A ground fault on the secondary side of a transformer will be seen as a line to line load on the primary side, this is why a GFP on the primary side will not function when a fault on the secondary side happens, lets say you have a 480/277 service with GFP on the mains, then you have a 480-208/120 transformer to feed some receptacle loads, a ground fault on the 208/120 will not trip the 480 GFP since it will only see a phase to phase load.
