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Automatic reset circuit breakers

Aviation Mechanic Powerplant (AMP) practice question 12 of 13 on Engine Electrical Systems. ACS element AM.III.F.K4 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic is selecting circuit protection for an aircraft circuit and finds an automatic reset circuit breaker in the parts bin. What does the handbook say about this device?

  1. AThey may be used if they are trip-free.
  2. BThey should not be used as circuit protection.
  3. CThey are preferred for all high-current circuits.
Show the answer and explanation

Correct answer: B. They should not be used as circuit protection.

  • A — incorrect Trip-free describes a breaker that opens regardless of the position of its operating control; that property does not make an automatic reset breaker acceptable.
  • B — correct The handbook states that automatic reset circuit breakers, which reset themselves periodically, should not be used as circuit protection devices in aircraft.
  • C — incorrect No preference of this kind is given; circuit protector ratings are matched to the conductor, and self-resetting breakers are excluded from use.

BACKGROUND

Conductors should be protected by circuit breakers or fuses located as close as possible to the electrical power source bus, and the equipment manufacturer normally specifies which protector to use. The protector must open the circuit before the conductor emits smoke, which means its time-current characteristic has to fall below that of the conductor it protects. All resettable circuit breakers should open the circuit in which they are installed whenever an overload or fault exists, regardless of the position of the operating control; such breakers are called trip-free. Automatic reset circuit breakers reset themselves periodically and should not be used as circuit protection devices in aircraft.

STUDY TIP

Separate the two terms: trip-free is a required property of resettable breakers, while automatic reset is a type that is not used.

KEY POINTS

  • Protectors go as close as possible to the power source bus.
  • Resettable breakers must be trip-free.
  • Automatic reset breakers are not used for aircraft circuit protection.

SOURCES

  • amp-powerplant/ch04.txt "Circuit Protection Devices"
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MORE ON ENGINE ELECTRICAL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.