Partial discharges are the very fast movements of small amounts of charge within localised parts of the dielectric (rather than an arc between the conductor and the shield) of a piece of high voltage equipment.

Partial discharge activity is normally sporadic (stochastic) unlike corona activity which is more consistent. PD activity will typically occur above a voltage (called the inception voltage) and will disappear below another voltage (extinguishing voltage). These voltage levels are not necessarily equal but their relative magnitudes compared to the operating voltage as well as each other are important.

Partial discharge activity would typically indicate potential for failure of the equipment at the point of the partial discharge. Of interest to the tester is the magnitude of the PD, the frequency of the PD and the position of the PD. Cable joints and terminations are normally associated with some PD activity. Some cable types generate a lot of PD activity under normal operating conditions.

Partial discharge equipment is typically operated in very noisy environments where nearby electromagnetic activity is easily measured by the sensitive pd detectors designed to detect the small PD currents. A number of techniques have been developed to filter the extraneous noise from the partial discharges.

Partial discharge is particularily useful as a diagnostic technique at commissioning to check whether the joints have been manufactured properly.

Partial discharge techniques have not been shown to be useful in detecting water ingress.