NUREG/CR-6597 

SAND99-1320

Results and Insights on the 

Impact of Smoke on 

Digital Instrumentation and 

Control 

Sandia National Laboratories 

U.S. Nuclear Regulatory Commission 

Office of Nuclear Regulatory Research 

Washington, DC 20555-0001
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NUREG/CR-6597 

SAND99-1320

Results and Insights on the 

Impact of Smoke on 

Digital Instrumentation and 

Control

Manuscript Completed: September 2000 

Date Published: January 2001 

Prepared by 

T. J. Tanaka, S. P. Nowlen 

Sandia National Laboratories 

Albuquerque, NM 87185-0748 

C. Antonescu, NRC Project Manager 

Prepared for 

Division of Systems Technology 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001 

NRC Job Code W6051

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Internet: bookstore.gpo.gov Phone: (202) 512-1800 Fax: (202) 512-2250 

Mail: Stop SSOP, Washington, DC 20402-0001 

ISBN 0-16-050737-5
Previous reports in series:

NUREG/CR-6476, SAND96-2633, Tina J. Tanaka, Steven P. Nowlen, and Dennis J. 

Anderson, "Circuit Bridging of Components by Smoke," Sandia National 

Laboratories, Albuquerque, NM, October 1996. 

NUREG/CR-6543, SAND97-25, Tina J. Tanaka, "Effects of Smoke on Functional 

Circuits," Sandia National Laboratories, Albuquerque, NM, October 1997.

ii
Abstract

Smoke can cause interruptions and upsets in active electronics. Because nuclear 

power plants are replacing analog with digital instrumentation and control systems, 

qualification guidelines for new systems are being reviewed for severe environments 

such as smoke and electromagnetic interference. Active digital systems, individual 

components, and active circuits have been exposed to smoke in a program sponsored 

by the U.S. Nuclear Regulatory Commission. The circuits and systems were all 

monitored during the smoke exposure, indicating any immediate effects of the 

smoke. The major effect of smoke has been to increase leakage currents (through 

circuit bridging across contacts and leads) and to cause momentary upsets and 

failures in digital systems. This report summarizes two previous reports and 

presents new results from conformal coating, memory chip, and hard drive tests. 

The report describes practices for mitigation of smoke damage through digital 

system design, fire barriers, ventilation, fire suppressants, and post fire procedures.

iii
CONTENTS ......................................................................................................... Page 

Abstract ................................................................................................... 111 

Executive Summ ary ................................................................................ xi 

Acknowledgments ............................................................................ xiii 

Abbreviations ........................................................................................ xiv 

1 INTRODUCTION .......................................................................... 1 

1.1 RATIONALE FOR THE PROGRAM ....................................................................... 1 

1.2 HISTORY OF THE PR OGRAM .............................................................................. 1 

2 PROGRAM RESULTS .................................................................... 7 

2.1 BACKGROUND STUDIES .................................................................................... 7 

2.1.1 Literature Review ...................................................................................... 7 

2.1.2 Smoke Scenarios .................................................................................... 10 

2.2 SNL TESTING METHOD .................................................................................. 11 

2.2.1 Fuels Burned .......................................................................................... 15 

2.2.2 Digital Equipment Tested ...................................................................... 18 

2.2.3 Smoke Measurements ............................................................................ 20 

2.2.4 Humidity measurements ......................................................................... 23 

2.2.5 Temperature measurements .................................................................. 27 

2.2.6 Chemical Analysis .................................................................................. 27 

2.3 SUMMARY OF PREVIOUSLY REPORTED SMOKE EXPOSURES ............................ 30 

2.3.1 Digital System Tests .............................................................................. 30 

2.3.2 Component Packaging and Circuit Bridging ....................................... 34 

2.3.3 Chip Technology Tests ............................................................................ 42 

2.3.4 Functional Circuits ................................................................................ 45 

2.4 RECENT SMOKE TEST RESULTS ...................................................................... 50 

2.4.1 Conformal Coating of Functional Circuit Boards ................................. 50 

2.4.2 Digital Throughput ................................................................................ 61 

2.4.3 Memory Chips (SRAM, EPROM and Non Volatile) .............................. 62 

2.4.4 Hard Disks .............................................................................................. 68 

2.4.5 Electrical Measurements ........................................................................ 74 

3 FIRE PROTECTION AND THE MITIGATION OF SMOKE 

DAM AGE ............................................................................................ 85 

3.1 OVERVIEW .......................................................................................................... 85 

3.2 PHYSICAL FIRE PROTECTION FEATURES ................................. 85 

3.2.1 The Classical Fire Protection Strategy ................................................. 85

V
3.2.2 Current USNRC Fire Safety Regulations and the Smoke Damage Issue 85 

3.2.3 Choice of Suppression Agents ................................................................ 87 

3.2.4 Avoidance of Known Hazards and Vulnerabilities ............................... 88 

3.2.5 F ire B arriers .......................................................................................... 89 

3.2.6 Localized Encapsulation ........................................................................ 91 

3.2.7 General Ventilation Features ................................................................ 91 

3.2.8 Local Ventilation Features .................................................................... 94 

3.3 FIRE PROTECTION ANALYSIS TOOLS .............................................................. 96 

3.3.1 Fire Risk Assessment .............................................................................. 96 

3.3.2 Fire M odeling ........................................................................................ 97 

4 POST-FIRE RECOVERY .............................................................. 99 

4.1 INTRODUCTION ............................................................................................... 99 

4.2 STEPS TO REDUCE SMOKE DAMAGE ............................................................... 99 

4.3 ENVIRONMENTS THAT CONTRIBUTE TO SMOKE DAMAGE .................................. 100 

4.3.1 H igh H um idity ......................................................................................... 100 

4.3.2 H igh Voltage ............................................................................................. 101 

4.4 CLEANING M ETHODS ....................................................................................... 101 

4.4.1 NRL Method of Cleaning Electronic Equipment Exposed to Smoke ...... 101 

4.4.2 Swedish Institute of Production Engineering Research Method of Cleaning Smoke-Contaminated Electronics ........................................................ 102 

4.4.3 Commercial Cleaning of Electronics ........................................................ 103 

5 CONCLUSIONS .............................................................................. 105 

5.1 SUM M ARY OF INSIGHTS .................................................................................... 106 

5.1.1 Environmental Stress Characteristics of Smoke ..................................... 106 

5.1.2 Methods of Smoke Protection ................................................................... 107 

5.1.3 Q ualification Tests ................................................................................... 108 

5.2 UNADDRESSED QUESTIONS .............................................................................. 108 

5.3 RECOMMENDATIONS FOR FUTURE WORK ......................................................... 109 

5.3.1 Validation and improvement of fire and smoke transport models ......... 109 

5.3.2 Develop models for different classes of electronic equipment .................. 110 

5.3.3 Improve methods of smoke protection ...................................................... 110 

6 R eferen ces ....................................................................................... 111 

Figures 

Figure 1. This program exposed a range of components and samples to smoke ............... 2 

Figure 2. These electrical components were tested at SNL ............................................... 4 

Figure 3. Electrical targets and chemical samplers were determined the effect of smoke on 

general electrical devices and measured the smoke environment ............................... 5

vi
Figure 4. The small smoke exposure chamber was built to the dimensions of ASTM 

E05.70.21 draft smoke corrosivity test and used for the memory chip, electrical, hard 

disk and digital throughput tests ................................................................................ 12 

Figure 5. All of the smoke tests other than the multiplexer tests were performed in an 

environmental chamber to control initial temperature and humidity ....................... 13 

Figure 6. The large smoke exposure chamber was used for testing the digital safety system 

and com ponent tests ......................................................................................................... 14 

Figure 7. Fuel was loaded in the combustion chamber in tray below the stainless steel 

chimney (large rectangular shape at top center) ....................................................... 17 

Figure 8. Quartz radiant heat lamps heated the fuel in the combustion chamber and smoke 

flowed up the chimney to the smoke chamber ............................................................ 17 

Figure 9. Smoke was filtered from smoke chamber to determine the smoke density .......... 21 

Figure 10. Optical extinction coefficient Vs Mass density for PVC smoke ....................... 22 

Figure 11. Humidity/temperature gage included in the smoke exposure chamber ...... 23 

Figure 12. Temperature of the humidity probes was higher near the mass vs. conductivity 

boards than near the test electronics ........................................................................... 24 

Figure 13. Relative humidity in two locations in the smoke chamber ............................. 24 

Figure 14. The mixing ratio decreased as dry nitrogen was added to the smoke chamber 

between the time that the lamps were turned off and the chamber vented (0.25-1.0 hr). .............. . . . ................. .............. ..................................................................................... 26 

Figure 15. The conductance of the 500 V dc biased mass vs. conductivity board increased 

slightly after the chamber was vented (>1.0 hr) .......................................................... 26 

Figure 16. The temperature in the smoke chamber varied with position, T1-T3 are near the 

top of the chamber, T4 is at the bottom, and T6 is midway between top and bottom of 

ch am ber ............................................................................................................................. 27 

Figure 17. The host computer controlled two multiplexer backplanes for the multiplexer 

tests ................................................................................................................................... 3 1 

Figure 18. Fiber optic modules were tested in different orientations ............................. 33 

Figure 19. The fiber optic module that was open failed ................................................... 33 

Figure 20. SMT chip packages are much smaller than PTH chip packages for the same type 

of device ............................................................................................................................. 35 

Figure 21. The component packaging tests featured both PTH and SMT component 

p ack ages ............................................................................................................................ 36

vii
Figure 22. Surface insulation resistance was measured on an interdigitated comb pattern, IPC-B-24, developed by the Institute for Interconnecting and Packaging Electronic 

C ircu its .............................................................................................................................. 3 6 

Figure 23. The component packaging tests were performed in the large smoke chamber.. 37 

Figure 24. Resistance between leads of surface mounted ceramic package during a smoke 

exp osu re ............................................................................................................................ 3 8 

Figure 25. The time of failure increased with the spacing between leads for different 

com pon ents ....................................................................................................................... 40 

Figure 26: The time to failure due to smoke exposure is longer for acrylic-coated boards 

than uncoated or chassis housed boards ..................................................................... 41 

Figure 27. The chip technology test circuit introduced a variable shunt resistance into a 

logic circuit to simulate the effect of smoke ................................................................ 44 

Figure 28. The functional board tested various types of circuits to determine their relative 

v u ln erability ..................................................................................................................... 4 7 

Figure 29. The small smoke chamber was loaded with four functional boards and an IPC

B-24 board for the coating tests ................................................................................. 54 

Figure 30. For the PTH components in the HVLC circuit, parylene and polyurethane were 

th e best coatings ............................................................................................................... 56 

Figure 31. The parylene, acrylic, and epoxy coatings worked best on the HVLC circuit with 

SM T com pon ents .............................................................................................................. 57 

Figure 32. Resistance of the HCLV circuit. The SMT circuit resistance increased when 

smoke was added, but the PTH remained unchanged ............................................... 58 

Figure 33. The coupling increased between transmission lines while smoke was in the 

chamber, but the coated board remained constant at 50 MHz .................................... 59 

Figure 34. Throughput boards tested digital transmission ............................................ 61 

Figure 35. SRAM and EPROM memory chips were tested ............................................... 64 

Figure 36. The EPROM on the left was exposed to smoke; the EPROM on the right has not 

been exposed to smoke..."'. ................................................. 65 

Figure 37. This memory chip board is darkened by smoke ............................................. 66 

Figure 38. The 3.3V SRAM failed when 10 or more grams of fuel were burned ............. 67 

Figure 39. Hard drive test tested only the hard drive ..................................................... 70 

Figure 40. Hard disk drives showed some corrosion after testing .................................... 70

viii
Figure 41. These disk drives did not fail during the smoke test ....................................... 72 

Figure 42. The inside of the disk drives remained clean ................................................. 73 

Figure 43. Perforated parallel plates were used to measure the effect of smoke between 

vertical plates with DC and AC bias voltages ............................................................. 75 

Figure 44. The optical extinction coefficient decreased after the fire more quickly than the 

500 D C V conductance ................................................................................................. 76 

Figure 45. 500 DC V parallel plates at beginning of test are clean .................................. 77 

Figure 46. After a short time, the parallel plate starts to collect smoke, notice the encircled 

w hisker of sm oke .............................................................................................................. 77 

Figure 47. 4 minutes into the smoke test, a substantial amount of smoke has gathered on 

the parallel plates ............................................................................................................. 78 

Figure 48. After 12 minutes, a thick bridge of smoke shorts the parallel plates together... 78 

Figure 49. The conductance between the 50 DC V parallel plates also falls slower than the 

optical extinction coeffi cient ......................................................................................... 79 

Figure 50. Both the real and imaginary parts of admittance at 30MHz are affected by 

sm ok e ................................................................................................................................ 80 

Figure 51. The mass vs. conductivity board was designed with an interdigitated comb 

p attern .............................................................................................................................. 8 1 

Figure 52. The mass vs. conductivity board was placed in a holder to provide bias and 

protect the board from air currents after smoke collection ........................................ 82 

Figure 53. Smoke distribution on the mass vs. conductivity boards is more even on the 5 

D C V biased board ............................................................................................................ 82 

Figure 54. The conductance remains higher on the 5 V biased board that has a more even 

distribution of sm oke ........................................................................................................ 83 

Figure 55. Smoke collected by conductivity board was roughly proportional to the amount 

of fuel burned and not very dependent on the DC voltage applied ............................. 84 

Tables 

Table 1. Tests Perform ed ................................................................................................. 5 

Table 2. Nuclear Qualified Low-Power Cables Burned in Smoke Exposures .................. 16 

Table 3. Ratio of Optical Extinction Coefficient to Smoke Mass Density ......................... 21 

ix
Table 4. Chemical analysis of filters placed in smoke chamber (pg/filter) ....................... 28 

Table 5. Ion Chromatography Results (pg/cm 2) ..................................................................... 29 

Table 6. Critical Resistances for Failure from Smoke ..................................................... 45 

Table 7. Smoke exposure conditions for the functional circuit tests ............................... 48 

Table 8. Summary of uncoated functional circuit worst case results ............................... 48 

T able 9. C oating Types ............................................................................................................ 50 

Table 10. Coating Properties ............................................................................................. 52 

Table 11. Coating Board Test Plan .................................................................................... 53 

Table 12. Momentary Failures (out of 9 boards) .............................................................. 58 

Table 13. Parameters measured for first memory chip test series .................................. 66 

Table 14. Hard disk test conditions .................................................................................. 71 

Table 15. H ard disk results ............................................................................................... 72 

Table 16. Smoke Deposition Guidelines ............................................................................... 100

x
Executive Summary

This report summarizes and draws insights from a study of the effects of smoke on 

digital instrumentation and control equipment. Several important conclusions can 

be drawn from project findings. Based on the investigation of smoke susceptibility 

and the resulting understanding of key failure mechanisms, it is clear that smoke 

can result in adverse consequences. However, there is no practical repeatable 

testing methodology, so it is not feasible to assess smoke susceptibility as part of 

environmental qualification. As a result, the most reasonable approach to 

minimizing smoke susceptibility is to employ design, implementation, and 

procedural practices that can reduce the possibility of smoke exposure and enhance 

smoke tolerance. In particular, current fire protection methods are an appropriate 

preventative approach, employing isolation and detection practices. Additionally, 

post-event recovery procedures can mitigate the extent of smoke damage. Finally, 

there are design choices and implementation practices that can reduce equipment 

susceptibility to smoke exposure, such as chip packaging and conformal coatings. 

Smoke can result in immediate failure of electronic equipment through increased 

leakage currents. These currents are caused primarily by circuit bridging by 

charged smoke particles in the air and smoke deposits on surfaces. Other effects 

are possible, such as metal corrosion from acidic smoke and coating of contacts with 

a non-conducting material; however, these effects are slower to cause failure in 

digital circuits since the circuits have few thin metal contacts or moving parts. The 

circuits that are most susceptible to smoke have high voltage and high impedance. 

Circuits can be protected from smoke by conformally coating the printed circuits 

and by controlling the movement of smoke. The coatings themselves, however, are 

typically flammable. The best classes of conformal coatings have been found to be 

parylene, polyurethane, and acrylic (dipped). Other coatings, such as epoxy and 

silicone, did not protect as well. Ventilation and reduction of humidity after a fire 

are very important for preserving electronics. Location may reduce the chances of 

smoke exposure if equipment is not located where fires are likely to occur and fire 

barriers are used. 

Circuit boards can be thoroughly cleaned and refurbished after a fire. They are best 

cleaned with a detergent and water mixture instead of a halogenated solvent. 

Conformally coated boards, however, are not easy to clean as the smoke may 

penetrate the top surface of the coating. Cleanliness is usually measured by 

chloride concentration. 

At this time, addressing smoke exposure as a part of environmental qualification is 

not feasible since there are no testing standards available. A systematic repeatable 

smoke exposure test does not exist and is not likely given that smoke production is 

difficult to control and measure. As an alternative, the most reasonable approach is

xi
to use smoke-tolerant circuits and technologies, protective fire barriers, and 

conformal coatings to minimize the risk of smoke damage for digital circuits.

xii
Acknowledgments

The authors wish to thank the following people for their contributions to this 

project: Christina Antonescu of the U.S. Nuclear Regulatory Commission for her 

help in initiating, planning, and supporting this study; Nathan Siu of the U.S. 

Nuclear Regulatory Commission for support of the high-voltage AC studies; Dennis 

J. Anderson and Dennis Huffinan for planning the experiments, specifying the 

equipment model and statistically analyzing the results; Specialty Coating Inc., for 

their suggestions on conformal coatings and prompt application of these coatings to 

the functional boards; D. Michael Ramirez, Edward Baynes, John Garcia, and Bob 

Nichols for their technical support; students Lizdabel Morales, Andrea Hirst, Karla 

Waters, Paul Lucero and Michael Dandini for writing programs, analyzing data, 

and sessions of cutting and pasting; and Ruth Haas for her steadfast editing. Kofi 

Korsah and Richard T. Wood of Oak Ridge National Laboratory collaborated on the 

selection of digital equipment and drawing conclusions on the results. 

The authors also wish to thank Stephen J. Martin for his steadfast support and 

technical contributions.

xiii
ACRS 

ASTM 

BNL 

CMOS 

COTS 

CSPE 

Do 

DOE 

ECL 

EMJIRFI 

EPR 

EPDM 

EPROM 

FACT 

FAST 

FRXLPE 

HCLV 

HDR 

HF TL 

HSD 

HVLC 

I&C 

IPC 

KfK 

LPF 

LRSTF 

NAND 

NEMA 

NIST 

NRL 

ORNL 

PGA 

PTH 

PVC 

RH 

SIR 

SMT 

SNL 

SRAM 

STD 

TOC 

TTL 

USNRC 

V dc 

XLPE

xiv

Abbreviations

Advisory Committee for Reactor Safeguards 

American Society for Testing and Materials 

Brookhaven National Laboratory 

complementary metal-oxide semiconductor 

commercial off-the-shelf 

chlorosulfonated polyethylene 

Optical extinction coefficient 

Department of Energy 

emitter-coupled logic 

electromagnetic interference/radio frequency interference 

ethylene propylene rubber 

ethylene propylene diene monomer 

erasable programmable read-only memory 

advanced CMOS 

advanced Schottkey TTL 

fire-resistant cross-linked polyethylene 

high current low voltage 

Heiss Dampf reactor 

high-frequency transmission line 

high-speed digital 

high voltage low current 

instrumentation and control 

Institute for Interconnecting and Packaging Electronic Circuits 

Kernforschungszentrum Karlsruhe 

low-pass filter 

low-residue soldering task force 

Not-AND 

National Electrical Manufacturer's Association 

National Institute of Standards and Technology 

Naval Research Laboratory 

Oak Ridge National Laboratory 

pin grid array 

plated-through hole 

polyvinyl chloride 

relative humidity 

surface insulation resistance 

surface-mounted technology 

Sandia National Laboratories 

static random access memory 

standard deviation 

transistor outline can 

transistor-to-transistor logic 

United States Nuclear Regulatory Commission 

volts direct current 

cross-linked polyethylene
1 INTRODUCTION

1.1 Rationale for the 

Program 

Nuclear power plants are replacing 

analog instrumentation and control 

(I&C) equipment with digital I&C 

equipment. Changes in equipment 

raises concerns on how new equipment 

will react to abnormal severe 

environmental conditions. One 

concern is how smoke from a fire will 

affect these new control systems. It 

was widely thought that smoke would 

cause long-term degradation (over 

several weeks) in performance; 

however, not much concern was 

focused on the immediate effects of 

smoke on the equipment during a fire. 

The Office of Nuclear Reactor 

Research of the US Nuclear 

Regulatory Commission (USNRC) had 

several motives in funding this 

program, including a user need letter 

from the Office of Nuclear Regulatory 

Regulation requesting information on 

the effects of smoke. In addition, the 

Advisory Committee for Reactor 

Safeguards (ACRS) requested that 

smoke be included in the investigation 

of environmental qualification. 

1.2 History of the Program 

In March 1994 the USNRC initiated a 

program at Sandia National 

Laboratories (SNL) to determine the 

impact of smoke on advanced 

instrumentation and controls. This

program was a small part of a larger 

program at Oak Ridge National 

Laboratory (ORNL) on the 

environmental qualifications for 

microprocessor-based I&C.' The 

ORNL program included 

environmental qualification for 

electromagnetic interference,2 

temperature, humidity, and smoke. 3 

A third laboratory, Brookhaven 

National Laboratory (BNL), was also 

involved in this effort by determining 

the relative risk posed by various 

environmental hazards for digital I&C 

and hence for a nuclear power plant.4 

This program on the impact of smoke 

helped support these other programs 

by determining the necessity of 

including smoke in the risk 

assessments (BNL) and by producing 

smoke environments for testing the 

experimental digital safety system 

(ORNL). 

At the time the USNRC program was 

started, there had been very few 

controlled tests to determine how 

electronics would behave while 

exposed to smoke from a fire. 

However, there were data to indicate 

that smoke causes substantial 

problems. For example, a fire in 

Hinsdale, IL in 1985 caused 

widespread smoke damage to 

electronics in a telecommunications 

central office.5 The major focus of 

research based on this fire was (1) 

estimating the amount of soot or 

chemical deposits that can cause long

term damage to electronics, (2)

1
determining which types of cable 

insulation and jackets will produce the 

least damaging chemicals, and (3) 

developing methods to control the 

movement of smoke. Most of the 

concern about the electronics was 

focused on determining if the 

equipment could continue to operate 

after a fire or, if not, be could be 

salvaged (generally involves cleaning 

off soot deposits). The salvage 

question is important to insurance 

companies. 

Nuclear power plants require 

continuous control of and feedback 

from the reactor. The ability to 

salvage equipment after a fire is not 

an adequate indication of damage 

because smoke might cause equipment 

malfunction during a fire. Tests on 

active electronic equipment were 

required to ensure that the function of

such equipment during a fire is 

unaffected, or if affected, will fail in a 

safe manner. Since very few other 

industries besides nuclear power are 

so concerned about active, continuous 

control over a process, SNL assumed, 

and later verified, that there would be 

little data on the function of 

equipment during a fire and proposed 

to do some tests. Figure 1 shows some 

of the smoke measurement targets 

and electronic equipment tested in 

this program. 

A testing program such as this 

required background studies to 

determine the answers to several 

questions: 

- What is known about the 

effects of smoke on 

electronics?

Figure 1. This program exposed a range of components and samples to 

smoke.

2

Co0

U
What type of tests should 

be performed? 

What equipment should 

be tested to best serve 

the needs of the USNRC? 

A literature review of past fires and 

test methods yielded little information 

on the performance of electrical 

equipment during a fire. In 

particular, very little information was 

available on digital equipment. There 

have been several smoke tests 

developed to determine which 

materials, if burned, would produce 

the most corrosive gases. The relative 

merits of the different materials were 

determined by either the acidity of the 

smoke produced, or the amount of 

metal that was lost from standard 

exposure targets through corrosion 

when exposed to the smoke. The 

method of producing the smoke and 

the targets varies by test, and each 

test has its proponents. None of these 

standard tests, however, measured the 

performance of an active digital 

system. 

Of these various standard test 

methods, the American Society for 

Testing and Materials (ASTM) draft 

standard to produce smoke by a 

radiant heat method was determined 

to be the method most adaptable to 

exposing electronic equipment. This 

method, coincidentally, was also 

selected by a study performed by 

Hughes Associates for the Department 

of Energy (DOE) to determine the 

"immediate" effects of smoke on 

electronics, but no tests were 

performed.6 SNL also determined 

what smoke scenarios are likely for

nuclear power plants. These scenarios 

were determined by reviewing past 

fires and reports on the amount of fuel 

in electrical cabinets and rooms. 

Along with this program, the USNRC 

requested ORNL to investigate how 

digital safety systems would be 

affected by electromagnetic and radio 

frequency interference (EMI/RFI), 

temperature, humidity, and smoke.3 

To satisfy this request, ORNL 

designed and built an experimental 

digital safety system that contained 

elements of different proposed 

commercial digital safety systems. 

Typical safety systems contain four 

trip channels. This safety system 

emulated one channel out of four that 

would be likely for a real reactor, but 

it also contained a computer that 

simulated three other channels of the 

system and monitored the reaction of 

the safety channel. Smoke exposure 

tests of this system were performed 

using SNL facilities and smoke was 

shown to cause upsets to the safety 

system. 

SNL has studied the effects of smoke 

on electrical components and systems 

of three levels of complexity: digital 

systems with communications, 

functional analog circuits, and 

component packaging. Most of these 

systems and components are shown in 

Figure 2. The components and 

systems that were chosen to test were 

picked from a wide array of possible 

designs and configurations. The 

components tested fulfilled these 

qualifications: 

- The component would be 

expected to play an

3
important role in an 

advanced nuclear power 

instrumentation and control 

system. 

- The component was 

suspected to be vulnerable to 

smoke. 

- The component might be 

used in multiple circuits 

within advanced control and 

instrumentation systems. 

- The component may be 

tested simply in isolation 

and is not dependent on 

complicated simulation 

hardware to be tested. 

Besides components, changes in 

physical properties were measured. 

These changes included optical 

extinction coefficient, loss of metal,

loss of insulation resistance on printed 

circuit boards and between vertical 

parallel plates, mass of smoke 

suspended in the air, and chemical 

composition of the smoke and deposits. 

These measurements aided in 

determining the process by which the 

smoke affected the electrical 

components. The measurements 

evolved, as it became apparent that 

electrically, the most significant effect 

was the loss of insulation resistance. 

Hence, at the end of the program, the 

most important gage of the effect of 

the smoke was the change in 

conductance between insulated 

conductors. The physical property 

measurements were mainly funded by 

the Sandia LDRD program as this 

could be classified more as research 

into basic physical phenomena rather 

than testing of components that were 

likely to be used in nuclear power

Figure 2. These electrical components were tested at SNL.

4

Co2-

U
Table 1. Tests Performed

Smoke exposure tests 

Multiplexer (a digital system) 

ORNL Experimental Digital Safety Channel 

Component packaging 

Functional Circuits 

Conformal Coatings 

Digital Throughput (SNL sponsored) 

Memory Chips 

High Voltage AC 

Hard Disk Drives 

Memory Chips (non-volatile SRAMs)

plants. Figure 3 shows some of the 

targets and samplers used to measure 

the effects of smoke on basic 

electronics and smoke properties. 

The order in which these tests were 

performed is shown in Table 1. Each 

series of tests were devised to 

determine either the smoke tolerance 

of a component or measure smoke 

effects. 

This is the final report of the program 

to study the impact of smoke on digital 

I&C equipment. This report provides 

a brief summary of the project, results 

and insights that can be drawn from

Date of tests

December 1994 

June 1995 

February 1996 

November 1996 

March 1997 

June 1998 

December 1998 

March 1999 

June 1999 

Oct/Nov 1999

Report completion

August 1995

July 1996 

October 1996 

October 1997 

Section 2.4.1 (this report) 

Section 2.4.2 (this report) 

Section 2.4.3 (this report) 

Section 2.4.5 (this report) 

Section 2.4.4 (this report) 

Section 2-4.3 (this report)

this program, a summary of how the 

present fire protection rules and 

regulations can prevent smoke from 

damaging equipment, and a summary 

of recovery methods that should be 

practiced after a fire. It also includes 

the results of some previously 

unreported tests on how different 

types of conformal coatings can protect 

equipment, the effect of smoke on 

high-voltage AC circuits, and how 

memory chips and hard disks will 

react to smoke.

