August 2014

NASA/TM2014-218512 (Corrected Copy)

NESC-RP-13-00869

 

International Space Station (ISS) 

Plasma Contactor Unit (PCU) Utilization Plan 

Assessment Update

Amri Hernandez-Pellerano 

Goddard Space Flight Center, Greenbelt, Maryland

Christopher J. Iannello/NESC 

Langley Research Center, Hampton, Virginia

Henry B. Garrett, Andrew T. Ging, Ira Katz, and R. Lloyd Keith

Jet Propulsion Laboratory, Pasadena, California

Joseph I. Minow, Emily M. Willis, and Todd A. Schneider 

Marshall Space Flight Center, Huntsville, Alabama

Albert C. Whittlesey

Jet Propulsion Laboratory, Pasadena, California

Edward J. Wollack

Goddard Space Flight Center, Greenbelt, Maryland

Kenneth H. Wright

University of Alabama in Huntsville, Huntsville, Alabama
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August 2014

NASA/TM2014-218512 (Corrected Copy)

NESC-RP-13-00869

 

International Space Station (ISS) 

Plasma Contactor Unit (PCU) Utilization Plan 

Assessment Update

Amri Hernandez-Pellerano 

Goddard Space Flight Center, Greenbelt, Maryland

Christopher J. Iannello 

Langley Research Center, Hampton, Virginia

Henry B. Garrett, Andrew T. Ging, Ira Katz, and R. Lloyd Keith

Jet Propulsion Laboratory, Pasadena, California

Joseph I. Minow, Emily M. Willis, and Todd A. Schneider 

Marshall Space Flight Center, Huntsville, Alabama

Albert C. Whittlesey

Jet Propulsion Laboratory, Pasadena, California

Edward J. Wollack

Goddard Space Flight Center, Greenbelt, Maryland

Kenneth H. Wright

University of Alabama in Huntsville, Huntsville, Alabama
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7115 Standard Drive

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443-757-5802

Acknowledgments

The NESC team would like to acknowledge the following individuals who 

provided their time to answer questions and/or contribute information to this 

effort: Steven Koontz, Johnson Space Center (JSC), Ronald Galvez (JSC), 

Mathew Scudder (JSC/Boeing), Doug Hamilton (Wyle), Tamra George 

(JSC/Hamilton), Orbital Sciences Corporation (OSC) EVA Tool Personnel (JSC), 

Raymond Kaminski (JSC/Boeing), and Penni Dalton, Glenn Research Center 

(GRC).

The NESC team would also like to thank the following peer reviewers for their 

thorough review and commentary: Tim Brady (JSC/Systems Engineering Office 

(SEO)), Bob Kichak (GSFC), Dawn Emerson (GRC/NESC Chief Engineer 

(NCE)), Mike Patterson (GRC), Scott West (JSC/NCE), Steve Gentz 

(MSFC/NCE), and Rob Boyle (JSC/ Crew and Thermal Systems Division of JSC 

Engineering, see Appendix J).

The use of trademarks or names of manufacturers in the report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration.
ERRATA

NASA/TM‒2014-218512

Issue Date: 07/21/2015

On each page of body of report in NESC header, Version number changed from 1.0 to 2.0.

Revised Section 4.0, Executive Summary, page 11, by adding two additional paragraphs at the 

end to reference the new Appendix K, pp. 226-294.

Block 18 of Report Documentation Page adjusted to include additional pages (from 230 to 299).
NASA Engineering and Safety Center 

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International Space Station (ISS) Plasma Contactor Unit (PCU)

Utilization Plan Assessment Update

July 10, 2014
NASA Engineering and Safety Center 

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Report Approval and Revision History 

NOTE: NOTE: This document was submitted to the NESC Director for out-of-board approval

on July 21, 2015, for configuration control.

Approved: Original Signature on File 7/22/15

NESC Director Date

Version Description of Revision Office of Primary 

Responsibility Effective Date

1.0 Initial Release Dr. Christopher 

Iannello, NASA 

Technical Fellow for 

Electrical Power, KSC

7/10/14

1.1 Roeschel references changed from 

year 2010 to 2013. (No Signature 

Required)

Dr. Christopher 

Iannello, NASA 

Technical Fellow for 

Electrical Power, KSC

8/28/14

2.0 Revised Section 4.0, Executive 

Summary, by adding 2 additional 

paragraphs at the end to reference the 

addition of Appendix K.

Dr. Christopher 

Iannello, NASA 

Technical Fellow for 

Electrical Power, KSC

7/16/15
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Table of Contents

Technical Assessment Report

1.0 Notification and Authorization.....................................................................................................7

2.0 Signature Page................................................................................................................................8

3.0 Team List........................................................................................................................................9

3.1 Acknowledgements.............................................................................................................9

4.0 Executive Summary.....................................................................................................................10

5.0 Assessment Plan ...........................................................................................................................12

6.0 Problem Description and Proposed Solutions...........................................................................13

6.1 Problem Description Summary.........................................................................................13

6.2 Background Information...................................................................................................14

6.2.1 ISS and the Ionosphere/Plasma Environment...................................................................14

6.2.2 ISS Power System and Spacecraft Charging ....................................................................14

6.2.3 Charged Particle Collection: Ions, Electrons, and FP......................................................14

6.2.4 Mitigating ISS Spacecraft Charging – PCUs....................................................................15

6.2.5 Potentials Generated by Magnetic Induction....................................................................15

6.2.6 Insulating Surfaces, Anodized Components, and Capacitors...........................................15

6.3 Detailed Problem Description...........................................................................................16

6.3.1 ISS Charging.....................................................................................................................16

6.3.2 Plasma Shock Hazard for EVA Astronauts......................................................................17

6.3.3 PCUs.................................................................................................................................17

6.3.4 Hazard Classification and Protection Systems..................................................................18

6.3.5 FPMU ...............................................................................................................................19

6.3.6 PIM3.0 Charging Model...................................................................................................20

6.3.7 Electrical Shock Hazard Scenarios...................................................................................22

6.3.8 Hazard Circuit Associated with Negative Charging.........................................................22

6.3.9 Hazard Circuit Associated with Positive Charging...........................................................29

6.3.10 Shock Hazard Probabilities...............................................................................................30

6.3.11 Approach to EVAs Without a Two-Fault Tolerant Hazard Control.................................32

6.3.12 Data Supporting NESC Recommendations ......................................................................34

7.0 Data Analysis................................................................................................................................34

7.1 Shortcomings in the Space Weather Forecast Planning that Limits its Utility for 

Forecasting........................................................................................................................34

7.1.1 Persistence of Conditions Assumption is Not Accurate ...................................................35

7.1.2 Dependency on Benign Solar Cycle is Unreliable............................................................35

7.1.3 Use of the Climatological Model – IRI is Inadequate ......................................................37

7.1.4 Missing Short Term Changes in the Plasma Environment: Geomagnetic Storm 

Activity .............................................................................................................................37

7.1.5 Inconsistencies in Input Parameters..................................................................................38

7.1.6 Limited Validation Studies...............................................................................................38

7.2 FPMU Role in the Forecast: Criticality and Alternate Data .............................................39

7.2.1 Ambiguity in Dataset Requirements.................................................................................39

7.2.2 Alternatives for Ionospheric Data.....................................................................................40
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7.2.3 FPMU Reliability..............................................................................................................41

7.2.4 FPMU Design Life Limitations Compared To PCU ........................................................41

7.2.5 Spare FPMUs – EVA Deployment...................................................................................42

7.2.6 FPMU Power Supply Limitations.....................................................................................42

7.3 Limitations of the ISS Charging Model PIM3.0...............................................................43

7.4 PIM3.0 Charging Model in the Critical Path to EVA.......................................................44

7.5 Example of PIM3.0 Error Estimate ..................................................................................45

7.6 Types of Charging Events.................................................................................................50

7.7 Estimate of Likelihood of Auroral Charging for ISS........................................................60

7.8 PCU Capability to Maintain the ISS Near to Space Plasma Potential..............................62

7.8.1 PCU IV Characteristic versus FP Mitigation....................................................................62

7.8.2 PCU Operational Life .......................................................................................................64

7.9 EMU Exterior Metal Parts................................................................................................66

7.10 Reassessment of the Positive Voltage EVA Hazard.........................................................72

7.11 Features of the Current Path from the ISS-EMU-Plasma Circuit versus the Shock 

Hazard...............................................................................................................................80

7.11.1 Electrical Current Path from the ISS through the Astronaut to the Plasma through 

Multiple Layers of Insulation............................................................................................80

7.11.2 Effects of Coincidental EMU Insulation Failures.............................................................84

7.12 Shunt Array FDIR.............................................................................................................84

7.12.1 FDIR Operation ................................................................................................................84

7.12.2 Risks for High Negative Potential Peaks..........................................................................86

7.13 The Negative FP Limit .....................................................................................................87

7.13.1 Origin of the ‒40V ISS Charging Limit ...........................................................................88

7.13.2 Plasma Safety Hazard Identification and Risk Acceptance at ‒45.5V Charging 

Levels................................................................................................................................90

7.14 Review of the ISS-NCR-232G: Lack of Two-fault Tolerance to EVA Crew Shock 

in the Low Earth Orbit Plasma Environment....................................................................92

7.15 Examination of ISS-PRA-12-56: PRA for Shock Hazard ................................................93

7.15.1 Additional PRA Review ...................................................................................................93

8.0 Findings, Observations, and NESC Recommendations............................................................96

8.1 Findings ............................................................................................................................96

8.2 Observations...................................................................................................................100

8.3 NESC Recommendations ...............................................................................................101

9.0 Alternate Viewpoint...................................................................................................................103

10.0 Other Deliverables.....................................................................................................................103

11.0 Lessons Learned.........................................................................................................................103

12.0 Recommendations for NASA Standards and Specifications..................................................103

13.0 Definition of Terms....................................................................................................................103

13.1 ISS PCU Report Definition of Terms.............................................................................104
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14.0 Acronyms List ............................................................................................................................107

15.0 References...................................................................................................................................109

16.0 Appendices..................................................................................................................................113

Appendix A. Human Current Safety Limits.........................................................................................114

Appendix B. Overview of Plasma Shock Hazard to EVA Crew .........................................................115

Appendix C. ISS-NCR-232G Review..................................................................................................129

Appendix D. Tools and EMU Hardware Presentation .........................................................................138

Appendix E. Additional EMU Pictures................................................................................................152

Appendix F. FDIR Reference Emails..................................................................................................183

Appendix G. Maximum Magnetic Induction Potential Along ISS Truss.............................................198

Appendix H. International Space Station Electrical Power Systems Training Manual ISS EPS 

TM 21109 (Section 2.3.4)...............................................................................................200

Appendix I. International Space Station (ISS) Plasma Contactor Unit (PCU) Utilization Plan 

Assessment Update: Key Points Summary.....................................................................210

Appendix J. EMU Team Email...........................................................................................................224

Appendix K. Current Flow to EMU in Electrical Contact with +15V ISS Surface .............................226

List of Figures

Figure 6.3-1. ISS Potential with Respect to the Local Ionosphere Plasma. ...........................................17

Figure 6.3-2. PCU Installed on ISS ........................................................................................................18

Figure 6.3-3. FPMU Probes and Layout.................................................................................................19

Figure 6.3-4. Location of the PCUs and the FPMU at the ISS...............................................................20

Figure 6.3-5. 2010 Comparison of Potential Calculations with PCUs On and Off ...............................21

Figure 6.3-6. Example of FPMU Data showing the Effect on the Peak Chassis Potential (i.e., FP) 

when the PCUs are On and Off.........................................................................................21

Figure 6.3-7a. Circuit Element Definitions used in Circuits #1, #1a, #2, and #3.....................................23

Figure 6.3-7b. Circuit Diagram showing Solar Array Current Collection and related Charging of 

the ISS...............................................................................................................................24

Figure 6.3-7c. Circuit Diagram showing the Scenario where an EVA is being Conducted.....................25

Figure 6.3-7d. Circuit Diagram showing the Scenario where a Direct Electrical Connection is 

established between the Charged ISS Chassis and the EMU (space suit). .......................26

Figure 6.3-7e. Circuit Diagram showing the Scenario where an Arc occurs on an Anodized 

Component of the EMU (space suit). ...............................................................................27

Figure 6.3-8. Positive Potential Electrical Current Path through the Crew Member. ............................30

Figure 6.3-9. EMU Suit External Metal Locations [ISS-NCR-232F, Attachment 5, 2012]...................31

Figure 6.3-10. Modifications to the MMWS (“tool belt”) [ISS-NCR-232F, Attachments 5 

and 7, 2012] ......................................................................................................................32

Figure 6.3-11. Comparison of Hazard Control Approaches.....................................................................33

Figure 7.1-1. Predictions of Solar Cycle 24 Sunspot Maximum............................................................36

Figure 7.5-1. Scatter Plot of PIM3.0 Voltage Calculations versus FPMU FP Measurement.................46

Figure 7.5-2. Plot of the (PIM3.0 calculation – FPMU Measurement) Difference versus FPMU 

FP Measurement. ..............................................................................................................47

Figure 7.5-3. Histogram of PIM3.0, FPMU Measurement Difference for 1V bins ...............................48

Figure 7.5-4. Data in Figure 7.5-2 Re-plotted with Color Code to indicate Points that lie inside 

1-, 2-, and 3- Boundaries and Points that lie beyond the 3- Boundary....................49
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Figure 7.5-5. FPMU versus PIM3.0 Calculation....................................................................................50

Figure 7.6-1. ISS Solar Array Charging .................................................................................................51

Figure 7.6-2. Detail of Sunlight Unshunt Rapid Charging Event...........................................................53

Figure 7.6-3. ISS FPMU Charging Event Summary ..............................................................................54

Figure 7.6-4. Positive Charging Events..................................................................................................56

Figure 7.6-5. ISS Auroral Charging .......................................................................................................57

Figure 7.7-1. Absolute Probability of Encountering a Large Energy Flux Event/Aurora as a 

Function of corrected Geomagnetic Latitude and Local Time for a Satellite...................61

Figure 7.7-2. The ISS Orbit Track over 24 hours and the Locations (red) of DMSP Charging 

Eventsof less than -100V ..................................................................................................61

Figure 7.8-1. Plasma Contactor Emission Current Measured in a Ground Test Chamber as a 

Function of Voltage ..........................................................................................................64

Figure 7.8-2. Xenon Usage Projections..................................................................................................65

Figure 7.9-1. EMU Photo .......................................................................................................................70

Figure 7.9-2. EMU Photo .......................................................................................................................71

Figure 7.9-3. Suited Astronaut: EMU Upper Part..................................................................................72

Figure 7.10-1. Circuit where the Astronaut is ~15V Positive with Respect to the Surrounding 

Plasma...............................................................................................................................73

Figure 7.10-2. Example of an Exposed Section of the Stainless Steel Wrist Bearing Ring directly 

above the Blue Anodized Ring .........................................................................................74

Figure 7.10-3. Electrical Equivalent Circuit.............................................................................................75

Figure 7.10-4. Plasma Current Collection for Spherical (3-DIM), Cylindrical, and Planar Probes.........76

Figure 7.10-5. Diagram of FPMU in its Deployed State with indicated Dimensions .............................77

Figure 7.10-6. An NLP Electron Current as a Function of Voltage .........................................................77

Figure 7.10-7. EMU Currents Post MMWS Modification [ISS-NCR-232F, Attachment 8, 2012].........78

Figure 7.11-1. Safety Tether showing the Insulating Fabric Section at the End ......................................81

Figure 7.11-2. EMU showing the Fabric Section of the Safety Tether and How all the Anodized 

Rings are Covered by the Suit Fabric. ..............................................................................82

Figure 7.11-3. Circuit Paths from ISS Chassis Ground to the Astronaut Inside the EMU.......................83

Figure 7.12-1. ISS NiH2 Battery ORU .....................................................................................................85

Figure 7.12-2. FP Data from the FPMU...................................................................................................87

List of Tables

Table 7.6-1. Charging Events  0V and  ‒45V...................................................................................56

Table 7.9-1. EMU Metal Entry Points Summary..................................................................................68
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Technical Assessment Report

1.0 Notification and Authorization

The NASA Engineering and Safety Center (NESC) received a request to support the Assessment 

of the International Space Station (ISS) Plasma Contactor Unit (PCU) Utilization Update. This 

assessment was co-led by Dr. Christopher Iannello, NASA Technical Fellow for Electrical 

Power, and Ms. Amri Hernández-Pellerano, NASA Electrical Power Technical Discipline Team 

(TDT) member. The NESC conducted an earlier assessment of the use of the PCU in 2009 

(NESC Request #07-054-E

1

) [NASA, 2009]. The objective for that assessment was to evaluate 

whether leaving PCUs off during non-extravehicular activity (EVA) time frames presented any 

risk to the ISS through assembly completion. Dr. Steven Koontz asked the previous assessment 

be extended to include the following possible additions to the PCU utilization plan:

– Nominally leaving the PCUs off during EVA if pre-EVA hazard severity 

measurements and short-term ionospheric environment forecasts support that 

decision.

– Disabling the EVA shunt fault detection, isolation and recovery (FDIR) logic and the 

supporting operational hazard controls if two PCUs are in discharge during the EVA.

– Possible long-term marginalization of the ISS EVA-312 shock hazard report so that 

no active hazard controls are required.

The key stakeholders for this assessment were Dr. Steven Koontz and the ISS Program (ISSP).

 

1

NESC-RP-07-054/NASA/TM-2010-216683
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2.0 Signature Page

Submitted by: 

Team Signature Page on File – 7/29/14

Ms. Amri Hernandez-Pellerano Date

Significant Contributors: 

Dr. Christopher J. Iannello Date Dr. Henry B. Garrett Date

Mr. Andrew T. Ging Date Dr. Ira Katz Date

Mr. R. Lloyd Keith Date Dr. Joseph I. Minow Date

Ms. Emily M. Willis Date Mr. Todd A. Schneider Date

Mr. Albert C. Whittlesey Date Dr. Edward J. Wollack Date

Dr. Kenneth H. Wright Date

Signatories declare the findings, observations, and NESC recommendations compiled in the 

report are factually based from data extracted from program/project documents, contractor 

reports, and open literature, and/or generated from independently conducted tests, analyses, and 

inspections.
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3.0 Team List

Name Discipline Organization

Core Team

Christopher Iannello NASA Technical Fellow for Electrical Power KSC

Amri Hernandez-Pellerano EPS TDT/PMAD and Power GSFC

Linda Anderson MTSO Program Analyst LaRC

Dan Burbank Mission Specialist JSC

Michael Engle Flight Operations JSC

Henry Garrett Space Environments & Effects Scientist JPL

Ira Katz Charging Specialist JPL

Lloyd Keith NESC Chief Engineer JPL

Shannon Melton Medical Team JSC/Wyle

Kathy Messersmith Aerospace Flight Systems JSC

Joseph Minow Flight Vehicle Space Environment MSFC

Jack Rasbury Medical Team JSC/Wyle

Eduardo Roeschel Safety & Mission Assurance JSC

Todd Schneider Basic Properties of Materials MSFC

Jason Vaughn Basic Properties of Materials MSFC

Albert Whittlesey Electromagnetic Engineer JPL

Emily Willis Flight Vehicle Space Environment MSFC

Edward Wollack Research Astrophysicist GSFC

Kenneth Wright Research Scientist: Plasma-Body Interactions UAH

Consultant

Andrew Ging Mission Operations JPL

Administrative Support

Linda Burgess Planning and Control Analyst LaRC/AMA

Melinda Meredith Project Coordinator LaRC/AMA

Erin Moran Technical Writer LaRC/AMA

3.1 Acknowledgements

The NESC team would like to acknowledge the following individuals who provided their time to 

answer questions and/or contribute information to this effort: Steven Koontz, Johnson Space 

Center (JSC), Ronald Galvez (JSC), Mathew Scudder (JSC/Boeing), Doug Hamilton (Wyle), 

Tamra George (JSC/Hamilton), Orbital Sciences Corporation (OSC) EVA Tool Personnel (JSC), 

Raymond Kaminski (JSC/Boeing), and Penni Dalton, Glenn Research Center (GRC).

The NESC team would also like to thank the following peer reviewers for their thorough review 

and commentary: Tim Brady (JSC/Systems Engineering Office (SEO)), Bob Kichak (GSFC), 

Dawn Emerson (GRC/NESC Chief Engineer (NCE)), Mike Patterson (GRC), Scott West 

(JSC/NCE), Steve Gentz (MSFC/NCE), and Rob Boyle (JSC/ Crew and Thermal Systems 

Division of JSC Engineering, see Appendix J). 
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4.0 Executive Summary

The International Space Station (ISS) vehicle undergoes spacecraft charging as it interacts with

Earth’s ionosphere and magnetic field. The interaction can result in a large potential difference 

developing between the ISS metal chassis and the local ionosphere plasma environment. If an 

astronaut conducting extravehicular activities (EVA) is exposed to the potential difference, then 

a possible electrical shock hazard arises.

The control of this hazard was addressed by a number of documents within the ISS Program

(ISSP) including Catastrophic Safety Hazard for Astronauts on EVA (ISS-EVA-312-4A_revE). 

The safety hazard identified the risk for an astronaut to experience an electrical shock in the 

event an arc was generated on an extravehicular mobility unit (EMU) surface. A catastrophic 

safety hazard, by the ISS requirements, necessitates mitigation by a two-fault tolerant system of 

hazard controls. Traditionally, the plasma contactor units (PCUs) on the ISS have been used to 

limit the charging and serve as a “ground strap” between the ISS structure and the surrounding 

ionospheric plasma. 

In 2009, a previous NASA Engineering and Safety Center (NESC) team evaluated the PCU 

utilization plan (NESC Request #07-054-E) with the objective to assess whether leaving PCUs 

off during non-EVA time periods presented risk to the ISS through assembly completion. For 

this study, in situ measurements of ISS charging, covering the installation of three of the four 

photovoltaic arrays, and laboratory testing results provided key data to underpin the assessment. 

The conclusion stated, “there appears to be no significant risk of damage to critical equipment 

nor excessive ISS thermal coating damage as a result of eliminating PCU operations during nonEVA times.” 

In 2013, the ISSP was presented with recommendations from Boeing Space Environments for 

the “Conditional” Marginalization of Plasma Hazard [Mikatarian, R., et al., 2013]. These 

recommendations include a plan that would keep the PCUs off during EVAs when the space 

environment forecast input to the ISS charging model indicates floating potentials (FP) within 

specified limits. These recommendations were based on the persistence of conditions in the 

space environment due to the current low solar cycle and belief in the accuracy and completeness 

of the ISS charging model. Subsequently, a Noncompliance Report (NCR), ISS-NCR-232G, 

Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth Orbit Plasma Environment, 

was signed in September 2013 specifying new guidelines for the use of shock hazard controls 

based on a forecast of the space environment from ISS plasma measurements taken prior to the 

EVA [ISS-EVA-312-AC, 2012]. 

This NESC assessment re-evaluates EVA charging hazards through a process that is based on 

over 14 years of ISS operations, charging measurements, laboratory tests, EMU studies and 

modifications, and safety reports. The assessment seeks an objective review of the plasma 

charging hazards associated with EVA operations to determine if any of the present hazard 

controls can safely change the PCU utilization plan to allow more flexibility in ISS operations 

during EVA preparation and execution. 
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The following approach was used:

1) Review shock hazard-related data as provided in the measurements from the floating 

potential measuring unit (FPMU) aboard the spacecraft and other ISSP sources.

2) Compare the ISS charging model output versus FPMU measurement.

3) Review existing ISS documentation related to shock hazards and controls.

4) Provide preliminary analysis and data observations related to the shock hazard severity, 

available controls, and forecast tool capabilities. 

The NESC recommends continuing the catastrophic hazard assumption and the use of three 

controls for the typical two-fault tolerant hazard control during all EVAs regardless of FP 

predictions or EVA location. These recommendations include the use of the two PCUs in 

discharge for EVAs and propose the ISS/EVA team evaluate the use of the low probability of 

contact (which includes the isolation features in the ISS-suit-crew path) as the third control while 

discontinuing the use of the solar array wing shunt fault detection, isolation and recovery

(FDIR). In addition, it is recommended that the Plasma Interaction Model version 3 (PIM3.0)

“predictions” (i.e., forecast) be constrained to planning purposes and not be used to determine 

the use of active hazard controls. Refer to Appendix I for a Summary of Key Points from this 

assessment.

The NESC team’s work in this report considers the positive charging hazard to EVA crew and 

assumes a single galvanic contact case the team deemed most concerning. Other contact cases 

might result in higher current collection. However, those require multiple simultaneous galvanic 

contacts and therefore these were considered worst-on-worst, hence overly conservative.

At the completion of this report, the requester specifically asked that the team go back and 

evaluate this worst current collection case regardless of the additional conditions for the situation 

to arise. Appendix K provides the current collection of individual pieces of the system that 

might contribute and can be used to consider randomly generated cases, including the multiple 

galvanic contact cases previously considered overly conservative. The ISS Program can use this 

data to assess severity of the individual contact cases, and coupled with accurate probabilistic 

risk assessment (PRA), determine quantitatively, which is the driving risk case based on the 

product of severity and likelihood.
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5.0 Assessment Plan

This assessment started with the assembly of a team that included plasma physicists, space 

environment scientists, EVA safety specialists, medical team specialists, system engineers, 

power system engineers, and administrative support. 

The plan was divided (according to the request) into three main re-phrased questions: 

(1) Is it acceptable for PCUs to be off during EVAs? 

(2) Can the FDIR be disabled if two PCUs are in discharge?

(3) Is it acceptable to conduct an EVA without active shock hazard controls?

Several key documents and presentations related to the use of controls and environment 

“forecasting” were reviewed to understand the hazard and available controls and guidelines. For 

example, these included the ISS-EVA-312-AC (1/26/2012): Electric Shock to EVA Crew 

Resulting from EMU Arcing in Plasma [ISS-EVA-312-AC, 2012]; the ISS-NCR-232F 

(1/26/2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth Orbit Plasma 

Environment [ISS-NCR-232F, 2012] and the ISS-NCR-232G (9/2013) [ISS-NCR-232G, 2013]. 

Data from the FPMU, International Reference Ionosphere (IRI), and calculations from the 

PIM3.0 were reviewed to understand the forecast limitations and the types of charging events at 

the ISS. The known magnitudes of these charging events and the ISS FP levels were considered 

in the assessment.

Documents were reviewed and direct communication was established with ISS power 

engineering to understand the FDIR basic functionality. Data related to charging events due to 

shunting or unshunting solar arrays were considered. Alternatives to the use of the FDIR were 

considered based on the shock hazard severity, the likelihood of completing an electrical circuit

current path, possible conditions affecting the ISS power positive state during the FDIR use, and 

available information related to the validation of the process.

The following is an outline of the assessment plan: 

1. Basis of PCU as a control

i. Proposed forecast adequacy to determine PCU control utilization review 

hazards [ISS-EVA-312-AC, 2012]

ii. Review the forecasting process

1. Sources

2. Limits

3. Proposed changes

iii. Review of PIM3.0 charging model adequacy for forecasting

1. Prediction capabilities

2. Error bars

3. Accuracy of prediction for the FP

a. Magnitude of values

b. Forecast time length: Can it accurately predict 2 to 3 or up 

to 14 days?
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c. What boundaries shall be in place based on what can and 

cannot be predicted?

iv. FPMU role and criticality

v. Review assumptions for the probabilistic risk assessment (PRA)

2. Evaluate the FDIR

i. Two-fault tolerance requirement

1. Two operational PCUs are considered single fault tolerant

2. For two-fault tolerance the ISS-NCR-232F list the two PCUs and 

have the FDIR (solar array shunt control algorithm) as third control

ii. Risks

1. Is there a risk to the ISS power configuration?

2. Are there risks of large negative events with the array shunting?

3. How reliable is the system? Is it programmed for every EVA?

4. Severity of hazard if one PCU fails

3. Hazard controls marginalization

i. Recommended analyses

1. Worst-case positive and negative potentials

2. How much electrical circuit path current collection is realistic for a 

positive EMU charging?

a. Compare to medical limits

3. Evaluate charging events

a. Eclipse exit normal charging

b. Eclipse exit rapid charging event

c. Auroral charging

d. Array unshunt in sunlight

The NESC team did not evaluate the EMU systems (i.e., electrical systems and instruments) to 

understand their susceptibility to the assessment hazards. In addition, the analysis in this 

assessment focused on the present ISS configuration and did not attempt to address the effects of 

possible configuration changes (e.g., future Russian solar arrays).

6.0 Problem Description and Proposed Solutions

6.1 Problem Description Summary

The ISS vehicle undergoes spacecraft charging as it interacts with Earth’s ionosphere and 

magnetic field. The interaction can result in a large potential difference developing between the 

ISS metal chassis and the local ionosphere plasma environment. If an astronaut conducting an 

EVA is exposed to the potential difference, then a possible electrical shock hazard arises.

This assessment evaluated the approach and methodology adopted by the ISSP, which relies on 

modeling to determine if hazardous charging conditions exists. The modeling was contrasted 

with the use of active charge mitigation devices (i.e., PCUs), which are in place on the ISS and 
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directly limit the potential difference between the ISS and the ionosphere plasma when they are 

operational. 

6.2 Background Information

6.2.1 ISS and the Ionosphere/Plasma Environment

The ISS orbits the Earth at an altitude of approximately 400 km. In this orbit, the ISS is 

continually moving through Earth’s ionosphere and magnetic field. The ionosphere, which is a 

plasma environment, is made up of a superheated gas in which the neutral atoms are converted 

into charged particles via ionization. The principle constituents of the ionosphere plasma are 

electrons (i.e., negatively charged particles) and oxygen ions (positively charged particles). 

Since the ionosphere plasma is comprised of charged particles, the interaction with the ISS can 

occur because of direct collisions or as a result of electrostatic attraction/repulsion. As opposite 

charges attract (e.g., positive attracts negative), like charges repel (e.g., positive repels positive). 

