Internal arc-proof switchgear: arc modelling and active protection

According to the current 298 IEC Standards, internal arc tests are not mandatory but are a basic requirement for users who are aware of personnel safety needs. Over twenty years experience in the design of arc-proof switchgear has led to a construction with little difference from nonarc-proof switchgear. Therefore, a logical consequence would be internal arc-proof switchgear as the standard solution. The additional costs of internal arc-proof solutions are due to research and development costs as numerous and expensive destructive type tests are necessary for the many different versions and service conditions. For these reasons, it is necessary to reduce to a minimum the tests and to avoid the empirical approach which was used a few years ago for the development of this type of switchgear. To this purpose, ABB SACE has conceived a series of calculation methods which allow the behaviour of the switchgear to be foreseen during internal arc conditions starting right from the design phase. The main characteristic of the internal arc-proof switchgear is that, in the presence of an arc, the external manifestations must be contained to the utmost extent; this can be obtained by guaranteeing structural strength and by containing the temperature of the gas which could strike the personnel. To foresee the structural behaviour of the switchgear and the thermal characteristics of the exhaust gases it is necessary to calculate the pressure developed inside the switchgear in the presence of an electric arc.