Figure 3. Electrical targets and chemical samplers were determined the 

effect of smoke on general electrical devices and measured the smoke 

environment.

5

C03
2 PROGRAM RESULTS

This section reviews smoke test 

results and draws conclusions from 

this program. It contains sections on 

the background studies, a general 

description of how SNL performed the 

smoke tests, a summary of previously 

reported smoke tests, and results from 

the recently completed tests on 

conformal coatings on functional 

circuit boards, memory chips, digital 

throughput boards, and hard disks. 

The details of tests performed before 

March 1997 have been published in 

either "Circuit Bridging of 

Components by Smoke", NUREG/CR

64767 or "Effect of Smoke on 

Functional Circuits", NUREG/CR

6543.8 Further conclusions on 

environmental qualifications of 

microprocessor-based systems are 

found in Technical Basis for 

Environmental Qualification of 

Microprocessor-based Safety-related 

Equipment in Nuclear Power Plants", 

NUREG/CR-6479. 9 

2.1 Background Studies 

The background studies of this project 

are reported in the appendices of 

reference 7 on these subjects: (1) a 

review of the public literature on 

smoke damage, (2) the conditions 

expected during a fire in a nuclear 

power plant, and (3) the type of 

equipment that would be tested for 

this project. This section summarizes 

the results from the first two reports. 

The types of equipment that were 

tested are reviewed in Section 2.3.

2.1.1 Literature Review 

As a preliminary step for this project, 

the public literature was reviewed in 

1995 to determine the effects of smoke 

on electrical equipment. 7 This 

included searches of FIREDOC (a 

database administered by the 

National Institute of Standards and 

Technology), the nuclear power plant 

fire event database,'" and various 

other databases, such as those 

administered by Compendix and the 

DOE. The review yielded few tests of 

electrical equipment reliability during 

smoke exposure, although accidental 

fires and smoke corrosivity testing 

(material tests) are the subjects of 

many smoke-related articles. 

Smoke is known to cause considerable 

damage to electrical equipment. 

Estimates of equipment losses due to 

smoke range from 90 to 95% of losses 

due to fire. 5 Smoke losses are 

recognized by insurance companies as 

well as federal agencies such as the 

DOE. 6 

The DOE has been concerned about 

the effects of fire on electrical 

equipment such as computers and 

control systems. Since the SNL 

background studies were performed, 

the DOE has published a very 

complete literature review on the 

thermal and nonthermal effects of 

fires on electrical equipment. 6 This 

review recommends some of the same 

types of tests that have been 

performed in this program and calls 

them tests of immediate failure.

7
Thus, the needs of the DOE overlap 

those of the USNRC. Active 

monitoring of equipment during a 

smoke test is more involved than 

analysis of equipment before and after 

exposure to smoke. 

Smoke from fires can cause loss of 

control and instrumentation during 

the fire. During the Hinsdale fire, the 

fire alarms were intermittent 

indicating that power or signal cables 

could have been destroyed or smoke 

could have caused shorts. The 

intermittent phenomena have not 

been well investigated, mainly 

because most industries do not need 

information on equipment 

performance during a fire. In a 

nuclear power plant, such failures 

(intermittent or continual) during a 

fire could cause the loss of control or 

the loss of critical information within a 

plant. 

Most of the damage analysis has been 

done after, not during, a fire. 

Companies who do post-fire recovery 

work reduce losses by quickly reducing 

the humidity around smoke-exposed 

equipment and cleaning the 

equipment to remove soot and smoke 

deposits that can corrode the 

equipment. Equipment exposed to 

smoke corrodes because smoke 

contains acidic gases. The amount of 

acid depends on the type and quantity 

of material burned. To reduce the 

corrosion of electronic equipment by 

fires, the telecommunications industry 

has promoted different testing 

methods to determine which cables 

will produce the lowest amount of 

acidic smoke.

Several smoke failure mechanisms are 

predicted in the literature. These 

include loss of contact metal, an 

increase in leakage currents between 

exposed contacts, loss of conduction of 

mechanical contacts, and loss of fine 

motion of electromechanical systems 

such as hard disks and chart 

recorders. Loss of contact metal 

occurs because the metallic material 

combines with chlorine or oxygen to 

form salts or rust, which are not as 

conductive as the purer metal. 

Leakage currents increase through the 

formation of paths of ionic chemicals 

between the insulated conductors. 

Mechanical contacts loose conductivity 

as they get coated with soot and 

mechanical systems that depend on 

fine motion and narrow tolerances 

may fail because particulate matter 

can inhibit motion. Despite all of 

these hypothesized mechanisms, very 

few controlled tests had been 

performed prior to those in this 

program to compare the likelihood of 

failures from the various 

mechanisms.7 

Smoke Test Methods 

Many different smoke test methods 

have been endorsed by different 

standards organizations throughout 

the world." The primary purpose of 

the smoke test methods have been to 

determine which materials, when 

burned, would produce the least 

harmful smoke. The standards vary 

in their method of producing and 

collecting the smoke and determining 

how harmful the smoke is. The three 

main indications of damage to 

electronics were measurements of 

smoke acidity, metal loss (variation in

8
weight or resistance of thin metallic 

deposits), or increased leakage 

current. Although acidity and metal 

loss measurements are aimed at 

detecting failures from loss of 

conductor material, leakage current 

measurements consider a separate 

failure mode, the increase of 

conduction between insulated 

conductors (e.g., the creation of "smoke 

circuit paths"). 

The leakage current measurements 

have been adopted from studies'of the 

breakdown of equipment from 

exposure to polluted air. 12 13 14 

Electronic equipment has been found 

to fail because of increased leakage 

currents (shorts) when exposed to 

airborne pollutants. Surface 

insulation resistance measures circuit 

bridging for a printed circuit board, 

and is performed on parallel, non

intersecting conductive traces on a 

printed wiring board.15 

Manufacturers of printed wiring 

boards use surface insulation 

resistance to determine how clean 

their processes are. The best 

assemblies are clean and have high 

surface insulation resistance. Air 

pollutants and soldering residue 

reduce the insulation resistance and 

increase leakage currents in 

electronics. Leakage current 

measurements are being adopted to 

evaluate smoke, 16 17 and may allow 

smoke corrosivity measurements to 

correlate better to actual damage to 

electronics. 

Our literature search has found two 

tests on actual electronic equipment, 

Jacobus in 198618 and Bridger in 

1994.19 Jacobus exposed various

electrical equipment, including strip 

chart recorders, an electronic 

counters, amplifiers, mechanical 

relays, switches, and power supplies, 

to a full-scale cabinet fires. Only the 

switches, relays, power supplies, and 

amplifiers were monitored 

continuously during the fire. Leakage 

currents were blamed for failure of the 

electronic counter as a result of 

corrosive action of chlorides in 

particulates deposited on the circuit 

boards. The strip chart recorders 

failed mechanically due to particulates 

deposited on pen slider mechanisms. 

An amplifier experienced errors, 

which was explained as a thermal 

drift problem because it returned to 

normal after the test was over. Some 

of the relays and switches had 

increased contact resistance at low 

voltages. Chlorides generated in the 

fire were combined with particulates 

and less hydrogen chloride vapor was 

in the air than previously predicted by 

small-scale tests. 

Bridger exposed switchgear and tested 

the conductivity of various switchgear 

(but not leakage current) after 

exposure to smoke from different types 

of cables. These measurements were 

performed immediately after the 

exposure and several weeks after the 

exposure. The resistance of the 

mechanical contacts increased with 

time after the fire. Bridger found that 

non-halogen based cables were 

superior to halogen-based cables 

because there was less smoke, the 

smoke was less corrosive and there 

were fewer toxic effects from the non

halogenated cables.

9
2.1.2 Smoke Scenarios 

A second background study was 

conducted on the types of fires to be 

expected in nuclear power plants and 

to what smoke conditions the 

electronics would be exposed. This 

background study is published as 

appendix A of reference 7. Since 

smoke can be generated under many 

different conditions and environments, 

a limited number of test variables that 

could cover a range of smoke scenarios 

were considered. Three aspects of 

smoke from fires were considered: 

quantity of smoke, quality of smoke, 

and duration of exposure. 

Quantity refers to the amount of 

smoke per volume of affected air. 

These values were determined from 

estimates of how much fuel (i.e., 

plastic and materials) is available in a 

control panel and how much volume 

there is in either a control room, 

general room, or within an electrical 

cabinet. No ventilation systems were 

considered because active ventilation 

would decrease the smoke density 

within a room with a fire. Quantities 

considered in this project ranged from 

fuel loads of 3 g/m 3 to 200 g/m 3. All of 

the quantities were scaled by air 

volume. However, at the time of this 

report, it was unclear whether volume 

or exposed surface area should scale 

the tests. Volume scaling implies 

smoke density is predominant while 

surface area scaling implies smoke 

deposition is of primary importance. 

Later tests have shown that 

significant effects of smoke are a 

result of the smoke density in the air, 

thus, it is better to scale by volume 

than surface area. (Section 2.4.5)

Quality refers to the composition of 

the smoke and interactions with fire 

suppressants. The chemical 

composition of smoke is highly 

dependent on the composition of the 

fuel, how it is burned, and how far the 

fire is from the equipment. The 

distance of the equipment from the 

fire is important because particulates 

and certain gases may not be 

transported equally to all sites. Many 

different fuel materials are available 

in nuclear power plants, including 

plastic (cable insulation), liquid fuels 

such as diesel oil, paper and wood. 

The fuel expected to be the most 

damaging is plastic because it 

contains a large amount of chlorine, 

which when burned produces 

hydrochloric acid. Throughout this 

project, cable insulation has been 

burned because it is a representative 

fuel and is the most corrosive. 

Humidity and CO 2 were added to 

simulate the effect of using fire 

suppressants. 

The exposure duration was addressed 

in two ways: (1) the length of time the 

equipment could be exposed to a 

smoke-filled environment and (2) the 

length of time equipment would be 

required to be operational after a fire. 

Based on typical fire scenarios, we 

developed a standard test procedure 

that included 15 minutes of burn time 

and an additional 45 minutes of 

exposure before the smoke was vented. 

The equipment was monitored for a 

total of 24 hours from the beginning of 

the exposure. The total monitoring 

time was determined by the amount of 

time that was judge to be necessary to 

stabilize a power plant after a fire 

event.

10
A third background study on the types 

of electronics to be tested was 

published as Appendix E of "Circuit 

Bridging of Components by Smoke", 

reference 7. This background study 

was published at the beginning of the 

program. Subsequently, our reasons 

for testing different equipment 

changed as we discovered how the 

smoke was causing failures in 

electronics. Results found in our 

experiments indicated what types if 

equipment were more likely to fail and 

the course of experiments were 

realigned as results became available 

to test the weakest links that were 

likely to be in a safety system. The 

information that follows in Section 

2.2.2 on equipment that was tested 

throughout this program is a more 

accurate summary of the work done 

for the project. 

2.2 SNL Testing Method 

The needs of the nuclear power 

industry for smoke tests are different 

from those of most other industries. 

The former requires active control and 

monitoring of a potentially dangerous 

facility during and immediately 

following a fire, whereas other 

industries need to recover either data 

or equipment only well after the fire is 

extinguished. This need was a 

primary reason for performing smoke 

exposure tests at SNL. 

Fire tests are somewhat variable 

under the best of conditions. Small 

changes in the environment, such as 

the layout of the fuel and the heat flux 

that ignites the fire, may change the 

quality and quantity of the smoke that

is produced. Because the smoke can 

be so variable, it is important to 

expose as many targets and make as 

many measurements as possible 

during each exposure to reduce the 

uncertainty of the results. 

Smoke production techniques vary 

widely. To compare failure modes and 

conditions, we needed to produce 

smoke consistently and in a manner 

that is like a true fire. We developed a 

procedure that was modeled on a 

smoke corrosivity draft standard using 

the radiant heat method as described 

in the draft ASTM E05.21.70 

committee standard. 20 This 

production method had several 

features important for real-time 

testing. 

1. This procedure is a static production 

method-all of the smoke that is 

produced is contained in the smoke 

exposure chamber rather than 

continuously vented or sampled. This 

represents a worst-case scenario of all 

possible smoke being contained within 

a limited volume and insures that the 

exposure represents an accurate 

smoke sample comparable to that of a 

real fire. 

2. The exposure volume is large 

enough to include electronic 

equipment, such as computer boards. 

3. The burning conditions can be 

controlled and varied. For dynamic 

tests, the only method of exposing 

electronics to the smoke is to pump air 

through the fire and onto the sample. 

No low-oxygen tests can be performed 

with a dynamic test.

11
Using the ASTM draft standard as a 

basis, a smoke test procedure was 

developed. Several criteria from the 

draft standard have been followed in 

this test procedure: the configuration 

and size (0.2 m3 ) of the exposure 

chamber, the combustion unit, radiant 

heat lamps, method of measuring heat 

fluxes, and duration of smoke 

exposure and monitoring. Figure 4 

shows the smoke chamber built to the 

ASTM E05.21.70 draft standard. As 

in the draft standard, the humidity 

(75% RH) and temperature (23.9 0C or 

75 OF) were controlled before and after 

the exposure; however, because the 

smoke production takes place in an 

enclosed volume, the temperature and 

humidity were not controlled during 

the 1-hour exposure period. For the 

first tests on the multiplexer board, 

the humidity and temperature were

not well controlled, but starting with 

the test on the digital safety channel, 

the entire smoke chamber was placed 

inside a large walk-in environmental 

chamber that could control these 

parameters. Figure 5 shows the 

smoke chamber inside of the 

environmental chamber. 

The digital safety system was too 

large to place in the standard ASTM 

exposure chamber, so SNL built a 

chamber that was five times larger in 

volume (lm 3 ) and had four combustion 

chambers instead of one. This larger 

chamber was used for two sets of tests: 

(1) the digital safety system and (2) 

the component packaging tests in 

which the protection provided by a 

personal computer chassis was being 

investigated. The smaller standard 

configuration was used for the other

Figure 4. The small smoke exposure chamber was built to the dimensions 

of ASTM E05.70.21 draft smoke corrosivity test and used for the memory 

chip, electrical, hard disk and digital throughput tests.

12

Co0-t
Figure 5. All of the smoke tests other than the multiplexer tests were 

performed in an environmental chamber to control initial temperature 

and humidity.

tests because the cleanup was faster 

and less cable was burned in the 

smaller chamber. This larger 

chamber is shown in Figure 6. 

Most of the smoke measurements were 

automated by computer and logged. 

Because of the range of equipment 

tested, one computer was generally 

devoted to the test equipment and a

separate computer controlled the 

ignition lamps and recorded the 

automated measurements such as 

temperature and extinction coefficient. 

With each new piece of equipment to 

be monitored, the programming for 

one computer would change, whereas, 

only moderate changes were made to 

the program that controlled ignition 

and measured the environment.

13

Co06
Figure 6. The large smoke exposure chamber was used for testing the 

digital safety system and component tests. 

14 

Ce"S
2.2.1 Fuels Burned 

With the exception of some limited 

tests with polyvinyl chloride (PVC) 

cable, most of the cables burned for 

this program have been nuclear

qualified cables. For the first tests on 

the multiplexer boards, only one cable 

type at a time was burned, but for the 

remainder of the tests, a mixture of 

cables based on the frequency of plant 

use was burned. Polyvinyl chloride is 

widely acknowledged to cause 

increased corrosion from smoke and, 

while it does not ignite as easily as 

materials with no chlorine, it will 

nevertheless burn vigorously and 

produce a lot of smoke. Although 

power plants do not currently use PVC 

cable, they have not removed all such 

cables from the plants and these 

cables are therefore available as fuel 

for a cable fire. Therefore component 

packaging tests included 

approximately 5% PVC in the mixture 

of fuels to determine if a small amount 

of PVC would contribute significantly 

to electrical failure. Generally, the 

mixture did not include PVC, however, 

most of these cables use halogens such 

as chlorine or bromine as a fire 

retardant. For the functional board 

and coating tests, the fuel was 

analyzed by Swartzkopf Laboratories 

(Woodside, NY) for heat of combustion 

and chemical analysis of the ashes. In 

general the cable materials produce 

2.4 x 107 J/kg and contained 23% ash 

(mostly carbon), 0.95% Br, 12.60% Cl, 

and 0.49% F.

Manufacturers include halogens in 

cable formulae because they reduce 

material flammability. Halogens tend 

to combine with the same chemicals as 

oxygen during a fire, hence preventing 

fires by excluding oxygen. Halogens 

are blamed for increasing smoke 

corrosivity and damage to electronics 

because when halogenated cable 

bums, they produce strong acids, such 

as hydrochloric acid. Therefore some 

organizations such as the British 

Navy and many the 

telecommunications companies in 

Europe do not allow the use of 

halogenated cables. However, many of 

the non-halogenated cables are more 

flammable than halogenated ones. 

For the ORNL experimental digital 

safety system, pieces of cable from 

Table 2 were stacked in a tray and 

burned with the conductor material in 

it. For tests starting with the 

component packaging tests, the cable 

insulation and jacket was stripped 

from the conductor and ground to 

make a mixture of cables from Table 2. 

The fuel was mounded in a tray inside 

of the quartz combustion chamber 

(Figure 7) located under the smoke 

exposure chamber. Figure 8 shows 

the combustion chamber during a 

smoke test. Most of the light is a 

result of the quartz lamps as they heat 

the fuel. A pilot spark, gas lighter, or 

electrical coil provided an ignition 

source for flaming tests, but was 

omitted during smoldering tests.

15
Table 2. Nuclear Qualified Low-Power Cables Burned in Smoke Exposures

Cable name 

Rockbestos Firewall III 

Anaconda Flameguard 1kV 

Brand Rex XLPE 

Okonite Okolon 

Kerite HTK 

Rockbestos Coax (le) 

Raychem 

Dekoran Dekorad 

BIW 

Kerite FR

Insulation 

FRXLPE 

EPR 

XLPE 

EPR 

unk* 

unk 

XLPE 

EPDM 

EPR 

unk

*unk=unknown material, not specified by manufacturer.

16

Jacket 

CSPE 

CSPE 

CSPE 

CSPE 

unk 

unk 

XLPE 

CSPE 

CSPE 

unk
M

Figure 7. Fuel was loaded in the combustion chamber in tray below the 

stainless steel chimney (large rectangular shape at top center).

Figure 8. Quartz radiant heat lamps heated the fuel in the combustion 

chamber and smoke flowed up the chimney to the smoke chamber. 

17

cO-7
2.2.2 Digital Equipment Tested 

A wide variety of equipment has been 

exposed to smoke through this 

program. Most of the electronics that 

were exposed were actively monitored 

throughout the smoke exposure. The 

components and assemblies that were 

tested are shown in Figure 1, with the 

components on the left and the smoke 

effects test assemblies on the right. 

Starting with the components, top left 

is the multiplexer board, then to its, 

right is the empty chip board, the 

chips tested for critical resistance, the 

functional board, the digital 

throughput board. On the second row 

are four memory chips, the hard disk 

drive and the non-volatile memory 

chip. From the bottom of the electrical 

effects devices are two absorption 

tubes to measure the acid generated in 

the smoke, the Rohrback probe, and 

the surface insulation resistance 

board. Side by side are the silver 

membrane filter to measure the smoke 

in the air and the humidity sensor. 

Above these are the mass vs. 

conductivity board in its holder, and 

the parallel plates to test voltage 

breakdown. 

The first equipment tested, 

commercial off-the-shelf (COTS) 

multiplexer boards, converted digital 

signals to 4-20 mA DC current output 

and back again to digital data. A 

serial cable to a personal computer 

placed outside the exposure chamber 

connected the boards. The software 

for this board was written by ORNL 

and did not have any method to 

recover from time-out errors. 

Therefore, when smoke-induced time

out errors stopped the monitoring

program data collection was 

interrupted. 

The second system tested, the 

experimental digital safety system 

(not shown), was also developed by 

ORNL out of COTS parts. This was a 

three-node networked computer 

system that emulated a safety channel 

for a nuclear power plant. The first 

computer-the process multiplexing 

unit-performed signal conditioning, 

data acquisition, and multiplexing of 

process signals. The data was then 

passed onto the second computer-the 

trip logic computer-via a fiber 

distributed data interchange (FDDI) 

network. These trip logic units 

simulated channel trip logic elements 

and performed voting logic 

calculations. The third computer 

monitored the performance of the 

entire system. Reference 3 provides 

further details of the system. 

The third set of tests was performed 

on COTS component packages. 

Circuit-bridging tests measured 

leakage currents generated by the 

addition of smoke to empty component 

packages (no devices installed), plastic 

chip packages, and surface insulation 

resistance patterns. In addition, 

complementary metal-oxide 

semiconductor (CMOS) memory chips 

were measured for function and 

performance before and after the 

exposures and an optical isolator chip 

was continuously monitored. Both 

surface-mounted technology (SMT) 

and plated-through-hole (PTH) 

packages were tested. The spacing 

between these components varied from 

0.43 to 1.0 mm.

18
The fourth set of tests was performed 

on functional circuit boards developed 

by the Low-residue Soldering Task 

Force (LRSTF).2 1 The boards were 

developed to determine how different 

low-residue soldering practices would 

affect the quality and cleanliness of 

circuit boards. This board contained 

different types of circuits constructed 

of COTS and milspec components. 

These circuits featured different 

properties such as high voltage low 

current (HVLC), high current low 

voltage (HCLV), high frequency (HF), 

and high-speed digital (HSD). These 

different circuits allowed the 

determination of which failure 

mechanisms were the most likely, 

including increased leakage currents, 

increased rise and fall times, solder 

joint faults, and increased stray 

capacitance. 

A fifth series of tests determined the 

relative merits of conformal coatings 

on boards developed by the LRSTF. 

Several different conformal coatings 

were used to protect electronics from 

smoke exposures. These included a 

sprayed-on acrylic (used for the 

component packages only), brushed-on 

polyurethane, a dipped polyurethane, 

silicone, epoxy, acrylic, and parylene. 

The last six coatings listed were used 

to protect the functional circuit 

boards. 

A sixth set of test determined if smoke 

would upset the transmission of 

digital data through common 

communications connectors. These 

connectors included the subminiature 

D nine-pin connector, commonly used 

for serial ports; the subminiature D 

25-pin connector, commonly used for

parallel ports, and the RJ-45 

connector, commonly used for 

Ethernet connections. This test series 

was sponsored by the LDRD program 

and included wood, jet fuel, and PVC 

cable as fuels. 

A seventh set of tests determined if 

smoke would affect the performance of 

erasable-programmable read-only 

memory (EPROM) and static random 

access memory (SRAM). These tests 

compared chip performance for 

different amounts of smoke. They also 

measured the leakage currents for a 

given voltage application and the 

voltage level for current injection. The 

EPROM chips were preprogrammed 

with data, and the data were read and 

checked to determine performance. 

The SRAM chips both recorded and 

read data. Non-volatile SRAM chips 

with internal lithium batteries were 

also tested in the smoke. 

An eighth set of tests determined if 

smoke would interrupt hard disk 

access in the presence of smoke. The 

disks were used for both reading and 

writing while in the smoke 

environment. Hard disks were tested 

because hard disks are essential 

equipment in most computers, and 

they can cause severe interruption in 

computer performance if they are 

faulty. 

Throughout the smoke tests, electrical 

measurements of surface insulation 

resistance, conductance of 

freestanding parallel plates and other 

effects of smoke on circuits were 

measured. These measurements were 

performed because we needed a basic 

understanding of how smoke would

19
effect an electrical circuit in general. 

Early tests also included a metal loss 

measurement and chemical analysis. 

The literature review indicated that 

metal loss was an important measure 

of the effect of smoke on equipment 

and that the amount of chlorides 

deposited indicated the likelihood of 

failure. These measurements were 

discontinued because they were not 

direct measures of electrical problems 

that were likely to cause failures. 

The equipment tested ranged from 

entire computers to component 

packages. This range of complexity 

allowed analysis of the reaction of a 

system to smoke and determination of 

individual causes of failure. The 

networked system tests identified 

likely system failures, while the 

component and functional board 

testing allowed determination of the 

causes of failure modes. The results of 

these tests are summarized in 

Sections 2.3 and 2.4. 

2.2.3 Smoke Measurements 

The smoke conditions were measured 

using physical means; optical 

extinction coefficient, fuel mass loss, 

mass of smoke in the air, and the 

density of soot deposited on flat 

surfaces were measured. These 

measurements were useful in 

determining the smoke and other 

environmental conditions of the test. 

For example, the smoke density, how 

much fuel was burned how much 

smoke was deposited on surfaces, 

temperature and humidity. Because 

of test-to-test variability with the 

same initial conditions, these smoke 

measurements allowed us to compare

results for equivalent conditions. 

Sometimes smoke tests with the same 

initial test conditions result in 

different smoke measurements and in 

different electrical outcomes. 

Chemical analysis of the soot was 

performed before the LDRD program 

started and has yielded some chlorides 

and sulfates as standard ionic 

byproducts of the burning cables. No 

chemical analysis was followed up 

after the coating tests because of the 

limited scope of the program. 

Fuel mass loss 

Fuel mass loss indicates how 

completely a fuel is burned. In 

general, the higher the heat flux, the 

faster and more completely the fuel 

burned. The fuel mass was tracked by 

two methods, the fuel in a tray was 

weighed before and after each test, 

and a low-resolution measurement 

was made during the test using a load 

cell. A drawback of the load cell 

method was that the load cell was 

temperature sensitive so 

measurements during the flaming 

period, when the lamps were on are 

not very accurate. 

Optical extinction coefficient 

The optical extinction coefficient was 

measured with a He-Ne laser system. 

The laser beam was split and part of 

the laser light was monitored with a 

silicon photodiode detector while the 

rest of the laser light is collimated 

onto a fiber optic bundle. The fiber 

bundle directed the laser light through 

the smoke enclosure and then a second 

fiber optic bundle directed the light 

back to a silicon photodiode detector.

20
The transmission through the 

chamber was measured by comparing 

the signals from the monitor detector 

and the transmission detector. Since 

the objective was to measure the 

smoke in the air, nitrogen gas was 

used to purge the smoke from optical 

surfaces in the smoke chamber. The 

extinction coefficient, Do, was then 

calculated using the Lambert-Beer 

law: 

D, =-ln(I/I0 )/t. 

The ratio of I/lo is the ratio of the 

transmission while the smoke is in the 

chamber to the transmission before 

the smoke is added to the chamber 

and t is the distance the beam passes 

through the smoke (10 cm). 

Mass density 

Comparisons between the optical 

extinction coefficient and the mass 

density help ensure that the amount 

of smoke is known and can be 

compared with fire modeling codes. 

During the smoke exposure, four 

samples of air were drawn through 

silver 0.8 p mesh membrane filters to 

measure the smoke mass density. The 

air samples were drawn directly above 

the optical extinction coefficient 

measurement. The filters were 

weighed before installation and again 

after the smoke exposure. Figure 9 

shows the smoke collected on the 

silver membrane filter. The amount of

air that was filtered was determined 

by measuring the airflow through the 

filters before and after the smoke test. 

Each sample was drawn for 30 s at 

rates that were less than 10 L/min 

(0.166 L/s). The optical extinction 

coefficient was proportional to the 

mass density as shown in Figure 10. 

Table 3 compares the ratios of optical 

to mass density for different cable 

fuels in this program.

Figure 9. Smoke was filtered from 

smoke chamber to determine the 

smoke density. 

Table 3. Ratio of Optical 

Extinction Coefficient to Smoke 

Mass Density 

Cable Ddp 

Fuel (ULnmg-cm) 

PVC 0.038 

Ground mixed 0.026 

(see Table 2) 

Brand rex 0.022

21

cot,

U
Do=0. 0 3 8 p E 

0 

0 

LU 

C'

0 

x

O

0 12 3 4 

Mass density (mg/L) 

Figure 10. Optical extinction coefficient Vs Mass density for PVC smoke

Mass deposition 

The mass of surface deposited soot 

was measured with a quartz crystal 

microbalance. A quartz crystal, 

patterned with gold contacts was 

connected to an oscillator circuit that 

drives the crystal at its resonant 

frequency. When smoke deposits on 

the surface, the resonant frequency 

decreases (as in mass loading of a 

spring), and the mass can be 

measured as the test is run. The 

disadvantage of this device is that 

temperature changes affect the 

resonant frequency of the crystal;

therefore, corrections must be made 

for temperature changes. Although 

the quartz crystal is cut to reduce the 

frequency dependence on temperature, 

temperature changes also affected the 

oscillator circuit, changing the 

resonant frequency. Also, soot does 

not deposit on all surfaces equally, 

temperature and surface polarities are 

known to influence soot deposition. 

