchge

ar

Undetermined Fire in the non-safety 7.2kV 

switchgear room. CH 

187 Undetermined Plant 

Personnel

Control 

Cabinet Undetermined

Smoke from Unit 3 condensate 

demineralizer control panel. Power 

supply in the panel was unplugged to 

extinguish the fire.

PC 

188 Lightning 

Strike

Plant 

Personnel

Power 

Control 

Cabinet

Undetermined

Lightning strike caused a fire in a 

power control center. De-energizing 

bus supplying power extinguished the 

in the power control center.

PC 

203 Undetermined Plant 

Personnel MCC Undetermined Two MCCs burned CH 

D-6
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

206 Missing 

Component

Plant 

Personnel Breaker Breaker

Fire in recirculation motor generator 

field breaker caused by missing 

extension piece for the center phase 

shorting bus. This allowed the field to 

be continuously shorted during 

operation.

PC 

211 Undetermined Plant 

Personnel MCC CPT Control Power Transformer failure in MCC

U (NCPC) 

219 Undetermined Roving Fire 

Watch MCC CPT Control Power Transformer failure in MCC PC 

224 Human Error Plant 

Personnel MCC Undetermined

Electrical fault in 480V MCC cubicle 

caused by human error during 

maintenance/cleaning

U (NCPC) 

253 Breaker 

Failure

Plant 

Personnel

Switchge

ar Trip Coil Breaker failed to open causing excessive current in trip coil.

U (NCPC) 

254 Undetermined Plant 

Personnel MCC Undetermined MCC electrical overload U (NCPC) 

303 High 

Resistance

Plant 

Personnel

Control 

Cabinet

Fuse 

Disconnect

Plant heater boiler control cabinet on 

fire caused by high resistance 

connection in the 60 amp fuse 

disconnect. Cabinet doors were 

found open with flames coming out of 

the cabinet and paint burning off of 

the top.

PC 

320 Breaker 

Failure

Plant 

Personnel MCC Breaker

Feeder breaker tripped when operator 

attempted to start ‘B’ main chill water 

pump. Local breaker was observed 

to be on fire and had not tripped.

PC 

381

Breaker 

cooling fan 

failure

Control 

room 

Instrumentation / 

annunciator

MCC Cooling fan

Aux cooling equipment fan motor 

shorted out with fan motor assembly 

on fire.

PC 

411 Water 

intrusion

Plant 

personnel

Breaker 

box Breaker Breaker box failure caused by water intrusion

U (PCCH) 

D-7
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

517 Transformer 

fault

Control 

room 

annunciator 

& Smoke 

Alarm

UPS Transformer

Fire in ERFDADS computer 

uninterruptable power supply. 

Apparent cause was a turn to turn 

fault in the top winding. Vibration, 

temperature, and age are contributing 

factors to this failure.

CH 

520 Inverter fault

Control 

room 

annunciator 

& Smoke 

Alarm

UPS Unknown Fire in ERFDADS Inverter PC 

588

Ground fault

Control 

room 

annunciator 

& smoke 

alarm

Switchge

ar

Unknown Ground fault on 480V SWGR CH 

10338 Breaker fault Plant 

Personnel MCC Breaker

Failure on Demand. During start of 

pump, breaker flashed and resulted in 

small fire in cubicle with door forced 

open.

PC 

20264 MCC Coil fault Plant 

personnel MCC Coil Smoke observed coming out of MCC. Hold in coil overheated.

U (NCPC) 

20267 Breaker fault Plant 

Personnel MCC Undetermined Breaker malfunction U (NCPC) 

20268 Overheated 

component

Plant 

Personnel MCC CPT Control Power Transformer overheated

U (NCPC) 

20269 Undetermined Plant 

Personnel

Electrical 

Lighting 

Panel

Undetermined Electrical Lighting Panel Failure U (NCPC) 

20270 Transformer 

Failure Fire Watch MCC Transformer MCC Breaker Transformer failure U (NCPC) 

20272 Relay failure Plant 

Personnel

Electrical 

Panel Relay Electrical Panel Relay U (NCPC) 

D-8
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

20273 Trip coil failure Plant 

Personnel

SwitchgearBreaker trip 

coil

Heavy smoke was observed in the 

Train ‘A’ switchgear room caused by 

a faulted trip coil.

U (NCPC) 

20275 Overheat Plant 

Personnel MCC CPT

Control power transformer burned up 

causing the diesel generator lube oil 

heater MCC to smoke.

U (NCPC) 

20276 Breaker Plant 

Personnel

Switchgear

Undetermined RCP breaker cubicle U (PCCH) 

20282 Overheat Plant 

Personnel MCC CPT

Operator saw smoke coming from an 

MCC for the MISV hydraulic pump; 

transformer fault.

U (NCPC) 

20287 Overheat Roving Fire 

Watch MCC CPT Control power transformer overheat U (NCPC) 

20295 Overheat Plant 

Personnel MCC CPT Control power transformer overheat U (NCPC) 

20302 Ground fault Plant 

Personnel MCC Undetermined Ground fault on main and/or reserve feed breakers cause fire

U (NCPC) 

20312 Switch Smoke 

alarm

Switchgear

Switch EDG roto test switch damaged and 

failed causing a fire

U (NCPC) 

20325 Chemical Spill Plant 

Personnel

Heat 

trace 

wiring

Heat trace 

wiring Acid spill on heat trace wiring U (NCPC) 

20328 Undetermined Smoke 

alarm

Electrical 

DistributionUndetermined Sudden electrical distribution panel 

failure with smoke.

U (NCPC) 

20329 Relay fault Plant 

personnel

Switchgear Relay Relay stuck in intermediate position U (NCPC) 

20334 Breaker Plant 

Personnel MCC Breaker MCC Breaker U (NCPC) 

20346 Breaker Plant 

Personnel MCC Breaker Breaker in 4kV room CH 

20356 Internal Short Plant 

Personnel MCC Light bulb Short in light bulb U (NCPC) 

D-9
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

20357

Human 

Interaction, 

Improper 

Maintenance

Plant 

Personnel MCC

MCC internals 

fell on power 

phase

Ground fault inside a 

nonsafety-related MCC caused by

improperly equipment restored to 

service. Internal plane cover not 

properly secured and fell during 

investigations and caused ground 

fault.

U (PCCH) 

20362 High 

Resistance

Other 

Equipment 

Failure

MCC Insulation / 

fuse block

Insulation burned off of one lead to 

motor starter contactor and fuse block 

severely melted. Termination screw 

loose on starting input terminals.

U (NCPC) 

20382 Undetermined Plant 

Personnel

Switchgear

Undetermined Switchgear failure U (NCPC) 

30276

PCB fault Plant 

Personnel

Emergency 

Lighting

Power 

transformer

Emergency Lighting Battery Box 

Failed during annual inspections. 

Power transformer inside the box was 

observed to have sparked and 

caused a fire. Failure caused by bad 

charging board and one bad cell.

PC 

30281 Procedure 

Error

Plant 

Personnel

Control 

Panel Insulation

During testing of synch switches for 

Main Xfmr, Emergency Aux Xfmrs 

and Main generator, insulation failed 

because of excessive applied voltage 

(230,000volts applied to a 120V 

synch bus).

CH 

30338 Inadequate 

PM

Control 

Room 

Instrumentation / 

Annunciation

Control 

Panel Panel Blower Panel blower (fan) failure. Blower found to be full of dust and dirt. PC 

30478 Relay failure Plant 

Personnel Control Relay Condensate demin panel fire and smoke from affected relays (3). PC 

D-10
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

30513 Overheat Fire Alarm Control CVT

Constant Voltage Transformer inside 

rod action control cabinet in back 

panels of MCR ignited combustible 

materials located inside transformer 

housing

PC 

30522 Undetermined Fire Alarm Control Undetermined RBCCW cathodic protection cabinet 

fire. PC 

30578 Undetermined Plant 

Personnel

Power 

Supply Undetermined

Fire reported in electrical box 

associated with power supply for the 

cask handling crane. Damage limited 

to heat shrink tubing on a connector.

PC 

50467

Breaker fault Plant 

Personnel

Switchgear Closing Coil

Breaker found to be smoking. 

Breaker removed and found closing 

coil was frozen in the close position.

U (NCPC) 

50473 Water 

intrusion

Equipment 

trouble 

alarm

Electrical 

Panel Relay

Small fire discovered in electrical 

panel while investigating burning odor 

while responding to alarm from same 

electrical panel. Flames and smoke 

were observed emanating from relay. 

Failure was a result of water intrusion 

from HVAC condensate drain line.

PC 

50784 Relay 

misalignment

Plant 

Personnel

Control 

Cabinet Relay

During relay testing, the relay began 

to smoke. During de-energization 

activates, the relay caught fire. Fuses 

were pulled and CO2 was used to 

extinguish. Suspected cause was a 

slight misalignment of the relay and 

contact structure.

PC 

50811 Relay failure

Control 

Room 

Instrumentation / 

Annunciator

Control 

Cabinet HFX relay

Received numerous alarms in control 

room related to FP filter low-flow 

alarm. Found FP pump tripped and 

pressure drop. Smoke observed in 

room. Investigation found HFX relay 

burning. Extinguished with portable.

PC 

D-11
Table D-1. Evaluation and Description of Bin 15 Events 

Fire 

ID Fire Cause Detected by

Cabinet 

Type

Ignition 

Component Description Severity Class

50874 Breaker failure Plant 

personnel

Switchge

ar Trip Coil

During shutdown of Recirc MG set the 

field breaker failed to open. Trip coil 

smoking and on fire. Fire 

extinguished and fuses pulled.

PC 

D-12
D-13

D.2 Quantification of the Time Available for Operator Response

As shown in Figure 2-2, the fire growth profile is made up of several stages. Of importance is 

the early stage of the fire referred to as the “Incipient” stage. Although many fires exhibit this 

incipient stage, it is not typically modeled in performance-based design, primarily because of the 

variability of its duration. This variability is caused by several reasons; including deviations in 

material properties, ignition source, and configuration, just to name a few. Additionally, for fires 

which do display an incipient stage, testing has shown that aspirating smoke detecting systems 

configured to provide very early warning fire detection (VEWFD) do not detect the component 

degradation at its onset, but sometime after the first quartile of the incipient stage duration. 

Thus, to quantify the time available for operator response, operating experience was reviewed 

to identify quantitative information to support a better understanding of either the incipient stage 

duration, or time available between ASD VEWFD alert and fire conditions. To focus this study, 

several assumptions must be made.

The assumptions used for the characterization of the available time estimates in this project are

as follows:

1. Limited to electrical enclosures found in nuclear power plants

a. Excluded equipment includes components such as, motors, pumps, MG sets, 

diesel generators, air compressors.

i. These components are excluded, not because they do not have the 

potential to exhibit an incipient stage, but because of the limited scope 

and resources to develop an understanding of the failure modes and their 

associated duration, which can contribute to risk significant fire scenarios.

2. Limited to component fires that have a sufficiently long duration (incipient stage) such 

that operator response has the potential for enhanced warning and thus enhanced 

suppression capabilities.

a. The duration curve developed DOES NOT model the incipient duration for all 

fires, just those fires that exhibit an incipient duration greater than approximately

30 minutes.

1

It should be noted that prior to the effort undertaken by this project, the state of knowledge

related to understanding the duration of component degradation and associated fire signature 

from such degradation, was limited. One of the primary reasons for said limitation is because of

the fact that, in many instances, the early degradation phases do not affect circuit or system 

functionality. As such, the early degradation conditions are not explicitly explored, with the 

exception of those plants that have a periodic inspection process which employs the use of 

thermal imaging cameras or similar technologies. Even so, these technologies and the

frequency of inspection may not be completely successful in identifying these early 

degradations with such finality that corrective actions can be taken to remedy the potential 

failure of the component(s). Even when such actions are successful in identifying degrading 

components and removing/repairing them, there is not tracking system to document a 

representative incipient stage duration.

 1 The 30-minute minimum was chosen based on a factor of 2 on the maximum use of 15 minutes for plant 

personnel (fire brigade) response to fire locations. The factor of 2 was based on the assumption that the 

incipient fire location may be more difficult to locate because of the nature of the fire signature exhibiting low 

amounts of smoke and possibly invisible smoke.
D-14

Numerous efforts were made to obtain relevant operating experience to support quantification of 

the incipient stage duration. The process involved identifying detailed timing information from all 

available sources, including: fire events database, industry provided operating experience, and 

national laboratory fire experience. This evaluation is based on operating experience, test 

results, and in one case the application of engineering judgement.

Operating Experience

The Updated EPRI Fire Events Database was reviewed for Bin 15 “Electrical Cabinets” Fire 

Events. A summary of those events that contribute to the fire ignition frequency are presented 

above in Section D.1. Of those events, only a limited number are associated with low-voltage 

(<250V) equipment, with the majority of the events (approximately 80 percent) involving 

low-voltage switchgear (480 or 600V), load control centers, or distribution panels, etc.

In reviewing all the low-voltage (<250V) Electrical Panel events classified as “Challenging,”

“Potentially Challenging,” and for “Undetermined,” zero events were identified as non-power 

distribution-type equipment in scenarios for which timing information associated with the 

duration of the incipient stage was provided.

