What is Partial discharge?

When an insulation system has localised imperfections, electrical discharges can occur between two conducting electrodes which do not completely bridge the gap between the electrodes (hence ‘partial’). The partial discharges usually begin where the insulating material contains voids or cracks within it, or at the boundary between different insulting materials. Such discharges are low energy electrical activity but can happen many times per second, each time causing further deterioration in the insulting material(s).

Partial discharge can therefore rapidly degrade insulation to the point at which electrical breakdown occurs, often with serious consequences. This is why testing for and monitoring partial discharge is so important and valuable: it’s a warning signal. How early a signal, depends on how often you check.

Does it always occur in older equipment?

No - but partial discharge in recently fitted equipment is an indicator of poor quality insulating materials, poor workmanships (especially in terminations and cable joints) and even poor design.

What signs are created by partial discharge?

When partial discharge occurs within a gas, it shows itself as a cluster of micro-arcs, creating a corona effect (hence ‘corona’ is the term often used to describe such partial discharges). These are more visible in lower light conditions and can often also be heard, as a crackling or hissing sound (though a stethoscope or highly sensitive microphone is needed).

When partial discharge occurs within paper-insulated high voltage cables, they initially show as small pin holes, progressing to tree-like carbonised patterns (though note that ‘treeing’ is a common sign of partial discharge in solid polymer dielectrics). These trees partially conduct, leading to further charring. Complete dielectric failure of the cable and electrical explosion tend to follow.

Are some materials less susceptible to partial discharge?

Yes. Non-organic dielectrics tend to fare better. Thus partial discharge is relatively rare where glass, mica and porcelain are used, compared with organic and polymer dielectrics.

Partial discharge can occur in applications as low as 300V. To find out more about partial discharge and our testing for it, please contact us on +44 (0) 1299 251758