Free Engineering Tool

Motor Insulation Resistance Interpreter

Interpret megger test results per IEEE Std 43. Calculate minimum acceptable insulation resistance, temperature-corrected values, polarization index, and trending recommendations.

IEEE 43 Megger Test Polarization Index

Interpretation

Overall Assessment

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Minimum Acceptable (IEEE 43)

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Corrected IR @ 40°C

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Temperature Correction Factor

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Polarization Index (PI)

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Recommended Test Voltage

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Trending Recommendation

Minimum Insulation Resistance (IEEE 43)

Per IEEE Std 43, the minimum recommended 1-minute insulation resistance at 40 °C is:

R_min = kV + 1 (MΩ)

Where kV is the rated voltage in kilovolts. For a 4160V motor: R_min = 4.16 + 1 = 5.16 MΩ.

Temperature Correction

Insulation resistance approximately doubles for every 10 °C decrease in temperature. To normalize to 40 °C:

R₄₀ = R_T × C_T, where C_T = 0.5^((40 − T)/10)

For a reading at 25 °C: C_T = 0.5^((40−25)/10) = 0.5^1.5 = 0.354. So a 500 MΩ reading at 25 °C becomes 177 MΩ at 40 °C.

Polarization Index (PI)

PI = IR₁₀min / IR₁min

PI Value	Insulation Condition
< 1.0	Dangerous — do not energize
1.0 – 2.0	Questionable
2.0 – 4.0	Good
> 4.0	Excellent

Recommended Test Voltages (IEEE 43)

Motor Rated Voltage	DC Test Voltage
< 1000 V	500 V DC
1000 – 2500 V	500 – 1000 V DC
2501 – 5000 V	1000 – 2500 V DC
5001 – 12000 V	2500 – 5000 V DC
> 12000 V	5000 – 10000 V DC

⚠️ Important: A single IR reading is less meaningful than a trend. Always compare with previous readings taken under similar conditions. A sudden drop (>50%) is more alarming than an absolutely low but stable value.

Motor Insulation Resistance (Megger) Interpreter | IEEE 43

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Interpretation

Minimum Insulation Resistance (IEEE 43)

Temperature Correction

Polarization Index (PI)

Recommended Test Voltages (IEEE 43)