AIR SEPARATION IN MV CABLE BOXES

Air at normal temperatures and pressure has an intrinsic breakdown strength of 3MV m¯¹.This means that if two electrodes spaced 10mm apart provide a uniform field and a voltage is applied between them, there will be a breakdown of the air and an arc will form at 30kV.

In MV cable boxes there is rarely a uniform field as the earth plane is usually a flat sheet and the electrode is a point configuration. This means that the field lines are much closer around the electrode and hence the breakdown strength of the air is much lower. In the above case the voltage required to reach breakdown will be as low as 10kV.

Air separation is more of a problem in three core termination boxes as single core type usually have larger separation between the bushings. The problem is normally remedied with the use of Bushing Boots, either heat shrink type or cold applied, or separation shields.

Jointing Technologies sales staff can advise on recommended separation distances and methods of dealing with the issue. Contact us today and terminate your separation worries!