What is switchgear?

Switchgear is essential equipment used in electricity world and present in electrical power systems, it used to control, protect and isolate any electrical device or electrical network under consideration. It consists mainly of electrical disconnecting switches equipped with fuses or may be circuit breakers So it is impossible to find any electricity system without switchgears.

Switchgear is a must to increase the electricity supply reliability as it clear the faults downstream as soon as the fault occurs in the electrical networks.
At the earlier time our electricity power station used very simple knife switches as the power and voltage levels was small, after that power and voltage level increased rapidly making manually opening operated switches are so dangerous especially when we deal with the loaded electricity circuits.
Consequently anew generation of switchgear has been manufactured to allow large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, metering and protection devices in the electricity systems.
The technology has been improved till we can now used switchgear for voltages up to 1100 k.v as in power substation which step up or down the voltage.
We use electrical switchgear in both the high and low voltage side to control the high rated power transformer and also the downstream electricity distribution circuits.

What are the main types of switchgears?

Switchgear enclosure contain a number of equipment's that allow it to interrupt fault currents with thousands of amperes , the primary component   that interrupt the fault current is called the circuit breaker .
Circuit breaker is carefully designed to disconnect the electrical circuit under faulty conditions, it contains a mean to quench the arc resulting from the on /off switching, and according to this mean we have four types of circuit breaker:
1-Oil Circuit breaker: depend on the oil to quench the arc.
2-Gas Circuit breaker: this type stretch the arc by using magnetic field then use (SF6) gas to quench this stretched arc.
3-Vacuum Circuit breaker: this type also stretch the arc by small amount and at or near the zero current, the circuit breaker will interrupt the current as the arc is not so hot at this moment. Vacuum circuit breakers are almost used in modern medium-voltage switchgear up to 35 k.v.
4-Air Circuit breaker: this type use the magnetic force or the compressed air to quench the arc and usually able to terminate all current flow very quickly typically between 30 ms and 150 ms depending on the age and construction of the circuit breaker.

How can we classify switchgears?

Many details of switchgears like rating and specifications are set by several of standards, for example IEEE and ANSI standards are used in North America and IEC are frequently used in the reset of the world.
We can classify switch gear according to the following parameters:
(1) Current rating.
(2) Interrupting rating which means the maximum short circuit current that the circuit breaker can safely interrupt.
(3) Voltage rating as following:
(a)       Low voltage "less than 1 kV AC".
(b)      Medium voltage " 1 kV up to approximately 75 kV (AC)".
(c)       High voltage "75 kV up to approximately 230 kV AC".
(d)       Extra high voltage "more than 230 kV".
(4)  Insulation around the contact points:
"Oil - Gas (SF6) - Vacuum - Air ", as referred above.
(5)  Construction type:
"Indoor – Outdoor – Marine – Industrial – Draw out elements (removable with a tool) - Fixed elements (not removable) – Metal enclosed – Nonmetal enclosed".
(6) IEC degree of internal separation:
(a)     No Separation.
(b)     Busbars separated from other functional units.
(c)     Terminals for external conductors separated from busbars.
(d)     Terminals for external conductors separated from functional units but not separated from each other.
(e)     Terminals for external conductors separate from their associated functional unit.
(f)       Terminals for external conductors separated from each other.
(7) Interrupting device:
"Fuses – Air circuit breaker – Oil circuit breaker – Vacuum circuit breaker – SF6 circuit breaker ".
(8) Operating method:
"Manually operated – Motor operated – Solenoid operated".
(9) Type of current:
"Alternating current – Direct current".
(10) Application:
"Distribution System - Transmission system".

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