As the name suggests both relays are used to sense the earth's fault in the system. Earth Fault relay is designed to sense earth fault anywhere in the system after relaying point.
There could be two ways of connecting the overcurrent relay to sense earth faults in the system. The first way of connecting is to do the residual connection of all three phases current transformer and connect the relay at the residual point.
Modern numerical/microprocessor-based relays are able to internally calculate neutral current from 3-phase current input.
The second way to connect the Earth fault relay is to have separate CT at the natural grounding terminal of the system which will detect Earth fault current directly from the system and Operate on the same.
Restricted Earth fault
The restricted earth fault scheme of protection is used to identify the fault in the fault in a limited section of the power system. Mostly used to sense faults in the star side of the transformer.
The restricted earth fault relay compares the measured Neutral current of the neutral Current transformer with the neutral current measured from 3-phase current transformer inputs. If both the neutral current is of the same value that indicates that the fault is outside our concern area, and it won't operate and will allow the normal earth fault relay to take action. If both the neutral current does not match, then it indicates a fault in the concerned part and the REF relay will operate on the same.