Restricted earth fault (REF) relay is a unit protection 
intended for one winding of a transformer (neutral winding) 
the input of which is from a neutral bushing CT and in some 
applications, balanced by the output of three line CTs in 
the Y-winding directly connected to neutral. This provides 
accurate sensing of zero-sequence leakage currents for a 
fault within its zone of protection, i.e., on the windings. 
Whereas, standby EF relay is a back-up overcurrent ground 
protection usually connected on the HV side and its main 
purpose is to provide local back-up protection in case for 
a failure of main protections of the LV feeder/line prots., 
bus prots., and the transf. main protections.