Thursday, February 4, 2010


A Switchgear is an apparatus that is used to switch, control and protect electrical circuits and equipment. It is essentially used with switching and interrupting currents either under normal and abnormal conditions. Switchgear is a term used to refer to combinations of switching devices and their interaction with associated control, protective and measurement systems.
They are designed to be able to clear safely any fault which may occur in the power system. They can range from domestic 240V to industrial supply of 11kV and 132kV.In industries they are used to supply current to motors, heating equipments and capacitors. There are normally two types of switchgear assemblies namely Outdoor and Indoor Switchgear assemblies. Outdoor assemblies are mounted in fenced areas and interconnected using air insulated bus-bars. Indoor assemblies are coupled together by the switchgear manufacturer within a metal enclosure...

In most switchgear systems there is need for providing adequate insulating material for proper insulation purposes. The insulating material used is air, solid insulating material (PVC, Vulcanized Rubber), insulating oil and bitumen compound. Modern switchgears use an inert dielectric gas known as Sulphur Hexafluoride (SF6)...

Essential features of a Switchgear
1. Complete Reliability – When a fault occurs in a power station the switchgear must operate to protect the equipment.
2. Absolute certain discrimination – When a fault occurs in any section of the power station the switchgear should isolate the faulty system from the healthy system.
3. Should have manual controls – In case of electrical failure the desired operation should be operated manually.
4. Fast Operation – The switch gear should operate at a high speed to protect sensitive electrical components and instruments.
5. Provision for instruments – The switchgear should have instruments to measure current or voltage quantities.

The Switchgear Equipments:
         Air Break Switch
This is a switch that is designed to open a circuit under load. Opening a circuit will obviously result in an arc and therefore to quench this arc, arching hones (pieces of metal between which the arc is formed) are spread further and further apart. This consequently lengthens the arc and it is cooled and interrupted. It is used outdoor such as supplying industrial load from a main transmission line or feeder.
Isolator or Disconnecting Switch
This is designed like a knife to open a circuit under no load. It is used on both sides of circuit breakers so that the maintenance can be undertaken safely. They should never be opened when the circuit is live.
Oil Switches
In this type of switch the contacts are submerged in oil and opened under oil. This is to cool and quench the arc formed. They are used for high voltages and heavy current carrying capacities.
A fuse is a short piece of wire or strip that melts down when excessive current beyond the rated flows through it for a given time. The fuses used in switchgears are for large currents. However with the advent of modern circuit breakers fuses are losing their use slowly because some cannot be replaced soon after blowing.
Circuit Breakers

This is a safety electrical which can open or close a circuit under normal and abnormal conditions and on or no load.
A relay is a device which detects a fault and sends the info to the circuit breaker for quick interruption and the current is cut.
Just as the name suggests, isolators are special switches that separate electrically one part of a circuit to another. They are commonly used with circuit breakers. An isolator will safely render a conductor dead by switching it off completely. Normally when maintenance is to be done on one part of a circuit, one should never approach the conductors until the isolators are in ON position.

This is a device that does the same task as that of a switch but the only difference is that it is capable of performing very frequent switching operations as may occur. In industrial processes it is used where frequent starts and stops are required. They are also applied in remotely operating motors.


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