A 120 V, single-phase motor in an industrial fan application makes a loud buzzing noise and trips its overcurrent device almost immediately when energized. You perform an insulation resistance test with a megohmmeter between the motor's line terminals and its frame, and read 0.5 MΩ. What is the most probable cause of the tripping?

Question: A 120 V, single-phase motor in an industrial fan application makes a loud buzzing noise and trips its overcurrent device almost immediately when energized. You perform an insulation resistance test with a megohmmeter between the motor's line terminals and its frame, and read 0.5 MΩ. What is the most probable cause of the tripping?

Answer options: ✅ A ground fault in the motor windings.

• The motor is severely overloaded.
• An open circuit in the motor windings.
• A faulty starting capacitor.

Correct answer: A ground fault in the motor windings.

Explanation: A megohmmeter reading of 0.5 MΩ (500 kΩ) to ground is below the typical acceptable threshold of 1 MΩ for this voltage class, indicating weak or compromised insulation. This suggests a ground fault, where current is leaking from the windings to the motor frame, causing the overcurrent device to trip. Overloading would generally not result in such a low insulation resistance reading to ground. An open circuit would prevent current flow, and a faulty starting capacitor would prevent the motor from starting but not typically cause an immediate trip due to a ground fault.