Polarisation index is the ratio of the insulation resistance measured at ten minutes to the value at one minute. Dielectric absorption ratio uses sixty and thirty seconds instead. Both are designed to remove the influence of absolute resistance, which varies enormously with machine size, temperature and humidity.
For modern insulation systems, a PI above 2.0 is generally accepted as satisfactory. However, machines with very high absolute resistance — above 5 GΩ at one minute — can produce low PI values simply because the measurement is dominated by noise rather than genuine absorption current. In these cases the absolute value, not the ratio, is the meaningful figure.
Temperature correction is essential when trending. Insulation resistance approximately halves for every 10 K rise, so uncorrected readings taken at different ambient temperatures cannot be compared. Record winding temperature alongside every test and normalise to 40 °C before trending.
Used consistently, PI and DAR give early warning of moisture ingress and contamination long before an insulation failure occurs. Used in isolation, without temperature context or trend history, they generate unnecessary alarm.
Downloads
- Temperature Correction Chart0.4 MB PDF
