Aicraft avionics

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Avionics WorkshopOttawa, Ontario, 2004

Ahmed El Goueli


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ARC FAULT CIRCUIT BREAKERS

The issue of aircraft wiring safety has received widespread attention in

recent years, highlighted by the unfortunate TWA 800 and Swissair 111tragedies. As a result of these incidents and other concerns, the issue of

wiring safety has been taken up by OEMs, regulatory agencies, the

military, and is being addressed as part of the Aging Transport Systems

Rulemaking Advisory Committee (ATSRAC).


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Protection Against Arc Fault Accident

Wire Selection

Wire Installation

Wire Maintenance

Arc Fault Circuit breaker


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Definition

Aircraft circuit breakers have historically been the best available protection for

aerospace wiring. These circuit interruption devices are used protect the aircraftwiring from overheating or melting. During arcing faults these currents are often

small, short in duration and well below the overcurrent time protection curve

designed into these breakers.

An arc-fault circuit interrupter (AFCI) is a device intended to provide protection from

the effects of arc-faults by recognizing characteristics unique to arcing and byfunctioning to de-energize the circuit when an arc-fault is detected


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How it works ???

AFCI technology monitors the electrical circuit for arcing events that are indicative of potentially unsafe

wiring conditions and could result in fires or loss of electrical circuit functionality.

The AFCB trips in about five milliseconds, greatly reducing the chance of damaging the rest of the

surrounding wiring, other equipment and structure.

Laboratory tests have shown that AFCI breakers detect faults and trip faster than aircraft circuit

breakers and are significantly faster at detecting arcing faults in aircraft wiring.


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DESIGN

A robust AFCI algorithm should not rely on current level to determine an arcing event, but should

distinguish the arcing event based on the arcing content of the signal

The AFCI must be designed such that:

1.To distinguish such a wide variety of normal steady state and transient load characteristics

2.To recognize normal steady state and transient load conditions

3.Be packaged in an integral device and can be interchangeably installed with existing CB.


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DESIGN

There are a number of undesirable arcing conditions that have

been identified that the arc fault detection algorithm should be

required to protect against. These arcing conditions can be

manifested in arcing between parallel conductors, to ground,or in series along the conductor.


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Parallel Arcs

Parallel arcing occurs when there is a arc between two wires or wire-to-frame and the current is limitedby the impedance of the voltage source, the wire, and the arc. Parallel arcing is generally morehazardous than series arcing. The energy released in the arc is much higher with temperatures inexcess of 10,000 Deg. F. This causes pyrolyzation or charring of the insulation, creating conductivecarbon paths and ejecting hot metal that is likely to encounter flammable materials.

Series Arcs

Series arcing may begin with corrosion in pin-socket connections or loose connections in series withthe electrical loads. Loose terminal lugs, misarranged or cross-threaded electrical plugs, brokenconductor strands inside a wire are also possible sources. The amount of power from a series arc is farless than in a parallel fault. Series arcing is particularly treacherous because the arc current remainswell below the trip curve of the Mil-Spec aircraft breaker. These arcs cause load voltage drops andheating of the wire, pin and sockets, or terminal lugs.This heating can lead to component failure andignition source.


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Proof of Concept

1. Loose Terminal Test.

2. Guillotine or Point Contact Test.

3. Carbonized Path Test.

4. Wet Arc or Salt Water Drip Test.


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Tripped AFCI Breaker withindication of thermal overload

Tripped AFCI Breaker with

indication of arcing fault


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ADVANTAGES OF AFCB

1. AFCBs provide a simple means for mitigating the effects of electrical arcing

before the arc develops into a serious condition, preventing potentially

catastrophic damage to electrical wire bundles.

2. AFCBs will limit the energy expended by the arc fault thereby reducing the

potential to ignite surrounding materials (i.e., wire insulation, lint, cleaning

fluids, etc.) and the associated smoke and fire that can occur.

3. A tripped AFCB will identify on which wire the fault is located.


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CONCERNS

1. Present AFCB prototypes do not provide a means for monitoring the

performance of the AFCB functional status. In the absence of an active status

indication, some AFCB surveillance programs may be necessary to provide

assurance that the AFCBs are fully functional.

2. The rate of AFCB tripping versus thermal circuit breakers. Upon installation of

AFCBs, the rate of AFCB trips will be greater than that of the thermal breakers

being replaced.

3. The problem of determining the location of the arc fault along the length of the

circuit.


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Summary

While further technology development is ongoing for AC and DC applications. AFCI technology

for aircraft requires a robust algorithm that provides:

1.Fast response to true arcing events

2.Does not result in nuisance trips

3.Ability to discriminate between normal and transient load signatures and arcing events

4.Ability to sense arcing events in the presence of masking loads

5.Immunity to filters, EMI, cross-talk, and other circuit noise

6.Low level arc sensitivity

There are numerous issues that need to be addressed to satisfy the wiring system application

and protection requirements on commercial and military aircraft. Moreover, the detection levels

and performance requirements must be such that these new products provide a truly enhanced

circuit protection function in the aircraft wiring system.


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Transport Canada Position

Awaiting maturity of the product development, specific projects in Canada utilizingAFCBs are being addressed by issue papers. The IPs provide clarification of the

criteria and TC policy for means of compliance expected with the applicableregulations.

Upcoming Milestones

SAE Committee is developing an industry standard for minimum operatingspecification for AFCBs.

FAA is developing associated TSO and advisory circular in parallel with the SAEactivity.

TC will adopt the FAA policy or issue an AMA if needed.


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Questions ?

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