Switchgear is used to connect and disconnect electric power supplies and systems. It is a general term which covers the switching device and its combination with associated control, measuring, protective and regulating equipment, together with accessories, enclosures and supporting structures.
Switchgear is applied in electrical circuits and systems from low voltage, such as domestic 220/240 V applications, right up to transmission networks up to 1100 kV. A wide range of technology is needed, and this chapter is split into the four main subdivisions of low voltage, medium voltage (distribution), high voltage (transmission) and dc.
The main classes of equipment are as follows:
● disconnectors, or isolators
● fuse–switch combinations
● circuit breakers
● earthing switches
A disconnector is a mechanical switching device which in the open position provides a safe working gap in the electrical system which withstands normal working system voltage and any overvoltages which may occur. It is able to open or close a circuit if a negligible current is switched, or if no significant change occurs in the voltage between the terminals of the poles. Currents can be carried for specified times in normal operation and under abnormal conditions.
A switch is a mechanical device which is able to make, carry and interrupt current occurring under normal conditions in a system, and to close a circuit safely, even if a fault is present. It must therefore be able to close satisfactorily carrying a peak current corresponding to the short-circuit fault level, and it must be able to carry this fault current for a specified period, usually one or three seconds.
A fuse and a switch can be used in combination with ratings chosen so that the fuse operates at currents in excess of the rated interrupting or breaking capacity of the switch.
Such a device is known as a ‘fuse switch’ if the fuseholder is also used as part of the main moving contact assembly, or a ‘switch fuse’ if the fuse is a separate and static part of an assembly which includes the switch connected in series.
Figure 7.1 shows schematically the various combinations of fuse, switch and disconnector that are available.
A circuit breaker is a mechanical switching device which is not only able to make, carry and interrupt currents occurring in the system under normal conditions, but also to carry for a specified time and to make and interrupt currents arising in the system under defined abnormal conditions, such as short circuits. It experiences the most onerous of all the switching duties and is a key device in many switching and protection systems.
An earthing switch is a mechanical device for the earthing and short-circuiting of circuits. It is able to withstand currents for a specified time under abnormal conditions, but it is not required to carry normal service current. An earthing switch may also have a short-circuit making capacity.