FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A technician has installed a new wire bundle for a flight control system and must verify there is no short or degraded insulation between the conductors and the airframe before applying power. Which test instrument is designed for this insulation resistance check?
- ADigital multimeter set to the ohms function
- BWheatstone bridge
- CClamp-on ammeter
- DMegohmmeter (megger)
Show answer & explanationAnswer & explanation
Correct answer: D. Megohmmeter (megger)
A megohmmeter applies a high test voltage to measure very high resistance values (in megohms), which is necessary to properly evaluate wire insulation integrity; a standard multimeter's ohms function cannot generate sufficient voltage to reveal insulation breakdown.
Why the other options are wrong
- A. A standard multimeter uses a low test voltage inadequate for testing high-resistance insulation.
- B. A Wheatstone bridge is used for precise low-resistance measurements, not insulation testing.
- C. A clamp-on ammeter measures current flow in an energized circuit, not insulation resistance.
Megohmmeter (Megger)
A test instrument that applies a high DC test voltage to measure very high resistance values, used to check the insulation resistance of wiring and windings.
- Applies test voltages far higher than a standard multimeter
- Measures resistance in megohms
- Used before energizing new or repaired wiring installations
Memory trick: Megger = 'Mega volts to catch mega problems' in insulation.