How to Megger Test a Motor: Step-by-Step Guide

Guides 2026-04-08 8 min read

A megger test (insulation resistance test) is the single most useful diagnostic you can perform on an electric motor. It tells you whether the motor's winding insulation is healthy, degraded, or failed. Every maintenance electrician should know how to do this — and more importantly, how to interpret the results.

Warning

Safety first: Always lock out and tag out the motor and disconnect it from the VFD or starter before megger testing. A megger applies high voltage (typically 500V or 1000V DC) to the windings. Verify zero energy with a voltmeter before connecting the megger.

What You Need

Insulation resistance tester (megger) — 500V DC for motors up to 480V, 1000V DC for medium voltage
Multimeter for verifying zero energy
Motor leads disconnected from the drive/starter
Clean, dry conditions (moisture will affect readings)

Step-by-Step Procedure

Lock out / tag out the motor circuit. Verify zero energy with a multimeter at the motor terminals.
Disconnect the motor leads from the VFD or starter. You're testing the motor and its cable, not the drive.
Set the megger to the correct test voltage: 500V DC for motors rated 480V and below, 1000V DC for higher voltages.
Connect the megger's ground lead to the motor frame (a clean, unpainted surface). Connect the line lead to one motor phase lead (T1/U).
Press the test button and hold for 60 seconds. Record the reading at 60 seconds — this is your insulation resistance value in megohms (MΩ).
Repeat for each phase to ground (T1→GND, T2→GND, T3→GND).
Test phase-to-phase: T1→T2, T2→T3, T1→T3. These readings should be relatively equal.
After testing, discharge each winding to ground before reconnecting (the megger charges capacitance in the winding).

How to Interpret the Results

The IEEE 43 standard and general industry practice give these guidelines for motor insulation resistance (phase-to-ground, 500V DC test, at 40°C ambient):

Above 100 MΩ — Excellent. Motor insulation is in great shape.
10–100 MΩ — Good. Normal for older motors or humid environments. No action needed.
2–10 MΩ — Marginal. Schedule a rewind or replacement. Monitor frequently.
1–2 MΩ — Poor. Plan replacement soon. Do not put back in service without investigation.
Below 1 MΩ — Failed. Motor insulation has broken down. Do not energize.

Tip

Temperature matters: insulation resistance halves for every 10°C increase. If you're testing a hot motor, the readings will be lower than a cold motor. IEEE 43 provides correction factors, or simply test the motor at ambient temperature for the most reliable baseline.

When to Megger Test

After any VFD overcurrent (F012, oC) or ground fault (F013, GF) trip
Before re-energizing a motor that has been sitting idle for months
After a motor has been exposed to moisture (flood, washdown, outdoor rain)
As part of annual PM on critical motors
Before and after a motor rewind to verify quality

Common Mistakes

Testing with the motor connected to the VFD — the megger voltage can damage the drive's output stage
Not waiting 60 seconds — capacitive charging in large motors takes time to stabilize
Testing in humid conditions without accounting for surface leakage
Forgetting to discharge windings after the test — stored charge can shock you

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