USCG Exam PracticeElectrical measuring devices and instrumentation
A megohmmeter (megger) is used instead of a standard ohmmeter to test motor insulation because:
- A. The megger's low test voltage prevents damage to delicate insulation materials
- ✓ A standard ohmmeter's low internal voltage is insufficient to stress insulation and reveal high-resistance leakage pathsCorrect
- C. The megger measures resistance in ohms rather than megohms, giving finer resolution
- D. A standard ohmmeter requires the circuit to be energized, which is unsafe for insulation testing
Why B is correct
A megohmmeter applies a high DC test voltage, typically 500–1000 V, which is necessary to stress the insulation and reveal leakage paths that a standard ohmmeter's low battery voltage cannot detect. The result is read in megohms, not ohms.