Partial Discharge is a localized electrical discharge that can occur at voids, gaps and similar defects in medium and high voltage cable systems. If not properly addressed, partial discharge will erode the cable insulation, usually forming a tree-shaped pattern of deterioration (electrical tree) and eventually result in complete breakdown and failure of the cable or accessory.

Testing involves applying a voltage conducive to partial discharge and then directly or indirectly measuring the discharge current pulses using calibrated PD sensors. Characteristics of partial discharge depend on the type, size and location of the defects, insulation type, voltage, and cable temperature.

The PD test is known to detect small imperfections in the insulation such as voids or skips in the insulation shield layer, however, PDs must be present in order to detect any PD. Measurements can be made on newly installed and service-aged cables to detect any damage created during the installation of new cable, or in-service degradation of cable insulation due to PDs.