Internal faults in distribution switchgear-where are we now and where are we going?

The requirement for internal fault qualification of distribution switchgear was first introduced through IEC 298 in 1981. Since then, manufacturers have embraced the need to concentrate on this aspect of switchgear performance and a number of methods for mitigating against internal faults are now well established. The paper discusses these which include lowering the risk of occurrence of a fault through careful choices of materials and design approach, pressure relief for tanks and chambers and the use of various types of fault detectors to trigger circuit-breakers or fault diverters. Attention is also be paid to the design of buildings for housing switchgear to optimise their response under fault situations. Progress has been made in refining methods of test, particularly in respect of tests for early development and for building assessment; these methods are presented. Theoretical methods for predicting pressure rise are discussed briefly, looking at the applicability of the various approaches rather than dealing with one particular approach in detail. In the UK, the industry´s viewpoint is expressed by EATS 41-26 which adds specific requirements for conditions and methods of test. Manufacturers have been able to satisfy these requirements; the paper discusses how this has been achieved and presents the views of a manufacturer and a user on approaches currently in use. The paper also examines alternative options for the future together with assessments of the suitability of each of these in various types of application