This insulation test should be repeated regularly over the years in order to evaluate the ageing of the insulation. In contrast to the classic tangent-delta measurement, this does not measure how much current leaks through the insulation, but to what extent partial discharges occur within the insulation.

Every insulation has areas where voltage can build up and then discharge at a certain level. This is the so-called apparent charge. Such areas can be e.g. air inclusions in the insulation, sharp edges aligned with the earth potential, connection faults between insulation and earth potential or faulty anti-coronaprotection. Every partial discharge is measured and documented, i.e. the level of the charge and the time of discharge within the course of the voltage are determined.

Any concentration at specific positions within the course of the voltage and the form of the graphically depicted measurement cluster (see diagram above) provide information about the type and position of the partial discharges in the high voltage winding, so that possible sources of faults in the insulation are made clear.

Partial discharge measurement is useful from a measurement voltage of 4000 V upwards. Significant discharges do not normally take place below this value.

This type of measurement is increasing in significance in the evaluation of high and medium voltage insulation.