An over-current relay has a single input in the form of AC current. The output of the relay is a normally open contact, which changes over to closed state when the relay trips. The relay has two settings: time setting and the plug setting. The time setting decides the operating time of the relay while the plug setting (comes from the electromechanical over-current relay) decides the current required for the relay to pick up. In these relays we have to insert a shorting plug in a plug setting bridge, so as to change the number of turns of the operating coil to get a particular pick value. The block diagram is as shown in figure 2.1
Fig. 2.1 Block Diagram of a Over-current Relay
The plug setting multiplier, PSM is defined as follows:
Irelay = The current through the relay operating coil &
PS = The plug setting of the relay
The value of PSM tells about the severity of the current as seen by the relay.
PSM < 1 - Normal load current is flowing
PSM > 1 - The relay is supposed to pick up and higher values indicate how serious the fault is.
E.g.:- Let us consider a 1.0 A relay (i.e. a relay with current coil designed to carry 1.0 A on a continuous basis) whose plug has been set at 0.5 A i.e. at 50 %. Assume that for a certain fault the relay current is 5.0 A. The relay therefore is said to be operating at a PSM of 5.0 / 0.5 = 10.