An example of electrostatic attraction of charged particles is the solar arrays on the ISS. The 

solar arrays are made up of silicon solar cells with an exposed edge. When illuminated by 

sunlight, the cells produce electrical power and achieve a positive voltage. Electrons in the 

plasma near a solar cell will be pulled towards the solar cell due to electrostatic attraction. Some 

fraction of the electrons attracted to the cell will be collected by the cell (since it is an electrical 

semi-conductor) and result in spacecraft charging.

6.2.2 ISS Power System and Spacecraft Charging

The ISS power system was electrically configured as a negative ground system. To understand 

this configuration, a solar array can be treated as a simple battery. The negative terminal of this 

“battery” is connected to the ISS aluminum (Al) structure (or chassis) and the positive terminal is 

immersed in the ionosphere plasma. Accordingly, if electrons in the plasma are collected by a 

positively biased solar cell, they will ultimately accumulate on the ISS chassis as part of the 

negative ground power system arrangement. To characterize the amount of charge that might 

accumulate on the ISS chassis, an electrical reference point must be defined. On Earth’s surface, 

this reference point is Earth Ground. For the ISS, it is not practical to use Earth Ground as a 

reference. Instead, it is easier to choose the local plasma environment around the vehicle as the 

electrical reference point or “plasma ground.” Using this convention, the potential difference 

(voltage) that develops between the ISS chassis and the local plasma can be described. In the 

scenario where the solar cells collect electrons, which end up on the ISS chassis, a negative 

voltage developing on the chassis with respect to the local plasma can be described. 

6.2.3 Charged Particle Collection: Ions, Electrons, and FP

A corollary to the electron collection scenario is ion collection. Exposed metal surfaces on the 

ISS chassis that are negatively biased with respect to the local plasma can collect ions 

(i.e., positive charges). In the spacecraft-charging arena, it is understood that equilibrium 

potential must be arrived at where the ion current collection balances the electron current 

collection. Known as the FP, it is dependent on the amount of ion collection area, the electron 

collection area, and the mass and energy of the electrons and ions in the plasma. For the 
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ionosphere plasma, the ions are massive compared to the electrons and the ions have very little 

thermal energy. Ions are mostly collected as a result of the ISS colliding with them – which is 

called RAM collection. The electrons, however, are very light and have a modest thermal 

energy, so they interact with all the surfaces on the ISS and can be easily collected by positively 

biased conductive (or semi-conductive) surfaces. Combining all of these factors, one finds that 

the typical equilibrium FP of the ISS chassis is a negative potential. 

6.2.4 Mitigating ISS Spacecraft Charging – PCUs

When the design decision was made to use high-voltage (+160 volts (V)) solar arrays on the ISS, 

scientists and engineers familiar with the ionosphere plasma environment predicted that the ISS 

would experience significant spacecraft charging. To limit the ISS chassis charging due to solar 

array electron current collection, the spacecraft charging design team in the early 1990s 

recommended the use of PCUs. The PCUs would act as an effective “ground strap” to the local 

plasma. The PCUs operate by creating a plasma bridge between the ISS chassis and the 

ionosphere plasma. They move the excess charge accumulated on the ISS chassis back into the 

ionosphere, thereby minimizing any spacecraft charging. Thus, the ISSP developed and 

deployed two robustly designed PCUs. Each PCU was rated to continuously emit as much as 

10 amps of accumulated charge back into the ionosphere and respond to changes in the ISS 

current collection in a fraction of a second. The PCUs were designed and verified such that ISS 

chassis potential would never go more negative than -40V when the PCUs were operating.

6.2.5 Potentials Generated by Magnetic Induction

Charging on the ISS chassis is actually a combination of current collection by charged surfaces 

(described above) and induced potentials created by magnetic induction. The magnetic induction 

occurs as a result of the long metallic ISS truss structure moving through field lines in the 

Earth’s magnetic field. Like a wire in a conventional electric generator, the ISS develops a 

potential difference (voltage) across its length as it moves through a magnetic field. The formula 

that governs the induction voltage is induced = v × B • L, where v is the spacecraft velocity 

vector, |B| is the magnetic field strength, and |L| is the length of the conductor. This equation is 

actually a vector equation, which means that the orientation of the conductor with respect to the 

magnetic field is important. Often referred to by the shorthand “v cross B”, the magnetic 

induction potential can have a net magnitude as high as about 38V (see Appendix G) measured 

from truss tip to truss tip. Thus, the potential that is created by magnetic induction (v × B • L) 

across the ISS is a function of position along the truss. 

6.2.6 Insulating Surfaces, Anodized Components, and Capacitors

Like most other spacecraft, the ISS is made up of a wide variety of materials, including 

electrically conductive and electrically insulating materials. When electrically insulating 

materials or dielectric materials are exposed to the ionosphere plasma environment, their surface 

can become electrically charged. An important example of an insulating material charging on 

the ISS is the case of the micrometeoroid and orbital debris (MMOD) shields. The MMOD 

shields are anodized Al. The anodizing process creates a significant thin oxide layer on the Al
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surface (for corrosion protection). The Al metal is a good electrical conductor, the anodization 

layer, however, is a good electrical insulator (dielectric). The MMOD shields form the outer 

surface of the ISS pressurized modules. The Al metal in the MMOD shields is attached to the 

chassis and the oxide layer (anodized coating) is exposed to the ionosphere plasma. This 

arrangement can be described in electrical circuitry terms as a parallel plate capacitor. 

Recall that a parallel plate capacitor is a device made up of two electrically conductive plates 

separated by a dielectric material. To characterize the MMOD shields as a capacitor: one of the 

capacitor plates is the Al metal, the dielectric material between plates is the anodization layer, 

and the other “plate” is the plasma. Given the large amount of surface area associated with the 

MMOD shields and the significant thin anodization layer, it turns out that the capacitance of the 

ISS modules can be quite large – on the order of milli-Farads [Carruth, 2001]. 

Three important features of capacitors are:

1. Charge Storage – a large capacitance translates to a capacity to store a large amount of 

charge.

2. Direct Current (DC) Blockage – only changing or pulsed currents can pass through a 

capacitor.

3. Pulse Discharge – shorting across the plates of a charged capacitor or dielectric 

breakdown can produce a large pulse of current out of the capacitor.

Given the large capacitance of the ISS MMOD debris shields, it can be expected that a great deal 

of charge can be stored and, in turn, sourced as a large current pulse when the capacitor terminals 

are shorted. An electrical arc across a capacitance is equivalent to shorting the capacitor with a 

switch.

Of the many external surfaces on ISS that can be characterized as capacitors, three areas figure 

prominently in this assessment: 1) the main ISS structure capacitance associated with the 

MMOD shields, 2) the solar array capacitance, and 3) the EMU capacitance. It should be noted 

that the capacitance associated with the MMOD shields is very large compared to the solar array 

and EMU capacitances. 

Reference:

1. Carruth, Jr., M.R., et al. (2001): “ISS and Space Environment Interactions without Operating 

Plasma Contactor,” AIAA-2001-401, Aerospace Sciences Meeting and Exhibit, 39th, Reno, 

Nevada, January 9-11, 2001.

6.3 Detailed Problem Description

6.3.1 ISS Charging

The conditions that generate a plasma hazard on ISS arise when a difference in potential 

develops between the ISS chassis and the surrounding ionosphere plasma, which is the defined 

electrical reference point. The two sources that create this potential difference (voltage) are: 

(1) electron current collection on the high voltage (+160V) solar arrays which drives the ISS 
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chassis to negative potentials, and (2) the magnetic induction voltage generated across the long 

truss structure as it moves through the Earth’s magnetic field. 

The electron collection on the solar array occurs when the solar array is illuminated by sunlight 

and connected to the power system. The array output can be short-circuited through an operation 

known as shunting. If an array is shunted, the electron current collection from the plasma does 

not charge the ISS chassis since it is also short-circuited. The magnetic induction voltage 

generated across the length of the truss changes depending on the orientation of the truss to the 

magnetic field. 

6.3.2 Plasma Shock Hazard for EVA Astronauts

The plasma hazard occurs when an astronaut conducting EVAs is exposed to the potential 

difference between ISS and the local plasma as a result of an electrical connection being made to 

the EMU (spacesuit). The magnitude and the nature of the hazard condition are dependent on 

the astronaut’s location along the vehicle as well as some vehicle operations (e.g., PCU on/off,

solar array state, etc.). Figure 6.3-1 provides a pictorial representation of the ISS spacecraft 

charging that results from solar array current collection and magnetic induction (i.e., v × B • L). 

Figure 6.3-1 shows that the v × B • L voltage is distributed along the truss such that one end of 

the truss can be at a more positive voltage than the other end. 

Figure 6.3-1. ISS Potential with Respect to the Local Ionosphere Plasma. The ISS potential is a 

combination of solar array current collection and magnetic induction (or v × B • L).

6.3.3 PCUs

To dramatically reduce the negative charging that occurs on the ISS chassis due to solar array 

electron collection, the PCU was developed for the ISS. The PCU acts as an effective “ground 

strap” to the local plasma. The purpose of this device is to mitigate the negative charging hazard 

by returning excess charge accumulated on the ISS chassis back to the ionosphere plasma. This 

provides mitigation to the negative FP hazard by keeping the station chassis potential more 

positive than -40V. There are two independently powered and controlled PCU systems installed 
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on the ISS and together they provide a single-fault tolerant control against the negative FP

hazard (Figure 6.3-2). A third unit is in storage at the ISS. For operational description of the 

PCU, see Appendix H for reference to Section 2.3.4 of the International Space Station Electrical 

Power Systems Training Manual ISS EPS TM 21109 [Anon., 2004].

Figure 6.3-2. PCU Installed on ISS (Source: NASA)

Reference:

1. Anon. (2004): ISS Electrical Power Systems Training Manual, ISS EPS TM 21109, Mission 

Operations Directorate, Space Flight Training Division, NASA Johnson Space Center, 2004.

6.3.4 Hazard Classification and Protection Systems

Given that the plasma hazard is an electrical shock hazard for an EVA astronaut, it has been 

classified as a catastrophic hazard. In this classification, a two-fault tolerant hazard control must 

be employed. To meet the two-fault tolerant requirement, the ISSP has employed two PCUs and

an automatic array shunting algorithm referred to here as solar array shunt FDIR, or just FDIR. 

The solar array FDIR algorithm is enabled after the two PCUs are in discharge. If the FDIR 

detects that one of the two PCUs have failed, the algorithm will shunt solar arrays (refer to the 

B9-908 document, “Plasma Hazard Mitigation during EVA”). Appendix F provides information 

received from the electrical power system (EPS) hardware operator in relation to the FDIR. To 

support the ISS power demands, ground commands to unshunt the arrays may occur any time, in 

or out of sunlight. However, to reduce the RAM electrical current collection, the commands are 

issued after the corresponding array is off-pointed from the velocity vector by >105 degrees. No 

more than two arrays can be unshunted and auto-tracked while being less than 105 degrees from 

the velocity vector.

The EVA pre-planning efforts involve a short-term forecast where environment measurements 

are taken 14 days prior to the EVA (per ISS-NCR-232G). The ionosphere plasma environment 

measurements are made with the FPMU. The PCUs are off when the FPMU measurements are 

made so corresponding ISS potentials are indicative of the conditions uncontrolled by the PCUs.

Calculations of the ISS chassis potential are made by using FPMU data in the empirical model 

PIM3.0. Configurations of the solar arrays resulting in calculated FPs more positive than 

-40V are acceptable and within the limits. In the event of a PCU failure, if the solar array 

management necessary to maintain the ISS in a “power positive” mode produces a chassis 
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potential more negative than -40V, then additional safety risk will be accepted up to the level of 

-45.5V. The hazard limit for the negative potential was set as -40V [ISS-EVA-312-AC, 2012]. 

However, side notes included on the ISS-provided overview presentations suggest an increased 

risk acceptance level for arc occurrence has been established to tolerate potentials as negative as 

-45.5V (1/14/2009 ISS Safety Review Panel (SRP)). However, the rationale for this move has 

not been documented in any reference this team has uncovered.

Reference:

1. ISS-EVA-312-AC (2012): Electric Shock to EVA Crew Resulting from EMU Arcing in 

Plasma, 1/26/2012.

6.3.5 FPMU

The FPMU is a multi-probe instrument designed to measure: (1) FP, (2) plasma density, and (3) 

electron temperature from the ISS local ionospheric environment (see Figure 6.3-3). Refer to 

Figure 6.3-4 for the location of the PCUs and FPMU through the ISS assembly. The FPMU was 

installed with the goal to use its data for refinement and validation of the ISS spacecraft charging 

models and to determine the severity and frequency of ISS charging events.

Figure 6.3-3. FPMU Probes and Layout
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Figure 6.3-4. Location of the PCUs and the FPMU at the ISS. The FPMU has been in two different 

locations on ISS over the course of its lifetime.

Reference:

1. Wright, et al. (2008): “Charging of the ISS as Observed by the FPMU: Initial Results,” IEEE 

Transactions on Plasma Science, Vol. 36, No. 5, October 2008.

6.3.6 PIM3.0 Charging Model

The initial PIM charging model was developed by Science Applications International 

Corporation and Boeing Space Environments and is currently maintained by Boeing Space 

Environments. The latest revision of the PIM3.0 is used to calculate the ISS chassis potential 

and includes various processes such as: 1) the magnetic induction potentials due to motion of the 

vehicle through the Earth’s geomagnetic field; 2) the charging due to solar array and other 

current collection processes from the ionosphere plasma; and 3) PCU effects. Figure 6.3-5 

shows examples of the calculated potentials on the ISS using the PIM3.0 model. Figure 6.3-5 

also shows that the use of PCUs affects the potential distribution across the vehicle. Figure 6.3-6 

shows the effectiveness of the PCU at controlling the chassis potential (i.e., potentials with PCU 

on versus potentials with PCU off). The PCUs keep the ISS within the -40V limit when the 

PCUs are in discharge. 
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Figure 6.3-5. 2010 Comparison of Potential Calculations with PCUs On and Off [Kramer, et al., 

2010]

Figure 6.3-6. Example of FPMU Data showing the Effect on the Peak Chassis Potential (i.e., FP) 

when the PCUs are On and Off 

Reference:

1. Kramer, L.; Hamilton, D.; Mikatarian R.; Thomas J.; and Koontz, S. (2010): “Positive 

Voltage Hazard to EMU Crewman from Currents through Plasma,” Proc. 4th IAASS 

Conference ‘Making Safety Matter’, Huntsville, Alabama, USA, 19–21 May 2010 (ESA SP680, September 2010).
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6.3.7 Electrical Shock Hazard Scenarios

The electrical shock hazard associated with EVAs is based on a situation in which an electrical 

circuit is established that could inject an electrical current into a crew member inside an EMU. 

Critical to the establishment of a shock hazard is the fact that EMU crew members wear a 

cooling garment against their skin, which quickly becomes soaked in perspiration as an EVA 

commences. The close confines of the EMU, combined with the layer of perspiration that covers 

the crew member’s body, results in a situation where there is electrical contact between the crew 

member and the metal components used at several locations in the EMU construction. Thus, if 

electrical current flows through an EMU, there will be a parallel path through the crew member’s 

body, which represents a hazardous situation for the crew member (i.e., a shock hazard). The 

severity of this hazard ranges from a small shock on the skin that causes the astronaut to be 

startled, to a catastrophic situation in which current flows through the astronaut’s thoracic cavity 

and causes defibrillation or arrest of the heart (see Appendices A and B). 

Two charging scenarios on the ISS must be assessed to determine if they give rise to an electrical 

shock hazard: 

1) Negative charging 

2) Positive charging 

Given that astronaut safety is at stake, the most conservative approach is taken to assess the 

electrical circuit associated with each charging scenario. Specifically, the circuit that is 

evaluated is the one that can lead to electrical current flow through the astronaut’s thoracic 

cavity. This circuit is created when current enters a lower portion of the EMU (i.e., the waist 

area), and then flows through crew member’s body and exits at a point in the upper portion of the 

EMU (e.g., the neck area). 

6.3.8 Hazard Circuit Associated with Negative Charging

In the case of a negative charge being applied to the EMU, the hazard that arises is from current 

flow due to an electrical discharge (arc) on an anodized Al component somewhere on the EMU. 

With a crew member in a perspiration-soaked garment that is in electrical contact with portions 

of the EMU, as current flows through the EMU to an arc site, a portion of the current can flow 

through the crew member’s body. The arcing scenario associated with negative potentials on the 

ISS and applied to the EMU can be visualized in Figures 6.3-7a through 6.3-7e. The choice to 

separate the negative charging hazard circuit into several circuit diagrams was made to not only 

illustrate how the situation develops, but to also indicate that multiple events must occur 

simultaneously in order for the actual hazard to be created.
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Figure 6.3-7a. Circuit Element Definitions used in Circuits #1, #1a, #2, and #3 (below)
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Figure 6.3-7b. Circuit Diagram showing Solar Array Current Collection and related Charging of 

the ISS. In steady state, the ISS chassis potential (or FP) adjusts to achieve current balance, such 

that the Ion Current = Electron Current.
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Figure 6.3-7c. Circuit Diagram showing the Scenario where an EVA is being Conducted. There is 

no direct electrical connection between the charged ISS chassis and the EMU (space suit).
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Figure 6.3-7d. Circuit diagram showing the scenario where a direct electrical connection is 

established between the charged ISS chassis and the EMU (space suit). In this situation, there is a 

small electron current that flows from the ISS chassis to the EMU. Only a small amount of electron 

current flows to the EMU due to limitations in RAM ion current collection on CEMU.
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Figure 6.3-7e. Circuit diagram showing the scenario where an arc occurs on an anodized component 

of the EMU (space suit). A large current of electrons flows through the EMU to the arc site. With a 

crew member inside a perspiration-soaked garment in the EMU, there is electrical contact between the 

crew member and various EMU metal components. Current flowing to the arc site can follow a 

parallel path through the crewmember. Arc current magnitude can exceed 10 amps as electrons in the 

arc plasma neutralize nearby anodized surfaces on the ISS vehicle (one side of CISS). If a fraction of 

the arc current flows through the crew member, a significant hazard occurs. The source of the large 

arc current is the CISS, which is a very large capacitor.

Summary – Negative Charging Hazard

The shock hazard associated with negative charging on the ISS vehicle is by a situation in which 

an electrical discharge (arc) forms on an EMU component. The simple application of a negative 

charge on the EMU does not create a hazard. The application of a negative charge on the EMU 
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must result in an arc occurring before the hazardous situation arises. The crew member in an 

EMU is inside a perspiration-soaked garment, which provides electrical contact between the 

crew member’s body and metal components that make up the EMU. Therefore, if current flows 

in an EMU due to an arc occurring on an external component, the crew member’s body may be 

subjected to current flow as it represents a parallel path for a portion of the arc current. 

The formation of an arc on an EMU requires the simultaneous occurrence of multiple events, 

which means the likelihood of an arc occurring is very low. As depicted in Figures 6.3-7a 

through 6.3-7e, for an arc to occur on an EMU, the following must happen: 

1) The ISS vehicle must experience spacecraft charging to negative potentials, as shown in 

Figure 6.3-7b. 

2) A bare metal component on an EMU must make electrical contact with a bare metal 

component on the ISS chassis. This situation is shown in Figure 6.3-7d. 

3) Anodized Al components on the EMU must develop a potential difference across their 

anodization (oxide) layers (i.e., negative charge on the surface against the Al metal and 

positive charge on the surface exposed to the plasma). In Figure 6.3-7d, the potential 

across the anodization layer is represented by the electrical charges on the element 

“CEMU.”

4) An anodization (oxide) layer must breakdown and generate an arc. In Figure 6.3-7e, the 

arc on an anodized component is shown as the lightning bolt across the element “CEMU.” 

5) Charge from the ISS vehicle must flow through the EMU to the arc site – which means

the EMU must remain electrically connected to the ISS chassis throughout the charging 

and arcing process. In Figure 6.3-7e, the large current through the arc site is provided by 

the capacitance of the ISS vehicle represented by “CISS.” 

If such a set of events were to occur, and an arc was generated on an EMU, the astronaut inside 

the EMU would most likely experience an electrical shock as electric charges move from the ISS 

chassis through the EMU into the arc site and return to the local plasma environment. With the 

crew member in electrical contact with EMU metal components, due to the perspiration-soaked 

garment covering the crew member’s body, some of the arc current can split into the parallel 

path created by the crew member’s body. Because the United States (U.S.) modules on the ISS 

are constructed in a manner that results in a large effective capacitance, the magnitude of current 

flow (charge movement) through an EMU arc site is possibly very large (>10 amps). If only a 

small fraction of the arc current goes through the crew member’s body, a potentially catastrophic 

situation can be created.

A key to all electrical shock hazards associated with the EMU is that bare metal on the EMU 

must make electrical contact with bare metal on the ISS vehicle in order to charge the EMU. 

Due to the nature of the construction of the EMU and ISS vehicle, it is very unlikely that an 

electrical contact can be established, let alone maintained, for the time period required to 

establish a hazardous charging situation (negative or positive).
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6.3.9 Hazard Circuit Associated with Positive Charging

A crew member inside an EMU is in electrical contact with the metal surfaces in the EMU 

because the crew member’s body is covered in a perspiration-soaked garment (the liquid cooling 

and ventilation garment (LCVG)). The crew member can, therefore, become part of an electrical 

circuit in which current can flow and a shock can be delivered. In the case of the negative 

charging hazard (described in Section 6.3.8), an arc generates the hazardous situation. In the 

case of positive charging of the EMU, current flow in the EMU (and the crew member’s body), 

can occur as the capacitance of the EMU is charged by electron current from the local plasma. A 

bare metal component on the EMU must contact a bare metal component on a positively charged 

section of the ISS vehicle. In this scenario, the EMU metallic structure becomes positively 

charged and electrons are attracted to the external surfaces of the EMU. Anodized components 

of the EMU act as capacitors and can be collectively treated as a single capacitance “CEMU.” It is 

possible that as the capacitance of the EMU charges due to electron current from the plasma, the 

crew member’s body, that is part of the EMU circuit, will be impacted by the current flow. The 

positive charging hazard, therefore, is initiated when the EMU metallic structure charges to 

positive potentials with respect to the local plasma.

While the PCUs are in discharge, the ISS is grounded close to the center of the station where the 

units are located. At precisely the PCU location, the potential is around -10V since there is a 

10V drop across the device. The difference in potential across the truss due to v × B·L is on the 

order of ~38V. With the PCUs on, a maximum positive potential is on the order of 10V

(accounting for the PCU potential drop) can be seen as the calculated FP in Figure 6.3-5. 

Considering the positive potential electrical current path (Figure 6.3-8), the hazard is from the 

electron current collection during charging of the EMU capacitance (i.e., the capacitance due to 

external anodized components). The plasma impedance for collecting electrons when the 

potential is positive is high, thus limiting the electrical current in the path. This current lasts on 

the order of 1 microsecond (ms) and it is in the order of 1 milliampere (mA). See Section 7.10 of 

this report for details. The hazard control documents ISS-EVA-312-AC and the ISS-NCR232F/G do not specify a positive potential or electrical current collection limit. References to 

electrical current threshold for human reaction can be found in Appendices A and B of this 

report.
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Figure 6.3-8. Positive Potential Electrical Current Path through the Crew Member.

Note the crew member’s body can become part of the electrical circuit due to contact of the 

perspiration-soaked cooling garment covering the crew member with internal metal structures in the 

EMU.

6.3.10 Shock Hazard Probabilities

The probability of a shock hazard developing during an EVA involves the probability of large 

chassis potentials developing combined with the probability of completing the electrical current 

path through the EMU. The ISS Probabilistic Risk Assessment for Shock Hazard, ISS-PRA-12-

56 (May 17, 2013), lists the simultaneous events for a shock hazard to occur and reports the 

probability as 6.72E-06, which can be improved to 9.44E-08 with additional isolation to the 

operational bioinstrumentation system (OBS). 

Fundamental to the establishment of both the negative and positive shock hazard circuit are the 

following two conditions:

1) The crew member’s body must be in electrical contact with exposed metal inside the 

EMU at two separate locations.

2) An electrical connection must be made between ISS structure and the EMU (i.e., exposed 

metal on the exterior of the vehicle must connect to/touch exposed metal on an exterior 

surface of the EMU). 

With respect to the first condition, the crew member’s body is covered with a LCVG, which 

quickly becomes soaked with perspiration as an EVA begins. The wet LCVG increases the 

likelihood of electrical contact between the crew member’s body and metal components on the 

interior of the EMU. Figure 6.3-9 shows the locations of possible metal contact in the EMU suit. 

Electrical connection between exterior bare metal surfaces on the ISS vehicle and the EMU is a 

low probability condition due to the prolific use of anodized Al on both the vehicle and the 

EMU. (Recall that anodized coatings are electrically insulating). To further decrease the 

probability of bare metal contact between the exterior surfaces on the vehicle and the EMU, 
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isolation features were implemented (circa 2009) into the EMU’s Modular Mini Workstation 

(MMWS) exposed metal (refer to ISS-NCR-232F, Attachment 7 and Appendix D). Kapton®

film was placed between the Al baseplate and the stainless steel receptacles 

(Figure 6.3-10) and hard anodized washers were used to isolate conductive paths through the 

fasteners. These modifications were validated through ground testing [Castillo, 2010], which 

included isolation and mechanical stress tests.

Figure 6.3-9. EMU Suit External Metal Locations [ISS-NCR-232F Attachment 5, 2012]
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Figure 6.3-10. Modifications to the MMWS (“tool belt”) [ISS-NCR-232F, Attachments 5 and 7,

2012]

References:

1. Castillo, M. (2010): Modular Baseplate Assembly/Body Restraint Tether/Handrail Electrical 

Continuity Test, dtd. 05/04/10.

2. ISS-NCR-232F (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, 1/31/2012. Attachment 5.

3. ISS-NCR-232F (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, 1/31/2012. Attachments 5 and 7.

6.3.11 Approach to EVAs Without a Two-Fault Tolerant Hazard Control

The negative potential limits and hazard controls discussed so far are referenced in the ISS-NCR232F. This version of the “Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment” document was the active hazard control guidelines at the start of this 

assessment. However, on September 2013, a new version, G, of the document was signed. 

Figure 6.3-11 summarizes the differences between the two guidelines as well as the 

recommendations from this assessment relative to the controls. In summary, the new guidelines 

(1) extends the “short-term” forecast to 14 days prior to an EVA, (2) updates the FP risk 

acceptance limit to -45.5V, and (3) provides guidelines for the use of controls based on the 

14-day FP calculations from the forecast. The extension of the forecast based on the 
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environment measurements taken 14 days prior to the EVA is based on the environments 

persistence of condition (the environment today is the same as it would be in 14 days) and the

solar cycle predictions remaining “benign” at least through Solar Cycle 25 which extends 

through 2030 (ISS mission). Because operation of the PCUs increases the magnitude of the 

positive potential at certain points on the vehicle, the new control guidelines are biased towards 

not operating the PCUs (i.e., PCUs not in discharge). 

The new NCR document establishes the following for controls based on the calculation of the 

ISS potentials 14 days in advance of an EVA:

1) When the 14-day forecast calculates FP more positive than -45.5V, then:

- for EVAs inboard the solar alpha rotary joint (SARJ), use of the PCUs is optional 

and the array shunt FDIR is not required. 

2) When the 14-day calculates FP more negative than -45.5V, then: 

- for EVAs inboard of the SARJ, use the two PCUs in discharge with the array 

shunt FDIR enabled.

3) Because of the positive FP outboard of the SARJ when the PCU is on, the PCU will not 

be used for EVAs outboard of the SARJ.

4) If the PCUs are required due to extreme negative potentials, the ISS will be placed in the 

y-axis in the velocity vector (YVV) orientation to mitigate the positive hazard.

5) If the YVV orientation is not possible, then the ISS-PRA-12-56 low probability of shock 

hazard (which includes the isolation modifications to the MMWS) will be used as 

justification against the hazard. Additional isolation to the OBS would be added.

Figure 6.3-11. Comparison of Hazard Control Approaches
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6.3.12 Data Supporting NESC Recommendations

Section 7.0 presents the supporting information for the NESC recommendations to revise the 

new guidelines [ISS-NCR-232G, 2013]. However, the NESC team recommends a combination 

of controls different from the earlier version [ISS-NCR-232F, 2012] of the guidelines. The 

recommended hazard control plan is to use the two PCUs in discharge for all EVAs regardless of 

location, and the EMU isolation features, which predict a low probability of contact, as the three 

controls. As for the positive potential hazard, the NESC position is that it is not a threat even 

under the worst-case positive potential (+15V) and the maximum exposed metal area in the 

EMU.

References:

1. ISS-NCR-232G (2013): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, NCR-20264-R7, 18 September 2013.

2. ISS-NCR-232F (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, 1/31/2012. 

7.0 Data Analysis

The use of active controls to prevent the shock hazard (e.g., PCUs and shunt array FDIR) was 

evaluated based on the data and analyses presented in this section. The recent ISS Safety team’s 

proposed control-use approach triggered questions related to the adequacy of the forecast and the 

tools associated with the output calculations and limits for FP subsequently used for safetycritical decisions. The various FP scenarios and events were considered and examples are 

provided below. These examples of charging events were considered along with the solar array 

shunt FDIR operations to identify non-characterized issues during the array management. 

Several aspects of the PCU utilization were studied to determine reasons that would merit the 

discontinuation or limitation of the PCU use. The PCU adequacy to support the ISS mission (up 

to 2030) was considered from the capability and reliability perspective. The positive ISS truss 

FP bias introduced when the PCUs are in discharge seems to have been a factor against its use. 

Therefore, the electrical current collection scenario under the positive FP conditions was 

analyzed with the purpose to understand the severity of the positive potential hazard. 

Other aspects studied in this assessment involve the probability of completing an electrical

current path from the ISS through the EMU suit through the crew member. This condition was 

studied considering the isolation layers in this path that include most recent modifications to the 

suit-tool configuration.

