walz - arcing flammability factors

7/27/2019 Walz - Arcing Flammability Factors

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ARCING FLAMMABILITY FACTORSMichael F. Walz Aging

Electrical Systems Program Manager

Cesar A. Gomez

Aging Electrical Systems Engineer

Airport and Aircraft Safety Research and Development Division

William J. Hughes Technical Center

Atlantic City International Airport

I. ABSTRACTAircraft safety relies in part on the design, integration, installation, maintenance, and proper performance of

the electrical wiring interconnect system (EWIS) throughout an aircrafts operational life. Aircraft are designed

with redundant flight critical systems to improve safety and tolerance to faults. An important aspect of the EWISdesign is the ability of the wiring to safely react to failures during normal and abnormal aircraft conditions. The

EWIS design should prevent the propagation of the effects of electrical faults to other independent power sources

and critical systems, which can happen due to aircraft operation, maintenance, and human factors.

The Federal Aviation Administration (FAA) conducts EWIS research on arc damage assessment,separation and segregation of electrical systems, and quantification of damage to adjacent electrical and non-

electrical systems during EWIS failures. The Arc Fault Evaluation Laboratory (AFEL) at the William J. HughesTechnical Center is used to perform research in this field. The AFEL is currently conducting a test program to

evaluate damage tolerance on the EWIS and adjacent structures and systems due to fires and electrical faults.

This paper presents an assessment of wire flammability thresholds and damage tolerance of the EWIS andcascading effects due to fires and electrical arcing inside the aircraft. Experimental variables include material types,

dust and lint accumulation, and circuit protection. This study also presents data on the potential of the variables to

increase or reduce the potential of flammable events. The effects of dust and lint on chafed insulation were included

in the assessment to determine if these contaminants contribute to creation of a carbonized path and subsequent

electrical arcing in the presence of fire. Results of this study will assist the development of guidelines for EWIShazard mitigation and design criteria.


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II. INTRODUCTIONThe Federal Aviation Administration (FAA) is conducting research on the damage tolerance of the

Electrical Wiring Interconnection System (EWIS) under the stress of an aircraft fire. The research is being

conducted in the Arc Fault Evaluation Laboratory (AFEL) at the William J. Hughes Technical Center in Atlantic

City, New Jersey. EWIS separation and segregation, circuit protection, and material selection are important aspects

of wiring system design; and properly applied increase the ability of the EWIS to adequately mitigate failure effectsduring normal and abnormal aircraft conditions. This project is developing data on EWIS damage tolerance in a fire

condition and the ability of the EWIS to safely transmit power, avoiding catastrophic damage and improving aircraft

safety. The results presented in this paper will assist the development of EWIS design guidelines that can improve

EWIS safety when exposed to an aircraft fire condition.

III. BACKGROUNDAircraft safety relies in part on the design, integration, installation, maintenance, and proper performance of

the EWIS throughout an aircrafts operational life. An important aspect of the EWIS design is the ability of the

wiring to safely react to failures during normal and abnormal aircraft conditions. Separation and/or segregation ofEWIS and electrical equipment are important factors for ensuring that the functional and physical integrity of critical

systems is maintained after a failure in the EWIS. Aircraft are designed with redundant flight critical systems to

achieve acceptable margins of safety. Separation is a measure of distance and addresses physical hazards resulting

from potential electrical failure modes. Adequate separation is a function of the physical and electrical attributes ofthe wires in the bundle and the hazard potential of a failure at any given point along the length of the bundle.

Segregation involves the physical segregation of wires into separate bundles.

The FAA and aircraft manufacturers and suppliers are constantly evaluating new materials and their

resistance to fire. The Code of Federal Regulations (CFR) part 25 Appendix F mandates two flammability tests forelectrical wire: the 60-degree test, and the vertical test. All wire used on aircraft must pass these tests (There are

exceptions for special conditions). Improved fire resistance enhances the ability of the EWIS to resist damage

during an aircraft fire. Current test standards and research by the FAA and industry on wire flammability are

conducted without power applied to the wire and the insulation in pristine condition. While this gives a good

indication of how the wire insulation material will react in a fire there are other considerations that must be takeninto account to fully understand the hazard potential to an aircraft.

