Over load current, and fault current discrimination can be done more accuratly and precisely by using these type of relays between phase to neutral (R,Y,B) and neutral to ground connection, for large machines, equipments and power systems.

In some situations the short circuit current could be less than load current.

Consider a synchronous condenser (for simplicity non salient ignoring controls

such as field forcing and transients only steady state) the with synchronous

reactance xs    we get  ss operation leading power factor rated current

  Eg = Vt - Ir*xs   where Eg is generated voltage and Vt is network or Bus

voltage . Notice Eg is less than Vt. When a short occurs at the terminals

assuming Eg is kept constant the steady state Ishort =Eg/Xs is less than Ir

so we need to shift the operating characteristic of OCR to lower values.Of course

this is just an example and in an actual system there would be many other

aspects  such as speed of CB's  transient short cct currents etc to take into

account.

Cheers

Thank you Narasim Rames Sir for your valuable answer.