SOLUTION

Inverse-time relays

The fundamental property of these relays is that they operate in a time that is inversely proportional to the fault current, as illustrated by the characteristic curves shown in fig. Their advantage over definite-time relays is that, for very high currents, much shorter tripping times can be obtained without risk to the protection selectivity. Inverse time relays are generally classified in accordance with their characteristic curve that indicates the speed of operation; based on this they are commonly defined as being inverse, very inverse, or extremely inverse. Inverse-time relays are also referred to as inverse definite minimum time or IDMT overcurrent relays.

The definite time characteristic is simple. The time of operation is constant for any current above the pickup value, e.g. a relay is set to operate at 5 A (CT secondary current) and set at 0.5 sec definite time. The relay operation time will be 0.5 sec for 5, 10 or 20 A whatever is the current above the set current.