The design of distribution substations to accommodate switchgear internal arc faults

Distribution substations within urban areas of the United Kingdom usually consist of a ground mounted MV/LV transformer which is controlled by a ring main unit (RMU). Cable termination on most of these RMUs is by means of bitumastic filled cable end boxes. The failure of one such cable box in a brick built substation caused an explosion with significant structural damage to the building. The roof had lifted, some of the brickwork had been demolished and the doors had been thrown nearly ten metres. There was no injury but the incident served to focus attention on the possibility of there being injury at some time in the future. The investigation sought both to reduce the risk of future failure and the effect of that failure in terms of building damage and this paper reports on the second aspect