The directional over current relay is an over current relay operates in on direction of current flow and blocks in the opposite direction. Three condition must be satisfied for operation the previous two conditions which are current magnitude and time delay must be considered and the 20 third condition which is added in this type is the directionality.
The directionality of current flow is identified using voltage as a reference of direction. The connection diagram of current circuit and voltage circuit of directional over current. relays are as shown in figure (15). Almost, this type is installed on the low side of power transformer.
