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.

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