Hence measurements on these 

quartz/gold surfaces may not be 

indicative of how much smoke deposits 

on other types of surfaces, such as 

printed circuit boards.

22

0.16 

0.12 

0.08

0.04 

0
2.2.4 Humidity measurements 

The conductivity of soot is highly 

dependent on the relative humidity 

(RH) in the smoke chamber. The SNL 

smoke exposure tests have been 

performed with no control over the 

humidity other than the beginning 

and the end of the smoke test. Since 

conductivity depends on humidity and 

is a major concern in the reliability of 

electronics, humidity can be an 

important parameter to measure 

during a test. 

Near the end of the program, small 

RH gages were included in the smoke 

chamber, one near the electronics and 

one near the mass vs. conductivity 

measurement boards. These gages 

included a platinum resistance 

temperature detector (RTD) so that 

both RH and temperature can be 

measured and other measurements of 

humidity can be derived. Figure 11 

shows the RH/temperature gage that 

was placed in the smoke chamber to 

give readings throughout the smoke 

exposure. RH is the ratio of the mass 

of water vapor in the air to the mass of 

water vapor in saturated air (if the air

Figure 11. Humidity/temperature 

gage included in the smoke 

exposure chamber.

is saturated, it contains the maximum 

mass of water vapor for air of that 

temperature). The mixing ratio is the 

ratio of the mass of water vapor to the 

mass of dry air and is independent of 

temperature of the air. 

At the beginning of the smoke test, the 

humidity level was controlled by the 

environmental chamber, which 

surrounds the smoke exposure 

chamber, to approximately 75% RH. 

As the radiant heat lamps are turned 

on, the air in the smoke chamber was 

heated (Figure 12). The RH decreases 

with increase of air temperature 

because hot air can sustain more 

water vapor than cool air (Figure 13). 

Dry nitrogen was also added to the air 

near the smoke opacity measurement 

throughout the test. Dry nitrogen was 

used to sweep smoke from the optics of 

the smoke opacity measurement and 

prevent lenses from darkening from 

soot. 

After 30 seconds the fuel began 

burning in the smoke chamber. 

Burning fuels that contain hydrogen 

(most fuels) creates water because the 

hydrogen molecules from the fuel 

material combine with oxygen to form 

H20. After 15 minutes, the lamps 

were turned off and the temperature 

of the smoke chamber dropped. At 

this point the RH increased slightly 

due to temperature effects, but 

because the nitrogen was added, the 

RH does not recover the original level 

when the temperatures return to the 

starting temperature.

23

CocA
60

&50 

40 

E 

I- 30

20

0 0.5 1 1.5 2

Time (hr)

Figure 12. Temperature of the humidity probes was higher near the mass 

vs. conductivity boards than near the test electronics.

80

-60 

E 

*40 

a)

20

0 0.5 1 1.5 2

Time (hr)

Figure 13. Relative humidity in two locations in the smoke chamber

24

CIO
One hour after the lamps were turned 

on, the smoke chamber was vented. 

Smoke was exhausted from inside of 

the smoke chamber to outside of the 

building with a hood and moist, 75% 

RH air replaced the smoke. Venting 

with high humidity air overcomes the 

dry nitrogen purge and the RH in the 

chamber returns to the RH of the 

environmental chamber. Most of the 

soot has precipitated from the air and 

has now attached to surfaces within 

the smoke chamber (including the 

surface of the RH sensor.) 

Relative humidity is the ratio of water 

vapor to saturation water vapor. The 

mixing ratio is the mass of water 

vapor compared to the mass of dry air. 

The mixing ratio (Figure 14) is 

dependent on the temperature and 

pressure, since the air pressure 

determines how many kg of dry air is 

in a volume. 22 The mixing ratio shows 

the effect of increased water vapor by 

burning and diluting the water vapor 

by adding nitrogen. The mixing ratio 

was calculated by estimating the 

ambient air pressure in the laboratory 

(approximately 1800 m above sea 

level) and calculating the mixing ratio 

of saturated air. Note that the 

humidity gage closest to the nitrogen 

feed by the mass-vs.-conductivity 

boards drops faster after the lamps

are turned off and remains lower than 

the gage that was further from the 

nitrogen feed until venting at 1 hour. 

Conductivity of soot-laden boards 

depends on both the suspended soot 

and the humidity. When the lamps 

are on and the optical extinction 

coefficient was high, the conductivity 

was dominated by smoke that is 

landing on the circuit boards. Later, 

after most of the smoke has deposited 

on the surfaces, changes in RH 

determine the changes in the 

conductivity measured by the mass vs. 

conductivity boards. Figure 15 shows 

the conductivity of the 500 V dc biased 

board. The initial peak occurs while 

smoke was being generated and was 

in the air. After the initial peak, the 

conductivity decreases as the smoke in 

the air settled, and the RH decreased 

due to the addition of dry nitrogen. 

The nitrogen feed was located directly 

above the conductivity boards. After 1 

hour, plastic plates were placed over 

these boards to protect the soot from 

being cleared by venting the air and 

the smoke chamber was vented. The 

increases in conductivity at this time 

could be due to the increase in RH. 

Thus, the conductivity boards reacted 

to both the changes in humidity and 

the smoke density.

25
35 

30 

X 25 

-0 20 

09 

IM ci15

10

0 0.5 1 1.5 2

Time (hr)

Figure 14. The mixing ratio decreased as dry nitrogen was added to the 

smoke chamber between the time that the lamps were turned off and the 

chamber vented (0.25-1.0 hr).

1.0E-05

1.0E-06

to 

C 

C, C 'U 

U 

C 

0 

U

1 0E-07

1.OE-08

1.OE-09

1,OE-10

0 0.5 1 1.5 2

Time (hr)

Figure 15. The conductance of the 500 V dc biased mass vs. conductivity 

board increased slightly after the chamber was vented (>1.0 hr).

26

CII
2.2.5 Temperature measurements 

The temperature and humidity of the 

smoke chamber were controlled before 

and after the smoke exposure, but 

during the fire and for 45 minutes 

after the fire while the smoke is 

contained in the chamber, the 

temperature and humidity are 

uncontrolled. The temperature of the 

air within the smoke exposure 

chamber is measured throughout with 

type K thermocouples. The 

thermocouples were located in various 

positions inside of the smoke chamber 

and three outside of the chamber. 

Figure 16 shows the measured

70

60

S50 

0.40 

E 

1!

30

20

temperature from the tests from some 

of the thermocouples.

2.2.6 Chemical Analysis 

The burned mixed cable fuel, soot, and 

smoke were chemically analyzed 

during the early stages of the 

program. When the fuel was burned, 

some of it becomes gaseous; some 

deposits as soot on surfaces of the 

smoke chamber, and some remains as 

ashes. In general 50% of the mass of 

the fuel is lost as either gaseous 

products or soot. Section 2.2.1 

reported the byproducts of the fuel. 

The ashes were analyzed for metals

0 0.5 1 1.5

Time (hr)

I--T1 -T2 -T3 -T4 -T6 I

Figure 16. The temperature in the smoke chamber varied with position, T1

T3 are near the top of the chamber, T4 is at the bottom, and T6 is midway 

between top and bottom of chamber.

27

ci?-

-

2
semi-quantitatively. The predominant 

metals were silicon, titanium, lead, 

iron and magnesium. 

Soot was collected in the smoke 

chamber by placing ashless filter 

paper on the floor of the smoke 

chamber. The filter papers were 

analyzed for halide and sulfide 

content, ionic compounds that have 

been found to increase the 

conductivity of insulators. 7.6 pg/cm 2 

of chloride, 19 pjg/cm 2 of bromide, and 

31 pg/cm 2 of sulfate were measured 

from the soot samples taken during 

the coating tests where 40 g of fuel 

were burned in the flaming mode in 

the small, 200 L smoke chamber. Soot 

deposits can vary in an actual firecold surfaces attract more deposition 

and hot surfaces repel soot. Surface 

charge can also increase deposition, 

When the fuel was burned in a 

flaming mode, less chloride was These 

filters were burned and analyzed 

using ion chromatography to 

determine how many Cl, Br, and SO 4 

ions were deposited at these sites. As 

shown in Table 4, the Cl and Br ion 

levels were fairly consistent deposited

in the soot than when burned in the 

smoldering mode. This effect is shown 

in Table 4 taken from the functional 

tests. When the flux level was 

25 kW/m2, the fire was smoldering 

rather than flaming, as in the 50 

kW/m2 heat flux fires. There was an 

unexplained presence of fluoride, a 

third halogen that was detected in the 

chemical analysis, on the unexposed 

filer. 

One of the standard measurements for 

smoke corrosivity has been the 

amount of chloride ions deposited on 

the electronics. To get some idea of 

how much smoke was available for 

deposition, filter papers were placed 

on the bottom of the smoke chamber 

on two sides of the functional boards: 

between the boards and the chimney 

(the front), and on the other side of the 

boards from the chimney (the back). 

throughout the tests, but the SO 4 

measurements varied widely. It is 

unclear if this is a result of poor 

analysis or actual differences in each 

burn.

Table 4. Chemical analysis of filters placed in smoke chamber (tg/filter). 

Fuel level Flux leve 

(g/m•3 ) (kW/m 2) Chloride BromidE Fluoride 

3 25 95.5 2.1 0 

3 50 24 5.5 0.25 

25 25 92.6 96.6 0 

25 50 73.0 66.9 1.45 

50 25 147 181 1.25 

50 50 95.4 165 0 

Unexposed filter 1 3.9 0 0.7

28
Gas samples were obtained by 

evaluating a vacuum tight bottle and 

allowing gases from the chamber to 

flow into the bottle during the fire. 

Gas samples were analyzed for argon, 

CH4, CO, C02, 02, and N2. The CO 

and CO 2 levels increased and N2 and 

02 fractions decreased as more fuel 

was burned as would be expected from 

burning a carbon based fuel. Typical 

levels for C02 were 2-3% and 1% of

CO, depending on the burning mode. 

Smoldering fires produce more CO. 

Sorption tubes obtained samples of 

ionic compounds in the smoke in the 

component tests. Smoke was drawn 

through sorption tubes that were 

specifically developed to absorb ionic 

species. Chloride, bromide and sulfate 

were detected in the sorption tubes, all 

species that are known to increase 

leakage currents on circuit boards.12

Table 5. Ion Chromatography Results (•ggcm 2) 

Front Back 

Test no. Br C1 SO 4 Br Cl S04 

1 22.2 10.4 3.2 22.9 11.8 13.6 

2 19.4 9.0 48.8 20.0 9.3 58.6 

3 32.0 13.1 1.1 31.4 13.2 9.6 

4 18.0 6.9 61.5 19.1 7.2 5.1 

5 19.7 5.9 113.1 21.7 6.4 2.3 

6 21.5 8.3 18.5 24.0 9.8 1.4 

7 17.7 7.3 0.6 19.4 8.0 18.8 

8 17.1 7.8 NDa 19.4 8.9 5.9 

Average 20.9 8.6 35.2 22.3 9.3 14.4 

STD 4.8 2.3 42.0 4.1 2.3 18.8 

a ND = not detected

29
2.3 Summary of Previously 

Reported Smoke 

Exposures 

This section summarizes results that 

were reported in reference 3, 7, and 8 

on the digital system tests, component 

testing, and functional board testirg. 

2.3.1 Digital System Tests 

The digital system tests were smoke 

exposures of microprocessor-based 

systems-either a computer or a bcard 

connected to a computer-and 

included digital communication 

between computers or 

microprocessors. 

Multiplexer Tests 

Multiplexing is one of the primary 

advantages of a conversion to 

advanced digital safety systems. 

Multiple control and sensor signals 

can be transmitted on a single cable. 

Three preliminary smoke exposure 

tests were performed on Analog 

Devices model 6BP04-2 multiplexer' 

boards that were connected to a 

computer by a serial port (RS232 

connection). The multiplexer boards 

consisted of circuit board back planes, 

which contained mounting slots for 

different plug-in units that could 

convert digital-to-analog (D/A) or 

analog-to-digital (A/D) signals. The 

back planes used serial ports to 

communicate between the multiple 

plug-in units to a computer that could 

control and log data from the different 

units. The multiplexing action 

results from transmitting several 

different signals from the plug-ins

through the same serial port. This 

multiplexer model was chosen because 

this Analog Devices product contains 

many of the components that an 

advanced digital safety system might 

include, such as AID and D/A units 

and an on-board microprocessor. This 

particular product was also simple to 

program and control with a personal 

computer and was small enough to be 

tested in our small smoke chamber. 

A schematic of the multiplexer test 

setup is shown in Figure 17. DC 

current signals generated on a D/A 

plug-in unit located on a multiplexer 

board outside of the smoke chamber 

were transmitted through a pair of 

conductors to a 50 Q resistor on a 

multiplexer board located inside of the 

smoke chamber. The A/D plug-in 

inside of the smoke chamber converted 

the DC analog current into digital 

signals. The multiplexer board inside 

of the smoke chamber transmitted the 

digital signals out through its serial 

port back to the computer located 

outside of the smoke chamber. For 

each test, either Brand Rex XLPE, 

Anaconda Flameguard, or Belden PVC 

cables were burned for a total of three 

different burns. Each test was 

performed at a high smoke exposure 

level of 75 g of fuel/m3 of air. 

Humidity was expected to lead to 

failures by increasing circuit bridging 

and metal loss. Unfortunately, in this 

experiment, the humidity level was 

difficult to control since the humidity 

was added by means of a portable 

steamer, as the environmental 

chamber was not yet available. The 

humidity could not be controlled 

better than 15% in this manner.

30
Host computer

Figure 17. The host computer controlled two multiplexer backplanes for 

the multiplexer tests.

Of the three cables burned, two tests 

did not show any failures, but the PVC 

cable caused intermittent failures. 

The multiplexer board that was 

exposed to PVC smoke failed 

intermittently twice-once while the 

cables burned and once when 

humidity was being added after the 

exposure. When the board failed, the 

program that controlled and logged 

data from the boards stopped and 

could not be restarted until the 

personal computer was re-booted. 

Thus, the amount of time that the 

board was malfunctioning was not 

known.

The tests showed that smoke could 

cause failures in digital electrical 

equipment during the first 24 hours of 

exposure. It was hypothesized that 

intermittent shorts from soot that 

bridge conductors caused the failures 

in the circuits. Circuit bridging is the 

most likely cause because the failures 

were intermittent and most other 

failure modes, such as metal loss,

would be expected to be continuous 

rather than intermittent.

These tests contributed to the general 

knowledge of smoke testing and 

indicated likely failure scenarios. Few 

of the physical and chemical 

measurements from this test showed 

significant differences between 

conditions that caused failure and 

those that did not cause failure. The 

only measurement to show an 

outstanding difference between the 

PVC smoke and the other exposures 

that did not cause failure was taken 

with the Rohrback probe, which 

measures metal loss. These tests 

indicated the need to improve software 

for the next experiment on the 

experimental digital safety system 

because the test program stopped 

when the failures occurred. A better 

testing program would accumulate 

data and record it even if the 

communication was temporarily 

interrupted.

31

cr- 3
ORN7L Experimental Digital 

Safety System 

Digital safety systems for nuclear 

power plants have the potential to be 

radically different from the present 

analog systems. For example, the 

digital safety system is expected to 

convert data from the analog 

transducers in various locations in the 

power plant to digital signals, and 

relay the signals back to the control 

room, where decisions on plant control 

can be made. The digital signals can 

be multiplexed so that all transducer 

signals may be transferred back to the 

control room on the same network 

cables instead of using separate cables 

for each signal. One concept that may 

not change is the idea of multiple

channel (e.g., two out of four) voting to 

determine if the plant should be 

tripped. 

Because of functional and 

technological differences from analog 

systems, digital safety systems as a 

whole were studied to determine what 

environmental conditions could cause 

failure in an example system. ORNL 

assembled an experimental digital 

safety system from COTS parts that 

duplicated some of the features of 

advanced digital safety systems. The 

COTS parts included personal 

computers (of an industrial variety), 

fiber optic modules (FOMs) and the 

same type of multiplexer boards as 

used in the first test series, the Ana]og 

Devices boards with D/A and A/D 

units. This system was exposed to 

various environmental stressors and 

the results were reported in Reference

3. The environmental stressors 

included high humidity, 

electromagnetic interference, high 

temperature, and smoke. 

The smoke exposures were performed 

at SNL. Subsystems of the 

experimental digital safety channel 

were subjected to various levels of 

smoke that approximated credible 

control and general room fire 

scenarios (a control panel fire and a 

small in-cabinet fire) while the system 

was in operation. These levels ranged 

from a low of 3 g/m 3 to 200 g/m 3. 

Before, during, and after each of these 

tests, the system was monitored to 

check that it worked. Several fire 

suppression simulations were included 

in the tests. This included the 

addition of humidity in the form of 

steam and CO 2 from a fire 

extinguisher. The monitoring period 

extended for up to 18 hours after the 

start of the exposure because this is a 

reasonable period to assess post event 

survivability. 

It is noteworthy that the computers 

under test exhibited no permanent 

failures or serious upsets such as 

processor lockups resulting from 

deposition of smoke particles, 

although soot was spread throughout 

each chassis by the computer's fan. 

However, communication link errors 

were observed at all levels of smoke 

density, ranging from a few network 

retransmissions at low smoke 

densities to serial communication 

time-out errors at higher smoke 

densities. The FOMs were enclosed in 

a plastic case and when exposed to 

smoke in this manner did not have 

any failures, but when the FOMs were

32
opened and their printed circuit 

boards exposed, the FOMs were found 

to be vulnerable to smoke deposition. 

Figure 18 shows the FOMs before the 

smoke exposure and Figure 19 shows 

the FOMs after the smoke exposure. 

The severity of the communication 

link errors generally increased as the 

smoke concentration increased. The 

results of the high humidity (85% RH)

tests showed that humidity may be an 

important factor in creating temporary 

shorts, and its adverse effect on digital 

boards is likely to increase with the 

severity of the smoke exposure. The 

CO 2 had very little effect on the 

equipment, although the temperature 

in the chamber dropped drastically.

Figure 18. Fiber optic modules were tested in different orientations.

Figure 19. The fiber optic module that was open failed.

33

CiI1
Once the system was exposed to 

smoke, the system tests in a nonsmoke 

environment were no longer free of 

communication link errors. This 

occurred even after the computer was 

cleaned with a Freon-based electronics 

cleaner. This behavior underscores 

the potential difficulty of thoroughly 

ridding a previously exposed board of 

all residual smoke particulates 

through cleaning and may point to the 

need to replace all exposed circuit 

boards after a fire as a matter of 

policy. These computers were cleaned 

with a halogenated cleaner similar to 

Freon. This may not have been the 

optimal method for cleaning. 

Cleaning is discussed further in 

Section 4. 

Conclusions from the system tests 

In summary, the experimental digital 

safety system was more smoke 

tolerant than the multiplexer boards 

because of its use of networked 

communication rather than serial 

communication. The networked 

communication system included more 

error checking and the ability to re

send the data if the data was not 

received correctly. The effect of the 

smoke on the network was to delay 

communication at the low levels of 

smoke and cause serious time-out 

errors at the higher smoke 

concentrations. Plastic cases provide 

smoke protection of the printed circuit 

boards for the FOMs, but without 

these cases the FOM printed circuit 

boards are vulnerable to smoke. High 

humidity and high smoke 

concentration increases the likelihood 

of failure, and electrical circuits 

cannot be adequately cleaned with a

Freon-based cleaner after a smoke 

exposure. 

2.3.2 Component Packaging and 

Circuit Bridging 

The digital system tests showed that 

smoke could cause intermittent 

failures of electronics. Circuit 

bridging was suspected to cause these 

failures, but, because of the complex 

nature of the system tests, this 

suspicion could not be proved. To 

pinpoint the cause of intermittent 

failures in smoke, the component tests 

were designed to measure short-term 

smoke effects such as circuit bridging 

in typical component packages and 

under varied environmental factors to 

determine which factors were most 

significant in increased circuit 

bridging (i.e. shorts). The factors that 

were studied can be divided into three 

categories: smoke generation factors, 

component technology and packaging, 

and circuit board protection with 

coatings or enclosures. Measuring 

leakage currents and converting those 

currents to resistance in ohms tested 

the likelihood of circuit bridging. Loss 

of surface insulation resistance can 

cause problems in many components 

and circuits; for example if the 

resistance between the supply voltage 

and input signal drops, a false signal 

may be received by the device input 

from the voltage supply. 

Test Component packages and 

circuit boards 

Discreet components are packaged for 

two methods of mounting on circuit 

boards, plated through hole (PTH) and 

surface-mounted technology (SMT).

34
For PTH components, the circuit 

board must be drilled for the leads, the 

component leads inserted into the 

board, and the leads soldered to the 

underside of the board. For SMT 

components, the component package is 

soldered directly to the front side of 

the board. In large scale operations, 

this is accomplished by adhering all of 

the components to the face of the 

board, then passing the board through 

a wave solder machine, which heats 

the solder pads on the boards and 

makes contact to the component. The 

older PTH mounting results in larger 

component packaging than the SMT 

for the same device as shown in Figure 

20. Mass produced boards are more 

frequently populated with SMT 

components because they are faster to 

build on a large scale and can be 

packed more densely, resulting in 

smaller circuit boards. 

A test circuit board was designed for 

use with empty chip packages to

measure the effect of bridging of leads 

and traces on circuit boards by smoke 

(Figure 21). Both PTH and SMT 

components were placed on these 

boards to compare the effect of lead 

spacing and density on circuit bridging 

by smoke. Adjacent leads were biased 

with either 5 V de or grounded, and 

the leakage currents between the 

leads were monitored. Surface 

insulation resistance measurement 

boards, designed by the IPC, were 

included in these tests to measure the 

effect of circuit bridging for printed 

circuit traces also by measuring 

leakage currents for a range of bias 

voltages (Figure 22). These boards 

included 4 identical interdigitated 

comb patterns. Alternating traces on 

the IPC surface insulation resistance 

boards were either grounded together 

or biased with 5, 50 or 160 V dc. 

Leakage currents were measured for 

the biased patterns and the leakage 

current for the grounded pattern was 

measured for with a 5 V dc bias.

Figure 20. SMT chip packages are much smaller than PTH chip packages 

for the same type of device. 

35

C15

L
Figure 21. The component 

packaging tests featured both 

PTH and SMT component 

packages.

Figure 22. Surface insulation 

resistance was measured on an 

interdigitated comb pattern, IPC

B-24, developed by the Institute 

for Interconnecting and 

Packaging Electronic Circuits.

Two chips with functional components 

were included in these tests, optical 

isolators and 16K memory chips. The 

role of the optical isolator is to provide 

a directional gate that allows pulses to 

pass from input to output, but not 

from output to input. They also limit 

the voltage of a pulse that is passed 

through the gate. The optical isolator 

performs this function by converting 

electrical input to an optical signal, 

then converting the optical signal back 

to an electrical signal all within a 

small chip. A square wave pulse was 

transmitted through the optical 

isolator and the rise time, delay and 

amplitude of the transmitted pulse 

was measured. 

16 K memory chips in both 

hermetically sealed ceramic packages, 

and plastic packages were tested in 

these smoke tests. The 16K memory 

chips were biased with 5 volts, but no 

signals were applied to the chips. 

These were tested before and after 

each test to measure functionality. 

Component packaging test setup 

Many factors determine a fire 

environment and the smoke it 

produced, such as the availability of 

oxygen, fuel, humidity level, and fire 

size. To determine which were the 

most significant, 27 tests were run, 

and some factors were varied. The 

humidity levels that were compared 

were between 30% and 70% RH. High 

and low fuel loads were compared. 

The high load corresponded to burning 

100 g/cm 3 and low fuel loads to 3 

g/cm 3. Two burning modes were 

compared, flaming (at 50 kW/m2

) and 

smoldering (at 25 kW/m2). The burn

36

r,'1(
mode would be expected to change the 

smoke products in two ways: different 

chemical products can be produced at 

different temperatures, and the mass 

loss rate of the fuel is slower if the fire 

is smoldering. The fuel mixture was 

varied slightly, some of the fuel 

included 5% PVC because of the great 

concern regarding PVC and metal loss 

failures of electronics. CO 2 in the form 

of a fire extinguisher was added to 

some of the tests as well as galvanized 

metal to determine if galvanic salt 

solutions would form and drip on 

electronics. 

For each of the 27 tests, 4 chip 

packaging boards and 4 SIR boards 

were tested using different 

configurations to determine the effect 

of circuit protection. These 

configurations included: inside of a PC 

chassis with a fan, face up in the 

smoke chamber, face up in the smoke

chamber with a sprayed on acrylic 

coating, and outside of the smoke 

chamber (Figure 23). Leakage current 

measurements from all of the chips on 

the boards and the SIR patterns were 

collected every 3 minutes. The 

memory chips and optical isolators 

were placed inside of the smoke 

chamber, but were not protected with 

either coating or chassis. 

Results of component packaging 

and circuit bridging 

Resistance between leads on the 

component packages dropped when 

exposed to smoke as shown in Figure 

24. To further analyze this data, the 

leakage current measurements were 

averaged over two periods: (1) the 

period when the smoke was in the 

chamber and (2) after the smoke was 

vented. Equations were derived to 

model the resistance as a function of 

the different factors. Factors that

Figure 23. The component packaging tests were performed in the 

large smoke chamber. 

37

cli

M
0.5 1.0 1.5 2.0 

Time(hrs)

Figure 24. Resistance between leads of surface mounted ceramic package 

during a smoke exposure.

were most significant were determined

by their frequency of occurrence in the 

equations. The most significant factors 

were humidity, fuel level, and burn 

mode.

The tests showed that the synergistic 

effects of smoke and humidity are 

higher than for humidity alone. For 

low fuel loads, the resistance generally 

dropped during the smoke exposure 

but recovered after the smoke was 

vented. This recovery was not often 

the case with the high fuel loads; once 

the circuit was shorted, it never 

recovered. The burning mode (flaming 

vs. smoldering) did not affect the 

resistance as much as the humidity or 

fuel amounts; however, the flaming

mode did cause the components to 

short slightly faster. Soot analyses for 

Cl, Br, and SO4 show a low correlation 

with burn mode. The amount of fuel 

burned is determined by both fuel and 

oxygen availability. If a large amount 

of fuel was burned by smoldering, only 

20% of the fuel was consumed (80% of 

mass left after burning), while if a 

small amount of fuel was burned in 

either mode or a large amount of fuel 

was burned by flaming, approximately 

40-50% of the fuel was burned. The 

large fuel load did not seem to have 

enough time to be consumed if 

smoldered since the lamps were on for 

only 15 minutes. Smoke deposited a 

film on the surface of the electronics, 

which was black and powdery if the

38

1E+14

1E+12

1E+10 

S 1E+06

I E+04

1 E+02

0.0
fuel was burned in the flaming mode 

with adequate oxygen and was white 

and oily if produced by smoldering. If 

so much fuel is burned that the flame 

extinguishes due to lack of oxygen, the 

film is black and oily. Nowlen 23 

reports that on large-scale cable fire 

tests, black oily deposits are common. 

The insignificant factors that were 

tested included presence of PVC, 

addition of CO 2 as a fire suppressant, 

and inclusion of galvanic metal in the 

smoke chamber. PVC smoke has been 

found by other researchers to be a 

significant cause of metal loss, but 

metal loss is not a significant 

electrical failure mechanism unless 

the loss is significant as in equipment 

exposed to the smoke and soot for an 

extended period (months). Only a 

small proportion of the fuel was PVC 

in the high fuel load tests, and with 

the high fuel load, many of the 

components shorted without PVC. 

There is also little correlation between 

the Cl deposition found by chemical 

analysis and the presence of PVC. 

Although the other cable materials did 

not contain PVC, some had high 

proportions of Cl and Br, which are 

typically used as fire retardants. No 

other cable materials were singled out 

for study in these tests. 

The addition of CO2 as a fire 

suppressant did not affect the 

resistance adversely, supporting 

results from the ORNL smoke 

exposures. The addition of CO 2 may 

be beneficial to the electronics by 

cooling the room and blowing away 

some of the soot deposits. These 

findings are also supported by tests on 

the effect of CO 2 on computers.'z

Galvanic metal was included in the 

smoke test because earlier reports 

showed that it could cause additional 

failures of electronics by combining 

with smoke and humidity to create 

galvanic salt solutions, which then 

drip on electronics." Galvanic metal 

is often used in electronic enclosures. 