When the review was expanded to include all Electrical Panel equipment types, which are 

classified as “Challenging,” “Potentially Challenging,” or “Undetermined,” only one event was 

identified in which timing information associated with the duration of the incipient stage was 

presented. This event was FID 161 and indicated that approximately 30 minutes elapsed 

between time of breaker trip and operations finding a small flame within the enclosure. The 

event description is presented as follows:

‘D’ control rod drive mechanism (CRDM) fan tripped. Approximately 30 minutes 

later, operations locally opened the breaker after identifying a strong odor and 

that the breaker associated with CRDM fan was smoldering. Upon opening the 

cabinet a “6-inch flame” was observed.

Although the event report is unclear regarding what caused the initial tripping of the CRDM fan 

and how long before the breaker trip the failing component had been degrading, it does suggest 

an approximately 30 minute reference point between an initiating event and detection (by plant 

personnel) of a small fire.

One event out of the ~70 events used to determine the fire ignition frequency for electrical panel 

provides a very weak basis with large uncertainty for quantifying the duration of the incipient 

stage generically for all of the varieties, vintages, and combinations of electrical components 

found in operating NPPs. Thus, in an attempt to develop a generic prediction of the duration of 

NPP electrical cabinets, other sources of information were sought.

The first source included reviewing the “Non-Challenging” events from the EPRI fire events 

database. Although use of this information is not ideal because these events are not used to 

quantify risk, they may provide some insight into the quantification effort. Upon this review,

two events were identified as having timing information associated with the duration of the 

incipient stage. The first “Non-Challenging” event was FID 10647, which indicated that a fire 

occurred in a control cabinet for a reactor building chiller that, because of high outside air 

temperature, ran under a full-load condition over the course of an afternoon. The following 

three apparent fire causes and contributing factors were identified:
D-15

(1) Power cable was inadequately sized to 208 Amp service while the minimum electrical 

circuit load ampacity for this load was 403.2 Amp.

(2) Breaker failed to trip during high current conditions

(3) Experienced poor power-block phase connection (high resistance).

Although the event description does not provide a specific time frame during which the chiller 

unit was operating at full-load, or in excess of the ampacity limits of a 4/0 AWG power cable 

conductor, it did specify that the failure occurred over an afternoon. Webster’s Dictionary 

defines an afternoon as “the day from noon until sunset” (Webster's II New College Dictionary, 

3rd Ed., 2005). Thus, depending on the time of year and location within a time zone, the 

definition of an afternoon could vary from 5 to 9 hours. So, for this example, the average 

afternoon length was assumed to be 7 hours.

The second “Non-Challenging” event was FID 50836, and identified a computer inverter fire. In 

this event, detailed timing information was provided as follows. At 0500, the Unit 2 computer 

inverter was started up per operating procedures, with normal startup and loading indications. 

At 0555, a fire alarm was received. At 0558, the operator reported smoke and fire emitting from 

a U2 computer inverter. Loss of power to the computer bus resulted in the loss of most 

MCR U2 trending displays. Consequently, this event shows that in less than 58 minutes 

the equipment transitioned into flaming conditions.

The last source sought for insights was recent U.S. NPP events in which ASD systems were in 

the area. These events, which occurred in 2013 and 2014, included ones captured in the fire 

events database.

2013 event: On August 7, 2013, at 23:30, an operator was dispatched to Aux Bus E to 

investigate an equipment trouble alarm. Four minutes later the operator identifies that there is a 

ground fault on the 6.9kV/480V Aux Bus E. At 23:38, an IFD zone located in an electrical 

cabinet 20 feet way notifies an alert condition. Two hours and forty-two minutes later an 

explosion occurs involving the Aux Bus 1E2. Therefore, the IFD provided 2.7 hours of 

advanced warning before ignition; in this case ignition is considered to be the occurrence of the 

explosion. Information from this event was taken from the following source, Beasley, K., 

08-08-2012 HNP Fire Event on 1E2 Bus. Retrieved July 29, 2014 from Fire Protection 

Information Forum Archives: http://www.nei.org/Conferences/Conference-Archives/FireProtection-Information-Forum-Archives.

2014 event: The event occurred because of an internal fault in the 480V load center station 

service transformer 1D2. The event started at 07:51, with the MCR receiving electrical 

equipment trouble alarms associated with 1D2. At 08:32, it was confirmed that grounds were 

present on both sides of the transformer. At 09:00 a single zone of the IFD notified an alert 

condition, and at 09:02, the same IFD zone indicated an alarm condition. At 09:29, the resistor 

associated with the ground relay was glowing red, and at 09:31 it was determined to deenergize 1D2. At 10:07, local plant personnel identified smoke emanating from the 1D2 

transformer cubicle and a determination was made to trip the reactor, which was at 75 percent 

power. At 10:24, a confirmation was made that no more smoke was emanating from the 

1D2 transformer. Report never identified if flaming conditions occurred.

Regardless of the severity classification of this event, it provided timing information associated 

with the time between ASD response and fire, if it is assumed that when the plant was shut 
D-16

down a fire was eminent. Because of the limited amount of data, it was decided to keep this 

event data. So, given the IFD notified the MCR of an alert condition at 09:00, if we assume that 

ignition occurred at 10:07, there was approximately 1.12 hours of advanced warning before

assumed flaming conditions.

During the public comment period for the draft version of this report, an event occurred involving 

an ASD VEWFD system. Summary information of the event was provided as part of the 

comments received and a follow-up site visit was performed by several NRC staff.

2015 event: A description of this event was presented at the NEI Fire Protection Information 

Forum and transmitted to the NRC during the public comment period for the draft report. This 

event began at 19:52 on August 1, 2015 when the first of two alerts was received from a 

VEWFD system installed within an electrical cabinet. Operator and technician responded and 

reported no unusual conditions. Thirteen minutes after the alert, the annunciator was cleared. 

At 23:08 the same day the same system issued a second alert. Again no unusual conditions 

were found and the system alert annunciator was cleared for the second time 22 minutes after 

the second alert. At 23:34 the VEWFD system was declared non-functional because of multiple 

alerts and no local abnormal conditions detected using a portable VEWFD system. A 

continuous fire watch was established and a work request to investigate further was initiated. 

[Ref. 2015 NEI Fire Protection Information Forum Slides titled, “Harris Incipient Detection 

Success,” Bob Rhodes] Subsequently, an overheated control switch was found within the 

protected cabinet using a thermal imaging camera. The control switch was associated with the 

control power to the “C” steam generator power operated relief valve. The maximum 

temperature reading of 65 °C (149 °F) was observed with the thermal imaging camera. On

August 3rd at 7:28am a work order was completed and on August 6th, the control switch was 

replaced. 

Based on the description of the event, the in-cabinet VEWFD system provided approximately 35 

hours of advanced warning from the first alert until the overheated control switch was found and 

over 95 hours until the control switch was replaced. In comparison to the previous event 

descriptions, this event was significantly longer, but did not lead to any adverse conditions (fire, 

smoke, reactor trip). An important question to answer regarding the applicability of this event 

relates to its severity classification. Because this event did not lead to a fire, the question of 

assigning a severity classification is purely speculative. That is, if actions were not taken to 

eliminate the abnormal condition, the component may have developed into a fire that would be 

characterized as potentially challenging or greater and would contribute to fire PRA ignition 

frequency; alternatively, it may have never resulted in a fire (via self-mitigation or other means) 

and would not be considered a challenging fire of importance to the fire PRA quantification.

The event appears to meet all the criteria for one of the standard override criteria developed to 

support the updated fire events database, “Individual Sub-Component Failures Not Resulting 

in Flaming Combustion” (Ref. “The Updated Fire Events Database: Description of Content 

and Fire Event Classification Guidance,” EPRI 1025284, Palo Alto, 2013.). Events meeting 

this classification were categorized as “Not Challenging” (NC) because they did not indicate 

the potential for development of a spreading fire. Typical examples of conditions meeting 

this override include overheating of; motor windings, terminal blocks, single printed circuit 

cards, light or light sockets, indicators, control switches, relays, motor contactors, etc.

These types of NC fires were prevalent within the database prompting the creation of a 

generic override criteria. Fires that do not indicate the potential for development of a 

spreading fire are not be considered to be of importance to fire PRA quantification and 
D-17

therefore are screened out of frequency. The point being made is that current methods 

to quantify fire risk only consider events that have the potential to challenge plant safety. 

This event description also identified that the plant personnel were not successful in locating 

the source of the system alert using the handheld VEWFD system (sometimes referred to as a 

“sniffer”). These insights question the assumptions and estimates of the human error 

probabilities presented in the body of this report. Other insights from this event include a 

discussion of how close this component was to an ignition threshold. Most 

materials/components found in an NPP have ignition temperatures above 200 °C (392 °F). 

Thus, 65°C (149°F) is well below the ignition temperature cited here. 

Regardless of the unknowns from this event, the authors of this report determined that this 

event should be considered in the time available estimates. However, the large difference 

between the other event durations (mean of 1.4 hours) and this event (90 hours) would result in 

a distribution that is influenced more by this long event (due to the use of a single parameter 

distribution). As such, a surrogate value was used to characterize this event, and is suggested 

to be used for any subsequent events that an ASD VEWFD system and operator response 

prevent a fire. The surrogate value represents the mean of an exponential distribution that has 

a 10th percentile (first sampling point from Section 10) that is feasible. The time used for this 

feasibility is equal to the cumulative time from “alert” for a successful identification of cabinet for 

the 10 cabinet case, i.e., 30 minutes. Based on this approach a time available estimate of 4.75 

hours is used. For any future updates to the time available curves, this value is recommended 

to be used as a surrogate incipient stage duration for instances where transition to flaming 

conditions does not occur (unable to determine severity classification) and the time between the 

VEWFD system alert and end of event is more than 4.75 hours. For events that do not 

transition to flaming and are less than 4.75 hours, the actual time duration should be used. This 

approach is recommended until such time that sufficient operating experience data is collected 

and can be analyzed to better understand these systems performance to challenging type fires 

per the fire PRA characterization.

This completes the available quantitative information regarding the length of the incipient stage

for electrical components found in U.S. NPPs through operating experience. However, other 

sources of timing information were sought, including NASA, Canada NPPs, National 

Laboratories, and the NAVY. One event was provided by a National Laboratory where resistors 

overheated in two isolation chassis drawers of a 125Vdc power supply rack for a Particle Beam

Fusion Lab Z shot machine. The system uses software to control hardware (programmable 

logic controller). During an experiment in 2011, the shot was aborted, however an error in the 

software failed to shut down the power supply to two of the charging capacitor circuitry. This 

resulted in overheating of the resistor network to ground. From time of test abort to 

identification of overheating resistors was 59 minutes. Visible thermal damage was observed 

on the exterior of the electrical enclosure (discolored paint, soot deposits) and the resistor 

drawers were a complete loss.

Table D-2 provides a summary of this available operating experience duration information. 

The ”Time Available” estimates are based off of the incipient stage operating experience 

discussed above and adjusted using the data from the NIST testing. The resulting exponential 

distribution fit to the “Time Available” data is shown in Figure D-1 and Table D-3. Software 

packages were used to fit the data and evaluate the goodness of fit. An exponential distribution 

provided the best fit and made seemed reasonable for the small sample of data available. The 

single parameter “lambda” of the exponential is estimated by the inverse of the arithmetic mean 

of the data. 
D-18

λλ = �1

nn �xx

nn

=1

�

−1

where xi represents an individual data point of the data set x1, ..., xn and n represents the total 

number of samples. The cumulative distribution function for an exponential is,

1 − ee−λλλλ

where t is time in hours.

If sufficient additional data becomes available in the future, any update to this analysis should 

evaluate the reasonableness of this distribution and the continuity with those fires that are fast 

developing and not included in the data set.

Table D-2. Summary of Incipient Stage Duration and Time Available for Operator 

Response

Event

Incipient 

Stage

(Hours)

Time Available for Operator Response (Hours)

CC LS-SS ION PHOTO

EPRI FEDB 161 0.5 *0.26 *0.13 *0.17 *0.04

EPRI FEDB 50836 0.9 *0.47 *0.23 *0.31 *0.07

SNL z-machine 0.98 *0.51 *0.26 *0.33 *0.08

2014 Event 1.12 *0.56 *0.73 *0.17

2013 Event 2.75 *1.38 *1.80 *0.42

EPRI FEDB 10647 7 *3.64 *1.82 *2.38 *0.56

2015 Event Ŧ 4.75 *2.38 *3.11 *0.73

Lambda 0.518 1.036 0.793 3.382

* estimate based on operating experience adjusted with experimental results (mean time to detection)

and assumption of linear component degradation

Ŧ basis for estimate discussed in Appendix D
D-19

Figure D-1. Exponential distributions fit to data for the time available for operator 

response following an ASD alert for different detection technologies tested
D-20

The distributions presented in Figure D-1 and Table D-3 are based on the system response at 

the sensitivities discussed in Appendix B.4. This information is used in Section 10 to develop 

split fractions to support human error probability estimation for specific time periods. If NPP 

systems use other port sensitivities, applicable data could be used to develop new distributions 

for those settings. The following provides equations to estimate the sampling points. 

