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Insulation resistance test: reading corrected to 40 °C

Correct a megger reading on a motor to 40 °C per IEEE 43-2013, compare it with the minimum for the winding and get the polarization index. Free, instant.

  • Instant result
  • Formula and standards shown
  • On desktop and phone

Pre-filled example

A low-voltage random-wound motor with thermoset insulation, reading 500 MΩ after 1 minute and 1,200 MΩ after 10 minutes, winding at 20 °C.

MΩ
°C
kV
MΩ

Result

381MΩResistance corrected to 40 °C

Insulation resistance at 40 °C: 381 MΩ, compliant

Verdict
Compliant
Required minimum
5.0MΩ
Correction factor Kt
0.762
Polarization index
2.40
Minimum index
2.0
Index verdict
Compliant
Test voltage, min
500 V
Test voltage, max
500 V
  • Caution : Condition indicator, not an acceptance criterion.
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Insulation readings kept test after test

Here, the calculator is free and the result is gone when you close the page. In Reglova, it is attached to the work order on the right machine: you find it again at the next breakdown, and on audit day.

The CMMS software comparison, our gaps included
  • Attached to the work order with its inputs: the server recalculates it, it stays verifiable.
  • On the technician's phone, even without a network.
  • In the machine's history, for the next breakdown.
  • The result becomes a checklist step of the work order.
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Reglova · Calculations · Work order WO-AT01-04219
Silent video, 33 s: from the work order for an oil leak on a shear cylinder, the tightening torque of an M12 bolt, then "Attach to the work order". A step to tighten it to 93.1 N·m lands in the checklist (step text from the French demo data). Watch the video

When to run a motor insulation resistance test

  1. 01

    Decide whether a motor left idle in a damp room can be energized again.

  2. 02

    Track a motor's insulation reading after reading, at the same reference temperature.

  3. 03

    Write the verdict in the work order report.

Good to know

This is a condition indicator, not an acceptance criterion. The trend matters more than a single reading: a drop from one reading to the next, at comparable temperature, warns before a breakdown.

IEEE 43 temperature correction and polarization index

A winding's insulation resistance falls as temperature rises: a reading is only comparable once corrected to 40 °C. The calculator applies the IEEE 43-2013 correction, compares the result with the minimum required for the winding type, suggests the test voltage and calculates the polarization index from the 1-minute and 10-minute readings.

Formula · Reference documents

R40 = Kt·RT ; thermoset: Kt = exp[−B·(1/(T + 273) − 1/313)], B = 1,245 below 40 °C, 4,230 above ; thermoplastic: Kt = 0.5^((40 − T)/10) ; PI = R10/R1

IEEE 43-2013, § 6.3 and tables 1 to 3

Inputs and accepted ranges
InputUnitAccepted rangeExample
Resistance at 1 minMΩ0.001 to 10,000,000500 MΩ
Winding temperature°C10 to 6020 °C
Insulation system2 optionsThermoset
Rated voltagekV0.01 to 300.4 kV
Winding3 optionsRandom-wound or under 1 kV
Thermal class4 optionsF
Resistance at 10 minMΩ0.001 to 10,000,0001,200 MΩ
Calculated results
  • Resistance corrected to 40 °CMΩ
  • Verdict
  • Required minimumMΩ
  • Correction factor Kt
  • Polarization index
  • Minimum index
  • Index verdict
  • Test voltage, minV
  • Test voltage, maxV
What the calculator flags
  • Condition indicator, not an acceptance criterion.
  • Above 5,000 MΩ at 1 min, the polarization index is not assessed.

Megger tests planned as preventive work orders

  • Breakdowns reported by QR code
  • Work orders on the phone, even without a network
  • Mandatory inspections reminded by email
  • Audit file as a PDF, in one click
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