Aviation Investigation Report A98H0003

2.13.1  Circuit Breaker Technology

The arced in-flight entertainment network (IFEN) power supply unit (PSU) cables were protected by conventional CBs typical of those used in the remainder of the aircraft and throughout the aviation industry. Two of the PSU cables (exhibits 1-3790 and 1-3791) had arcing events that did not trip the associated CB. It is most likely that the CBs did not trip because the electrical characteristics of the arcs were outside the defined Time versus Current curve.

Conventional aircraft CB technology can provide protection against hard short-circuit faults, but is limited in that it does not adequately protect against the full range of arc faults.

Industry and government are presently engaged in various research and development efforts aimed at designing a CB that will detect and react to the full range of known arc fault events, including short duration arc faults that typically occur outside the defined Time versus Current curve of traditional CBs. The resulting Arc Fault Circuit Breaker (AFCB) is being designed to surpass the protection provided by traditional thermal CBs. While the new circuit protection devices will mitigate the wire damage from successive arcing events on the same wire, ignition of surrounding flammable material may still occur from the initial arcing event.

Theoretically, had an AFCB been available to protect the IFEN PSU cables, the initial arcing events on exhibits 1-3790 and 1-3791 would have been detected by the AFCB, and the device would have tripped and de-energized the cable.

While the proposed AFCB certification tests will result in improved arc-fault detection capabilities and response times, as written they will not certify the AFCB's ability to prevent the ignition of flammable materials by arcing phenomena. Given the existence of flammable materials used in aircraft construction, it would be prudent to establish AFCB certification criteria based on limiting the arc energy to a level below that necessary to ignite any materials likely to be used in aircraft. Although such testing standards have been incorporated for residential arc fault circuit-interrupters, draft certification test requirements for aircraft AFCBs do not include such criteria.[117]

[117]    Underwriters Laboratories Inc. UL 1699 Standard for Safety for Arc Fault Circuit-Interrupters dated 26 February 1999.

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