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
- Current transformer (CT)
- 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.
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- Auot-reclosing
- Contactor
- HRC fuse
- Protective scheme
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