The Zn in the metal will combine with 

Cl in the smoke to produce ZnC!2. 

ZnC12 readily combines with humidity 

and forms a thick liquid. Although a 

greasy film formed on the galvanic 

metal that was suspended in the 

smoke exposure chamber, the film 

never accumulated enough water to 

drip. Instead, the metal piece formed 

a surface upon which some of the 

smoke deposited. The overall effect 

was to reduce deposition on the 

surface of the electronics and reduce 

the negative effect of the smoke. On 

some high-humidity tests, water 

appeared to be collecting at the base of 

the PC chassis. This water did not 

affect any of the components. Overall, 

these tests were found to be of an 

inadequate scale to properly assess 

the importance of this factor. 

Resistance measurements on the comb 

patterns indicate that patterns are 

more affected by smoke if the applied 

voltage is higher. At higher voltages, 

comb patterns have higher leakage 

rates before the smoke is applied and 

continue to produce more leakage 

current throughout the exposure. 

Visually, soot tends to accumulate 

more around the high voltage 

patterns. The 160-V pattern was 

observed to be arcing during the 

smoke exposure.

39
Comparison of the functionality of the 

16-K memory chips showed that the 

ceramic packages were more robust 

than the plastic packages in a smoke 

environment. The higher failure rate 

of plastic packages may be due to 

penetration of the plastic by humidity 

rather than shorting outside of the 

packages because the spacing between 

leads is similar in the plastic and 

ceramic packages tested. Most 

common digital electronics, however, 

use inexpensive plastic packages. 

Hermetically sealed packages are 

significantly more expensive and are 

not typically available unless used for 

military applications. 

Since high fuel load, high humidity, 

and a flaming fire are the most 

difficult conditions for maintaining

high resistance among leads, the 

results of the tests performed with 

these conditions were investigated 

further. For many components, smoke 

exposures with these conditions 

resulted in interlead resistance below 

the recording levels considered 

reliable (<103 W). The amount of time 

required to reach a resistance level 

less than 103 9 was averaged for four 

tests, all of which had high fuel load, 

high humidity, and a flaming fire and 

are presented in Figure 25. The 

longer it takes to short the component, 

the more smoke tolerant it is. 

Packages with larger lead spacing 

tended to take longer to short than 

closely spaced lead packages. 

Comparisons of the bare chip packages 

showed that the transistor outline can 

(TOC) takes the longest to short, while

* Surface mounted EPlated-through hole

E TOC

0.3

Ceramiclt Pack 

* Ceramic LC* SOlO

0.6 

Spacing between leads (mm)

E 

Plastic• Ceramic 

DIP

0.9 1.2

Figure 25. The time of failure increased with the spacing between leads 

for different components. 

40

clcý

36:00 

28:48 -

221:36 

0 

Z 14:24 

E 

07:12 

00:00

0
the small-outline integrated chip 

package shorted sooner. The spacing 

between leads for these packages can 

be compared in Figure 25. Both the 

lead spacing and separation of the 

TOC are larger than those of the 

small-outline integrated chip are. In 

general, surface-mounted technology 

SMT packages were more closely 

spaced than PTH packages, and PTH 

packages were better able to maintain 

a high resistance. 

Figure 26 compares different 

protective treatments-an acrylic 

coating* and enclosure in a fan

ventilated chassis-on different types 

of component packaging. The acrylic

coated ceramic leadless chip carrier 

and the chassis-enclosed ceramic dual

1:30 7 - m-.- IFh•lI

E 

E 

E 

Pin-line package (DIP) were not 

included because the instrumentation 

wires to these components broke. In 

general, acrylic-coated chip packages 

took longer to short than either the 

bare or chassis-enclosed packages. 

The chassis-enclosed packages shorted 

in less or equal time than the bare 

packages, except for the ceramic 

leadless chip carrier. The acrylic 

coating improved leakage resistance 

somewhat and hence performance. 

Bare components exposed directly to 

the smoke performed the same or 

better than bare components inside a 

fan-ventilated chassis. The fan tended 

to collect soot into clumps and fling 

them onto the boards. The bare 

boards in the chassis were coated with 

larger clumps of soot than the bare

1:00 

0:30

0:00

Ceramic Ceramic Ceramic Transistor Plastic Small Plastic 

Leadless DIP Flat Pack Outline DIP Outline Leaded 

Chip Can Integrated Chip 

Carrier Chip Carrier

Figure 26: The time to failure due to smoke exposure is longer for acrylic

coated boards than uncoated or chassis housed boards.

41
boards outside of the chassis, which 

were coated with a more even layer of 

soot. 

Conclusions from the Component 

Tests 

Several conclusions can be drawn from 

the results of these circuit-bridging 

tests. They include: 

1. Smoke causes circuit bridging in 

components. Circuit bridging 

increases leakage currents and can 

cause failures because stray currents 

cause errors in digital circuitry. 

2.The most significant factors that 

affect circuit bridging are the amount 

of smoke, humidity level, and burning 

mode. Other factors such as the use of 

CO 2 as a fire suppressant and the 

presence of small amounts of PVC are 

not as important. The test for 

galvanic metal was not of adequate 

scale to provide conclusive results. 

3. Although surface deposits cause 

some circuit bridging, the leakage is 

highest while smoke is in the air. 

4. Failure thresholds from circuit 

bridging are dependent on type of 

circuit, device technology, chip 

packaging, and circuit layout. 

5. Smoke can randomly short or 

partially short the contacts of a 

component, and the component 

reaction depends on the impedance of 

the input or output of the device and 

which contacts are shorted. 

6. Conformal coatings add some 

protection to circuits against bridging 

due to smoke.

7. Mechanical protection may also 

protect circuits, but the presence of a 

ventilation fan may negate this effect. 

A fan may draw more smoke into the 

electronics and cause more smoke 

bridges 

2.3.3 Chip Technology Tests 

The component packaging tests 

showed that smoke causes circuit 

bridging, which results in increased 

leakage currents. Increased leakage 

currents can cause digital failure by 

shorting a high to a low output, and 

thus transmitting the wrong signal 

through a circuit. As smoke is neither 

a perfect conductor, nor a perfect 

insulator, the effect of the smoke is 

more like a resistor placed in the 

circuit rather than a good conductor. 

In-situ measurement of resistance 

changes in smoke was difficult 

because of the wide range of values 

during a smoke test. Typical clean 

circuit boards maintain resistances of 

approximately 1012 Q between traces. 

When smoke is added the resistance 

values may drop below 103 Q2, but 

typical values were in the range of 106 

Q2. We found that measurements of 

resistance ranges from 1012 to 102 Q to 

be difficult during our smoke tests 

because of time limitations. Although 

test equipment can be used in 

autorange mode, this mode is 

significantly slower and peak smoke 

measurements were lost. As our early 

measurements showed that most of 

the resistance measurements were 

greater than 103 Q the later 

measurements were limited to values 

above 103 Q.

42
The amount of leakage current needed 

to cause digital failure depends on the 

type of logic component. The chip 

technology tests measured the amount 

of conductance (in terms of resistance) 

that would cause false data in 

different types of logic chips. Rather 

than exposing the logic chips to 

smoke, the effect of increased 

conductivity that the smoke would 

have caused was simulated using a 

variable shunt resistor. 

Test chips selected 

A variety of logic chip technologies 

were chosen to test for their 

vulnerability to increased leakage 

currents. At the time of the tests 

(1996), two types of transistor-to

transistor logic (TTL) were widely 

available and were tested: Fast 

Schottky (FAST) and Low Power 

Schottky (LS). The advantage of TTL 

chips is that they are fast and simple 

to use in circuits. (Ground is a low 

state, 4.5 V and higher is a high state, 

and the chips are biased with 5 V). At 

publication time of this report the LS 

version is still available, but the FAST 

is no longer widely manufactured. A 

wide variety of complementary metal

oxide semiconductor (CMOS) gates 

were also available at the time: 

standard metal gate (MG), high

performance silicon gate (HC), and 

advanced CMOS (ACT). The 

advantage of these chips is their small 

size and low power usage. The CMOS 

gates can be biased at different DC 

voltages and their high states vary 

according to the bias level of the chips. 

These CMOS chips are still available. 

The fastest chips that were widely 

available were the emitter-coupled

logic (ECL) chips, which at the time 

were available in the 100K or 10H 

models. Other varieties of ECL chips 

are now available. The advantage of 

ECL chips is their speed, but they use 

more power than the CMOS and are 

more difficult to install in a circuit 

than the TTL chips. A list of chips 

that were tested is included in Table 

6. 

Test setup for chip technology 

tests 

For simplicity, a steady-state test was 

designed to determine the critical 

surface insulation resistance below 

which a logic gate would provide 

faulty logic values. Motorola logic 

AND and OR devices, packed 4 to a 

package were tested on a prototype 

board. Figure 27 shows a sample 

circuit diagram where the outputs of 

two gates are shorted with a variable 

resistor. The resistance was started at 

a high value, and the value was 

reduced until the output of the circuit 

changed. The resistance value that 

resulted in a change of output was 

determined to be the critical 

resistance below which the circuit 

would fail. 

Results 

The results are shown in Table 6. The 

components that tolerated the lowest 

resistance had the highest output 

drive current. Components with a 

high tolerance to resistance loss are 

more tolerant of smoke. The 

components with high tolerance 

included the FAST (TTL) chips. 

Higher supply voltage on the HC 

(CMOS) chips improved smoke

43
Figure 27. The chip technology test circuit introduced a variable shunt 

resistance into a logic circuit to simulate the effect of smoke.

tolerance. Standard CMOS 

components were the least tolerant to 

a decrease in resistance, especially at 

low power supply voltages. Resistance 

values of 103 Q would definitely cause 

faulty logic for CMOS gates. Other 

gates such as TTL and HC would be 

more resistant to smoke. Although 

only Motorola components were used 

in these tests, the specifications from 

other companies indicate that similar 

technology families should react in a 

similar fashion to smoke. 

Chip technology conclusions 

The failure resistance varied between 

chip technologies and was highly 

correlated with the output current 

drive. This is especially evident for 

the standard CMOS, where the drive 

current changes with supply voltage. 

Gates with higher current drives can 

better supply the current necessary to 

maintain a voltage level despite a 

shunt. For the CMOS family, higher 

supply voltages correspond to higher 

drive current and lower input 

impedance. CMOS gates with higher 

supply voltage will be more resistant

to the effects of smoke. The circuit

bridging smoke exposure tests only 

measured down to a resistance of 1000 

2; this level of resistance would cause 

metal gate CMOS failure, but other 

components would not be likely to fail. 

The circuit bridging tests can be 

viewed as providing conservative 

estimates of smoke levels that would 

cause errors in logic circuits. 

Note that in these experiments, 

measurements were made using 

steady state values. The effect of 

smoke on fast switching circuits was 

not taken into account. The increased 

conductance of circuit boards by smoke 

can slow switching, because some of 

the current can be drained from the 

normal circuit path and thus logic 

changes that are expected to occur 

when a particular voltage level is 

attained would be slower. If a range 

of conductance of the circuit board 

were defined as likely to cause failure, 

this test would show an absolute 

upper level. Any circuit containing 

these types of chips that were exposed 

to smoke to cause these levels of 

conductance would always fail.

44
Table 6. Critical Resistances for Failure from Smoke 

Logic family Supply voltage Output drive current (mA) Critical resistance 

(VI) (2) 

LS(TTL) +5 -0.4/8 100 

FAST(TTL) +5 -1.0/20 31 

Standard CMOS +5 +/-0.88 1220 

Standard CMOS +10 +/-2.25 605 

Standard CMOS +15 +/-8.8 490 

HC (CMOS) +2 +/-0.02 120 

HC (CMOS) +4.5 +/-4.0 68 

HC (CMOS) +6 +/-5.2 56 

FACT (CMOS) +3 +/-12 30 

FACT (CMOS) +4.5 +/-24 30 

FACT (CMOS) +5.5 +/-24 25 

100K (ECL) -5.2 50-4 load 105 

looK (ECL) -5.2 50-4 load 107

2.3.4 Functional Circuits 

The component packaging and chip 

technology tests showed that circuit 

bridging is a likely mode of digital 

circuit failure for standard CMOS 

circuits. Although most of the logic 

and decision making circuits of an 

advanced digital safety system would 

be primarily CMOS or TTL type 

components, the rest of the circuit, 

especially the sensors or actuators, 

could be very different. The functional 

circuit tests compared the effect of 

smoke on very different simplified 

circuits. The test target was a circuit 

board designed by the Low Residue 

Soldering Task Force (LRSTF) to 

study the effects of no-clean solders. 

These tests showed (1) the 

predominant effects of smoke on 

electronics, (2) which types of circuits 

are most vulnerable and (3) the 

conditions under which circuits are 

most affected. The advantage of 

exposing simplified circuits is that the

effect on each circuit is easier to 

determine than if the circuit had more 

functions and components. 

Test boards for functional circuit 

tests 

Smoke may cause three different, 

immediate effects on electrical 

circuits: (1) it may lower resistance by 

acting like a shunt; (2) it may increase 

resistance by attacking solder joints or 

adding debris to connectors; and (3) it 

may increase stray capacitance. 

These effects were tested on the 

functional circuit board designed by 

the LRSTF (shown in Figure 28). The 

LRSTF designed this board to test 

extremely different types of circuits 

and measure how these circuits react 

to different levels of cleanliness. Since 

the physical differences between PTH 

and SMT components are significant, 

both of these types of components are 

included on the boards. As smoke 

exposure can be considered an

45
extreme case of soiling a printed 

circuit board, the LRSTF test board 

was adopted for this smoke program to 

compare the effects of smoke on the 

different circuits. 

The high-voltage low-current (HVL-C) 

circuit, leakage current 

measurements, high-frequency 

transmission line (HFTL), and high

speed digital (HSD) circuits 

determined effects of smoke as an 

added shunt. Both the HVLC circuit 

and the leakage current 

measurements were on circuits with 

high (>50 MIV) resistance. When 

smoke acts as a shunt in these 

circuits, overall impedance drops and 

current through the circuit increases. 

The HFTL consists of two parallel 

transmission lines whose cross talk 

from one transmission line to the 

other is measured. For a clean circuit 

board there is high impedance to any 

cross talk, but when smoke is added, 

the impedance can be lowered and 

more cross talk results. The HSD 

circuit consists of a series of NAND 

gates that propagate a signal pulse. 

The action of a shunt resistor can add 

a delay to this signal. The technology 

of these gates are TTL (FAST), 

however, and as shown in Section 

2.3.4, are not expected to be very 

vulnerable to smoke. 

The high current low-voltage (HCLV) 

circuit indicated the effect of increased 

resistance. This circuit had very low 

initial resistance (<3 ohms) and was 

good for measuring any increases in 

resistance. 

The low-pass filter (LPF) determined 

the effect of stray capacitance. This

circuit contained both capacitors and 

inductors and is designed to cut off 

frequencies higher than 1 GHz. In 

reality, the PTH circuit cuts off at 4 

GHz, and the SMT circuit cuts off at 8 

GHz, even though they contain the 

same component values. The PTH 

circuit has a lower cut off frequency 

because the components are larger 

and the stray capacitance is larger. 

Smoke might also increase 

capacitance and increased capacitance 

should change the cut-off frequency of 

the filter.

46
Functional circuit smoke test 

conditions 

The functional circuit tests were 

performed in the small smoke 

exposure chamber. Four functional 

boards were tested at one time, one 

coated with an acrylic spray and one 

uncoated board were placed inside the 

smoke chamber and one coated and 

one uncoated board were placed 

outside of the smoke chamber as 

controls. Since the component 

packaging tests showed some smoke 

effects although they were outside of

the smoke chamber, the control boards 

were wrapped in plastic to protect 

them from stray smoke that escaped 

the smoke exposure chamber. Six 

smoke exposure conditions were tested 

as shown in Table 7, where the fuel 

indicated is ground mixed cable in 

grams of fuel provided for the small 

200-L smoke exposure chamber. All 

smoke tests were conducted at a 

starting humidity of 75% as that was 

the worst case found in the previous 

component packaging test conditions.

Figure 28. The functional board tested various types of circuits to 

determine their relative vulnerability. 

47

C7-O
All of the circuits were individually 

measured before and after the smoke 

test. The fuel was placed in the 

combustion chamber and the humidity 

was increased to 75% RH. The lamps 

were turned on to the required heat 

flux for 15 minutes, and then the 

boards were allowed to sit in the 

smoke for 45 minutes with the lamps 

off. The smoke was then vented and 

the boards were allowed to continue to 

be monitored for 23 hours. 

Throughout the smoke test the boards 

were monitored every 3 minutes using 

a computer controlled process.

Functional circuit test results 

The high fuel load and high heat flux 

levels caused the most smoke effects. 

Smoke effects that resulted from 

increased conductance of smoke were 

highest during the smoke exposure 

and fell after the smoke was vented 

from the chamber. Smoke effects that 

resulted in increased resistance 

increased throughout the monitoring 

period. These results are summarized 

in Table 8. More details on these tests 

and comparisons between the control, 

coated and uncoated boards can be 

found in reference [8].

Table 7. Smoke exposure conditions for the functional circuit tests.

Fire Scenario 

Large cabinet fire in control 

room 

General Area fire 

Equipment in small cabinet 

fire

Condition Fuel /air ratio 

(g/mý')

1 

2 

3 

4 

5

3 

3 

25 

25 

50 

50

Heat flux (kW/m2 ) 

25 (smoldering) 

50 (flaming) 

25 

50 

25 

50

50

Table 8. Summary of uncoated functional circuit wo 

Circuit Smoke effect

HYIILC PTH and SMT 

HCLV PTH 

HCLV SMT 

HSD PTH 

HSD SMT 

LPF PTH and SMT 

HFTL 

PTH solder pad leakage 

SIR

(greatest change) 

R changed from 50 MQŽ to < 1 

M ý? 

No change 

Voltage needed to produce 1 A 

changed by 0.034 V 

Fall time changed from 2.9 to 

3.2 ns 

Fall time changed from 3.7 to 

4.3 ns 

No change 

Increase of 23 db 

R changed from 1013 Q to 107 Q 

R changed by 10W

rst case results. 

LRSTF failure criteria 

AR>5 MQ 

AV > 0.365 V 

Fall time >7 ns 

Change > 5 db 

Change >5 db 

R< 5 x 105 t2 

Comparison only

48

6
Conclusions from functional 

circuit tests 

The predominant smoke effect was 

lowered resistance (increased 

conductance), and the most vulnerable 

circuits were those that had high 

input impedance. Clean printed 

wiring boards have resistances higher 

than 1012 Q between traces. When 

smoke lowers electrical resistance in 

the vicinity of a circuit, the relatively 

low impedance of the smoke will 

bridge a high-impedance circuit. 

Thus, the circuits that were most 

affected were those that reacted to 

reduction in surface insulation 

resistance (i.e., added shunt current 

paths), such as the HVLC circuit. For 

the HVLC circuit, virtually all of the 

current was transmitted through the 

smoke and soot deposits instead of 

through the circuit when the fuel level 

was higher than 25 g/m3. However, 

smoke did not affect the HSD circuit; 

the shunt resistance was not low 

enough to affect the FAST (TTL) logic 

chips in the HSD circuits. FAST 

(TTL) chips have a high tolerance to 

smoke because they have low 

impedance and high output current. If 

the HSD circuit contained a CMOS 

chip instead, it might have been 

damaged just as the memory chips in 

the earlier component tests were, 

because CMOS chips have lower 

output current drive. 

The increased conductance induced by 

smoke was highest during the smoke 

exposure and was reduced by venting. 

Therefore, smoke and not just surface 

deposition causes increased 

conductivity by orders of magnitude. 

Values of resistance for the smoke can

be estimated to be in the MQ range 

and hence are significant for high 

impedance circuits and insignificant 

for low impedance circuits. Results 

from tests earlier in this program 

showed that a fuel level greater than 

100 g/m3 caused increased 

conductivity that remained after the 

smoke was vented. This implies that 

for higher smoke densities, the 

increased surface deposition will cause 

lingering effects. The tests showed 

that a polyurethane coating helped 

considerably to prevent smoke from 

increasing conductance or damaging 

solder joints. The polyurethane acts 

like an insulator, keeping the smoke 

from shorting the components. 

The resistance of an uncoated HCLV 

SMT circuit increased by 1 to 2% when 

it was exposed to fuel levels of 25 g/m 3 

or greater, but the PTH circuits and 

all coated circuits remained stable. 

For these low impedance devices, 

smoke did not lower resistance around 

the circuit enough to cause circuit 

bridging. Instead, smoke increased 

the resistance by corroding solder 

joints. These types of changes are 

more apparent in low-impedance 

circuits. Venting the smoke did not 

decrease these effects; however, they 

made relatively small changes in 

circuit performance-less than 2% in 

the worst case. Smoke did not cause 

any obvious change in the low-pass 

filter circuit, indicating that there was 

little change in stray capacitance.

49
2.4 Recent Smoke Test 

Results 

This section describes tests that were 

not previously published in this 

NUREG/CR series,7 ,

8 the conformal 

coatings, digital throughput, memory 

chip and hard disk drive tests. 

2.4.1 Conformal Coating of 

Functional Circuit Boards; 

Conformal coatings are plastic 

materials that are either sprayed, 

brushed, or vacuum deposited on 

electronic circuit boards to protect 

them from contamination. During 

component package tests, an acrylic 

conformal coating on the boards 

significantly reduced the adverse 

effects of smoke. The study with 

functional circuits coated with 

polyurethane also showed that

conformnal coatings can add 

considerable protection to circuits. 

This section describes a series of 

smoke tests on the functional board to 

compare the performance of five types 

of coatings. 

Types of coatings tested 

While the conformal coatings available 

on the market number in the 

hundreds, there are five major 

categories of coatings: acrylic, epoxy, 

parylene, polyurethane, and silicone. 

One representative from each of these 

five coating categories was included in 

these tests as shown in Table 9. 

Choices of coatings to be used for a 

product are usually based on the 

ability to rework the board, thermal 

conductivity, electrical characteristics, 

cost, and ease of application.

Table 9. Coating Types 

Coating tvpes Brand Product Application thickness 

(mils)

How applied

Acrylic 

Epoxy 

Parvlene 

Polyurethane 

Silicone

Humiseal 

Enmibar 

Union Carbide 

Conap 

Dow

1B-31 

UV 1244 

Type C 

CE-1155 

3-1765

2.5 

2.5 

0.75 

2.5

Dipped 

Dipped 

Vacuum deposited 

Dipped 

Dipped

50
Test setup 

A selection of different coatings was 

applied to LRSTF functional circuit 

boards before smoke exposures under 

high-fuel, high-humidity conditions to 

determine how different conformal 

coatings compared. A single product 

from each category was applied to the 

boards. The coatings were supplied by 

Specialty Coating Inc.,* who suggested 

which coatings would best represent 

each category. 

The acrylic coating, Humiseal 1B-31, 

is a single-component; fast drying 

coating that can be cured at room 

temperature. This coating was 

deposited by dipping the board in the 

acrylic liquid and allowing it to dry. 

The epoxy coating, Envibar UV1244, 

is also a single-component coating. 

This coating was applied by dipping, 

but it was cured under UV radiation 

(it does not cure unless heated to 160 

°C). Parylene, Union Carbide type C, 

was the most difficult and expensive 

to apply because it is vacuum 

deposited onto the boards, but it has 

the advantage of being a very tough 

coating for the application thickness. 

Vacuum deposition creates a very even 

coating because the deposition is not 

dependent on gravity or on the 

direction of a spray, but is like a cloud. 

Conap polyurethane is a single

component liquid that can be applied 

by dipping and air-dried. The 

polyurethane used in the coatings

tests was a different brand than that 

used in the functional board tests 

discussed in Section 2.3.5. 

The Dow Coming silicone is a one-part 

self-vulcanizing elastomer that can be 

applied by dipping. Silicone can be 

applied thicker than any of the other 

conformal coatings and remains 

flexible after curing. The silicone

coated boards had a tacky feel. 

Thermal conductivity and application 

thickness determine how much heat 

will be dissipated through the 

conformal coating and are important 

considerations in the design of a 

system. All of the conformal coatings 

have high volume and surface 

resistance. The values of electrical 

and physical characteristics of these 

coatings are presented in Table 10. 

The coating manufacturer provided 

these characteristics.

* Specialty Coating Systems, 5707 

West Minnesota Street, Indianapolis, 

IN, 46241

51
Table 10. Coating Properties

Property Acrylic Epoxy

Dielectric 2.5 at 1 2.0 -)at 1 

constant at MHz MHz 

fieq ue n cy 

Dissipation 0.01 0.0139 

factor at 1 MHz 

Dielectric 5500 V/rn 

strength for for 2-mil 

given thickness 1ilm 

Moisture vapor 9.96 

transmission 

( g-rail/ 100 mn--d ) 

Volume (bulk-) 7 -<! 

resistivity (Q2

cm) 

Surface 800 x 10o 4 x 10 

resistiviIty (ý2) 

Dielectric 7500 V iat 

hreakdo, n 2 miI 

voltage 

Dielectric > 1500 V 

withstand voltaire 

Thermal 

conductivity 

(W/m/ C) 

Water absorption

Parylene Poly-urethane

2.95 at 1 

MHz 

0.01" 

5600 V/mil 

for I-rail film

ill

3.5 at 100 

MIH-z. 3.21 at 

1 MHz 

0.0162 

3000 V/mil 

for 2 mil film

Silicone 

2.53 at 100 

Hz 

1100 V/11i1l

0.21

8.8 x 10l 

400 V at I 

mil

1.18x Ix o 

5.66 x 10

1.0 x 10o

1500 V

0.08 0. 13

52

<0. I -
Coating Test Conditions 

The conformal coatings were expected 

to provide some protection for the 

LRSTF functional circuit boards; 

hence the boards were exposed to 

more aggressive rigorous testing than 

in functional circuit tests. Therefore, 

a large amount of fuel, 40 g, was 

burned in the small exposure 

chamber. The fuel load was 200 g/m3. 

The humidity level before and after 

the exposure was 75% RH. Because of 

limitations in multiplexer channels, 

only four boards could be exposed and 

monitored at one time. Figure 29 

shows the four boards in their vertical 

orientation for the smoke test. To 

account for variations among tests, 

uncoated boards were used as controls. 

For each test, at least one board was 

uncoated. The testing plan is 

presented in Table 11. Because the 

smoke tests were variable, we 

included 12 bare "control" boards in 

the test plan. No two boards coated

with the same material were tested at 

one time, and the placement of the 

boards on a particular data channel 

was varied in case the data channel 

had problems (for example, soot can 

get into the connection or a connector 

can fail). All of the 29-pin connectors 

were protected with black electrical 

tape. 

The smoke tests were conducted using 

the same measurement setup as the 

functional board tests. Nine active 

measurements as well as four leakage 

measurements were monitored for 

each coated or bare board. Two 

surface insulation resistance (SIR) 

boards were included in each test, one 

in a horizontal position and one in a 

vertical position. Of the four combs on 

each SIR board, two were biased at 5 

and two at 30 V dc. The leakage 

currents from these boards were 

determined by measuring the voltage 

drop across a ballast resistor.

Table 11. Coating Board Test Plan

Test No. Channel 1 Channel 2 Channel 3 Channel 4

Bare 

Polyurethane 

Epoxy 

Bare 

Acrylic 

Parylene 

Epoxy 

Bare

Parylene 

Silicone 

Bare 

Bare 

Polyurethane 

Epoxy 

Bare 

Acrylic

Acrylic 

Bare 

Parylene 

Parylene 

Silicone 

Bare 

Polyurethane 

Silicone

Bare 

Bare 

Bare 

Acrylic 

Bare 

Polyurethane 

Silicone 

Epoxy

53

1 

2 

3 

4 

5 

6 

7 

8
Figure 29. The small smoke chamber was loaded with four functional 

boards and an IPC-B-24 board for the coating tests. 

54

CSI
Before each test, the circuits were 

measured to determine a baseline 

value using the manual measurement 

box (pretest). The boards were then 

connected to an automatic 

measurement system and 

measurements taken every 3 minutes 

throughout the smoke exposure. For 

analysis, the automatic measurements 

were separated into four time periods: 

before the fire started (preburn), while 

the lamps were on (burn), after the 

lamps were turned off, but before the 

chamber was vented (soak), and after 

the chamber was vented until the end 

of the 24-hour data-taking period 

(vent). The boards were then placed 

back into the manual measurement 

box and a final manual measurement 

made (post-test). 

