A technician is comparing conventional thermal circuit breakers to arc fault circuit breakers (AFCBs) being installed in a new transport-category aircraft wiring system. What key capability distinguishes an AFCB from a conventional thermal circuit breaker?
- AAn AFCB trips only when current exceeds its rated continuous value for an extended time
- BAn AFCB is designed exclusively for use in low-voltage instrument circuits
- CAn AFCB can detect and interrupt intermittent arcing faults that may not draw enough current to trip a standard breaker
- DAn AFCB resets automatically after tripping without technician action
Show answer & explanationAnswer & explanation
Correct answer: C. An AFCB can detect and interrupt intermittent arcing faults that may not draw enough current to trip a standard breaker
Arc fault circuit breakers use electronic circuitry to analyze current waveform signatures and detect the characteristic pattern of arcing faults, such as those from chafed or damaged wire insulation, even when the fault current is too low or too brief to trip a conventional thermal breaker, helping prevent electrical fires from wiring damage.
Why the other options are wrong
- A. This describes conventional thermal overcurrent protection, not the distinguishing feature of an AFCB.
- B. AFCBs are used in various aircraft power circuits, not exclusively low-voltage instrument circuits.
- D. Trip-free circuit breakers, not AFCBs specifically, are designed so they cannot be reset while the fault persists, but they are not automatic-reset devices.
Arc Fault Circuit Breaker (AFCB)
An electronic circuit protection device that detects the distinctive current signature of arcing faults and trips to prevent wiring-related fires, even when current levels are below a conventional breaker's trip threshold.
- Uses electronic waveform analysis to detect arcing
- Protects against intermittent/low-current arc faults from chafed wiring
- Increasingly required in modern transport-category aircraft wiring
Memory trick: AFCB = the breaker that listens for the crackle of an arc, not just the heat of overload.