What Is the Polarization Index PI? Why Is R600/R60 More Reliable Than Absorption Ratio?

On the basis of absorption ratio DAR, HZJY-30KV can also automatically measure and calculate polarization index PI. If DAR is a "fast rough judgment" of insulation state, PI is a more in-depth "fine diagnosis" - the two use different time scales and are applicable in different situations.

Definition of polarization index PI

PI (Polarization Index)=R600/R60

The ratio of insulation resistance at 600 seconds (10 minutes) to insulation resistance at 60 seconds.

HZJY-30KV automatically records the resistance values of R15, R60, and R600 at three times, calculates and displays DAR and PI simultaneously at 600 seconds, without the need for operator intervention or manual time recording throughout the process.

The reason why PI is more reliable than DAR

DAR compares 15 seconds and 60 seconds, with a shorter time window. The measurement results are sensitive to environmental conditions (temperature, humidity) and the charging status of the capacitor at the start of the test. Sometimes, there may be significant differences in DAR between different measurements taken by the same device.

PI uses 60 seconds and 600 seconds, with longer time spans. The capacitor charging current has already decayed, and the impedance changes during the long-term absorption process are compared. It is less affected by initial conditions, resulting in more stable and repeatable results.

For large equipment (such as large motors and transformers), PI is a more suitable indicator due to its large capacitance and long absorption process duration; For small devices, DAR is sufficient.

Judgment criteria for PI

PI judgment of insulation for large rotating electrical machines (refer to IEEE 43 standard):

PI ≥ 4: Excellent insulation

2 ≤ PI<4: Good insulation

1 ≤ PI<2: Suspected insulation, attention should be paid

PI<1: Severe insulation degradation, not recommended for continued operation

For transformer windings and cables, different regulations have corresponding PI reference values, but the basic logic is consistent: the higher the PI, the drier and healthier the insulation; PI close to or below 1, insulation has significantly deteriorated and needs to be dealt with immediately.