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Insulation resistance (IEEE 43)

Enter the megohmmeter readings and the winding temperature to see whether the insulation of a motor or generator meets IEEE 43 and how it has evolved between inspections.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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Leave the 30 s or 10 min reading at 0 if it was not taken. Test with the machine off, disconnected and grounded; after the test, keep the winding grounded for at least 4 times the voltage application time.

Inspection history (paste from Excel)

One row per test: date; 1-minute resistance in MΩ; winding temperature in page units; PI (optional). Each reading is corrected to 40 °C (104 °F) to show the trend.

See what was read
—1-minute resistance corrected to 40 °C
—

Test absorption curve

Measured resistance and resistance corrected to 40 °C during the test, with the recommended minimum. Log scales.

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Trend of corrected resistance

History readings corrected to 40 °C, exponential trend line and recommended minimum.

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Method, assumptions and references

Equations used

Correction to 40 °C: R40 = Kt · RT, Kt = 0.5^((40 − T) ÷ 10) (T in °C)
Polarization index: PI = R10min ÷ R1min; absorption ratio: DAR = R60s ÷ R30s
Minimum at 1 min and 40 °C (IEEE 43-2013, Table 3): kV + 1 MΩ; 100 MΩ; 5 MΩ. Minimum PI (Table 2): 1.5 class A; 2.0 classes B, F and H
Leakage current: I (µA) = V ÷ R (MΩ); trend: ln R40 = a + b · year, yearly change = e^b − 1

Assumptions and limits

  • Temperature correction by doubling every 10 °C is the IEEE 43 approximation and works best near 40 °C. To compare inspections, always test at a similar temperature and voltage.
  • With 1-minute resistance above 5000 MΩ (corrected), IEEE 43 notes that PI may not indicate insulation condition.
  • The absorption ratio is not part of IEEE 43; the ranges shown are field practice published by Megger.

Standards and technical documents

  • IEEE Std 43-2013: Recommended Practice for Testing Insulation Resistance of Electric Machinery
  • IEC 60034-27-4:2018: Rotating electrical machines — Measurement of insulation resistance and polarization index of winding insulation
  • ANSI/EASA AR100: Recommended Practice for the Repair of Rotating Electrical Apparatus
  • Megger: A Stitch in Time — The Complete Guide to Electrical Insulation Testing

U.S. regulations

  • NFPA 70E: Standard for Electrical Safety in the Workplace
  • NFPA 70B: Standard for Electrical Equipment Maintenance
  • OSHA 29 CFR 1910.333: selection and use of electrical work practices

Frequently asked questions

What is the minimum insulation resistance of a motor?

Per IEEE 43-2013, the 1-minute reading corrected to 40 °C (104 °F) should be at least 100 MΩ for modern form-wound windings and 5 MΩ for low-voltage random-wound motors. For older machines use kV + 1: 5.16 MΩ for a 4160 V motor.

Why correct the reading to 40 °C (104 °F)?

Because resistance halves for every 18 °F (10 °C) of warming. A motor reading 1800 MΩ at 86 °F equals 900 MΩ at 104 °F; without correction, readings taken on hot and cold days cannot be compared.

How do I calculate the polarization index?

Divide the 10-minute reading by the 1-minute reading. With 5200 MΩ and 1800 MΩ, PI is 2.9, above the 2.0 minimum for class F. A PI below 1 points to wet or contaminated insulation.

What megger voltage should I use?

Per IEEE 43 Table 1: 500 V for windings up to 1000 V; 500 to 1000 V up to 2500 V; 1000 to 2500 V up to 5000 V; 2500 to 5000 V up to 12 kV. A 4160 V motor is tested at 2500 V.