7.1 Shortcomings in the Space Weather Forecast Planning that Limits its 

Utility for Forecasting 

The proposed strategy for forecasting ISS charging levels 14 days in advance of an EVA as 

described in ISS-NCR-232G has technical issues. The strategy involves forecasting space 

weather conditions and using the forecast conditions as input to the PIM3.0 charging model. The 
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issues with the forecasting process must be addressed before the strategy is used by the ISSP for 

making safety critical decisions regarding EVAs.

7.1.1 Persistence of Conditions Assumption is Not Accurate

No sophisticated space weather modeling technique is being used in the 14-day space weather 

forecast. The plasma electron density (Ne) and plasma electron temperature (Te) “forecast” is a 

simple persistence of conditions method based on the assumption that space weather conditions 

in 14 days will be the same as on the day the FPMU measurements are obtained. FPMU 

measurements are obtained on a reference day about 14 days in advance of a scheduled EVA and 

used to document the current Ne and Te values along the ISS orbit. The FPMU data are then 

compared to output from a statistical version of the IRI model to determine which statistical 

estimate for Ne and Te deviations at 1, 2, and 3 levels (where  is the standard 

deviation) about the IRI model best represents the measured FPMU data. The selected statistical 

IRI model output is used to generate Ne and Te values along the ISS orbit that are input to the 

PIM3.0 charging model to predict ISS charging 14 days in advance of the EVA.

The National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction 

Center (SWPC) (the Federal entity chartered with providing official U.S. government space 

weather forecasts) only issues 3-day forecasts of solar flare activity and geomagnetic storm 

conditions which could impact ionosphere electron density and temperature conditions 

(http://www.swpc.noaa.gov/wwire.html#swxdaypre). 

NOAA SWPC does provide a 45-day forecast of geomagnetic Ap and solar F107 indices 

(http://www.swpc.noaa.gov/ftpdir/latest/45DF.txt) that could be used to provide the predicted 

F107 values required to run the IRI model. However, no guidance is provided in ISS-NCR232G or the plasma hazard assessments available to the study team for review [Hartman, 

2013a,b; Schmidl, 2013b] as to how future F107 values are obtained for use in the generating the 

plasma hazard assessments.

References:

1. Hartman, D. (2013a): Plasma Hazard Relief Assessment for US EVA 22, ISS-HOU-ENVWAH-130032, 25 June 2013.

2. Hartman, D. (2013b): Plasma Hazard Relief Assessment for US EVA 23, ISS-HOU-ENVWAH-130035, 2 July 2013.

3. Schmidl, D. (2013b): Plasma Hazard Relief Assessment for US EVA 21, ISS-HOU-ENVWDS-110018, 10 May 2013.

7.1.2 Dependency on Benign Solar Cycle is Unreliable

ISS-NCR-232G provides a statement that “the Space Environments community has concluded 

based on the downward trend of recent Solar Cycles that the environment will remain benign at 

least through Solar Cycle 25 which extends through 2030.” It is not clear from the document 

what group the term “Space Environments Community” is intended to represent. The general 

consensus of this NESC team is that, based on the poor results from the solar physics community 

in predicting the low activity state of the current Solar Cycle 24, it is unlikely there is any 
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physical basis for making quantitative predictions of activity levels through the end of Solar 

Cycle 25.

The ability of the solar physics community to forecast solar activity for a complete solar cycle in 

advance is limited at best. Figure 7.1-1 from Pesnell [2008] shows a collection of predictions for 

the annual averaged sunspot number (Rnn) at the peak of Solar Cycle 24, which were all made 

before Solar Cycle 24 started. 

A few of the prediction techniques gave values close to the local maximum of R = 67 that was 

observed in February 2012 [Biesecker et al., 2013]. However, a number of the predictions are 

lower than the observed maximum in 2012 and most of the predictions are significantly higher 

than the observed maximum. Some of the predictions even give values in the range of R = 180 

with error bars extending over R = 200. Such high values typify the solar cycle maxima from 

past cycles, thereby demonstrating that pre-Solar Cycle 24 predictions varied from historic lows 

to typical highs. Predictions of Solar Cycle 25 activity using some, or all, of these same 

techniques will likely result in the same large range of predicted activity levels. Additional work 

is required before forecasts of solar activity in future cycles can be claimed with any real 

accuracy [Pesnell, 2008, 2012].

Figure7.1-1. Predictions of Solar Cycle 24 Sunspot Maximum

Colored bars show the wide range of Solar Cycle 24 sunspot maxima values obtained from different 

prediction techniques [Pesnell, 2008].
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References:

1. Pesnell, W.D. (2008): “Predictions of Solar Cycle 24,” Solar Phys., 252, 209-220, 2008.

2. Biesecker, D.; Balch, C.; and McIntosh, S. (2013): “Where is Solar Cycle 24? Did it happen 

already? Is There More to Come?” Presented at the NOAA 2013 Space Weather Workshop, 

Boulder, Colorado, 16-19 April 2013.

3. Pesnell, W.D. (2012): “Solar Cycle Predictions (Invited Review),” Solar Phys., 281, 

pp. 507-532, 2012. 

7.1.3 Use of the Climatological Model – IRI is Inadequate

Use of the IRI model to generate the input values required for the PIM3.0 charging model 

calculations is problematic in two significant areas. First, the IRI model itself is only a monthly 

average climatology model not intended for use in predicting changes in ionospheric Ne or Te 

values over shorter time periods. Second, the model is incapable of predicting the full range of 

environments responsible for ISS charging, including auroral electron flux and plasma depletions 

at low latitude eclipse exit where the strongest ISS charging to date has been observed. IRI 

models only the ambient background plasma conditions within the ionosphere and contains no 

model for the physics of energetic auroral electrons that are responsible for auroral charging. 

The eclipse exit rapid charging events that represent some of the largest ISS charging observed 

to date (in the -40 to -67V range) have been shown to occur in plasma density depletions at 

high latitudes and in dawn density depletions in the equatorial region. IRI does provide some 

representation of the low plasma density in high latitude ion troughs, but regularly 

underestimates their magnitude. The physics for dawn density depletions is not included in the 

IRI model.

7.1.4 Missing Short Term Changes in the Plasma Environment: Geomagnetic Storm 

Activity

Examples of the plasma hazard assessment’s provided to the ISS program before each EVA 

reviewed for this study [Hartman, 2013a,b; Schmidl, 2013b] do not include information on the 

current state of geomagnetic activity, which is a significant issue. Geomagnetic storm activity 

tends to deplete the ionosphere of plasma density and increase the electron temperature. These 

changes actually serve to suppress ISS solar array charging because the reduction in electron 

density reduces the amount of electron current to the solar cells and the higher electron 

temperature increases charging of the cover glass material on the solar cells. This, in turn,

increases the barrier potentials and reduces the amount of electron current that reaches the solar 

cell. Measurement of the ionospheric Ne and Te values during a geomagnetic storm period will 

give values representative of suppressed ISS charging conditions. Once a geomagnetic storm 

ends—typically on time scales of 12 to 24 hours—the electron density and temperatures recover 

to the pre-storm values, which will result in higher charging levels. Obtaining the reference data 

for the 14-day forecast during a geomagnetic storm period almost certainly guarantees the 

charging environment will be underestimated for the time period of an EVA. There is no release 

documentation suggesting that this effect has been considered in development of the 14-day 
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forecast products and no documented plans to deal with FPMU data obtained during disturbed 

periods when reference data may under represent environment in 14 days. 

References:

1. Hartman, D. (2013a): Plasma Hazard Relief Assessment for US EVA 22, ISS-HOU-ENVWAH-130032, 25 June 2013.

2. Hartman, D. (2013b): Plasma Hazard Relief Assessment for US EVA 23, ISS-HOU-ENVWAH-130035, 2 July 2013.

3. Schmidl, D. (2013b): Plasma Hazard Relief Assessment for US EVA 21, ISS-HOU-ENVWDS-110018, 10 May 2013.

7.1.5 Inconsistencies in Input Parameters

An additional issue identified with use of the forecast tools is an inconsistency in the use of 

different versions of the IRI models. The plasma hazard reports reviewed for this study

[Hartman, 2013a, b; Schmidl, 2013b] indicate the IRI-2011 model is used for the plasma hazard 

assessment. However, the plasma variability model that is used to obtain the 1, 2, and 3

level deviations in the Ne and Te values about the IRI model output was derived from comparing 

satellite data with the IRI-2001 model [Minow, 2004]. No evidence was presented to 

demonstrate that the statistical variability levels for Ne and Te values derived from the older IRI2001 model are still applicable to the newer IRI-2011 version of the ionospheric climatology 

model.

References:

1. Hartman, D. (2013a): Plasma Hazard Relief Assessment for US EVA 22, ISS-HOU-ENVWAH-130032, 25 June 2013.

2. Hartman, D. (2013b): Plasma Hazard Relief Assessment for US EVA 23, ISS-HOU-ENVWAH-130035, 2 July 2013.

3. Schmidl, D. (2013b): Plasma Hazard Relief Assessment for US EVA 21, ISS-HOU-ENVWDS-110018, 10 May 2013.

4. Minow, J.I. (2004): “Development and Implementation of an Empirical Ionosphere 

Variability Model,” Adv. In Space Res., 33, 887-892, 2004.

7.1.6 Limited Validation Studies

Reliability of the technique to give predicted ISS charging levels that are not exceeded during an 

EVA period would depend critically on the ionosphere exhibiting very low levels of Ne and Te 

variability over the forecast period. The ISS-NCR-232G argues that these conditions are met for 

the current Solar Cycle 24 because the lower than typical solar ultraviolet/extreme ultraviolet 

output has resulted in a depressed solar cycle with hotter electron temperatures that limit 

charging. What is required to test this prediction technique, however, is not an argument based 

on high electron temperatures, but rather comparisons of measured Ne and Te values on a 

reference day to those observed on the forecast day 14 days later. The only available material 

showing such a validation study [Hartman, 2013c] is limited to comparing FPMU Ne and Te
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measurements at approximately 7-, 14-, 21-, and 30-day intervals from a single reference day 

measurement in the two time intervals Greenwich Mean Time (GMT) 2011/120-150 and 2013

GMT 20-100. It is not clear why such a limited set of validation comparisons have been 

attempted since there are numerous long periods of FPMU data from recent years that can be 

used to conduct more extensive comparisons for validation. In addition, there are alternative 

data sets such as ground-based ionosonde measurements that can be used to test 14-day forecasts 

for periods of a year or more. 

No complete verification of the ability to predict ISS potentials 14 days in advance has been

demonstrated. The validation studies for the 14-day forecast and PIM3.0 charging modeling 

strategy available for review have only shown that Ne and Te values have not significantly 

changed over 14 days for a few limited time periods. No attempt to forecast the ISS charging 

levels and then compare the measured potentials after 14 days to validate that charging values 

never exceed the forecast was provided to the NESC team. As a result, pieces of the forecast 

technique appear to work at least for a couple of isolated time periods, yet there has been no full 

validation study to demonstrate the technique.

Reference:

1. Hartman, D. (2013c): Extension of Plasma Forecasting, Boeing Space Environments, 2013.

7.2 FPMU Role in the Forecast: Criticality and Alternate Data

FPMU data are critical to the 14-day plasma hazard forecast approach because FPMU Ne and Te

measurements are used to constrain which statistical set of IRI-2001 statistical model output will 

be used as input to the PIM3.0 charging model calculations of the ISS potentials. In order to 

provide the plasma hazard forecast, a source of Ne and Te data to constrain the ionosphere model 

is required. A review of the NCR proposing to replace PCU operations with the plasma hazard 

forecast approach [ISS-NCR-232G, 2013] shows no explicit contingency procedure that outlines 

what to do if recent FPMU data are not available. 

Reference:

1. ISS-NCR-232G (2013): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, NCR-20264-R7, 18 September 2013.

7.2.1 Ambiguity in Dataset Requirements

The NESC team does note that in at least three of the plasma hazard relief assessment reports 

that were available to the team for review [Hartman, 2013a,b; Schmidl, 2013b], a statement is 

included indicating “if sufficient FPMU data are not available, then +2  results may be used. In 

that case, the Space Environments team will provide those results.” If this vague statement is the 

contingency procedure intended to be followed when FPMU data are not available, then explicit 

information needs to be added describing what constitutes sufficient FPMU data and what 

conditions will require discontinuing use of the plasma hazard forecast process. For example, 

what total amount of data is the minimum required for the assessment? What quality of data is 

acceptable (and what metric is used for the quality assessment)? Which instrument(s) provide 
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the data for the analysis? Can data from any of the FPMU instruments be used or must the data 

come from a specific instrument? Details of this procedure must be documented in the NCR for 

review and concurrence by the ISSP to assure that inadequate FPMU data are not being used in 

the process. 

References:

1. Hartman, D. (2013a): Plasma Hazard Relief Assessment for US EVA 22, ISS-HOU-ENVWAH-130032, 25 June 2013.

2. Hartman, D. (2013b): Plasma Hazard Relief Assessment for US EVA 23, ISS-HOU-ENVWAH-130035, 2 July 2013.

3. Schmidl, D. (2013b): Plasma Hazard Relief Assessment for US EVA 21, ISS-HOU-ENVWDS-110018, 10 May 2013.

7.2.2 Alternatives for Ionospheric Data

The FPMU is not the only source of ionospheric Ne and Te data. Alternative sources of 

ionospheric Ne and Te data should be evaluated by the ISSP for use as a contingency option for 

characterizing the plasma environment should FPMU data not be available. One possible 

example is the approximately 500 to 1000 electron density profiles provided by the 

FORMOSAT-3/COSMIC satellite constellation distributed over a wide range of latitudes and 

longitudes [Rocken, et al., 2000; Schreiner, et al., 2007; Anthes, 2011]. Another source of 

ionospheric plasma density data is the maximum F2-region electron density routinely measured 

by a global network of ionosonde stations and distributed by the NOAA SWPC every 30 to 60

minutes [NOAA, 2014]. ISS orbital altitudes are typically above the F2-peak where the electron 

density is less than the F2-region peak values so this data would characterize the worst-case 

electron density for ISS charging. Finally, availability of data from the Global Assimilative 

Ionospheric Model (GAIM), or other full physics ionosphere models, should be evaluated for use 

in providing Ne and Te along the ISS orbit. GAIM is of particular interest because the model 

output is constrained by real-time data from a number of sources including ionosondes and 

satellites.

References:

1. Rocken, C., Kuo, Y.H.; Schreiner, W.; Hunt, D.; Sokolovskiy, S.; and McCormick, C. 

(2000): “COSMIC system description,” Terr. Atmos. Oceanic Sci., 11, 21–52.

2. Schreiner, W.; Rocken, C.; Sokolovskiy, S.; Syndergaard, S.; and Hunt, D. (2007): Estimates 

of the Precision of GPS Radio Occultations from the COSMIC/ FORMOSAT-3 Mission, 

Geophys. Res. Lett., 34, L04808,doi:10.1029/2006GL027557.

3. Anthes, R. A. (2011): “Exploring Earth’s Atmosphere with Radio Occultation: Contributions 

to Weather, Climate and Space Weather,” Atmos. Meas. Tech., 4, 1077–1103, 

doi:10.5194/amt-4-1077-2011.

4. NOAA (2014), Space Weather Prediction Center, URL: 

http://www.swpc.noaa.gov/ftpdir/lists/iono_day/README, accessed 21 May 2014.
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7.2.3 FPMU Reliability

A number of issues related to FPMU data availability and reliability must be considered when 

deciding whether to discontinue PCU operations in favor of the plasma hazard forecast approach. 

The FPMU was designed and built as Class 3 electronics (for a 3-year operational life). 

Reliability is provided by (a) redundant Ne and Te and FP measurements from multiple 

measurement techniques and (b) spare FPMU units to replace a failed unit [Swenson and 

Thompson, 2002]. Three flight and two engineering/qualification units were delivered to NASA 

with the assumption that flight units would be replaced with a spare when operational units 

failed. FPMU Serial Number 3 was deployed on the ISS during an EVA on August 3, 2006 with 

the first data received the same day. This same unit continues to serve as the operational FPMU 

instrument on the ISS and has collected data for approximately 709 days during the period 

starting August 3, 2006, and ending October 1, 2013, (the last time the data collection statistics 

were updated), representing 1.9 years of powered instrument operations over an on-orbit time of 

7.2 years. The data collection time is only approximate (within a few days) since it was obtained 

from a count of daily file folders generated by the FPMU ground station and not a detailed 

measure of the actual instrument operations time. FPMU operations are typically limited to 

about 100 days a year although operations in 2012 exceeded 130 days with no operational FPMU 

issues. 

Reference:

1. Swenson, C.; and D. Thompson (2002): “FPMU Systems Overview,” presented at FPMU 

CDR, February 19-20, 2002.

7.2.4 FPMU Design Life Limitations Compared To PCU

Any recommendation to discontinue the use of the PCUs in favor of a process requiring FPMU 

data should balance the remaining life expectancy of the operational FPMU unit and the two 

flight spares against the expected life of the PCUs. PCUs were designed for long-term use in the 

space environment and finding 7 (F-7) in this report demonstrates the two operational PCUs have 

adequate xenon gas and a hardware design life to support their use past 2028. In addition, a third 

spare PCU unit is located on board the ISS with a full tank of xenon gas and a hollow cathode 

that has seen little use. 

In contrast, the cumulative design life for an FPMU unit is only three years and long-term 

reliability of the instrument is based on redundant measurements from the multiple probes and 

replacing failed units with flight spares. As of March 1, 2014, the operational FPMU unit will 

have been exposed to the space environment for 7.6 years, exceeding the cumulative 3-year life 

requirement for a single unit by 4.6 years. Limited life items used in the design of an FPMU 

include the cleaning lamp in the wide‐sweep langmuir probe (WLP), and a thermal switch used 

in the survival heater. The cleaning lamp in the WLP sphere is not an issue because the cleaning 

lamp is no longer used in FPMU operations. The survival heater is required to run continuously 

when FPMU is outside the vehicle to protect the electronics from extremely cold temperatures. 

The thermostats are expected to cycle every 270 minutes and are rated for 10,000 cycles for a life 
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of 5.1 years [Utah State University, 2002]. The current operational unit has successfully 

exceeded the survival heater rating by 2.5 years. Additionally, radiation damage to the 

electronics is an issue because the FPMU was not built using radiation hardened parts. The ISS 

radiation design environments (Space Shuttle Program (SSP) 30512) indicate the 1-year total 

ionizing dose, due to trapped protons and electrons in silicon behind 5 mm of Al-equivalent 

shielding, is approximately 123 radiation absorbed dose (rad) [Space Station Program Office, 

1994]. Radiation sensitive components with this amount of shielding could exceed a total 

ionizing dose of 1000 rad in the next year based on the SSP 30512 specification, a benchmark

where commercial parts not selected for tolerance to radiation environments begin to show 

degradation. SSP 30512 is a conservative design environment so the as-flown radiation dose is 

certainly lower, but a more thorough analysis of potential radiation effects on the FPMU is 

warranted to determine what additional time remains for the operational unit before replacement 

with the flight spare is required.

While the FPMU currently in operation on the ISS has exceeded the design life, it has not been 

shown that the remaining spare flight units can be expected to operate for a similar period 

beyond the design life. Should the operational FPMU fail in 2014, the conservative assumption 

is the two flight spares will last the 3-year design life and can be expected to support the 

proposed plasma hazard forecast process only until 2020. If the two units last for periods 

approaching the flight experience of the FPMU operating on ISS, then the plasma hazard process 

could possibly be supported by FPMU data until 2028.

References:

1. Utah State University (2002): SDL 2002, FPMU Limited-Life Items List, SDL/02-037, 

Space Dynamics Laboratory, Utah State University, 7 February 2002.

2. Space Station Program Office (1994): Space Station Ionizing Radiation Design Environment, 

Revision C, 3, SSP 30512, June 1994. 

7.2.5 Spare FPMUs – EVA Deployment

Two additional FPMU flight units provide a backup to the operational unit on ISS. An FPMU 

(Serial Number 5) is stored on-board the ISS for use as a pre-positioned flight spare, but an EVA 

will be required to replace a failed unit. This EVA would have to be conducted without the 

benefit of FPMU data and the plasma hazard forecast process although operation of the PCUs 

during this EVA would mitigate the negative charging hazard. The third FPMU flight unit 

(Serial Number 2) is located in bonded storage at the Kennedy Space Center (KSC), but would 

require a flight to the ISS.

7.2.6 FPMU Power Supply Limitations

The use of the FPMU depends on availability of the television camera interface controller 

(TVCIC) and its power supply, because the TVCIC provides power to FPMU and the link 

between the FPMU and the ISS data telemetry system. The power supply in the TVCIC box 

currently in use with FPMU was launched with a known reliability issue [Kichak et al. 2009; 

Mikatarian, 2010] resulting in periodic shut down. Power cycling of the FPMU/TVCIC 
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combination is occasionally required to re-establish FPMU data flow. Spare TVCIC power 

supplies with a new design to correct the supply failure mechanism exists on-orbit, but the details 

for refurbishing the TVCIC with a spare power supply would need to be worked out should a 

failure occur. Finally, note that options presented as part of the ISS-NCR-232 update do not 

show contingency plans against a TVCIC power supply failure.

References:

1. Mikatarian, R.(2010): Operation of the FPMU to support plasma hazard assessments, 

FINAL-ShortVer-2010-04-13-SSPCB-FPMU-Requirements-revK.pdf, April 13, 2010.

2. Kichak, R., E. Young, C. Pandipati, and R. Cooke, International Space Station (ISS) External 

Television (TV) Camera Shutdown Investigation, NASA TM-2009-215572, NESC-RP-06-

49/06-001-E, February 2009.

7.3 Limitations of the ISS Charging Model PIM3.0

Calculated values of the maximum ISS eclipse exit potential obtained from PIM3.0 charging 

model using measured FPMU Ne and Te at the time of the potential maxima are not the same as 

the maximum ISS potential measured by the FPMU at eclipse exit. Discrepancies between the 

measured data and PIM3.0 modeled data show that there are deficiencies in the model and use of 

FPMU data that limit the accuracy of the output. These limitations and sources of error need to 

be identified, documented, and communicated to the critical decision makers as part of meeting 

the NASA-STD-7009 Standard for Models and Simulations requirements [NASA, 2013a].

The NESC team identified a number of limitations and sources of error in PIM3.0, which 

resulted in discrepancies between measured and modeled data. Fundamental issues with the 

physics-based algorithms used in the code include: 

 Analytical approximations used in the numerical solutions for the potential barriers in the 

gaps between solar cells (solar array electrical current collection model).

 Assumption that every solar cell and solar array string collects the same electrical current.

 Use of a static (equilibrium) charging algorithm independent of ISS capacitance that 

cannot predict rapid charging events.

 Use of single capacitance in time-dependent charging algorithms that oversimplifies the 

physics of ISS charging and fails to model fast transient charging (i.e., rapid charging 

events).

The issue with the charging algorithms included in the PIM3.0 charging model is fundamental to 

whether the code will be able to predict the full range of charging behavior observed on the ISS. 

PIM3.0 in its current state is only capable of modeling the relatively slow change in ISS potential 

at eclipse exit, but fails to correctly model the rapid charging events observed at eclipse exit and 

when solar arrays are unshunted in sunlight. Refer to Section 7.6.

In addition, errors in input data used to run the model or configuration data used to constrain the 

ISS electrical current collection processes will also impact the model results. A number of these 

errors include: 
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 Uncertainties in FPMU Ne and Te input data (due to FPMU data reduction errors).

 Timing of FPMU data chosen for the PIM3.0 charging model input relative to the 

charging peak maxima.

 Errors in knowledge of (or values used for) solar array angles, ISS flight attitude, ISS 

velocity.

 Variations in ion collection area (free parameter adjusted to obtain best results).

While it may not be necessary to fully characterize each of these sources of error in the PIM3.0

charging model output, an error bound at some appropriate statistical level should be computed 

and applied to the PIM3.0 charging model output when used in safety assessments.

Reference:

1. NASA (2013a): NASA-STD-7009 Standard for Models and Simulations, July 10, 2013.

7.4 PIM3.0 Charging Model in the Critical Path to EVA

The Columbia Accident Investigation Board report [CAIB, 2003] and NASA’s response to it, 

contained in the “A Renewed Commitment to Excellent” report [NASA, 2004], both emphasize 

that modeling and simulation (M&S) used as a basis for critical decisions must meet certain 

standards to ensure the credibility of the results and that analytical results derived from M&S are 

assessed and properly conveyed to those making critical decisions. NASA responded to the 

findings in CAIB, 2003 and PB2005-10096, 2004, by establishing a minimum set of 

requirements and recommendations for use of M&S to support critical decisions and published 

them in NASA-STD-7009, 2013. The requirements and recommendations contained in the 

standard are intended to address one or more of the following eight objectives:

1. Identify best practices to ensure that knowledge of operations is captured in the user 

interfaces (e.g., users are not able to enter parameters that are out of bounds).

2. Develop a process for tool verification and validation, certification, verification, 

revalidation, and recertification based on operational data and trending.

3. Develop a standard for documentation, configuration management, and quality 

assurance.

4. Identify any training or certification requirements to ensure proper operational 

capabilities.

5. Provide a plan for tool management, maintenance, and obsolescence consistent with 

M&S environments and the aging or changing of the modeled platform or system.

6. Develop a process for user feedback when results appear unrealistic or defy explanation.

7. Include a standard method to assess the credibility of the M&S presented to the decision 

maker when making critical decisions (i.e., decisions that affect human safety or mission 

success) using results from M&S.

8. Assure that the credibility of M&S meets the project requirements.

NASA-STD-7009 defines a critical decision as “those technical decisions related to design, 

development, manufacturing, ground, or flight operations that may impact human safety or 

mission success, as measured by program/project-defined criteria.” A decision to discontinue 
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the use of PCUs as a redundant hazard control to guard the safety of crew members during EVA 

(based on the results of the plasma hazard forecasts and PIM3.0 modeling of ISS shock hazards) 

meets the NASA-STD-7009 definition of a critical hazard. However, the PIM3.0 charging 

model, when used to provide results in support of a critical decision, falls short of the 

requirements and recommendations contained in the NASA-STD-7009 in almost every regard. 

Examples of the more serious PIM3.0 shortcomings include (but are not limited to):

 The limitations of the PIM3.0 are not explicitly known by the decision makers. 

 User’s manual and parameter definitions for the PIM3.0 code are not available.

 The configuration files that provide the PIM3.0 model input and control how the model is

run are not documented in the pre-planning proposed procedure. No documented process 

exists to constrain the content of the configuration files assuring the model is used the 

same every time it is run.

 The model has not been independently peer reviewed.

 There is no process identified to update the PIM3.0 charging model to include physical 

changes to the station configuration.

 There is no clearly documented validation, verification, or certification process.

 Uncertainty in the model results are not documented and applied to model output.

The NESC team finds that the PIM3.0 charging model should not be in the critical path for EVA 

safety decisions as it lacks the pedigree associated with NASA standards for M&S. 

References:

1. CAIB (2003): Columbia Accident Investigation Board, Volume 1, August 2003.

2. NASA (2004): A Renewed Commitment to Excellence: An Assessment of the NASA Agencywide Applicability of the Columbia Accident Investigation Board Report, PB2005-100968, 

January 2004.

7.5 Example of PIM3.0 Error Estimate 

Results from the PIM have been stated and used with a high level of accuracy. Sometimes 

PIM3.0 results are reported to the tenth of volt and in some cases reported to the hundredth of 

volt. Kramer et al. (2010) [Kramer, et al., 2010] in a contributed paper to a conference state that 

“The EVA worksite voltage exposure, as seen in Figure 6 and Figure 9, using Boeing-developed 

capability incorporated into the PIM3.0 is accurate.” The authors attribute any error in the 

PIM3.0 results on inputs to the model. No evidence of PIM3.0 validation is referenced for their 

statement. No evidence of a validation exercise has been provided to the NESC team. In a 

hazard situation (e.g., the EVA scenario), statements implying “no error” should be regarded 

with skepticism. Scatter plots of PIM3.0 and FPMU visually do not support using PIM3.0

results to an accuracy of 0.1V. This note is to derive an error based on available PIM3.0 results 

as compared with FPMU data. The result of this method is not offered as the final value, but as 

motivation for the ISS Environments team to derive a value that is vetted and approved within 

the ISSP. 
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Figure 7.5-1 shows a scatter plot of PIM3.0 versus FPMU data. The time period that spans the 

data is day 188 of 2007 to day 105 of 2013. For each FP measurement of the FPMU, the 

simultaneous FPMU density and temperature measurements were input to the PIM3.0 with 

output calculation appropriate for the FPMU location. Note this data period includes both 

locations of the FPMU on the truss (i.e., S1 Truss from August 3, 2006, to November 21, 2009, 

and the P1 truss from November 21, 2009, to the present). The number of data points is 2164. 

Comparing charging events included the file used to generate Figure 7.5-1 [Boeing, 2013] with a 

Marshall Space Flight Center (MSFC) study of eclipse exit charging events [Wright, et al., 

2009], the NESC team determined that not all of the rapid charging events are included in the 

Boeing, 2013 data set. Nevertheless, fundamental information about an error bar to associate

with a PIM3.0 calculation can be obtained from this data set. 

Figure 7.5-2 shows a plot of the difference between the PIM3.0 calculations and the FPMU FP

measurement versus the FPMU measurement. As noted in the figure, ~74 percent of the data 

show a positive difference. Note that the values shown in Figure 7.5-1 are negative. A positive 

difference indicates that the PIM3.0 calculation is less negative, meaning that the model is 

underpredicting the FP of the ISS frame. In a hazardous situation (e.g., EVAs are treated), 

underpredicting should be viewed with concern.

Figure 7.5-1. Scatter Plot of PIM3.0 Voltage Calculations versus FPMU FP Measurement. The 

dashed line represents a one-to-one correspondence; i.e., a slope of 1. 

Additional rapid charging events exceeding 

-45V are missing from the plot.
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Figure 7.5-2. Plot of the (PIM3.0 calculation – FPMU Measurement) Difference versus FPMU FP

Measurement. Dashed line indicates no difference between the model and measurement. 

