Polarization Index Calculator (PI & DAR Insulation Test)

How to use the polarization index calculator

Apply the test voltage with an insulation resistance tester and record the readings at 30 seconds, 1 minute and 10 minutes. Enter them and the calculator returns the polarization index and the dielectric absorption ratio, each with a condition rating.

Polarization index formulas

PI = R_{10 min} ÷ R_{1 min}  ·  DAR = R_{60 s} ÷ R_{30 s}

Worked example

Motor winding readings at 1,000 V DC: 800 MΩ at 30 s, 1,000 MΩ at 1 min, 2,500 MΩ at 10 min:

• PI = 2,500 ÷ 1,000 = 2.50 — good (above the IEEE 43 minimum of 2.0)
• DAR = 1,000 ÷ 800 = 1.25 — questionable on its own, which is why the 10-minute test is preferred

Interpreting PI and DAR

Notes

• Insulation resistance roughly halves for every 10 °C rise — correct readings to 40 °C before comparing with past tests; PI is a ratio and is less sensitive to temperature.
• Discharge the winding fully after the test, for at least four times the test duration.

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