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Switch derating for a lamp circuit

Aviation Mechanic Airframe (AMA) practice question 21 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.K7 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA 28-volt lamp circuit carries a continuous load current of 2 amperes. Applying the derating factor for that load, what nominal switch rating is required?

  1. A16 amperes
  2. B8 amperes
  3. C4 amperes, twice the load
Show the answer and explanation

Correct answer: A. 16 amperes

  • A — correct The derating factor for a lamp load in a 28-volt system is 8, and 2 amperes multiplied by 8 gives a 16-ampere nominal switch rating.
  • B — incorrect This uses the derating factor itself as the answer instead of multiplying it by the 2-ampere continuous load.
  • C — incorrect A factor of 2 is the value listed for a resistive load in a 28-volt system, not for a lamp load.

BACKGROUND

The nominal current rating stamped on an aircraft switch is the continuous current the switch can carry with its contacts closed. Switches must be derated from that nominal rating for high rush-in circuits, inductive circuits, and motors. Incandescent lamps are a high rush-in load because they can draw an initial current 15 times greater than the continuous current, which can burn or weld the contacts when the switch is closed. The standard derating factors in a 28-volt system are 8 for a lamp, 4 for an inductive load, 3 for a motor, and 2 for a resistive load. Multiplying the continuous load current by the factor for that load gives the nominal switch rating to select.

STUDY TIP

Multiply the continuous current by the derating factor rather than subtracting anything, and memorize the 28-volt lamp factor of 8 as the largest of the four.

KEY POINTS

  • Nominal switch rating equals continuous current times the derating factor.
  • The 28-volt lamp factor is 8, the highest of the derating factors.
  • Lamps are derated heavily because inrush can be 15 times normal current.

SOURCES

  • sources/ama-airframe/ch09.txt "Switches"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.