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.
Interpretation
Minimum Insulation Resistance (IEEE 43)
Per IEEE Std 43, the minimum recommended 1-minute insulation resistance at 40 °C is:
Where kV is the rated voltage in kilovolts. For a 4160V motor: R_min = 4.16 + 1 = 5.16 MΩ.
⚠️ Scope of the kV + 1 rule: IEEE Std 43 recommends kV + 1 MΩ for windings made before about 1970. For most form-wound windings made after about 1970 the recommended minimum is 100 MΩ, and for random-wound stators rated below 1 kV it is 5 MΩ. For modern form-wound machines, judge the corrected IR against 100 MΩ rather than the kV + 1 value used below.
Reference note: IEEE Std 43-2013 is Inactive-Reserved (since 2024); revision project P43 is in progress.
Temperature Correction
Insulation resistance approximately doubles for every 10 °C decrease in temperature. To normalize to 40 °C:
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 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.
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Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
