When failure occurs on any part of the system it must be quickly detected and disconnected from  the system for two reasons. Firstly if fault is not cleared it may cause unnecessary interruption of  service to the customers. Secondly the rapid disconnection prevents the damage and spreading of  fault into the system. The detection of fault and disconnection of a faulty section can be achieved  by relays in conjunction with circuit breakers.
The protective relay is a device, which detects the fault and initiates the operation of the circuit  breaker to isolate the defective element from the rest of the system. Relays detect the abnormal  condition by  constantly measuring  the  electrical  quantities  that  are different under normal  and  fault conditions. The electrical quantities, which may change under fault conditions, are voltage  current,  frequency and phase angle. After detecting  the  fault  the relay operates  to close  the  trip  circuit of the breaker.
A typical relay circuit is shown in fig 2. This diagram shows one phase of a 3-phase system for  simplicity. The relay circuit connections are divided into three parts:
i.  First part  is  the primary winding of a  current  transformer, which  is connected  in  series  with the line to be protected.
ii.  Second part consists of secondary winding of ct and relay operating coil.
iii.  Third part is the tripping circuit, which may be either ac or dc. It consists of a source of  supply, trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit occurs at point F on the transmission line, the current flowing increases to  an enormous value. This results in a heavy current flow through the relay coil causing the relay  to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making circuit  breaker open and isolating the faulty section from rest of the system.

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