Directional Sensitive Earth Fault

I hope you know what is sensitive earth fault. If not, let me try to explain that first. You know, conventionally, earth fault relays used to have a minimum pick-up current setting of 10% of the relay rated current. But, with the advent of modern numerical relay technology, it is now possible to have a minimum setting of 5% or even 2% of the relay rated current - in an earth fault relay. Such a relay is called "Sensitive Earth Fault Relay" - as the relay is sensitive to fault currents as low as 2% or 5%.

Now, coming to the directional part of it! You see, in parallel connected systems, if there is an earth fault in the incoming side of one of the sources, then the other source will feed into this fault & trip, thereby creating a total black-out. This has to be sensed and only the faulty portion isolated. This is achieved by a directional earth fault relay. As the name suggests, this relay will respond only if the fault current is more than the setting AND it flows in the prescribed direction. If the current flow is not in the prescribed direction, then this relay wouldn't operate, even if the fault current is more than the setting.

Wheras in a non-directional earth fault relay, the relay will respond, as long as the fault current is more than the setting made in the relay, irrespective of the direction of the fault current flow.

To conclude, non-directional relays are used in stand-alone single source-fed systems (as well as in dual fed systems), whereas directional relays are used ONLY in dual fed, parallel operated systems.

Hope it is clear now!