Summary

Insulation Resistance Measurements: Megohm, Polarization Index, and Dielectric Absorbtion Ratio

This page is an overview of the iTIG insulation resistance (IR) measurements. For details about IR measurement methods, including types of IR measurements and iTIG IR measurement techniques, see Insulation resistance (IR) measurement methods.

Why are Megohm, Polarization Index (PI), and Dielectric Absorption (DA) or Dielectric Absorption Ratio (DAR) measurements used?

Megohm Measurement

An insulation resistance measurement is used to diagnose breakdown in wire insulation integrity. The IR measurement is used to measure the winding’s insulation resistance (IR) to ground in megohms. When windings are dirty, insulation resistance is lowered. The megohm measurement result indicates how dirty, contaminated or wet the windings are. Dirty, contaminated, and wet states are all referred to as dirty. Typically, the lower the megohm value is, the dirtier the windings are. If the megohm measurement is below an acceptable level, the motor should be scheduled for reconditioning.

Important: A megohm measurement result can indicate whether insulation is weak at or below the recommended test voltage, so it also tells the test operator whether the windings can be over-voltage tested with a hipot or surge test.

Polarization Index (PI) and Dielectric Absorption (DA) Measurements

PI and DAR measurements are ratios calculated based on megohm measurements performed at different times. The PI calculation computes the polarization index as a ratio of the megohms measured at 10 minutes to the megohms measured at 1 minute. The DAR calculation computes the dielectric absorption ratio (DAR) as a ratio of the megohms measured at 1 minute to the megohms measured at 30 seconds. PI is most commonly used with form coil machines, and usually includes a megohm time series graph with multiple recorded megohm values. In addition to megohm data, this graph can provide information about the condition of the insulation system and indicate whether the insulation is brittle or delaminated. DAR and PI measurement results are useful to track over time in maintenance and reliability programs.

High Accuracy Insulation Resistance (IR) Megohm Test 
iTIG megohm screen with DAR displayed at left.

How Megohm, PI, and DAR Measurements Work

The iTIG features a highly accurate leakage current measurement. with accuracy of 2% and resolution of 10pA.  Insulation Resistance Measurement Methods contains more details about how megohm, PI, and DA measurements are conducted.

A voltage potential is applied to the windings. The voltage and resulting current to ground is measured, and the resistance in megohms (1,000,000Ω = 1megohm) is calculated using Ohm’s law.

For a megohm measurement on a used winding, often most of the leakage current is surface currents running in the dirt on the outside of the windings. The ground insulation certainly can be weak with conduction current flowing through it to ground. The insulation resistance can be burned or otherwise damaged, leading to a low megohm number. There can also be a slowly decaying insulation absorption current. However, in many cases a low megohm measurement reading is the result of dirty windings. Therefore, the insulation resistance or megohm measurement is sometimes referred to as the “dirt test.”

Polarization index (PI): The PI measurement is mainly used on form wound motors and generators. Megohm data is recorded over the 10 minutes, and the resulting graph and PI ratio can provide additional information about the winding insulation resistance as indicated above. The number of recorded data points can be programmed in the iTIG motor tester.

iTIG DC IR and Hipot Testing Specifications
iTIG model kV 4kV 6kV 12kV 12kV-H 15kV-H
Max resistance 8 TΩ 12 TΩ 24 TΩ 24 TΩ 30 TΩ
Min resistance 0.25 MΩ 0.25 MΩ 0.25 MΩ 0.25 MΩ 0.25 MΩ
Max output voltage 4 kV 6 kV 12 kV 12 kV 15 kV
Voltage accuracy 2% 2% 2% 2% 2%
Current resolution 10 pA 10 pA 10 pA 10 pA 10 pA
Current accuracy* 2% 2% 2% 2% 2%
Current trip-out 10–2,000 μA 10–2,000 μA 10–2,000 μA 10–2,000 μA 10–2,000 μA
*Offset: ±0.4 nA (preliminary)

The dielectric absorption (DA) measurement is used to calculate the dielectric absorption ratio (DAR). DAR is the ratio of the megohm measurement at 1 minute divided by the megohm measurement at 30 seconds (IR1m/IR30s). The PI is the ratio obtained from the megohm measurement at 10-minutes divided by the megohm measurement at 1-minute. The DA measurement is typically used as an alternative to the PI measurement. When the leakage current measured during a PI measurement stabilizes within 1 minute, as often happens with more modern insulation, the resulting value is 1. If a conventional PI calculation results in a value of 1, but ratio data is required, the DAR should be used instead. A variant of the PI measurement, the DA measurement uses Equation A.1 from IEEE Std 43-2013, Annex A, Variants in Polarization Index1.

1. “IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery,” in IEEE Std 43-2013 (Revision of IEEE Std 43-2000), vol., no., pp.1-37, 6 March 2014, doi: 10.1109/IEEESTD.2014.6754111.