How does overcurrent relay work

function

A overcurrent relay that is operate when current exceed a pick up value of relay setting.

overcurrent protection can protect the system or equipment from excessive current it is create through phase to phase fault or phase to ground fault or any other fault occur in the system

working principle

In an overcurrent relay there would be a essentially a current coil.

when normal current flows through this coil the magnetic effect generated by the coil is not sufficient to move the moving element of the relay.

In  this condition restraining force is greater than the deflecting force.

But when the current through the coil increases, the magnetic effect is increases after the certain level of current deflecting force is generated by magnetic effect of the coil this force is more than the restraining force.

so moving element start the moving and change the contact position from normal open to normal close position.

Types of over current relay

instantaneous over current relay

definite time over current relay

inverse time over current relay

(1)instantaneous over current relay

The relay operates as soon as the current in the coil is higher than the pick up value of the current.

There is no intentional time delay applied.

but there is always an inherent time delay which we cannot avoid practically.

operating time of the instantaneous relay is of the few milliseconds

(2) definite time over current relay

The relay is created by applying intentional time delay after crossing pick up value of the current.

This relay can be adjusted to issue a trip output at an exact amount of time after it is pick up..

so it has a time setting adjustment and pick up adjustment.

(3) inverse time over current relay

In this relay time of operation is inversely varies with input current.

if the fault is sever it will clear the fault faster.