What type of circuit breaker may not be used in an aircraft?

• A. Automatic resetting type
• B. Manual resetting type
• C. Thermal-magnetic type
• D. Magnetic only type

Answer: (A)

In aircraft electrical systems, protection must force crews to acknowledge and address faults. An automatic resetting circuit breaker can reopen after tripping and restore power to a circuit that has a fault, which may hide a developing problem, allow continued overheating or arcing, and increase the risk of electrical fire. Because of this safety concern, automatic resetting types are not used in aircraft.

Manual resetting breakers require a deliberate action to reset after a trip, ensuring the fault is investigated. Thermal-magnetic breakers combine a thermal element for overload protection with a magnetic element for instantaneous short-circuit trips, providing versatile protection. Magnetic-only breakers respond rapidly to high current to interrupt severe short circuits.