I have 9ea 480v circuits feeding approximately 129 400w MV fixtures.  Four of the circuits indicate current carrying conductors are somehow connected to ground.  Indeed when we meter one side of the 2 pole circuit we get 480v not 277 and the other side registers zero.

It is inconceivable that this could happen in so many circuits because in all my years in 277v/480 Y circuits it has happened maybe twice where a ground was inadvertantly connected to a feeder or phases were crossed.  This is the second time I have dealt with 480volt Delta lighting and had no problems before.  Any suggestion as to how to look for the problem.  Resistance testing shows no short circuits between feeder wires and ground.