Remember that 'ground' potential is _not_ the same everywhere. The two supply legs might be at 120V relative to the supply ground reference, but at a different voltage relative to a different ground reference.

This would require both 1) bad bonding between the supply and the point that you are measuring and 2) something introducing current into the ground so that there is different ground potential at your measurement point. Perhaps there is a high impedance ground fault on the third leg, dumping current into some grounded electrode, presenting a shock hazard and killing a few worms, but not anything that anyone has yet noticed [Linked Image]

Perhaps the panel at the source is properly bonded to neutral, but the grounding electrodes have dissolved, and the bond to the receptacle box is also toast. In this case your source is only kinda-sorta grounded, through the impedance of whatever ersatz 'grounding electrodes' happen to be bonded to the panel. In this case, 'ground' at the panel will be right at the center tap potential, but elsewhere in the system, the potential to ground might not be as well defined.

-Jon


If there is bad ground bonding between the source and the point of measurement, then 'stray voltage' could