Insulation Resistance Testing
Insulation resistance testing (megger testing) checks the integrity of motor and cable insulation. Learn how to test, interpret results, and when to act.
What is Insulation Resistance Testing?
Insulation resistance testing (commonly called megger testing) measures the resistance of the insulation between motor windings and the frame, or between cable conductors and earth. It applies a high DC voltage (typically 500V or 1,000V) and measures the leakage current, calculating resistance.
Good insulation has very high resistance (megohms). Degraded insulation has lower resistance, indicating that the insulation is breaking down and may fail, causing a short circuit or ground fault.
How to Test
- Isolate the motor or cable from the supply. Lock out and tag out.
- Discharge any stored capacitance (especially in cables) by shorting the conductors to ground.
- Disconnect any electronic devices (VFDs, surge protectors) that could be damaged by the test voltage.
- Connect the megger: one lead to the winding (or conductor), the other to the frame (or earth).
- Apply the test voltage for 1 minute. Record the resistance reading.
- Test each phase to ground and phase to phase (for 3-phase motors).
- After testing, discharge the windings by shorting to ground. Stored DC charge can give a shock.
Interpreting Results
| Reading (500V test) | Assessment | Action |
|---|---|---|
| Above 100 megohms | Excellent | No action needed |
| 10 to 100 megohms | Good | Acceptable. Continue trending. |
| 1 to 10 megohms | Questionable | Investigate. May be moisture or contamination. Dry and re-test. |
| 0.1 to 1 megohm | Poor | Insulation is degraded. Plan motor rewind or cable replacement. |
| Below 0.1 megohm | Critical | Do not energise. Immediate rewinding or replacement required. |
Moisture affects readings
Motors that have been unused for extended periods, especially in humid environments, absorb moisture into the insulation. This lowers the megger reading without permanently damaging the insulation. Dry the motor (space heaters or low-voltage heating) and re-test before condemning it.
Polarisation Index (PI)
The Polarisation Index (PI) is a more sophisticated insulation test. It is the ratio of the 10-minute insulation resistance reading to the 1-minute reading. PI = R(10 min) divided by R(1 min).
Good insulation has a PI above 2.0. A PI below 1.0 indicates degraded insulation. The PI test takes 10 minutes but provides more reliable information about insulation condition than a single reading.
Tip: Trend insulation resistance over time. A gradual decline is more significant than a single low reading. Test motors annually and record results. PlantLogica's asset records can store these test results and automatically flag motors with declining insulation resistance for attention.