ElectroHub

Insulation Resistance (Megger) Calculator

A megger reading means nothing until it is corrected for temperature. Insulation resistance roughly halves for every 10 °C rise, so a winding read at 20 °C shows four times what it will show at 40 °C. That is the correction people skip, because it is the one that makes the result worse.

Insulation resistance (megger test)

A megger reading means nothing until it is corrected for temperature. Insulation resistance roughly halves for every 10 °C rise, so IEEE 43 refers every reading to 40 °C before comparing it with anything.

Acceptance rule: 1 MΩ per kV, minimum 1 MΩ

Polarisation index (optional)

Corrected to 40 °C
Minimum acceptable
Test voltage
Passes — 53 MΩ against a 1 MΩ minimum.
  • Measured at 25 °C, the reading is corrected downward to 53 MΩ. A cold winding flatters itself — this is the correction people forget, because it makes the result worse.
  • The correction doubles per 10 °C, so it grows fast. Twenty degrees off reference is a factor of four in either direction, which is larger than most of the margins being argued about.
Common trap: Testing a machine cold and calling it healthy is the failure this correction exists to stop. A winding read at 20 °C shows four times the resistance it will show at 40 °C — so a 150 MΩ reading on a cold motor is really 37.5 MΩ, and a 100 MΩ acceptance limit fails it.
Acceptance values from IEEE 43-2013 for rotating machines and the 1 MΩ-per-kV rule used by IS 732 for installations; test voltages from IEEE 43 Table 1. A pass is a minimum, not a certificate — the trend across successive tests tells you far more than any single reading.

The formula

R₄₀ = R_measured × 2^((T − 40)/10) ; PI = R at 10 min / R at 1 min ; installation minimum = 1 MΩ per kV

R₄₀
resistance referred to the 40 °C reference, MΩ
T
winding temperature when the reading was taken, °C
PI
polarisation index — below 1 dangerous, 2 to 4 good
V_test
DC test voltage: 500 V below 1 kV, 5 kV at 11 kV (IEEE 43 Table 1)

Worked example

150 MΩ measured on a 415 V installation at 25 °C.

  1. Correction factor = 2^((25 − 40)/10) = 2^−1.5 = 0.354
  2. R₄₀ = 150 × 0.354 = 53.0 MΩ
  3. Minimum for a 415 V installation = 1 MΩ
  4. Test voltage for a winding under 1 kV = 500 V DC

53 MΩ corrected, comfortably above the 1 MΩ minimum — but the same 150 MΩ on an 11 kV machine at 20 °C corrects to 37.5 MΩ and fails a 100 MΩ limit.

Where you'll use it

Every megger test in a machines lab and every pre-commissioning check on a motor, cable or transformer. The trend across successive tests says far more than any single reading, and a failed test usually means moisture rather than damaged insulation — dry the winding and repeat before condemning it.

The laws behind it

Parts this applies to

Resistor (fixed)Thermistor (NTC / PTC)Photoresistor (LDR)Resettable Fuse (PTC / Polyfuse)Temperature Sensor (thermocouple/RTD/IC)Humidity Sensor (DHT etc)Gas / Air-Quality Sensor (MQ)Load Cell / Strain Gauge

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