An electrician is checking a three-phase, 600 V motor using an insulation resistance tester (megohmmeter). With all three motor leads disconnected from the starter and isolated from each other and ground, the electrician tests between each lead and the motor frame. The readings are: Lead 1 to frame: 20 MΩ; Lead 2 to frame: >500 MΩ; Lead 3 to frame: 15 MΩ. What could these readings indicate?

Question: An electrician is checking a three-phase, 600 V motor using an insulation resistance tester (megohmmeter). With all three motor leads disconnected from the starter and isolated from each other and ground, the electrician tests between each lead and the motor frame. The readings are: Lead 1 to frame: 20 MΩ; Lead 2 to frame: >500 MΩ; Lead 3 to frame: 15 MΩ. What could these readings indicate?

Answer options:

• The motor has a winding-to-winding short.
• The motor is in excellent condition and ready for service. ✅ Lead 1 and Lead 3 likely have insulation degradation or contamination.
• The megohmmeter battery is low and needs replacement.

Correct answer: Lead 1 and Lead 3 likely have insulation degradation or contamination.

Explanation: Significant differences in insulation resistance readings between phases, especially values like 15 MΩ and 20 MΩ compared to >500 MΩ, suggest insulation degradation or surface contamination (e.g., dirt, moisture) on Lead 1 and Lead 3. A winding-to-winding short would be detected by testing between phases, not phase to ground. Excellent condition would show consistently high readings.