What is restricted earth fault relay protection and How is it provided?

A restricted Earth Fault (REF) means an earth fault from a restricted/localized zone of a circuit. The term “REF protection method” means not to sense any earth faults outside this restricted zone. REF is a type of “Unit Protection“ applied to transformers or generators and is more sensitive than the method known as differential protection. An REF relay works by measuring the actual current flowing to earth from the frame of the unit. If that current exceeds a certain present maximum value of milliamps (mA) then the relay will trip to cut off the power supply to the unit. Differential protection can also be used to protect the windings of a transformer by comparing the current in the power supply’s neutral wire with the current in the phase wire: if the currents are equal then the differential protection relay will not operate; if there is a current imbalance then the differential protection relay operates. However, REF protection is also applied to transformers in order to detect ground faults on a given winding much more sensitively than differential protection can do.