To get an idea of error from the PIM3.0 calculations, the distribution of occurrence of the modelmeasurement difference data was plotted. The histogram (blue line) in Figure 7.5-3 shows the 

number of occurrences in 1V bins versus the PIM3.0 calculation-FPMU measurement difference. 

Overlaid on the histogram is a Gaussian curve (dashed red line) defined in Eqs. (1) and (2). 

Gaussian = A0 exp-(Z*Z)/2, where Eq. (1)

Z = (x-A1)/A2 Eq. (2)

A0 was chosen to match the largest amplitude of the distribution. A1 was chosen to match the 

location of the peak in the distribution. A2 is the standard deviation and was determined by 

requiring that 68 percent of the distribution lie within one standard deviation of A1. Visual 

examination of the distribution (blue curve) indicates that it is not quite Gaussian in shape. The 

exercise here is to demonstrate a non-zero error and the use of a Gaussian distribution is 

sufficient to do this. 
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Figure 7.5-3. Histogram of PIM3.0, FPMU Measurement Difference for 1V bins. The red-dashed 

line overlay is a Gaussian curve-fit. 

Figure 7.5-4 shows how the data points fall into the 1-, 2-, 3-, and beyond 3 bands. The various 

data bands are colored-coded. The 2.5V difference bias in the data is denoted as the dashed line. 

The various rapid charging event data points are not known with certainty in this plot, but it is 

speculated that these events are the points denoted in red that lie outside the 3 band and solicit

inclusion in an updated PIM.
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Figure 7.5-4. Data in Figure 7.5-2 re-plotted with color code to indicate points that lie inside 1-, 

2-, and 3- boundaries and also points that lie beyond the 3- boundary. The dashed line is the 

2.5V bias inherent in the PIM. 

A suggested method to use for the purpose of deriving a FP value calculated by PIM3.0 is the 

following. First, consider the data plotted in Figure 7.5-5 as FPMU versus PIM3.0 calculation. 

The y-axis in this case could be considered a “prediction.” The data were curve-fit to Eq. (3). 

Y = A + BX, with Eq. (3)

 A = -7.89 and B = 0.67

The best linear fit is marked by the black line. Also shown in Figure 7.5-5 are the 1- (±4V) 

boundary lines and the 2- (±8V) boundary lines drawn parallel to the best linear fit line. 

If a plasma environment (i.e., density and temperature) is input to PIM, then a calculated value 

for a particular location is determined. This calculated value should be processed through 

Eq. (3) to obtain a corrected value. Note the difference between the black centerline and the 

green 1 boundary lines for a given PIM3.0 value approximately ±3V. Once the corrected 

PIM3.0 value is obtained, an error of ±3V for the 1 case should be assigned. In considering the 

2 case, an error of ±6V should be assigned. The risk posture of the ISSP should determine what 

amount to include of standard deviations.
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Figure 7.5-5. FPMU versus PIM3.0 calculation. The y-axis can be interpreted as a prediction 

based on a given environmental input. 

Recommendation: The ISS Environments team should obtain a voltage error to assign to any 

PIM-calculated value and refrain from stating such calculated values to an accuracy of less than 

1V.

References:

1. Kramer, L.; Hamilton, D.; Mikatarian R.; Thomas J.; and Koontz, S. (2010): “Positive 

Voltage Hazard to EMU Crewman from Currents through Plasma,” Proc. 4th IAASS 

Conference ‘Making Safety Matter’, Huntsville, Alabama, USA, 19–21 May 2010 (ESA SP680, September 2010).

2. Boeing (2013): PIMVar1_results.xls file, 2013a. 

3. Wright, et al. (2009): Wright, K.H., FPMURapidChargingEvents_KHW_2009Mar02.xls file, 

2009.

7.6 Types of Charging Events

Figure 7.6-1 shows examples of the three basic types of negative charging events, due to solar 

array interactions with the plasma environment, which have been identified in FPMU data. The 

PCU was not operating during any of the charging events shown in the figure and the potentials 

refer to the ISS potential measured by the FPMU floating potential probe at the location of the 

FPMU instrument. Voltages at other locations on the truss will be shifted by the appropriate 

Green: 1 lines

Red: 2 lines

Prediction error ≈ +/- 3V
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v × B  L inductive potential at the time the data was obtained. Potentials due to normal 

charging (Figure 7.6-1a) are generally in the range of -20V to -30V, but the duration of the 

charging events may last for many minutes to 10s of minutes [Wright, et al., 2007]. Normal 

charging is the most commonly observed type of ISS eclipse exit charging event.

Figure 7.6-1. ISS Solar Array Charging

Charging events identified to date due to solar array interactions with the plasma environment 

include (a) normal eclipse exit charging, (b) eclipse exit rapid charging events, and (c) rapid 

charging events in sunlight following array unshunt operations. 
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Rapid charging events at eclipse exit (Figure 7.6-1b) are characterized by increases in potential 

over time scales of seconds followed by a rapid decrease in potential over a few seconds. While 

many rapid charging events remain within the -20 to -40V range, some of the largest eclipse 

exit charging events observed on the ISS have been rapid charging events with potentials in the 

-40 to -67V range [Craven, et al., 2009; Minow, et al., 2010]. Rapid charging events are less 

common than normal charging, and appear to be correlated to eclipse exit conditions with low 

plasma densities (less than 3 × 1010 m-3) [Craven, et al., 2009]. 

Finally, a class of rapid charging events (Figure 7.6-1c) occur when fully shunted solar arrays are 

unshunted in full sunlight [Minow, et al., 2010]. Sunlight unshunt rapid charging events are 

transient events reaching the maximum potential within one FPMU sample period 

(7.8 milliseconds (msec)) followed by a rapid decrease in potential on times scales of 20 to 

150 msec. Sunlight unshunt rapid charging events were first observed on GMT 2010/155 and 

over the period GMT 2010/205-212 during a set of 36 experiments in which all 8 ISS solar 

arrays were fully shunted for about 3 minutes following eclipse exit. Then each array wing was 

unshunted at 1-second intervals resulting in a set of eight charging peaks (Figure 7.6-2). Two 

additional events were observed on GMT 2013/130 when array power manipulation activities 

associated with the ammonia pump repair required shunting the 2B array and unshunting in 

sunlight. The largest recorded ISS negative charging events fall in this category. Maximum 

potentials for 288 of the 289 sunlight unshunt rapid charging event charging peaks observed to 

date are more negative than -45V, 265 events are more negative than -60V, and 16 events are 

more negative than -90V. Two charging events on GMT 2010/209 reached -95V and are the 

largest negative charging events observed to date on the ISS. Sunlight unshunt rapid charging 

events have been observed in all cases where FPMU data is available following unshunt of a 

solar array in sunlight. 
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Figure 7.6-2. Detail of Sunlight Unshunt Rapid Charging Event

The time scale for the sunlight unshunt rapid charging event from Figure 7.6-1 is expanded to 

better show the rapid rise time and decay of each of the eight events. Rise time from background 

to maximum potential is 7.8 msec and the charging peaks decay within ~100 msec. The array 

responsible for each charging peak is indicated and the highest negative charging observed to date 

on the ISS are the events from the 3B and 4B arrays. 

Figure 7.6-3 provides a summary of the ISS eclipse exit charging levels and examples of the 

most extreme negative and positive charging events observed to date. PCUs were off for all of 

the events in the summary so it provides examples of the range of charging that can be observed 

when PCUs are not used to control the vehicle potential. The figure and analysis of the data used 

to generate it highlights three important findings. First, ISS charging is variable with 

approximately 95 percent of the observed charging events remaining within 0 to -45V. Second, 

FPMU data provide a record of a number of ISS charging events more negative than -45V

contradicting the ISS-NCR-232G that states “FPMU measurements since 2007 have indicated no 

ISS charging in excess of -45V.” Third, positive potentials are not due solely to PCU operations 

so discontinuing use of the PCUs will not eliminate exposure to positive potentials.

1A 1B 2A 2B 3A 3B 4A 4B
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Figure 7.6-3. ISS FPMU Charging Event Summary 

(Top Panel) Colored symbols indicate the maximum potential in individual charging events as a 

function of time. (Bottom Panel) The F107 index is a measure of the solar 10.7 cm radio flux (in 

solar flux units) showing the phase in solar cycle.

The majority of the data points in Figure 7.6-3 are from a Boeing study [Boeing, 2013] of the 

maximum negative ISS frame potential due to solar array charging following eclipse exit. The 

data cover 2,164 orbits during the period starting 2007/188 through 2013/105. The ISS potential 

measurements at the FPMU location are adjusted for the v × B • L potential difference between 

the measurement location and the ISS centerline (black symbols), starboard Truss tip (red 

symbols), and port truss tip (blue symbols). 

The Boeing data were checked against a MSFC study of rapid charging events during the period 

from 2007/027 through 2009/037 [Wright, et al., 2009] to determine if all rapid charging events 

more negative than 45V in the MSFC study are in the Boeing data set. Seven events were 

identified that exceed -45V (green symbol) that are missing from the Boeing study. These 

values are from the original location of the FPMU on the starboard Truss and have not been 

adjusted by the v × B • L potential to the locations used in the Boeing study. However, they can 

be directly compared because all seven events occur near the geographic equator where v × B • L

effects along the Truss are small. A total of 2,171 eclipse exit charging events are available 

including the 2164 from the Boeing 2013 study [Boeing, 2013] and the seven additional events 

from Wright, et al. 2009. Table 7.6-1 provides a summary of the eclipse exit charging events 

from these studies providing the number of events more negative than -45V and the number of 

events more positive than 0V. 
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Figure 7.6-3 also includes all 287 of the sunlight unshunt rapid charging events (yellow symbols) 

from the 2010 solar array charging experiments and the two on GMT 2013/130 during the 

ammonia pump repair activities. Charging events more negative than -45V are summarized in 

Table 7.6-1. Two important points regarding sunlight unshunt rapid charging events are worth 

emphasizing: (1) the FDIR process currently used to protect the crew in case of a PCU failure 

during an EVA automatically shunts all eight solar arrays if one of the PCUs is not operational, 

and (2) the ISS can operate on batteries for only a limited amount of time. Ground control will 

have to unshunt a subset of the arrays to restart the solar array electrical current collection 

sometime after the FDIR has been activated. Present flight rules provide no guidelines on when 

to unshunt the arrays, so there is a risk the operation could be implemented in sunlight, and 

expose the EVA crew to the large sunlight unshunt rapid charging events. Developing a new 

flight rule to require the array unshunts to be implemented during night or discontinuing use of 

the FDIR will eliminate this risk.

Finally, transient positive charging events were also observed on the ISS with maximum 

potentials often reaching some 10s of volts. Figure 7.6-4 shows examples of three positive 

charging events from GMT 2010/208. The largest event exceeded 0V for over 200 msec,

reaching a maximum potential of approximately +55V. This is the largest positive charging 

event that has been identified in the FPMU data to date. Additional examples of four transient 

positive charging events can be seen in Figure 7.6-1c. Maximum potentials from 21 positive 

charging events (orange symbols) are included in Figure 7.6-3 and Table 7.6-1, including the 

record event from GMT 2010/208. No attempt was made to identify all positive charging events 

in the FPMU data records. The values shown in Figure 7.6-3 only provide examples for the 

range of positive potentials that have been seen on a few dates. The ISS environments 

community currently has no explanation for origin of these events. It is worth noting, however, 

that discontinuing use of the PCUs will not protect an EVA crew from the transient positive 

charging events since the examples included in Figures 7.6-3 and 7.6-4 were all observed when 

the PCUs were not operating. 
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Figure 7.6-4. Positive Charging Events

Example of the positive charging events including the largest observed to date reaching 

approximately +55V. 

Table 7.6-1. Charging Events  0 and  ‒45V

Study, location All 

Events

Events < -45V Events > 0V

Boeing, starboard 2164 8 50

Boeing, port 2164 27 77

Boeing, center 2164 1 0

Wright et al., FPMU starboard 7 7 0

Sunlight unshunt, FPMU port 289 288 0

Positive transients 21 --- 21

The NESC team emphasizes that Figure 7.6-3 does not represent a complete record of all 

charging on the ISS or even a carefully designed statistical study of ISS charging using a subset 

of eclipse exit charging data. The ISS potentials are available only when FPMU is operating and 

data are available through live telemetry downlink. The period starting 2007/188 through the 

end of 2013/105 represents approximately 33,117 ISS orbits (based on orbit numbers between 
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the first two-line element set on 2007/188 and the first two-line element set on 2013/106). Only 

2,171 eclipse exit charging events are included in the study from this period, which is 

approximately 6.6 percent of the orbits during the study period. 

FPMU operation periods are selected for the engineering purpose they support including PCU 

operation verification, PIM3.0 charging model studies, plasma hazard analysis for EVA, 

verifying charging contributions due to visiting vehicles, payload science support, international 

ionosphere World Day periods, and space weather charging studies. No attempt was made to 

optimally distribute the FPMU operations to best sample the widest range of eclipse exit 

conditions in order to obtain a statistically unbiased set of charging data. 

Figure 7.6-5 is the first example of auroral charging observed on ISS. The event was captured 

while the FPMU was running in support of Space Transportation System-123 mission activities 

at the ISS and automated transfer vehicle docking operations. This charging event cannot be due 

to solar array electrical current collection because the arrays are not biased at night. Night 

charging events on the ISS typically are observed at high latitudes during geomagnetic storms 

consistent with an auroral electron source for the charging currents.

Figure 7.6-5. ISS Auroral Charging

Two ISS orbits showing short periods of solar array charging at eclipse exit and entry 

superimposed on the v x B • L potential oscillation due to the motion of ISS across the Earth’s 

magnetic field. The -37V charging peak just before 08:00 UT is auroral charging at high northern 

latitudes in the middle of the night. This was the first and to date the largest auroral charging event 

observed on ISS. 
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Auroral charging of the ISS vehicle chassis is well-controlled by the PCUs since they are capable 

of discharging currents up to 10 amps while extreme auroral electrical current densities are 

typically on the order of 10-5ampere per meter-squared (A/m)

2

to 10-4 A/m2[Cho, et al., 2012]. 

Electrical current collection from the most extreme auroral conditions should not exceed the 

capability of the PCUs to discharge the charging current because most of ISS is covered by 

insulating materials with a relative small area of conductor exposed to the space environment. 

For this reason, auroral contributions to the ISS frame charging have never been considered a 

risk for EVA as long as the PCUs are operating. 

However, the situation is quite different if the plasma hazard forecast process is used instead of 

the PCUs to protect the crew from arcing hazards during EVA because auroral charging cannot 

be predicted using the IRI model and PIM3.0 analysis. IRI is a climatology model which only 

treats the low energy (~0.1 eV) charged particles that comprise the bulk of the ionosphere 

plasma, but does not provide information on the currents of high energy (~1000s to 10,000s eV) 

electrons responsible for auroral charging. Even if the auroral particle flux information was 

available, the PIM3.0 does not include a module for incorporating the contributions of auroral 

currents to ISS charging. A decision to discontinue PCU use for protecting EVA astronauts from 

arcing hazards in favor of the plasma hazard forecast process will leave the crew exposed to 

negative charging hazards due to the ISS frame charging by the aurora. 

Figure 7.6-5 is not only the first auroral charging event observed on the ISS, but is also the 

largest. The ISS potential increase due to the auroral electron current is more negative by about 

17V than the background -20V due to v × B • L. In this case, the net charge at the location of 

the FPMU where the charging was measured is -37V, which does not exceed the -45.5V limit. 

However, a similar -17V charging event would result in violations of the -45.5V safety limit for 

any part of the ISS structure with a potential more negative than -28V. Such a violation might 

occur, for example, when the ISS exits eclipse at high latitudes where aurora is present such that 

auroral charging is coincident with the eclipse exit charging. It can be estimated what kind of 

conditions might lead to these safety violations by consulting the data set used to generate Figure 

7.6-3 to see how often charging events with potentials of -28V or more have been observed. 

There are 55 eclipse exit charging events with negative potentials more negative than -28V at 

the ISS centerline; 444 more negative than -28V at the ISS port Truss tip and 760 more negative 

than -28V at the ISS starboard Truss tip. Each of these events would result in potentials on the 

ISS reaching or exceeding the -45V safety limit. No analysis has been presented by the ISSP to 

evaluate the risk trade involved in discontinuing the use of PCUs, which currently control this 

risk and using the plasma hazard forecast process that is incapable of predicting auroral 

charging threats.

While -17V was used in the preceding discussion, there is no reason at this time to believe that 

auroral charging could not result in higher potentials. Sampling of auroral charging by FPMU

has not been extensive due a number of factors. First, FPMU is operating only in campaign 

mode for limited amounts of time so auroral events may be missed. There has been some effort 

in recent years to target FPMU operations to capture auroral charging data [Minow, et al., 2010, 
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2012; Minow and Parker, 2013], but there are still geomagnetic storm periods that are missed 

due to constraints on operation of the instrument. Second, there is a sampling bias due to the 

position of an ISS orbit relative to the location of the aurora. Even during geomagnetic storm 

periods when aurora moves closer to the equator, the ISS may not encounter the aurora because 

there is a local time dependence on the maximum magnetic latitude along the orbit where aurora 

is more likely to be encountered. If the highest magnetic latitudes along the orbit are not at the 

right longitude, then the ISS is unlikely to encounter strong auroral electron particle flux 

regardless of the strength of the aurora. Finally, auroral activity sampled since FPMU started 

operations on the ISS has not been that strong because FPMU started operations in late 2006 as 

the last solar cycle was ending, through the geomagnetic quiet period between the previous and 

current solar cycle, and through the current relatively low activity solar cycle. The result is that 

only nine periods during geomagnetic storms with auroral charging have been observed through 

April 2013 [Minow and Parker, 2013] with one or two additional periods observed later in 2013. 

References:

1. Wright, Jr., K.H.; Swenson, C.; D. Thompson, D.; A. Barjatya, A.; S. L. Koontz, S.L.; T. 

Schneider, T.; Vaughn, J.; Minow, J.; Craven, P.; Coffey, V.; Parker, L.; and Bui, T. (2007): 

“Initial results from the Floating Potential Measurement Unit aboard the International Space 

Station,” 10th Spacecraft Charging and Technology Conference, Biarritz, France, June 18-21, 

2007.

2. Craven, P.D; Wright, Jr., K.H.; Minow, J.I.; Coffey, V.N.; Schneider, T.A.; J.A. Vaughn, 

Ferguson, D.C.; and Parker, L.N. (2009): Survey of International Space Station Charging 

Events, AIAA-2009-0119, 47th AIAA Aerospace Sciences Meeting and Exhibit, Orlando, 

Florida, January 5-8, 2009. 

3. Minow, J.I.; Wright, Jr., J.H.; Chandler, M.O.; Coffey, V.N.; Craven, P.D.; Schneider, T.A.; 

Parker, L.N.; Ferguson, D.C.; Koontz, S.L.; and Alred, J.W. (2010): Summary of 2006 to 

2010 FPMU Measurements of International Space Station Frame Potential Variations, 11th

Spacecraft Charging Technology Conference, Albuquerque, New Mexico, 20 – 24 

September 2010.

4. Boeing (2013): PIMVar1_results.xls file, 2013

5. Wright, et al. (2009): Wright, K.H., FPMURapidChargingEvents_KHW_2009Mar02.xls file, 

2009.

6. Cho, M., K. Saito, and T. Hamanaga (2012): “Data Analysis of the Polar Plasma 

Environment for Spacecraft Charging Analysis,” Acta Astronautica, 81, 160-173, 2012.

7. Minow, J.I.; Pettit, D.R.; and Hartman, W.A. (2012): “Space Weather Monitoring for ISS 

Space Environments Engineering and Crew Auroral Observations,” Abstract IN31D-05 

presented at 2012 Fall Meeting, AGU, San Francisco, California, December 3-7, 2012.

8. Minow, J. I., and L.N Parker (2013): “Space Weather Monitoring for ISS Geomagnetic 

Storm Studies,” Space Weather Workshop (invited), Boulder, Colorado, 16-19 April, 2013.
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7.7 Estimate of Likelihood of Auroral Charging for ISS

As discussed in previous inputs to the ISS EVA Charging Study, the aurora have been observed 

to cause charging of bodies in low altitude orbit from -100 to -2,000V. Such charging events 

are relatively infrequent, as discussed below. However, the events are typically of short duration 

(e.g., ~10 seconds to 1 minute typically) and up to 3 minutes on one occasion (Minow, private 

communication). Solar lighting, seasonal variations in the ionospheric density, geomagnetic 

activity, and plasma wake shadowing are known to contribute to the event likelihood. For 

EVAs, the main requirements are the presence of the ISS in the auroral zone, the encounter with 

an auroral arc, and the shadowing (i.e., from sunlight and the ionospheric plasma) of the 

astronaut. One method to estimate the probability of the astronaut experiencing an auroral 

charging event is a Monte Carlo simulation taking into account these variable conditions. 

However, for the purposes of this assessment, such a detailed analysis is not appropriate. Rather, 

a first order estimate on the upper limit of the hazard was derived.

Upper Bound on Auroral Charging Hazard

As shown in Figure 7.7-1 [Evans, 2012], the auroral zone forms a roughly ellipsoidal pattern 

around the Earth’s magnetic poles oriented in local time. The maximum probability of 

encountering an auroral arc (assumed here to occur in the form of roughly longitudinal arcs 

~60 km in latitudinal thickness) is 0.01 (for a 1-degree × 8-arc minute bin in Figure 7.7-1) near 

65 degrees geomagnetic latitude and 21 hours local time. The equatorward extension of the 

auroral zone is ~60 degree-geomagnetic. This corresponds to geographic latitude of ~49 degrees

as the Earth’s magnetic field is inclined ~11 degrees to the geographic pole. Thus, the ISS needs 

to be both poleward of ~49 degree-geographic latitude and approximately in the longitude 

sectors near ~70 degrees W (North Pole) and ~110 degrees E (South Pole). This “auroral 

charging” region, in geomagnetic coordinates, is marked by the red ellipse in Figure 7.7-1. 

Figure 7.7-2 [Anderson, 2005] shows the locations of observed Defense Meteorological Satellite 

Program (DMSP) auroral charging events superimposed on the ISS orbit (Note: the DMSP data 

are skewed because of various data collection and temporal selection issues) in geographic 

coordinates—the blue rectangles mark the approximate regions of ISS charging. A simple 

estimate of the ISS orbit indicates that it has a probability of ~0.16 to 0.18 of being poleward of 

49 degrees for a given orbit. A similar analysis gives a fractional probability of ~0.13 for the ISS 

to be within the longitude range of the auroral zone. Since the two events are independent of 

each other, the probability of being in the auroral charging region is given by the product of their 

respective probabilities. That is, the ISS will likely “on the average” encounter the lower edge of 

the auroral charging zone with a probability, PE, of (0.13 × 0.16 =) ~0.02 during an orbit. 

Further, Evans estimates that at least one 10-second duration active auroral arc will be 

encountered with a probability of 0.1 for a single orbit crossing the auroral zone [Evans, 2012]. 

Since the ISS skirts the equatorward edge of the auroral zone rather than passing through it, it is 

assumed that a more conservative estimate of the probability, PAC, of encountering a 10-second 

duration arc would be between 0.0001 to 0.001 for a single ISS passage through the auroral zone 

based on Figure 7.7-1.
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Figure 7.7-1. Absolute probability of encountering a large energy flux event/aurora as a function of 

corrected geomagnetic latitude and local time for a satellite. Latitude scale is from 45 to 90 degrees

magnetic [Evans, 2012]. The red ellipse marks the approximate region where one would expect to 

see ISS charging.

Figure 7.7-2. The ISS orbit track over 24 hours and the locations (red) of DMSP charging events of 

less than -100V [Anderson, 2005]. The rectangles mark the approximate regions where one would 

expect to see ISS charging.

If an EVA were ~6 hours, then that would be ~4 orbits. Assuming that the ISS will encounter 

the auroral zone twice (North or South) during a single orbit, the total probability, PT, for 4 orbits 

would be given by PT = PE*(1-(1-PAC)

2*4) ~0.02*2*4*PAC for a probability of PT ~ 1.6 × 10-5

to 
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1.6 × 10-4for an arbitrary 6-hour EVA. Mitigating this concern, however, would be the 

previously mentioned issues of the astronaut being in both sunlight and plasma shadows. The 

solar wind conditions can be used to give up to a 40-minute warning of pending auroral activity 

or the use of Kp (or even “looking out the window” to see if aurora are in progress) to either 

terminate or abort an EVA further limiting the threat of auroral charging.

Conclusion

While it is strongly encouraged to carry out a much more thorough “Monte Carlo” analysis of the 

likelihood of encountering a 10-second duration auroral arc, the preceding estimates put an upper 

bound on the probability of 1.6 × 10-5to 1.6 × 10-4for a 6-hour EVA. Seasonal, solar cycle, and 

“shadowing” issues will further significantly change the estimate—the latter requirement for 

“shadowing” will greatly reduce the number, but currently there is no way to estimate that factor 

as it is “mission-dependent.” Finally, terminating or avoiding EVAs based on forecasting or 

monitoring of auroral conditions could be used to further limit the concern of auroral charging.

References:

1. Evans, D. S. (2012): “A Study of Intense Auroral Electron Precipitation Events,” Space 

Environments Laboratory, NOAA, Boulder, Colorado, personal communication, 2012.

2. Anderson, P. (2005): “Spacecraft Charging Hazards In Low-Earth Orbit,” 9

th Spacecraft 

Charging Technology Conference, 2005.

7.8 PCU Capability to Maintain the ISS Near to Space Plasma Potential 

7.8.1 PCU IV Characteristic versus FP Mitigation

The PCUs make EVA safer for the astronaut under negative conditions including rapid charging 

events and frame charging due to aurora. The discussion below shows that the PCU has the 

ability to maintain the ISS chassis potential within 15V of the local space plasma for all 

conceivable conditions because the plasma contactor can emit electron currents two orders of 

magnitude greater than the largest emission currents observed to date on the ISS. The PCU is 

capable of emitting currents greater than the sum of all possible plasma currents to the station, an 

extreme “worst-on-worst” upper bound. The PCU is capable of controlling the ISS potential for 

all planned future ISS configurations.
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The maximum electron current that the ISS could possibly collect is when all the array surfaces 

were facing the ram in the highest density ionosphere and the total array solar cell area were 

collecting as if it were entirely exposed conductors. This is an extreme upper bound because the 

solar cells top surfaces are insulating cover glass. The solar array has eight wings, with each 

wing having two flexible blankets with solar cells. The blankets consist of 82 live panels with 

200 8-cm × 8-cm cells. Thus, the mathematical upper bound on the electron collecting area is 

less than 1700 m2.

The electron thermal current, jth, is a function of the plasma electron temperature, T, and 

density, n:

where me is the electron mass and echarge is the charge on an electron. The electron current 

collected in worst-case ionosphere environment is around 20 ampere (A).

This is an unrealistically high worst (maximum collecting area) on worst (maximum electron 

current density) upper bound on the electron current. This upper bound is almost 40 times larger 

than the largest PCU currents observed to date, 0.575 A PCU 1 + PCU 2, measured on orbit

[Koontz, 2013 private communication].

Prior to flight, the plasma contactor hollow cathode was subject to a 28,000-hour life test in a 

vacuum chamber [Sarver-Verhey, 1997]. The test was conducted at 12A emission current. As 

shown in Figure 7.8-1, the PCU I-V trace is essentially vertical at 10A. Hollow cathodes of 

essentially the same design are qualified for and routinely run in electric propulsion thrusters at 

more than 13A continuous emission current. For brief periods, several minutes at a time, the 

PCU hollow cathode is able to emit more than 20A without damage (Goebel2).

 

2 Goebel, Dr. Dan, JPL Hollow Cathode Expert.
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Figure 7.8-1. Plasma Contactor Emission Current Measured in a Ground Test Chamber as a 

Function of Voltage

The NESC team’s conclusion is that PCU is capable of emitting orders of magnitude greater 

electron currents than has been needed to date on the ISS and that the device is capable of 

handling the even worst-case upper bound electron currents from worst-case environments.

References:

1. Koontz, Steve (2013): “PCU_emission_currents_2010_through_2013.xlsx,” private 

communication.

2. Sarver-Verhey, T.R. (1997): “28,000 Hour Xenon Hollow Cathode Life Test Results,” IEPC97-168, 25th International Electric Propulsion Conference, Cleveland, Ohio, August 24-28, 

1997.

7.8.2 PCU Operational Life

The on-orbit PCUs both satisfy the two necessary conditions for long life. First, both PCUs have 

enough xenon to last well past 2028. The chart in Figure 7.8-2 from “Plasma Contactor Unit 

(PCU) – Status,” [Kaminski and Scudder, 2013] was used to estimate that each EVA uses about 

65 gram (gm) of xenon.
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Figure 7.8-2. Xenon Usage Projections

Kaminski and Scudder’s lowest estimate of remaining xenon is that PCU2 may have 37 pounds 

remaining in the tank, about a pound less than shown in Figure 7.8-2. Based on the charts in that 

presentation and the worst-case assumption of 14 EVAs per year, the remaining xenon will last 

an additional 18 years, or through 2031. In the calculation below, the xenon mass used for an 

EVA, MEVA, is estimated from the slope in Figure 7.8-2. MPCU2 is the estimated mass of xenon 

remaining in the PCU2 tank.

The second requirement is that the PCU hardware, in particular the hollow cathode assembly, has 

sufficient life to process the xenon remaining in the tank. The low side of the nominal xenon

flow rate is 6 standard cubic centimeters per minute. The lowest rate can be chosen because it 

requires the longest hollow cathode life.
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From this, calculate the total time per EVA that the plasma contactor is operated.

From Figure 7.8-2, it is estimated the PCU1 was loaded with 48 kilograms of xenon. The PCU 

would have to operate for 22,000 hours to process that much xenon.

