Aircraft Electrical System Safety Considerations: Arc Fault Circuit Interrupters 2000-01-2121
The power distribution architecture in today’s aircraft typically use electromechanical circuit protection. These circuit breakers use a bimetal element to mimic the thermal impact on the wire and insulation caused by the carrying of current by the wire. Current running through the breaker heats the bimetal until it reaches its thermal set point and initiates a mechanical movement that opens the circuit breaker. As a safety and convenience feature, the breaker can be thought of as a re-settable fuse. Existing circuit breakers are quite effective in fulfilling their primary role of protecting wire and cable from damage due to current overloads. However, it has long been recognized that merely modeling the I2R heating has not totally eliminated wire insulation, cable, and load damage. Evidence of the problem can be gleaned from the continued progression of wire insulation systems and today’s concerns about an aging aircraft fleet. One type of electrical fault, for which today’s circuit protection was not designed, is arcing faults. The arcing phenomenon can be caused by a number of factors that damage wire insulation such as mechanical damage, environmental degradation, and thermal stress. These can lead to insulation failures and ultimately arcing. Arcing is very destructive and often does not provide enough thermal heating to trip convention circuit breakers. This will require the development of a new generation of circuit protection to be developed that addresses this problem. The paper examines existing aerospace power distribution and circuit protection as well as how it interrelates to the arcing phenomenon, demonstrating the need for Arc Fault Circuit Interrupters (AFCI). These AFCI devices must maintain the benefits of proven circuit protection while providing the supplemental protection from arcing faults.