Page 87 - Basic PD Theory

References

Test setup
Connections - if practical it is recommended to isolate the phases and test each phase individually. This allows for phase
comparisons. In practice, the insulation resistance of a single-phase is approximately twice the values obtained when the
entire winding is tested together.
Instrument - use a true IR (megohm meter) with a voltage selector and well-regulated voltage output

Polarization Index
The polarization index, PI, is the slope of the insulation resistance plotted over time. The
theory is to determine whether the leakage current (dependent upon Contamination) or the  Polarization Index : PI
absorption current (property of the insulation) dominates the curve. = IR 10 min / IR 1 min

Recommended Minimum Values
of Polarization Index*

Class A 1.5
Class B 2.0
Class F 2.0
Class H 2.0

Figure 64. Typical insulation resistance measurements for three different insulation systems.
If it is the leakage current, then the winding is wet or dirty. If it is the absorption current, then the winding is clean and dry.
Because the PI value is normally obtained when the winding is at the same temperature and humidity it is less affected by
ambient conditions than the insulation resistance value.
NOTE: If the one minute insulation resistance is above 5,000 megohms, the calculated P.I. may not be meaningful. In such
cases, the P.I. may be disregarded as a measure of winding condition. The Polarization Index test is not applicable to
synchronous generator field windings.

IR/PI Interpretation
The insulation resistance, IR, is an indication of the total current. This value trended over time may give indications as to the
presence of cracks (increase in conductance current, I G) or aging (increase in absorption current, I A). Generally, the trend of the
IR value is erratic because of the effects of winding temperature and humidity. However, some minimum limits for suitability
for return to service are suggested here.

Recommended Minimum Insulation Resistance Values at 40°C
(all values in megohms)
IR 1(min) = kV + 1 for most windings made before about 1970, all field windings, and
others not described below
IR 1(min) =100 for most DC armature and AC stator windings built after about
1970 (form-wound coils)
IR 1(min) =5 for most machines with random-wound coils and form-wound coils
rated below 1kV

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