Partial discharge is a small electrical discharge that occurs inside a void, impurity or defect in cable insulation without completely bridging the space between conductor and screen. Each discharge is tiny, but repeated discharges slowly erode the insulation around the defect and can eventually lead to breakdown. The effect is of most concern in medium and high voltage cables, where the electrical stress in the insulation is high.
Partial discharge is measured in picocoulombs (pC), and a lower figure means a cleaner insulation system. Voids are reduced by extruding the conductor screen, XLPE insulation and insulation screen together in one pass, which also keeps the interfaces clean. APAR states that its MV/HV XLPE cables are made by simultaneous triple extrusion and are suited to critical applications requiring partial discharge values below 2 pC.