Table D-3. Exponential Distribution Values for Time Available for Operator Response 

by Smoke Detection Systems

Time 

(Minutes)

Complementary Cumulative Distribution Function

ASD CC ASD LS and SS ION Spot PHOTO Spot

0 1 1 1 1

1 0.99 0.98 0.99 0.95

2 0.98 0.97 0.97 0.89

3 0.97 0.95 0.96 0.84

4 0.97 0.93 0.95 0.80

5 0.96 0.92 0.94 0.75

6 0.95 0.90 0.92 0.71

7 0.94 0.89 0.91 0.67

8 0.93 0.87 0.90 0.64

9 0.93 0.86 0.89 0.60

10 0.92 0.84 0.88 0.57

11 0.91 0.83 0.86 0.54

12 0.90 0.81 0.85 0.51

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

30 0.77 0.60 0.67 0.18

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

60 0.60 0.36 0.45 0.03

Lambda 0.5185 1.0355 0.7928 3.3818
D-21

Figure D-2. Illustration of sample point to split fraction relation of cumulative 

distribution function curve for time available.

First split fraction (SF1) is fixed at 0.10. The corresponding time is calculated as;

tt10tth = �

−1

λλ � ∙ ln(1 − 0.1)

The second split fraction (SF2) corresponds to the portion of the CDF between t10th and 30 

minutes (0.5 hours), and is calculated as;

SS 2 = ee−λλ∙ 10tth − ee−λλ∙0.5

The third split fraction (SF3) corresponds to the portion of the CDF between 30 and 60 minutes 

(0.5 and 1 hour), and is calculated as;

SS 3 = ee−λλ∙0.5 − ee−λλ

The last split fraction (SF4) corresponds to the portion of the CDF beyond 60 minutes (1 hour) 

and is calculated as;

SS 4 = ee−λλ

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5 4

CDF

Time (Hours)

SF1

SF2

SF3

SF4
D-22

D.3 Evaluation of Enhanced Fire Suppression Fire Events

This section documents the review of fire events from the fire events database where an 

operator was present in the room of origin when a flaming condition began. These results were 

used to develop a new suppression curve. The results are summarized in Table D-3, which 

contains several fields described below.

Fire ID Record number from EPRI Fire Events database

Fire Cause Identifies apparent cause of event

Detected by Identifies how the event was detected 

Cabinet Type Identifies the type of cabinet where the event occurred

Description Provides summary of event

Table D-3. Evaluation of Enhanced Fire Suppression Fire Events

Fire 

ID Fire Cause Detected by

Cabinet 

Type Description Suppression Time (min)

83.1 Ground Fault Plant 

Personnel Power

Smoke was discovered in the 

back boards area of the control 

room by a security officer 

performing an hourly fire watch 

tour. Smoke was emanating 

from the Emergency Lighting 

Uninterruptible Power Supply 

and the Essential Lighting 

Distribution Panel. Cause was 

short circuit current in plant 

ground system because of

inadequate grounding 

procedures. Fire was selfextinguished by removal of 

power by opening AC breaker in 

ELDP.

9

161 Undetermined Plant 

Personnel MCC

‘D’ control rod drive mechanism 

(CRDM) fan (1-HV-F-37D) 

tripped. Approximately 30 

minutes later operations locally 

opened the breaker after 

identifying a strong odor and that 

the breaker associated with 

CRDM fan was smoldering. 

Upon opening the cabinet a “6-

inch flame” was observed.

At the time of fire discovery the 

Operations fire brigade members 

were in the area. When the 

source was found the fire was 

immediately extinguished and 

reflash watch set

5
D-23

Table D-3. Evaluation of Enhanced Fire Suppression Fire Events

Fire 

ID Fire Cause Detected by

Cabinet 

Type Description Suppression Time (min)

253 Breaker 

Failure

Plant 

Personnel Switchgear

Breaker failed to open causing 

excessive current in trip coil. 

Operations crew were present 

within room during failure. 

10

20270 Transformer 

Failure

Fire 

Watch MCC MCC Breaker Transformer failure detected by fire watch 1

20272 Relay failure Plant 

Personnel

Electrical 

Panel

Electrical panel relay fire. 

Detected by security guards 

within the fire area.

4

30276 PCB fault Plant 

Personnel

Emergency 

Lighting

Emergency Lighting Battery Box 

Failed during annual inspections. 

Power transformer inside the box 

was observed to have sparked 

and caused a fire. Failure 

caused by bad charging board 

and one bad cell.

An indication existed of the smell 

of smoke in the Control Room. 

Upon investigation found 

Emergency Lighting Battery Box 

# E7-12 on the East Wall of the 

Control Room, behind 1C614, 

with smoke coming out.

2

E-1

APPENDIX E

LITERATURE REVIEW

This appendix provides a summary of literature reviewed in support of this project. All 

summaries are from publically available documents. The literature is from a variety of sources, 

including the National Fire Protection Research Foundation, academia, vendors, conference 

proceedings, and journal articles. The summaries are provided in chronological order. At the 

end of this section, additional reading material that was reviewed is identified, but not 

summarized below.

E.1 Custer, R.L.P., Bright, R.G., “Fire Detection: The State-of-the-Art,” U.S. Department of 

Commerce, National Bureau of Standards, June 1974.

This report, although dated, provides a substantial amount of information related to 

fire detection technologies with consideration of fire signatures, detection modes, test 

methods, performance requirements, and code requirements. For those with limitedto-no knowledge of fire detection systems operations, review of the report is strongly 

recommended.

Important concepts gathered from this report, applicable to the current study include 

a description of submicrometer particle counting detectors; ambient condition effects 

on detector response; and an emphasis on the importance of detector maintenance. 

Relevant information is communicated in the main body of this report.

E.2 Meacham, B.J., “Factors Affecting the Early Detection of Fire in Electronic Equipment 

and Cable Installations,” May 1992. 

This article provides an overview of the factors that influence fire detection, such as 

fuel characteristics; compartment configuration; environmental configurations; and 

maximum allowable fire size at time of detection; and guidance on selecting 

appropriate devices. The focus is on telecommunication facilities. The first factor 

discussed is the fuel and fire characteristics, which consist of circuit, component, or 

interconnecting wiring, which can produce considerable combustion products, but 

have a relatively low-energy output, and may never transition to a flaming fire. Loss 

history has shown that small, low-energy fires are a serious problem and make early 

detection difficult because of weak plume strength. Weak plumes and low ambient 

temperatures can cause insufficient smoke transport to the uppermost level of a 

ceiling-height enclosure.

E.3 Gottuk, D.T., McKenna, L.A., “Response Time Comparison of Spot vs. Aspirated Laser 

Smoke Detection,” September 1999. 

Hughes Associates, Inc. performed 56 full-scale tests for Pittway Systems 

Technology Group, comparing the response time of laser-based Notifier VIEW spot 

detection systems to Vision Systems laser aspirating smoke detectors (ASDs), which 

were exposed to a variety of smoke sources and ventilation conditions within a 

telecommunications facility. 
E-2

An operating Bell Canada switch center was selected as the test site (100 ft. wide, 

180 ft. long [18,000 sq.ft.], 15.5 ft. high. Five smoke sources were evaluated, 

including BSI 6266 wire; Bell Canada wire; BSI 6266 chemical smoke test; internal 

printed wire board; and conductive heating of EPDM cable insulation. The sources 

were evaluated at ten different locations with variable source heights and two 

different ventilation conditions. Normal ventilation conditions consisted of three 

recirculation air conditioning units, operating at a combined 25844 cubic feet per 

minute (cfm), serving the DMS switching center, and a general HVAC unit serving 

the larger toll/frame area at 12,618 cfm. Reduction ventilation conditions consisted 

of shutting down two of the three recirculation air conditioning units. Several 

variations of detector configuration were also used to evaluate any performance 

differences. The VIEW spot detectors were arranged in two redundant loops. One 

loop used detector spacing at 200 ft2 while the other loop spaced the detectors at 

400 ft2. VIEW detectors were also placed on the return air grills, and detectormounting angles were varied among tests to evaluate the effect on system response. 

Two laser ASDs were installed in the ceiling area-wide configuration, and one laser 

ASD was installed on the return air grills of the recirculation system.

Several key insights were identified from this testing, namely the following:

• Comparable performance between the laser ASD and 200 ft2 VIEW systems 

was demonstrated, while the 400 ft2 VIEW system showed decreased 

performance compared to either the 200 ft2 VIEW or laser ASD systems.

• Return air grill detection showed no clear difference between the laser ASD 

and the spot VIEW detectors. For the VIEW system, the return air grill 

configuration responded to fewer sources than the 200 ft2 system and was 

slower to respond; however, the returns did detect 3 additional fires not 

detected by the area-wide VIEW system.

• Considerable variability existed in the alarm times from a given detector 

system because of changes in source type and location. Even for tests in 

which conditions were the same, response time varied.

• ASD configured to both ceiling area-wide and return air grills were unable to 

detect most of the BSI wire tests (13 of 19 tests undetected). Additionally, 

the majority of the Bell wire tests were undetectable by both ASD and VIEW 

systems.

• Air flow monitoring tests showed that the ASD system did not issue a 

supervisory alarm until 66 percent to 76 percent of the sampling holes were 

blocked, while the vendor default settings for low airflow warning were 

determined or found to occur at 30 percent reduction in normal airflow. Thus, 

air flow reduction does not correlate to sampling hole blockage for the 

particular ASD system tested.
E-3

E.4 Geiman, J., Gottuk, D., “Alarm Thresholds for Smoke Detector Modeling,” Fire Safety 

Science Proceedings of the Seventh International Symposium, 2002.

This paper illustrates an evaluation of the use of smoke optical densities outside a 

detector as a criterion for predicting smoke detector response. The Temperature 

Rise Method was not evaluated because of the highly questionable accuracy. The 

study applied three full-scale experimental data sets that used optical density meters 

located in close proximity to some of the detectors (ion and photo). This paper 

concludes that determining the precise alarm times is not currently possible with the 

large number of variables that not only exist, but influence smoke detector response. 

Using nominal detector sensitivities as an alarm threshold, leads to only about 20 

percent of the alarm predictions corresponding to actual detector alarms. In most 

cases, the use of the nominal sensitivity will result in predicting alarms before they 

actually occur. One exception is ionization detector response to flaming fire 

conditions, which corresponded with predicted alarm conditions 50 percent of the 

time. Using an alarm threshold of 0.14OD/m (9.4 %/ft obscuration) provides a 

relatively high level of confidence in predicting detector alarms, but will typically 

predict alarm response times that are potentially longer than would actually occur.

E.5 Nicholson, J., “Looking Up: NFPA 805, Performance-Based Standard for Fire Protection 

for Light Water Reactor Electric Generating Plants,” NFPA Journal, May/June 2003.

This article provides a high-level overview of how performance-based fire protection 

application of NFPA standards have improved safety at Canadian nuclear power 

plants (NPPs). The use of performance-based, over prescriptive requirements,

facilitates the use of (their) equivalencies in making more realistic analyses of Bruce 

Power Plants safe shutdown capability. The Bruce implementation of performancebased initiatives was deemed to be equivalent to the Canadian Standards 

Association N-293. As part of the plant upgrades, Vesda Air Aspiration Systems 

were installed.

E.6 Tieppo, Eddie, “Very Early Warning Aspirated Smoke Detection in Nuclear Power 

Facilities,” October 2003.

This paper provides a high-level overview of design and benefits of VEWFD systems 

over conventional spot-type detectors. Maintenance considerations for ASD systems 

are also presented with discussion of detector unit, filter, and pipe network 

maintenance concerns. This paper concludes with a discussion of several 

applications used in Canadian NPPs in areas such as the main control room, 

equipment rooms, cable tunnels, and generators halls.

E.7 Vision Systems, “White Paper: Using Air Sampling Smoke Detection to Protect MissionCritical Facilities from Fire,” 2004.

This paper provides a high-level overview of the importance of protecting missioncritical facilities, such as essential financial installations, from the effects of smoke 

damage on their electronic computer systems. More and more, the risks associated 

with data center fires are caused by increased energy consumption from modern 

computers, and the need to use high-ventilation, flow rate air conditioning to cool the 

computer electronics. Consequently, such high air flow environments, using 

conventional spot-type detectors becomes ineffective. However, it is not only 
E-4

potential fire damage from which the data centers need to be protected. Smoke 

contamination of electronic equipment, even as small as 16 micrograms per square 

centimeter, can cause corrosion and long term effects and 30 microgram per square

centimeter can result in short-term effects.

The paper also provides an overview of how ASD systems work; common 

application in typical data center environments; ASD system design characteristics 

that are important to consider in ensuring the EW or VEW detection; coverage area;

and sensitivities. The paper also establishes that in-cabinet and integratedequipment detection enable an excellent VEWFD solution for the following reasons:

1. Sampling is performed closest to the source.

2. Addressability is enhanced as compared to ceiling or air return type 

detection.

3. Likelihood of smoke/fire damage to other equipment is minimized.

4. Background noise (dust/smoke) within enclosures is relatively consistent.

5. Numerous detector set points made a staged response replicable/possible.

E.8 Vision Fire & Security, “Development of Performance Equivalency Methodology for 

Detection and Suppression System Integration,” 2006.