The plasma contactor hollow cathode assembly was qualified prior to flight with a 28,000-hour 

life test in a vacuum chamber [Sarver-Verhey, 1997]. Since then, similar hollow cathode 

assemblies have operated for long periods without any difficulty. The NASA Solar Technology 

Application Readiness (NSTAR) ion thruster has two similar cathodes. The NSTAR Extended 

Life Test was run for 30,000 hours before ending due to programmatic constraints [Sengupta, 

et al., 2004]. During the Deep Space 1 flight mission, the NSTAR thruster accumulated 16,265 

hours before the mission ended [Rayman, 2003]. The neutralizer hollow cathode used on the 

NASA Evolutionary Xenon Thruster (NEXT) ion thruster is the same design as the ISS PCU. 

The NEXT thruster recently successfully completed a 48,000-hour life test [NASA, 2013b]. 

This is more than twice the worst-case required hollow cathode life.

References:

1. Kaminski, R.; and Scudder, M. (2013): Boeing ISS EPS, September 2013.

2. Sarver-Verhey, T.R. (1997): “28,000 Hour Xenon Hollow Cathode Life Test Results,” IEPC97-168, 25th International Electric Propulsion Conference, Cleveland, Ohio, August 24-28, 

1997.

3. Sengupta A.; Brophy J.R.; Anderson J.R.; and Garner C. (2004): “An Overview of the 

Results from the 30,000 Hour Life Test of the Deep Space 1 Flight Spare Ion Engine,” AIAA 

Paper 2004-3608, 40th Joint Propulsion Conference, Ft. Lauderdale, Florida, July 11-14, 

2004.

4. Rayman, M.D. (2003): “The Successful Conclusion Of The Deep Space 1 Mission: 

Important Results Without a Flashy Title,” Space Technology 23, Nos. 2-3, p. 185, 2003.

5. NASA (2013b): Press Release 13-193 “NASA Thruster Achieves World-Record 5+ Years of 

Operation.” June 24, 2013.

7.9 EMU Exterior Metal Parts

This section summarizes the EMU (U.S. suit) exterior metal parts that may pose an entry point 

into the astronaut’s body either by direct contact with a charged metal surface (of the ISS) or a 

plasma contact so that two of them can cause or permit electrical current flow in the astronaut’s 

body.
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Table 7.9-1 lists the name, material, coating/covering, isolation ohms, probability of failure and 

comments for every entry point. The table title permits listing a material (i.e., stainless steel or 

Al) and a coating (e.g., anodize or paint or uncoated stainless steel), but that information is 

difficult to find and is not listed here.

It can be seen in Section 7.10 that the specific materials are not used in the calculations and they 

are not listed in Table 7.9-1.

Table 7.9-1 contains word descriptions of the relative importance of the various items 

listed. Refer to Figures 6.3-9 and 6.3-10 to illustrate the listed entry points. “Covering flap” 

means that the suit material covers the named metal, and there will be little or no plasma 

contact. Superscripts refer to the references presented after the table.

The NESC team decided to use the neck ring as the plasma contact entry and the waist ring as 

the ISS conductive entry to provide a current path through the thorax of the astronaut for worstcase calculations. The NESC team considered that the International Space Station (ISS) 

Probabilistic Risk Assessment (PRA) EVA Shock Update and Summary has assumed contact with 

all possible external metal contact points [Duncan, 2013]. These have all been covered with 

flaps of material as can be seen in the various photos of the EMU. The contact point material 

(anodize or paint or stainless steel) does not matter.

 Isolation of MMWS Components:

o Implementation to reduce electrical current paths 

 Isolation of interface receptacles using non-conductive materials: 

o Kapton® film acts as a dielectric membrane between Al baseplate and stainless 

steel receptacles 

o Hard anodized washers are used to isolate conductive paths through fasteners 

 Testing and validation: 

o Isolation checks 

o Mechanical stress test 
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Table 7.9-1. EMU Metal Entry Points Summary

# Name Material

(Coating & 

Covering)

ISS Contact?

Per NESC team 

usage

Pfail2

(Ranking

for ISS 

contact)

Plasma 

Contact?

Comments,

References

and NESC 

team action

1 Scye Bearing1 SB2 covering flap Less likely than 

waist bearing

0.00000

(8, 11)

2 Arm Bearing1 AB2 covering flap Less likely than 

waist bearing

0.00025

(5)

3 Wrist Bearing1

(or Wrist Ring)

covering flap Less likely than 

waist bearing

0.006

(1)

4 Waist Bearing WB2/

D-Rings1

covering flap 0.003

(4)

Considered 

most likely as 

plasma 

contact 

(is sometimes 

“waist ring”, 

or WR)

5 Thigh 

Disconnect1TD2

covering flap Less likely than 

waist bearing

0.005

(2 & 3)

6 Ankle 

Disconnect1AD2

covering flap Less likely than 

waist bearing

0.001

(6 & 7)

7 Body Seal Closure-

(BSC)/MMWS 

Connection1

covering flap BSC equally likely 

to waist bearing

0.001

(6 & 7)

Considered 

most likely as 

plasma 

contact

MMWS 

isolated1

8 Neck Ring1NR2 Less likely than 

waist bearing

0.00000

(8, 11)

Reference 2 

states 

probability of 

ISS contact is 

0.00000; and 

plasma 

contact to NR 

lower than 

BSC or WR.

9 Helmet Purge 

Valve1HPV2

covered with 

white

0.00000

(8, 11)

No outside

exposure

10 CCA3 no outside 

exposure

N/A 0.00000

(8, 11)

No outside 

exposure

11 *OBS/DCM2insulated and 

electrically 

isolated

N/A 0.005

(2 & 3)

>50 

megohms per 

Ref 3

12 (not EMU)

Any ISS damaged 

anodize2

Most of the 

ISS exterior 

metal has been 

anodized

Consider only 

direct contact for 

EMC/ astronaut 

hazard 

assessment

0.01

(rank is high)

N/A One part of 

EMU must 

touch this for 

Damage 

probability
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# Name Material

(Coating & 

Covering)

ISS Contact?

Per NESC team 

usage

Pfail2

(Ranking

for ISS 

contact)

Plasma 

Contact?

Comments,

References

and NESC 

team action

13 (not EMU)

Any ISS exposed 

stainless steel

Lots of bits and 

pieces (nuts & 

bolts; solar 

array 

tensioners and 

other unlisted 

items.

Consider only 

direct contact for 

EMC/ astronaut 

hazard 

assessment

0.01

(rank is high)

N/A One part of 

EMU must 

touch this for 

damage 

probability

1. ISS-NCR-232F (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth Orbit Plasma 

Environment, 1/31/2012, Boeing ISS System Safety, Joseph E. Thomas, originator. Pages have Tracking 

Number: (blank), “International Space Station Safety Noncompliance Report (NCR),” Date: 1/26/12.

2. Duncan, G. (2013): Document DRD-MAPI-SA-06-ISSPRA-12-56, EVA Shock Update and Summary, 

International Space Station (ISS) Probabilistic Risk Assessment (PRA) Trade Study – Long Form, ISS-PRA12-56 (Probability Risk Assessment Doc), Prepared by Gary Duncan, dated May 17, 2013.

3. Castillo, M.; PPT “Modular Baseplate Assembly/Body Restraint Tether/Handrail Electrical Continuity Test”, 

ONE EVA, 05/04/10.

The Material column contents are only outlined as rough descriptions of the coating and covering. The probability 

of failure is shown in the Pfail column as contained in ISS-PRA-12-56.

*OBS/DCM (Display and Control Module) is located above the BSC with the MMWS. 

Figure 7.9-1 shows the communications carrier assembly (CCA) [Duncan, page 7, Figure 2, 

2013]. The CCA is a fabric cap worn by the astronauts with microphones and speakers for use 

with the radio. It allows hands-free radio communications within the suit. It seems to have no 

external connections, but the ISS-PRA-12-56 implies that there is a connector that is exposed on 

the outside of the EMU. The CCA probability of contact with the ISS chassis is rated in the ISSPRA-12-56 as 0.00000. Figure 7.9-1 also shows visible wrist rings before attaching gloves.
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Photo Courtesy NASA

Figure 7.9-1. EMU Photo. Note CCA –a cap with microphone and speakers. Note visible wrist 

rings before attaching gloves.

Figure 7.9-2 is another EMU photo that also shows the exposed wrist ring before the covering 

flap is positioned over the ring. Note on the right that the covering flap leaves no exposed wrist 

ring for contact with space plasma; other details can also be seen.

Figure 7.9-3 is a photo of a suited astronaut. Additional EMU pictures to support this section are 

included in Appendix E.
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Photo Courtesy NASA

Figure 7.9-2. EMU Photo
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Figure 7.9-3. Suited astronaut: EMU upper part (picture source unknown). Some details of wrist, 

EMU tether lower right and equipment/tool tether (right) shown.

Reference:

1. Duncan, G. (2013): Document DRD-MAPI-SA-06-ISSPRA-12-56, EVA Shock Update and 

Summary, International Space Station (ISS) Probabilistic Risk Assessment (PRA) Trade 

Study – Long Form, ISS-PRA-12-56 (Probability Risk Assessment Doc), Prepared by Gary 

Duncan, dated May 17, 2013.

7.10 Reassessment of the Positive Voltage EVA Hazard

The upper bound electron collection currents that could flow through an astronaut as a result of 

low positive potentials are less than 1 mA. This is an order of magnitude lower than the lowest 

currents in ISS-NCR-232F [Kramer, et al., 2010], and may not be hazardous. 

The changes to the EMU outlined in the “NESC_ISS_Shock_EVA_Actions.pptx” (provided in 

Appendix D) have eliminated almost all electrical current paths for electrons collected from the 

ionosphere to flow through the astronaut’s torso to the ISS structure ground [Roeschel, 2013]. A 

single, highly improbable electrical current path has been identified. Maximum currents through 

this path for both solar max and solar min are shown to be less than 1 mA. Based on these 

calculations, it is suggested revisiting the question of whether plasma currents from low positive 

voltages are an EVA hazard. The analysis does not consider whether a hazard exists when there 

are large negative potentials on the ISS and the plasma contactor is not operating and merely 

assumes these conditions for conservatism.
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Analysis

Under normal conditions, since the astronaut’s tether has an insulating segment, there is no 

electrical contact between the astronaut and potentials on the ISS [Roeschel, 2013]. However, in 

the unlikely case that the tether is in electrical contact with a ring on the EMU, then electrical 

current could flow through the astronaut to another anodized ring exposed to the plasma and 

back through the plasma, as shown in Figure 7.10-1. 

Figure 7.10-1. Circuit where the Astronaut is ~15V Positive with Respect to the Surrounding 

Plasma

This requires physical contact of the bare metal tether with a suit ring whose fabric cover has 

been inadvertently displaced (see Figure 7.10-2) on the stainless steel bearing ring, not an 

anodized ring. Electron current can be collected when the potentials on the EMU are positive 

with respect to the ionosphere plasma, such as those possible due to the station’s motion across 

the Earth’s magnetic field when the plasma contactor is operational. Following Kramer, et al., 

the extreme worst-case positive potentials possible in this scenario are the order of 

approximately +15V, and such potentials can only occur outboard of the SARJ [Kramer, et al., 

2010].
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Figure 7.10-2. Example of an exposed section of the stainless steel wrist bearing ring directly above 

the blue anodized ring [Roeschel, 2013]. Normally both rings are covered by suit fabric.

Electrically, the circuit is represented schematically in Figure 7.10-3. Positive potential 

generated at the end of the truss by the ISS’s orbital motion is carried to the suit by the tether 

contacting the waist ring. This positive potential goes through the astronaut’s torso and appears 

on the exposed, anodized neck ring. This scenario also assumes there are flaws in the 

anodization on the interior of the suit, and electrical current flows through sweat-soaked 

garments through the torso, not around it. Calculations below exclude electrical current 

collection by modular base plate because the data presented by Castillo (provided in 

Appendix D) showed it was electrically isolated from the rest of the EMU.
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Figure 7.10-3. Electrical equivalent circuit. The only metal surface always exposed to the 

ionosphere is the neck ring, which has an insulating coating. (EMU figure from ISS-NCR-232F).

The electron current that flows is limited by the electron current collected from the ionosphere 

by the exposed section of the EMU neck ring. To estimate the collection, approximate the 

exposed area of the ring as a strip 1-inch wide by 1-foot long. 

The simplest estimate of the current an object can collect from a plasma is to assume that every 

electron that conservation of angular momentum would not prevent from being collected is 

collected. This an upper bound, called “Orbit Limited Collection” to the actual currents 

collected by complex objects in a dense plasma where potentials on nearby dielectric materials, 

shadowing by other objects, and space charge effects can dramatically limit the current. For 

symmetrical conductors floating in space, three different expressions, shown in Figure 7.10-4, 

can be used to estimate the orbit limited upper bound current depending on the relative 

dimensions of the object. In the figure, the abscissa is the potential on the object divided by the 

electron temperature and ordinate in the plot labeled “Current” is the current density to the object 

divided by the electron thermal current density. For a long, thin object (e.g., the neck ring), the 

cylindrical probe approximation is appropriate.
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Figure 7.10-4. Plasma Current Collection for Spherical (3-DIM), Cylindrical, and Planar Probes

[Hamilton and Kramer, 2007]

Following J.E. Allen, “Probe Theory – The Orbit Motion Approach,” Physical Scripta. Vol. 45, 

497-503, 1992, the collected current in terms of the one-sided electron thermal current density, 

the area of the collecting surface, and the applied dimensionless potential can be written [Allen, 

1992]. 

 

k T

eV

j A

k T

eV

m

k T I n r l e

p

th cylinder

p

cylinder p

  

















 

















 



 



2

1

1

2

1

2

2

2

1/ 2 1/ 2

where the one-sided electron thermal current is defined as

Fortunately, the ISS FPMU [Wright, et al., 2008] has a cylindrical probe, narrow‐sweep 

langmuir probe (NLP), with dimensions similar to that of an anodized ring. One way to test the 

cylindrical formula’s applicability is to compare the calculated current using it with the actual 

electron current measured by the NLP.

The NLP, shown in Figure 7.10-5, is one of the instruments on the ISS FPMU.
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Figure 7.10-5. Diagram of FPMU in its Deployed State with Indicated Dimensions 

[Wright, et al., 2008]

The NLP is a gold-plated cylinder with a radius of 1.43 cm and length of 5.08 cm. Its area is 

0.005 m2, a little over half the team’s estimate of the exposed anodized area of the neck ring. An 

electron current collection curve is shown in Figure 7.10-6.

Figure 7.10-6. An NLP Electron Current as a Function of Voltage 

[Wright, et al., 2008]
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Using the parameters in the figure and the cylindrical orbit limited collection formula the 

electron collection at the highest potential is 31 (microampere) μA about 50 percent higher than 

the 20 μA measured current shown in Figure 7.10-6.

As expected, the current to the probe, Iprobe, calculated using orbit limited theory is larger than 

the measured probe current because orbit limited theory, as discussed above is an upper bound. 

More accurate formulations that take into account the effect space charge (finite Debye length) 

would reduce the calculated current. 

Using worst-case solar max plasma environment and worst-case v × B • L potential from in ISSNCR-232F (Attachment 8), the upper bound, orbit-limited collection current is less than 1 mA.

The currents listed in ISS-NCR-232F/Attachment 8 for these conditions are as much as 50 times 

greater than this upper bound value (see Figure 7.10-7).

Figure 7.10-7. EMU Currents Post MMWS Modification [ISS-NCR-232F, Attachment 8, 2012]
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Discussion

The calculation above gives very much lower electrical current values than used in ISS-NCR232F for the particular case of the plasma contactor operating, the station experiencing a 

maximum v × B • L potential, and the astronaut at the extreme end of the truss. The calculated 

current, while significantly low, is probably an extreme overestimate because it is for a 

conducting cylinder sticking out in a plasma without any surrounding dielectrics to impede 

electron collection.

As is pointed out by Kramer, et al., 2008, because of surrounding dielectrics, the ISS solar array 

does not collect like the simple, orbit-limited theory. The ISS solar arrays collect much less 

electrical current than the model above would have predicted. For the solar array, the dielectrics 

reduced the electrical current by more than an order of magnitude. It can be expected dielectric 

on the suit as the same order of reduction in EMU currents compared with the upper bound 

calculated above.

Another issue is the duration of the current. The value above is for the peak electrical current to 

the neck ring surface. Since the outer surface of the neck ring is insulating, it acts as a capacitor 

(Figure 7.10-3). In the calculation below, it can be assumed the coating is thin anodization. This 

is a worst-case for the charging time because it was assumed that a very thin coating and 

anodization has a very high dielectric constant.

The collected current reduces the voltage that is seen by the plasma. For the values above the 

timescale for the current to flow is less than one millisecond. Over that timescale, the average 

current is about half the calculated peak current.

With respect to DC collection on the EMU, two factors combine that virtually eliminate any 

hazard from this type of collection. First, applying the appropriate plasma models (as discussed 

above) significantly reduces calculated current collection. Second, the electrical isolation of the 

MMWS (i.e., tool belt) radically reduces the area of exposed bare metal on the EMU. The 

reduction in collection areas is described in “NESC_ISS_Shock_EVA_Actions.pptx” (provided 

in Appendix D) [Roeschel, 2013]. Combining realistic current collection scenarios with a very 

small area of exposed bare metal on the EMU will result in an extremely small DC current 

collected by the EMU. 
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The accuracy of the analysis above includes many assumptions and approximations. It is beyond 

the scope of this task to perform a more accurate and detailed investigation. However, the above 

analysis shows that even a calculation that assumes that the neck ring collects like a cylinder 

floating in the ionosphere, rather than a sphere, marginalizes any astronaut hazard due to 

v × B • L-induced positive potentials when the PCU is operating. Accounting for the nearby 

dielectric suit surfaces and the actual EMU geometry will further reduce the currents collected. 

A more thorough investigation is warranted and will surely reduce the potential hazard from 

positive current collection.

References:

1. Kramer, L.; Hamilton, D.; Mikatarian R.; Thomas J.; and Koontz, S. (2010): “Positive 

Voltage Hazard to EMU Crewman from Currents through Plasma,” Proc. 4th IAASS 

Conference ‘Making Safety Matter, Huntsville, Alabama, USA, 19–21 May 2010 (ESA SP680, September 2010).

2. Roeschel, E. (2013): “NESC_ISS_Shock_EVA_Actions.pptx,” 6-25-13.

3. Hamilton, D. and Kramer, L. (2007): “Simple Transient Circuit Simulation of EMU 

Touching Solar Array (Bird on a Wire....NOT!!!!),” page 37, August 29, 2007.

4. Allen, J.E. (1992): “Probe Theory – The Orbit Motion Approach,” Physical Scripta. Vol. 45, 

497-503, 1992.

5. Wright, et al. (2008): “Charging of the ISS as Observed by the FPMU: Initial Results,” IEEE 

Transactions on Plasma Science, Vol. 36, No. 5, October 2008.

6. ISS-NCR-232F (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, 1/31/2012, Boeing ISS System Safety, Joseph E. Thomas, 

originator. Pages have Tracking Number: (blank), “International Space Station Safety 

Noncompliance Report (NCR).” Date: 1/26/12.

7.11 Features of the Current Path from the ISS-EMU-Plasma Circuit versus

the Shock Hazard

7.11.1 Electrical Current Path from the ISS through the Astronaut to the Plasma through

Multiple Layers of Insulation

The identified hazard is the possible flow of electrical current through an astronaut’s torso. The 

voltage that drives the electrical current is the difference between the ISS chassis at the location 

of the astronaut and the potential of the ambient ionosphere. If the PCU is not operating, this 

potential difference can be driven by a combination of the orbital motion of the station through 

the Earth’s magnetic field (v × B • L) and by the exposed electrical potentials on the ISS 160V

solar arrays. The NESC team found that there are several specific features of the EVA suit –

tether – tool system, each designed to interrupt the circuit. For electrical current to flow through 

an astronaut requires a simultaneous failure of several of these features. Below, the electrical 

circuit current path from the station is followed through the astronaut to the ambient ionosphere 

and identifies the four or five features in series that are designed to stop electrical current flow.
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The astronaut is attached to the ISS by an 85-foot safety tether. One end of this conducting 

tether is clipped to rings on the ISS. It is the NESC team’s understanding that the tether 

attachment point rings have an insulating anodized coating. This is the first break in the circuit.

The end of the tether attached to the astronaut has several inches of non-conducting fabric 

specifically designed to insulate the astronaut from the ISS potential as shown in Figure 7.11-1 

(all photos from Eduardo Roeschel, “NESC_ISS_Shock_EVA_Actions.pptx.”). See 

Appendix D.1 [Roeschel, 2013]. 

Figure 7.11-1. Safety Tether showing the Insulating Fabric Section at the End

The tether is attached to the waist ring on the EMU, as shown in Figure 7.11-2. Notice the fabric 

end of the tether connecting to the EMU waist ring.
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Figure 7.11-2. EMU showing the Fabric section of the Safety Tether and how all the Anodized 

Rings are Covered by the Suit Fabric

The EMU suit has waist, elbow, and wrist rings made of anodized Al and bearing rings of 

stainless steel. For the tether to transmit the station potential to the astronaut, the conducting 

tether would have to contact one of the stainless steel bearing rings where the covering fabric has 

been moved. The tether contacting the MMWS base plate is not a hazard, since the MMWS base 

plate is electrically isolated from the rest of the EMU.

An alternative path is for an EMU ring to make direct electrical contact with the ISS. In order 

for this to occur, the fabric cover must be moved, the stainless steel bearing ring has to either 

contact the ISS at a location where the anodization has been removed, or there is an exposed 

stainless steel fastener. See Section 7.9.

This still would not complete the circuit. If the sequence of events above were all to occur 

simultaneously, the ISS potential would be on a suit ring. There is a high probability that the 

astronaut’s perspiration would support a conducting path to the astronaut’s torso. Perspiration 

could then also make a conducting path to the neck ring. 

For negative ISS potentials, the circuit is then completed by ions from the ionosphere 

accumulating on the exterior insulating neck ring surface, charging it to the local ionosphere 

potential (see Figure 6.3-7d). The hazard comes from currents that would flow from a 

breakdown across that insulating surface (see Figure 6.3-7e). 
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For the positive potentials, the electrical current path is by electron collection on the insulating 

surface of the neck ring. As shown in Section 7.10 of this report, the magnitude of this path is 

limited to less than 1 mA for 1 ms.

In summary, to generate an arc that is hazardous to an astronaut, the station has to be at high 

negative potential and there must be a complete electrical circuit current path from the ISS 

chassis through the astronaut to the ionosphere to exist long enough for an arc to occur. To 

establish the electrical circuit associated with the negative charging hazard requires the following 

events to happen simultaneously during an EVA:

1. at a location on the ISS where the anodized layer has worn through;

2. the tether clip contacts the attachment ring;

3. the conducting tether ahead of the insulating fabric section is in contact with a suit ring;

4. where the fabric cover has pulled back;

5. the ring is made of stainless steel, not anodized Al;

6. there is enough perspiration for a low resistance path; 

7. the astronaut is in contact with the neck ring.

The circuit parts (items 1 through 7) are shown schematically in Figure 7.11-3. The probabilities 

are gross estimates. The “ISS Not Anodized” and “Contact Neck Ring” are based on the NESC 

team’s interpretation of DRD-MAPI-SA-06-ISSPRA-12-56 [ISS-PRA-12-56, 2013]. The 

purpose of Figure 7.11-3 is to show how many insulation failures must occur simultaneously inseries to establish the negative charging hazard circuit.

Figure 7.11-3. Circuit Paths from ISS Chassis Ground to the Astronaut Inside the EMU 

References:

1. Roeschel, E. (2013): “NESC_ISS_Shock_EVA_Actions.pptx” 6-25-13.

2. Duncan, G. (2013): Document DRD-MAPI-SA-06-ISSPRA-12-56, EVA Shock Update and 

Summary, International Space Station (ISS) Probabilistic Risk Assessment (PRA) Trade 

Study – Long Form, ISS-PRA-12-56 (Probability Risk Assessment Doc), Prepared by Gary 

Duncan, dated May 17, 2013.
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7.11.2 Effects of Coincidental EMU Insulation Failures 

The NESC team reviewed the “Shock to EVA Crewman due to Negative ISS Potential” in DRDMAPI-SA-06-ISSPRA-12-56. The hazard consist of the joint probability that the ISS chassis 

ground is at a potential more negative than ‒45V with respect to the local ionosphere and that 

there is a circuit path that connects the astronaut to the ISS chassis ground. 

Potentials more negative than ‒45V can only occur when no PCU is operating. In ISS-PRA-12-

56, it is estimated that with the PCU off, the probability of a “negative potential situational 

condition factor” occurring during an EVA is 0.0137. In Figure 7.11-3, the NESC team

estimated the probability of a complete circuit from ISS ground to the astronaut within the EMU 

as 10-6. This low value comes from the EMU modifications designed to prevent the shock 

hazard as described in Roeschel, “NESC_ISS_Shock_EVA_Actions.pptx.”

As discussed in Section 7.13.1, the probabilities in Figure 7.11-3 are crude estimates for 

illustrating the point that many insulation failures must occur simultaneously. The combined 

probability of the negative charging (0.0137) environment occurring and the circuit closure (10-6) 

is about 1 in 10 million, a much lower probability reported in ISS-PRA-12-56:

“The probability of the negative shock hazard is about 1 in 250,000.”

7.12 Shunt Array FDIR

7.12.1 FDIR Operation

FDIR algorithms are used in the ISS system to detect that a fault condition has occurred, confine 

the fault, and execute a recovery process (ISS EPS TM 21109) [Anon., 2004]. The array shunt 

FDIR is enabled, as a third shock hazard control, after the two PCUs are verified to be in 

discharge mode prior to the start of an EVA. The PCU will remain in this mode as long as the 

anode current is greater than 0.5A. Below 0.5A, the PCU returns to its startup routine. Five

parameters are monitored for the PCUs: (1) plasma current, (2) anode voltage, (3) cathode 

heater voltage, (4) tank and tube temperature, and (5) tank and tube pressure. The PCU has its 

own FDIR, which reacts to the loss of or low discharge consequently setting the corresponding 

fault indicators. 

When enabled, the array shunt FDIR will monitor the PCU fault indicators. In the event of one 

PCU failure during or prior to an EVA (Plasma Hazard Mitigation during EVA, B9-908), the 

FDIR will shunt all active solar arrays. The EVA might continue with no more than two arrays 

unshunted while oriented less than 105 degrees from the velocity vector. These allowed arrays 

are determined as part of the pre-planning FP analysis. Subsequently, in order to maintain ISS 

power balance, arrays will be unshunted when needed, but after the panel is oriented more than 

105 degrees from the velocity vector. To remain power positive, the unshunting must occur on 

the order of 10s of minutes after the FDIR response.

It is undesirable to keep the arrays shunted because extended battery discharge will occur on the

order of 1 hour, which shortens the cell life. As of 2004, it takes an average of 51 minutes of 
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battery discharge (all arrays shunted) to deplete down to the maximum design depth-of-discharge 

of 35 percent [Dong, 2004; Dalton, 2004] (calculated from information in these references).

The ISS battery capacity total is 192 kilowatt hours (kWh), with 24 batteries at 8 kWh each3,4

[Boeing, 2009; Space Systems/Loral, 1998]. One battery consists of two orbital replacement 

units (ORU) electrically in series. See Figure 7.12-1. 

Figure 7.12-1. ISS NiH2 Battery ORU

The FDIR was activated 3 times since it has been in use. The validation of the array shunting 

FDIR seems to have been limited. Below is a summary of the three on-orbit events:

(1) 2006/348:19:50 – In preparation for 12A.1 EVA 2 the FDIR was inadvertently actuated 

during a Node1 multiplexer/demultiplexer (MDM) transition. 2 of 3 deployed arrays 

shunted, 1 did not; root cause of arrays not shunting isolated to a timing issue between 

MDMs. (The software timing error was analyzed and fixed under SCR 35596.)

(2) 2006/348:22:56 – During 12A.1 EVA 2, PCU1 was intentionally commanded to 

“standby” for assembly operations. PCU not in “discharge” mode is one of the triggers 

to shunt the solar arrays. All arrays were shunted by the FDIR.

(3) 2006/350:22:38 – During 12A.1 EVA 3, PCU2 was intentionally commanded to 

“standby” for assembly operations. All arrays were shunted by the FDIR.

 

3 http://www.boeing.com/assets/pdf/defense-space/space/spacestation/components/docs/S6.pdf

4 http://sslmda.com/downloads/products/ispacest.pdf
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References:

1. Anon. (2004): ISS Electrical Power Systems Training Manual, ISS EPS TM 21109, Mission 

Operations Directorate, Space Flight Training Division, NASA Johnson Space Center, 2004.

2. Dong, S., et al. (2004): International Space Station Nickel-Hydrogen Extended Battery 

Discharge Model Analysis, 2004.

3. Dalton, Mark P. (2004): International Space Station Nickel-Hydrogen Batteries Approached 

3-Year On-Orbit, 2004.

4. Boeing (2009): “Discovery to Transport Last U.S., Boeing-built Starboard Truss Segment to 

Space Station,” January 2009. http://www.boeing.com/assets/pdf/defensespace/space/spacestation/components/docs/S6.pdf

5. Space Systems/Loral (1998): http://sslmda.com/downloads/products/ispacest.pdf

7.12.2 Risks for High Negative Potential Peaks

The array off pointing prior to unshunting is performed to reduce the RAM electrical current 

collection. However, the magnitude of the rapid charging event created during the unshunting 

(even in wake) has not been characterized.

The use of the FDIR presents risks for high negative potential peaks of short duration if, during 

required power restoration following the FDIR, any array is unshunted in sunlight. The results 

of on-orbit experiments conducted in 2010 on days 155 and 205 through 212 revealed large

negative potentials, up to ‒95V, when an array was unshunted in daylight while facing in the 

RAM direction with PCUs off. The duration of the peaks observed was approximately 10 ms. 

