USCG Exam PracticeMaintenance, machine tools, drawings, and tables
A megohmmeter (megger) is used in engine-room electrical maintenance. Which of the following correctly describes the proper procedure for its use?
- A. Connect in parallel across the energized circuit and read resistance directly from the scale
- B. Connect in series with the load while the circuit is energized to measure insulation resistance
- ✓ De-energize and discharge the equipment before and after the test; the instrument applies a high DC voltage to read insulation resistance in megohmsCorrect
- D. Apply 120 VAC from the ship's service bus through the megger to the winding under test
Why C is correct
DOE-HDBK-1011 Vol.4 §14-1 states that the megohmmeter applies a high DC test voltage (commonly 500–1000 V) to read insulation resistance in megohms, and that equipment must be de-energized and discharged before and after the test. Connecting a megger to an energized circuit is incorrect and dangerous.