In an overcurrent relay, we use both directional as well as non - directional relays. Directional really is connected towards the load side of the line which will trip if overcurrent flows from the load towards the source and nondirectional relay is connected at the source side which will trip if a fault occurs irrespective of the direction of the current.
Nowadays we use multiple conductors or transmission lines forming a ring structure so as to avoid interruption of power supply to the customers in case of a fault in any line. Therefore, when any of the transmission lines undergo a fault, the nondirectional really at the source side will automatically trip and separates the source from the fault point but the load side is still connected to the fault point.
Due to other transmission lines present in the system the power will still reach to the customers. As the fault is still present therefore another transmission line will also feed this fault point and thus system might get damaged. Therefore, the directional really that we placed at the load side will detect the reverse flow of current and thus trips and avoids other transmission lines feeding the fault location.