COMPONENTS OF SWITCHGEAR

Switchgear has got mainly four components which are as follows:

• Fuse
• Lightning arresters
• Circuit breakers
• Relays

The various components along with some other equipments used have been described as follows :

Fuse

Fuse is a piece of metal which interrupts the short circuit current. It is the cheapest and simplest current interrupting device for protection of the equipment upto 600 V against overloads and/or short circuits. The fuse is expected to allow the flow of normal working current safely without overheating and during short circuit/overloads it gets heated upto the melting point rapidly. The materials used normally are tin, silver, zinc, lead, copper, aluminium etc. The breaking capacity of the fuse is the maximum current that can safely be interrupted by the fuse. This should be higher than the prospective short-circuit current.

Isolators

Isolators are known as disconnecting switches, which operates under no-load conditions. They are not manufactured for any making or breaking current capacity. They do not have an arc quenching system within it. Isolators main functions is to disconnect a particular part after the current interruption from that particular part. These switches are generally used for the repair or replacement of a circuit breaker. It acts as a switch that disconnects the required part from the circuit where the repair has to be done.

Lightning Arrester or Surge Diverter

The device/equipment used for the protection of the power system against voltage surge is called “surge diverter” or “lightning arrester”. It is connected between the line and earth to divert the incoming high voltage wave to earth. A lightning arrester consists of non-linear resistor blocks connected in series with the series gap as shown below in figure. When a high voltage wave reaches the surge diverter, it sparks-over and provides a conducting path of relatively low impedance between the line and earth so that the resulting current flows to the earth. As soon as the voltage again comes to normal value, it stops the flow of current.

“Thyrite arresters” are the most common and is mostly used for the protection against high and dangerous voltages.

Surge Absorber

The device which reduces the steepness of the wave front of a particular surge and thus minimises the danger due to over voltage is known as “surge modifier” or “surge absorber”

Circuit Breaker (CB)

Circuit breakers are mechanical (or automatic now a days) devices designed to close or open contact members, thus closing or opening an electrical circuit under normal or abnormal conditions. Automatic circuit breakers for the protection of electrical circuits are equipped with a trip coil connected to a relay or other means which are used to open the breaker automatically under abnormal conditions (i.e. over current etc.).