I have an old fixture that was rewired ......When we hung it and turned it on it tripped the ocpd.....What we found,on the light, was that there was continuity between what we thought was the hot and the ground and no continuity between gounded conductor and the ground......What I think happened is that however rewired the light tied the ground and what they thought was the nuetral together......Is this a common practice when rewiring old lights?