Explainer video · TEMISA

High megger reading but a damp motor: the polarization index (IEEE 43)

Short answer

Yes. If insulation is clean and dry, leakage current drops over time; with moisture or dirt it stays high. The polarization index divides the 10-minute insulation resistance reading by the 1-minute one, and IEEE 43 recommends a minimum of 2 for most insulation. If it comes out lower, the winding is cleaned and dried before starting.

Key points

A high 1-minute reading doesn't prove dry insulation.
Clean, dry insulation shows leakage current falling over time.
PI = 10-minute reading divided by 1-minute reading.
IEEE 43 recommends at least 2 for most insulation.
Below that, clean and dry before starting.

Transcript

Why can your megger read thousands of megohms and the motor still be damp? It's not the one-minute number, it's how it changes over ten. If the insulation is clean and dry, leakage current drops over time; with moisture or dirt, it stays high. Like a dry sponge versus a wet one: from the outside they look the same. The polarization index divides the 10-minute reading by the 1-minute one, and IEEE 43 recommends a minimum of 2 for most insulation. But careful, if it comes out lower, it's cleaned and dried before starting. So it's not the high reading, it's the trend. Did yours pass the polarization index? Comment MEGGER and we'll tell you what to check.

Frequently asked questions

How is polarization index calculated?

Insulation resistance at 10 minutes divided by the reading at 1 minute.

What minimum does IEEE 43 recommend?

An index of 2 for most insulation systems.

What if it is lower?

Clean and dry the winding before starting. TEMISA tests and dries motors; email ventas@temisa.mx.

Video with images and narration generated with AI.

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