Chapter 13 - Switchgear

Abstract

This chapter concentrates on the description of various switching phenomena under different practical circuit conditions and then relates these basic principles to the different switchgear designs currently available in the market. In particular it is intended to assist the reader in specifying switchgear for particular applications. Switchgear is a general term covering switching devices and their combination with associated control, measuring, protective, and regulating equipment. The term covers assemblies of such devices and equipment with associated interconnections, accessories, enclosures, and supporting structures intended for use in connection with transmission and distribution networks. A circuit breaker is intended to switch both load and short circuit currents. Unlike a fused device it enables supplies to be quickly restored after operation on short circuit and is the most expensive form of switchgear. It is not primarily intended for frequent operation although vacuum and SF6 breakers are more suited to load switching duties than older switchgear types. A contactor is operated other than by hand and is intended for switching loads under normal and overload conditions. It is designed for frequent operations but has a limited short circuit current carrying and switching capability. It is therefore often backed up by fuses or a circuit breaker. A disconnector provides in the open condition a specific isolating distance. It has only a very limited current switching capability and is not intended for frequent use or for breaking full load current.