FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsEasy
A technician is selecting circuit protection for a new avionics installation. Which characteristic is required of an aircraft circuit breaker but is not necessarily found in a fuse?
- AIt must be trip-free so it cannot be held closed against an electrical fault
- BIt must open the circuit automatically when current exceeds a rated value
- CIt must be reset by replacing it with a new unit after tripping
- DIt must be made of a fusible link that melts under overcurrent
Show answer & explanationAnswer & explanation
Correct answer: A. It must be trip-free so it cannot be held closed against an electrical fault
Aircraft circuit breakers are required to be 'trip-free,' meaning the breaker will open the circuit on an overload even if the operator holds the switch/handle in the closed position. Fuses simply melt open on overcurrent and offer no trip-free holding feature, and unlike fuses they are reset by the crew, not replaced.
Why the other options are wrong
- B. True of both fuses and breakers, so it doesn't distinguish them.
- C. Breakers are reset, not replaced; fuses are replaced.
- D. This describes a fuse, not a breaker.
Trip-Free Circuit Breaker
A circuit breaker designed to open an overloaded circuit automatically regardless of the position of the operating control.
- Cannot be held closed by the switch during an overload
- Protects wiring from overheating even if crew tries to override it
- Required by 14 CFR for most aircraft circuit protection devices
Memory trick: 'Trip-Free' Frees the circuit even if you fight it