How to megger a motor safely

An insulation tester (megger) puts hundreds of volts on the windings. Done wrong, it can hurt you or destroy a drive. Done right, it takes ten minutes.

Safety first. Follow your site's lockout/tagout procedure and electrical safety rules. Only qualified persons should do this work. Never defeat, jumper or bypass any safety device or interlock to make a test easier — if a procedure seems to need that, stop and ask.

1. Lock out and tag out

Open the disconnect that feeds the motor circuit, apply your own lock and tag, and try the start to confirm it is isolated.

2. Verify zero energy

Live-dead-live: check your voltmeter on a known live source, test every phase-to-phase and phase-to-ground at the motor terminals, then check the meter on the known source again.
Wait for drive capacitors to discharge. A variable-frequency drive's DC bus can stay charged for minutes after power is off. Wait the time on the drive label and measure the DC bus before touching anything.
Check for other energy: a motor that can be turned by its load (a fan, a falling axis, a pump with flow) can generate voltage. Block it.

3. Disconnect the drive leads

Never megger through a drive. The test voltage can destroy the drive's output transistors. Disconnect the motor leads (U, V, W) from the drive or starter and keep them apart. Also disconnect any electronics on the motor circuit (encoders, thermistors, surge suppressors) per the manufacturer.

4. Test

Choose the test voltage for the motor rating (commonly 500 V DC for low-voltage motors — follow your site standard and the motor documentation).
Test each lead to ground (PE/frame) for the set time, typically 1 minute.
Optionally test the cable and the motor separately to find which one is bad.

5. Read the result

Hundreds of megohms: healthy. Falling towards a few megohms: moisture or ageing — investigate and trend it. Below about 1 MΩ: do not run it; dry out or repair. Your site or the motor maker may set different limits.

6. Discharge and restore

Let the tester discharge the winding (it holds a charge after the test). Reconnect the leads correctly, remove your lock only when everyone is clear, and prove the motor runs.

Practise the whole sequence, including the traps, in the Technician calls of Service Call: PLC Trainer.

← All help articles · Watch a job being played