Results of coatings tests 

The HVLC test measured the 

resistance across a 50-MQ, high

resistance circuit with a 300-V dc bias. 

The data were analyzed by averaging 

data from the four time periods as well 

as the manual measurement taken 

before and after the test. All 

measurements were converted into 

resistances. The HVLC circuit reacted 

in two ways to smoke. For most of the 

control boards, the measured 

resistance dropped as a result of the 

smoke providing alternative current 

paths between the 300 V dc applied to 

the circuit. However, the resistance of 

some of the PTH circuits increased 

during the burning phase, as shown in 

Figure 30. This did not occur for the 

SMT circuit (Figure 31), so this must 

indicate a difference between how 

SMT and PTH components are 

constructed or connected to the circuit

rather than how close the traces are 

placed, because these distances were 

closely matched in the two designs. 

The PTH circuit showed two different 

failure modes, one in which the 

resistance increased (open-circuit 

fault) and one where it decreased 

(short-circuit fault). After the test, 

many of the control board circuits 

recovered almost back to normal. 

The HCLV circuit on the functional 

circuit board is a low-resistance circuit 

(1.5 Q) designed to study corrosion of 

solder joints and traces. Smoke 

increased the resistance of the 

uncoated HCLV SMT circuits, but did 

not affect the PTH. This effect has 

been observed during the functional 

circuit tests on HCLV circuits. All of 

the coatings did a very good job of 

protecting the circuits from this 

failure mechanism. Typical data are 

shown in Figure 32. 

The HSD circuit was a series of NAND 

gates built with SMT and PTH 

components using FAST TTL 

technology. TTL technology uses more 

power than CMOS technology and also 

has more output current drive. A 

square wave pulse was passed through 

a series of gates to determine if the 

smoke would cause the pulse to be 

shorted or delayed. 

Because of the higher smoke 

exposures, the HSD circuit failed in 

tests 3 and 5 with the bare boards, 

whereas they had not failed in 

previous tests. The increased leakage 

currents caused a drop in the output 

from the 5-V dc power supply that 

drove the HSD circuits. Test 3 was 

unusual because the plug for the

55
Burn Soak Vent

Figure 30. For the PTH components in 

polyurethane were the best coatings.

the HVLC circuit, parylene and

56

CIt100

a. 

S 

U 

C S 

w 

50 

S 

S 

0 I

C 

4

0 o

Pretest Preburn Postlest
U 

100 

-- Bare 

"--Acrylic 

- - - -Epoxy 

-... -Parylene 

- - Polyurethane 

-- Silicone 

r 

a 

0IO 

50 - - --- - •-+ 

CM 

~~100 

> ~.40 

774 

Pretest Preburn Burn Soak Vent Posttest 

Figure 31. The paryiene, acrylic, and epoxy coatings worked best on the 

HVLC circuit with SMT components.

57
1.5

1.49

•, 1.48 

0 

tr "t• 1.47

1.46

1.45

-1 0 1 2 3 4 5

Time (hr)

Figure 32. Resistance of the HCLV circuit. The SMT circuit resistance 

increased when smoke was added, but the PTH remained unchanged.

combustion chamber fell out and as a 

result the burn took place at a higher 

oxygen level than before. The reason 

test 5 was different is unknown. 

Because all of the HSD circuits were 

connected to the same 5-V de power, 

all of the data from the HSD for the 

two tests were eliminated because we 

cannot be sure if the anomaly was a 

result of the control board causing the 

short or a coated board. Comparisons 

between the automatic and manual 

measurement systems are not valid 

for this circuit because of differences 

in cable length.

Two types of changes can be observed 

in the HSD circuits: catastrophic 

failure of the HSD circuit where the 

circuit quits pulsing, and a more 

subtle increase in the fall times of the

circuit. Besides the tests that we 

eliminated because of power supply 

problems (which can also be 

considered catastrophic failures), 

catastrophic failures occurred in the 

control PTH circuits. These mostly 

occurred in the vent period, but were 

momentary in nature. Table 12 shows 

the momentary failures that occurred 

in the remaining 9 bare boards.

Table 12. Momentary Failures 

(out of 9 boards)

Time Period PTH SMT

During fire 0 0 

After fire 7 1

58

cett

SI
Comparisons of the PTH fall times 

between pretests and post-tests 

(manual measurements) show that fall 

times increased from 1.6 ns to 2.2 ns. 

The SMT control fall times measured 

at pre- and post-tests were almost 

unchanged. The SMT seemed to be a 

more rugged component and not as 

susceptible to failure. The conformal 

coatings tended to protect the circuits 

very well. 

The transmission line coupling was 

measured at three frequencies: 50 

MHz, 500 MHz, and 1 GHz. Of these 

three, the most understandable 

reaction to smoke is exhibited by the 

50-MHz data. When transmission 

lines are coated, the coupling between 

them tends to decrease. When smoke 

is added to uncoated transmission 

lines, the 50-MHz coupling increases,

as shown in Figure 33. This is not the 

case for the other frequencies; instead, 

the coupling decreased when the 

smoke was in the chamber. The 

coated transmission lines were 

unaffected by smoke, and the coatings 

worked very well for the transmission 

lines 

The high-frequency low-pass filter 

measurements showed a slight effect 

of the smoke on the control boards, but 

very little on the coated boards. The 

high-frequency-filter throughput at 

250 MHz drifts down during the 

smoke exposure on the uncoated 

boards. 

All coatings protected the pads and 

gull wing areas of the board. Pin grid 

array (PGA) sockets could not be 

compared because they were not

0 

E 

I5 

AO 

I I FE 

.5

Uncoated

Coated

0 2 

Time (hr)

Figure 33. The coupling increased between transmission lines while smoke 

was in the chamber, but the coated board remained constant at 50 MHz. 

59

C26

0

-I4

-0) 

S 

=-20 

0 

& -40

-50

-1 t 3 4 5
uniformly treated with a conformal 

coating. The parylene coating worked 

very well. This measurement is 

similar to that for the HVLC because 

it is a measurement of leakage 

current, but since it is performed aL a 

lower voltage, this does not 

differentiate the good coatings from 

the bad as well as the HVLC. 

Conclusions from coatings tests 

Overall the coatings did a very good 

job of protecting circuits from smoke, 

especially for failure by increased 

leakage currents. This was not true, 

however, for the HVLC circuits. 

Sometimes the H-ILC PTH circuit 

failed by increasing (open circuit) 

rather than decreasing (short circu lt 

resistance, as was expected. For this 

failure mode, the coating had very 

little effect. As with earlier functional 

board tests (Section 2.3.5), HVLC was 

the most affected. This was the circuit 

that separated the effects of coatings. 

The two best coatings were parylene 

and polyurethane, followed by acry]ic, 

silicone, and epoxy. The epoxy coating 

did not seem to recover after testing as 

the others did. 

Overall, the coatings protected the 

boards against decreased resistance; 

however, for the PTH circuit, some of 

the coated boards showed increased 

resistance, just as they had for the 

uncoated PTH circuits. These coatings 

included the parylene, acrylic, and 

silicone coatings. The least benefit 

was provided by silicone, followed by 

the epoxy coating. 

At pretest, the epoxy-coated SMT 

circuit had a lower resistance than

normal. The epoxy coating could have 

required a longer cure time, for during 

the preburn period these circuits 

returned to normal. During the burn 

and soak stages, the silicone and 

epoxy coatings again were the worst 

coatings, followed by the acrylic 

coating. After the vent period and 

after the test, the epoxy was the worst 

coating. 

Because more smoke was added than 

for the functional board tests, new 

measurement problems occurred. In 

particular, the HSD circuit failed on 

tests 3 and 5. The same power supply 

was being used for all HSD circuits 

and when it shorted on one of the 

boards, it caused all other HSD 

circuits to fail because the voltage was 

not high enough to power the HSD 

chips. It is assumed that the power 

supply was shorted on one of the 

uncoated control boards. On test 3 

this seems to have caused permanent 

damage on all of the chips, but it did 

not damage the coated boards in test 

5. 

In general, the coating tests at 200 

g/m• 3 caused significantly more failures 

of the uncoated functional boards than 

the earlier functional circuit tests at a 

maximum of 50 g/m3. This was 

especially apparent in the HSD 

circuits. The HF TL circuit. still 

recovered after the smoke was vented. 

Thus, the smoke in the air is much 

more significant than deposits for 

transmission line coupling.

60
2.4.2 Digital Throughput 

Types of digital connectors tested 

The connectors typically used for 

serial signal transmission, parallel 

signal transmission, and Ethernet 

transmission for standard personal 

computers are the D-subminiature 9

pin (DB-9), the D-subminiature 25-pin 

(DB-25), and the network modular 

(RJ-45) connectors, respectively. To 

test these connectors, signals from a 

personal computer were routed 

through connectors placed in the 

smoke exposure chamber. A printed 

circuit board was manufactured that 

would wire three pairs of connectors (a 

pair of each type) so signals entering 

from one connector would be 

transmitted straight through to its 

pair. All of the connectors were 

through-hole-soldered connectors. The 

printed circuit board was placed in the 

smoke chamber so those through-hole 

pins were uppermost, exposing the 

contacts to the most smoke deposition. 

Figure 34 shows the digital 

throughput board with the D

subminiature 9 and 25 pin, and the 

RJ45 connectors. 

Test Setup 

To test the connectors, appropriate 

digital signals were transmitted 

through the connectors in the smoke 

chamber. For serial communication, a 

bit-error rate test was performed on a 

serial communications port on a 

personal computer. To test parallel 

communications, an IOMEGA Zip 

drive was placed at the end of a

Figure 34. Throughput boards 

tested digital transmission. 

parallel port connection after the 

connecting cable was passed through 

the smoke chamber. Data were 

written to and read from the Zip drive, 

located outside of the smoke chamber. 

To test Ethernet communications, the 

network communications between two 

computers were monitored as data 

was transmitted between computers. 

Results of digital throughput tests 

A total of 19 tests were made on the 

digital throughput connectors. The 

fuel used for the tests varied, PVC, 

Douglas fir, and jet fuel were burned 

through the various tests. The fuels 

were varied because other smoke 

measurements were tried during these 

tests including optical extinction 

coefficient and conductance between 

vertical parallel plates with a DC bias. 

These tests were the first conducted 

after we assembled a new computer 

and acquisition system, so some of the 

recording problems could be attributed

61

C_ z (
to the problems of the acquisition 

system. The throughput boards were 

unaffected by smoke. A simple test 

with a variable resistor indicated that 

the resistance between leads had to 

drop below 200 ohms before any of the 

throughput signals would be 

interrupted. 

The surface insulation measurements 

(with 5 V dc bias) have been 

inconsistent as have the 5Vdc biased 

parallel plate. These voltages may be 

too low to attract soot. The AC biased 

plates have shown some changes in 

admittance at frequencies above 3 

MHz. 

Conclusions of digital throughput 

tests 

Although the digital connectors were 

exposed to some of the highest levels 

of smoke in our series of tests, none of 

the digital connectors failed. An 

evaluation of the likelihood of failure 

using a variable resistor in place of 

smoke showed that the conductance 

through the smoke must be very high, 

about 10-2 S, before the typical 

connector fails with this application. 

This level of conductance is 

approximately the same value as the 

admittance of typical cables. For 

example, a typical coaxial cable has 

impedance of 50 ý2. The admittance of 

such a cable is 1/50 t? or 2 x 10-2. 

Since most of our measurements of' 

smoke conductance indicate that a 

typical value with a high level of 

smoke is about 10-6 S, the smoke levels 

would not be expected to cause failure 

of the digital connectors.

2.4.3 Memory Chips (SRAM, 

EPROM and Non Volatile) 

Digital systems contain many 

different components, but a common 

component is the memory chip. These 

chips store both programs and data. 

Memory chips are often based on 

CMOS technology because CMOS 

technology draws little power and has 

high noise immunity. CMOS has also 

been shown to be more vulnerable to 

smoke than other technologies because 

it has a low output current drive. 

When digital circuits are exposed to 

smoke, the current is divided between 

the intended circuit and alternative 

circuit paths created by the smoke. 

Because the current is split, the chip 

switches more slowly or may not 

switch at all. The lower the current 

drive, the more slowly the chip 

switching will take place. 

CMOS memory chips were tested in 

two separate series. First, Static 

Random Access Memory (SRAM) and 

Erasable Programmable Read Only 

Memory (EPROM) were tested in a 

smoke environment. At a later date 

the non-volatile SRAMI was tested. 

Different smoke concentrations were 

used to determine a tentative failure 

threshold and failure mode. The 

previous tests had shown that digital 

equipment could fail when the smoke 

concentration is high, and then 

recover as the concentration drops. 

Therefore, it was important to detect 

failures during a fire, rather than 

after the smoke has settled or vented 

as we had for the 16 K memory chips 

during the component packaging tests. 

This information could be used to help 

estimate the failure threshold of

62
digital systems because memory chips 

are assumed to be one of the most 

vulnerable components in a digital 

system. 

Types of memory chips tested 

Memory chips have evolved by 

increasing the amount of data they 

can store while decreasing in physical 

size and power requirements. Three 

standard memory chips in use today 

are the static random-access memory 

(SRAM), the dynamic random-access 

memory (DRAM), and the erasable 

programmable read-only memory 

(EPROM). SRAMs can store data as 

long as power is supplied. If the 

power is removed the data is lost. To 

supply power continuously, the non

volatile SRAM contains a lithium 

battery so that data can be stored for 

9-10 years without an outside power 

supply. DRAMs (commonly used as 

random access memory-RAM--in 

personal computers) also require a 

power supply to store data, but their 

data must also be refreshed at regular 

intervals. DRAMs are slower than 

SRAMs, but also less expensive. 

EPROM's, another form of non-volatile 

memory can store data even though 

the power supply is removed. 

However, they can only store and be 

read with the power on. Some 

EPROM's can only be written to one 

time, but others may be 

reprogrammed after exposure to UV 

light. The advantage of EPROM's is 

that they can be used to store the start 

up program for a microprocessor so 

that it could be restarted after it is 

powered down.

There were many choices of memory 

chips to test, but decisions were made 

by determining what kinds of chips 

are being used in designs of advanced 

safety systems and what kinds and 

capacity of chips could be tested with 

the equipment on hand. Korsah 3 

suggested that certain EPROM's and 

SRAM's would be common in 

advanced safety systems such as 

AP600 (Westinghouse), the ABWR 

(General Electric), and the System 80+ 

(Combustion Engineering) systems. 

Project mangers at the NRC who 

found that these chips were likely to 

be installed in digital safety systems 

suggested the non-volatile SRAM. 

The number of pins in a random 

access memory chip is determined by 

the amount of memory addresses and 

how many bits of data are stored per 

address. For example, if 8 bits of data 

are stored per address, 8 pins are 

required to either input or output the 

data. Each bit of data is either a 

logical 1 or 0. Pins identified as DQ 

serve as both the input and output 

data connections. Two to the power of 

the number of address input pins (A) 

determines the number of memory 

addresses. Hence, for a 128K X 8-bit 

memory chip (one megabyte), the 

number of memory addresses is 17 

(2"7 = 131,072 = 128 x 1024). In addition 

to these pins to select an address and 

input and output data, each memory 

chip needs an input power supply pin 

(Vc, or Vdd), a ground pin (V__s), chip 

enable (E), write enable (W), and 

output enable (G). 

The chip must also have power on the 

supply pin (V~c or Vdd) and be 

grounded through Vs. to operate. If

63
the power or ground is disconnected, 

the data will be lost. To read data 

from a particular address, the correct 

voltage is placed on the address pins 

(A0, A1, _.A), the chip enable pin (E) 

is set to a low state (because of the 

overbar sign, enabling this pin is done 

by setting the logical value to 0), the 

output enable pin (G) is set to a low 

state, and the data is read off of the 

DQ pins. To write data to a particular 

address, the correct voltagesare 

placed on the address pins, E is set to 

a low state, W is set to a low state, 

and data, in the form of various 

voltage levels, are applied to the DQ 

pins. 

In the first series of memory chip 

tests, two-one Mbit SRAM's (128KX 8 

bit) were tested, the MCM6226BB and 

the MCM6926A, both manufactured 

by Motorola. The MCM6226BB 

operates on 5V while the MC6926A 

operates on 3.3V. These two standard 

voltages are typical of SRAM's, but 

more equipment is now being designed 

to use the lower 3.3V because there is 

less power used and the temperatures 

of the equipment can be lower. These 

two chips had similar packages, 

surface-mounted plastic-bodied with J

shaped leads common known as SOJ 

packages. Each package had 32 pins, 

in the case of the MCM6226BB chip, 

two pins had no connection and two 

pins are connected to chip enable, E 

and E2 . The MCM6926A chip has two 

pins connected to V.s and Vdd. 

The Advanced Micro Devices EPROM, 

AM27C256, was tested also in the first 

series of tests. This chip comes in 28

pin dual-in-line (DIP) and 32-pin 

plastic-leadless chip carrier (PLCC)

packages. This chip is a 256 kb 

memory chip (32,768 x 8 bit), thus it 

contains 15 address, 8 data, 1 output 

enable, 1 chip enable, 1 voltage 

supply, 1 ground, and 1 program 

voltage input pins. The AM27C256 

came in two different packages, the 

28-pin dual-in-line (DIP) package, and 

the 23-pin plastic-leadless chip carrier 

(PLCC). Both packages were tested. 

The EPROM chips were programmed 

prior to installation on a printed 

circuit board. The SRAM and EPROM 

chips are shown in Figure 35.

Figure 35. SRAM and EPROM 

memory chips were tested. 

The SRAM's and programmed 

EPROM's were individually mounted 

on printed circuit boards as shown in 

Figure 36. The chips were soldered on 

the board rather than mounted in a 

socket so that the circuit board would 

be similar to a board in a computer. 

The printed circuit board traces 

connected the pins on the memory 

chips to a connector for testing. The 

printed circuit board was designed to

64

cTl
accept all of the memory chips, 

although they used four different chip 

packages. Because of the general 

features of the board, when one 

memory chip was mounted, other 

solder pads were unoccupied. 

Potentially, the unoccupied solder 

pads could short. To reduce shorting 

of the unoccupied solder pads, they 

were coated with a polyurethane 

conformal coating that had been 

tested earlier in the program and 

shown to be very protective. 

In the second series of memory chip 

tests, a Dallas Semiconductor 8 Mbit 

non-volatile memory chip was tested, 

DS1265Y. This chip was packaged in 

a 36-pin DIP package with a minimum 

10-year lithium battery to supply 

power to store data. Similar to the 

SRAM's tested in the first series, these 

chips have 8 data input/output pins, 

20 address pins, one chip enable, one 

write enable, one output enable, power

Figure 36. The EPROM on the left 

was exposed to smoke; the 

EPROM on the right has not been 

exposed to smoke.

and ground. The normal operating 

bias is 5 V. These chips were not 

preprogrammed and were mounted on 

a separate printed circuit board than 

the ones designed for the first series of 

tests. 

Test setup 

In the first series of tests two types of 

tests were performed on the chips in 

the smoke chamber: "functional and 

timing tests" and "parametric 

measurements." The functional and 

timing tests measure if the chip can 

record and output data from all cells 

in the memory chip without errors and 

indicate how long the chip takes to 

make data available. Parametric 

measurements test to see if certain 

parameters change as the chip is 

exposed to smoke. The parametric 

measurements include a standby 

current measurement (current drawn 

by the supply voltage pin), a current 

leakage measurement (a standard 

voltage is applied to the pin and the 

current measured), and current 

injection test (a standard current is 

injected and the voltage level 

measured). Two chips were exposed to 

the smoke at a time, and all of the 

chip tests were repeated at 30-s 

intervals. 

The HP 82000 chip tester was 

programmed to measure the 

performance, access time, and voltage 

and current specifications. For the 

SRAM, a pattern of 1 and 0 states was 

written into all the addresses and then 

read out to determine functionality. 

For the EPROM, the pattern was only 

read, as the pattern was programmed 

in prior to testing. The parameters of

65

crL9,
Table 13. Parameters measured for first memory chip test series. 

Parameter Symbol Parameter Description 

VOH Output HIGH Voltage 

VOL Output LOW Voltage 

ILI Input Load Current 

ILO Output Leakage Current 

IccI Vec Active Current 

tcE Chip Enable to Output Delay

the memory chips were then tested. 

Typical parameters are listed in Table 

13. The chip tester can test up to 64 

connections at one time. Each test 

exposed two EPROM or two SRAM 

chips. The functionality test 

frequency depends on the load time for 

the chip tester and is approximately 

30 seconds to test two chips. Each test 

was performed for 2 hours; for the first 

hour the chips would be immersed in 

smoke, and the next hour the smoke 

was vented. To test for the worst case, 

the boards were mounted with the 

solder pads up as in Figure 37. 

For the tests of the non-volatile 

SRAM, a logic analyzer was used to

Figure 37. This memory chip 

board is darkened by smoke.

write data to the memory chip. The 

data was then read from the memory 

chip to determine if the smoke would 

cause any errors. The power was then 

turned off for 1 second and the data 

was read again to determine if the 

battery back up was working properly. 

A mixture of cable material was 

burned at 50 kW/m2 to create different 

levels of smoke density. The staring 

humidity level was 75% RH, but the 

heat of the fire and the addition of N2 

to purge the laser optical 

measurement system decreased the 

RH during the test. The RH was 

restored to 75% upon venting. 

Results of memory chip tests 

The smoke caused the memory chips 

to fail functionally. As shown in 

Figure 38, the 3.3V SRAM chip failed 

each of the four times it was tested 

with more than 10 g of fuel burned, 

but did not fail for when less fuel was 

burned. The 5 V SRAM failed half of 

the time when more than 10 g of fuel 

was burned, but not for less fuel. The 

EPROM's failed once each at low fuel 

levels, but did not fail for higher 

levels. All of the chips were tested 

outside of the smoke chamber several 

weeks after the smoke exposure, and

66

C29
they all performed well. The failure of 

the memory chips was a temporary 

failure that occurred during or 

immediately after exposure to smoke. 

Failures occurred at different times 

during the tests depending on the type 

of chip. A majority of the failures 

occurred after the smoke was vented. 

When the smoke is vented, the test 

samples have a coating of soot on 

them, but no more is being deposited, 

the environment is held at 75 OF and 

at 75% RH, and air is circulated inside 

of the smoke chamber. Measurements 

taken in the second series of memory 

chip tests indicated that RH drops 

significantly during the test while the 

lamps are on and nitrogen is used to 

purge the laser optics. As the failure 

seems to occur during those times that

both smoke is high and the humidity 

is high, we believe that both 

conditions must occur to cause failures 

in the 3.3V SRAM chips. 

The failures for lower smoke levels for 

the EPROM's are suspect data. The 

failure of the PLCC EPROM only 

occurred 2 times out of the 800 or 

more functional tests performed 

during that test. The DIP EPROM 

failed more regularly, however, the 

first failures occurred before the 

smoke was added to the chamber, so 

the test setup may have been flawed 

to begin with. After this test where 

the DIP EPROM failed even before the 

smoke was added (the second test), 

new cables were used to connect each 

new set of chips to be tested. This 

eliminated the possibility of the cable

6 

5 

04 

-3 

z3 

0

* Passed 

U Failed

"8 C. 10CC 

6 7 10-12

Fuel Burned (g) 

Figure 38. The 3.3V SRAM failed when 10 or more grams of fuel were 

burned.

67

p
from a previous test influencing the 

outcome of the next test. 

The parametric measurements did 

vary during the test, depending on 

how much smoke was introduced. As 

expected, leakage currents increased 

during the fire and were observed 

during the current leakage 

measurement. However, changes in 

leakage current did not correlate with 

most of the failures. Instead, the 

current on the supply pin varied 

drastically during chip failure. When 

the 3.3 V SRAM chips failed they 

tended to increase in current. The 

normal operating current was 

approximately 4 to 6 mA, but when 

these chips failed their supply current 

increased to greater than 10 mA. 

When the 5 V SRAM chips failed, their 

operating currents dropped to half of 

their normal value. As the EPROM's 

failed only once each, no trend has 

been assigned to when they failed. 

The non-volatile SRAM's did not fail 

even though they were exposed to 

smoke created by 30 g of cable mixtuire 

in the 200 L small smoke chamber 

(150 g/m ). 

Conclusions from the memory 

chip tests 

Low voltage (3.3 V) SRAM's were 

found to be the most vulnerable to 

smoke. Higher bias voltages decrease 

the likelihood of failure as was 

predicted by the chip technology tests. 

EPROM's and non-volatile SRAM's 

were very smoke tolerant. Failures of 

the SRAM's occurred when two 

conditions were present, high 

densities of smoke and high humidity.

As the high humidity was present for 

only part of the test, the failures were 

intermittent; all of the chips that 

failed during the test recovered after 

enough venting. This behavior can 

account for the failures of the digital 

systems where there was also 

intermittent failure (Section 2.3.1). 

2.4.4 Hard Disks 

Purpose of hard disk tests and 

selection criteria 

Hard disks are standard equipment in 

most personal computers, providing 

long-term storage of the majority of 

data on a computer. In normal 

operation the hard disk is accessed 

frequently, often several times a 

minute. When hard disks fail, the 

entire computer can halt, waiting for 

the hard disk to be accessed correctly. 

Thus, unimpaired hard disk 

performance is essential to computer 

performance. 

Hard disks consist of a stack of 

magnetically coated disks mounted on 

a spindle. Each side of the disks is 

accessed by its own read-write head, 

which floats on an air stream above 

the swiftly rotating disk. The hard 

disk is operated at atmospheric 

pressure and it is the presence of air 

that allows for correct operation. 

Typical distances between the head 

and disk are only 0.0003 mm. 26 If dirt 

or smoke were to deposit on the disk, 

the heads could be easily fouled and 

the data could be destroyed. The air 

inside of the hard disk drive is heavily 

filtered to prevent destruction by dust 

particles, and the disk is sealed 

(typically with adhesive tape) so that

68
outside air does not easily penetrate. 

Smoke exposure has the potential to 

disrupt the operation of the hard disk 

and thereby a computer system. 

For a personal computer in 1999, 

there are two common types of hard 

drives, those that operate off of 

enhanced intelligent drive electronics 

(EIDE) buses, and those that operate 

on small computer systems interface 

(SCSI) buses. Since the EIDE bus is 

more prevalent at this time, this type 

of disk was selected for testing. A 

series of smoke exposures were 

performed on standard EIDE, 8.4 

Gbyte hard drives,* a typical size for 

computers at the time of testing. 

Hard disk drives are rapidly 

increasing in capacity, however their 

physical design has not changed 

significantly. 

Hard disk test setup 

Five hard drives were exposed to 

different smoke conditions to 

determine if they would be susceptible 

to failure. Because only the hard drive 

was to be tested, not the computer, 

only the hard drive was placed inside 

of the smoke chamber. The computer 

was enclosed in a plastic bag outside 

of the smoke chamber to reduce the 

likelihood that smoke that leaked from 

the smoke chamber could cause any 

failures of the computer. The hard 

drive is shown in the smoke exposure 

chamber in Figure 39, and a close-up 

of the drive is shown in Figure 40. In 

later tests the hard drive was placed 

* Western Digital Caviar 28400 drive

upside down from the standard 

orientation because more electronic 

circuitry would be exposed to the 

smoke in this orientation. The hard 

drive was connected with the 

computer by an 18" (45 cm) data cable 

and a power cable extension. The data 

cable was limited to this length 

because of specifications by the hard 

drive manufacturer. 

The monitoring scheme was a simple 

read and write test. A string of text 

was stored on the hard drive and then 

recalled from the hard drive. The 

string that was read back from the 

drive was compared to the original 

string. The result of the comparison, a 

pass or failure to recall the same 

string as was written, was recorded. 

Preliminary experiments showed that 

if some of the data or power supply 

lines that were connected to the hard 

drive were shorted, the computer on 

which the hard drive was mounted 

would stop operating. If the computer 

that was recording the failures was 

directly connected to the smoke

exposed hard drive, smoke could have 

shorted some of the data or power 

lines and stopped the computer. In 

that case, hard drive failures could 

have prevented failures from being 

recorded. Thus, a second computer, 

located outside of the environmental 

chamber, but connected by an 

Ethernet network, both monitored and 

recorded failures, and transmitted 

data to write on the smoke-exposed 

hard drive. Data was written to and 

read from the hard drive over the 

network. The data read from the hard 

drive was compared to the data 

written to the hard drive to determine 

if the smoke caused any failures.