An extensive literature search on the effects of carbonization of the wire insulation and contamination on

the EWIS was performed. The results of the search did not yield any comprehensive conclusions. During an

aircraft fire the wire insulation carbonizes; creating additional unintended conduction paths. It can also createexternal induced ionization of air. During a fire, ionization gases may be present, reducing the dielectric constant of

air; thus resulting in an increased likelihood of electrical arcing [3]. In addition, carbonization creates additional

potential arcing paths in a wire bundle which can cascade into a serious hazard condition. Aircraft malfunctions

created by such an event can also lead to increased crew workload on the flight deck and upset the crews situationalawareness.

The flammability characteristics of dust and lint contaminants found on aircraft are not fully understood.

The accumulation of contaminants and their effect on exposed wire conductors in the EWIS as a result of some high

temperature source, the effect on the EWIS of a sustained and undetected fire, and the potential of thesecontaminants to aid the propagation of fire need to be studied.

This research assesses the ability of the EWIS to safely conduct power while exposed to a flammable eventand explores the effects of lint and debris during these events.


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III. OBJECTIVEThe goal of this research is an assessment of the damage tolerance of the EWIS under the stress of aircraft fire,

including the effect of variables such as contamination, material types, and circuit protection to increase or decrease

the hazard potential.

IV. TEST SET UP

Three test methods were used in the course of this study: the Carbonization Flammability Test, the ArcingBurn-Through Test, and the Contamination Arcing Test. Test results were recorded on a Nicolet Vision digital

recorder, stop watch, digital video, and a high-quality digital camera. Digital photographs were taken before andafter the event and used for assessment. The power source for all tests was a 400 Hz, three-phase, 60KVA, 115 volt

generator. Table 1 list the wire types used in the tests. The lint and debris contaminants obtained from varioussources and all contain material commonly found in aircraft lint accumulations. The fire sources included a foam

block soaked in Heptane and an improvised Heptane gas lamp. All experiments were performed using conventional

aircraft thermal circuit protection and arc fault circuit protection.

Table 1. Material Types

Material

Polyimide Mil-DTL-81381/11-20

PVC Mil-W-5086/1-20

PTFE AS22759/9-20PTFE AS22759/34-16

A. The Carbonized Flammability Test

The objective of this test is to assess the damage tolerance of powered aircraft wire insulation while

exposed to a flammable event. The set up consists of a two wire bundle where one wire is connected to a 400Hz

Single phase generator, a circuit breaker and a constant light load. The other wire in the bundle is connected to theaircraft return. The power wire is monitored electrically through out the test cycle. A schematic of the test set up is

shown below.

Phase B

400 Hz

Wires under test

The test is initiated upon application of a Heptane flame to the bundled wire as shown in figure 1. Therewere three different methods that were used to apply the flame on the wire. These are the Heptane candle, a small

Heptane soaked foam block, and a large Heptane soaked foam block. These methods were used to achieve

concentration of the hottest part of the flame on the wire. The time to burn-thru is considered to be when the

monitored power wire shows excessive current or the electrical conductor is exposed. If any of these conditions was

met the lapsed time was recorded.


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Exposed ConductorArcing Bundle

Wire Under Test

Figure 2. Arcing Burn-through Test

C. The Contamination Arcing TestThe objective of this test is to determine if the dust and lint contamination assists the creation of a

carbonized path to start an arcing event in the presence of fire. A two wire bundle is prepared by creating a 1mm

gap in the insulation exposing the conductor on each of the wires as shown in figure 3. A small amount of dust andlint (1 to 2mm) is placed at the exposed conductor area of the bundle as shown in figure 4. The bundle under test is

powered by two independent phases of the generator or a single phase and aircraft return. The time is monitored

from the moment the Heptane flame is applied to the exposed conductor area until electrical monitoring identifies

conduction between the wires of the bundle. The schematic of the test set up is shown below.

400 Hz

Wires under testPhase B

The Heptane candle was used in this testing because it allow for a controllable application of the flame on

the exposed conductors. For comparison purposes the tests were done with clean exposed conductors first and thenwith the applied contamination. Polyimide wires were used because its greater performance during the carbonized

flammability tests above the others. This facilitates the observation of the dust and lint carbonization properties

without any intrusion from the polyimide insulation.


