it can vary,it doesnt vary on TT or TNS from DNO because it is rodded and earthed throughout,but a TNS from a genny is magically different than that of DNO?
you are talking about an installation that has no rod at star point.i have always stated that a rod must be installed at star
so it cannot have a common earth point,
if the genny is not earthed, (possibly on wheels/a lorry) then it will be insulated, and therefore have absolutely no reference to general earth, it would almost be similar to TNCS without the S. both N & E are the same potential.
if genny is on trailer and insulated by rubber wheels, then it could be very dodgy... trailer to ground would be IT, but installation may be TN