Restricted earth fault relays are differential relays connected to provide sensitive protection for equipment against ground faults. The most common usage is for the protection of delta-wye transformers with a resistance grounded neutral.One CT in the transformer neutral is balanced against the neutral of three phase CTs in the transformer output connection (or bushing CTs in the transformer circuit breaker) and connected in parallel. The REF relay is connected across the two CT leads - this relay is a high impedance type, operating on fault voltage developed across the CT leads for an internal fault. For an external earth fault, the zero sequence current in both sets of CTs is balanced and there is no fault voltage developed on the CT leads.The REF scheme is a high speed differential protection, with instantaneous operation for internal faults and high stability (depending on setting) for external faults.By extension, an unrestricted earth fault relay is a relay connected to a single CT or set of CTs to measure zero sequence current. This is a normal time delayed overcurrent relay, which must be delayed to coordinate with other relays downstream. This would normally be a non-directional relay, but could be directional depending on the application .Standby earth fault protection Relay connected to the neutral CT. Normally a kind of back-up protection with a relative long delay or many times a long-time inverse curve. Need to coordinate with downstream E/F relays.