Recommended Practice for Electrical Equipment Maintenance, states that insulation breakdown is the number one cause of electrical failures. In an electrical asset, partial discharges provide an early indication of dielectric irregularities that can lead to insulation breakdown.
Partial discharges are electrical sparks in gas voids within the insulation when the voltage is high enough. The spark generates a fast current pulse which travels through the stator winding. The larger the Pd pulse, the higher the current pulse that reaches the winding terminals.
Partial discharge tests are probably the best means for assessing the insulation condition without a visual inspection. By measuring the PD activity of a stator an indication of the health of the winding can be obtained.
A partial Discharge test will be done using a Partial Discharge Analyser at normal running conditions without disturbing the supply.