The measurement setup and measurement method are equivalent to the insulation resistance measurement. However, the measurement duration is 10 minutes, whereby the measurement points are output after one minute and after 10 minutes. This method is used to determine the additional possible charge increase of the model capacitor. If one compares the insulation value after 10 minutes with the insulation value after one minute, one obtains a factor that provides information about the insulation quality, contamination degree of the stator core and winding overhang or simply the ageing of the insulation.

Recording these measured values for monitoring value trends enables planned shutdowns for replacements or repair of the machine.

Evaluation of the PI measurement
Once a PI value has been determined, it still has to be suitably evaluated. A PI value under 1.5 indicates a very damp or extremely dirty insulation, which can be the case after long operation due to the environmental conditions. In the range between 1.5 and 2.5, the insulation can be evaluated as damp or contaminated. In both these cases, cleaning or drying the machine is recommended.

If a PI value between 2.5 and 4.0, or even higher, is measured, then no immediate revision is necessary. This obviously indicates a dry and clean insulation or a very dry and clean insulation.