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Saturday, 22 February 2020

Over Current Relay- Working Principle, Types, Drawbacks

Over Current Relay


  • This is another type of relay which is used to protect the transmission lines from faults. In this relay there is a setting that save the current value and protects the line if it goes high.
  • Over current relay is the relay which operates at the value of current is greater than the setting time of relay.
  • In this over current relay there is one current operated element is present. So the protection against the over current or exceeds current in the line is the essential protection for the device.
  • Over current protection is directed entirely to the clearance of the fault although with the setting usually adopted some measure of overload protection is obtained.  


Working principle of Over Current Relay

  • In over current relay the current coil is given to it. At the normal current flowing through this coil, the coil generates magnetic effect but that not sufficient to move the moving element of the relay.
  • At this condition restraining force is greater than the deflecting force.
  • When the current increases then the magnetic field also increases and at some level deflecting force that generated by the magnetic effect is increases and crosses to the restraining force.
  • Because of this the moving elements of relay starts moving to change the position of relay. 

Operation OverCurrent Relay
Operation Over Current Relay

Types of Over Current Relay

There are three types of over current relay.

1. Definite current or instantaneous over current relay

2.Definite time over current relay

3. Inverse time over current relay


1. Definite current or instantaneous over current relay
  • Instantaneous relay sense the over current in the circuit and gives tripping command to operate the circuit breaker.
  • This relay takes no time in giving tripping command once the over current sense. So the operating time delay is zero.
  • In the construction of over current instantaneous relay magnetic core is wound by a current coil. The iron pieces are attached by hinge support and restraining springs in the relay.
  • When the excess current flowing through the coil, NO contact remains open  that attractive force pull and the NO contacts gets closed.
  • The operating time of the instantaneous over current relay is of order few milliseconds. 

2.Definite time over current relay

  • It is made with intentional time delay after crossing the pick up value of the current.
  • It is provided with the time setting adjustments and also with pick up adjustment.

3. Inverse time over current relay

  • In the magnitude of the current the operation of relay depends.
  • The current increases the relay takes minimum time to trip the circuit. Inverse Time Relay also referred to as Inverse Definite Minimum Time(IDMT).
  • Inverse means higher the current value lesser the time taken for the relay to trip the circuit. Current in the line and time taken for the relay to trip the circuit breaker follow an inverse proportionally.

Drawbacks Of Relay

  • The continuity in the supply cannot be maintained at the load end in the event of fault.
  • Time lag is provided which is not suitable in short circuits.
  • Not suitable for long distance transmission.
  • Difficult to co-ordinate and requires changes with the addition of load.


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