USCG Exam PracticeElectrical measuring devices and instrumentation
A megohmmeter is the preferred instrument for testing motor insulation rather than a standard ohmmeter primarily because:
- A. The megohmmeter measures resistance in series with the circuit while energized
- ✓ A standard ohmmeter's low test voltage is inadequate to stress insulation and reveal degradation, whereas the megohmmeter applies a high DC test voltage (typically 500–1000 V)Correct
- C. The megohmmeter measures AC insulation resistance, which a standard ohmmeter cannot do
- D. A standard ohmmeter would trip the motor's overcurrent protection during the test
Why B is correct
Insulation resistance must be measured under sufficient voltage stress to reveal moisture, contamination, or incipient breakdown. A standard ohmmeter's internal battery voltage is far too low to stress the insulation meaningfully; the megohmmeter applies 500 V or 1000 V DC for a valid insulation-resistance reading in megohms.