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Figure 3. 1mm Gap Exposed Conductor

Figure 4. 2mm of Dust and lint at Exposed Conductor

V. RESULTSA. The Carbonized Flammability Test

The results for the carbonized flammability test are shown in table 2. The polyimide insulation exhibited

greater fire resistance than the other wire types and did not experience a breach into the conductor in any of the test

trials. One polyimide sample was exposed to the Heptane flame for more than 30 minutes without breaching. It ispossible that the Heptane flame temperatures were not sufficiently high for the carbonization of the insulation to

create an arc over event. PVC insulated wire had the least fire resistance. After 45 seconds the PVC insulation was

completely consumed and the conductor was breached. For AS22759/9 the flame was applied for 20 minutes and at

the completion of the test both conductors were exposed. Particular to the AS22759/9 test was the absence of a

large arc flash over during the test. Instead much scintillation (mini arcs) was seen until the wire insulation wasconsumed and the flame was consumed.


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Figure 6. Inoperable Wires After Arcing

B. Arcing Burn-Through TestThe tests results for the Arcing Burn-Through test are shown in Table 3. The time to breach to any

conductors was recorded as the amount of arcing half cycles needed to breach to any of the generator phases and

ground. An arcing half cycle is defined per AS5692 ARC Fault Circuit Breaker (AFCB), Aircraft, Trip-Free Single

Phase 115 VAC, 400 Hz-Constant Frequency. Table 3 shows the number of arcing half-cycles incurred by each of

the wires before the first breach; consequently breaches after resetting the circuit breaker are explained for each of

the tests and can be seen on the waveforms recorded.

Table 3. Test results for the Arcing Burn-Through Test

Material

Test

Number

Arcing Half Cycles To

First Breach

Test 22 229Polyimide Mil-DTL-81381/11-20

Test 23 294Test 18 Breached to Ground

PVC Mil-W-5086/1-20Test 19 85

PTFE AS22759/9-20 Test 24 150

PTFE AS22759/34-16 Test 25 66

In test number 22, phases A and C were connected to the polyimide wire under test. Phase B and ground

where connected to the arcing wire. The first breach occurred after 229 arcing half cycles from Phase B to C after

which the breaker was reset. After multiple resets the conductor breached from Phase B to C until the wire was burntto an open condition. Phase A was never breached.

For test number 23, phases A and C were connected to the polyimide wire under test. Phase B and ground

where connected to the arcing wire. The first breached occurred after 294 arcing half cycles from B to C. Aftermultiple resets phases A, B, and C were breached. The resulting damage is shown in figure 7.


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Figure 7. Arcing Burn-through test on Polyimide Wire

For test number 18, phases A and C were connected to the PVC wire under test. Phase B and ground wereconnected to the polyimide arcing wire. Only one arcing event was recorded prior to exposing the conductor on the

grounded wire of the PVC bundle. The thermal breaker did not trip. For Test 19 under the same configuration, thefirst breach occurred after 85 arcing half cycles from B to C. The thermal Breakers on phases A and C did not trip.

The conductor was exposed as seen in figure 8.

Figure 8. Exposed PVC conductorFor test number 24, phases A and C were connected to the PTFE wire under test. Phase B and ground were

connected to the arcing wire. The first breach occurred after 150 arcing half cycles from B to A. After one reset of

the breaker on Phase B, all phases were breached. The circuit breakers were reset until the polyimide arcing wirewas consumed or was separated from the wire under test. After review of the damage the PTFE was rendered

inoperable, both conductors on the wire bundle were breached.

For test number 25, phases A and C were connected to the PTFE wire under test. Phase B and ground were

connected to the arcing wire. The first breach occurred after 66 arcing half cycles from B to A. After three resets ofthe circuit breaker all the phases were breached and the arcing polyimide wire separated. After review of the damage


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both conductors of the PTFE wire bundle were breached.

Each circuit in the test was powered through an arc fault circuit breaker in line with each of the electricalphases. The arc fault circuit breaker interrupted the tests within four arcing half cycles. Per AS5692 an arc fault

circuit breaker must remove power if it detects 8 arcing half cycles in a 100 milliseconds time period. The resulting

damage to the wire under test was minimal and there was no breach to any of the electrical phases. A recorded arcfault waveform can be seen in figure 9.

Figure 9. Arc Fault Waveform

The position of the polyimide arcing wires was significant. When they were on the side of the tested

bundle the time to breach was greater and the damage effect was less than when the arcing wire was underneath. The

adhesive tape used to hold the two bundles together became flammable during some tests creating additional

damage to the insulation. The arc tracking characteristics of polyimide cause the arc event not to be in a constantposition to the wire tested.

