A maintenance electrician is using a megohmmeter (megger) to test the insulation resistance of a 600 V motor that has been sitting idle for several months in a damp environment. The motor leads are disconnected from the supply. They test between each winding and the motor frame, and between the windings themselves. What would be an acceptable minimum insulation resistance reading for this motor in megohms (MΩ)?

Question: A maintenance electrician is using a megohmmeter (megger) to test the insulation resistance of a 600 V motor that has been sitting idle for several months in a damp environment. The motor leads are disconnected from the supply. They test between each winding and the motor frame, and between the windings themselves. What would be an acceptable minimum insulation resistance reading for this motor in megohms (MΩ)?

Answer options:

• 0.1 MΩ
• 5 MΩ
• 100 MΩ ✅ 1 MΩ

Correct answer: 1 MΩ

Explanation: A common rule of thumb for acceptable insulation resistance is 1 MΩ plus 1 MΩ for each 1000 V of nameplate operating voltage, or simply 1 MΩ for equipment rated up to 1000 V. For a 600 V motor, 1 MΩ would be considered a minimum acceptable value, especially given potentially damp conditions. Readings below this suggest compromised insulation.