USCG Exam PracticeHand/power tools, measuring instruments, and maintenance
A megohmmeter (megger) is the preferred instrument for testing motor insulation aboard a vessel primarily because it does which of the following?
- A. Measures true power while accounting for power factor
- ✓ Applies a high DC test voltage to read insulation resistance in megohmsCorrect
- C. Measures current without breaking the circuit using a clamp
- D. Confirms phase sequence before connecting a motor to the bus
Why B is correct
An ordinary ohmmeter's low voltage is inadequate for insulation testing. The megger applies a high DC test voltage, typically 500 V or 1000 V, and reads insulation resistance in megohms, making it the primary tool for detecting moisture, contamination, or breakdown in motor, generator, and cable insulation.