Vision Fire & Security Pty Ltd. developed a detection equivalency method, which 

resulted in development of an “application tool” to allow ASD systems to be specified 

as an alternative to the conventional spot-type detection design for applications in 

which detection is used for suppression system actuation. The study focused on the 

use of ASD systems in challenging environments, such as areas with high airflow, 

very low or non-thermal energy fire hazards, and dense equipment layout as a 

solution for risk management and business continuity. The Fire Dynamics Simulator 

(FDS), a fire modeling computational fluid dynamic tool developed by NIST, was 

used to determine the appropriate ASD alarm level to establish an equivalent or 

better level of performance as compared to the benchmark for conventional 

spot-detection.

This work identified that, for an equivalent performance, ASD requires higher 

sensitivity settings in rooms where (1) the airflow is higher, (2) the room size is 

increased, and (3) the ceiling height is increased. For the application using an ASD 

system to supplement or replace conventional spot-type detectors, the report 

concludes that with proper alarm settings (based on airflow characteristics and room 

physical dimensions), detection performance can be enhanced, and more consistent 

fire size at time of suppression can be achieved. The report does not appear to take 

into consideration the uncertainness associated with the FDS modeling, nor does it 

explicitly describe the basis equivalency calculation method.

E.9 Fabin, T.Z., Gandhi, P.D., “Smoke Characterization Project,” April 2007.
E-5

The Fire Protection Research Foundation, along with the Underwriters Laboratories,

undertook a smoke characterization study to more fully characterize the products of 

flaming and non-flaming combustion for materials found in common residential 

settings. Small-scale cone calorimeter, intermediate scale calorimeter and UL 

217/268-type fire room tests were conducted to evaluate smoke characteristics for 

natural, synthetic, and multi-component materials in both flaming and non-flaming 

combustion modes. Quantities such as mass-loss rates; heat and smoke release 

rates; smoke particle size and count distribution; effluent gas composition;

combustion mode effects; smoke alarm response; and smoke stratification were 

characterized. Although the report provides a thorough evaluation for the 

combustion modes and materials tested, most of the materials are not commonly 

found in NPP applications, making the application of these results uncertain.

E.10 Collier, P.C.R, Whiting, P.N., “Timeline for Incipient Fire Development: Study Report No. 

194,” 2008.

The Building Research Levy funded work to evaluate incipient fire development of 

furniture fires with regard to the modeling of the incipient stage in performance-based 

design fires. Using test data, and literature and statistical evaluations, the 

researcher noted that the incipient fire development period is variable and its 

duration was dependent on intensity and location of the ignition source. Because of

the variability in the incipient stage, the report recommends no allowance for incipient 

fire development in performance-based applications. Although furniture fires are not 

realistic surrogates for electrical cabinet fires, the variability of the duration of the 

incipient stage is a characteristic likely applicable to electrical cabinet fires.

E.11 Milke, J.A., et al., “Validation of a Smoke Detection Performance Prediction 

Methodology,” Volumes 1-4, October 2008.

The Fire Protection Research Foundation sponsored a research project to evaluate 

the current capabilities of the computational fluid dynamic (CFD) code FDS to predict 

smoke detector activation in response to relatively low energy incipient fire sources. 

This work was performed by the University of Maryland and Underwriter 

Laboratories, and is documented in a four-volume report titled, “Validation of a 

Smoke Detection Performance Prediction Methodology(Milke, Mowrer, Brookman, & 

Gandhi, 2008).” Volume 1 documents the characterization of the heat and smoke

release of eight incipient fire sources that were selected for the project; Volume 2 

provides a detailed description of the large-scale room fire tests. Volume 3 

evaluates the smoke detector response, and Volume 4 compares the experimental 

results with the predictions of FDS simulations.

Modeling of smoke detector response typically uses one of three methods, namely, 

temperature rise, critical velocity, or optical density. However, these methods do not 

address the operational principles of the detectors resulting in uncertainty in the 

detector response modeling approximations. Eight smoke sources were 

characterized by performing small-scale tests using the IMO intermediate scale 

calorimeter at UL in Northbrook, IL. The smoke sources used include: shredded 

office paper, polyurethane (PU) foam with micro-fiber fabric, printed circuit board, 

and an ABS plastic computer case, which was used to create flaming smoke 

sources, while PU foam with micro-fiber fabric, ponderosa pine sticks, cotton lined 

fabric, and polyvinyl chloride (PVC)-insulated wire were used for smoldering smoke 
E-6

sources. The primary smoke signature of interest was the obscuration of visible 

light, but particle count density, mean particle diameter, CO and CO2 production 

data were also collected.

The interesting results from this small-scale testing, as it relates to NPP smoke 

sources, are the characterization of the PVC-insulated wire test. The PVC test 

followed the procedure outlined in Annex B, “Performance Test Procedure for Very 

Early Warning and Early Warning Fire Detection Systems” of NFPA 76, using the 

North American Wire Test outlined in Table B.2.1 of that standard. However, the UL 

test deviated from the Annex procedure in that the test duration lasted 60 seconds,

instead of the standard specified 30 second duration. The results indicated that the 

average maximum smoke release rate of 0.10 m2/s occurred just after 60 seconds;

the maximum mean particle diameter of 0.135 micron occurred just before 120 

seconds; and the average maximum particle count density of 200,000/cc occurred at 

approximately the same time.

The large-scale testing included unventilated and ventilated conditions. The 24 

unventilated tests were conducted in the UL 217/268 room, measuring 10.8 m by 6.6

m and 3.0 m tall. The rooms were instrumented with photocell/lamp assemblies;

measurement ionization chamber units; spot-type smoke detectors (both ionization 

and photoelectric types); thermocouples; thermocouple trees and velocity probes. 

The 64 ventilated large-scale tests were conducted in different room measuring 

7.2 m wide, 7.2 m long (51.8 m2) by 9.0 m high. This enclosure was equipped with 

an injection-type mechanical ventilation system, with two ceiling air injector diffusers, 

and four transfer grills providing air exhaust to the plenum space above the 

ventilated test room. The ventilated room experiments consisted of testing three 

ventilation rates, namely 0, 6, or 12 air changes per hour (ACH). The ventilated 

room tests included the same instrumentation as the unventilated tests, with the 

addition of three ASD single-zone systems, having three sampling ports per zone 

(two sampling ports within the test room, one port sampling from outside the test 

room). The intent of including the ASD systems was to collect information to 

evaluate the prediction capabilities of FDS. Unfortunately, the data files associated 

with the ASD system could not be synchronized with the other data files. 

Evaluation of the data/feedback indicates that the point type “smoke detector 

response(s) appear to be strongly dependent on the specific characteristics of the 

smoke and, in some cases, on the detector technology.” The researchers then 

evaluated the detector response based on the 80th percentile values of obscuration 

and suggested nominal guidelines for detector response, depending upon the 

ventilation conditions, fire characteristics, (flaming vs. smoldering) and in some 

cases, detector technology. 

Table E1. 80th Percentile Obscuration Level at Detector Response

Smoke Source Ventilation Detector Type

Photoelectric Ionization

Flaming With 5%/ft 8%/ft

Without 8%/ft

Non-Flaming With 1 – 2.5%/ft

Without 10%/ft 12%/ft
E-7

The report classified the smoke sources as “incipient fire sources,” considering that 

half the sources used produced their smoke signatures during flaming combustion;

the basis for classifying these smoke sources as “incipient fire sources,” is 

questionable given that all were identified as either smoldering or flaming. In 

addition, no clear distinction was made as to the portion of the overall smoke release 

rate profile that corresponds to the incipient stage. Although these tests provided 

useful data for characterizing the effluent from the various fire sources and for 

evaluating the smoke prediction capabilities of FDS, because the results were not 

reported for the ASD system tested, little can be drawn from these results regarding 

the response of an ASD VEWFD system.

E.12 Xtralis, “IT/Server Room Fire Test Demonstrations – VESDA & photoelectric 

conventional point detectors, Technical Report,” October 2008.

Xtralis provides a case study from demonstration tests conducted at their Test 

Facility IT/Server room. Smoke tests were conducted in various locations within the 

room to demonstrate an ASD smoke detector’s early warning (EW) and very early 

warning fire detection (VEWFD) capability. The ASD applications tested included 

ceiling, return air grill, air duct, and cabinet configurations. The ASD system results 

were compared to conventional spot-type detectors that were included in the tests.

The test facility measured 11.0 m wide x 6.5 m deep x 3.0 m high for a total floor 

surface area of 71.5 m2. The room ventilation conditions were representative of 

those found in telecommunication facilities, namely, clean, cool air is introduced 

through the floor, and exhaust via the air return grill located on a wall. 

Individual Xtralis VLC detectors were used for cabinet, ceiling, air return grill and duct 

detection with conventional photoelectric spot-type detectors set to 1.4 %/m

obscuration (Mode 1) located adjacent to the ASD sampling points, except for the 

cabinet applications. An alarm control panel was used to log all detectors’ alarm 

times. Fire/smoke tests included overheated PVC wire, overheated resistor, and 

smoldering smoke pellet tests were also conducted. The duration of the overheated 

cases did not exceed 10 seconds. Seven different locations throughout the facility 

were used to locate the fire/smoke generation specimens.

Results indicated that for the short-duration fire/smoke sources tested and the 

locations of the EW and VEWFD systems, performed better than the conventional 

spot-type detectors. These tests did not provide any information on system 

performance for slowly degrading electrical components or timing information for 

operator response to such incipient stages.

E.13 Xtralis, “Warehouse Fire Detection Test Results - ASD vs Point (spot-type) vs Beam 

Detectors, Case Study,” April 2008.

Case study presents results from a series of demonstration tests conducted in 

Victoria University’s Warehouse. Three small fire tests were conducted to illustrate 

the benefits of EW and VEW detection capabilities. Comparisons were drawn 

between ASD EW and ASD VEW, as well as conventional spot-type and beam 

detectors.
E-8

The warehouse was 43 m long, 12 m wide for a gross floor area of 516 m2, and had 

a ceiling height of 8.5 m at the central pitch. A single ASD pipe was installed along 

the ceiling ventilated ridge and contained six sampling holes spaced 7.2 meters

apart. Several alarm thresholds were used for the ASD system, however only the 

Alert (~0.1 %/ft obscuration [0.3 percent per meter obscuration]) and Fire 1 (~0.37

%/ft obscuration [1.2 percent per meter obscuration]) alarm times were reported. 

Thus the Alert threshold was twice as sensitive and the Fire 1 threshold was almost 

1⁄2 as sensitive as the minimum sensitivity for an alert per NFPA 76. Six optical spottype detectors were also placed along the ceiling ridge vent adjacent to each ASD 

sampling point. The spot-type detectors had a sensitivity of 1.4 %/m obscuration. 

Smoke sources included liquid heptanes (100 ml), timber, and smoke pellets and 

were all located in the same area under the center line of the sloped ceiling, between 

the farthest two sampling points of the ASD system, and directly below the projected 

beam detector line of sight; positioning of the warehouse rollup door(s) also varied.

The results indicated that the ASD system performed better than the conventional 

and projected beam. For tests in which conventional spot-type detectors responded, 

the ASD system responded on average 52.2 seconds before the spot-type detector. 

Of the 21 tests conducted, the ASD systems responded with an alert 19 times, while 

the spot-type responded only 4 times, and the beam detector only responded in one 

test. The report concludes that the ASD system performed better than the other 

systems compared and supports the use of ASD systems in challenging warehouse 

type environments that promote smoke dilution because of high ceiling heights, 

stratification and natural ventilation conditions. However, it should be noted that,

unless the report contains a typographical error, the 4.6 %/ft obscuration [1.4 %/ft

obscuration] sensitivity of the spot-type detector is outside the bounds of UL 268 

(0.5-4.0 %/ft obscuration) and would not be cited on the listing.

E.14 Milke, J., et al., “Guidelines for Estimating Smoke Detector Response,” Suppression and 

Detection Research and Applications Symposium (SUPDET 2009), Orlando, FL

This paper summarizes the work completed in Volume 3 of the “Validation of a 

Smoke Detection Performance Prediction Methodology,” sponsored by The Fire 

Protection Research Foundation. See Section E.11 for a full description of this work.

E.15 Miller, J., “Analyzing Photo-electric smoke detector response based on aspirated smoke 

detector obscuration,” Masters of Science Thesis, University of Maryland 2010.

Following the completion of the fire protection research foundation work, the data 

was analyzed by a student at the University of Maryland to improve the obscurationlevel response accuracy of spot-type photoelectric smoke detectors. This work is 

documented in a Master of Science thesis titled, “Analyzing Photo-Electric Smoke 

Detector Response Based on Aspirated Smoke Detector Obscuration” (Miller, 2010). 

