What Partial Discharge Is
Partial discharge (PD) is a small electrical discharge that occurs within a localised region of an insulation system without fully bridging the gap between conductors. It typically starts at a void, crack, contaminant, or sharp edge in or on the insulation where the electric field is locally concentrated beyond what that spot can withstand. Each individual discharge is tiny, but repeated over months or years it gradually erodes the surrounding insulation.
Why It Matters for Switchgear
Medium-voltage switchgear relies on solid insulation (epoxy resin bushings, cast components, cable terminations) and air or gas clearances to hold off system voltage. Partial discharge is one of the main mechanisms by which that insulation quietly degrades over time — and because it is progressive, an insulation system can appear perfectly healthy in an ordinary visual inspection or a basic insulation resistance test right up until it fails.
Common Causes
- Voids or cracks in cast insulation introduced during manufacture, or developed through thermal cycling and mechanical stress.
- Surface contamination and moisture that create tracking paths across insulation surfaces, particularly in humid or dusty environments.
- Poorly made cable terminations where stress control at the cable end has not been properly applied.
- Loose or damaged components that create sharp edges concentrating the electric field.
How It Is Detected
PD activity can be detected through several methods including transient earth voltage (TEV) sensors, ultrasonic detectors that pick up the acoustic signature of discharge, and dedicated online or offline PD measurement systems. Many operators use these techniques as part of a condition-based maintenance programme, running periodic surveys and trending results over time rather than relying on a single reading.
If your switchgear insulation components need replacement, browse VCB and switchgear spares or send your equipment details for a quote.