Directional overcurrent relaying is necessary for multiple source circuits, when it is essential to limit relay tripping for faults in only one direction. It would be impossible to obtain correct relay selectivity through the use of a non-directional overcurrent relay in such cases.
If the same magnitude of the fault current could flow in either direction at the relay location, coordination with the relays in front of, and behind, the nondirectional relay cannot be achieved except in very unusual system configurations. Therefore, overcurrent relaying is made directional to provide relay coordination between all of the relays that can see a given fault.
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