In this work, the ASD data files were analyzed against the other light obscuration 

measurements made during the testing, such that the data files could be 

synchronized. Then, the spot-type detector response was compared to the 

obscuration measurements made by the ASD system. The results concur with the 

conclusions made in the original test program, with the exception of non-flaming fires 

without ventilation; in this case, the student’s analysis indicated that the photoelectric 

detector performed better (responded at lower obscuration values) than the original 

report suggested (range of 0.4 to 5 %/ft obscuration). 
E-9

Although the results compare well with the conclusions from the original report, 

uncertainties associated with averaging the ASD system response, unknown system 

offset time, filter effects, and smoke transport lag, were not explicitly quantified, and 

complicated the analysis. Even so, the results of this work parallel the conclusions 

from the original report suggesting a relatively large deviation in the expected 

obscuration level of 2.5 percent at photoelectric detector response. Unfortunately, 

because the ASD system was used as a measurement device, rather than setup as 

a detector, little can be drawn from this work regarding its performance. However, 

the underlying conclusion re-enforces the basic fire detection principles that are 

applicable to ASD detectors, which state that smoke characteristics, smoke 

transport, and detector characteristics directly impact the performance of a detectors 

response compared to its listing.

E.16 Zaworski, J., Laramee, S., O’Conner, D.J., “Comparative Testing of Various Detection 

Technologies,” Schirmer Engineering Corporation, 2010.

Schirmer Engineering witnessed and reported the results of a series of fire tests 

conducted in a warehouse environment for axonX. The work was commissioned by 

axonX to evaluate the relative performance of their video image fire and smoke 

detection (VID) against numerous flame and smoke detection technologies. The test 

series consisted of 21 different fire scenarios which were repeated three times each 

for a total of 63 tests using five technologies (ASDs, projected beam smoke 

detection, spot-type ionization and photoelectric, and VID). 

An ASD sampling pipe was located near the ceiling (~18ft) and ran the length of the 

room with three sampling ports spaced approximately 20ft from each other. The 

default ASD detector sensitivities were used with the “Fire 1” and “Fire 2” set points 

used for comparison (~0.18 %/ft obscuration and 1.2 %/ft obscuration, respectively, 

at each sampling point). Seven different fire sources ranging from smoldering to 

flaming fires, were used. Of interest to the NRC/NIST project was the overheated 

smoldering wire source, which consisted of a bundle of Type NM-B 14/3 cables 

wrapped around a heating element energized for 20 minutes per test. The fire 

sources were placed in three different locations within the room.

Results from this test series indicated that, in general, for warehouse type 

applications, the VID response fastest form most fire sources typically followed by 

the ASD and Ion spot detectors. For the low-energy overheated wire and smoldering 

wood tests, the VID responded in the 298 – 455 second range, while the ASD, beam, 

and Ion detectors responded in the 805 – 1016 second range. Given the sensitivity 

settings for the ASD systems, physical arrangement, and fire source characteristics,

these results are reasonable.

E.17 Gottuk, D., et al., “Validation of Modeling Tools for Detection Design in High Air Flow 

Environments, Final Phase I Report,” The Fire Protection Research Foundation, August 

2012.

The Fire Protection Research Foundation sponsored an effort to examine the 

applicability of using computer modeling tools for modeling smoke detection system 

designs in high airflow rate environments. This report documents the identification of 

modeling requirements, potential computer models, gaps in knowledge, and 
E-10

development of a research program to address these gaps. Seventeen model 

requirements and eight models were identified. The gap analysis identified the 

following four gaps: 

1) Specification of the fire and smoke input

Specification of the rate of smoke production for incipient fires and 

the smoke production and heat release for flaming fires is 

considered a modeling gap. Two aspects for this gap include

existence of applicable measured data, and the methodology for 

specifying the inputs. The capability to predict the ignition and 

growth of fires of real world objects remains the subject of academic 

research.

2) Smoke transport

Soot deposition upon walls and equipment is a known challenge related 

to smoke detection system design and modeling. Deposition of soot 

reduces the concentration of soot in the air, and acts to delay detection 

response. Soot deposition is primarily caused by thermophoresis 

(thermal gradients), electrophoresis (electric fields), and impactation 

(sharp turns in air streams near obstructions). This all suggests 

deposition is more significant inside electrical equipment than outside.

3) Smoke detection performance 

The ability to correlate conditions predicted by a model at a smoke 

detector/ASD sampling port to an alarm condition within the detector is a 

significant gap. There are no established correlations for predicting alarm 

response for ASD systems at either low or high air flow environments. 

ASD test data indicates that correlations will be highly dependent upon 

the specific detector model.

4) Large-scale integral test data set

Limited data exists for HVAC flows and cooling effectiveness of 

IT/telecom facilities. An IT/telecom facility-specific set of large-scale tests 

would serve as a validation benchmark for determining the suitability of a 

specific model.

In the development of a test plan to address these gaps in knowledge, the authors 

identified the following issues, which are applicable to NPP fire scenarios in electrical 

equipment enclosures:

• The duration and amount of smoke released from electrical equipment will be 

dependent upon the size of the heat source, the orientation of the material 

with respect to the heat source, and the amount of material exposed.

• Significant fire development occurs when pyrolysis/smoldering combustion 

transitions to flaming. Although it is generally a goal to detect fires before

flaming conditions, there are practical issues that can prevent successful 

intervention at the early stage. These include smoke levels not visible to site 

personnel, resulting in a source being difficult to find, and fires that occur 

because of energetic events, and result in relatively fast fire growth.
E-11

• Smoke production from an incipient circuit board fire may impact several 

other circuit boards and pieces of electrical equipment, before being 

enveloped into the primary air flow. In this scenario, soot deposition on the 

impacted equipment may reduce the overall smoke concentration that is 

transported to a detection site, compared to the same source in the open.

• Given the range of cabinet and ventilation configurations, it may be 

necessary to evaluate several potential configurations with sources at 

different locations within a cabinet. Total soot deposition may be significantly 

different for source in the bottom of a cabinet that has continuous vertical 

ventilation openings exposed to external ventilation conditions (smoke flows 

out of the cabinet), compared to a source at the bottom of the cabinet with 

exhaust openings at the top of the cabinet (smoke must flow up through the 

cabinet.

• Several detection devices used complex algorithms that continuously monitor 

ambient conditions and adjust detection levels accordingly. 

An understanding of the operation of such software, the optional and default 

settings, and the manufacturers recommendations are needed to ensure that 

the device meets performance goals.

Additional Readings:

i. Litton, C.D., “Laboratory Evaluation of smoke detectors for use in underground mines,”

Fire Safety Journal, pp. 387-393, Vol. 44, 2009.

ii. Xtralis, “Designing & Deploying Aspirating Smoke Detection in Harsh Environments,”

SFPE Webinar, http://magazine.sfpe.org/webinar/designing-and-deploying-aspiratingsmoke-detection-asd-harsh-environments.

iii. Gallucci, R.H.V., Iqbal, N., Frumkin, D., Metzger, D., Barrett, H., “Credit for Very Early 

Warning Fire Detection in Fire Probabilistic Risk Assessment,” ANS Risk Management, 

2009.

iv. Ludewig, F.A., “Application of Cloud Chamber Techniques to Fire Detection,”

Environment One Corporation, Schenectady, NY.

v. Schifilti, R.P., “Applications of Fire Detection, Alarm and Signaling Systems in Industrial 

Occupancies,” Fire Protection Engineering Magazine, 

http://magazine.spfe.org/content/applications-fire-detectoin-alarm-and-signalingsystems-industrial-occupancies, 2013.

vi. Gottuk, D.T., McKenna, L.A., “Spot and Aspirated Laser Smoke Detection in 

Telecommunications Facilities,” Fire Technology, pgs. 147-178, Vol. 38, 2002; 

Proceedings, AUBE ‘01, 12th International Conference on Automatic Fire Detection, 

2001.

vii. AirSense Technology, Ltd., “Aspirating Smoke Detection, A Brief Guide for the 

Designer,” 1999.
E-12

viii. Xtralis, Dinaburg, J., Meikle, P., Vayeda N., “Testing & Verification of ASD Technology in 

Different Environments,” SPFE Webinar, 2012.

ix. NRC, “NUREG/CR-1798, Acceptance and Verification for Early Warning Fire Detection 

Systems – Interim Guide,” 1980.

x. Fissan, H., Otto, E., Dixkens, J., “Particle Transport in Sampling Lines used in Fire 

Detection Systems,” Fire Safety Journal, pgs. 205-215, Vol. 29, 1997.

xi. Xtralis, “Validation of NIST FDS Model with VESDA Detector Smoke Measurement,”

Doc. 11124-01, 2013.

xii. Xtralis, White Paper, “Very Early Warning Aspirating Smoke Detection in Nuclear 

Facilities,” Doc. 18434-02.

xiii. Xtralis, White Paper, “The Complementary Application of Very Early Warning Fire 

Detection and Reliable Gas Monitoring in Diverse Environments,” Doc.18620_01.

xiv. Air Intelligence, “Industry Applications, High Ceilings,” 2009.

xv. FireTEC, “A Comparison of Aspirated Smoke Detectors, Honeywell FAAST Detectors 

and Xtralis VESDA VLF (laserfocus) Conducted by Packer Engineering, Inc. and the Fire 

Testing and Evaluation Center at The University of Maryland,” College Park, MD.

xvi. Parks, L.L., Kushler, B.D., Scrapiglia, M.J., McKenna, L.A., Budnick, E.K., Watts, J.M., 

“Fire Risk Assessment for Telecommunications Central Office,” Fire Technology, Vol. 

34, No. 2, 1998.

xvii. Floyd, J., Overholt, K., Ezekoye, O., “Soot Deposition and Gravitational Settling 

Modeling and the Impact of Particle Size and Agglomeration,” Proceedings of the 

Eleventh International Symposium, Fire Safety Science, 2014.

xviii. Gottuk, D., Mealy, C., Floyd, J., “Smoke Transport and FDS Validation,” Proceedings of 

the Ninth International Symposium, pp., 129-140, International Association for Fire 

Safety Science.

xix. Tinsley, A., Gorbett, G., Hanks, C., “Comparative Assessment of VLC-505 & FAAST 

8100, Return Air Duct and Warehouse Testing,” Eastern Kentucky University, 2011.

xx. Vision Systems, “Cabinet Detection Aspirating Smoke Detection,” 2000.

xxi. NRC, “NUREG/CR-0488, Nuclear Power Plant Fire Protection Fire Detection 

(Subsystems Study Task 2),” 1979.

xxii. Brookman, M.J., “Smoke Characterization of Incipient Fire Sources for FDS Modeling,”

Master of Science Thesis, University of Maryland, College Park, MD 2008.

xxiii. Johnson, P., “Very Early Warning Fire Detection for High Tech/High Value Facilities,”

SFPE Symposium, Fire Protection and Suppression, 1987.
F-1

APPENDIX F

QUICK REFERENCE FOR PARAMETERS USED IN 

RISK SCOPING STUDY

The purpose of this appendix is to provide a quick reference to the parameter estimates 

developed and presented in Part II of this report. This appendix was developed based on 

comments received on the draft version of this report. These comments related to the effort 

required to quickly and efficiently locate parameters estimate that are dispersed in different 

sections of the report. Tables F-1 to F-6 repeat the information found in the body of this report. 

Table F-1. Fraction of Fires that Have an Incipient Stage - Parameter ‘α’

Category

Incipient Stage Detectable by 

VEWFD System Total # 

Events

Fraction (alpha)

Mean [lower/upper] Yes No Undetermined

Power 

Cabinets# 16.5 16 22.5 55 0.50[0.36 / 0.64]

Low Voltage 

Control 

Cabinets

6 2 5 13 0.28

[0.08 / 0.54]

# Power cabinets include electrical distribution electrical enclosures such as motor control centers, load centers, 

distribution panels, and switchgear

Table F-2. Unreliability and Unavailability – Parameter “β”

Parameter Mean (per device-year) 5th / 95th percentile

ASD Unreliability 1.6×10-3 9.7×10-4 / 2.3×10-3

ASD Unavailability 2.0×10-3 3.0×10-4 / 3.7×10-3

Total Unreliability & 

Unavailability (β) 3.6×10-3 8.6×10-4 / 6.3×10-3
F-2

Table F-3. ASD VEWFD System In-Effectiveness – Parameter “τ”

Application Cabinet/Room

Ventilation Detector Type Mean (τ)

[5th/95th percentile]

In-Cabinet

Natural 

and

Forced <100

cabinet ACH

ION Spot 1.0×10-1

[6.8×10-2 / 1.4×10-1]

PHOTO Spot 7.7×10-1

[7.1×10-1 / 8.3×10-1]

SS 2.6×10-1

[2.1×10-1 / 3.2×10-1]

ASD CC 2.7×10-3

[1.1×10-5 / 1.0×10-2]

ASD LS1 5.3×10-1

[4.5×10-1 / 6.5×10-1]

ASD LS2 1.9×10-1

[1.4×10-1 / 2.4×10-1]

Forced ≥100

cabinet ACH

ION Spot 7.9×10-1

[6.5×10-1 / 9.0×10-1]

PHOTO Spot No Data

SS No Data

ASD CC 1.9×10-2

[7.8×10-5 / 7.3×10-2]

ASD LS1 No Data

ASD LS2 3.7×10-1

[2.2×10-1 / 5.2×10-1]

Area-wide, Air Return 

Grill HVAC in room

ASD CC 3.0×10-1

[1.7×10-1 / 4.5×10-1]

ASD LS2 5.2×10-1

[3.6×10-1 / 6.7×10-1]

Area-wide, Ceiling Any

ION Spot 8.1×10-1

[7.3×10-1 / 8.9×10-1]

PHOTO Spot 8.7×10-1

[8.0×10-1 / 9.3×10-1]

SS 5.7×10-1

[4.7×10-1 / 6.7×10-1]

ASD CC 3.2×10-1

[2.3×10-1 / 4.2×10-1]

ASD LS2 1.1×10-1

[4.6×10-2 / 1.8×10-1]
F-3

Table F-4. Human Error Probability (HEP) for ASD VEWFD Cloud Chamber Detectors –

Parameters “μ” and “ξ”

Detector Type

Total Human Error Probability Estimates

MCR (μ) Field Operator (ξ)

In-Cabinet Area-Wide

ASD VEWFD 

Cloud Chamber 

(CC)

1×10-04

4.6×10-04

1.0◊

Ionization spot 

(ION) 1.7×10-02

ASD VEWFD Light 

Scattering (LS) 

and VEWFD Light 

Scattering 

Sensitive Spot 

(SS)

4.6×10-02

◊

 the variability of area-wide applications does not permit the development of a generic field operator HEP. 

