Using a Megger (Insulation Tester) for HVAC Troubleshooting

What is a Megger?

• A megger is a tool designed to push high voltage through the windings of a compressor to detect insulation breakdown.

• By applying this higher voltage, the megger tests for leakage to the compressor casing, identifying potential short circuits to ground.

• The process involves:

  • Connecting one lead of the megger to the winding.

  • Connecting the other lead to a good grounding point, such as the copper discharge pipe or another clean, unpainted metal surface on the compressor.

• Important Tip: Paint can act as an insulator, so be sure to scratch off any paint to ensure a solid connection.

When Should You Use a Megger?

A megger is best used when troubleshooting a compressor that is tripping a breaker or blowing a fuse — typically due to a short circuit.

Steps for Using a Megger:

1. Verify the Compressor is the Issue:

   • Isolate the compressor by disconnecting it completely.

   • Run the system without the compressor connected — if the unit runs without tripping, the compressor is likely the culprit.

2. Testing with the Megger:

   • Connect one megger lead to a compressor winding.

   • Connect the other lead to the copper discharge pipe or another metal ground point.

   • The megger will indicate if there’s a short to ground.

Important Considerations & Notes:

• Open Winding Compressors:

  • These compressors may still test fine with a megger, even if they have other issues.

• Scroll Compressors:

  • Scroll motors are pressed tightly into the case, which can sometimes cause slight voltage leakage.

  • This may trigger a "bad" reading on certain meggers (like the Supco M500) when the compressor is actually fine.

• Winding-to-Winding Testing:

  • Meggers are not intended to test winding-to-winding resistance.

  • Since windings are connected, this test is ineffective and won’t provide meaningful results.

Key Takeaways:

• Use a megger primarily to diagnose short circuits to ground in compressors.

• Always isolate the compressor first to confirm it's the source of the electrical fault.

• Be aware of false positives, especially with scroll compressors.

• Avoid using a megger for winding-to-winding testing — it won’t yield useful results.

By adding a megger to your troubleshooting toolkit, you can quickly and accurately diagnose compressor electrical issues, saving time and reducing guesswork.