C. The Contamination Arcing TestPolyimide wires were used because of the higher fire resistance exhibited during the carbonized

flammability tests. The Heptane candle was used in this testing because the application of the flame on the exposed

conductors was more controllable. Only a few tests were conducted to validate the test method for future studies.However the preliminary results show that dust and lint contamination greatly affected the carbonization polyimide

with a minimum amount applied. Two types of tests were done, one on pristine exposed polyimide conductors and

the other with dust and lint applied at the exposed conductors.

For the pristine samples the flame was applied for over 10 minutes with no arc over flash recorded, the time

was dependant on the fuel extinguishing from the candle. Then two types of lint and dust were applied to the

exposed conductors, one was typical dryer lint and the other was collected from the interior of an aircraft. Thecarbonization was recorded as an arcing flash over. For the dryer lint the time to carbonization was 51 seconds, for


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the aircraft lint the time was 1:36 seconds. The wires were rendered useless as result of the arc over flash as seen on

figure 10. Variation in the type of lint and dust may have a significant affect on the carbonization process. The

flame position and temperature play an important role in the time to carbonization and damage severity.

Figure 10. Damage from Arc Over Flash


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VI. CONCLUSIONSDuring a fire event, unintentional electrical connections can be made on the EWIS leading to increased damage,

degraded situational awareness, flight deck overload, and unpredictable reactions from the electrical system, and a

degraded level of aircraft safety. Preliminary results show that lint and dust can provide an electrical path that can

lead to an arcing event. The composition of the lint and dust plays a significant role in the degradation of properties.

Arc fault circuit breakers (per current specification) significantly reduce the chance of evolving more wires in thebundles, therefore avoiding cascading damage and failure of the wire bundle. In addition testing results show that:

The Heptane flame did not generate a sufficiently high flame temperature for the carbonization of thepolyimide insulation to create an arc over event. The Heptane flame does not provide sufficient ionizationof the air between exposed conductors to facilitate arcing.

A wire breech during a fire as well as the time to breach is dependent on the flame temperature and the typeinsulation material.

Polyimide type insulation is more resistant to deterioration of the insulation during an open fire event thanother common wire types.

The time to breach was significantly faster with arcing than open flame. The temperature during an arcingevent, while much higher than the n-Heptane flame, still required repeated arcs and reset of the circuit

breakers to breach the wires.

If an electrical fault does occur and the circuit breakers are reset, it can lead to greater hazard potential ofthe EWIS.

Arcing events are capable of compromising a greater range of material types due to the range of highertemperatures these materials can experience during the event..

VII. RECOMMENDATIONSStudy the effects of lint and dust on the EWIS.Study the flammability properties of contaminants on the EWIS.

Study the chemical properties of lint and dust from aircraft.


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VIII. REFERENCES1. Cahill, P., Evaluation of Fire Test Methods for Aircraft Thermal Acoustical Insulation,

FAA Report DOT/FAA/AR-97/58, September 1997.

2. Cahill, P., An Evaluation of the Flammability of Aircraft Wiring,FAA Report DOT/FAA/AR-TN04/32, December 2004.

3.

Dunki-Jacobs, J.R., The Escalating Arcing Ground-Fault Phenomenon, IEEE Trans. Ind. Appl. IA-22,1156-1161 (1986).

4. Babrauskas, V., How do Electrical Wiring Faults Lead to Structure Ignitions?, pp. 39-51 in Proc. Fireand Materials 2001 Conf., Interscience Communications Ltd., London (2001).

5. AS5692 ARC Fault Circuit Breaker (AFCB), Aircraft, Trip-Free Single Phase 115 VAC, 400 Hz-ConstantFrequency.

6. http://ntsb.gov/aviation/aviation.htm7. http://www.tsb.gc.ca/en/reports/air/2002/A02O0123
http://ntsb.gov/aviation/aviation.htmhttp://www.tsb.gc.ca/en/reports/air/2002/A02O0123http://www.tsb.gc.ca/en/reports/air/2002/A02O0123http://www.tsb.gc.ca/en/reports/air/2002/A02O0123http://www.tsb.gc.ca/en/reports/air/2002/A02O0123http://ntsb.gov/aviation/aviation.htm

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