Restricted earth fault relays are used for the protection of equipment against earth faults these are differential relays it is mainly used for the protection of the delta-wye transformers. Earth faults can occur at any time so we need to take precautions for it. Earth fault is the fault between the conductor and the earth, it can also cause by the insulation breakdown if any fault occurs, it will cause the short circuit current to flow through the system and this current will return through the equipment or the earth and it will make the equipment damage and this can also cause the interruption of the supply

It is used in power transformers to detect the internal earth faults of the transformer, in this each phase of the transformer is connected together and the common terminals are connected to the secondary of the neutral transformer, if there is any unbalancing between the three phases of the transformer a resultant unbalanced current will flow through the secondary terminals of the current transformer, and an unbalanced current will also flow through the neutral and thus a secondary current is formed in the neutral current transformer in this scheme the terminals of the current transformer is connected to the neutral in a certain way that the unbalanced current in the neutral and the phase will oppose to each other and if both of them has equal amplitude then there won’t be any resultant current


If an internal fault occurs the secondary current in the current transformers doesn’t have any other way to go thus it will flow through the relay and the relay will trip if the current exceeds the limit of the relay. Voltage-dependent resistors are used for the protection of the current transformer in case of heavy internal faults if the fault is in outside the transformer then the earth fault protection won’t operate because the fault in the outside zone will be neutralized the current in the neutral pumped by the REF relay will meet the current generated from the faulted phase in the current transformer and both of them are cancelled and it won’t operate outside the zone and it is called the stability of the REF relay

Restricted earth fault is the protection of the system from earth faults in the restricted section of the system not outside the restricted section

It won’t sense any faults outside the restricted area

Restricted earth protection is a kind of protection which is used to protect the transformers or the generators

It works by measuring the current flow to earth from the unit if it exceeds the preset maximum value then the relay will trip and the power supply is cut off