Definition Of Some Terms For Substation

8. Definition of some important terms 

• Circuit breaker 

is a device capable of breaking an electric circuit under normal and abnormal condition such as short circuits

• Isolator (Disconnecting switch)

is a switching device which can be opened or closed only under no-current condition. It provides isolation of a circuit for the purpose of maintenance.

• Earthing switch

is a switch which connects a conductor to earth so as to dicharge the charges on the conductor to earth. Earthing switches are generally instaleed on the frames of the isolator.           

• Relay

is an automatic device which closes its contacts when the actuating quantity/quantities reach(s) certain predetermined values.

• Current transformer (CT)

is a transformer whose current ratio is generally high (e.g. 500A/5A) and its volt-ampere capacity is relatively low ( e.g. 50 VA) as compared with that of power transformers.

• Potential transformer (PT), Voltage transformer (VT) 

is a transformer whose volt-ampere capacity is low (e.g. 100 VA) and its voltage ratio relatively high (e.g. 132 KV/100V). The protective relays are connected in the secondary circuits of CT's and VT's

•  Lighting arresters (surge arresters)

It is an equipment connected between the conductor and ground to discharge the excessive voltage to earth

• Fault clearing time

Is is the time elapsed between the instant of fault occurance and the instant of final arc extinction, in the circuit-breaker . It is usually expressed in cycles. One cycle of 50 HZ system is equal to 1/50 second. The fault clearing time is the sum of the relay time and the circuit-braeker time.

• Auot-reclosing

It is automatic closing of the circuit-breaker after its opening. It is provided to restore the service continuity after interrupting a transient fault. High voltage circuit-breraker, used for controlling overhead transmission lines, are provided with such a feature.

• Contactor

It is a switching device capable of making, carrying, and braking electric current under normal and overload condition

• HRC fuse

high rupturing capacity cartridge fuses is used for over-current protection in low voltage and medium voltage circuits.

• Protective scheme

It is a selected set of protective systems which protect one or tow components of the power system against abnormal condition, e.g. generator protection scheme, transformer protection scheme, etc.