During the experiments, all eight arrays were forced to remain shunted via ground commanding 

as the station entered insolation. Approximately 3 minutes into insolation, the arrays were 

commanded to unshunt one at a time. FP data from the FPMU was recorded, as shown in Figure 

7.12-2. These potential peaks were present each time the commanded unshunt was performed, 

with minor variations in peak potential and peak duration. The experiments were limited to 

unshunting arrays in daylight at the beginning of the orbital day with the arrays facing in the 

RAM direction and PCUs off. More experimentation should be done to determine the nature of 

potential peaks at other times during the daylight portion of the orbit, at various array angles, and 

with PCUs on and off. Characterization of potential peaks with arrays pointing at >105 degrees 

from RAM is particularly important because it is the minimum pointing angle required during 

post-FDIR power recovery. Currently, there is no other data to support potential peaks when 

unshunting at >105 degrees. The FDIR is not a good hazard control strategy considering it could 

cause charging in excess of the defined hazard limit. It is unknown if these potential peaks are a 

hazard considering their short duration (~10 ms) because the defined hazard limit does not 

specify a time duration.
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Figure 7.12-2. FP Data from the FPMU

7.13 The Negative FP Limit

EMU Limit

The latest version of the Hazard Report (ISS-EVA-312-AC) and the NCR (ISS-NCR-232G) do 

not explicitly state a requirement for the FP limit of the EMU. A review of past NCRs and EMU 

documentation by the NESC team indicates that the EVA Office adopted ‒40V as the FP limit 

for the EMU in 2002. The adoption of the ‒40V level for the EMU appears to have occurred as 

a result of testing in 2001 at MSFC. Specifically, in 2001, a set of arc tests on EMU samples 

was performed. A statistical analysis of 10 samples was performed. These data were presented 

in an American Institute of Aeronautics and Astronautics (AIAA) conference paper in January 

2002 [Schneider, 2002]. The statistical analysis indicated a median arc voltage value of ‒74V 

with a standard deviation () of 8.1V. In April 2002, the statistical summary was presented by 

Hamilton Sundstrand (the EMU manufacturer) to an ISS/EVA panel [Gworek, 2002]. In that 

presentation, it was noted that ‒40V represented 4.2 standard deviations (4.2) from the median 

arc voltage. According to the presentation, the 4.2 value represented a 0.01-percent chance of 

arcing at ‒40V. 

ISS Vehicle Limit

The negative FP limit associated with the ISS vehicle was established to be ‒40V after a limited 

number of arcing tests in 1991 described in Section 7.13.1. However, per the ISS-NCR-232G, 

Block 13: “The largest accepted charging violation is ‒45.5V.” This safety margin reduction is 

justified, in the same NCR: “At the 1/14/09 SRP, a risk acceptance point of ‒45.5V was agreed 

upon by the Panel as a final non-negotiable limit for the negative potential. It was believed that 

the risk of increase in voltage was within the realm of engineering judgment acceptance.”
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The ISS-NCR-232F/G document accepts an increased risk associated with an EMU possibly 

encountering a section of ISS charged to ‒45.5V for scenarios involving a PCU failure as 

described by the following statement:

“In order to stay within previously accepted charging exceedances, OCAD #1 00006 

specifies that only pairs of arrays which result in charging levels lower in magnitude 

than ‒45.5V, per attachment 1, may be excluded from shunting and allowed to 

autotrack following a PCU failure.”

References: 

1. Schneider, T.A.; Carruth, M.R., Jr.; and Hansen, H.J. (2012): “Minimum Arc Threshold 

Voltage Experiments on Extravehicular Mobility Unit Samples,” AIAA Paper 2002-1040, 

40th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 14-17, 2002.

2. Gworek, P.; and Hansen, H. (2002): “EMU – Plasma Arc Update”, Presentation to CCB, 

Hamilton Sundstrand Space Systems International, Inc., April 3, 2002.

7.13.1 Origin of the ‒40V ISS Charging Limit

The ISS, like many spacecraft, uses Al metal in the construction of most of its structural 

components – due to the lightweight nature of Al. To avoid corrosion issues, Al is anodized. 

The anodizing process creates an oxide layer on the Al surface, which protects it from corrosion. 

The oxide layer is a dielectric layer (i.e., electrically insulating). In the case of ISS, the MMOD 

shields form the outer shell of the spacecraft and are in contact with the ionosphere plasma 

environment. The MMOD shields are made of Al metal and are anodized. In fact, a special 

anodization process was used to protect the MMOD shields on the ISS, which represent a large 

fraction of the vehicle’s surface area. The special anodized coating was needed to obtain 

thermo-optical characteristics, which would keep the MMOD shields relatively cool compared to 

standard anodized Al components. The special anodizing process used on the ISS MMOD 

shields resulted in an extremely thin anodization (oxide) layer, with thickness on the order of 

1.3 microns.

Early in the design process for the ISS, a solar array power system was adopted which operates

at 160V. The power system also employs the standard negative ground scheme, whereby the 

negative terminal of the power system is attached to spacecraft chassis (i.e., the Al metal hull), 

which includes the MMOD shields. Recognizing that the 160V solar arrays would interact with 

the ionosphere plasma, NASA personnel in the field of spacecraft charging predicted that the ISS 

vehicle would experience negative charging on the Al metal hull, followed by positive ion 

collection on the RAM facing anodization layers. The result would be a large electric field 

developing across a very thin dielectric layer. In the event the electric field exceeded the 

dielectric strength of the anodization (oxide) layer, an electrical discharge (arc) would form and 

damage the anodization layer. In a related scenario, if a micrometeoroid particle were to impact 

a charged anodization layer, it could precipitate an arc. 

Therefore, in the 1990 to 1991 time period, M. Ralph Carruth Jr., and Mr. Jason Vaughn, from

MSFC, conducted a test campaign to determine if an arc would be generated on a negatively-
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charged anodized Al plate immersed in a plasma environment in the event that the anodized 

layer was struck by a micrometeoroid particle. The tests sought to determine if there was a lower 

charging limit such that an arc would not be generated in the event of a particle strike.

All of the tests were conducted at Auburn University using a “hypervelocity gun” as a source of 

fast micrometeoroid particles. The tests were very time consuming to set up and execute. A 

plasma comparable to the ionosphere plasma had to be created and maintained at high vacuum, 

an anodized sample had to be charged to a specific voltage, and the high energy hypervelocity 

gun had to be successfully fired during the time window when the plasma and charging 

conditions were as desired.

The complexity and long set up times associated with the Carruth and Vaughn test led to only a 

limited number of successful shots. Details about this test campaign appear to be captured in the 

following reference: “Minutes from the Joint Meeting of the Electrical Grounding Tiger Team 

and the Electrical Power System Working Group for Development of the Decision Package 

for SSF Electrical Grounding,” Fairview Park, OH, August 5-7, 1991, (Carruth and Vaughn),

pp. 172-181. Unfortunately, this reference appears to have had only a limited distribution and,

unfortunately, the NESC team has not found a copy of this report.

Fortunately, both Carruth and Vaughn still work at MSFC and can be consulted about their 

recollection of the test campaign. According to Mr. Vaughn in a December 3, 2013, e-mail to 

Todd Schneider:

What I remember was we ran several tests with single MMOD shots at each level. We 

started at ‒150V and went down in steps of ‒25V. We definitely saw an arc at ‒75V, but 

it did not appear to be a full discharge of the cap [capacitor]. With one shot at ‒50V, we 

did not produce an arc. Because we did not see an arc at ‒50V, we asked for more 

resources to generate more data and better statistics at ‒50V. However, at that time in 

the investigation and all the data pointing to needing a PC [plasma contactor], the 

program management decided to solve the problem with the addition of the PC [plasma 

contactor].

From past discussions between Carruth and Schneider during ISS Plasma Charging Tiger Team 

activities in 2000, the ISSP (in 1991) asked Carruth to recommend a safe charging level for 

anodized ISS structural elements. Based on the limited data at ‒50V, Carruth recommended 

adopting a 10V margin and suggested ‒40V. 

The origin of the ISS ‒40V limit, therefore, is based on the work of Carruth and Vaughn in the 

early 1990s to determine a voltage whereby a charged section of the ISS MMOD shield would 

not arc in the event that section was struck by a micrometeoroid particle. 

In parallel with the work at MSFC to investigate arc initiation was an effort to determine the 

expected vehicle charging levels due to the interaction between the high voltage solar arrays and 

the ionosphere plasma environment. In 1991, at the Lewis Research Center (LeRC) – later 

renamed Glenn Research Center – Carolyn Purvis, Dale Ferguson, and David Snyder made 

measurements of the current collection of ISS solar array (or photo-voltaic array) panels in a 
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representative ionosphere plasma. Their measurements showed that the ISS solar arrays were 

capable of collecting large electron currents from the ionosphere plasma, which pointed toward 

ISS vehicle charging levels that could reach ‒140V.

Recognizing the need to limit ISS vehicle charging in order to minimize arcing on anodized 

surfaces, the LeRC team developed an active charge control system. Michael Patterson (LeRC) 

used the setup built by Purvis, Ferguson, and Snyder to demonstrate that a hollow cathode device 

could be deployed on ISS, which would actively control/limit the charging on the ISS chassis to 

‒40V. The hollow cathode system demonstrated by Patterson became known as the PCU and 

formal implementation of the PCU was approved in April 1992 [Moorehead, 1992].

At the time the ‒40V limit was established, the primary concern about the ISS spacecraft 

charging was related to the potential damage that an arc could do to the anodization layer on a 

MMOD shield. If enough arcs occurred on the MMOD shields, the thermo-optical properties of 

the anodization layer on the MMOD shield could be significantly altered, which would result in 

taxing the ISS cooling systems. In other words, in the 1990s the problem associated with arcing 

on ISS was a vehicle-level problem, and not a personnel safety problem.

Reference:

1. Moorehead, R.W., Deputy Director (1992): Space Station Freedom Program and 

Operations, communication to Work Packages 1-4 Directors, dated April 3, 1992.

7.13.2 Plasma Safety Hazard Identification and Risk Acceptance at ‒45.5V Charging 

Levels

In September of 2000, Schneider and Carruth, conducted a test (at MSFC) to determine if 

components of the EMU would arc if they were charged in the presence of a plasma 

environment. This test was triggered by the work of members in the ISS PCU Tiger Team, who 

recognized that it might be possible for the EMU to become charged to the same potential as the 

ISS vehicle metal chassis. The results of the test by Schneider and Carruth showed that EMU 

components (e.g., the display and control module) could indeed arc. In fact, an arc at ‒68V was 

recorded for an anodized component of the display and control module in the MSFC plasma test 

chamber.

Thus, a shift occurred in the ISS spacecraft charging community from concern about arc damage 

on the ISS vehicle to a possible electrical safety hazard for an astronaut conducting a spacewalk 

wearing an EMU.

In the 2000 to 2001 time period, studies by the EMU suit manufacturer Hamilton Sundstrand and 

NASA’s EVA teams confirmed that electrical pathways did exist which would allow for the 

EMU suit to reach the same charging level as the ISS vehicle. So, if the ISS vehicle was 

experiencing charge levels of ‒80V, then it would be possible for the EMU to also charge to 

‒80V. Since the EMU did contain anodized components exposed to the ionosphere plasma, the 

possibility existed that electrical discharges (arcs) could develop on those EMU components via 

dielectric breakdown. Unfortunately, since the astronaut is in contact with metal components 

inside the EMU, due to conduction via the perspiration-soaked cooling garment covering their 
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bodies, there is a possible safety concern that emerges since the astronaut’s body is part of an 

electrical circuit in which an arc is occurring. That is, an electrical shock hazard could becreated 

for an astronaut inside an electrically charged EMU suit.

To help in determining the safe operating limits for EMU charging, Harold Hansen (Hamilton 

Sundstrand) and Todd Schneider (NASA/MSFC) conducted an arcing test in 2001 using 

anodized Al samples prepared using the same processes as EMU components. The full test 

description and results can be found in Schneider, et al., 2002.

The 2001 test by Hansen and Schneider showed that within the limitation of a dataset that 

included only 10 samples, a statistical fit to the data indicated that ‒40V represented a 

0.01-percent probability of generating an arc on an anodized EMU component. Using the test 

results, combined with the previously defined vehicle limit, it appears that in 2002 the EVA 

Safety teams adopted ‒40V as the safe limit for EMU charging. This limit was then applied to 

the creation of EVA reports and hazard documents.

As the EMU plasma shock hazard represented a possibly catastrophic hazard for the astronaut, it 

was mandated that a two-fault tolerant safety system be used to protect the astronaut on EVA 

from the plasma charging hazard. To meet this two-fault tolerant requirement, two PCUs were 

operated during an EVA and solar arrays were shunted (i.e., power production from the array 

was stopped by shorting the output of the array). This ensured that the ISS charging would be 

more positive than ‒40V. 

By 2007, a better understanding of the ISS spacecraft charging had emerged with the availability 

of measured vehicle charging data from the FPMU, which was deployed on the ISS in 2006. 

Using a modeling capability based on empirical data, the Boeing Company projected a scenario 

in which a ‒45.5V charging level could be reached on the ISS in the event that no active charge 

control device (e.g., PCU) was operating, but the solar arrays were producing power (i.e., not 

shunted). This marks the first time an exception was made to allow for a scenario in which an

EVA astronaut might be exposed to charging levels exceeding the ‒40V charging limit. The 

exception is documented in the “ISS Safety Noncompliance Report,” ISS-NCR-203 Rev. B, The 

Boeing Company/Space Exploration/International Space Station, September 19, 2007 [Boeing, 

2007]. It should be noted that this NCR was intended only to cover the ISS build stage 10A 

(October 2007 to February 2008).

The exception documented in the ISS-NCR-203 Rev. B is actually a worst-case scenario in 

which two PCUs fail and solar array shunting is not allowed due to the need to maintain a 

minimum safe power level onboard the ISS. The -45.5V level is actually a model prediction 

using the Boeing Company’s ISS charging model.

In 2009, using the ISS-NCR-203 Rev. B as a precedent, a SRP apparently agreed to once again 

accept the risk associated with an EVA continuing after predicted potentials could reach -45.5V. 

Recall that the following statement documents that decision: 
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“At the 1/14/09 SRP, a risk acceptance point of ‒45.5V was agreed upon by the Panel as 

a final non-negotiable limit for the negative potential. It was believed that the risk of 

increase in voltage was within the realm of engineering judgment acceptance.”

This statement appeared in ISS-NCR-232F, “Lack of Two-fault Tolerance to EVA Crew Shock in 

the Low Earth Orbit Plasma Environment,” The Boeing Company/Space Exploration/

International Space Station, Joseph E. Thomas (Document Originator), 1/26/2012, page 6

[Thomas, 2012].

It is important to distinguish between a risk acceptance of ‒45.5V and a safety limit of ‒40V. 

The risk acceptance does not change the established safety limit, rather it allows for an EVA to 

continue with only single-fault or zero-fault tolerant hazard controls and increased safety risks.

The ‒40V charging limit for both the vehicle and the EMU was based on test data. In the case of 

the vehicle, the test data focused on micrometeoroid impact induced arcing. In the case of the 

EMU, the testing was on dielectric breakdown of samples produced by Hamilton-Sundstrand, the 

vendor who constructed the EMU. Thus, it seems prudent that changes to the established 

charging limits, for either the vehicle or the EMU, would be accompanied by new test data that 

makes a compelling case.

Reference:

1. Boeing (2007): “ISS Safety Noncompliance Report,” The Boeing Company/Space 

Exploration/International Space Station, ISS-NCR-203 Rev. B, September 19, 2007.

2. Thomas, J.E. (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, The Boeing Company/Space Exploration/ International Space 

Station, Joseph E. Thomas (Document Originator), ISS-NCR-232F, 1/26/2012.

7.14 Review of the ISS-NCR-232G: Lack of Two-fault Tolerance to EVA 

Crew Shock in the Low Earth Orbit Plasma Environment

The ISS-NCR-232G is the governing document to define the use of controls during EVAs, lack 

of three controls (two-fault tolerance), or failures of controls. In addition to the discrepancies in 

the control approach, as defined in Section 6 of this document, the NESC team agreed there are 

inconsistencies and other general statements to address in the reviewed documentation. For 

example:

o The document implies that independent Space Environment Scientists are in agreement 

with “the environment will remain benign at least through Solar Cycle 25 which extends 

through -2030.” The NESC team does not agree this is widely accepted.

o There are obvious inconsistencies, relating to the ISS safe FP limits. Currently, both 

‒40V and ‒45.5V are referenced. If ‒40V is the limit and the ‒45.5V includes the 

accepted risk, it needs to be explicitly stated. The tolerance of the calculations also needs 

to be taken into consideration.

o There is no safety limit specified for the positive potential (if considered a hazard) and 

electrical current collection.
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o The use of the “short-term” expressions is misleading when referring to a 14-day forecast 

since a 1-day prior to forecast might be in order.

o There is no coherent list of all possible electrical current entry points into the astronaut 

via the EMU’s external metal contact points, especially when also looking at the PRA‐

12‐56.

Refer to Appendix C for the complete review of the ISS-NCR-232G.

7.15 Examination of ISS-PRA-12-56: PRA for Shock Hazard

At the request of the NESC team, a Jet Propulsion Laboratory (JPL) PRA expert briefly reviewed 

the information provided by the NESC team, the ISS-PRA-12-56 document. He recommended 

that the PRA material, as it exists, needs an in-depth review. This conclusion is based on the 

following observations:

 The event sequence diagrams, event trees, and fault trees (a) lack direct provenance to the 

experiential evidence used to derive and quantify them, and (b) need to be reviewed for 

completeness (i.e., determine whether any potentially risk significant events/phenomena 

are omitted). 

 As much of the probabilistic data originally resulted from expert opinion, the data need to 

be either verified by comparison with physical data or physics‐based models, or have the 

uncertainty assigned to the probabilities expanded to include variations among cognizant 

experts.

 The ISS PRA report describes a model and data, which were quantified using the 

Systems Analysis Programs for Hands-on Integrated Reliability Evaluations (SAPHIRE)

software tool. The fidelity of the model and data to the physics of EVA shock need to be 

reviewed—the intent should be to perform a broad, “horizontal” review followed by 

selected “vertical” slices.

 It is not clear if the negative case considers the PCU “on” (study categorizes the PCU as a 

positive hazard contributor) or if the analysis takes into consideration the suit changes or 

if the changes reduced the hazard posed by the PCU.

7.15.1 Additional PRA Review

A review of the PRA documentation package provided [1, 2] revealed lapses in clarity and detail. 

The methodology and underlying assumptions provided are insufficient to enable duplication of 

the stated findings. In general, the document would benefit from an editorial review of its 

detailed content. However, several areas could potentially benefit from additional detail. In its 

present form, the PRA does not meet commonly held standards for technical rigor [3]. Selected 

examples follow in the interest of increasing the level of clarity and potential value of the 

presentation:

 Terminology and labeling between documents greatly reduces clarity of presentation ([1], 

page 8): “+Transient Capacitive Discharge Hazard” is referred to as “AC Shock” in PRA 

[1, page 14]. Similarly “+DC Hazard” is labeled “DC Shock” [1, page 13], etc. To find 

these details one would need to be quite familiar with the contents of the PRA. Use of a 
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summary with common nomenclature between documents [1, 2] could greatly help the 

clarity of presentation. A variation on Table 1 of PRA [1] would be preferable to relying 

on text to convey findings. This would allow the reader to more readily inter-compare 

relevant magnitudes. Why are probabilities for a “single crew member” provided in [1, 

page 8, see “PRA Updates and Results”]? In other sections, mean probability for “two 

crew members” are stated. If there is a compelling reason why this is done it should be 

clearly stated, if not, if one should consider simplifying to a common case and language. 

This would improve readability of the text for decision makers. At best, the current 

narrative formats used in [1,2] are challenging to decipher and time consuming if one 

wished to compare in detail. 

 Clarity ([1], page 8): The term “baseline” needs to be clearly defined. See comment 

below regarding “baseline” definition used in PRA.

 Documentation of methodology ([2], page 11-12): Contact probabilities (Table 3-4) 

justification largely unstated – some appear larger than one might expect given present of 

insulating material. Have these tables been updated to reflect suit modification? Text 

presently states: “...reliability data used to populate the events in model originally 

generated in 2008 using expert judgment....” Table 2 provides point estimate for 

“negative potential situational condition factor = 0.0137” – what is the uncertainty in this 

value? Stated uncertainties in contact probabilities all have “uncertainty 7 log normal”—

unclear what this means here – is this the standard deviation, error factors [7, pages 78-

79], or other? Unclear why all uncertainties have the same magnitude. 

Comment: At a minimum, it would appear to be of value to reveal these inputs, document 

rationale, and where the possible link to physical measurement is (e.g., contact 

probability of anodized-anodized Al surface ~ 0.01, etc.). Unclear from documentation 

provided and hard to tell if values represent “opinion” or physical observation. Similar 

comments hold for the uncertainties used in simulations.

 A contact probability of zero is equivalent to “not credible” [2]. Inclusion in this form 

merely tends to complicate model topology and distract from clarity of presentation. 

Reader is left pondering why such events are present beyond indicating that they have 

been considered.

Comment: The results presented in the document cannot be uniquely reproduced from the 

explanation provided. Would be of value to compute and explicitly document product of 

values leading to max-min probabilities. Such a crosscheck would bound the expected 

order of magnitude and validate of the detailed simulation described [2, pages 16-62]. 

Such an exercise could potentially lend physical insight and credibility to the modeled 

results. 

 Unclear [2, Figures 6-8]: 2- versus 1-Crew probabilities appear to scale with exposure 

time. This makes physical sense, however, inclusion merely graphically confuse data 

presentation. Why is this information detail desirable to present? If it is – maybe of 

value clearly state desired conclusion.
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 Unclear [2]: “...for a shock hazard, several events must occur simultaneously...” latter 

stated “...model does not depend on chronological sequence of events....” Observation 

if “simultaneous” – how could sequence matter within such a logical framework? Given 

the underlying circuit topology assumed, the threat must occur at the same time a path is 

present. 

 Acronym “OBS” does not appear to be defined – from context reader might assume 

“Operational Bioinstrumentation System.” Similarly, acronym “CCA” does not appear to 

be defined – from context – appears to be an electrical connector interface or similar. 

Recommend checking documents [1,2] for definition of all acronyms.

 Unclear [2]: “The risk of loss of crew (LOC) for a single EVA, but the baseline ISS PRA 

EVA model is presented for comparison only; it should be noted that the EVA shock 

hazard is the probability an EVA crew would experience a shock and imply LOC....”

Unclear why stated “baseline” is relevant if suit modification has occurred and in use? 

Why would one not treat the modified suit as the baseline and merely state improvement 

over prior art in passing? Alternatively, need to provide context and logic for stated 

baseline.

 Executive Summary [2]: The potential is one parameter of interest here – the magnitude 

of the electrical current that can be sourced by the threat is the other – would be of value 

to briefly discuss both aspects.

 Typesetting ([1], page 6-7): “...These five events are discussed individually below.” Six 

items are enumerated. Document could benefit from careful editorial review. 

References:

1. ISS-NCR-232G (2013): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, NCR-20264-R7, 18 September 2013.

2. Schmidl, D. (2013a): EVA Shock Update and Summary PRA Report ISSPRA-12-56, NASA 

JSC, May 7, 2013, DRD-MAPI-SA-06-ISSPRA-12-56.

3. Stamatelatos, M. and Dezfuli, H. (2011) Probabilistic Risk Assessment Procedures Guide for 

NASA Managers and Practitioners, Second Edition, 2011, NASA/SP-2011-3421.

4. Smith, C.L. and Wood S.T. (2011a): “Systems Analysis Program for Hands-on Integrated 

Reliability Evaluations (SAPHIRE) Version 8, Volume 1: Overview and Summary,” 2011, 

United States Nuclear Regulatory Commission, NUREG/CR-7039, Vol. 1.

5. Smith, C.L. and Wood S.T. (2011b): “Systems Analysis Program for Hands-on Integrated 

Reliability Evaluations (SAPHIRE) Version 8, Volume 1: Technical Reference,” 2011, 

United States Nuclear Regulatory Commission, NUREG/CR-7039, Vol. 2.
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8.0 Findings, Observations, and NESC Recommendations

8.1 Findings

The following findings were identified:

F-1. There are numerous shortcomings in the space weather forecast planning used on the ISS 

that limits its use for 14-day (or any) forecasting: 

– Ne and Te “forecast” is a simple persistence of condition methods based on the 

assumption that conditions in 14 days will be same as on day the FPMU 

measurements are obtained.

– Validation of Ne and Te environment forecast is based on data from two limited time

periods.

– IRI model used to project measured data into future is a monthly average climatology 

model incapable of predicting the full range of environments responsible for ISS 

charging.

– No complete verification of ability to predict ISS potentials 14 days in advance has 

been demonstrated. 

– Plasma hazard assessment report does not include information on current state of 

geomagnetic activity—no documented plans to deal with FPMU data obtained during 

disturbed periods when reference data may under represent the charging environment 

present in 14 days at the time of EVA. 

– Assumption that solar activity will remain benign through next solar cycle into ~2030 

has no basis in current ability of solar physics community to predict future solar 

activity.

F-2. The FPMU is an integral part of the proposed forecast process; however, there is no 

explicit contingency procedure when FPMU data is not available.

– FPMU data are critical since Ne and Te values from the IRI statistical model are 

constrained by FPMU measurements in determining which set of IRI values are used 

as inputs to the PIM3.0 charging model calculations for the plasma hazard forecast 

approach.

– A spare FPMU unit is available on board the ISS, but will require an EVA to replace 

a failed unit.

F-3. Comparisons between calculations of the ISS potentials by PIM3.0 using the actual 

ionospheric environment with the real-time FPMU measurements has identified 

deficiencies. Potentials more negative than ‒45V have been measured on the ISS. The 

ionosphere forecast and PIM3.0 models are not capable of predicting these large 

potentials (see Figure 7.6-3).

Limitations and sources of error in the PIM:

– Analytical approximations used in the numerical solutions for the potential barriers in 

the gaps between solar cells (solar array electrical current collection model).
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– Assumption that every solar cell collects the same electrical current.

– Uncertainties in FPMU Ne and Te input data (due to FPMU data reduction errors).

– Timing of FPMU data chosen for PIM3.0 input relative to charging peak.

– Errors in knowledge of solar array angles, ISS flight attitude, and ISS velocity.

– Variations in ion collection area (free parameter adjusted to obtain best results).

– Use of static (equilibrium) charging algorithm cannot predict rapid charging events.

– Use of single capacitance in time-dependent charging algorithms oversimplifies the 

physics of the ISS charging and fails to model fast transient charging.

– The IRI and PIM3.0 models do not contain the appropriate physics to predict auroral 

charging.

F-4. The proposed usage of PIM3.0 puts this model in the critical path to EVA, yet fails to 

meet the NASA modeling standards imposed after the Columbia tragedy.

– The CAIB report and NASA’s response to it emphasizes that various aspects of 

ensuring credibility of modeling results gets conveyed to critical decision makers 

relying on those results.

– PIM3.0 fails to meet the minimum requirements:

 The limitations of the PIM3.0 are not explicitly known by the decision 

makers.

 User’s manual and parameter definitions for the PIM3.0 code are not 

available.

 The configuration files for the use of PIM3.0 are not documented in the preplanning proposed procedure. These will constrain how the model is used 

every time.

 The model has not been independently peer reviewed.

 There is no process identified to update PIM3.0 to include physical changes to 

the station configuration.

 There is no clearly documented validation, verification, or certification 

process. 

– This model should not be in the critical path if it lacks the pedigree associated with 

above mentioned standards. 

F-5. The PCU maintains the ISS near to space plasma potential, even under poorly 

characterized charging events like rapid charging events.
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– Under the worst-case conditions, the PCU has the capability of sourcing enough 

electrical current to keep the ISS close to the plasma potential. 

– The PCU has demonstrated emission to 10 A in ground testing. (See Figure 7.8-1.)

– The largest PCUs electrical current measured on orbit is 0.575 A.

F-6. The added positive potential caused by operating the PCUs introduces negligible 

additional electrical current collection in the EMU in light of the recent EMU electrical 

isolation modifications, even outboard of the SARJ.

– An analysis was performed by the NESC team of plasma current collection by the 

EMU due to positive ISS potentials with the PCU on. 

– Electron plasma currents have been recalculated accounting for modifications to the 

EMU including those that isolate equipment. 

• Currently, there is no DC condition due to the isolation of the MMWS 

since it is no longer an exposed conducting path.

– The NESC team calculation used the orbit-limited cylindrical electrical current 

collection model, which is more applicable than the more conservative orbit-limited 

spherical electrical current collection model.

– An analysis performed by the NESC team showed that the previous calculations of 

thorax electrical current levels used to determine that low positive potentials are a 

hazard, were more than an order of magnitude too large. 

– Electrical isolation of the MMWS has greatly reduced the probability of any potential 

hazard due to DC conditions.

F-7. The PCU has adequate supply of xenon gas and the hardware (hollow cathode) has 

demonstrated life in space to support its use at the ISS past 2028.

– Both PCUs satisfy the two necessary conditions for long life:

• There is enough propellant to run the PCUs past 2031.

– No PCU hardware component has been identified to limit the operational life shorter 

than 2024.

– In flight hollow cathode experience, DS1, demonstrated >16,000 hours (Test 

Readiness Level 9). Currently on the Dawn spacecraft, the three thrusters and their 

hollow cathodes have a combined >35,000 hours of operation.

F-8. The modified suit acts as a hazard control by disrupting the electrical current path from 

the ISS through the astronaut to the plasma through multiple layers of insulation.

– There are several specific features of the EVA suit – tether – tool system each 

designed to interrupt the circuit.
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– For electrical current to flow through an astronaut requires a simultaneous failure of 

several of these features.

F-9. The low likelihood of occurrence of coincidental EMU insulation failures in the ISSEMU-plasma circuit necessary for electrical current flow through the astronaut torso 

supports its use as a control.

– The FP as a known hazard is controlled by the insulated EMU-tool system as 

supported by the calculated low probability of a shock hazard, which considers the 

environment and the electrical current path.

– The NESC team’s preliminary estimates from the circuit path probability suggest that 

the suit insulation reduces the probability of shock hazard to less than 1 in 107.

