Polarization Index and Dielectric Absorbation Test
What do they tell you?
The polarization index (PI) and Dielectric Absorption (DA) ratios indicate the condition of the insulation system of the motor and power circuit. Both of these tests use ratios of measurements of insulation resistance taken at two different times. The PI is the ratio of the reading taken at 10 minutes and divided by the reading taken at 1 minute. The DA is the ratio of the reading taken at 60 seconds divided by the reading taken at 30 seconds.
There are three different currents that flow through an insulator when a voltage potential is applied. Since the RTG test measures the voltage and current to calculate insulation resistance, all of these currents must be taken into account.
First, the "charging current" starts out high and drops to nearly zero after the insulation has been charged to full test voltage. This is normally negligible after the first few seconds of the test..Second the "absorption current" also starts out high and drops off. The majority of this current dissipated after one minutes, but continues to decay for up to 5 to 10 minutes.Finally the "conduction" or "leakage current" is a small, mostly steady current which becomes a factor after the absorption current drops to a negligible value. This current should remain steady for the remainder of the test.
As the motor accumulates dirt and as the insulation ages and cracks, the PI and DA ratios decrease. Dirt accumulates based on the operation and the environment of the motor. The insulation cracks as a function of heat and aging of the motor.
Because of the effects of each if these varying currents, the resistance to ground measured by any insulation tester varies with the amount of time the voltage is applied to the insulation. In order to trend or compare insulation RTG values, the charge time for all tests MUST be the same. If the charge time is not the same, the trend or comparison may not be valid.
Finally, the charging developed by these three different currents does not dissipate immediately when the voltage is removed at the end of the test. The insulation system must be allowed to discharge sufficiently between resistance to ground tests in order to obtain accurate results. A rule of thumb states that a motor takes four times the amount of charge time to discharge.
Why is this important?
Resistance-to-ground readings involve three different current components: charging, absorption and leakage. The PI test allows the charging and absorption currents to decay so that only actual leakage current is measured. As a voltage is continuously applied, healthy insulation slowly polarizes and the absorption current diminishes. This causes a steady rise in resistance until the majority of the current is from the small amount leaking to ground. In poor insulation, leakage current is high enough to overshadow the lowering absorption current and provide little increase in the resistance over time.
