Environmental Influence of Gravity and Pressure on Arc Tracking of Insulated Wires Investigated

Momentary short-circuit arcs between a defective polyimide-insulated wire and another conductor may thermally char (pyrolize) the insulating material. The charred polyimide, being conductive, can sustain the short-circuit arc, which may propagate along the wire through continuous pyrolization of the polyimide insulation (arc tracking). If the arcing wire is part of a multiple-wire bundle, the polyimide insulation of other wires within the bundle may become thermally charred and start arc tracking also (flash over). Such arc tracking can lead to complete failure of an entire wire bundle, causing other critical spacecraft or aircraft failures.

Unfortunately, all tested candidate wire insulations for aerospace vehicles were susceptible to arc tracking. Therefore, a test procedure was designed at the NASA Lewis Research Center to select the insulation type least susceptible to arc tracking. This test procedure addresses the following three areas of concern: (1) probability of initiation, (2) probability of re-initiation (re-strike), and (3) extent of arc tracking damage (propagation rate). Item 2 (re-strike probability) is an issue if power can be terminated from and reapplied to the arcing wire (by a switch, fuse, or re-settable circuit breaker). The degree of damage from an arcing event (item 3) refers to how easily the arc chars nearby insulation and propagates along the wire pair. Ease of nearby insulation charring can be determined by measuring the rate of arc propagation. Insulation that chars easily will propagate the arc faster than insulation that does not char very easily.

A popular polyimide insulated wire for aerospace vehicles, MIL-W-81381, was tested to determine a degree of damage from an arcing event (item 3) in the following three environments: (1) micro-gravity with air at 1-atm pressure, (2) 1g with air at 1 atm, and (3) 1g within a 10^-6 Torr vacuum.

bar chart

Comparison of MIL-W-81381 insulated wire (20 AWG), in each environment of interest, with respect to the distance the arc travels in 16 sec.

The micro-gravity 1-atm air was the harshest environment, with respect to the rate of damage of arc tracking, for the 20 AWG (American Wiring Gauge) MIL-W-81381 wire insulation type . The vacuum environment resulted in the least damage. Further testing is planned to determine if the environmental results are consistent between insulation types and to evaluate the other two parameters associated with arc tracking susceptibility.

Lewis contact: Thomas J. Stueber, (216) 433-2218 (E-Mail: stueber@lerc.nasa.gov)
Headquarters program office: OSMA
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Last updated April 17, 1996, by nancy.amman@lerc.nasa.gov
TITLE:


Evaluation of Kapton Pyrolysis, Arc Tracking, and Flashover on SiOx-Coated Polyimide Insulated Samples of Flat Flexible Current Carriers for SSF
AUTHOR(S):
Thomas J. Stueber and Chris Mundson

REPORT DATE:
April 1993

FUNDING NUMBERS:
WU-474-46-10 C-NAS3-25266 0PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES):
Sverdrup Technology, Inc. Lewis Research Center Group 2001 Aerospace Parkway Brook Park, Ohio 44142

PERFORMING ORGANIZATION REPORT NUMBER:
E-7706

SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES):
National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135-3191

REPORT TYPE AND DATES COVERED:
Final Contractor Report

SPONSORING/MONITORING AGENCY REPORT NUMBER:
NASA CR-191106

SUPPLEMENTARY NOTES:
Project Manager, Bruce A. Banks, Power Technology Division, (216) 433-2218.

ABSTRACT:
Kapton polyimide wiring insulation has been found to be vulnerable to pyrolization, arc tracking, and flashover when momentary short-circuit arcs have occurred on aircraft power systems. Short-circuit arcs between wire pairs that pyrolize the polyimide resulting in a conductive char between conductors that may sustain the arc (arc tracking). Furthermore, the arc tracking may spread (flashover) to other wire pairs within a wire bundle. Polyimide Kapton will also be used as the insulating material for the flexible current carrier (FCC) of Space Station Freedom (SSF). The FCC, with conductors in a planar type geometric layout as opposed to bundles, is known to sustain arc tracking at proposed SSF power levels. Tests were conducted in a vacuum bell jar that was designed to conduct polyimide pyrolysis, arc tracking, and flashover studies on samples of SSF's FCC. Test results will be reported concerning the minimal power level needed to sustain arc tracking and the FCC susceptibility to flashover. Results of the FCC arc tracking tests indicate that only 22 volt amps were necessary to sustain arc tracking (proposed SSF power level is 400 watts). FCC flashover studies indicate that the flashover event is highly unlikely.

SUBJECT TERMS:
Polyimide pyrolysis

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Technology update
Electrical wiring protection system

Eaton Corp. has received a contract from two federal agencies for the development of electrical wiring protection systems for both commercial and military aircraft. The $1 million contract from the FAA and U.S. Navy calls for Eaton to adapt the company's proprietary arc fault circuit interrupter (AFCI) technology to the miniature circuit breakers in an aircraft's 400 Hz electrical system.

The company will develop 20 AFCIs, and the associated test procedures for use on aircraft such as the DC-9, operated by both the military and commercial airlines. Initial target applications are for nonflight-critical circuits but, according to Eaton, the technology could be used throughout the entire aircraft. "More importantly, is the expectation that AFCI will lead to early warning of arcing originating in aircraft electrical circuits and provide a new level of safety for passengers and aircraft flight crews," said Alexander Cutler, Eaton President and COO.

Intermittent electrical arcing on aircraft could be flash points for fires. Electrical arcing occurs as a result of mechanical wear, environmental effects, and thermal stress on wire insulation. AFCIs use integrated electronics to diagnose when arcing or "jumping" occurs in a wiring system, then act immediately to shut down the circuit. Because most aircraft wiring is hidden or not easily accessible, remote detection is considered a particularly important safety feature.

Frank Bokulich

Aerospace Engineering June 2000  

read more on AFCI's here

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The addition of Arc Fault Technology to existing circuit protection represents a major step forward in the evolution of residential safety devices. The AFCI device trips when arc fault detected Parallel Arcing Faults Denotes an arc fault between the line
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What is an arc fault ? What is an arc fault Circuit Interrupter? An AFCI uses electronics to recognize the current and voltage characteristics of arcing faults, and interrupts the circuit when the fault occurs.
45% http://www.geindustrial.com/industrialsystems ...

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