Answer Posted / akshit limbasiya

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 preset 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.

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