Switchgear is responsible to connect and disconnect the electric power supplies and systems. Switchgear is a general term which uses to covers the switching device and its various combinations with some associated control, measuring all parameter, protective and regulating equipment, together with accessories, enclosures and supporting structures. Switchgear mainly applied in electrical circuits and systems from low voltage like domestic 220/240V applications to transmission networks up to 1100 kV. To set up this control system a wide range of technology is needed and this chapter is split into the four main subdivisions of low voltage, medium voltage (Distribution System), high voltage (Transmission System) and DC.
Now the main classes of equipment are as follows:
Disconnector: A disconnector is a kind of mechanical type switching device which in the open position provides a safe working gap in the electrical system which withstands the normal working system voltage and any kind of over voltages which may occur. Disconnector is able to open or close a circuit system 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 system and under abnormal conditions.
Fuse and Switch: A switch is a kind of mechanical device which is able to make, carry and interrupt current occurring under normal conditions in a circuit system and to close a circuit safely even if a fault is present in system. Switch 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 of time and usually it should take one or three seconds.
A fuse and a switch can be used in combination system with chosen ratings so that the fuse operates at currents in excess of the rated interrupting or breaking capacity of the switch during fault. This kind of device is known as a
<strong>Fuse Switch</strong> if the fuse holder is also used as part of the main moving contact assembly or a
<strong>Switch Fuse</strong> if the fuse is a separate and static part of an assembly which includes the switch connected in series.
This figure shows schematically the various combinations of fuse, switch and disconnector that are available.
Circuit Breaker: A circuit breaker is a mechanical type switching device which is not only able to make contact, carry and interrupt currents occurring in the circuit system under normal conditions but also to carry for a specified period of time and to make and interrupt currents arising in the system under defined type of abnormal conditions like short circuits. Circuit Breaker use to experiences the most onerous of all the switching duties and it is a key device in many important switching and protection systems.
Earthing Switch: An earthing switch is a mechanical type device for the earthing and short-circuiting of circuits. Earthing switch is able to withstand currents for a specified time under the abnormal conditions but it is not required to carry normal service current. An earthing switch may also have short circuit making capacity.
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