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  Thermal Relay  Circuit Breaker Tripping Schemes  Classification of protective relays  Protective Relay

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Protective Relay

A protective relay is a device which initiates the circuit breaker to cut the faulty circuit from the healthy system. It acts just like a ‘ silence guard’, when any fault occurred in a system, the relay operates to close the trip circuit of the breaker and the breaker disconnects the faulty circuit.

A typical relay circuit is shown in fig. There are three parts in the relay circuit :-

  1. First part is, the primary winding of a current transformer (C.T ) which is connected in series the line.
  2. Second part , secondary winding of C.T is connected in series with relay operating coil.
  3. Third part is the tripping circuit which may be either a.c and d.c . source of supply , trip coil of the circuit breaker and relay stationary contacts are connected in series connection.

Now describe , how the relay disconnects the faulty circuit . Suppose when a short circuit occurs at a point F, on the line , the huge current  flows in the line. Hence , a heavy current  flow through the relay coil, causing the relay to operate by closing its contacts. Result complete the tripping circuit of the breaker and making the circuit breaker open.



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