WiremanWireman · Reading megger and winding test results: troubleshooting for electricians · 2023 NEC ·
Reading megger and winding test results
What a spot reading, a polarization index and winding resistances tell you about a motor.
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How it works
An insulation tester (megger) puts a high DC voltage between the windings and the frame and turns the small leakage current into a megohm reading. Good insulation reads high and keeps climbing as it polarizes; damp, dirty or damaged insulation reads low and stays flat. One number means little alone: compare it to the IEEE 43 minimums, correct it to 40 °C, and trend it against earlier tests on the same motor.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Which reading do you want to understand?
- One-minute reading to groundCorrect the reading to 40 °C. Where does it fall?
- Below 5 MΩHas the motor been wet, sat idle in a damp place, or is it dirty or oily?
- YesMoisture or contamination
- No, it’s been clean and dryTest: Test the motor and cable separatelyExpect: Both at or above the minimum.
- Cable reads low, motor goodCable insulation fault
- Motor reads lowMotor winding insulation failure
- 5 MΩ or more, but far lower than past testsAbove minimum but falling
- 5 MΩ or more, similar to past testsInsulation passes
- Below 5 MΩHas the motor been wet, sat idle in a damp place, or is it dirty or oily?
- Polarization index (10 min ÷ 1 min)What is the polarization index?
- 2.0 or higherInsulation condition is good
- 1.5 to 2.0Marginal polarization index
- Below 1.5Has the motor been wet, sat idle in a damp place, or is it dirty or oily?
- YesMoisture or contamination
- No, it’s been clean and dryTest: Test the motor and cable separatelyExpect: Both at or above the minimum.
- Cable reads low, motor goodCable insulation fault
- Motor reads lowMotor winding insulation failure
- Winding resistance between leadsTest: Compare the three winding resistancesExpect: All three within about 5% of each other.
- Within about 5%Windings are balanced
- One pair more than 5% offWinding resistance unbalance
- One pair open (OL)Open winding
- One-minute reading to groundCorrect the reading to 40 °C. Where does it fall?