A switchgear is a generic term which includes all the switching devices associated with power system protection. It also includes all devices associated with control, metering and regulating of electrical power systems. Assembly of such devices in a logical manner forms switchgear. In other words systems used for switching, controlling and protecting the electrical power circuits and different types of electrical equipment are known as switchgear. This is very basic definition of switchgear.
We are all familiar with low voltage switches and re-wirable fuses in our home. The switch is used to manually open and close the electrical circuit in our home and electrical fuse is used to protect our household electrical circuit from over current and short circuit faults.
In same way every electrical circuit including high voltage electrical power system needs switching and protective devices. But in high voltage and extra high voltage system, this switching and protective scheme becomes complicated one for high fault current interruption in safe and secure way. In addition to that from commercial point of view every electrical power system needs measuring, controling and regulating arrangement. Collectively the whole system is called switchgear and protection of power system. The electrical switchgear has been developing in various forms.
Switchgear protection plays a vital role in modern power system network, right from generation through transmission to distribution end. The current interruption devices are called circuit breaker. The circuit breakers can be operated manually as when required and it can also be operated autometically during over current and short circuit or any other faults in the system by sensing the abnormality of system parameters. These power system parametrs can be current, voltage, frequency, phase angle etc. The circuit breaker senses the faulty condition of system through protection relays and these relays are again actuated by faulty signal normally comes from current transformer or voltage transformer.
The switchgear has to perform the function of carrying, making and breaking the normal load current like a switch and it has to perform the function of clearing the fault in the power system. In addition to that, it also has the provision of metering and regulating the various parameters of electrical power systems. Thus the switchgear includes circuit breakers, current transformers, voltage transformers, protection relays, measuring instruments, electrical switches, electrical fuses, miniature circuit breaker, lightning arresters or surge arresters, electrical isolators and other associated pieces of equipment.
Electric switchgear is necessary at every switching point in the electrical power system. There are various voltage levels and hence various fault levels between the generating stations and load centers. Therefore various types of switchgear assembly are required depending upon different voltage levels of the system. Besides the power system networks, electrical switchgears are also required in industrial works, industrial projects, domestic and commercial buildings.