USCG Exam PracticeElectrical troubleshooting and maintenance
A megohmmeter is applied conductor-to-ground on a de-energized motor winding. The reading is significantly lower than the previous recorded value. What does this indicate?
- A. The motor winding resistance has increased, indicating an open circuit developing
- B. The motor is drawing excessive current during normal operation
- ✓ Moisture, contamination, or insulation breakdown is present and a ground fault may be developingCorrect
- D. The ground-detection system at the switchboard has malfunctioned
Why C is correct
DOE-HDBK-1011 Vol.4 §15-3 states that low or falling megger readings warn of moisture, contamination, or insulation breakdown before it becomes a ground fault. A declining trend in insulation resistance is a pre-fault warning, not an indication of an open circuit or switchboard malfunction.