As such, an HEP of 1.0 is assumed. If a scenario specific analysis were conducted following the process 

outlined in Section 10, the result of such an analysis may support using a value other than 1.0.

Table F-5. Enhanced suppression parameters

Parameter Estimate Suppression Rate 

Parameter (λ) Guidance

π1

Scenario 

Dependent# 0.324

Use main control room suppression rate 

for manual suppression from EPRI 

3002002936 / NUREG-2169 and the 

scenario dependent time to damage 

value to calculate enhanced suppression 

parameter.

π2

Scenario 

Dependent# 0.194

 Use occupied room suppression rate 

developed in this report for manual 

suppression and the scenario dependent 

time to damage value to calculate 

enhanced suppression parameter. # the estimate mean is dependent on the time to damage calculation. This parameter will vary per scenario 

based on fire modeling inputs used
F-4

Table F-6. Conventional Suppression Parameters

Parameter Applicability Guidance

η1

In-cabinet & 

Area-wide

Used to credit redundant detection and suppression (automatic 

and manual) system response. Solve conventional detection 

and suppression event tree (See Figure F-1) and sequences 

F-N in Table F-7.

η2

In-cabinet & 

Area-wide

Used to credit VEWFD system prompt response (time to 

detection = 0 minutes). Solve conventional detection and 

suppression event tree (Figure F-1) and sequences F-I

(Table F-7).

η3

In-cabinet & 

Area-wide

Used to credit automatic detection and suppression system 

response only. Parameter is estimated from automatic 

detection and suppression system unreliability estimate, 

including any dependencies.

Fire

Automatic Manual

Sequence

End 

State

Detection Suppression Detection Fixed Fire Brigade

FI AD AS MD MF FB

1.0 F OK

G OK

H OK

I NS

J OK

K OK

L OK

M NS

N NS

Figure F-1. Conventional detection suppression event tree
F-5

Table F-7. Conventional Detection Suppression Event Tree Outputs

Sequence Detection Suppression

F Automatic detection by

• Heat detectors

• Smoke detectors

Fire suppression by an automatically actuated fixed 

system 

G Fire suppression by a manually actuated fixed system

H Fire suppression by the fire brigade

I Fire damage to target items 

J Delayed detection by

• Roving fire watch

• Control room verification

Fire suppression by an automatically actuated fixed 

system 

K Fire suppression by a manually actuated fixed system

L Fire suppression by the fire brigade

M Fire damage to target items 

N Fire damage to target items 

G-1

APPENDIX G

VEWFD SYSTEM DATA COLLECTION

This report concluded (see Sections 14.1 and 15) that consistent reporting criteria and collection 

of data on VEWFD system performance could support a better understanding of their risk 

benefit. This appendix identifies (see boxes and Table G-1) the types of information that could 

be collected to support this objective.

System unavailability, unreliability and nuisance alarm rate data could be reported every 5 or 

10 years. This would provide a snapshot of system performance across the industry.

Section 7.2 discusses how the unavailability and unreliability estimates are calculated, as used 

in this report. 

VEWFD system Unavailability

_____ Total system downtime during surveillance interval

_____ Surveillance interval

VEWFD system Unreliability

_____ Number of observed failure

_____ Total operating period in hours

Nuisance “Alert” Rate

_____ Number of Nuisance Alarms

_____ Total operating period in hours 

The nuisance “Alert” rate does not have a direct use in the risk scoping study presented in this 

report. However, the frequency of “Nuisance Alerts” could be useful for end users that may be 

considering the installation of these systems. It could also allow for a comparison across 

detectors at a site or across the industry to identify trends and outliers. This could identify the 

need to evaluate the acceptability of the system set points to the background noise (baseline). 

Tracking the system response to these nuisance alerts may provide insights as to what is the 

root cause of the nuisance alerts.
G-2

Specific VEWFD incident information

For each event where a VEWFD system “alert” occurred before a fire, explosion, or pre-flaming 

condition, provide the following;

Type of Detection : ______ In-Cabinet

______ Area-wide (Ceiling) 

______ Area-wide (Air Return Grill)

______ Other (please specify) _______________

Detection Technology : ______ ASD VEWFD Cloud Chamber

______ ASD VEWFD Light Scattering

______ ASD VEWFD Other (please specify) ____________

Port Sensitivity : ___________ % obsc. / ft.

___________ % obsc. / m.

___________ particles / cc

If port sensitivity is not known, detector sensitivity along with 

the number of sampling ports on the zone could be reported.

Detector Sensitivity : ___________ % obsc. / ft.

___________ % obsc. / m.

___________ particles / cc

# of sampling ports on “alert” zone : ___________

Time between initial “alert” and identification of degrading component

_______:________(Hours:Minutes)

Time between initial “alert” and de-energization of degrading component / cabinet

_______:________(Hours : Minutes)

Table G-1 identifies event timing information that, if consistently collected, could support 

advancements to the evaluation of ASD VEWFD system performance. The first column 

identifies the timing input, followed by the parameter identification to represent the timing input, 

followed by a brief description. The far right column identifies existing versus new information 

reporting based off of what is currently collected in the INPO ICES Fire Event Reporting system. 
G-3

Table G-1. VEWFD timing information collection

Timing 

Input

Parameter Description Reporting

Time of 

initial 

detection

talert

Time when first ASD VEWFD system “alert”

received in MCR.

Existing

Time of 

arrival at 

location

tarrival

Time when first responding plant personnel arrives 

at location of ASD VEWFD system “alert”

New

Time fire 

confirmed tfire

Time when fire was confirmed Existing

Time 

component 

identified

tidentify

Time when overheating component identified New

Time 

component 

de-energized

tpower_removed

Time when the power is removed from the 

component responsible for the ASD VEWFD 

system “alert”

New

Time of 

manual 

suppression 

actuation

tman-supp

Time when the manual fire suppression begins Existing

Time fire 

was under 

control

tcontrol

Time when the fire was controlled by suppression Existing

Time of fire 

brigade 

dispatched

tfb_dispatch

Time when the fire brigade was dispatched Existing

The information identified in Table G-1 and outlined with boxes above serve to support a 

quantitative assessment of operating experience. However, a narrative of the event could also 

help the reviewers of this information better understand the event. As such it is recommended 

that additional information on the event be provided in the event abstract, summary or as an 

attachment. With the limited information that is currently available, it is difficult to identify 

generically, the specific information that would be needed, but a narrative description answering 

the following questions may help.

“What happened?”

“Was there a fire?

“Was a fire mitigated (prevented)?”

Based on collected information identified in Table G-1 for each VEWFD system incident, the 

following important timing information can be determined.
G-4

Time available

The time available is the time between the first VEWFD system “Alert” and when a fire is either 

confirmed or the component is de-energized. It is calculated as;

tt aa aa aa = ttee ee − tt aa aa

where, tend is the lesser of tfire or tde-energize.

Response time

The response time represents the time between the VEWFD system “alert” being received in 

the MCR to the time when the first responding plant personnel arrive at the VEWFD system 

alert location. This is calculated as;

tt rrrr = tt aaaa aa aa − tt aa aa

Time required to locate source of VEWFD system “alert”

The time required to locate the source of the VEWFD system “alert” is dependent on a fire 

occurring before the first responding plant personnel arrival at the VEWFD system alert location 

or not. If a fire is confirmed when the first responding plant personnel arrives, then the time 

required to locate the source is zero:

tt ll = 0

When the first responding plant personnel arrives and no fire is present, the time required to 

locate the source of the VEWFD system “alert” is calculated as:

tt ll = ttii ii ii − tt aaaa aa aa

Time required to de-energize component or cabinet responsible for VEWFD system “alert”

The time required to de-energize the component responsible for the VEWFD system “alert” is 

the time between the identification of the component and when power is removed from that 

component (or cabinet). This is calculated as:

tt −ee ee eeee = tt pp _ rr − ttii ii ii

Time to suppress

The time to suppress the fire is the time between the start suppression to the time when the fire 

is controlled and is calculated as:

tt ss = tt − tt − ss
H-1

APPENDIX H

USER GUIDE FOR VEWFD EVENT TREE NONSUPPRESSION PROBABILITY CALCULATION TOOL

H.1 Objectives

This appendix provides instruction on the use of the electronic spreadsheets (Microsoft Excel)

that have been developed to help support estimation of the probability of non-suppression per 

the risk scoping study presented in Part II of this report. This appendix identifies the information 

needed to perform the calculation and guidance on how to use the electronic spreadsheets.

H.2 In-Cabinet Spreadsheet

The in-cabinet spreadsheet will estimate the probability of non-suppression for fire scenarios 

where smoke detection is located within an electrical cabinet (enclosure). The assumptions and 

limitations presented in Section 13 of this report apply to the use of this spreadsheet. The 

electronic spreadsheet is found on the CD enclosed on the back cover of this report and is 

also available on the NRC NUREG publication website.

H.2.1 Required input for in-cabinet spreadsheet calculation

The user must obtain the following information regarding the fire scenario under evaluation

before attempting a calculation using the in-cabinet spreadsheet:

(1) Type of in-cabinet smoke detection system

a. aspirated smoke detection (ASD)

b. spot-type

(2) Type of electrical enclosure (cabinet) being protected

a. control

b. power

(3) Technology of in-cabinet smoke detection

a. ASD VEWFD Cloud Chamber (CC)

b. ASD VEWFD Light Scattering (LS)

c. Spot-type VEWFD sensitive spot (SS)

d. Spot-type conventional Ionization (ION)

(4) Cabinet ventilation

a. naturally ventilated cabinet

b. forced ventilation less than 100 cabinet air changes per hour

c. forced ventilation greater than 100 cabinet air changes per hour

(5) Detection configuration

a. 1 to 10 cabinets only

(6) Redundant fire detection within room (if credited)

a. conventional smoke / heat (spot or linear)

b. ASD VEWFD

(7) Type of automatic suppression (if credited)

a. none

b. carbon dioxide
H-2

c. halon

d. wet pipe sprinkler system

e. deluge or pre-action sprinkler system

(8) Automatic suppression system dependence on detection (if credited)

(9) Fixed manual suppression failure probability (if credited)

The user must obtain the following values (point estimates - typically mean values) before 

attempting a calculation using the in-cabinet spreadsheet:

(1) Time to damage (in minutes) of the target of interest for the fire scenario.

(2) Time to detection (in minutes) of redundant fire detection system.

(3) Conventional non-suppression rate parameter for

a. Electrical

b. Main control room

H.2.2 Detailed Discussion on In-Cabinet Spreadsheet Input Parameters

The discussion that follows provides a step-by-step procedure for completing the electronic 

spreadsheets. Before entering any data into the sheet, it is recommended that the sheet be 

reset by selecting the black “Reset” button followed by the “Yes” radio box. This will ensure that 

previous entries do not become incorporated in to the current evaluation. The sheet is setup 

following a similar format as NUREG-1805, “Fire Dynamics Tools (FDTs),” spreadsheets. As 

such, the top portion of the sheet is where the input is entered, result is presented next to the 

red box entitled “Total Non-suppression Probability,” and supporting information such as the 

solved event trees are presented below the result.
H-3

Step 1: Project / Inspection Title

Input field allows user to provide a detailed description of the scenario being evaluated.

Step 2: Select In-Cabinet Detector Type

Drop down menu allows selection from one of three options;

- ASD Very Early Warning

- Spot

- User Defined

Selection of the detector type determines the point estimate to use for “β” representing the 

detection unit combined unreliability and unavailability. The basis for ASD very early warning 

point estimates is provided in Section 7.2. The estimate used for spot type detection are taken

from NUREG/CR-6850 (EPRI 1011989), “Fire PRA Methodology for Nuclear Power Facilities,”

estimated at 0.05.

• Select “ASD Very Early Warning” for fire scenarios consisting of ASD VEWFD systems 

installed within the electrical cabinet. This selection assigns a point estimate 3.6×10-3 for 

“β”.

• Select “Spot” for fire scenarios consisting of spot-type smoke detectors located within 

the electrical cabinet. This includes conventional ionization configured to at least a 1.0% 

obscuration / ft. sensitivity; as well as, sensitive spot-type detectors that are configured 

to meet the 0.2 % obscuration / ft. minimum sensitivity. This selection assigns a point 

estimates of 0.05 for “β.”

