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What is Switchgear? | Features, Components and Classification

What is Switchgear?

The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear. 

The term ‘switchgear’ is a generic term encompassing a wide range of products like circuit breakers, switches, switch fuse units, off-load isolators, HRC fuses, contactors, earth leakage circuit breakers (ELCBs), etc...

Components of Switchgear

A switchgear essentially consists of switching and protecting devices such as switches, fuses, isolators, circuit breakers, relays, control panels, lightning arrestors, current transformers, potential transformers, and various associated equipments. 

Some equipments are designed to operate under both normal and abnormal conditions. Some equipments are meant for switching and not sensing the fault.

During normal operation, switchgear permits to switch on or off generators, transmission lines, distributors and other electrical equipment. 

On the other hand, when a failure (e.g. short circuit) occurs on any part of power system, a heavy current flows through the equipment, threatening damage to the equipment and interruption of service to the customers. 

However, the switchgear detects the fault and disconnects the unhealthy section from the system.
Read more about Components of Switchgear

Evolution of Switchgear

The switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions. 
  • The tumbler switch with ordinary fuse is the simplest form of switchgear and is used to control and protect lights and other equipment in homes, offices etc. 

  • For circuits of higher rating, a high-rupturing capacity (H.R.C.) fuse in conjunction with a switch may serve the purpose of controlling and protecting the circuit. However, such a switchgear cannot be used profitably on high voltage system (33 kV) for two reasons. 

    • Firstly, when a fuse blows, it takes sometime to replace it and consequently there is interruption of service to the customers. 
    • Secondly, the fuse cannot successfully interrupt large fault currents that result from the faults on high voltage system. 
With the advancement of power system, lines and other equipments operate at high voltages and carry large currents. When a short circuit occurs on the system, heavy current flowing through the equipment may cause considerable damage. In order to interrupt such heavy fault currents, automatic circuit breakers (or simply circuit breakers) are used. 
  • A circuit breaker is a switchgear which can open or close an electrical circuit under both normal and abnormal conditions. Even in instances where a fuse is adequate, as regards to breaking capacity, a circuit breaker may be preferable. 
  • It is because a circuit breaker can close circuits, as well as break them without replacement and thus has wider range of use altogether than a fuse.

Essential Features of Switchgear

The essential features of switchgear are : 
  1. Complete reliability: With the continued trend of interconnection and the increasing capacity of generating stations, the need for a reliable switchgear has become of paramount importance. This is not surprising because switchgear is added to the power system to improve the reliabil-ity. When fault occurs on any part of the power system, the switchgear must operate to isolate the faulty section from the remainder circuit.
  2. Absolutely certain discrimination: When fault occurs on any section of the power system, the switchgear must be able to discriminate between the faulty section and the healthy section. It should isolate the faulty section from the system without affecting the healthy section. This will ensure continuity of supply.
  3. Quick operation: When fault occurs on any part of the power system, the switchgear must operate quickly so that no damage is done to generators, transformers and other equipment by the short-circuit currents. If fault is not cleared by switchgear quickly, it is likely to spread into healthy parts, thus endangering complete shut down of the system 
  4. Provision for manual control: A switchgear must have provision for manual control. In case the electrical (or electronics) control fails, the necessary operation can be carried out through manual control. 

Classification of Switchgear

Switchgear can be classified on the basis of voltage level in to the following
  1. Low voltage (LV) Switchgear
  2. Medium voltage (MV) Switchgear 
  3. High voltage (HV) Switchgear

1. Low Voltage Switchgear

Generally electrical switchgear rated upto 1 kV is termed as low voltage switchgear. 

The term LV Switchgear includes low voltage circuit breakers, switches, off load electrical isolators, HRC fuses, earth leakage circuit breaker, miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) etc i.e. all the accessories required to protect the LV system.

The most common use of LV switchgear is in LV distribution board.

2. Medium Voltage Switchgear

From 3 kV to 36 kV switchgear system is categorized as medium voltage switchgear or MV switchgear. 

These switchgears are of many types. They may metal enclosed indoor type, metal enclosed outdoor type, outdoor type without metal enclosure, etc. The interruption medium of this switchgear may be oil, SF and vacuum. 

The main requirement of MV power network is to interrupt current during faulty condition irrespective of what type of CB is used in the MV switchgear system. Although it may be capable of functioning in other conditions also.

A medium voltage switchgear, should be capable of,
  1. Normal ON/OFF switching operation.
  2. Short circuit current interruption.
  3. Switching of capacitive currents.
  4. Switching of inductive currents.
  5. Some special application.

3. High Voltage Switchgear

The power system deals with voltage above 36kV, is referred as high voltage. 

As the voltage level is high the arcing produced during switching operation is also very high. So, special care to be taken during designing of high voltage switchgear. 

High voltage circuit breaker, is the main component of HV switchgear, hence high voltage circuit breaker should have special features for safe and reliable operation. 

Faulty tripping and switching operation of high voltage circuit are very rear. Most of the time these circuit breakers remain, at ON condition, and may be operated after a long period of time. So CBs must be reliable enough to ensure safe operation, as when required. 

Switchgear Equipment

Switchgear covers a wide range of equipment concerned with switching and interrupting currents under both normal and abnormal conditions. It includes switches, fuses, circuit breakers, relays and other equipment. 

A brief account of these devices is given below. 

1. Switches

A switch is a device which is used to open or close an electrical circuit in a convenient way. It can be used under full-load or no-load conditions but it cannot interrupt the fault currents. When the contacts of a switch are opened, an arc is produced in the air between the contacts. This is particularly true for circuits of high voltage and large current capacity. 

The switches may be classified into 
  1. air switches 
  2. oil switches. 
The contacts of the former are opened in air and that of the latter are opened in oil.

(i) Air-break switch - It is an air switch and is designed to open a circuit under load. In order to quench the arc that occurs on opening such a switch, special arcing horns are provided. 

(ii) Isolator or disconnecting switch - It is essentially a knife switch and is designed to open a
circuit under no load. 

(iii) Oil switches. As the name implies, the contacts of such switches are opened under oil,
usually transformer oil. 

2. Fuses

A fuse is a short piece of wire or thin strip which melts when excessive current flows through it for sufficient time. It is inserted in series with the circuit to be protected. 

When a short circuit or overload occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the fuse element melts (or blows out), disconnecting the circuit protected by it.
Read more about fuse here What is a FUSE and How it Works?

3. Circuit Breakers

A circuit breaker is an equipment which can open or close a circuit under all conditions viz. no load, full load and fault conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions. For the latter operation, a relay circuit is used with a circuit breaker

4. Relays

A relay is a device which detects the fault and supplies information to the breaker for circuit interruption. 

When a fault occurs the relay contacts are closed and the trip coil of the circuit breaker is energised to open the contacts of the circuit breaker.
Read in detail How Protective Relays Work?