An industrial electrician is troubleshooting intermittent nuisance tripping of a ground fault circuit interrupter (GFCI) on a 120 V, 15 A circuit supplying a washdown pump in a wet location. After disconnecting the pump, the GFCI holds, but when the pump is connected, it trips almost immediately. A megohmmeter test between the pump's motor winding and its frame shows 0.8 MΩ. What is the most likely cause of the GFCI tripping?

Question: An industrial electrician is troubleshooting intermittent nuisance tripping of a ground fault circuit interrupter (GFCI) on a 120 V, 15 A circuit supplying a washdown pump in a wet location. After disconnecting the pump, the GFCI holds, but when the pump is connected, it trips almost immediately. A megohmmeter test between the pump's motor winding and its frame shows 0.8 MΩ. What is the most likely cause of the GFCI tripping?

Answer options:

• The GFCI is faulty and needs replacement.
• The neutral conductor is shared with another circuit. ✅ The motor winding insulation is degrading, causing leakage current to ground.
• The pump's power conductors are undersized for the motor load.

Correct answer: The motor winding insulation is degrading, causing leakage current to ground.

Explanation: A megohmmeter reading of 0.8 MΩ (800 kΩ) for a 120 V circuit indicates significant insulation degradation, allowing leakage current to flow from the winding to the motor's grounded frame. This leakage current, even if below the circuit breaker's trip rating, will be detected by the GFCI as an imbalance between the live and neutral conductors, causing it to trip. A healthy motor insulation should typically read in the millions of ohms.