The output is usually isolated on a CVT, so you may be getting wrong answers, which usually assumes one side of the line is grounded at the substation. You can verify the isolation by unplugging from the line, then use the ohms range on a meter between input and output, then output and frame, which should show infinite resistance. To check leakage, use a megger or other insulation tester, which should show 10's to 100's of megohms. Not much to go wrong on a CVT, other than resonating cap failure with age, but that is pretty rare as well...