The prevention and control of internal arcs in medium-voltage switchgear

One of the most important issues in the design of medium-voltage switchgear is personnel safety. Especially at the occurrence of an internal fault, very dangerous situations may arise. Manufacturers of medium-voltage switchgear have a large responsibility to achieve this safety. Safety is primarily achieved by preventing internal faults. Secondly, the effects of an internal arc have to be controlled in a safe manner in case an internal fault may still occur. International standards such as IEC 298 appendix AA give methods for testing switchgear on this subject. This paper deals with these two methods to guarantee personnel safety in respect to internal arcing. The possibility of an internal arc can be minimised by using cast resin as the insulating material for each phase. If, however, a gas or liquid is chosen as the insulating medium, it is important to control the effects of arcing faults. These effects are discussed both theoretically and practically. Some basic principles of the arcing process are pointed out, partly on the basis of a literature survey. Application of steel and copper is recommended in places where the arc is likely to have its footpoints. Furthermore, solutions resulting from theoretical analysis are related with internal arc tests. Based on these results, different safety regulations for the design of arc-resistant medium voltage switchgear are discussed