69
Figure 39. Hard drive test tested only the hard drive.

Figure 40. Hard disk drives showed some corrosion after testing.

70
Table 14. Hard disk test conditions. 

Hard Test Orientation Fuel level Nitrogen 

disk to start Purge 

1 Upright/Upside down 15 g + 15 g Yes 

(two exposures) 

2 Upside down 15 g Yes 

3 Upside down 30 g Yes 

4 Upside down 30 No 

5 Upside down 15 No

Six tests were performed with the 

hard drives. In the first test, the hard 

drive was placed in the standard 

configuration, aluminum side up, and 

15 g of mixed cable fuel was burned at 

50 kW/m2 of heat flux. Since no 

failure occurred, the same disk was 

placed upside down from the standard 

so that the printed circuit surface of 

the disk was upward. This disk was 

then exposed smoke from 15 g more of 

mixed cable burned at 50 kW/m2 of 

heat flux. Thereafter, each disk was 

exposed to either 15 or 30 g of fuel and 

with 50 kW/m 2 of heat flux as shown 

in Table 14. 

The humidity before and after the test 

was maintained at 75% _ 5%. As 

mentioned in the section on the laser 

data, nitrogen was added to the smoke 

chamber during the smoke exposure to 

prevent smoke from clouding the 

transmission measurement optics. 

The added nitrogen as well as the 

increased temperature of the smoke 

chamber combined to reduce the 

relative humidity of the smoke 

chamber. To check the effect with the 

most severe smoke case possible, for 

some of the tests the laser system was 

turned off and no nitrogen was added 

to the smoke chamber. This resulted

in higher humidity levels during the 

smoke exposure. 

The hard disk was monitored with the 

read and write test for 2 hours, which 

included 1 hour of smoke exposure and 

another hour while the smoke was 

vented and humidity was restored to 

the chamber. After the smoke 

exposures the hard disks were 

scanned for errors using a commercial 

product, Scantest. Scantest checks 

hard disks for corrupted data sites and 

reports the results. Because the hard 

drives were large compared to the 

operating system of the computer it 

was attached to, Scantest took up to 6 

hours to complete. This test was 

typically performed immediately after 

the smoke test or on the next day. 

Results of hard disk tests 

Results of the smoke monitoring test 

and Scantest are shown in Table 15. 

No hard disks failed during or within 

2 hours of the smoke exposure. Only 

one hard disk failed by showing errors 

during Scantest.

71
Figure 41 shows four of the hard 

drives after smoke testing in the 

orientation that they were tested.

Figure 42 shows the inside of one of 

the tested drives. No detectable 

smoke was found inside of the drive.

Table 15. Hard disk results 

Hard Disk Fuel Burned Active test result Scan test result 

1 22.2 g Passed Passed 

2 18.4 g Passed Some errors 

3 18.2 g Passed Passed 

4 18.7 Passed Passed 

5 10.7 Passed Passed

Figure 41. These disk drives did not fail during the smoke test. 

72

C'?'2.

-U
Figure 42. The inside of the disk drives remained clean.

73
Conclusions of hard disk tests 

The hard disks were very smoke 

tolerant. The two failure modes 

predicted for hard disks, particulate 

contamination and electrical shorting, 

did not cause any failures of the hard 

drives. Although disk 2 did show 

some bad sectors after the smoke test 

as revealed by Scantest, these bad 

sectors did not cause any problems 

during the smoke exposure. If a fire 

were to occur near a computer, 

commercial hard drives that are 

typically installed in personal 

computers could be expected to 

survive the smoke exposure at least 

for a day after the fire and data could 

be downloaded from the disks. No 

tests were performed on these drives 

beyond one day after the smoke 

exposure, so long term use of smoke

exposed hard drives may not be 

advised. 

2.4.5 Electrical Measurements 

Parallel plates and interdigitated 

combs were included in the smoke 

tests to provide a simple geometry 

whose electrical field can be 

approximated and whose leakage 

currents can be measured. Items with 

simple geometry, such as these, cart be 

more easily modeled to calculate field 

strengths that can be generalized for 

use with more complicated electrical 

equipment. The parallel plates model 

freestanding conductors, while the 

interdigitated combs model traces on 

surfaces of printed circuit boards.

Parallel plates DC and HF

Two types of leakage currents have 

been measured: the leakage between 

two freestanding vertical parallel 

plates and the leakage between 

interdigitated comb patterns printed 

on a circuit board. The parallel plate 

conductivity was measured to 

determine if the smoke in the air was 

causing the increased conduction. The 

plates were spaced 2.5 mm apart and 

were made of perforated stainless 

steel. The perforations allowed more 

transport of smoke between the plates, 

(Figure 43). Four pairs of plates were 

placed in the smoke chamber at a 

time, and each was biased with a 

different voltage, 500 Vdc, 50 Vdc, 5 

Vdc, and 1 Vac. The plates biased 

with dc voltages were connected 

electrically in series with a resistive 

circuit that allowed for measurement 

of leakage currents across the plates. 

The ac-biased plates were connected to 

a network analyzer, which measured 

the admittance of the plates for a 

range of high frequencies between 0.5 

and 30 MHz. 

Parallel plates AC 

AC voltage was applied to the same 

parallel plates as for the DC 

measurements. The steel plates 

spacing was adjust to distances 

between 3 and 25 mm apart for the 

tests. The plates were mounted near 

the top of the smoke chamber. 4.2 kV 

(RMS) was applied to the plates. 

Arcing was measured with a Pearson 

current probe on the high voltage 

conductor and the voltage measured

74
Figure 43. Perforated parallel plates were used to measure the effect of 

smoke between vertical plates with DC and AC bias voltages.

with a two-channel oscilloscope. Both 

the current and voltage waveforms 

were recorded when the plates arced. 

Some preliminary measurements of 

shorting voltage vs. humidity in the 

chamber showed little variation due to 

humidity. 

Ideal parallel plates are infinite in 

extent and perfectly flat. For ideal 

parallel plates, the field between the 

plates should decrease linearly with 

increasing separation. In our tests, 

plate separation was a significant 

factor in likelihood of arcing, although 

the variation was not linear. To study 

this, the voltage was raised slowly 

until the plates began to arc, then 

another separation distance was 

selected and the voltage raised again. 

When the plates began to arc, the 

arcing tended to be continuous. The 

ac power supply could supply up to 40

ma of current on a continuous basis, 

but if required to provide more, the 

voltage would drop. The arc current 

was high enough to lower the potential 

across the plates. Nevertheless, the 

arcing continued once a path was 

established because the hot, ionized 

air provided an easier path for arcing 

than normal. 

Results of the parallel plate 

experiments 

A surprising result was that the 

conductivity between parallel plates 

remained high although the optical 

extinction coefficient in the smoke 

chamber dropped drastically, 

indicating that there was very little 

smoke in the air. This result is plotted 

in Figure 44 for a 500 Vdc-biased pair 

of plates. A video recording of the 

plates in the smoke chamber shows

75

C 5-
I

the mechanism by which this occurs. 

Four frames from this recording are 

shown in Figure 45 to 48. The smoke 

is attracted to high-voltage surfaces 

and builds up fragile bridges between 

the parallel plates. These 

carbonaceous bridges conduct current 

much like carbon resistors. If the air 

is forcefully vented from the smoke 

exposure chamber, the carbon bridges

1.2E-06

1.OE-06

r 

0 

U 

CI 

V 

"C 

o 

V

8.OE-07

6.OE-07

4.0E-07

2,OE-07

O.OE+00

are destroyed and the conductivity 

falls. While smoke is in the air, air 

movement destroys some of the 

bridges, but more smoke is then 

attracted to the bridge formation. 

After the optical extinction coefficient 

drops, however, there is no more 

smoke to replace the bridges and the 

conductivity slowly falls.

0.16

0.14 .E 

0.2 

0.12 _, 

0.08 

0 

0.06 _4 

0.04 

0 2 

0.02 0

0

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Time (hr)

Figure 44. The optical extinction coefficient decreased after the fire more 

quickly than the 500 DC V conductance.

76

cS5
Figure 45. 500 DC V parallel plates at beginning of test are clean.

Figure 46. After a short time, the parallel plate starts to collect smoke, 

notice the encircled whisker of smoke.

77
Figure 47. 4 minutes into the smoke test, a substantial amount of smoke 

has gathered on the parallel plates.

Figure 48. After 12 minutes, a thick bridge of smoke shorts the parallel 

plates together.

78
A lot of soot collects on these plates. 

They tend to act like electro-static 

precipitators and attract soot 

particles. The soot is probably not 

evenly distributed in the smoke 

chamber, because high potentials will 

attract so much of the soot. Further 

discussion of these phenomena 

appears later in the section on the 

mass vs. conductivity boards. In this 

case, smoke to the right of the pair of 

plates is subject to one polarization of 

the electric field and the smoke to the 

left of the plates to the opposite field. 

The field strength is large in the 

general area and smoke is attracted to 

the plates. More soot should be 

attracted to the parallel plates when

1.20E-06

1.OOE-06

0 

U 

0 

C 

0 

U

8.OOE-07

6,OOE-07

4.OOE-07

2.OOE-07

O.OOE+0O

0 0.25 0,5 0.75

the voltage is high.

The conductivity on the 500 Vdc

biased plates was no higher than that 

on the 50 Vdc-biased plates at the 

peak of the smoke output Figure 49); 

however, the 500 Vdc bridges were 

more robust. Higher voltages have 

more force to maintain the bridges, 

but since the plates were the same 

size, the peak conductivity is similar 

because a maximum volume of soot 

could be attached to each plate. The 5 

Vdc-biased plates showed little 

increase of conductivity and probably 

did not have a strong enough field to 

attract soot and bridge the plates.

1 1.25 1.5 1.75

0.16

0.14 'E 

0.12 4) 

0 

0.1 

a 

0 

0.08c 

0 

0.06 " 

0.04 

0.02 ". 

0

0

2

Time (hr)

Figure 49. The conductance between the 50 DC V parallel plates also falls 

slower than the optical extinction coefficient.

79
The high frequency measurements 

yielded a change in admittance as the 

smoke was added. At high frequencies 

the changes were larger than for low 

frequencies. Figure 50 shows the 

change in admittance as smoke was 

added for 30 MHz. The parallel plates 

were set 2.5 mm apart. Smoke 

particles are semiconductive so the 

response of the circuit to smoke should 

be similar to the introduction of 

dielectric material between the 

parallel plates.

For a parallel plate capacitor, the 

capacitance is expressed as C = eA / d 

where A is the area of the plates, d is 

the distance between plates, and e is 

the dielectric constant. The resistance 

between two parallel plates is 

R = d / CA, where a is the conductivity 

of the air. These two formulas contain 

the geometric factor, Aid. Since the

parallel plates used in our 

experiments are perforated, the 

electric field lines will not yield the 

standard values of capacitance and 

resistance. However, we can assume 

that the field lines will be equivalent 

for the two formulations and hence, a 

geometric factor, g can be considered 

to be the same for both, i.e., C = eg and

R= 1lag.

The geometric factor can be 

determined when there is no smoke in 

the air. In that case, E = E. or the 

electrical permittivity of air, 8.85xlO- 12 

F/m. Thus, for a given capacitance 

measured in the air, C, can be 

determined. If the parallel plate can 

be modeled as a capacitor and resistor 

in parallel, the current from such a 

circuit would be: I = V/R + CV , where 

V is voltage and I is current. If we

1.5E-04 3.15E-03

1.0E-04 S.lOE-03

-5.0E-05

-1.OE-04

0

3.05E-03 >" 

C 

3.00E-03 *5 

E

2.95E-03

2.90E-03

0.5 1 1.5 2

Time (hr)

Figure 50. Both the real and imaginary parts of admittance at 30MHz are 

affected by smoke.

80

C 06

5.OE-05

C O.OE+±0
assume that V = V, c, a sine wave, 

then I = V/R + CjwV. Since the 

admittance, Y = I/V, then 

Re(Y)-- G = R =g and 

Lm(Y) B = Co) =co.

Surface insulation resistance 

(SIR)

Interdigitated comb patterns were 

placed in the chamber either faces up 

and parallel to the ground or 

horizontal and perpendicular with the 

ground. The comb patterns, shown in 

Figure 22 were connected to a 

resistive circuit similar to the de 

parallel plate circuits, biased with 5 

Vdc, and leakage currents were 

monitored in these circuits. These 

patterns (IPC-B-24 boards), developed 

by the Institute for Interconnecting 

and Packaging Electronic Circuits, 

measure surface insulation resistance 

and are used by the IPC to monitor 

the accuracy of the printed circuit 

manufacturing process. 2 7 Better 

processing should yield higher SIR 

values. Smoke and other 

contaminants decrease SIR values. 

Humidity tan influence contaminated 

boards.

Mass vs. conductivity 

measurements

An interdigitated pattern was 

designed for use as a surface to collect 

smoke for weighing and comparison to 

conductivity. The pattern has solder 

traces separated by 0.1" (0.25cm). The 

pattern is illustrated in Figure 51. 

Printed circuit boards with these 

traces were biased with 5, 50 and 500 

VDC and leakage currents were

measured as they were exposed in 

smoke. The substrate for these 

printed circuit boards were very thin 

so that the weight of the boards were 

below 1 g, but each printed circuit 

board had 30 cm 2 of surface area. The 

printed circuit boards were weighted 

before and after each smoke exposure 

to determine how much smoke would 

cause a given amount of shorting. All 

boards were placed in a horizontal 

position to get the maximum collection 

of soot possible. A board in its holder 

is shown in Figure 52.

Figure 51. The mass vs. 

conductivity board was designed 

with an interdigitated comb 

pattern.

The pattern of smoke deposition on 

the mass vs. conductivity boards 

depended on the applied bias voltage. 

As shown in Figure 53, 500 V and 50 

V biased boards collected soot around 

the conductors, while the 5 V biased 

board had a more even distribution. 

On the 500-V biased board, the area 

between conductors had little smoke 

deposition. Likewise, the 50 V biased

81

G"O
board had a smaller area of little 

deposition. The higher the bias 

voltage, the more uneven the smoke 

deposition. The higher-biased 

patterns had stronger electric field 

strengths and the stronger the electric

field, the more force on the smoke 

particles to flow toward the 

conductors. Hence, the higher-biased 

patterns had a more uneven 

deposition.

Figure 52. The mass vs. conductivity board was placed in a holder to 

provide bias and protect the board from air currents after smoke 

collection.

Figure 53. Smoke distribution on the mass vs. conductivity boards is more 

even on the 5 DC V biased board. 

82

Crl-
1.OE-06

"c' 1.OE-07 

U 

"0 

1 .0E-08 0

I.OE-09

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

Time (hrs)

I - mass cond5V --.-- 50 mass cond ---- mass cond 5oOV

Figure 54. The conductance remains higher on the 5 V biased board that

has a more even distribution of smoke.

The conductivity between traces 

varied throughout the smoke test as 

shown in Figure 54. The conductivity 

is highest when smoke is in the air, 3

4 minutes after the test starts. After 

most of the smoke has settled or 

vented, the conductivity drops the 

more for the 500 and 50-V biased 

boards than the 5-V biased board. The 

higher conductivity of the 5-V biased 

board after 1 hour (the smoke was 

vented from the smoke chamber by 

then) could be attributed to the more 

even distribution of smoke on that 

board.

Figure 55 compares the mass of 

deposition between boards for 

different amounts of fuel burned and 

at different bias voltages. The mass of 

smoke deposited does not vary

significantly with the bias voltage. 

Although higher electric fields are 

present near the surface of the boards 

when the bias voltage is higher, far 

away from the boards, the effect of the 

stronger electric field is cancelled 

because there are an even number of 

traces on the circuit board. However, 

the smoke mass collected on the board 

varies with the amount of fuel burned. 

When more smoke is in the air, more 

smoke is deposited.

The mass of soot collected on the mass 

vs. conductivity boards related well to 

the amount of fuel burned, but not to 

the conductivity measured near the 

end of the smoke exposure. This may 

be attributed to dependence of 

conductivity on the relative humidity.

83
1.2E-02

08.OE-03 

0A 

o A A 

0 

M AN 

W 4.OE-03 a 

O.OE+O 

0 2 4 6 8 10 12 14 16 18 20 

Fuel Burned (g) 

l*5V U50V AS6V 

Figure 55. Smoke collected by conductivity board was roughly 

proportional to the amount of fuel burned and not very dependent on the 

DC voltage applied. 

84

COJ3,

a
3 FIRE PROTECTION AND THE MITIGATION OF SMOKE DAMAGE

3.1 Overview 

There are a number of potential 

features of current fire protection 

practice that can be used or 

implemented to lend some additional 

protection against smoke damage. 

These include both direct features of 

physical fire protection, such as 

Appendix R (to 10CFR40) 

requirements, as well as predictive 

tools that can aid in assessing the 

level of smoke hazard associated with 

a particular installation and potential 

mitigation strategies. It is these 

measures that are the topic of this 

section. 

3.2 Physical Fire Protection 

Features 

3.2.1 The Classical Fire 

Protection Strategy 

One of the most effective means of 

preventing or minimizing smoke 

damage is to minimize the potential 

for and magnitude of the smoke 

production. In general fire protection 

practice, this can be accomplished 

through the classical approach to fire 

protection; the implementation of 

effective programs and features to 

minimize the occurrence of fires, and 

to quickly detect and suppress those 

fires that will inevitably occur. The 

design of digital equipment should not

neglect such concerns. While many of 

the fire protection features in this 

regard will be predetermined, the 

digital equipment design process may 

provide the opportunity to review and 

potentially enhance critical features of 

fire prevention, detection, and 

suppression practices. 

For example, implementation of 

enhanced rapid-response smoke 

detection systems may provide a 

significant benefit. Testing has 

demonstrated that smoke detectors 

located inside an electrical panel can 

detect the very early stages of an 

incipient electrical fire much more 

quickly and effectively than can 

general area smoke detectors that 

respond only after the fire has reached 

a stage of significant smoke 

production. The implementation of 

such features to address specific 

potential fire threats could greatly 

reduce the likelihood that significant 

smoke production might occur. 

Similarly, implementation of 

enhanced fire suppression systems to 

address the most likely fire threats in 

an area would also minimize the 

potential for smoke damage. 

3.2.2 Current USNRC Fire Safety 

Regulations and the Smoke 

Damage Issue 

The primary source for current 

USNRC fire safety regulations is 

Appendix R to 10CFR50. While not all

85
licensees are directly liable for 

compliance with the Appendix R 

requirements, all have implemented a 

fire protection program and the vast 

majority of these programs derive in 

large part from the Appendix R 

requirements. Hence, Appendix R 

will be used to represent the current 

"template" for fire safety regulation. 

There are aspects of Appendix R that 

will clearly impact the design and 

routing of digital instrumentation and 

control systems. For example, 

provisions for a fire protection 

program (Section II.A in Appendix R), 

fire prevention (Section II.C), fire 

fighting (Sections III.A-F and III.H-I), 

administrative controls (Section III.K), 

and barrier qualifications (Sections 

II.M and JI.N) will clearly reduce the 

fire hazard for digital systems just as 

they reduce the fire hazard for analog 

systems. However, it should also be 

recognized that Appendix R was not 

written with the explicit intent to 

address smoke damage. Rather, 

Appendix R was written with the view 

that fire damage was primarily a 

thermal problem. 

The focus of Appendix R is to ensure 

that at least one hot shutdown path is 

protected from the effects of any given 

fire (see Sections I and III.G.l.a). 

Where this cannot be achieved, such 

as in the main control room, an 

alternate shutdown capability is to be 

provided (see Sections II.D, III.G.3 

and III.L). (Separate and less 

restrictive requirements are set forth 

for cold shutdown equipment in 

Section I and III.G.l.b.)

Three acceptable means to protect one 

hot shutdown path are set forth in 

Section III.G.2; namely, (a) separation 

by 3-hour rated fire barriers, (b) 

separation by 20-feet of horizontal 

space free of combustibles with 

automatic suppression and detection, 

or (c) separation by 1-hour rated fire 

barriers with automatic suppression 

and detection. Any other measure 

found by the USNRC staff to provide 

an equivalent level of protection may 

also be accepted by exemption [10CFR 

50.48 (c)(6)]. Presumably, digital 

systems will be subject to these same 

requirements. 

The potential for direct thermal 

damage to digital components must be 

considered, and the separation criteria 

of Appendix R will provide some level 

of protection for the digital 

components in this regard. However, 

Appendix R will not ensure that 

smoke vulnerable components will not 

be threatened by the same fire, in 

particular, in cases where spatial 

separation has been relied upon to 

meet the Appendix R requirements. 

Note that the focus of the Appendix R 

requirement is placed on physical 

separation. The acceptable separation 

criteria noted above are not 

specifically intended to address smoke 

damage. Specifically, 20-feet of spatial 

separation has little or no significance 

with respect to smoke. While 

separation by a rated 3-hour or 1-hour 

barrier will provide some substantial 

barrier to smoke exposure, fire 

barriers are not specifically designed 

or tested for their ability to contain 

smoke. Rather, fire barriers are 

designed to prevent the spread of fire 

itself. Fire doors opened to allow

86
access for fire fighting and open 

hatchways will allow for the spread of 

smoke to adjacent areas. Smoke may 

also be spread through ventilation 

systems to various plant areas. 

Overall, smoke can easily traverse 

substantial distances. 

In other regards, Appendix R will 

clearly impact the digital system 

design. The requirement to maintain 

one hot shutdown path free of fire 

damage will still remain as a design 

criteria. As a result, some level of 

redundancy will need to be 

maintained in the digital design. For 

example, it would clearly be 

inappropriate to multiplex all of the 

redundant train signals for any given 

critical instrument reading onto a 

single transmission/communications 

link. Rather, some redundancy will be 

needed to ensure that loss of a single 

communications link would not 

compromise critical instrument 

readings. This will likely require 

component level redundancy and 

spatial separation of those redundant 

components. At least two sets of 

communications cables will be needed, 

and these cables would need to be 

protected from failure in a single fire. 

Similarly, at least two 

processing/transmission systems that 

are adequately separated may also be 

needed. The power sources for these 

systems will also need to maintain 

redundancy and separation. Finally, 

for areas like the control room where 

adequate separation cannot be 

maintained, provisions for alternate 

shutdown will also be needed.

3.2.3 Choice of Suppression 

Agents 

The choice of suppression agents could 

strongly influence the magnitude of 

smoke damage. In particular, the 

SNL tests and practical experience 

have shown that higher moisture 

levels contribute to significant 

increases in the level of smoke 

damage. Based on SNL tests, this 

appears to include both short-term 

and long-term damage. Hence, 

whenever possible the use of 

nonwater-based fire suppressants, 

such as carbon dioxide, would be 

preferred in areas containing digital 

components. Halon or a Halon 

replacement product may also be a 

possibility. However, the choice of 

such products should consider that 

Halon and some of the replacement 

products would produce corrosive 

decomposition products, especially 

including halogen acids such as HBr, 

in the presence of a heating source 

such as an open or smoldering fire. 

Note that this concern is not the same 

concern that digital components might 

be directly sprayed with water during 

a fire. While this may also be a 

concern, in the context of the smoke 

damage issue, the simple airborne 

moisture level (relative humidity) is a 

significant factor. The mere presence 

of high humidity levels (on the order of 

60% RH or more) is sufficient to 

sharply increase the damage potential. 

The lower the humidity levels, the 

lower the damage potential. Hence, 

avoiding the introduction of excessive 

moisture is a desirable strategy to 

minimize smoke damage.

87
In many situations, it may not be 

possible to make significant changes 

to the existing fire suppression 

capabilities. In others, a switch to a 

gaseous fire suppression agent may be 

impractical or inappropriate for a 

particular fire area or fire threat. It 

must also be anticipated that 

firefighters may eventually fall back 

on water hose streams as the ultimate 

suppression agent in almost any area 

of the plant. Putting out the fire will 

remain the number one concern. 

However, to the extent possible, the 

digital design should consider what 

suppression agents would be used in 

the fire area housing the digital 

components. For example, replacing 

water-based hand-held fire 

extinguishers with gaseous 

extinguishers in the area may be 

appropriate and desirable. 

Alternatively, if the designer is faced 

with a choice of which area a digital 

component is to be located in, an area 

protected by a gaseous suppression 

system might prove more favorable 

than one protected by a sprinkler or 

deluge system. 

3.2.4 Avoidance of Known 

Hazards and Vulnerabilities 

In implementing digital system 

upgrades, it is likely that the designer 

will have some latitude in determining 

where a specific component is to be 

placed within the plant. While this 

latitude may be limited by other 

design and equipment placement 

constraints, smoke damage should be 

considered. The likelihood and 

potential severity of the fire threats in 

the area should be considered. It may

well be possible to locate components 

to minimize the potential that fire will 

occur and threaten the equipment, 

and to increase the likelihood that 

those fires that do occur will be less 

severe. 

For example, putting digital 

components in a diesel generator bay 

or electrical switchgear room may be 

undesirable owing to the comparably 

high incidence of fire in such areas. 

Similarly, locating such components 

on upper-level decks overlooking a 

turbine hall may be poor practice 

because of the numerous potential fire 

sources in the area and the relatively 

high frequency of fires in such a large 

space. Such large open areas hold the 

potential for widespread and 

unchecked smoke dispersal under 

comparatively severe fire conditions. 

In making the placement decision, the 

designer must also balance the 

considerations of the external fire 

threats to the new components against 

the internal fire threat represented by 

the new components themselves. 

There is a limited database on fire 

incidents, and while the data are 

sparse, experience suggests that 

printed circuit boards can represent a 

significant fire threat in terms of both 

the initiation and the spread of fires. 

Thus, it is also appropriate for the 

designer to include the consideration 

of how these new components might 

alter the fire risk perspective in 

critical plant areas. For example, 

placing such components in a cable 

spreading room might be a highly 

desirable option from the standpoint of 

accessibility and convenience, but may 

introduce a potential fire threat into

88
this fire area that was not previously 

considered. The designer should be 

careful to ensure that a critical set of 

cables or equipment is not subjected to 

a new fire threat as a result of the 

digital upgrade process. Clearly, the 

use of low-flammability materials, 

including the connecting cables, 

conformal coatings, and to the extent 

possible, the digital components, will 

also minimize the fire threat 

introduced by the digital system. 

Another consideration is that the 

traditional approach to fire 

risk/hazards assessments based on 

subdividing fire areas into fire zones 

for analysis may not be appropriate 

when the smoke issue is considered. 

In fire protection terminology, a "fire 

area" is a strictly defined term and 

implies that the area in question is 

fully bounded by rated fire barriers 

("rated" is used here to imply 

performance consistent with the 

ASTM E119 fire barrier test 

standard). In contrast, a "fire zone" is 

typically defined more loosely as a 

subsection of a fire area that is 

expected to largely contain a fire. Fire 

zones will often include nonrated 

boundary elements, and may even 

include open doorways, penetrations, 

and/or hatchways between zones. 

This approach is generally based on 

the potential for the spread of flames 

and/or quantities of heat sufficient to 

cause thermal damage. In terms of 

smoke damage, these considerations 

are not sufficient. While an open 

hatchway in the ceiling of a 

compartment may not represent a 

realistic conduit for the spread of 

flames (due, for example, to the lack of 

proximate combustibles), it certainly

represents a ready conduit for the 

passage of smoke. 

The digital system design must take a 

broader view of the fire threat than 

that traditionally taken in a fire 

risk/hazards assessment. This view 

should include the realistic 

consideration of potential smoke 

movement and the placement of 

components accordingly, especially 

placement for redundant safety trains. 

For example, the 10CFR50 Appendix 

R requirement for the physical 

separation of redundant trains by a 

minimum of 20 horizontal feet with no 

intervening combustibles is based on 

concerns about thermal damage and 

flame spread. This provision will not 

mitigate smoke damage in any 

meaningful way. The digital design 

should take a more complete view of 

the potential fire threat. 

3.2.5 Fire Barriers 

Fire barriers will represent the 

ultimate line of defense against smoke 

spread. Such barriers include the 

primary structural elements such as 

walls, floors, and ceilings, but also 

secondary elements such as 

penetration seals, doorways, 

hatchways, and ventilation dampers. 

