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Polarization Index (PI)

This test is a more broad scope testing than just a simple insulation resistance (IR) test. This is a two part test done at a 1 minute, and a 10 minute interval. These readings are a ratio (divided by each other) to give a value which equals the polarization index absorption value. A PI value below 2 is not good and requires an investigation. Polarization can occur from 10 minutes to several hours so the ratio was needed in order to show the proper readings based over a period of time.

This test can point out issues in moisture, cracking and contamination. There are some DC machines though that will appear to have insulation values less than recommendation, but are perfectly normal due to their commutators and large end arm surface area.

Insulation Resistance (IR)
If you are testing insulation resistance you are really determining the (state or condition) of the conductor to ground. In our tests we inject voltage into the conductor or winding, and measure the leakage to ground. Depending on the sensitivity of the equipment, we can determine how sound the cable and/or windings are.

When testing occurs insulation resistance DECREASES with temperature. We know that with a conductor, resistance INCREASES with temperature, so it's exactly the opposite with the insulation. This is why it's important to keep the test within specified parameters for each round.

Also, having the equipment at the required temperature and dew point is normally done when it cools down. This would have to scheduled with PM (preventive maintenance) or during equipment shut down times.