Can you provide more information on the relays. I can't identify them from the numbers that you gave. Perhaps you can link us to their descriptions or data sheets.
Thank you for the clarification of your question. I personally do not know the answer but when I get some time I will do some research. Is it possible that we are comparing two different applications and one is for ground fault protection and the other is just using a residual ground current for sensing?
sir i work in fertilizer plant and in the plant, the MV motors are protected by 64 relay and transformer are protected by 51N relay.
the 64 relay current sensing is from a ring core CT and the 51N is connected between the star point of CT and the star point of 51 relay ( the 51 is over current relay and three 51 relay are connected in star connection)
64. Ground protective relay is a relay that functions on failure of the insulation of a machine, transformer or of other apparatus to ground, or on flashover of a dc machine to ground. Note: This function is assigned only to a relay which detects the flow of current from the frame of a machine or enclosing case or structure of a piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit.
It is not applied to a device connected in the secondary circuit or secondary neutral of a current transformer, or in the secondary neutral of current transformer connected in the power circuit of a normally grounded system.
As can be seen a relay connected in the Primary Neutral Circuit or in the residual path of the Phase CTs' secondary neutral MUST NOT be given the DEvice Function Number 64. It is generaly numbered 51N, as this can also respond to phase unbalance conditions, if the unbalance is more than the setting in this 51N Relay. And, this is possible with 3-Phase, 4-Wire distribution systems,with single phase distribution, if the single phase loads connected to the phases are not distributed and/or utilised evenly.
As regards 51G, it is a time delayed realy and is aptly called "Back-up" Earth Fault Relay. Whereas 64 shall be instamtaneous only.
