Motor Megger Testing Procedure
Objective
To check the health of motor insulation between:
Phase to Phase
Phase to Earth (Body)
Winding to winding (for higher voltage motors)
Test Equipment Required
Insulation Resistance Tester (Megger),
Digital Multimeter,
Discharge rod (for HV motors),
PPE (gloves, safety shoes, goggles) &
Lockout-Tagout (LOTO) kit.
Safety Precautions
Isolate motor supply completely.
Apply LOTO (Lockout Tagout).
Confirm no voltage using multimeter.
Discharge winding before and after testing &
Never touch terminals during test.
Recommended Megger Voltage Selection
| Motor Rated Voltage | Megger Test Voltage |
|---|---|
| Up to 250 V | 250 V DC |
| 250 - 500 V | 500 V DC |
| 500 - 1000 V | 1000 V DC |
| Above 1 kV | 2500 V / 5000 V DC |
Motor Megger Testing Procedure (Step-by-Step Procedure)
Step-1: Isolate Motor
Switch OFF breaker.
Remove cable connections if possible.
Ensure motor is not connected to VFD (disconnect VFD output).
Step-2: Clean Terminals
Remove dust, moisture, oil, or carbon deposits from terminal box.
Step-3: Phase-to-Earth Test
Connect:
- Megger Line (L) → U phase
- Megger Earth (E) → Motor body
- Press TEST for 60 seconds
Note reading
Repeat for V and W phases.
Step-4: Phase-to-Phase Test
Test between:
U – V
V – W
W – U
Step-5: Record Readings
Measure at:
15 sec
30 sec
60 sec (standard value)
Optional advanced tests:
Polarization Index (PI Test) &
Dielectric Absorption Ratio (DAR)
Insulation Resistance Values
Rule of Thumb:
Minimum IR (MΩ) = Rated Voltage (kV) + 1
| Motor Voltage | Minimum Acceptable IR |
|---|---|
| 415 V | ≥ 1.4 MΩ (preferably >10 MΩ) |
| 6.6 kV | ≥ 7.6 MΩ |
| 11 kV | ≥ 12 MΩ |
Healthy Motor Typically Shows:
1). New motor: >100 MΩ
2). Good motor: >10 MΩ
3). Warning: <5 MΩ
4). Dangerous: <1 MΩ
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