Step-by-Step How to Test Motor Insulation

Why a Procedure Beats Improvisation

Insulation testing on a 480 V three-phase motor is one of the highest-stakes routine tasks in plant maintenance. Done correctly it adds years of service life; done carelessly it electrocutes the technician or destroys a winding. The procedure below is the EK service-engineer standard and follows IEEE 43.

Step 1 — Isolate and Lock Out

De-energise the motor at the upstream breaker, lock out and tag out, and verify zero voltage with a separate multimeter. Disconnect the motor cables from the starter contactor — testing through the contactor will measure the contactor too and give meaningless readings.

Step 2 — Discharge Stored Charge

Connect a grounding strap to all three phase leads tied together and to the motor frame. Wait at least 5 minutes for large motors. Many fatal accidents involve a motor that 'should be' discharged but retained capacitive charge from a recent megger test.

Step 3 — Choose Test Voltage

Use the standard rule: 1000 V DC for motors rated 440–600 V; 2500 V DC for motors 1 kV to 5 kV. Higher-voltage motors require dedicated HV insulation testers and are outside the scope of routine work.

Step 4 — Connect the Tester

Connect the negative (earth) lead to the motor frame. Connect the positive (line) lead to one phase, with the other two phases grounded together. This isolates the test winding from the others.

Step 5 — Measure IR, DAR and PI

1. Apply the test voltage and start the timer.
2. Record the IR reading at 30 s and at 60 s.
3. Continue the test for 10 minutes; record IR at 10 min.
4. Calculate DAR = IR(60 s) ÷ IR(30 s). >1.4 = good; 1.0–1.25 = moisture or contamination.
5. Calculate PI = IR(10 min) ÷ IR(1 min). >4 = excellent; 2–4 = acceptable; <2 = service required.

Step 6 — Repeat for All Three Phases

Move the tester lead to phase 2, ground phases 1 and 3, and repeat. Then phase 3. The three sets should agree within 10 %; a large outlier indicates a winding-specific problem.

Step 7 — Discharge and Reconnect

Short the test lead to ground for at least the test duration (10 minutes here) before disconnecting. Restore the motor cable connections, remove lockouts, and document all six readings plus calculated PI/DAR in the maintenance log.

Trend Over Time

A single number means little; the trend is everything. Plot annual PI for each motor — a downward trend warrants intervention even if the absolute value is still in spec. EK motor insulation testing reports include the trend chart automatically.