Balance was the word I was going to use but didn't want to sound too far off base. Thanks for the clarificationA GFI measures the current going out of the Hot (Black) wire, and measures the current in the return (White) wire. These two must equil, (be in balance) or the GFI will trip. If the GFI goes out of balance, that would mean that current is being drawen through the black wire that is not returned through the white wire, so it is going to ground elseware (a path to ground, like your body), and it should trip at a small predetermined out of balance current.