F-10. The array shunting FDIR has not been validated and its use presents risks.

– There is a risk for high negative peaks (of short duration) when an array segment is 

unshunted in daylight after a FDIR response.

 Solar array unshunting can occur during EVA. Present flight rules provide no 

guidance when to unshunt arrays. 

 The peak magnitude of rapid charging events due to unshunting the array in 

wake (>105 degrees from RAM) is not known.

 Presents a potential risk to the ISS power balance.

• To remain power-positive, unshunting must occur on the order of 10s 

of minutes after FDIR’s response.

– The array shunting FDIR is considered a complicated algorithm potentially causing 

steady state power level issues as well as unknown and unexpected rapid charging 

events.

F-11. Use of the low-risk active hazard controls (e.g., PCUs) becomes optional in the ISS 

NCR-232G guidelines and depends on results from a “short-term plasma forecast” 

assessment issued prior to a planned EVA. The need for active hazard controls therefore 

depends on the ability of the higher risk “short-term plasma forecast” method to 

reliablypredict ISS floating potential prior to an EVA. 

– Reliability of the “short-term plasma forecast” (as described in the ISS-NCR-232G) is 

based on the assumption that low solar activity and benign charging conditions will 

continue for the balance of the current Solar Cycle 24 and all of Solar Cycle 25, 

allowing the persistence of plasma environments over time to characterize charging 

hazards. 

F-12. Discontinuing the use of PCUs in favor of the forecast is not the lowest risk option for 

mitigating EVA shock hazard.
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– Data shows PCUs fully capable of controlling any potential hazard. They are 

designed to be reliable and have the xenon needed to continue past 2028.

– The forecast cannot predict all the observed types of ISS charging. 

F-13. There is no written documentation provided as to what is considered a safe voltage level 

with respect to arc generation on an EMU suit. The value of ‒40V is referenced as a 

vehicle requirement. 

– While the ‒40V level appears to be used in safety assessments related to the EMU, no 

specific voltage requirement can be found which applies directly to the EMU.

– The current Hazard Report (ISS-EVA-0312-AC) does not provide a negative voltage 

level (with respect to the ionosphere plasma) which constitutes a safe operating limit 

for the EMU – in order to avoid arc generation.

– ISS-NCR-232F and ISS-NCR-232G discuss operation of the ISS vehicle with respect 

to a ‒40V required limit; however, these reports do not provide any specific 

references to safe voltage limits for the EMU suit.

F-14. There is no written documentation provided which justifies the “risk acceptance point” of 

‒45.5V for the ISS vehicle charging with respect to the ionosphere plasma. Furthermore, 

no information is provided as to the application to the EMU suit of this increased risk

level. 

– While it is made clear in ISS-NCR-232G that the ‒45.5V level was established at the 

“1/14/09 SRP,” no information is provided as to how much additional risk for arcing 

occurs when an EMU is charged to ‒45.5V as compared to ‒40V.

– Page 6 of ISS-NCR-232G contains the following statement: 

“At the 1/14/09 SRP, a risk acceptance point of ‒45.5V was agreed upon by the Panel 

as a final non-negotiable limit for the negative potential. It was believed that the risk 

of increase in voltage was within the realm of engineering judgment acceptance.”

– No information about the rationale used to support the ‒45.5V decision was found in 

all of the documentation reviewed by this NESC assessment team – including official, 

unofficial, and background reports and presentations.

F-15. There are inconsistencies between the released documented processes (e.g., in the ISSNCR-232G) and what is conveyed by the ISS Space Environment Community verbally or 

via email.

8.2 Observations

O-1. This assessment does not include scattered plots with the full set of ISS charging events 

nor sensitivity analysis of the floating potential calculations since there was limited 

information available on the PIM3.0 code. 

O-2. The limited information on the PIM3.0 restricted the ability to assess the code’s physics 

and capabilities.
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O-3. The NESC team did not evaluate the EMU systems (i.e., electrical systems, instruments) 

to understand their susceptibility to the study’s hazards.

O-4. The analysis in this assessment focused on the current ISS configuration and did not 

attempt to address the effects of proposed configuration changes, such as future Russian 

solar arrays. 

8.3 NESC Recommendations

The following NESC recommendations were identified and directed towards the ISS

Environments and EVA Safety teams unless otherwise identified:

R-1. The ISS-NCR-232G approach should be revised. The NESC team disagrees with the use 

of shock hazard forecasting based on environments and modeling to eliminate the PCU 

usage. (F-1, F-3, F-11, F-12)

R-2. Both PCUs should be operated in discharge during the entire EVA regardless of pre-EVA 

hazard severity measurements, short-term ionospheric environment forecasts, or location 

of the EVA. (F-5, F-6, F-7, F-12)

– This provides two of the required three controls to achieve two-fault tolerance.

R-3. Evaluate the use of the low probability of the ISS crew contact circuit path (per PRA and 

EMU modifications) as the basis for the third control to achieve two-fault tolerance 

instead of the FDIR. (F-8, F-9, F-10)

– This includes revising the PRA per preliminary analysis demonstrated in this 

assessment.

R-4. Reassess the severity of the positive potential hazard based on changes to the EMU 

configuration and the analysis provided in this report. (F-6, F-8, F-11)

– EMU “positive shock hazard” is the result of making unrealistic assumptions about 

plasma collection that model the EMU as a bare metal sphere floating in space 

connected with a wire to the ISS chassis ground, then claim that the actual 

configuration of the “EMU cannot be used as a hazard control” for this contrived 

“hazard.”

– If the floating positive potential is demonstrated and accepted as not a threat then 

YVV orientation as a control should be discontinued.

R-5. Perform a quantitative analysis to determine whether the rapid charging events exceeding 

‒45V constitutes a threat to crew during EVA. (F-3, F-10)

R-6. If the ISSP continues to use the 14-day forecast and PIM3.0 process, described in ISSNCR-232G, for EVA hazard control planning, then it is recommended to address the 

issues described below. (F-1, F-2, F-3, F-4, F-11, F-12)
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– The PIM3.0 code is an engineering tool and would need to be updated to meet the 

NASA software standards (NASA-STD-7009) if it is to be used for EVA safety 

critical decisions.

– PIM3.0 code should be peer reviewed, documented, and a user’s guide provided.

– The PIM3.0 input file should be documented to generate plasma hazard assessments 

in both the shock hazard control guidelines and plasma hazard assessments to assure 

configuration control when using the model.

– FP calculations should have error values assigned to them.

– The PIM3.0 should be updated to incorporate algorithms for simulating all measured 

data including rapid charging events if these are determined to be a hazard (R-5).

– Verify the ionosphere environment statistics derived from the IRI-2001 model are 

applicable to the IRI-2011 model.

– Forecasting based on persistence of ionospheric conditions is useful for long-term (14 

days) solar array configuration pre-planning, but this could also be accomplished 

using statistical models for range of expected conditions (including worst-case) 

– Alternative sources of ionospheric Ne and Te data (e.g., COSMIC Ne profiles, 

ionosonde Ne values, and GAIM model Ne and Te output) are available for use as 

contingency option for characterizing environment should FPMU data not be 

available.

R-7. The ISSP should complete a systematic study of all available FPMU data. This study 

should include information on the magnitude of charging events, changes in potential, 

rise and decay times, statistical ranges, and other details as required to fully characterize 

the charging events. (F-1, F-2, F-3, F-10)

– A yearly review of space weather status and the latest ISS measurements is 

recommended.

R-8. Develop procedures for terminating or avoiding EVA in the wake of the ISS during 

severe auroral events (e.g., capable of generating frame and surface potentials* in excess 

of ‒100 to ‒1000V). (F-1, F-3)

– Demonstrate the threat by independently verifying the effects of extreme auroral 

charging effects on the EMU. Ground tests have shown surface discharges on suit 

materials in simulated auroral conditions, but no tests have been done to determine if 

these will affect the EMU.

– Evaluate auroral charging effects during an EVA with PCUs turned on since the 

PCUs might not offer protection against these rare, but extreme, events in the ISS 

wake.

– Recommend monitoring geomagnetic indices (e.g., Kp or similar indices) and 

coronal mass ejections (CME) in real-time (at least 1 to 2 hours ahead of EVA). 

If likelihood of severe auroral activity at the ISS, delay or terminate EVA.
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– Conduct a thorough statistical analysis of likelihood of severe auroral arc at the ISS 

during EVA.

*PCUs mitigate the ISS frame charging, but will not reduce potentials on insulating 

surfaces 

R-9. Documentation related to EVA shock hazard control needs to be updated to be clear and 

specific in the following subjects. (F-11, F-12, F-13, F-14, F-15)

1) PCU utilization 

2) Disable FDIR 

3) Marginalization of positive hazard 

4) PRA 

5) EMU tools isolation 

6) Elimination of YVV 

– The ISS-NCR-232G document should be updated to correct inconsistencies, missing 

references, and other general statements.

– A complete document review is provided in Appendix C of this report.

9.0 Alternate Viewpoint

There were no alternate viewpoints identified during the course of this assessment by the NESC 

team or the NRB quorum.

10.0 Other Deliverables

No unique hardware, software, or data packages, outside those contained in this report, were 

disseminated to other parties outside this assessment.

11.0 Lessons Learned

No applicable lessons learned were identified for entry into the NASA Lessons Learned 

Information System (LLIS) as a result of this assessment.

12.0 Recommendations for NASA Standards and Specifications

No recommendations for NASA standards and specifications were identified as a result of this 

assessment.

13.0 Definition of Terms 

Corrective Actions Changes to design processes, work instructions, workmanship practices, 

training, inspections, tests, procedures, specifications, drawings, tools, 

equipment, facilities, resources, or material that result in preventing, 

minimizing, or limiting the potential for recurrence of a problem. 
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Finding A relevant factual conclusion and/or issue that is within the assessment 

scope and that the team has rigorously based on data from their 

independent analyses, tests, inspections, and/or reviews of technical 

documentation.

Lessons Learned Knowledge, understanding, or conclusive insight gained by experience 

that may benefit other current or future NASA programs and projects. 

The experience may be positive, as in a successful test or mission, or 

negative, as in a mishap or failure.

Observation A noteworthy fact, issue, and/or risk, which may not be directly within the 

assessment scope, but could generate a separate issue or concern if not 

addressed. Alternatively, an observation can be a positive 

acknowledgement of a Center/Program/Project/Organization’s operational 

structure, tools, and/or support provided.

Problem The subject of the independent technical assessment.

Proximate Cause The event(s) that occurred, including any condition(s) that existed 

immediately before the undesired outcome, directly resulted in its 

occurrence and, if eliminated or modified, would have prevented the 

undesired outcome.

Recommendation A proposed measurable stakeholder action directly supported by specific 

Finding(s) and/or Observation(s) that will correct or mitigate an identified 

issue or risk.

Root Cause One of multiple factors (events, conditions, or organizational factors) that 

contributed to or created the proximate cause and subsequent undesired 

outcome and, if eliminated or modified, would have prevented the 

undesired outcome. Typically, multiple root causes contribute to an 

undesired outcome.

Supporting Narrative A paragraph, or section, in an NESC final report that provides the detailed 

explanation of a succinctly worded finding or observation. For example, 

the logical deduction that led to a finding or observation; descriptions of 

assumptions, exceptions, clarifications, and boundary conditions. Avoid 

squeezing all of this information into a finding or observation.

13.1 ISS PCU Report Definition of Terms

Aurora

Transient displays of light, often displaying as moving curtains and rays, at high latitudes 

associated with geomagnetic disturbances.

Auroral region

Oval-shaped, high-latitude zone centered on the geomagnetic pole, in which aurora are 

most visible.
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Auroral activity

Usually refers to visible aurora and the particles that create them, but may also refer to 

electrical currents that flow in the auroral region. One measure of auroral activity is 

hemispheric power.

Auroral boundary

The high and low latitude edges of the auroral zone, typically 72 degrees (poleward) and 

62 degrees (equatorward).

Auroral precipitation

Ionized particles that fall, or are accelerated, into Earth’s atmosphere to create the aurora 

and aid in the flow of electrical current.

Coronal Mass Ejection

An eruption in the outer solar atmosphere that sends billions of tons of magnetized 

plasma clouds into interplanetary space. When traveling at high speeds these ejections 

create shocks in the solar wind. Earth-intercept of a CME is often followed by a 

geomagnetic storm.

Electron volt (eV)

A small unit of energy that is associated with a particle of a single charge, such as an 

electron or proton, moving through an electric potential of 1V. It is equivalent to 

1.602x10^-19 J. Highly energized particles may have energies of mega electron volts 

(MeV) or beyond. 

Energetic charged particles

Charged particles such as energetic electrons and energetic protons, and sometimes 

heavier ions, that have high enough energies to be moving at a significant fraction of the 

speed of light – at least 1 percent of the speed of light. These energetic particles can 

cause ionizing radiation damage spacecraft components and biological materials, such as 

DNA.

Energetic electrons

Electrons that are traveling much faster than ambient electrons in the space plasma and 

have the potential for causing ionizing radiation damage to spacecraft and astronauts. 

Glossary/energetic electrons

Energetic Protons

Protons that are traveling much faster than typical protons in the space plasma and have 

the potential for causing radiation damage to spacecraft and astronauts. 

Glossary/energetic protons

Geomagnetic Kp Index

The Kp-index is an indicator of the geomagnetic disturbance level in Earth’s mid- and

high-latitude magnetic field compared to a quiet day. 

Geomagnetic Storm/Space Weather Storm in the Earth’s Magnetosphere

Disturbances/Changes in Earth’s magnetic field due to changes in solar wind conditions 

typically lasting 3 to 6 days. 
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Kp Index

The Kp index indicates the magnitude of geomagnetic disturbance on a 0 to 9 scale, with 

zero being very quiet and 9 indicating a major geomagnetic storm. The index has a 3-

hour cadence. Higher values of Kp are associated with geomagnetic storming, the 

appearance of auroral lights at lower than normal latitudes, and stronger linkages between 

Earth’s upper atmosphere and magnetosphere. See also the “Kp Indices” Cygnet wiki 

page. 

Magnetosphere

The region of space dominated by the magnetic field of a star or planet. Earth’s 

magnetosphere takes on a tear-drop shape under the influence of the flowing solar wind.

Plasma

Plasma is a distinct phase of matter, separate from the traditional solids, liquids, and 

gases. It is a collection of charged particles that respond strongly and collectively to 

electromagnetic fields, taking the form of gas-like clouds. Since the particles in plasma 

are electrically charged (generally by being stripped of electrons), it is frequently 

described as an “ionized gas.” (http://physics.about.com/od/glossary/g/plasma.htm)

Space Weather

Describes the variable conditions in space, due to solar activity and the solar wind.
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14.0 Acronyms List

A Ampere

A/m2 Ampere per meter

AIAA American Institute of Aeronautics and Astronautics

Al Aluminum

B Magnetic Field Strength 

BRT Body Restraint Tether

BSC Body Seal Closure

CCG Capacitance of Solar Array Cover Glass

CEMU Capacitance of EMU Insulating Coating

CISS Capacitance of ISS Anodization

CAIB Columbia Accident Investigation Board

CCA Communications Carrier Assembly

CME Coronal Mass Ejection

DC Direct Current

DCM Display and Control Module

DMSP Defense Meteorological Satellite Program

EMU Extravehicular Mobility Unit

EPS Electrical Power System

EVA Extravehicular Activity

FDIR Fault Detection, Isolation, and Recovery

FP Floating Potential

FPMU Floating Potential Measurement Unit (operational on ISS from August 2006 

to present)

GAIM Global Assimilation of Ionospheric Measurements (ionosphere model)

gm gram

GMT Greenwich Mean Time

IGRF International Geomagnetic Reference Field

IRI International Reference Ionosphere (ionosphere model)

ISS International Space Station

ISSP ISS Program

jth Electron Thermal Current

JPL Jet Propulsion Laboratory

JSC Johnson Space Center

KSC Kennedy Space Center

kWh Kilowatt Hours

L Length of Conductor

LCVG Liquid Cooling and Ventilation Garment

LaRC Langley Research Center

LeRC Lewis Research Center

LOC Loss of Crew
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M&S Modeling and Simulation

mA Milliampere 

MDM Multiplexer/Demultiplexer

me Electron Mass

MMOD Micrometeoroid Orbital Debris

MMWS Modular Mini Workstation

mm Millimeter

ms Microsecond

msec Millisecond

MSFC Marshall Space Flight Center 

n Density

NCE NESC Chief Engineer

NCR Noncompliance Report

NESC NASA Engineering and Safety Center

NEXT NASA Evolutionary Xenon Thruster

NLP Narrow‐sweep Langmuir Probe (component of FPMU suite of plasma 

instruments)

NOAA National Oceanic and Atmospheric Administration

NRB NESC Review Board

NSTAR NASA Solar Technology Application Readiness 

O Oxygen

OBS Operational Bioinstrumentation System

ORU Orbital Replacement Unit

PT Total Probability

PCU Plasma Contactor Unit

PIM Plasma Interaction Model (Boeing/SAIC ISS charging model)

PRA Probabilistic Risk Assessment

rad Radiation Absorbed Dose

SAIC Science Applications International Corporation

SAPHIRE Systems Analysis Programs for Hands-on Integrated Reliability Evaluations

SARJ Solar Alpha Rotary Joint

SRP Safety Review Panel

SSP Space Shuttle Program

SWPC Space Weather Prediction Center (NOAA, source for space environment data)

T Temperature

TDT Technical Discipline Team

TVCIC Television Camera Interface Converter

U.S. United States

v Spacecraft Velocity Vector

V Volt

v × B • L Vector cross product of velocity and magnetic field
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WLP Wide‐sweep Langmuir Probe (component of FPMU suite of plasma 

instruments)

YVV Y-axis in the Velocity Vector

 Sigma

e Electron charge

ε0 Permittivity of free space

kB Boltzmann’s constant

λD Debye length

Ne Electron density

Te Electron temperature

Vf Floating potential

Vp Plasma potential

μA Microampere

15.0 References

1. Allen, J.E. (1992): “Probe Theory – The Orbit Motion Approach,” Physical Scripta. Vol. 

45, 497-503, 1992.

2. Anderson, P. (2005): “Spacecraft Charging Hazards In Low-Earth Orbit,” 9

th Spacecraft 

Charging Technology Conference, 2005.

3. Anon. (2004): ISS Electrical Power Systems Training Manual, ISS EPS TM 21109, 

Mission Operations Directorate, Space Flight Training Division, NASA Johnson Space 

Center, 2004.

4. Anthes, R. A. (2011): “Exploring Earth’s atmosphere with radio occultation: 

Contributions to weather, climate and space weather,” Atmos. Meas. Tech., 4, 1077–

1103, doi:10.5194/amt-4-1077-2011.

5. Biesecker, D.; Balch, C.; and McIntosh, S. (2013): “Where is Solar Cycle 24? Did it 

happen already? Is There More to Come?,” Presented at the NOAA 2013 Space Weather 

Workshop, Boulder, Colorado, 16-19 April 2013.

6. Boeing (2007): “ISS Safety Noncompliance Report,” The Boeing Company/Space 

Exploration/International Space Station, ISS-NCR-203 Rev. B, September 19, 2007.

7. Boeing (2009): “Discovery to Transport Last U.S., Boeing-built Starboard Truss Segment 

to Space Station,” January 2009. http://www.boeing.com/assets/pdf/defensespace/space/spacestation/components/docs/S6.pdf

8. Boeing (2013): PIMVar1_results.xls file, 2013a. 

9. CAIB (2003): Columbia Accident Investigation Board, Volume 1, August 2003.

10. Carruth, Jr., M.R., et al. (2001): “ISS and Space Environment Interactions without 

Operating Plasma Contactor,” AIAA-2001-401, Aerospace Sciences Meeting and 

Exhibit, 39th, Reno, Nevada, January 9-11, 2001.

11. Castillo, M. (2010): Modular Baseplate Assembly/Body Restraint Tether/Handrail 

Electrical Continuity Test, dtd. 05/04/10.
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12. Cho, M., K. Saito, and T. Hamanaga (2012): “Data Analysis of the Polar Plasma 

Environment for Spacecraft Charging Analysis,” Acta Astronautica, 81, 160-173, 2012.

13. Craven, P.D; Wright, Jr., K.H.; Minow, J.I.; Coffey, V.N.; Schneider, T.A.; J.A. Vaughn, 

Ferguson, D.C.; and Parker, L.N. (2009): Survey of International Space Station Charging 

Events, AIAA-2009-0119, 47th AIAA Aerospace Sciences Meeting and Exhibit, Orlando, 

Florida, January 5-8, 2009. 

14. Dalton, Mark P. (2004): International Space Station Nickel-Hydrogen Batteries 

Approached 3-Year On-Orbit, 2004.

15. Dong, S., et al. (2004): International Space Station Nickel-Hydrogen Extended Battery 

Discharge Model Analysis, 2004.

16. Duncan, G. (2013): Document DRD-MAPI-SA-06-ISSPRA-12-56, EVA Shock Update 

and Summary, International Space Station (ISS) Probabilistic Risk Assessment (PRA) 

Trade Study - Long Form, ISS-PRA-12-56 (Probability Risk Assessment Doc), Prepared 

by Gary Duncan, dated May 17, 2013.

17. Evans, D. S. (2012): “A Study of Intense Auroral Electron Precipitation Events,” Space 

Environments Laboratory, NOAA, Boulder, Colorado, personal communication, 2012.

18. Gworek, P.; and Hansen, H. (2002): “EMU - Plasma Arc Update”, Presentation to CCB, 

Hamilton Sundstrand Space Systems International, Inc., April 3, 2002.

19. Hamilton, D. and Kramer, L. (2007): “Simple Transient Circuit Simulation of EMU 

Touching Solar Array (Bird on a Wire....NOT!!!!),” page 37, August 29, 2007.

20. Hartman, D. (2013a): Plasma Hazard Relief Assessment for US EVA 22, ISS-HOU-ENVWAH-130032, 25 June 2013.

21. Hartman, D. (2013b): Plasma Hazard Relief Assessment for US EVA 23, ISS-HOU-ENVWAH-130035, 2 July 2013.

22. Hartman, D. (2013c): Extension of Plasma Forecasting, Boeing Space Environments, 

2013.

23. ISS-EVA-312-AC (2012): Electric Shock to EVA Crew Resulting from EMU Arcing in 

Plasma, 1/26/2012.

24. ISS-NCR-232F (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low 

Earth Orbit Plasma Environment, 1/31/2012. 

25. ISS-NCR-232G (2013): Lack of Two-fault Tolerance to EVA Crew Shock in the Low 

Earth Orbit Plasma Environment, NCR-20264-R7, 18 September 2013.

26. Kaminski, R.; and Scudder, M. (2013): Boeing ISS EPS, September 2013.

27. Koontz, Steve (2013): “PCU_emission_currents_2010_through_2013.xlsx,” private 

communication.

28. Kramer, L.; Hamilton, D.; Mikatarian R.; Thomas J.; and Koontz, S. (2010): “Positive 

Voltage Hazard to EMU Crewman from Currents through Plasma,” Proc. 4th IAASS 

Conference ‘Making Safety Matter’, Huntsville, Alabama, USA, 19–21 May 2010 (ESA 

SP-680, September 2010).

29. Kichak, R., E. Young, C. Pandipati, and R. Cooke, International Space Station (ISS) 

External Television (TV) Camera Shutdown Investigation, NASA TM-2009-215572, 

NESC-RP-06-49/06-001-E, February 2009.
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30. Mikatarian, R. (2010): Operation of the FPMU to Support Plasma Hazard Assessments, 

FINAL-ShortVer-2010-04-13-SSPCB-FPMU-Requirements-revK.pdf, April 13, 2010.

31. Mikatarian, R.; Schmidl, D.; Hartman, D.; Kramer, L. (2013): Plasma NCR 232 Update, 

SSPCB, Boeing Space Environments, October 1, 2013. Initial Draft: July 2013.

32. Minow, J. I., and L.N Parker (2013): “Space Weather Monitoring for ISS Geomagnetic 

Storm Studies,” Space Weather Workshop (invited), Boulder, Colorado, 16-19 April, 

2013.

33. Minow, J.I. (2004): “Development and Implementation of an Empirical Ionosphere 

Variability Model,” Adv. in Space Res., 33, 887-892, 2004.

34. Minow, J.I.; Pettit, D.R.; and Hartman, W.A. (2012): “Space Weather Monitoring for ISS 

Space Environments Engineering and Crew Auroral Observations,” Abstract IN31D-05 

presented at 2012 Fall Meeting, AGU, San Francisco, California, December 3-7, 2012.

35. Minow, J.I.; Wright, Jr., J.H.; Chandler, M.O.; Coffey, V.N.; Craven, P.D.; Schneider, 

T.A.; Parker, L.N.; Ferguson, D.C.; Koontz, S.L.; and Alred, J.W. (2010): Summary of 

2006 to 2010 FPMU Measurements of International Space Station Frame Potential 

Variations, 11th Spacecraft Charging Technology Conference, Albuquerque, New 

Mexico, 20 - 24 September 2010.

36. Moorehead, R.W., Deputy Director (1992): Space Station Freedom Program and 

Operations, communication to Work Packages 1-4 Directors, dated April 3, 1992.

37. NASA (2004): A Renewed Commitment to Excellence: An Assessment of the NASA 

Agency-wide Applicability of the Columbia Accident Investigation Board Report, 

PB2005-100968, January 2004.

38. NASA (2009): Assessment of International Space Station (ISS) Program Plasma 

Contactor Unit (PCU) Utilization, NESC Report NESC-RP- 07-054, May 14, 2009.

39. NASA (2013a): NASA-STD-7009 Standard for Models and Simulations, July 10, 2013.

40. NASA (2013b): Press Release 13-193 “NASA Thruster Achieves World-Record 5+ 

Years of Operation.” June 24, 2013.

41. NOAA (2014), Space Weather Prediction Center, URL: 

http://www.swpc.noaa.gov/ftpdir/lists/iono_day/README, accessed 21 May 2014.

42. Pesnell, W.D. (2008): “Predictions of Solar Cycle 24,” Solar Phys., 252, 209-220, 2008.

43. Pesnell, W.D. (2012): “Solar Cycle Predictions (Invited Review),” Solar Phys., 281, 507-

532, 2012. 

44. Rayman, M.D. (2003): “The Successful Conclusion Of The Deep Space 1 Mission: 

Important Results Without a Flashy Title,” Space Technology 23, Nos. 2-3, p. 185, 2003.

45. Rocken, C., Kuo, Y.H.; Schreiner, W.; Hunt, D.; Sokolovskiy, S.; and McCormick, C. 

(2000): “COSMIC system description,” Terr. Atmos. Oceanic Sci., 11, 21–52.

46. Roeschel, E. (2013): “NESC_ISS_Shock_EVA_Actions.pptx” 6-25-13.

47. Sarver-Verhey, T.R. (1997): “28,000 Hour Xenon Hollow Cathode Life Test Results,” 

IEPC-97-168, 25th International Electric Propulsion Conference, Cleveland, Ohio, 

August 24-28, 1997.

48. Schmidl, D. (2013a): EVA Shock Update and Summary PRA Report ISSPRA-12-56, 

NASA JSC, May 7, 2013, DRD-MAPI-SA-06-ISSPRA-12-56.
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49. Schmidl, D. (2013b): Plasma Hazard Relief Assessment for US EVA 21, ISS-HOU-ENVWDS-110018, 10 May 2013.

50. Schneider, T.A.; Carruth, M.R., Jr.; and Hansen, H.J. (2012): “Minimum Arc Threshold 

Voltage Experiments on Extravehicular Mobility Unit Samples,” AIAA Paper 2002-

1040, 40th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 14-17, 

2002.

51. Schreiner, W.; Rocken, C.; Sokolovskiy, S.; Syndergaard, S.; and Hunt, D. (2007): 

Estimates of the Precision of GPS Radio Occultations from the COSMIC/ FORMOSAT3 Mission, Geophys. Res. Lett., 34, L04808,doi:10.1029/2006GL027557.

52. Sengupta A.; Brophy J.R.; Anderson J.R.; and Garner C. (2004): “An Overview of the 

Results from the 30,000 Hour Life Test of the Deep Space 1 Flight Spare Ion Engine,” 

AIAA Paper 2004-3608, 40th Joint Propulsion Conference, Ft. Lauderdale, Florida, July 

11-14, 2004.

53. Smith, C.L. and Wood S.T. (2011a): “Systems Analysis Program for Hands-on Integrated 

Reliability Evaluations (SAPHIRE) Version 8, Volume 1: Overview and Summary,” 

2011, United States Nuclear Regulatory Commission, NUREG/CR-7039, Vol. 1.

54. Smith, C.L. and Wood S.T. (2011b): “Systems Analysis Program for Hands-on Integrated 

Reliability Evaluations (SAPHIRE) Version 8, Volume 1: Technical Reference,” 2011, 

United States Nuclear Regulatory Commission, NUREG/CR-7039, Vol. 2.

55. Space Station Program Office (1994): Space Station Ionizing Radiation Design 

Environment, Revision C, 3, SSP 30512, June 1994. 

56. Space Systems/Loral (1998): http://sslmda.com/downloads/products/ispacest.pdf

57. Stamatelatos, M. and Dezfuli, H. (2011) Probabilistic Risk Assessment Procedures Guide 

for NASA Managers and Practitioners, Second Edition, 2011, NASA/SP-2011-3421.

58. Swenson, C.; and D. Thompson (2002): “FPMU Systems Overview,” presented at FPMU 

CDR, February 19-20, 2002.

59. Thomas, J.E. (2012): Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth 

Orbit Plasma Environment, The Boeing Company/Space Exploration/ International 

Space Station, Joseph E. Thomas (Document Originator), ISS-NCR-232F, 1/26/2012.

60. Utah State University (2002): SDL 2002, FPMU Limited-Life Items List, SDL/02-037, 

Space Dynamics Laboratory, Utah State University, 7 February 2002.

61. Wright, Jr., K.H.; Swenson, C.; D. Thompson, D.; A. Barjatya, A.; S. L. Koontz, S.L.; T. 

Schneider, T.; Vaughn, J.; Minow, J.; Craven, P.; Coffey,V.; Parker, L.; and Bui, T. 

