What is meant by a restricted earth fault?

A Restricted Earth Fault (REF) refers to a method of fault protection that is applied to electrical system units, such as transformers or generators, that is more sensitive than the method known as Differential Protection (DP).

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.


REF protection is applied on transformers in order to detect ground faults on a given winding more sensitively than when only using differential protection.


Differential Protection is commonly 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 DP Relay will not operate. If there is a current imbalance then the DP Relay operates.


REF and DP protection devices operate in similar ways to a GFCI or RCD device, which combines both REF and DP functions inside the one single device.

For more information about GFCIs and RCDs, please see the answer to the Related Question shown below.