What is Restricted Earth Fault?

Restricted Earth Fault (REF) is a unit form of protection scheme which only looks a a particular defined zone of an equipment, all faults outside the defined zone should not be detected by REF. This zone of protection is defined by the position of your CTs in the circuit. For example when looking at a transformer, we can have LV REF which looks at the faults on the LV winding of the transformer alone and it has basically three line CTs and a neutral CT, with the secondaries connected such that under out of zone faults, there won't be any spill current to the relay, the relay only operating for in zone faults.

Regards,

if REF protection of a transformer is operating for out of zone faults what are the probable causes?

It could be the relay settings which are out of range, CT mismatch(ie all the 4 CTs should match) or the wiring which was not properly done( for exampe if any one of the CTs has the S1 and S2 connected in reverse with regards, to the others). You need to carry out stability tests and check your settings

Thanks. Settings seem to be in order. Can you please elaborate the stability tests?

regards.