A partial discharge is a localised electrical breakdown inside a void, a crack or a poorly fitted interface in the insulation, which does not bridge the two electrodes. Each discharge lasts nanoseconds and releases very little energy, but it repeats thousands of times per second and erodes the material step by step.
It matters because it is the degradation mechanism behind most insulation failures in transformers, switchgear, cables and rotating machines, and because it shows up months or years before the failure itself. Detecting it in time turns an unplanned outage into a scheduled intervention.
It is measured by several complementary routes: electrical coupling per IEC 60270 (in picocoulombs), acoustic emission with ultrasonic sensors, transient earth voltage (TEV) for metal-clad switchgear, and radio-frequency emission. In the field two of them are normally combined to pinpoint the source.