• Select “User Defined” to use a point estimate other than those estimated in the report. It 

is recommended that justification be provided to support any “User Defined” point 

estimates.

Step 3 : Select Cabinet Type

Drop down menu allows selection from one of three options;

- Low Voltage Control Cabinet

- Power Cabinet

- User Defined
H-4

Selection of the cabinet type determines the point estimate to use for “α” representing the 

fraction of fires that have an incipient phase of sufficient duration to support enhanced 

suppression. The basis for these point estimates is provided in Section 7.1 and Appendix D.

• Select “Low Voltage Control Cabinet” for fire scenarios contain components operating at 

a nominal voltage of 250 volts or less. Selecting “Low Voltage Control Cabinet” assigns 

a point estimate 0.28 for α. 

• Select “Power Cabinets” for fire scenarios involving electrical cabinets that have any 

component operating at voltages greater than 250 volts. Selecting “Power Cabinet”

assigns a point estimate 0.50 for α. 

• Select “User Defined” to use a point estimate other than those developed in this report. 

It is recommended that justification be provided to support any “User Defined” point 

estimates.

Step 4a: Select Cabinet Ventilation Configuration

Drop down menu allows selection from one of three options;

- Natural or Forced Ventilation up to 100 Cabinet ACH

- Forced Ventilation above 100 Cabinet ACH

- User Defined

Selection of the cabinet ventilation configuration in Step 4a, followed by the selection of the 

detector technology in Step 4b, determines the point estimate to use for “τ” representing the 

in-effectiveness of the system to detect the fire threat in its low-energy incipient stage. 

The basis for the in-effectiveness point estimates are presented in Section 7.2.3.

• Select “Natural or Forced Ventilation up to 100 Cabinet ACH” for fire scenarios 

consisting of electrical cabinets that are either naturally ventilated with louvers or grates 

on at least one side of the cabinet, or with mechanical forced ventilation (fans or 

blowers) that ventilate the cabinet at up to 100 cabinet air changes per hour. No point 

estimate is assigned until Step 4b is complete.
H-5

• Select “Forced Ventilation above 100 Cabinet ACH” for fire scenarios consisting of 

electrical cabinets that are mechanically ventilated with a ventilation rate above 100 

cabinet air changes per hour. No point estimate is assigned until Step 4b is complete.

• Select “User Defined” to use a point estimate other than those estimated in the report. It 

is recommended that justification be provided to support any “User Defined” point 

estimates.

Step 4b: Select Detection Technology

After Step 4a is complete, a drop down menu appears allowing for selection of the detection 

technology;

- Ionization Spot (ION)

- Photoelectric Spot (PHOTO)

- VEWFD Sensitive Spot (SS)

- ASD VEWFD Cloud Chamber (CC)

- ASD VEWFD Light Scatter 1 (LS1)

- ASD VEWFD Light Scatter 2 (LS2)

• Select “Ionization Spot (ION)” for fire scenarios consisting of ionization spot-type 

detectors located within the electrical cabinet on or near the cabinet ceiling. The 

ionization type detector should have an Alarm sensitivity set point of 1.0%obscuration / 

ft. or more sensitive. This selection assigns a point estimate of either 0.10 or 0.79 for 

“τ,” depending on the cabinet ventilation rate.

• Select “Photoelectric Spot (PHOTO)” for fire scenarios consisting of photoelectric spottype detectors located within the electrical cabinet on or near the cabinet ceiling. The 

photoelectric type detector should have an Alarm sensitivity set point of 2.1%obscuration 

/ ft. or more sensitive. This selection assigns a point estimate of 0.77 for “τ,” for naturally 

or forced ventilation up to 100 Cabinet ACH. Note that there is no data available from 

this testing program for the performance of a photoelectric spot detector within an 

electrical cabinet with forced ventilation greater than 100 ACH.

• Select “VEWFD Sensitive Spot (SS)” for fire scenarios consisting of VEWFD spot-type 

detectors located within the electrical cabinet on or near the cabinet ceiling. The 

VEWFD sensitive spot type detector should have an Alert sensitivity set point of 

0.2%obscuration / ft. above background or more sensitive. This selection assigns a 

point estimate of 0.26 for “τ,” for naturally or forced ventilation up to 100 Cabinet ACH 

Note that there is no data available from this testing program for the performance of a 
H-6

photoelectric spot detector within an electrical cabinet with forced ventilation greater than 

100 ACH.

• Select “ASD VEWFD Cloud Chamber (CC)” for fire scenarios consisting of aspirated 

smoke detections utilizing a cloud chamber technology and configured as a VEWFD 

system. The ASD sampling ports located within the electrical cabinet on or near the 

cabinet ceiling. The ASD VEWFD CC detector should have a port Alert sensitivity set 

point of 0.2%obscuration / ft. above background or more sensitive. This selection

assigns a point estimate of either 2.7×10-03 or 1.9×10-02 for “τ,” depending on the cabinet 

ventilation rate.

• Select “ASD VEWFD Light Scattering 1 (LS1)” or “ASD VEWFD Light Scattering 2 

(LS2)” for fire scenarios consisting of aspirated smoke detections utilizing a Light 

Scattering technology and configured as a VEWFD system. The ASD sampling ports 

located within the electrical cabinet on or near the cabinet ceiling. The ASD VEWFD LS 

detector should have a port Alert sensitivity set point of 0.2%obscuration / ft. above 

background or more sensitive. This selection assigns a point estimate of either

0.53 (LS1) / 1.9×10-01 (LS2) or 0.37 (LS2) for “τ,” depending on the cabinet ventilation 

rate. Note that there is no data available from this testing program for the performance 

of the LS1 detector within an electrical cabinet with forced ventilation greater than 100 

ACH.

Step 5: Select Human Error Probability for Main Control Room Response

Drop down menu assumes a main control room (MCR) human error probability of 1×10-04, but 

also allows for a user defined HEP estimate. If the “User Defined” option is selected, a point 

estimate different from the default can be entered. It is recommended that justification be 

provided to support any “User Defined” point estimates.

Step 6: Select Number of Cabinets

Selection of the number of cabinets assigns a HEP for the field operator response to the 

VEWFD system “alert” notification. The drop down allows selection from two options;

- Default Field Operator HEP [up to 10 Cabinets per zone]

- User Defined

• Use the default for fire scenarios consisting of ASD VEWFD systems that protect up to 

10 electrical cabinets (sometimes referred to as vertical sections). This assigns a HEP 
H-7

per the type of technology used (ASD VEWFD CC, ASD VEWFD LS, SS LS or ION 

spot). The basis for these estimates are presented in Section 10.6.

• Select “User Defined” to use a point estimate other than those estimated in the report. It 

is recommended that justification be provided to support any “User Defined” point 

estimates.

Step 7: Ensure Proper Non-Suppression Rate Parameters

The spreadsheet use the non-suppression rate parameters published in 2015 in NUREG-2169 

(EPRI 3002002936) titled, “Nuclear Power Plant Fire Ignition Frequency and Non-Suppression 

Probability Estimation Using the Updated Fire Events Database: United States Fire Events 

Experience Through 2009.” In that report, the electrical non-suppression rate parameter is 

estimated at 0.098, while the main control room non-suppression rate parameter is estimated at 

0.324. These are the default used in the spreadsheet. If other suppression rate parameters are 

used, enter those value(s) in the corresponding fields. It is recommended that justification be 

provided to support any “user defined” estimates.

Step 8: Enter Time to Target Damage Estimate

To conduct the detection/suppression analysis, a fire scenario time to target damage (in 

minutes) input is required. This estimate is typically based on fire modeling results. This 

estimate is entered in the yellow input box.

The time to target damage does not include any estimate for the incipient stage duration. 

Section 11 provides additional information.

Step 9: Enter Conventional Detection / Suppression Information

The last set of input required to calculate the non-suppression probability is related to 

conventional suppression and detection systems in the fire scenario protecting the target(s) of 

interest or the potential ignition sources. The input required is associated with redundant 

automatic fire detection and suppression systems, and any manual fire suppression capability. 

The input is entered by answering questions related to the redundant systems, user supplied 

input, and drop down menu selections.

The first question relates to any existing automatic fire detection system protecting the target(s) 

of interest and redundant from the in-cabinet fire detection system. For example, spot-type 

detection located at the ceiling of the room that has a different annunciator in the MCR would be 

considered a redundant automatic fire detection system, as would spot heat detectors, or a 
H-8

VEWFD ASD system. If there is no automatic fire detection system present, or no credit of that 

system is being taken, select the “No” check box and continue to the automatic suppression 

question.

If a redundant fire detection system is present, select the “Yes” check box. Once this is done, a 

drop down menu will appear. From the dropdown menu, select the type of redundant automatic 

fire detection system (VEWFD ASD, Spot-type Detection, or User Defined). This selection 

assigns an associated unreliability estimate (failure probability) to the redundant automatic fire 

detection system. If the “User Defined” option is selected, a failure probability for the redundant 

system should be entered in the box to the right. 

After the failure probability is assigned, a time to detection for the redundant automatic fire 

detection system must be entered. This time must be a positive value and entered in minutes. 

The next two questions relate to automatic fire suppression systems that are protecting the 

target(s) of interest. 

• If there is no automatic fire suppression system present, or no credit of that system is 

being taken, select the “No” check box. Selecting “No” will automatically answer “No” to 

the subsequent question regarding the suppression system dependency on fire 

detection. Continue to the manual fixed suppression question. 

• If there is an automatic fire suppression system present, select “Yes.” Once this is done, 

a dropdown menu will appear. From the dropdown menu, select the type of automatic 

fire suppression system (Carbon Dioxide, Halon System, Wet Pipe, Deluge, Pre-action 

sprinkler systems, or User Defined). This selection assigns an unreliability estimate 

(failure probability) to the automatic fire suppression system.

When crediting automatic suppression systems, the analyst 

must first determine that the automatic suppression will actuate 

prior to the predicted time to fire damage or else auto 

suppression fails.

• If the “User Defined” option is selected, a failure probability for the redundant system 

should be entered in the box to the right. 
H-9

Several types of automatic fire suppression systems are dependent on an automatic fire 

detection system for actuation. The failure probability estimates for these systems are listed 

below and taken from NUREG/CR-6850. If other systems are specified or if updated failure 

probabilities are desired to be used, select “User Defined” and enter the failure probability in the 

“Enter Value” field to the right.

- Carbon Dioxide : 4.0×10-02

- Halon System : 5.0×10-02

- Wet Pipe Sprinkler Systems : 2.0×10-02

- Deluge or Pre-action Sprinkler Systems : 5.0×10-02

The next questions ask if there is such a dependency and then assigns a failure probability to 

that automatic fire detection system for which the automatic suppression system is dependent 

upon. By selecting “Yes,” the spreadsheet assumes that the automatic suppression system is 

dependent on the redundant automatic fire detection system specified previously. If “No” is 

selected, go to the next question.

The last question in the conventional detection / suppression portion of the input fields is related 

to manual fixed suppression. If there is manual fixed suppression that will be credited for the 

fire scenario under evaluation, select “Yes” and then enter the Manual Fixed Suppression 

Failure Probability on the field to the right. Note that there is no default failure probability for 

manual fixed suppression. If “Yes” is selected, it is recommended that the user provide 

justification for the failure probability used. Otherwise, select “No.”

Step 10: Review Solution 

After all of the required input is entered into the spreadsheet, the solution will be presented as 

the total Non-Suppression Probability. 

The result is calculated from the sum of the failure end states (fire damage outside cabinet) from 

the in-cabinet event tree presented in Section 6.4. The solved event tree is provided below the 
H-10

result, including the supporting NUREG/CR-6850 Appendix P detection/suppression event 

trees. 
H-11
H-12

H.3 Area-Wide Spreadsheet

The Area-wide spreadsheet will estimate the probability of non-suppression for fire scenarios 

where ASD VEWFD systems have sampling ports either near the ceiling or across air return 

grilles. The assumptions and limitations presented in Section 13 of this report apply to the use 

of this spreadsheet. The electronic spreadsheet is found on the CD enclosed on the back cover 

of this report and is also available on the NRC NUREG publication website.

H.3.1 Required input for area-wide spreadsheet calculations

The user must obtain the following information regarding the fire scenario under evaluation 

before attempting a calculation using the in-cabinet spreadsheet:

(1) Type of area-wide smoke detection system

a. aspirated smoke detection (ASD)

b. spot-type

(2) Type of electrical enclosure (cabinet) being protected

a. control

b. power

(3) Technology of area-wide smoke detection

a. ASD VEWFD Cloud Chamber (CC)

b. ASD VEWFD Light Scattering (LS)

c. spot-type VEWFD sensitive spot (SS)

(4) Room ventilation (HVAC)

(5) Detection configuration

a. ceiling

b. air return grille

(6) Redundant fire detection within room

a. conventional smoke / heat (spot or linear)

b. ASD VEWFD

(7) Type of automatic suppression

a. none

b. carbon dioxide

c. halon

d. wet pipe sprinkler system

e. deluge or pre-action sprinkler system

(8) Automatic suppression system dependence on detection

(9) Fixed manual suppression

The user must obtain the following values (point estimates - typically mean values) before 

attempting a calculation using the in-cabinet spreadsheet:

(1) Time to damage (in minutes) of the target of interest for the fire scenario.