(2007): “Initial results from the Floating Potential Measurement Unit aboard the 

International Space Station,” 10th Spacecraft Charging and Technology Conference, 

Biarritz, France, June 18-21, 2007.

62. Wright, et al. (2008): “Charging of the ISS as Observed by the FPMU: Initial Results,” 

IEEE Transactions on Plasma Science, Vol. 36, No. 5, October 2008.

63. Wright, et al. (2009): Wright, K.H., FPMURapidChargingEvents_KHW_2009Mar02.xls 

file, 2009.
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16.0 Appendices

Appendix A. Human Current Safety Limits

Appendix B. Overview of Plasma Shock Hazard to EVA Crew

Appendix C. ISS-NCR-232G Review

Appendix D. Tools and EMU Hardware Presentation

D.1 NESC_ISS_Shock_EVA_Actions

D.2 Modular Baseplate Assembly/Body Restraint Tether/Handrail 

Electrical Continuity Test

Appendix E Additional EMU Pictures

Appendix F. FDIR Reference Emails

Appendix G. Maximum Magnetic Induction Potential Along ISS Truss

Appendix H. International Space Station Electrical Power Systems Training Manual ISS EPS 

TM 21109 (Section 2.3.4)

Appendix I. International Space Station (ISS) Plasma Contactor Unit (PCU) Utilization Plan 

Assessment Update: Key Points Summary

Appendix J. EMU Team Email 

Appendix K. Current Flow to EMU in Electrical Contact with +15V ISS Surface
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Appendix A. Human Current Safety Limits
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Appendix B. Overview of Plasma Shock Hazard to EVA Crew
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Appendix C. ISS-NCR-232G Review

REVIEWER/Date: Albert Whittlesey, JPL, 2/13/14,

member of NESC ISS Plasma Contactor Unit (PCU) Utilization Plan Assessment Team

(shortened to "NESC Team,” or usually "The Team" in this review).

COMMENTS TO:

REPORT NUMBER: ISS-NCR-232G

REPORT TITLE: Lack of Two-fault Tolerance to EVA Crew Shock in the Low Earth Orbit 

Plasma Environment

REPORT AUTHOR: The Boeing Company Space Exploration International Space Station,

DATE OF ISSUE: Sept. 27, 2013, signed by Scott I. Wolf

Background/Introduction:

Block 12: Section A. Applicable Requirement:

SSP 410001 System Specification for ISS

Paragraph 3.3.6.1.1.1 1 Catastrophic Hazard

The on-orbit Space Station shall be designed such that no two failures, or two operator errors 

(see 6.1 ), or one of each can result in a disabling or fatal personnel injury, or loss of one of the 

following: Orbiter or ISS.

SSP 41162 Safety Requirements for ISS

Paragraph 3.3.6.1.1.1 1 Catastrophic Hazards

The USOS shall be designed such that no two failures, or two operator errors (see 6.1 ), or one 

of each can result in a disabling or fatal personnel injury, or loss of the Orbiter or ISS.

No comment.

Block 13: Section B. Description of noncompliance: (specify how the design or operation does 

not meet the safety requirements):

NEGATIVE POTENTIALS, Cause 1.

I have difficulty reading and interpreting the cases involved and how to read and 

understand Attachment 1 as it supports the text in this Block/Section. In spite of that, I 

make the following observations.
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1. The second row of numbers is 38.8, 29.7, 25.7, and 33.5

From looking at Attachment 1, it looks like the second number should be 29.8:

33.8, 29.8, 25.7, and 33.5

It does not matter in a technical sense but it hinders my understanding of what I should be 

reading as support for the text.

2. Why are the two numbers 40.8 and 40.7 highlighted in red? It can't be because they 

exceed the 45.5V requirement. I think it is because they are between 40 and 45.5V, as 

noted in attachment 1. I think a yellow background highlight of the numbers would have 

been more meaningful. Also see "Block 15: Section D, paragraph 10: "At the 1/14/09 SRP, a 

risk acceptance point of -45.5V was agreed upon by the Panel as a final non-negotiable limit 

for the negative potential." There appears to be an inconsistency between -40V and -45.5V

as a hazard limit. Which is correct?

3. "The largest accepted charging violation is -45.5V.” 

Is it permissible to have a waiver based on a prior waiver? In that case, the 40.8 and 40.7V

would be permissible on a waiver basis.

4. Apparently the numbers shown in Attachment 1 were generated by PIM3.0. The NESC 

Team has difficulties with PIM3.0, based on the lack of documentation of the code itself, 

and the input parameters used for any given use of the code. I don't know if the numbers in 

attachment 1 are adequately thus documented in the attachment 1 reference, EID684-

13598, Rev. B (not examined).

POSITIVE POTENTIALS, Causes 2 & 3.

1. Attachment 2 notes that ISS potentials near the truss extremities can reach +11.9V per 

EID684-15543. The two paragraphs note that these voltages could "create a shock hazard,”

and there are "no certified controls to protect against this hazard.” Furthermore, this 

section states that "The EMU is not designed or certified to insulate against electric shock 

per hazard report EMU-018." Specifically, the "most likely path is between the Modular 

Mini Workstation (MMWS) and the Display and Control Module (DCM). We have been told 

repeatedly that most of the EMU metallic parts visible on the surface of the EMU are now 

carefully covered before and EVA and thus cannot be a current contact point. Additionally, 

the MMWS is isolated as a possible current flow path into the astronaut (see Attachment 5 

as an example). As the team understands it, the only metallic outer path into the astronaut 

either from a galvanic contact or from a plasma connection is the (anodized) neck ring or 
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other parts of the headgear, none of which permit a current path through the thorax of the 

astronaut, which is the most sensitive path for shock hazard.

2. "Bird on a Wire" (Kramer, 2007) uses certain curves to estimate the possible plasma 

current into exterior metal parts of the EMU (pages 37-38). The Team has examined these 

curves and found that alternate conservative estimation equations to estimate thorax 

current are more appropriate and have been validated by the FPMU, that substantially 

reduce the estimated currents from a neck ring and the current thus calculated current no 

longer exceeds the applicable safety limits (Katz, et al., 2013), even when positive voltages 

are as high as 15V (calculated only to +15V, but the positive voltage can be higher and still 

be safe).

Block 14: Section C, Reason requirement cannot be fulfilled:

NEGATIVE POTENTIALS, Cause 1.

See above for rationale why -40V can be exceeded.

POSITIVE POTENTIALS, Causes 2 & 3).

See above for rationale why Positive potentials will not be a problem.

Block 15: Section D Acceptance Rationale

NEGATIVE POTENTIALS, Cause 1.

1. The present depressed Solar Cycle 24 is limiting charging levels on ISS due to the hotter electrons collecting on 

the solar array cover glass and producing a potential barrier. This barrier prevents electrons from collecting in the 

solar array gaps and charging the ISS. The Space Environments community has concluded that, based on the 

downward trend of recent Solar Cycles, the environment will remain benign at least through Solar Cycle 25, which 

extends through 2030. FPMU measurements since 2007 have indicated no ISS charging in excess of -45V.

Prior attempts to predict the magnitude of any given solar cycle have failed sometimes to a 

great degree. Basing future estimates of the future charging of the ISS on this basis is folly. 

In any given solar cycle regardless of how strong it is, can have one or more large sunspots 

that can create huge ISS charging events, even if the cycle itself if generally low.

Making environmental ISS potential predictions on this basis for another 17 years is quite 

unwise.
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2. The ISS floating potential will be verified by a "short-term plasma forecast,” issued 14 days prior to a planned 

EVA. For the negative potential hazard for EVAs that are conducted entirely in-board of the SARJ. PCUs can be 

placed in discharge even though the ISS charging environment in the current

depressed solar cycle does not require it. Because the PCUs are optional, enabling the autoshunt FDIR is not 

required.

Assuming that the environment will be the same 14 days from what is today, although 

generally true, is not adequately true to always use that estimate as gospel for the actual 

day of a planned EMU. The Team agrees that a FDIR is not an appropriate control (but for 

other reasons).

3. The "short-term plasma forecast" assessments: (1) utilize planned EVA solar array positions, vehicle attitude,

etc. (2) use "short-term" in-situ ionospheric FPMU measured plasma properties to assess present state of 

ionosphere (e.g., to determine if it is a nominal or +1- 1 or 2 environment as compared to the International 

Reference Ionosphere II AI\ model\, and (3) are based on the assumption that the ionosphere will not undergo 

significant changes over a period of a few weeks (assumption confirmed with considerable FPMU data). In 

addition, the forecasting process includes space weather solar events (i.e., enhanced solar activity, CMEs, severe 

solar flares) and are addressed/monitored in real time.

This section does not include a reference as to where this forecast process is documented.

Utilizing the estimate that the ionosphere will not undergo significant changes over a 

period of a few weeks has not yet been verified to be a true assumption. The only way to 

properly verify the ISS state of charge on the day of the EVA is to use the FPMU. The 

forecasting process uses the data inputs as described above and then computes the ISS 

potentials based on a computer code called "PIM3.0.” PIM3.0 has been shown to have 

unexplained differences that are far beyond the 1 or 2 variance when the calculated 

potentials are compared to the FPMU measurements. At present, the PIM3.0 code as not 

been adequately validated (NASA coding standards for its use as a personnel hazard 

protection).

4. It should be noted that certain events that occur after the forecast is issued may invalidate the "short-term 

forecast" (e.g., the solar array plan changes, reboosts. Debris Avoidance Maneuvers). Also, if an event occurred 

that was not anticipated after the forecast was issued. The "short-term plasma forecast" would be declared Invalid. 

Reference Flight Rule 89-908 Plasma Hazard Mitigation During EVA.

The team agrees with this statement. The prior concerns still apply.

Paragraphs 5-9 in this section deal with "short-term" (meaning on the order of 14 days) 

"forecast predicts" (meaning calculations of today's ISS charging potentials). They suggest 

that for extraordinary circumstances raising the ISS potentials above those deemed 

acceptable, turning on the PCUs and executing the "autoshunt function" (meaning FDIR?) 

will be used to control ISS potentials. The team agrees that turning on the PCUs is 
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appropriate but disagrees that the FDIR is the appropriate third control for controlling ISS 

potentials to less than hazardous potentials.

Paragraph 10, stating that -45.5V has been accepted as a non-negotiable risk has been 

earlier noted that it is not consistent with an earlier implication that -40V is still the limit 

for non-hazardous ISS potentials.

Paragraph 11, the probabilistic risk assessment summary, is based on ISSPRA-12-56 and 

summary probability numbers are shown in Attachment 3. A PRA expert has examined -

12-56 at the request of The team and notes that at best, the -12-56 PRA is not adequately 

documented to determine whether its results are consistent with input assumptions, nor is 

there enough detail/transparency to verify the accuracy of the stated outcome 

probabilities.

DC POSITIVE POTENTIALS, Cause 2.

Placing the PCUs In discharge produces positive potential hazard in+ 10 to+ 12V range 

outboard of SAAJ (i.e., catastrophic hazard). Without PCUs in discharge potential at the truss,

tips may experience only + 1 to +2 volts.

The team has examined the basis for report's statement of "catastrophic hazard.” The team

has used newer plasma physics equations as reported separately in this report showing 

that more exact equations sometimes called "2D,” still with generous margins, show much 

lower possible plasma accumulation currents than were reported in "Positive Voltage 

Hazard...." (Kramer et al., Sept 2010). With as much as +15V potential on the ISS structure, 

the astronaut's EMU currents will be much less than the Kramer calculations show for 3V, 

and are nominally safe by the hazard curves of "Bird on a Wire" by Hamilton and Kramer, 

August 29, 2007, slides 11-17.

For the positive potential hazard, the PCUs will not be put into discharge for all EVAs out-board 

of the SARJ. The short-term forecast will be utilized to verify the ISS floating potential 

environment and in the event of hazardous charging levels that necessitate PCU use during the 

EVA. ISS will be maneuvered to a YVV attitude which eliminates the hazard. If YVV is 

undesirable for technical reasons or there is insufficient time to change ISS attitude and there is 

significant programmatic risk in delaying the EVA, the rationale below can be utilized:

The team disagrees with the rule to not put the PCUs into discharge. The team, by contrast, 

believes that the best policy is to put the PCUs into discharge during the full EVA. The team

recommends that the "short-term forecast" is OK for initial planning, but needs 

supplemental ISS charging determinations as the EVA nears and during EVA.
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In order for the circuit to be completed, several events must occur simultaneously: (1) The EVA 

crewmember must be at a positively charged location on the ISS truss; (2) The EMU must make 

galvanic contact with ISS; (3) The exposed bare metal of the EMU must be collecting charge 

from the ionosphere; (4) The crew must make galvanic contact with bare metal in the EMU 

interior; and (5) The overall circuit impedance must be low enough to allow a harmful current 

level. These five events are discussed individually below. {with AW's comment about each}

1) The VxB.L potential is only at outboard locations and varies with the orbit.

The team agrees.

2) The medical team assessed possible locations of electric shock on January 12-13, 2009, 

with a number of points of possible galvanic contact. They are shown in attachment 4.

The NESC team has been led to believe that very few, if any, of the stated possible locations 

of possible galvanic contact exist after suit modifications. The team supposedly has been 

provided with up-to-date information which is odd, since the NCR -232G is dated Sept 17, 

2013. This discrepancy needs investigation.

A Probabilistic Risk Assessment (PRA) (ISSPRA-12-56, May 7, 2013) was performed with 

the suspect metallic contact regions included as part of the relatively risk-ratings (before 

and after MMWS modification) shown in that document.

The team, again, believes that the galvanic contact regions assumed in the PRA are 

inappropriate and outdated and the PRA at the very least needs redoing with new 

assumptions. Additionally, the team had the -12-56 report reviewed by a PRA expert, who 

found its contents to be unreviewable due to lack of completeness. For human safety 

ratings, one would expect better.

3) 232G suggests that the Body Seal Closure, the Mini Workstation, the Body Restraint 

Tether, and the waist ring, all of which total collecting area sums to 0.8 m^2.

The team again finds this a large area, more closely fit by 0.3 m^2.

4) 232G assumes good galvanic contact inside the suit to the astronaut by sweat-soaked 

undergarments and LCVG.

The team agrees.
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5) 232G notes that the magnitude of current through a crewmember body depends on the 

body impedance.

The team agrees, and believes this is built into the safety limit curves in various locations.

6) 232G notes that the MMWS has been modified to isolate the MGA and swings from the 

baseplate, but only suggests "a significant reduction in the current level .....” See attachment 

5 for isolation modifications and Attachment 6 for the pre- and post-modification current 

levels.

The team notes that the -232G is not very clear, is difficult to read, and thus is subject to 

uncertainties. For one example, 3) above notes that a total collecting area was calculated to 

be 0.8 m^2, but the Tables in Attachment 6 have at most 0.3 m^2 collecting areas in the

tables, even for "before MMWS modification.” As another example, Attachment 4 has a 

diagram of "External EMU Metal Surfaces" that is not compatible with the separate text in 

the PRA -12-56 (which has at least two additional possible external ISS contacts: CCA 

Connector?-what is this?; and OBS/DCM). As a third example, we are told numerous times 

that the EMU has had numerous modifications, and yet in Attachment 5, only the two 

MMWS components are described. The report would have been better served if each of the 

9 external contact points in -232G Attachment 4 (11 external contact points used in the 

PRA -12-56 Table 3) had been listed in a table, showing the original non-isolated condition, 

and the post-isolation condition and what the improvement was (ohms before and ohms 

after), and when it was implemented.

TRANSIENT CAPACITIVE DISCHARGE POSITIVE POTENTIALS, Cause 3.

The likelihood of manifesting the +transient capacitive discharge current is comparable to that 

of the +DC current. Likewise, the MMWS modifications provide mitigation for this hazard as 

well as the +DC hazard by removing the largest and most likely contact point from the 

capacitance circuit. Further mitigation of this hazard in the Assembly Complete ISS 

configuration can be provided by taping the Operational Bioinstrumentation System (OBS) 

connections inside the EMU with Kaplan to electrically isolate the crewmember from the EMU 

single-point ground (Ref. CR EVA-01168).

The team has not heard specifically if the OBS connector inside the EMC is normally taped with 

Kaplan (sic) (?Kapton®). The team has been told that all possible galvanic connections on the 

outer surface of the EMU are covered with fabric flaps or are taped (with the possible exception 

of the neck ring or other head area connections). The team has not seen a specific list of regions 

that are non-compliant to the general claim of "no galvanic connections from outside the EMU to 

the astronaut.”
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PRA Updates and Results

The P6 lEA battery R&R task performed on Flights 2J/A and ULF4 represents a "worst-case" 

EVA from an exposure standpoint. The TCS jumper installations, venting and refill of the P6 

PVTCS radiator on Flight ULF6 represent a comparable level of exposure. For analysis 

purposes, it was estimated that approx. 80% of such an EVA would be spent outboard of the 

P1/P3 interface, i.e., 5:12 of a 6:30 EVA duration. This is reflected in the PRA calculations 

(Ref. ISSPRA-12-56). See Attachment 3 for the PRA event flow model and PRA results.

The team has not examined this situation and cannot comment.

The PRA was updated to account for the +transient capacitive discharge hazard as well as the 

mitigation provided by modifications made to the Modular Mini Workstation (MMWS) to 

electrically isolate it from the Baseplate/BSC. For the Assembly Complete ISS configuration, the 

PRA also modeled the mitigation provided by isolating the OBS connections inside the EMU to 

prevent contact with the crewmember. The results of the updated PRA are as follows (numbers 

are rounded):

+Transient Capacitive Discharge Hazard

(A) The mean probability of a shock event for 1 crew member on a single EVA is 5.11 E-05 (1-in19.573).

(B) The mean probability of a shock event for 1 crew member on a single EVA with the OBS 

isolated is 7.00E-07 (1-in-1.428.367).

+DC Hazard

(C) The mean probability of a shock event for 1 crew member on a single EVA is 4.75E-05 (1-in21,075).

(D) The mean probability of a shock event for 1 crew member on a single EVA with the OBS 

isolated is 6.63E-07 (1-in-1 .509,206).

The Baseline EVA Risk from all other hazard causes for 2 crew members on a single EVA is 

3.86E-05 (1-in-25.920).

The team obtained the services of a senior person with excellent PRA credentials and asked that 

person to review the -12-56 PRA.

That PRA expert did not have the time to adequately read and verify the total product. In fact, 

The team was only given a brief summary of the appearance of the document as it appeared to 

him. That report is provided in another section of The team's report (of which this section is a 

part). The summary was that it was difficult to properly track and validate the report's contents. 

However, it did not appear to provide total auditable verification of the results reported (numeric 
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probabilities and error bars of the occurrence of various events). Numeric outputs of the PRA -

12-56 are copied into the probabilities locate immediately above this paragraph.

Further, not to quote the PRA expert, but if the team or the ISS wishes to use this PRA (and -

232G quotes the PRA extensively to support its conclusions), then the PRA -12-56 should also 

have a good peer review to validate its assumptions (including basic probability assumptions) 

and proper use of the specific PRA computer code recognized by the Team's PRA expert).

In conclusion, while there are multiple current paths through the EMU/crewmember that can 

result in catastrophic effects if the circuit is established, modifications to external conductive 

EMU equipment have reduced the current associated with the +DC and +transient capacitive 

discharge hazards

The likelihood of occurrence is comparable to other previously accepted risks.

This conclusion, although weakly stated, is the same one reached by the team: a re-assessment of 

the risk during an EVA is much reduced because of the changed EMU suit design to isolate most 

of the possible current attachment paths into the body of the astronaut, and by comparison to 

other previously accepted risks.

The team's additional recommendations to operate the PCUs during an EVA (two hazard 

controls); and not use the EVA shunt FDIR logic (possible hazardous FDIR responses in some 

situations); and to treat the EMU's isolation modifications as a third hazard control, all are 

compatible with the -232G conclusion above.
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Appendix D. Tools and EMU Hardware Presentation

D.1 NESC_ISS_Shock_EVA_Actions

D.2 Modular Baseplate Assembly/Body Restraint Tether/Handrail 

Electrical Continuity Test
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D.1 NESC_ISS_Shock_EVA_Actions
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D.2 Modular Baseplate Assembly/Body Restraint Tether/Handrail 

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Appendix E. Additional EMU Pictures
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Appendix F. FDIR Reference Emails
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Appendix G. Maximum Magnetic Induction Potential Along ISS 

Truss

Inductive potential differences exist between two points on the ISS metallic structure due to 

motion of the vehicle across the Earth’s magnetic field. The magnitude of the potential 

difference induced between two points separated by a distance L is given by the vector equation

induced = (v x B) •L (1)

where v is the ISS velocity and B the Earth’s magnetic field strength at the location of ISS. 

Values of induced are small near the equator where the dominant component of the Earth’s 

magnetic field vector lies along the direction of the ISS truss (in the typical +/-XVV flight 

attitude) and the dot-product between v x B and the vector components of L along the Truss is 

small. The extremes in potential difference between the ends of the ISS truss due to magnetic 

induction will occur at high latitudes where geomagnetic field lines are steeply inclined relative 

to the Earth’s surface (and the ISS truss in typical flight attitudes), maximizing the v x B

components along the length of the truss. In this case, the vector equation can be reduced to the 

scalar form 

induced = vBrLT (2)

where the ISS velocity v is assumed to be parallel to the Earth’s surface, Br is the radial 

component of the Earth’s magnetic field, and LT is the length of the ISS Truss.

The ISS coordinates of the Truss tips are (D. Schmidl, personal communication, 2013):

 Starboard Truss Tip X = +0.73 meters

 Y = +47.15 meters

 Z = +0.73 meters

 Port Truss Tip X = +0.02 meters

 Y = -47.13 meters

Z = +0.73 meters 

The distance between the Truss tips is LT = (+47.15 meters + 47.13 meters) = 94.28 meters along 

the y-axis. The small contribution from the different locations of starboard and port tips in the xdirection has been neglected for this analysis.

Variation in ISS velocity as a function of altitude can be estimated from the equation for velocity 

of a circular orbit:

v = √μ⁄(z + RE) (3)

where  = 3.986x1014 m3/s2and z+RE is the geocentric radial distance of the circular orbit at 

altitude z above the mean Earth radius RE =6371 km. For example, ISS orbital velocity at an 

altitude of 400 km is 7673 m/s assuming the orbit is circular.
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Magnetic field intensity also varies as a function of altitude with the field intensity decreasing 

with increasing altitude. Numerical Br magnetic field component values are conveniently 

obtained from NASA’s Community Coordinated Modeling Center’s implementation of the 

International Geomagnetic Reference Field (IGRF) model 

(http://ccmc.gsfc.nasa.gov/modelweb/models/igrf_vitmo.php). 

The values used here are obtained from the IGRF model for the current year (2014) at latitudes 

of +51.6 degree in the northern hemisphere and -51.6 degree in the southern hemisphere. 

Because the magnetic field intensity varies with longitude, the model was run as a function of 

longitude between 0 degree longitude and 360 degree longitude in 1-degree increments to find 

the maximum value of the radial magnetic field component in order to estimate the worst case 

induction potential along the ISS orbit. 

Maximum IGRF Br magnetic field components in the northern and southern hemisphere and 

orbital velocity values from equation (3) as a function of altitude are listed in Table G-1 along 

with the corresponding magnetic induction potential between the ISS Truss tips computed from 

equation (2). The distance 94.28 meters is used in all calculations.

Table G-1. Maximum Induction Potential Between ISS Truss Tips

Altitude 

(km)

ISS 

Velocity 

(m/s)

Northern

Hemisphere

Southern Hemisphere

Br (nT) induced Br (nT) induced

330 7713 48046.1 34.9 55271.7 40.2

340 7707 47800.4 34.7 55002.5 40.0

350 7701 47556.3 34.5 54735.1 39.7

360 7695 47314.0 34.3 54469.4 39.5

370 7690 47073.3 34.1 54205.4 39.3

380 7684 46834.3 33.9 53943.2 39.1

390 7678 46596.9 33.7 53682.6 38.9

400 7673 46361.2 33.5 53423.8 38.6

410 7667 46127.0 33.3 53166.6 38.4

420 7661 45894.5 33.1 52911.1 38.2

Extreme inductive potential differences of approximately 40V between the tips of the ISS Truss 

may occur when the ISS orbital altitude is low. For example, ISS orbital altitudes were allowed 

to drop to approximately 335 km during 2001 and again in 2007. Mean ISS orbital altitudes in 

2014 have all exceeded 400 km with typical mean altitudes between 413 km and 418 km. A 

good estimate of the extreme inductive potential difference between the Truss tips for current 

ISS altitudes reported to the nearest volt is therefore 38V. 
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Appendix H. International Space Station Electrical Power Systems 

Training Manual ISS EPS TM 21109 (Section 2.3.4)
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Appendix I. International Space Station (ISS) Plasma Contactor 

Unit (PCU) Utilization Plan Assessment Update: Key Points 

Summary
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Appendix J. EMU Team Email 

From: "Boyle, Robert M. (JSC-EC511)" <robert.m.boyle@nasa.gov>

Date: June 13, 2014 at 5:08:18 PM CDT

To: "Hansen, Christopher P. (JSC-EC111)" <christopher.p.hansen@nasa.gov>, "Blanco, Raul A. 

(JSC-EC511)" <raul.a.blanco@nasa.gov>

Cc: "West, T. Scott (JSC-C105)" <timothy.s.west@nasa.gov>

Subject: RE: EMU/plasma shock hazard

I agree with conclusions in the executive summary and the recommendations in section 8. I reviewed 

sections 7.9 – 7.13 in detail, and had the following minor comments. They can be ignored if desired, it 

will not change the report conclusions.

In Table 7.9-1 the Body Seal Closure/MMWS Connection coating is noted as anodize. The parts are 

Stainless Steel. There is a caveat noting the coating data is suspect and not used in the calculations.

In Table 7.9-1 I don’t understand why the DCM and OBS are lumped in one row. Totally different 

hardware.

The baseplate and MWS probably protect the SS bosses (MMWS Connection) from contacting the 

tether, but the statement that the baseplate is isolated seems to ignore the exposed SS.

It was a very educational read. Thanks. Good job.

Rob

From: Hansen, Christopher P. (JSC-EC111) 

Sent: Tuesday, June 03, 2014 4:06 PM

To: Boyle, Robert M. (JSC-EC511); Blanco, Raul A. (JSC-EC511)

Subject: FW: EMU/plasma shock hazard

 

Here’s the NESC report on the PCU hazard. Scott West (NESC Chief Engineer for JSC) asked that we take 

a look at the EMU sections and let him know if we agree with them. 

Chris

 _____________________________________________

From: West, T. Scott (JSC-C105) 

Sent: Tuesday, June 03, 2014 10:19 AM

To: Hansen, Christopher P. (JSC-EC111)

Subject: RE: EMU/plasma shock hazard

Thanks Chris. I’ve enclosed the whole draft report, but sections 7.9 – 7.13 are the main sections to look 

at for the EMU in relation to the shock hazard. The appendices are also there with the EMU information 

that was presented to the assessment team that they used to do their analysis. And yes this was being 

looked at relative to running/not running PCUs and also enabling or not enabling the shunt FDIR. The 

first part of the report should help provide some context.
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A reply from team member Ira Katz to the below Rob Boyle comment.

The baseplate and MWS probably protect the SS bosses (MMWS Connection) from contacting the 

tether, but the statement that the baseplate is isolated seems to ignore the exposed SS.

From: Katz, Ira (353B) [mailto:ira.katz@jpl.nasa.gov] 

Sent: Tuesday, June 17, 2014 11:47 AM

To: Hernandez-Pelle, Amri I. (GSFC-5630); Schneider, Todd A. (MSFC-EM50); Moran, Erin (LARCC101)[TEAMS2]

Subject: RE: REPORT FINAL COMMENTS 

AmriI looked through the 2 suit presentations “Baseplate_-_BRT_Continuity_Test_Summary(1)” and 

“NESC_ISS_Shock_EVA_Actions” and the best I can interpret Rob Boyles comments are that there is 

some exposed stainless steel that actually can contact some of the metal, either anodized aluminum or 

stainless steel, inside the suit. However, they have a very small area, so they wouldn’t have a big effect 

on either the probability or current collection calculations. I think this metal was included in the Boeing 

PRA. Rob Boyles’s conclusion ““The baseplate and MWS probably protect the SS bosses (MMWS 

Connection) from contacting the tether,...” is basically our conclusion. I’d have to review the physical 

hardware with him in person if you need a better answer. I’m afraid this is the best I can do with emails 

and PowerPoint presentations.

Thanks - ira
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Appendix K. Current Flow to EMU in Electrical Contact with +15V 

ISS Surface
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Technical Assessment Report 

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NESC-RP13-00869

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ISS PCU Utilization Plan Assessment Update

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NESC Request No.: TI-13-00869
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

International Space Station (ISS) Plasma Contactor Unit (PCU) Utilization 

Plan Assessment Update

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Hernandez-Pellerano, Amri; Iannello, Christopher J.; Garrett, Henry B.; 

Ging, Andrew T.; Katz, Ira; Keith, R. Lloyd; Minow, Joseph I.; 

Willis, Emily M.; Schneider, Todd A.; Whittlesey, Albert C.; 

Wollack, Edward J.; Wright, Keneth H.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20454 NESC-RP-13-00869

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 16 - Space Transportation and Safety

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

The NASA Engineering and Safety Center (NESC) received a request to support the Assessment of the International Space 

Station (ISS) Plasma Contactor Unit (PCU) Utilization Update. The NESC conducted an earlier assessment of the use of the 

PCU in 2009. This document contains the outcome of the assessment update.

15. SUBJECT TERMS

International Space Station; Extravehicular activity; NASA Engineering and Safety Center; Plasma Contactor Unit; 

Extravehicular Mobility Unit

18. NUMBER

 OF 

 PAGES

299

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF 

 ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

May 2013 - July 2014

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

869021.05.07.09.37

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TM-2014-218512

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 08 - 2014