Megger Testing Guide
A practical guide to megger testing motors, cables, and switchgear. Step-by-step procedure, safety precautions, and interpretation of results.
Safety First
Megger testing involves high voltage
A megger applies 500V, 1,000V, or even 5,000V DC. This can give a severe shock. Always isolate the equipment from the supply, lock out and tag out, discharge stored capacitance before and after testing, and never touch the test leads while the megger is operating. Only trained personnel should perform megger testing.
Test Voltages
| Equipment Rating | Test Voltage | Minimum Acceptable Reading |
|---|---|---|
| Low voltage (up to 240V) | 500V DC | 1 megohm minimum (ideally above 100) |
| Medium voltage (415V to 1kV) | 500V or 1,000V DC | 1 megohm per kV of operating voltage |
| High voltage (above 1kV) | 1,000V to 5,000V DC | 1 megohm per kV of operating voltage |
Step-by-Step Procedure for Motor Testing
- Isolate the motor at the starter. Lock out and tag out.
- Disconnect the motor leads from the starter (or open the terminal box and disconnect the leads).
- Disconnect any temperature sensors (RTDs, thermistors) from the terminal box to protect them.
- Connect the megger positive lead to one motor phase (U1). Connect the negative lead to the motor frame (earth).
- Apply the test voltage for 1 minute. Record the reading.
- Repeat for each phase (V1, W1) to earth.
- Test phase to phase: U1 to V1, V1 to W1, W1 to U1.
- After each test, discharge the winding by connecting it to earth for at least as long as the test was applied.
- Reconnect all leads and sensors. Verify the terminal box is properly sealed.
When to Megger Test
- New motor commissioning: Baseline reading for future comparison. Verify insulation is intact after transport and installation.
- After long shutdown: Motors that have been idle for months may have absorbed moisture. Megger before re-energising. Dry if necessary.
- After water exposure: If a motor has been flooded or exposed to water, megger before energising. Low readings indicate moisture in the windings. Dry thoroughly.
- Annual maintenance: Trend the insulation resistance of critical motors annually. Declining readings indicate insulation degradation.
- After a fault trip: If a motor trips on overload or ground fault, megger to check for insulation damage before re-energising.
- Before installation of spare motor: Verify the spare motor's insulation is intact before installing it.
Never megger a VFD-connected motor while connected
Megger testing applies high voltage that will destroy the output semiconductors of a VFD. Always disconnect the motor from the VFD before megger testing. Connect the test leads to the motor terminals, not to the VFD output terminals.
Tip: If a motor meggers low (below 1 megohm) after being idle, don't condemn it. It may just be damp. Apply space heaters or run a low voltage through the windings to warm and dry the insulation. Re-test after 24 hours. Many failed motors are restored to service simply by drying.