(2) Time to detection (in minutes) of redundant fire detection system.

(3) Conventional non-suppression rate parameter for

a. Electrical
H-13

H.3.2 Detailed Discussion on Area-wide Spreadsheet Input Parameters

The discussion that follows provides a step-by-step procedure for completing the electronic 

spreadsheets. Before entering any data into the sheet, it is recommended that the sheet be 

reset by selecting the black “Reset” button followed by the “Yes” radio box. This will ensure that 

previous entries do not become incorporated in to the current evaluation. The sheet is setup 

following a similar format as NUREG-1805, “Fire Dynamics Tools (FDTs),” spreadsheets. As 

such, the top portion of the sheet is where the input is entered, result is presented next to the 

red box entitled “Total Non-suppression Probability,” and supporting information such as the 

solved event trees are presented below the result.
H-14

Step 1: Project / Inspection Title

Input field allows user to provide a detailed description of the scenario being evaluated.

Step 2: Select Area-wide Detector Type

Drop down menu allows selection from one of three options;

- ASD Very Early Warning

- Spot

- User Defined

Selection of the detector type determines the point estimate to use for “β” representing the 

detection unit combined unreliability and unavailability. The basis for ASD very early warning 

point estimates is provided in Section 7.2. The estimate used for spot type detection are taken

from NUREG/CR-6850 (EPRI 1011989), “Fire PRA Methodology for Nuclear Power Facilities,”

estimated at 0.05.

• Select “ASD Very Early Warning” for fire scenarios consisting of ASD VEWFD systems 

installed within the electrical cabinet. This selection assigns a point estimate 3.6×10-03

for “β”.

• Select “Spot” for fire scenarios consisting of spot-type smoke detectors located within 

the electrical cabinet. This includes conventional ionization, as well as, sensitive spottype detectors that are configured to meet the 0.2 % obscuration / ft. minimum 

sensitivity. This selection assigns a point estimates of 0.05 for “β.”

• Select “User Defined” to use a point estimate other than those estimated in the report. It 

is recommended that justification be provided to support any “User Defined” point 

estimates.

NOTE: If an air return grill application is being used, the failure probability of air handling 

unit should be considered as a contributor to the unreliability of the VEWFD system. Since 

the air return grill application is dependent on the HVAC, the HVAC system failure 

probability should be considered and included in the “β” point estimate. For this case, the 

“User Defined” option should be selected.
H-15

Step 3: Select Cabinet Type

Drop down menu allows selection from one of three options;

- Low Voltage Control Cabinet

- Power Cabinet

- User Defined

Selection of the cabinet type determines the point estimate to use for “α” representing the 

fraction of fires that have an incipient phase of sufficient duration to support enhanced 

suppression. The basis for these point estimates is provided in Section 7.1 and Appendix D.

• Select “Low Voltage Control Cabinet” for fire scenarios contain components operating at 

a nominal voltage of 250 volts or less. Selecting “Low Voltage Control Cabinet” assigns 

a point estimate 0.28 for α. 

• Select “Power Cabinets” for fire scenarios involving electrical cabinets that have any 

component operating at voltages greater than 250 volts. Selecting “Power Cabinet”

assigns a point estimate 0.50 for α. 

• Select “User Defined” to use a point estimate other than those developed in this report. 

It is recommended that justification be provided to support any “User Defined” point 

estimates.

Step 4a: Select Application

Drop down menu allows selection from one of three options;

- Air Return Grill

- Ceiling

- User Defined

Selection of the cabinet ventilation configuration in Step 4a, followed by the selection of the 

room ventilation configuration and detector technology in Step 4b, determines the point estimate 

to use for “τ” representing the in-effectiveness of the system to detect the fire threat in its low-
H-16

energy incipient stage. The basis for the in-effectiveness point estimates are presented in 

Section 7.2.3.

• Select “Air Return Grill” for fire scenarios consisting of ASD VEWFD systems protecting 

a room where the ASD sampling ports are located across the air return grill. No point 

estimate is assigned until Step 4b is complete.

• Select “Ceiling” for fire scenarios consisting of ASD VEWFD or spot-type VEWFD 

systems protecting the room where the ASD sampling ports or VEWFD spot-type 

detectors are located at or near the ceiling of the room. No point estimate is assigned 

until Step 4b is complete.

• Select “User Defined” to use a point estimate other than those estimated in the report. It 

is recommended that justification be provided to support any “User Defined” point 

estimates.

Step 4b: HVAC configuration

• If “Air Return Grill” was selected in Step 4a, the question “Is there HVAC in room?”

appears along with two radio button options. If the room under evaluation has heating, 

ventilation, and air-conditioning (HVAC) select “Yes.” If it does not, select “No.” 

Note that air return grill applications require active air handling and as 

such, if “no” is selected for that configuration, the application is assumed 

to be ineffective (i.e., β = 1). Drop down menu allows selection from one 

of three options;

• If “Ceiling” was selected in Step 4b, no HVAC question will appear and proceed to Step 

4c.

Step 4c: Select Detection Technology

Unless “User Defined” was selected in Step 4a, a “Select Detector Type” drop down box will 

appear that allows for the selection of the detector type used.

- VEWFD Sensitive Spot (SS)

- ASD VEWFD Cloud Chamber (CC)

- ASD VEWFD Light Scatter 2 (LS2)

• Select “VEWFD Sensitive Spot (SS)” for fire scenarios consisting of VEWFD spot-type 

detectors located at or near the ceiling of the room. The VEWFD sensitive spot type 

detector should have an Alert sensitivity set point of 0.2%obscuration / ft. above 
H-17

background or more sensitive. This selection assigns a point estimate of 0.57 for “τ.”

The estimate is only applicable for ceiling mounted SS detectors. The SS detectors 

were not evaluated in this program for their response when installed across an air return 

grill.

• Select “ASD VEWFD Cloud Chamber (CC)” for fire scenarios consisting of aspirated 

smoke detections utilizing a cloud chamber technology and configured as a VEWFD 

system. The ASD sampling ports located at or near the room ceiling or across the air 

return grill. The ASD VEWFD CC detector should have a port Alert sensitivity set point 

of 0.2%obscuration / ft. above background or more sensitive. This selection assigns a 

point estimate of either 0.32 or 0.30 for “τ,” depending on the application.

• Select “ASD VEWFD Light Scattering 2 (LS2)” for fire scenarios consisting of aspirated 

smoke detections utilizing a Light Scattering technology and configured as a VEWFD 

system. The ASD sampling ports located at or near the room ceiling or across the air 

return grill. The ASD VEWFD LS detector should have a port Alert sensitivity set point of 

0.2%obscuration / ft. above background or more sensitive. This selection assigns a 

point estimate of either 0.11 or 0.52 for “τ,” depending on the application. Note that 

there is no data available from this testing program for the performance of the LS1 

detector.

Step 5: Select Human Error Probability for Main Control Room Response

Drop down menu assumes a main control room (MCR) human error probability of 1x10-04, but 

also allows for a user defined HEP estimate. If the “User Defined” option is selected, a point 

estimate different from the default can be entered. It is recommended that justification be 

provided to support any “User Defined” point estimates.

Step 6: Develop a Human Error Probability for 1st Level Field Response (ξ)

The spreadsheet automatically assigns a 100% failure probability for the 1st level field response. 

As discussed in Section 10, the large variability in the area-wide applications could not support 

development of a generic HEP. This is not to say that one could not be developed and justified

for a plant specific scenario. If such a plant specific HEP were to be developed, it is 

recommended that a process such as that presented in Sections 9 and 10 be followed to 

support such an estimate. As with all other user defined inputs, it is recommended that 

justification be provided.

Step 7: Ensure Proper Non-Suppression Rate Parameters

The spreadsheet use the electrical non-suppression rate parameter published in 2015 in 

NUREG-2169 (EPRI 3002002936) entitled, “Nuclear Power Plant Fire Ignition Frequency and 

Non-Suppression Probability Estimation Using the Updated Fire Events Database: United 

States Fire Events Experience Through 2009.” In that report, the electrical non-suppression 
H-18

rate parameter is estimated at 0.098. In addition to this rate parameter, a new rate parameter 

was developed in this report to develop a non-suppression curve representing the field operator 

in the room responsible for the VEWFDS alert fails to promptly sup press the fire quickly enough 

to prevent damage to PRA targets outside the cabinet. Development of this parameter 

(λenh=0.194) is presented in Section 11.1. These are the default used in the spreadsheet. 

If other suppression rate parameters are used, enter those value(s) in the corresponding fields. 

It is recommended that justification be provided to support any “user defined” estimates.

Step 8: Enter Time to Target Damage Estimate

To conduct the detection/suppression analysis, a fire scenario time to target damage (in 

minutes) input is required. This estimate is typically based on fire modeling results. 

This estimate is entered in the yellow input box.

The time to target damage does not include any estimate for the incipient stage duration. 

Section 11 provides additional information.

Step 9: Enter Conventional Detection / Suppression Information

The last set of input required to calculate the non-suppression probability is related to 

conventional suppression and detection systems in the fire scenario protecting the target(s) 

of interest or the potential ignition sources. The input required is associated with redundant 

automatic fire detection and suppression systems, and any manual fire suppression capability. 

The input is entered by answering questions related to the redundant systems, user supplied 

input, and drop down menu selections.

The first question relates to any existing automatic fire detection system protecting the target(s) 

of interest and redundant from the area-wide VEWFD fire detection system. For example, spottype detection located at the ceiling of the room that has a different annunciator in the MCR 

would be considered a redundant automatic fire detection system, as would spot heat detectors, 

or second VEWFD ASD system (provided that it is independent of the primary system). If there 

is no automatic fire detection system present, or no credit of that system is being taken, select 

the “No” check box and continue to the automatic suppression question.

If a redundant fire detection system is present, select the “Yes” check box. Once this is done, a 

drop down menu will appear. From the dropdown menu, select the type of redundant automatic 

fire detection system (VEWFD ASD, Spot-Type Detection, or User Defined). This selection 

assigns an associated unreliability estimate (failure probability) to the redundant automatic fire 

detection system. If the “User Defined” option is selected, a failure probability for the redundant 

system should be entered in the box to the right. 
H-19

After the failure probability is assigned, a time to detection for the redundant automatic fire 

detection system must be entered. This time must be a positive value and entered in minutes. 

The next two questions relate to automatic fire suppression systems that are protecting the 

target(s) of interest. 

• If there is no automatic fire suppression system present, or no credit of that system is 

being taken, select the “No” check box. Selecting “No” will automatically answer “No” to 

the subsequent question. Continue to the manual fixed suppression question. 

• If there is an automatic fire suppression system present, select “Yes.” Once this is done, 

a dropdown menu will appear. From the dropdown menu, select the type of automatic 

fire system (Carbon Dioxide, Halon System, Wet Pipe, Deluge, Pre-action sprinkler 

systems, or User Defined). This selection assigns an unreliability estimate (failure 

probability) to the automatic fire suppression system. 

When crediting automatic suppression systems, the analyst must

first determine that the automatic suppression will actuate prior to 

the predicted time to fire damage or else auto suppression fails.

• If the “User Defined” option is selected, a failure probability for the redundant system 

should be entered in the box to the right. 

Several types of automatic fire suppression systems are dependent on an automatic fire 

detection system for actuation. The failure probability estimates for these systems are listed 

below and taken from NUREG/CR-6850. If other systems are specified or if updated failure 
H-20

probabilities are desired to be used, select “User Defined” and enter the failure probability in the 

“Enter Value” field to the right.

- Carbon Dioxide : 4.0×10-02

- Halon System : 5.0×10-02

- Wet Pipe Sprinkler Systems : 2.0×10-02

- Deluge or Preaction Sprinkler Systems : 5.0×10-02

The next questions ask if there is such a dependency and then assigns a failure probability to 

that automatic fire detection system for which the automatic suppression system is dependent 

upon. By selecting “Yes,” the spreadsheet assumes that the automatic suppression system is 

dependent on the redundant automatic fire detection system specified previously. If “No” is 

selected, go to the next question.

The last question in the conventional detection/suppression portion of the input fields is related 

to manual fixed suppression. If there is manual fixed suppression that will be credited for the 

fire scenario under evaluation, select “Yes” and then enter the Manual Fixed Suppression 

Failure Probability on the field to the right. Note that there is no default failure probability for 

manual fixed suppression. If “Yes” is selected, it is recommended that the user provide 

justification for the failure probability used. Otherwise, select “No.”

Step 10: Review Solution 

After all of the required input is entered into the spreadsheet, the solution will be presented as 

the total Non-Suppression Probability. 

The result is calculated from the sum of the failure end states (fire damage outside cabinet) from 

the area-wide event tree presented in Section 6.4. The solved event tree is provided below the 

result, including the supporting NUREG/CR-6850 Appendix P detection/suppression event 

trees. 
H-21
H-22






NUREG-2180

Final

Determining the Effectiveness, Limitations, and Operator Response for Very Early 

Warning Fire Detection Systems in Nuclear Facilities (DELORES-VEWFIRE)