The earth fault relay can be of the type that has one current transformer in each phase with the current transformers connected so that when the out of balance current exceeds a preset limit the relay trips. An alternative is the toroidal current transformer that has all the motor cables passing through the transformer, again, when the out of balance current exceeds a preset limit then the relay trips.
For LV (0-660Vac) systems the toroidal current transformer is cheaper than three separate current transformers and takes up less space. For higher voltage systems it is more practical to use three separate current transformers.
The earth fault relay should have a wide range of current and time settings so that a single type of relay may be used in every starter for each voltage range.
Earth fault relays must have two electrically separate contacts a normally closed when the earth fault relay is ‘on’ which opens when an earth fault occurs and a normally open which closes on earth fault. The normally closed contact is used in the fault circuit and the normally open in the SCADA circuit.