To address smoke damage concerns, 

the designer will need to consider how 

fire barriers can be utilized to protect 

critical components. Many aspects of 

this topic have already been discussed 

in Section 3.2.4 in relation to 

placement decisions. However, there 

are additional aspects of fire barrier 

elements that should be specifically 

recognized.

89
While the primary barrier elements 

(e.g., structural walls) will generally 

contain smoke effectively, the 

secondary elements (e.g., doors, seals, 

and dampers) may allow the passage 

of significant quantities of smoke from 

area to area. Consider, for example, 

that fire barriers are not generally 

designed as smoke barriers, but rather 

are designed to limit the spread of 

flames and heat. This is especially 

true for U.S. fire barrier testing 

standards compared with 

internationally accepted testing 

standards. Specifically, U.S. testing 

standards impose no pressure 

differential across a barrier element 

during testing, whereas international 

standards do. This means that certain 

types of rated barrier elements will 

allow the passage of significant 

quantities of smoke under realistic fire 

conditions in which pressure will build 

up modestly in the fire compartment 

due to the generation of hot 

combustion products. 

Even given these observations, taking 

advantage of fire barriers in the 

digital design can substantially 

minimize the smoke damage potential. 

For example, while smoke leakage 

through secondary barrier elements 

may be observed even when the 

barrier performs as intended, the real 

likelihood that this leakage might lead 

to significant smoke buildup in 

adjacent areas is small for many 

situations. Provided that the 

movement is limited to general 

leakage, the adjacent area would need 

to be relatively small in volume for 

significant buildup to occur.

Another consideration in this process 

is the potential for smoke movement 

through a barrier due to a failure of 

the barrier element or delayed 

activation of an active barrier. For 

example, a normally closed fire door 

may be blocked open or may be opened 

by the plant personnel in an effort to 

access the fire area. Similarly, a 

normally open fire door may fail to 

close. These conditions would allow a 

significant and prolonged passage of 

smoke through the barrier. Another 

case to consider is that significant 

passage of smoke may occur before a 

fire barrier closure mechanism is 

activated. This might involve doors or 

ventilation dampers in particular. 

Note, for example, that a "fusible link" 

is a common actuation device for fire 

protection features. This device is 

heat activated and may cause a 

significant delay between the onset of 

smoke spread and the actuation of the 

protective feature. 

All of these factors should be 

incorporated into the digital design 

strategy. However, even given the 

observation that significant smoke 

spread through a fire barrier is 

possible, it can be highly beneficial to 

take full advantage of fire barriers in 

the design of digital equipment. Even 

lesser barriers and obstructions may 

provide some protection against the 

spread of smoke, albeit perhaps only 

for limited time periods. This would 

include ceiling-level obstructions such 

as beams and the soffit over open 

passageways between compartments. 

These features might provide some 

additional benefit because a longer 

time for detection, suppression, and 

manual intervention would be

90
available and hence the probability of 

such intervention would be greatly 

increased. 

3.2.6 Localized Encapsulation 

The potential for smoke exposure may 

be minimized through localized 

encapsulation of the digital device. 

This is not in reference to conformal 

coatings (discussed elsewhere in this 

report), but rather to the placement of 

digital components in localized 

protective enclosures. For example, 

critical digital components could be 

placed in higher rated National 

Electrical Manufacturers Association 

enclosures for protection, or in 

enclosures with separate 

environmental controls (see related 

discussion on localized ventilation 

features in Section 3.2.8). Such 

measures may provide significant 

additional levels of smoke protection. 

There are numerous complicating 

features of such an approach that may 

limit its applicability. For example, 

allowances must be made for: 

"* maintenance access 

"* electrical access 

"* heat removal 

"* cost 

"* physical size constraints 

"* separation of redundant trains 

However, for at least some 

applications, localized encapsulation 

may represent a viable alternative. 

The overriding factor in this approach

would be to ensure that the component 

is not directly exposed to the smoke. 

This will require a level of 

environmental isolation well beyond 

that employed in general practice for 

electronic equipment. For example, 

the SNL tests found that simply 

housing the digital components in a 

computer chassis provided little or no 

protection. This is because the 

ventilation fan that is inevitably 

provided to remove heat from such a 

chassis circulates the smoke and 

compromises the potential protection 

(see related discussion on local 

ventilation features in Section 3.2.8). 

3.2.7 General Ventilation 

Features 

The tests at SNL have demonstrated 

important aspects of the smoke 

exposure problem that will be directly 

influenced by the ventilation systems 

servicing a given area. Hence, it may 

be possible to optimize the ventilation 

system's physical configuration and 

operating strategy so as to minimize 

the smoke damage potential. These 

options include, in particular, the 

prompt and effective removal of smoke 

from the fire area, the management of 

smoke transport, and the control of 

humidity levels. 

The SNL tests demonstrated that 

there are two modes through which a 

damaging smoke exposure can occur. 

The first is direct deposition of smoke 

onto the surfaces of a component. This 

is the classically considered smoke 

exposure problem. The SNL tests 

indicate that this is certainly a 

concern, especially in the context of 

long-term damage and degradation by

91
corrosion. However, the SNL tests 

also show that short-term damage 

may occur as a result of the mere 

presence of airborne ionized smoke 

particulates. Both of these damage 

modes can be effectively mitigated by 

prompt and efficient removal of smoke 

from the fire area, or by preventing 

smoke from spreading from adjacent 

areas into the area housing the digital 

equipment. 

Unfortunately, testing has 

demonstrated that common design 

practices for ventilation systems will 

not provide efficient smoke 

management or smoke removal from 

general plant areas28 . The most 

effective systems for smoke removal 

will have high flow rates in 

comparison with the room volume (one 

room air change per hour would be 

typical, with on the order of 10 air 

changes per hour a rough upper bound 

on typical ventilation rates). 

However, for efficient smoke removal, 

it is also important that the system be 

designed so that exhaust is taken from 

near the room ceiling and fresh 

incoming air is introduced near the 

floor of the space (to minimize mixing 

of the hot upper-layer gases and to 

take advantage of the natural 

buoyancy of hot fire products). 

It is not expected that the digital 

design process will typically provide 

significant opportunities to enhance 

the general ventilation system, 

especially in retrofitting existing sites. 

These systems tend to be deeply 

integrated into the structure of the 

plant and hence are often not 

amenable to significant change. 

However, the designer may have the

opportunity to select the area in which 

the components will be housed or may 

have the opportunity to make minor 

modifications to the ventilation system 

to enhance smoke removal. For 

example, given typical industrial 

ventilation configurations (inlet and 

outlet ports located high in the room), 

a simple extension of the inlet ports to 

floor level in the room might provide a 

significant smoke control benefit. It 

may also be possible to add extra

capacity smoke purge fans in the 

exhaust stream. A review of the 

configuration and capacity of the 

ventilation system would be 

appropriate and might provide an 

additional criterion upon which to 

base the placement decision. 

There is a clear advantage in housing 

digital components in humidity

controlled environments. If the 

humidity level in the area is low at the 

outset of the fire, then the potential 

for short-term damage will be reduced. 

After the fire, the availability of a 

ventilation system with the capability 

to reduce humidity back down to 

levels below which significant 

corrosion will occur will also help 

significantly in the post-fire recovery 

process. Hence, normal ventilation 

systems can be included in the digital 

design process to help minimize the 

smoke damage concerns. Placing 

components in areas with well

designed and higher capacity 

ventilation systems will help to 

minimize both the deposition of smoke 

and the exposure of the equipment to 

airborne smoke. 

Another aspect of the general 

ventilation system design to be

92
considered is the fire response 

strategy for the system. There are at 

least four different strategies that can 

be implemented in response to a fire. 29 

One strategy is to completely shut 

down the ventilation system, both 

inlet and outlet flow. This is the 

typical approach taken in practice 

today, and it is often implemented 

through ventilation duct dampers 

installed at area boundaries and fan 

power control circuits tied, for 

example, to in-duct smoke detectors. 

This strategy will tend to deprive the 

fire of needed oxygen and minimize 

the potential for fire products to 

spread through the ventilation system, 

but it also allows fire products (heat 

and smoke) to accumulate unchecked. 

A second strategy is to raise both inlet 

and outlet flow rates to maximum 

levels. This would be a typical 

response to a fire in a critical manned 

area such as the main control room. 

This approach will tend to minimize 

increases in room temperature and 

reduce the density of the smoke. 

However, it will also cause enhanced 

mixing of the smoke layer given 

typical ventilation configurations, and 

may actually result in faster descent 

rates for smoke layers (this is the rate 

at which the smoke layer develops 

from the ceiling downward). 

The third strategy is to shut down the 

inlet flow into the fire area and to 

maximize the outlet flow from the fire 

area. At the same time, the inlet flow 

rates into the adjacent areas are 

pushed to maximum levels, and 

exhaust flow from those adjacent 

areas is restricted or shut down. In 

this configuration, fresh air flows into 

the adjacent areas and moves toward

the fire area. Fire products are 

preferentially vented from the fire 

area. The potential for smoke to 

spread out of the area in which the fire 

occurs is minimized, and the potential 

for removal of smoke from the affected 

fire area is maximized. 

A similar but subtly different 

approach is to manage the airflow, not 

necessarily for the fire area, but 

rather for the protection of certain 

critical areas. In this case, the 

exhaust from a critical plant area is 

restricted or completely shut down in 

the event of a fire anywhere in the 

general vicinity of that area. At the 

same time, the inlet flow ventilation 

rate to the critical area is pushed to 

maximum levels. In this way, the 

potential for smoke to spread from 

adjacent areas into the critical area is 

minimized. Testing at the Heiss 

Dampf reactor (HDR) facility in 

Germany has illustrated that this 

method can be quite effective in 

controlling smoke migration into 

protected areas. 30 

While the last two strategies have a 

clear potential to contribute to an 

overall smoke management strategy, 

their implementation in existing 

plants will be problematic. These two 

strategies require the ability to realign 

ventilation systems in response to a 

fire that goes beyond the capabilities 

currently provided in typical power 

plant installations. This is likely to be 

more of a consideration in the design 

of new reactors (or other critical 

structures) than it will be in the 

retrofitting of existing reactors. 

Nonetheless, in some cases there may 

be an opportunity to optimize the

93
ventilation design strategy, and it 

should be taken advantage of. 

As a final note to this discussion of 

general ventilation systems and the 

smoke control issue, the design of 

digital equipment should recognize 

that ventilation systems are not in 

general designed with smoke 

management or mitigation in mind. 

In fact, ventilation systems may 

actually contribute to the spread of 

smoke to areas remote from a fire. 

For example, in at least one instance a 

ventilation system introduced carbon 

dioxide from a fire suppression system 

actuated in a ventilation fan area into 

the main control room during an 

emergency event. 31 A similar 

potential exists for smoke movement 

as well. Another aspect of this issue is 

that general ventilation filtration 

systems will quickly become clogged 

with smoke and will likely force a 

system shutdown in the event of fire 

(owing to excess pressure across the 

filter elements). This was observed 

repeatedly in the German HDR tests 

despite efforts to plan for and manage 

this behavior. Hence, while some 

benefit may be gained through 

optimization of the ventilation system, 

the robustness and longer-term 

performance of such measures may be 

suspect. The ventilation interactions 

can be both beneficial and detrimental 

to the smoke damage concerns. 

Careful assessment will be needed to 

properly balance these effects. 

3.2.8 Local Ventilation Features 

In addition to the design of general 

area ventilation systems, there is also 

the potential to implement localized

ventilation features to enhance the 

protection of critical digital 

components from smoke damage. 

These strategies might be especially 

useful in combination with the 

encapsulation strategy discussed in 

Section 3.2.6. 

One strategy that was discussed in the 

context of encapsulating the 

equipment was to place the critical 

components in an environmentally 

sealed panel. Electrical panels of this 

type can be readily obtained on the 

commercial market either with or 

without environmental controls (local 

air conditioning and/or air filtration). 

In effect, the digital components can 

be placed in an independent sealed 

environment. There are, however, a 

.number of potential disadvantages to 

this approach. The most obvious is 

that because the waste heat from the 

digital components may need to be 

removed, a sealed panel installation 

might require an independent local 

refrigeration system. This introduces 

yet another system into the plant that 

would require maintenance. The 

design would also need to assess the 

performance of the digital system and 

its effect on plant safety if the local 

environmental system fails (buildup of 

waste heat within the panel might 

eventually cause failure of the digital 

system; the question would be how 

long this would take). It would almost 

certainly be judged impractical if the 

importance of these environmental 

control systems elevated them to the 

level of a safety-grade system. 

Another obvious disadvantage is the 

added cost of these systems in 

comparison with generic electrical 

panels. It may also be prudent to

94
allow for some fire detection capability 

within the sealed environment. For 

critical applications, the additional 

expense may be warranted. For 

example, this might be one means of 

protecting one train of equipment from 

smoke damage in areas such as the 

main control room, where multiple 

equipment trains might converge, 

particularly if a 24- or even 72-hour 

survival time in the absence of the 

environmental control system could be 

demonstrated. 

A second less invasive strategy would 

be to provide filtered air flows into and 

out of the electrical panels of critical 

interest. As noted above, some 

mechanism for removing waste heat 

from the digital components will likely 

be needed. This strategy would allow 

for this through communication with 

the general enclosure volume, but only 

through a filtered local air handling 

system. That is, airflow could be 

provided through filtered, fan-driven 

inlets and through filtered outlets. 

An enhancement of this strategy 

might involve placing the inlets as low 

as possible in the panel to take 

advantage of the hot layer effect in an 

enclosure fire. This would, however, 

also make the filters more prone to 

collection of general dirt and dust from 

the ambient environment as well. 

While the technology required for this 

approach is not especially 

sophisticated, and should be readily 

available, it is likely to be unproven 

for smoke mitigation. As noted above, 

the German HDR tests illustrated 

quite clearly that high-efficiency 

ventilation filters were easily clogged 

by smoke. Some evaluation of the

performance of such filtration systems 

would likely be needed. In order to 

prevent rapid clogging, a cascading set 

of progressively finer filter elements 

may also be needed to ensure that the 

finer filters are not immediately 

clogged by the coarse smoke 

particulates. As with the full isolation 

strategy discussed above, the impact 

of loss of airflow on system 

performance and plant safety would 

have to be assessed. 

A final strategy would be to provide a 

direct connection between the 

electrical panels and the normal area 

ventilation system. This has already 

been implemented at some plants, in 

particular in the main control room. 

One such configuration involves the 

use of the electrical control panels, 

either individually or as a group, as a 

return air plenum for the ventilation 

system (see, for example, the 

ventilation configuration for the 

LaSalle main control room). That is, 

fresh air is introduced into the general 

area and exhaust air is taken from the 

electrical panels. Ventilation grills in 

the faces of the electrical panels allow 

air to flow from the general area into 

the panel. This allows the direct 

removal of waste heat from the panels 

using the normal ventilation system. 

This approach would ensure that the 

smoke from a developing panel fire 

was preferentially removed from the 

area, at least during the initial stages 

of the fire (the system would likely be 

overwhelmed within a short time if 

the fire continued to grow). However, 

in the event of a fire external to the 

panels, this approach would actually 

worsen the exposure problem by

95
drawing the smoke directly into the 

electrical panels. 

An alternative version of this strategy 

would be to provide a fresh air supply 

link from the normal ventilation 

system directly into the critical panels. 

In the event of fire, so long as the 

fresh air supply is maintained, the 

panel would be provided with some 

added protection from smoke 

intrusion. This would, however, tend 

to force smoke from a given panel fire 

more quickly into the general area. 

Individual panel dampers actuated by 

smoke or heat detectors could easily 

manage this. The results of the 

German HDR fire tests would provide 

useful design and optimization data in 

this regard (Kernforschungzentrum 

Karlsruhe developed a relationship for 

the pressure differential required to 

prevent smoke intrusion through an 

opening). 

3.3 Fire Protection Analysis 

Tools 

3.3.1 Fire Risk Assessment 

The problem of smoke damage is 

largely neglected in current fire risk 

assessments. At most, one can argue 

that certain aspects of the initial 

screening phases inherently include 

the potential for smoke damage. That 

is, in screening it is quite common to 

assume that given a fire in an area, all 

of the equipment in that area will fail. 

For some analyses, similar screening 

assessments are also made for 

combinations of fire areas that assume 

the spread of fire or fire damage 

beyond the compartment of fire origin.

Provided that adequate consideration 

is given to the potential extent of 

smoke spread, this would include the 

possibility that smoke was the cause 

of the equipment failures. Any 

scenario with a risk contribution 

above a certain threshold level given 

these conservative assumptions would 

be analyzed further. 

However, in the detailed 

quantification of surviving scenarios, 

this assumption of widespread damage 

is almost certain to be significantly 

relaxed. In particular, given current 

methods, the detailed quantification 

process focuses only on the issue of 

thermal damage. The extent of the 

thermal damage will typically be 

estimated using a computer fire model 

to predict the rates of fire growth and 

spread and the timing of equipment 

damage. This is then weighed 

probabilistically against the possibility 

that the fire would be suppressed 

before critical damage occurred. No 

direct consideration of smoke damage 

is currently included in this process. 

This is due to both a lack of proven 

methods of analysis and the general 

lack of knowledge regarding the 

vulnerability of plant equipment to 

smoke damage. 

Hence, fire risk assessment methods 

as they currently exist are not 

conducive to a detailed assessment of 

smoke damage. At most, a risk 

assessment might provide some coarse 

bounding estimates of the smoke 

damage risk. This could be 

accomplished by supplementing 

current screening methods to include a 

refined widespread damage 

assumption. That is, rather than

96
assuming that all equipment in an 

area is damaged, the assumption 

could be relaxed to include direct 

estimates of thermal damage (as per 

current practice) plus an assumed 

failure of all (or ifjustified, a subset) 

of the smoke-vulnerable digital 

components. This analysis would 

have to include consideration of smoke 

spread between fire areas or fire 

zones. This would allow for some 

conservative quantification of the fire 

risk associated with a given digital 

design and plant implementation. 

This type of analysis could be 

accomplished with relative ease given 

current methods. 

3.3.2 Fire Modeling 

In order to make a reasonable smoke 

damage prediction; several questions 

must be answered. These include: 

"* How much smoke is being 

generated in the fire as a function 

of time? 

"* Where does the smoke go once it is 

released from the fire (transport)? 

"* How much smoke accumulates in 

the immediate vicinity of the 

critical components? 

"• What is the threshold of damage 

for the components of interest due 

to airborne smoke particulates? 

"* At what rate is smoke deposited 

onto the surface of the component, 

and what is the deposition damage 

threshold?

"* How will suppression of the fire 

and post-fire recovery actions affect 

the smoke exposure? 

"* What is the relative humidity of 

the environment around the digital 

component? 

There are currently no known fire 

models that directly assess the 

potential for a fire to lead to smoke

induced equipment failures at any 

level of analysis. In particular, while 

some of the existing enclosure fire 

models do include predictions of smoke 

generation and spread, none of the 

current models includes any methods 

for assessing smoke damage to 

equipment. 

The current approach to smoke 

modeling requires that the user 

effectively specify the rate of smoke 

production for the scenario under 

analysis. That is, the current state of 

fire modeling does not support a priori 

predictions of smoke generation. Only 

in the most advanced fire models are 

smoke generation and kinetics models 

being implemented. These models 

remain in the development stages. In 

more accessible models the user 

specification will typically take the 

form of a fractional parameter 

(between 0 and 1) that sets the 

percentage of the fuel mass burned 

that is liberated from the fire as 

smoke particulate. Enclosure models 

can then track the airborne 

concentration of smoke as a mass per 

unit volume parameter by tracking 

smoke generation rates and the rate of 

smoke removal from the enclosure due 

to either natural or forced ventilation 

flows. This is a common capability in

97
current fire models. However, this 

does not provide all of the answers 

needed. The "missing links" in this 

process are (1) a submodel to assess 

the impact of airborne smoke on 

equipment, (2) submodels to predict 

the rates of smoke deposition onto 

component surfaces, and (3) data on 

the smoke damage thresholds for 

equipment. 

Given current modeling capabilities 

and limitations, it would be possible to 

perform only relatively crude and 

bounding calculations to assess the 

likelihood of smoke damage in a given 

fire scenario. In particular, it would 

be necessary to make bounding 

assumptions regarding smoke 

generation and transport as well as 

equipment damage thresholds. This 

would leave a high level of uncertainty 

in the results obtained. Coupling such 

analyses with the statistical methods 

of fire risk assessment (see Section

3.3.1) would clearly be appropriate. 

That is, little reliance should be placed 

on any individual analysis scenario, 

but a statistical weighing of many 

scenarios coupled with treatment of 

the inherent uncertainties could 

provide relevant insights. 

It is likely that advances in these 

capabilities will be forthcoming in the 

next decade through the development 

of a number of advanced fire physics 

models. These advanced models have 

not typically been used in nuclear 

plant risk assessments because of the 

high level of expertise required, the 

intensive level of model setup that is 

required for a given simulation, and 

the computing cost associated with 

these detailed simulations. However, 

the state of the art, the level of 

simplicity and user friendliness, and 

the cost of computing are all moving 

toward more acceptable levels.

98
4 POST-FIRE RECOVERY 

4.1 Introduction 

Fire causes permanent damage to 

electronics through excess heat and 

contamination from smoke. Since 

some smoke contains acidic 

compounds, it's important to reduce 

the corrosive action of these 

compounds as soon as possible. Many 

insurance companies have studied 

recovery from fires, and the long-term 

effects of smoke are well known. 32 

Since losses for a business not only 

include equipment but also losses in 

productivity, it is often better to 

refurbish equipment rather than 

replace it. This is especially true for 

specialized equipment that has a long 

lead-time for delivery. Depending on 

the equipment and the amount and 

type of smoke deposition, equipment 

may be cleaned and reused without 

concern for long-term effects.33 

Besides the equipment itself, for 

computer systems it is often very 

important to recover data. Data have 

been recovered from hard disks that 

have been exposed to fire.34 This type 

of work is best left to professional data 

recovery services. 

The amount of chlorides or sulfates 

deposited on the surface of electronic 

equipment has been the standard for 

determining whether the equipment 

should be cleaned or if it must be 

replaced. Table 16 shows some 

guidelines that have been found in the 

literature on cleaning electronic 

equipment. Claims have been made 

that up to 80 to 85% of the cost of

replacement equipment may be saved 

by swift response to a fire. 

4.2 Steps to Reduce Smoke 

Damage 

Common sense steps to reduce 

permanent damage to equipment are 

listed below: 

1. Remove the power from the 

electronics if it can be done safely. 

Many times removing the power 

will be necessary to put out the 

fire. Power on the electronics 

accelerates corrosion in two ways: 

(1) potential fields tend to attract 

more soot because the soot 

particles are charged and (2) 

potential fields between traces on 

printed wiring boards encourage 

dendridic growth of the metal. 

2. Vent out the smoke. Smoke 

contains ionized particles that can 

increase the chance of arcing. If 

the smoke is vented, it reduces the 

exposure of metal to acidic gases. 

3. Lower the humidity. Acid gases 

tend to act faster in higher 

humidity environments. Humidity 

increases leakage currents of 

contaminated printed wiring 

boards. 

4. Evaluate the equipment for smoke 

deposition. Evaluating the amount 

of chloride deposited generally does 

this. This will help determine 

what equipment should be cleaned

99
and what equipment should be 

scrapped. Do not waste time and 

effort on equipment that will not be 

usable after cleaning. 

5. Clean salvageable parts. Several 

commercial companies specialize in 

disaster recovery and can provide 

help. Recovering equipment can

reduce the monetary losses due to 

fire. A combination of detergent 

and water has been cited as the 

most effective cleaner rather than 

petroleum-based or halogenated 

solvents.

Table 16. Smoke Deposition Guidelines 

Deposition amount (gg C1/cm 2) 

Reference Should clean if greater than Replace if greater than 

Bellcore 25 31 93 

AREPA Benelux * 10-20 -

Elektronik Centralen * 10 

4.3 Environments that 

Contribute to Smoke 

Damage 

As shown in Section 2 of this report, 

high humidity and high voltage are 

both significant contributors to failure 

in smoke. 

4.3.1 High Humidity 

Humidity greatly increases the 

amount of damage that can result 

from smoke. When the humidity is 

high, water combines with the salts 

(such as ZnC12) formed from the 

interaction between acid gases in the 

smoke and the metals, such as lead 

and zinc that can be part of the 

mechanical structure or solder in the 

electronics. These salt solutions may 

be higher in conductivity than either 

the salt in a dry atmosphere or a 

higher humidity atmosphere without 

the smoke contamination.35 Thus,

100 

smoke works synergistically with 

humidity to increase leakage currents. 

The effects of smoke and dust 

contamination in a humid 

environment have been studied by 

Comizzoli, 36 Caudill,16 and Chapin 17 

on comb patterns. In general, for 

outdoor dust samples, the log of the 

leakage current varies linearly with 

the relative humidity. However, if the 

smoke sample contains a large 

fraction of graphitic carbon, as in the 

samples taken from the Kuwait oil 

fires, the leakage current is fairly 

constant, but higher, until the RH 

goes above 60%. Then the log of the 

leakage current increases linearly 

with RH.35 

Caudill and Chapin have found a wide 

variation in behavior of the leakage 

currents from comb patterns exposed 

to smoke from a range of cable 

materials. Some of these comb 

patterns had high leakage currents

100
throughout all humidity levels and 

some maintained a profile similar to 

outdoor dust samples. The materials 

that exhibited higher leakage 

currents, independent of humidity, 

were the cables with higher heat of 

combustion. We surmise that these 

cables burned more completely and a 

greater amount of smoke was 

deposited on the combs. 

4.3.2 High Voltage 

High voltage also contributes to higher 

leakage currents because high 

potentials attract more charged smoke 

particles to the surfaces. The higher 

accumulation of particles increases the 

leakage currents. In addition, most 

high-voltage circuits have high input 

impedance. This makes these types of 

circuits more vulnerable to increased 

leakage currents because a small 

change in impedance will cause a 

relatively higher effect on the overall 

circuit. 

4.4 Cleaning Methods 

Some of the earliest smoke tests by 

SNL and ORNL included reusing 

smoke-exposed equipment to save 

money and time (see Section 2.3.1). 

Although equipment was cleaned after 

each smoke exposure, the network 

system never worked quite as well 

after the first smoke exposure. The 

baseline tests showed that the 

equipment was not cleaned well 

enough. The equipment was cleaned 

with a halogenated degreaser (like 

Freon) rather than a water-based 

detergent. Water-based detergents 

are better able to remove the salt

deposits that are formed on metals 

exposed to acid gases than organic 

solvents. Salt dissolves in water 

better than in an organic degreaser 

such as Freon or benzene. 

4.4.1 NRL Method of Cleaning 

Electronic Equipment 

Exposed to Smoke 

Baker and Bolster (Naval Research 

Laboratory) detailed how to clean 

electronics that have been 

contaminated with seawater, oil, and 

smoke deposits. " This process was 

used on the USS Constellation after it 

caught fire and the fire was 

extinguished with seawater.38 

Although the process predates wide 

use of digital equipment, the 

chemicals used and the steps taken 

are well documented and the process 

should work on both analog and 

digital systems. The equipment 

described for this process is quite large 

and includes an ultrasonic tank that is 

4 x4 x 4 ft in size. The NRL steps for 

cleaning electronics after exposure to 

fire and smoke are: 

1. Alkaline presoak-Soak 2-5 min at 

120-160'F in a trisodium 

phosphate mixture of 2-3 oz of 

trisodium phosphate per gallon of 

water (1/2 cup of dishwashing 

machine detergent/ gallon of water) 

2. NRL emulsion ultrasonic wash

Ultrasonic cleaning for 2-20 min, 

depending on the size of the parts 

in the NRL cleaning emulsion (94 

vol.% dry cleaning solvent, Type II; 

5 vol.% fuel oil, diesel marine, Type 

I; and 1 vol.% surfactant such as 

polyethylene glycol 400 Dioleate),

101
diluted in a 50-50 mixture with 

water or a general-purpose 

detergent (Mil-D-16791E, Type I) 

mixture with water (1 oz per gallon 

of water.) 

3. Ultrasonic rinse-Rinse in plain 

water. 

4. Compressed air blow-Blow water 

from equipment at 10-20 psi 

5. Oven drying or spray-dry-dry in 

an oven at 120-160 'F or spray 

with water-displacing fluid such as 

1-butanol (n-butyl alcohol) 

6. Test equipment for function and 

cleanliness. 

