WHAT IS THE INSTANTANEOUS OVER CURRENT RELAYS?

INSTANTANEOUS OVER CURRENT RELAY: 

These relays work without an intentional time delay. Attracted armature type electromagnetic relays constructions are used as instantaneous overcurrent relays. In this relay, an armature moves into the field produced by a coil. An energising coil is wound over an iron core mounted on an iron frame. The armature is pivoted in order to make it free to move in line with the magnetic field. The movable contact moves with the movement of the armature and touches the field contact. There is no arrangement for time delay.

Instantaneous Over Current Relay 

In the AC power supply, there is a reversal of the polarity and zero crossing in every half-cycle. Thus, the flux in the attracted armature also passes zero in every half cycle as a result of which the armature being to reset and release itself slightly from the pole. A low resistance copper shedding loop is used to surround one part of the split magnetic pole in order to eliminate the effect of passing zero in every half cycle by the magnetisised armature flux. The induced eddy currents in the loop cause a phase lag in the flux passing through the loop as compared to that in the other part of the pole. There is always some hold on the force because the flux does not come to zero on the entire pole face.

During the design stage, due consideration should be given to the susceptibility of the relays towards any maloperation caused by transients and heavy mechanical vibrations.
These relays are useful for ensuring the feeder's protection, wherein current grading is feasible. These relay are also useful for protecting electrical equipment against short-circuits, especially in the case of high fault current values.