AC mains problem?

Question

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.

Phil Allison · Accepted Answer

** Nonsense - the US, domestic 120 volt AC supply is NOT balanced wrt earth.

Only US high energy outlets supplying 240 AC (at 60 Hz) are so balanced.

** Asked the tooth fairy for his/ her/its opinion yet ?

** So you are measuring that AC voltage with a DMM - right ? 10 or 11 Mohms load impedance - right ??

Go figure what effect 200 pF or so of stray transformer winding capacitance has when fed from 240 volts AC into that load.

200exp-12 x 2 x pi x 50 = 16 Mohms.

Make any sense now ?

Just ground the damn thing and make it safe and legal.

....... Phil

kreed · Answer

I would change the transformer anyway, the US is 60hz and AUS is

50hz. If the transformer isn't able to go to 50hz (and most US stuff I have seen, doesnt, even if it is intended for export) it will just run much hotter than it should which isnt good for it.

Earth the case properly and use an Australian 3 core cable and mains plug.

PS: The US 120v mains is NOT balanced, Their (non 3 phase) system is based on a centre tapped 120-0-120v pole transformer(s) and therefore only the 240v configuration is going to be balanced (and will still have a 120v potential to the neutral from EITHER side of the 240.).

(note: From what I have read in the past, their 3 phase supplies are much the same system as here, but are 208 not 415. They usually are only in industrial areas and not residential.)

Frank · Answer

something to read phase wire earth return

2468 · Answer

Thank you for the info.Was it really necessary to respond in such a rude way to a civilenquiry?

2468 · Answer

Thanks for the response and info - much appreciated.RGDS

Phil Allison · Answer

**  I always give any smug, top posting anonymous fool using a silly handle a hard time.     I see it as a matter of principle..........  Phil

Mr.T · Answer

You're new here obviously.handle And everyone else for that matter.Or the only thing that passes for principles as far as Phil is concerned :-)MrT.

AC mains problem?

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