DOE rules on post-fire recovery refer 

to the NRL method of cleaning. This 

method is also specified in MIL-STD

2110. 

4.4.2 Swedish Institute of 

Production Engineering 

Research Method of 

Cleaning Smoke

Contaminated Electronics 

Cider compared three printed circuit 

board cleaning methods for boards 

exposed to smoke from a fire composed 

of a combination of wood, PVC cable, 

cellular plastic, and FR-4 boards. 33 

The printed circuit boards contained 

both surface-mounted components and 

interdigitated comb patterns for 

surface insulation measurements. 

Dipping them into an acrylic 

conformal coating, Humiseal 1B31, the 

same coating SNL used in the coating 

tests conformally coated some of the

boards. The three treatment methods 

Cider compared were: 

1. A five-step process: 

a. Spray boards with cleaning 

agent (commercial alkaline 

cleaning agent, Euroclean F-42). 

b. Manually brush boards. 

c. Rinse with tapwater and 

deionized water. 

d. Dry with high-pressure air. 

e. Dry in vacuum at 40 'C. 

2. A three-step process: 

a. High-pressure spray at 50 'C for 

20 minutes with a mixture of 75% 

isopropanol and 25% water (in an 

automatic defluxing machine). 

b. Rinse with isopropanol. 

c. Dry with warm air. 

3. A six-step process: 

a. Rinse with tapwater. 

b. Dip into Euroclean F-42 and 

brush manually. 

c. Leach in cleaning solution 

(Euroclean F-42) for 10 minutes. 

d. Ultrasonic agitation for 10 

minutes in cleaning solution. 

e. Rinse with tap water and 

deionized water. 

f. Dry in pressurized nitrogen at 

80 'C for 1 hour.

102
The methods were compared by 

leaching the test boards in water and 

analyzing the water by ion 

chromatography for Pb, Si, Cl-, Br-, 

S042-, and organic carbon. Out of 

these different methods, the best was 

the six-step process with ultrasonic 

agitation and the worst was the three

step process using a high-pressure 

spray. The conformal coating 

protected the electronics, but made 

them more difficult to clean. 

4.4.3 Commercial Cleaning of 

Electronics 

Data are typically recorded on 

magnetic media of some type, either 

tape or hard disks. Read/write heads 

hover very close to the surface of 

magnetic disks, and although the hard 

disk assembly is located in a filtered 

box, smoke particles may enter and 

cause the heads to stick to the disk or 

may clog the space between the head 

and disk.34 If hard disks are exposed 

to smoke, it is important that the disk 

is not accessed until after it is cleaned 

because data can be lost. In general, 

many of the establishments who clean 

their electronic equipment after a fire 

feel that they may be able to recover 

very well and have a high level of 

confidence in their cleaned electronics 

if the cleaning is performed 

immediately and thoroughly. The 

decision to clean up the equipment 

may depend upon whether it is a 

commercial off-the-shelf item or a 

specialty item. 

Companies who clean electronic 

equipment may be found in the yellow 

pages or on the World Wide Web. The 

companies are usually listed under

"Disaster Recovery" or "Restoration," 

but the majority of these listings 

mostly deal with cleaning of buildings, 

carpets, and drapery after a fire.

103
5 CONCLUSIONS

This section summarizes the insights 

gained from this project, areas that 

have not been addressed but should 

be, and future work. Although not 

stated at the outset of the program, 

the overall goal of subjecting digital 

systems to smoke is to determine the 

amount of risk that smoke poses for 

equipment in a nuclear power plant 

and to identify methods by which that 

risk can be minimized or managed. 

Such a goal requires certain 

information, some of which may not be 

available. Risk assessments require 

knowledge about failure thresholds, 

failure modes (what the system will do 

when exposed to smoke), when failure 

takes place (immediately or after a 

few weeks), and failure probability. 

They also require knowledge on how 

often the equipment is likely to be 

subject to these failure thresholds, or 

in terms of smoke, fire frequency, 

amount of smoke generated, and 

smoke transport. In addition, during 

a fire, equipment is not subject to 

smoke only, but often to elevated 

temperatures and humidity. These 

stressors increase the vulnerability of 

digital systems because they interact 

synergistically. 

The main results found to date consist 

of preliminary information on how and 

when failures will occur, the basic 

underlying causes of failure, and 

methods of smoke protection. Insights 

from this project are important in 

determining how to proceed to an 

ultimate goal of determining or

minimizing risk. Although research 

issues remain to be addressed before 

susceptibility to smoke can be fully 

characterized and a comprehensive 

consensus approach to mitigating the 

consequences of exposure can be 

established, several important 

conclusions can be drawn from project 

findings. Based on the investigation 

of smoke susceptibility and the 

resulting understanding of key failure 

mechanisms, it is clear that smoke has 

the potential to be a significant 

environmental stressor that can result 

in adverse consequences. However, 

there is no practical, repeatable 

testing methodology so it is not 

feasible to assess smoke susceptibility 

as part of environmental qualification. 

As a result, the most reasonable 

approach to minimizing smoke 

susceptibility is to employ design, 

implementation, and procedural 

practices that can reduce the 

possibility of smoke exposure and 

enhance smoke tolerance. In 

particular, current fire protection 

methods are an appropriate 

preventative approach, employing 

isolation and detection practices. 

Additionally, post-event recovery 

procedures can mitigate the extent of 

smoke damage. Finally, there are 

design choices and implementation 

practices that can reduce equipment 

susceptibility to smoke exposure, such 

as chip packaging and conformal 

coatings. In the absence of consensus 

methods and practices for smoke

tolerant design and implementation,

105
the most effective approach is to 

rigorously adhere to the fire protection 

guidance given in Appendix R of the 

Code of Federal Regulations, Title 10, 

Part 50. 

5.1 Summary of Insights 

5.1.1 Environmental Stress 

Characteristics of Smoke 

How and When Smoke Causes 

Digital Equipment Failure 

When digital systems were tested 

(Section 2.3.1), the smoke caused 

errors during and within 24 hours of 

exposure. Most of the errors were 

diagnosed as errors in transmitting 

data from one computer or 

microprocessor to another. In general, 

the digital systems tested were 

designed to convert an analog to a 

digital signal and communicate the 

digital signal back to a main 

computer. There was no evidence of 

the smoke affecting the conversion 

from analog to digital; as long as the 

systems were communicating, these 

conversions were accurate. 

The digital systems failed 

intermittently, except for the case of 

very high fuel loads on the fiber optic 

modules, and networked systems 

merely retransmitted correct data. 

This was not the case for the serial 

communications port because once 

data were corrupted, the program 

halted, but including error handlers 

for these transmission errors could 

have compensated this for. It would 

be better in most situations that a 

digital system fails to communicate

instead of communicating erroneous 

information. 

Causes of Failure 

Smoke has been postulated to cause 

failure by many means: corrosion of 

metals, increased resistance of contact 

points, increased leakage currents, 

and impedance of small motion by 

smoke particles (i.e., motion of 

recording head on a disk drive). The 

results of the tests on the 

experimental digital safety system 

and the multiplexer imply that 

increased leakage currents or shorts 

are the most immediate problem. This 

is backed up by the tests on the 

functional circuit boards; current 

leakage is the most severe immediate 

reaction to smoke. Other effects have 

been noted during the functional 

board tests, but they were not 

significant. These include increases in 

resistance (open-circuit faults) from 

either component heating or 

breakdown of solder joints and 

connections. 

The highest leakage currents occur 

when the smoke is suspended in the 

air because smoke particles are 

attracted to electrically charged 

surfaces and build conductive bridges. 

Air currents easily break these 

bridges, but while smoke is 

suspended, a supply of particles is 

available to rebuild the bridges. 

Leakage currents can continue to be 

high after the smoke has been 

removed from the air if the soot bridge 

is supported by a surface, for example, 

on a printed circuit board.

106
The amount of smoke necessary to 

cause failure depends upon the 

technology that is being used. Analog 

signals typically provide a current 

level (e.g., 4-20 mA) or a voltage level 

(e.g., 0-10 V) as a means to transmit a 

signal. Such a signal would be 

difficult to short by means of smoke 

since such circuits usually are of low 

impedance (50 Q) and in general, the 

smoke has an impedance of 1000 ohms 

or higher. In the newer digital 

equipment, however, it may be easier 

to cause failures since a momentary 

short may disrupt communications. 

Smoke deposition measurements have 

been a typical method of determining 

electronic equipment failure. There 

are several "rules of thumb" about how 

much deposition, particularly of 

chloride, will cause smoke damage. 

For example, the Bellcore Company 

uses 93 mg/cm 2 as a measure of how 

salvageable equipment is. 25 Of 

course, after a fire, smoke deposition 

is all that is available as a measure. 

There is no way to determine how 

much smoke was in the air. 

5.1.2 Methods of Smoke 

Protection 

There are four basic methods for 

protecting electronics from smoke: (1) 

prevent fires, (2) control the 

movement of smoke, (3) protect the 

electronics by physical methods, and 

(4) locate components to minimize 

exposure to fire and smoke. Methods 

1 and 2 are addressed in fire 

regulations for nuclear power plants 

as discussed in Section 3. The third 

method can be difficult because all 

electronics use power and are a source

of heat. If electronics are enclosed to 

protect them from smoke, they can 

overheat. Fans that are used to cool 

electronics under normal operations 

can distribute smoke and soot 

throughout the chassis. It would be 

best in terms of smoke if the use of 

fans were avoided if possible. The 

fourth method is most amenable to the 

digital design process, but 

implementation options may be 

limited. 

Conformal coatings are a good way to 

protect electronics; however, 

connectors are generally not coated 

since they could not then provide an 

electrical connection. Use of coatings 

may also change flammability 

properties and may increase fire 

severity potential. Parylene is a very 

good coating, but it is expensive to 

apply, requiring vacuum deposition. 

Polyurethane and dipped acrylic 

coatings also performed very well in 

the SNL testing. Coatings should be 

selected carefully because they can 

cause problems if the underlying 

electronics are not cleaned before 

coating or if the coating starts to 

delaminate.39 Then contamination and 

humidity may be trapped under the 

coating and cause leakage current 

problems. 

Additional strategies for reducing the 

potential for smoke damage to digital 

systems include the following: 

- Identify the components of 

the digital system that are 

most vulnerable to smoke 

damage. Protection of these 

components could improve 

the overall system smoke

107
tolerance. 

- Connectors are not 

amenable to protection by 

conformal coatings. Identify 

and utilize connector 

configurations or designs 

with greater smoke 

tolerance. 

- Different chip technologies 

will have different smoke 

tolerances. Identify and 

utilize chip technologies with 

greater smoke tolerance. 

5.1.3 Qualification Tests 

At this time there are no standard 

tests for the effects of smoke on the 

operability of electronics. Current test 

standards that measure only metal 

loss are not adequate for this need. 

The development of such a test 

requires rigorous comparisons of test 

methods and results performed at 

different organizations. At present 

this process is in its infancy. 

Standard tests, such as in IEEE 323

1983, are performed for electronics 

under controlled temperature and 

humidity conditions. These tests 

typically include standard 

measurements of the environment. 

For smoke, there is no standard 

measurement that would convey all of 

the variables that can be created in a 

smoke environment. For example, 

many different chemicals are added to 

the environment; the balance of 02, 

CO, and C02 changes; and the 

particles in the air are charged and 

vary in size and mobility. Creating a 

repeatable smoke environment is also 

difficult. Tests using the same 

starting conditions had slightly 

different results.

At this time developing a standard 

test does not appear feasible. Such a 

test would require the participation of 

many organizations that could 

possibly use such a qualification test 

and many laboratories that could 

perform the test. Perhaps if there is 

enough interest in the subject, a 

standard could be developed in the 

future. 

5.2 Unaddressed Questions 

One question, which has not been 

explicitly addressed by the activities 

performed to date, is quantifying the 

fire risk contribution of smoke damage 

to electronics. In order to incorporate 

smoke damage into fire risk 

assessments additional information 

and tools will be needed. This 

includes the need to answer the 

following questions: 

- How do the test results 

gathered to date compare to 

the behavior in full-scale fire 

conditions? All of the tests 

describe here were 

performed under controlled 

small-scale test conditions. 

Real fires may present 

unique behaviors that have 

not been captured in these 

tests. One example is smoke 

interaction with galvanized 

metals, which has been 

observed in real fires but 

was not well captured in the 

small-scale tests. 

- What are the thresholds of 

damage due to smoke for the 

components that will be of 

interest to a fire risk

108
analysis? Can generic 

threshold values be 

established to cover broad 

classes of digital circuits and 

components? In order to 

estimate fire risk, one must 

be able to predict the fire 

conditions that will lead to 

the failure of specific 

components or classes of 

components. 

- Will smoke from a fire be 

transported to the site of the 

component of interest and in 

what quantities? A fire risk 

analysis must predict when 

environmental conditions 

will reach component 

damage thresholds. While 

some fire models do provide 

tools for the prediction of 

smoke generation and 

transport, their applicability 

to risk assessment has not 

been explored. 

- How will smoke-induced 

failures impact the operation 

of plant systems and 

components? In the tests to 

date the primary mode of 

faulting was temporary 

communication errors, but it 

is not clear whether this is 

the only potential fault mode 

of interest to risk analysis. 

Fire risk analyses must 

assess how digital circuit 

failures will affect plant 

systems and components. 

At present, fire risk assessments 

either assume that the electronic 

equipment malfunctions because the 

fire overheats cables leading to the 

equipment failure, or they assume

that any equipment within the same 

zone as the fire will be damaged. In 

the case of the first assumption, the 

damage may be underestimated 

because the smoke may spread to a 

wider area than the heat of the fire. 

In the case of the second assumption, 

the damage may be overestimated. 

Both overestimates and 

underestimates are undesirable in 

risk assessment work. 

5.3 Recommendations for 

Future Work 

In light of the unanswered questions, 

much future work must be done to 

adequately quantify the risk of using 

advanced digital systems in a nuclear 

power plant in the presence of smoke. 

In this sections we outline some of the 

work that will be needed to be done to 

truly determine this risk and to assure 

that future safety systems will not 

suffer from vulnerability to smoke. 

5.3.1 Validation and improvement 

of fire and smoke transport 

models 

Fire models now include methods to 

calculate how much smoke will be 

produced, although they are better at 

calculating the smoke from liquid 

fuels than solids. Fire models can also 

be used to calculate transport of the 

smoke due to ventilation. However, 

this transport should also include the 

effect of electrical fields that will 

increase transport to areas of high 

electric field density. Validation of the 

existing models and/or development of 

improved models are needed to

109
estimate how much smoke could be 

produced in the event of a fire. 

The fire models need to compare full

scale fires with small-scale fires as 

used in these tests. In these tests we 

have assumed that the smoke density 

that is produced can be scaled by 

volume, but we know from the high 

voltage tests that smoke does not 

distribute itself evenly. 

5.3.2 Develop models for different 

classes of electronic 

equipment 

The reliability of electronic equipment 

that is exposed to smoke is highly 

dependent on the type of equipment. 

Electronic equipment (such as 

transducers) produces many different 

types of signals using different 

electronic circuits. For example, for 

some transducers, the frequency of 

oscillation is the important parameter, 

while for others it is the current or 

voltage level. With so many different 

electronic properties that are 

important, assessing the reliability of 

this equipment may be an enormous 

task. 

Our data show that high impedance 

circuits are the most likely circuits to 

fail. If its highest impedance circuit 

can characterize equipment, then

failures can be predicted based on 

whether smoke is likely to create 

alternative circuit paths with similar 

levels of impedance. The risk of 

smoke causing failure can then be 

predicted from the smoke transport 

and the likelihood that smoke would 

create circuit paths of comparable 

impedance. This would establish 

damage thresholds for level of 

conductance to cause failure. 

5.3.3 Improve methods of smoke 

protection 

Conformal coatings have been 

suggested, as a method to protect 

electronics from smoke, however, some 

industries, such as 

telecommunications, will not use this 

method because of flammability 

issues. New coatings should be 

developed that are non-flammable, but 

can insulate electronics from smoke 

and dust. Another improvement 

would be to cool electronics without 

use of fans and air movement. Fans 

increase the exposure of electronics to 

airborne smoke and dust, and hence, 

can shorten the life of electronics 

through shorts even if they lengthen 

the life through cooling. Perhaps 

cooling fins or the like can be used 

instead of the fans.

110
6 References

1. K. Korsah, R.L. Clark, and R.T. Wood, Functional issues and environmental 

qualification of digital protection systems of advanced light-water nuclear 

reactors. Technical Report NUREG/CR-5904, ORNL/TM-12164, Oak Ridge 

National Laboratory, Oak Ridge, TN, 1994. 

2. S. W. Kercel, M.R. Moore, E.D. Blakeman, P.D. Ewing, and R.T. Wood, 

Survey of ambient electromagnetic and radio-frequency interference levels in 

nuclear power plants. NUREG/CR-6436, ORNL/TM-13171, Oak Ridge 

National Laboratory, Oak Ridge, TN, 1996. 

3. K. Korsah, Tina J. Tanaka, T. L. Wilson, Jr., and R. T. Wood, Environmental 

testing of an experimental digital safety channel. NUREG/CR-6406, 

ORNL/TM-13122, Oak Ridge National Laboratory, Oak Ridge, TN, 1996. 

4. M. Hassan and W.E. Vesely. Digital I&C systems in nuclear power plants; 

Risk screening of environmental stressors and a comparison of hardware 

unavailability with an existing analog system. NUREG/CR-6579, Brookhaven 

National Laboratory, Upton, NY, 1998. Draft. 

5. Joint Report of the Office of the State Fire Marshal (Illinois) and the Illinois 

Commerce Commission Staff. Hinsdale central office fire final report, 

prepared by Forensic Technologies International corporation, Annapolis, 

Maryland, March 1989. 

6. U.S. Department of Energy. DOE Handbook on Fire Protection, volume II. 

DOE-HDBK-1062-96, U.S. Department of Energy, Washington, DC, 1996. 

7. T.J. Tanaka, S.P. Nowlen, and D.J. Anderson, Circuit bridging of components 

by smoke. NUREG/CR-6476, SAND96-2633, Sandia National Laboratories, 

Albuquerque, NM, 1996. 

8. T. J. Tanaka, Effect of smoke on functional circuits. NUREG/CR-6543, 

SAND97-2544, Sandia National Laboratories, Albuquerque, NM, 1997. 

9. K. Korsah, Mahbubul Hassan, T. J. Tanaka, and R.T. Wood, Technical basis 

for environmental qualification of microprocessor-based safety-related 

equipment in nuclear power plants. NUREG/CR-6479, ORNITM-13264, 

Oak Ridge National Laboratory, Oak Ridge, TN, 1998.

111
10. W. T. Wheelis, User's guide for a personal-computer-based nuclear power 

plant fire database. NUREG/CR-4586, SAND86-0300, Sandia National 

Laboratories, Albuquerque, NM, 1986. 

11. S. L. Kessel, J. G. Bennett, Jr, C. E. Rogers, Corrosivity Test Methods for 

Polymeric Materials. Part 1-Radiant Furnace Test Method [11, Part 2

CNET TEST Method, Part 3-Modified DIN Test Method, Part 4-Cone 

Corrosimeter Method, and Part 5-A Comparison of Four Test Methods. J. of 

Fire Sciences, Vol. 12, No. 2. pp. 107-233. 

12. J. D. Sinclair, Corrosion of electronics: The role of ionic substances. Journal of 

the Electrochemical Society, 135 (30) 89C-95C, 1988. 

13. R.B. Comizzoli, R.P. Frankenthal, R.E. Lobnig, L.A. Psota-Kelty, G.A. Peins, 

D.J. Siconolfi, and J.D. Sinclair. Corrosion of electronic materials and devices 

by submicron atmospheric particles. Interface, fall, 26-36, 1993. 

14. W. H. Abbott, Corrosion still plagues electronic packaging. Electronic 

Packaging & Production, pages 28-33, 1989. 

15. D. Pauls, IPC cleaning and cleanliness test program phase 3: Water soluble 

fluxes. Part 1. Technical Report, Department of the Navy, Washington, DC, 

1992. 

16. L.M. Caudill, J.T. Chapin, R.B. Comissoli, P. Ghandhi, G.A. Peins, and J.D. 

Sinclair, Current state of fire corrosivity testing: Preliminary electrical 

leakage current measurements. In International Wire and Cable Symposium 

Proceedings, pages 432-437, 1995. 

17. J. T. Chapin, L.M. Caudill, P. Gandhi, and R. Backstrom. Leakage current 

smoke corrosivity testing comparison of cable and material data. in 

International Wire and Cable Symposium Proceedings, pages 184-192, 1996. 

18. M.J. Jacobus. Screening tests of representative nuclear power plant 

components exposed to secondary environments created by fires. NUREG/CR

4596, SAND 86-0394, Sandia National Laboratories, Albuquerque, NM, 1986. 

19. B.Bridger, D.W. Cooper, W.F. Powers, and L. B. McClung. Use of reduced 

emissions wire in switchgear equipments. In Proceedings of Industrial and 

Commercial Power Systems Conference, pages 127-137, 1994. 

20. S.L. Kessel, J.G. Bennett, Jr., and C.E. Rogers. Corrosivity test methods for 

polymeric materials. Journal of Fire Science, 12(2): 107-133, 1994.

112
21. R.L. Iman et al. Evaluation of low-residue soldering for military and 

commercial applications: A report from the Low-residue Soldering Task 

Force. SAND95-1060, Sandia National Laboratories, Albuquerque, NM, 1995. 

22. Adolf S. Jursa, Editor, Handbook of Geophysics and the Space Environment, 

Air Force Geophysics Laboratory, Air Force Systems Command, United 

States Air Force, 1985. 

23. S. P. Nowlen, A Summary of the USNRC Fire Protection Research Program 

at Sandia National Laboratories; 1975-1987, NUREG/CR-5384, Sandia 

National Laboratories, Albuquerque, NM, 1989. 

24. M. Arvidson and H. Persson. Koldioxides (C02) Patverkan p elektronik

sldckning med lga koncentrationer [carbon dioxide suppression systems

risk of damage to electrical equipment]. SP Report 1993:20, Swedish National 

Testing and Research Institute, Boras, Sweden, 1993. 

25. B. T. Reagor. Smoke corrosivity: Generation, impact, detection, and 

propagation. Journal of Fire Sciences, 10:169-179, 1992. 

26. H. P. Messmer, The Indispensable PC Hardware Book, Second Edition, 

Chapter 28, "Hard Disk Drives", Addison-Wesley Publishing Company, 

Workingham, England, 1995. 

27. Doug Pauls, "IPC Cleaning and Cleanliness Test Program Phase 3 Water 

Soluble Fluxes Part 1," Paper presented at the IPC 1992 Fall Meeting, 

October 12-15, 1992, Minneapolis, MN, Institute for Interconnecting and 

Packaging Electronics Circuits, Lincolnwood, IL, IPC TP-1043. 

28. S.P. Nowlen. Enclosure environment characterization testing for the base 

line validation of computer fire simulation codes. NUREG/CR-4681, SAND86

1296, Sandia National Laboratories, Albuquerque, NM, 1986. 

29. J.A. Milloy. Nuclear power plant fire protection-ventilation (subsystems 

study task 1). NUREG/CR-0636, SAND79-0263, Sandia National 

Laboratories, Albuquerque, NM, 1979. 

30. S.P. Nowlen. A summary of the fire testing program at the German HDR 

test facility. NUREG/CR-6173, SAND94-1795, Sandia National Laboratories, 

Albuquerque, NM, 1994. 

31. E.L. Jordan. Feedwater line break. IE Information Notice, No. 86-106, 

Supplement 2. U.S. Nuclear Regulatory Commission, Office of Inspection 

and Enforcement, Washington, DC, March 1987.

113
32. E. Schutz. Coping when disaster hits your business. Friendly Exchange, 

page 23, 1997. 

33. L. Cider. Cleaning and reliability of smoke-contaminated electronics. Fire 

Technology, 29:226-245, 1993. 

34. J. Hildreth. Coping with information infernos. Fire Prevention, 274:32-34, 

1994. 

35. J.F. Maguire. Letter on effect of humidity on SIR. Personal communication, 

October 1996. 

36. R.B. Comizzoli, J.P. Franey, T.E. Graedel, G.W. Kammlott, A.E. Miller, A.J. 

Muller, G.A. Peins, L.A. Psota-Kelty, J.D. Sinclair, R.C. Wetzel, and E.S. 

Sproles, Jr. The Kuwait environment and its effects on electronic materials 

and components. Journal of the Electrochemical Society, 139(7): 2058-2066, 

1992. 

37. H.R. Baker and R.N. Boster. Guide to NRL cleaning and salvaging 

techniques for reclaiming equipment contaminated with seawater, oil, and 

smoke deposits. Technical Report NRL Report 7563, Naval Research 

Laboratory, Washington, DC,1973. 

38. H.R. Baker, P.B. Leach, and C.R. Singleterry. Surface chemical methods of 

displacing water and/or oils and salvaging flooded equipment; Part 2-field 

experience in recovering equipment damaged by fire aboard USS 

Constellation and equipment subjected to salt-spray acceptance test. NRL 

Report 5680, Naval Research Laboratory, Washington, DC, 1961. 

39. Carl J. Tautscher. Contamination Effects on Electronics Products: A Basic 

Discussion of Common Contaminants, Their Origin, Effects, Removal, and 

Control, and Product Protection Against Contaminants from the Use 

Environment, Chapter 3-Contamination Sources: Assembly, Soldering, 

Cleaning, Coating, and Testing, Marcel Dekker, New York, 1991.

114
NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER 

(2-89) (Assigned by NRC, Add Vol., Supp.. Rev., 

NRCM 1102, BIBLIOGRAPHIC DATA SHEET and Addendum Numbers, if any) 

3201,3202 (See instructions on the reverse) NUREG/CR-6597 

2. TITLE AND SUBTITLE 

Results and Insights on the Impact of Smoke on Digital Instrumentation & Controls 3. DATE REPORT PUBLISHED 

MC" TH YEAR 

Oanuar 2001 

4. FIN OR GRANT NUMBER 

JON W6051 

5. AUTHOR(S) 6. TYPE OF REPORT 

Tina J. Tanaka Technical 

Steven P. Nowlen 7. PERIOD COVERED (inclusive Dates)

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (If NRC, provide Division, Office or Region, US. Nuclear Regulatory Commission, and mailing address; if 

contractor, provide name and mailing address.) 

Sandia National Laboratories 

P.O. Box 5800 

Mail Stop 1129 

Albuquerque, NM 87185 

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (If NRC, type 'Same as above': if contractor, provide NRC Division, Office or Region, U.S. Nuclear Regulatory 

Commission, and mailing address.) 

Division of Systems Technology 

Office of Research 

U.S. Nuclear Regulatory Commission 

Washinqton, DC 20555-0001

TO.- SUPPLEMENTARY NOTES 

C. Antonescu, NRC Project Manager 

11. ABSTRACT (200 words or less) 

Smoke can cause interruptions and upsets in active electronics. Because nuclear power plants are replacing 

analog with digital instrumentation and control systems, qualification guidelines for new systems are being 

reviewed for severe environments such as smoke and electromagnetic interference. Active digital systems, 

individual components, and active circuits have been exposed to smoke in a program sponsored by the U.S. 

Nuclear Regulatory Commission. The circuits and systems were all monitored during the smoke exposure, 

indicating any immediate effects of the smoke. The major effect of smoke has been to increase leakage currents 

(through circuit bridging across contacts and leads) and to cause momentary upsets and failures in digital systems. 

This report summarizes two previous reports and presents new results from conformal coating, memory chip, and 

hard drive tests. The report describes practices for mitigation of smoke damage through digital system design, fire 

barriers, ventilation, fire suppressants, and post fire procedures.

12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist researchers in iocating the report.) 13. AVAILABILITY STATEMENT 

Unlimited 

Fire, Smoke, Corrosion, Electronics 14. SECURITY CLASSIFICATION 

(This Page) 

unclassified 

(This Report) 

Unclassified 

15. NUMBER OF PAGES 

16. PRICE

NRC FORM 335 (2-89) SNL/6400/NRC Forms
Federal Recycling Program
ISBN 0-16-050737-5 

90000 

9780160 -507373--------
UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

WASHINGTON, - I- 20555-0001 

OFFICIAL BUSINESS 

PENALTY FOR PRIVATE USE, $300