Electric motor insulation resistance test (megger)

What’s inside

1. 1

   Isolate and lock out the motor

   Isolate the motor supply and apply locks and tags following your lockout / tagout procedure, then verify absence of voltage at the motor terminals.

   Action15 min

   Tools: Lockout devices, voltage tester

   Risks: Electric shock and arc flash; wear the PPE required for the supply.

2. 2

   Discharge the windings

   Connect each winding to earth to drain any stored charge, including from power factor correction capacitors, before touching the terminals.

   Action5 min

   Tools: Earthing lead

   Risks: Windings and capacitors can hold a dangerous charge after isolation.

3. 3

   Disconnect the motor from cables and electronics

   Open the terminal box and disconnect the supply cable. Never megger through a variable frequency drive, soft starter or other electronics: the test voltage destroys them. Disconnect surge capacitors and winding sensors, or test them separately.

   Action20 min

   Tools: Insulated hand tools

   Risks: Test voltage applied to drive electronics or sensors damages them beyond repair.

4. 4

   Record motor data and winding temperature

   Note the rated voltage and power from the nameplate, and measure the winding or frame temperature and the ambient humidity. Insulation resistance depends strongly on temperature.

   Measurement5 min

   Tools: Contact or infrared thermometer, hygrometer

5. 5

   Select the test voltage

   Choose the DC test voltage from the motor’s rated voltage, following IEEE 43 practice or your site standard. Typical guidance: 500 V DC for motors rated up to 1000 V, 500–1000 V DC up to 2.5 kV, 1000–2500 V DC up to 5 kV.

   Verification2 min

   Tools: Insulation resistance tester (megohmmeter)

6. 6

   Measure phase-to-earth insulation for 1 minute

   Connect the tester between the three phases tied together, or each phase in turn, and the motor frame earth. Apply the test voltage for 1 minute and record the value; where possible also record the 10-minute value to calculate the polarization index.

   Measurement10 min

   Tools: Insulation resistance tester, test leads

   Risks: Terminals and cables carry the test voltage during the test; keep everyone clear.

7. 7

   Measure phase-to-phase insulation

   Optional: where both ends of each winding are accessible, test between phases to detect inter-phase insulation weakness.

   Measurement10 minoptional

   Tools: Insulation resistance tester, test leads

8. 8

   Correct for temperature and compare with the trend

   Correct the reading to a common base temperature, usually 40 °C (resistance roughly halves for every 10 °C rise), and compare it with this motor’s previous results. A steady fall matters more than one value.

   Verification10 min
9. 9

   Check against the acceptance limit

   IEEE 43 minimum for the 1-minute value corrected to 40 °C: 5 MΩ for random-wound stators and for form-wound coils rated below 1 kV (most low-voltage motors); 100 MΩ for form-wound AC windings built after about 1970 (most medium-voltage motors); 1 MΩ plus 1 MΩ per kV of rated voltage only for windings built before about 1970 and for field windings. Use your site standard where it is stricter. Where measured, the polarization index should be 2 or more. Below the limit, do not return the motor to service: dry it out and retest.

   Verification5 min

   Risks: Energising a motor with failed insulation can cause an earth fault, fire or electric shock.

10. 10

    Discharge the windings again

    Earth the windings after the test for at least as long as the test voltage was applied, ideally four times longer, before touching the terminals.

    Action5 min

    Tools: Earthing lead

    Risks: The winding stores charge from the test voltage.

11. 11

    Reconnect and restore the motor

    Reconnect the cables to their marked terminals, tighten to the specified torque and close the terminal box with its gasket. Remove locks following your lockout procedure, restart and check the direction of rotation if connections were disturbed. Record all results.

    Action20 min

    Tools: Torque wrench, insulated hand tools

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