Free Engineering Tool
Insulation testing
Insulation Resistance Temperature Correction Calculator
Enter your megger reading and the winding temperature. Get the reading corrected to a common reference temperature so you can trend it honestly.
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Inputs
20°C is common for trending. 40°C is the IEEE 43 reference.
Readout
How it works
Insulation resistance falls as temperature rises. The field rule, used in IEEE 43 practice, is that the reading roughly halves for every 10°C rise and doubles for every 10°C fall. The correction factor is:
Factor = 2 ^ ((Ttest - Tref) / 10) · Corrected IR = Measured IR × Factor
Example: 25 MΩ measured at 40°C is 100 MΩ when referred to 20°C (factor 4.0). Log the corrected value, not the raw one.
Limits of this calculation
- It is an approximation. The true factor varies with insulation class and moisture, so use it for trending and screening, not for a legal pass/fail.
- Take the actual winding temperature, not the room temperature. A surface or infrared thermometer on the frame is a reasonable stand-in.
- Do not use it for PI or DAR. Those are ratios of readings in one test at one temperature, so temperature cancels out. Use the PI and DAR calculator.
Background reading: Insulation resistance temperature correction explained.
Report form: printable insulation resistance test report (PDF) that applies this correction for you.
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