How to Test Insulation Resistance (Megger Test) Step-by-Step
An insulation resistance test (often called a “megger” test) confirms that cable and equipment insulation is safe. This step-by-step guide shows you how to test insulation resistance correctly.
What You’ll Need
An insulation resistance tester (such as the Marmonix MIR 1000, MIR 510, or MIR 1020) rated for the required test voltage, plus PPE.
Step-by-Step
- Isolate and discharge the equipment before testing.
- Select the test voltage matched to the equipment rating.
- Connect the leads (line to conductor, earth to ground/frame).
- Apply the test and read insulation resistance in megohms.
- Read and hold; record PI or DAR for diagnostics if supported.
- Discharge and disconnect safely (use a tester with auto-discharge).
Interpreting Results
Higher is better. A common minimum guideline is 1 MΩ per kV of rating, but always follow the standard for the equipment. Falling readings over time indicate degrading insulation.