I purchased a mains powered late 70's vintage freq. counter from the U.S. It was wired for 110v, and after first verifying that it worked, I then rewired the trannie for 240v per manufacturer's instructions (tap shifing). Powered up and again verifying it was functional. The trannie simply supplies 9v AC to a board where it is then rectified etc.
Then when in the process of moving it while powered up, I got a zap when touching the case and another piece of earthed gear. I then measured 120v AC from the counter chassis to an earth. The counter only has a paired mains lead - i.e. no earth and the unit is not double insulated.
The counter had a couple of disc ceramics between each mains line and earth, and I removed these and also rewired the set in a more safe logical manner i.e. hot AC to fuse then to power switch, then to trannie, and the neutral direct to the trannie 0v point. As supplied for the U.S. balanced AC system it had the switch on one side, and the fuse in the other. With all this the chassis measured around 100v AC to ground. Suspecting a problem with the trannie, I then mounted it on insulated bushes but no change.
I've heard of problems with induced voltage in mains cables, but in my experience that has been with three core cables, when one end of the earth has been found floating.
If a three core cable is fitted it would send this 100v AC to earth - and that may be the appropriate fix - but an explanation of the cause of this problem would be appreciated.