INSTRUCTIONS FOR CIRCUIT BREAKER DESIGN

However strenuous the efforts to avoid design and maintenance quality lapses, their essentially random natures make them very difficult to eliminate. There are many reports of wiring loom damage where sustained arcing within/between looms occurred, or probably occurred, where CBs have failed to operate, or to operate in sufficient time to prevent serious wiring damage and, in some cases, loss of the aircraft. The four incidents reported here present such examples of sustained arcing. Electrical circuits are protected against electrical overheating of wires by thermal/mechanical types of circuit breaker. The 'thermal trip' type of circuit breaker is tripped, and thus the electrical circuit broken, by heat generated within the breaker from the current in excess of its rating. This is most suitable for a 'solid' and continuous short-circuit but less reliable for transient arcing faults, which develop high energy over a very short period of time insufficient to trip the circuit breaker. An 'intelligent' circuit breaker, which could directly replace the circuit breakers presently in widespread use, can recognise the rapid current and/or voltage signature associated with arcing faults. An extensive research programme has been sponsored entirely by the FAA, and has led to the development of such arc fault circuit breakers.

The findings of the ATSRAC research has shown that aircraft wiring does deteriorate with time. If wiring insulation material becomes damaged in some way, for example due to mechanical abrasion, so that the wire is exposed and a local external conductive path is available, then electrical arcing can occur. In response to previous incidents and accidents where arcing has been identified, and with regard to the development of arc-fault circuit breakers, the following recommendations are made: