Bangalore, India based Prok Devices Pvt Ltd offers Earth Fault Relay.

Details:
Prok Devices’ Earth Fault Relay is a current-sensing device which is tropicalised, professionally designed and tested for protection of generators, transformers and feeders. The Prok DV EFR can be employed with various earth fault protective schemes. The magnitude of the earth fault current depends on the fault impendence and invariably the fault impedance for earth fault is higher than that for phase faults, hence the earth fault current is low compared to the phase fault currents. 

The fault impedance depends on the system parameter and also on type of earthing.  The neutral may be solidly grounded, grounded through resistance or reactance. Figure shows how an earth fault can relay can be energised by a residual current, in this method the secondary CT currents, Ir, Iy, Lb of three different phase are connected in parallel. The vectorial sum of three current i.e., (Ir+Iy+Ib=0) is zero under normal condition. During the occurrence of earth fault the residual current is non-zero and when it exceeds the pick-up value (externally selectable by dip switch, pot meter on front fascia), the Earth Fault Relay trips. Theoretically during balanced load conditions the EFR carries no current hence its current, setting may be any value greater than zero, but in practice, such ideal system do not exist.