Step Down Transformer Grounding Question (kmpres)

Discussion in 'The Projects Forum' started by kmpres, Jan 24, 2016.

  1. kmpres

    Thread Starter New Member

    Jan 24, 2016
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    I have a similar question to the one Jace put forth in June 2008. In fact, I'll bet I live in the same country (Korea) that he does, or did when he wrote his question. I have a step-down transformer that reduces Korean mains 220VAC to US mains 110 VAC to allow the use of American appliances here. It gives a very perceptible shock when an appliance (microwave, toaster oven) attached to it is touched by one hand and the other hand touches my kitchen sink about two feet away. I measured this voltage and found 67 VAC between sink and appliance. Two other transformers made by the same company do not give shocks even when placed in the same location. OK, so I opened up the transformer case and found a three-wire cord entering the device but the green wire is literally cut off where the outer insulation is stripped. The other two wires (blue and brown) go to the fuse and lighted switch respectively. There is no case ground of any kind. In fact, the American-style duplex three-pronged outlet has had its ground socket pulled out. It's not there at all meaning that any three-pronged appliance attached to the transformer will not have its ground carried to earth. I suppose Korea ties their neutral to ground at the service box but the lack of ground carry-through to the end appliance concerns me, especially when appliances give very hefty shocks where water is near. There is, in fact, no ground wire at the 220V sockets even though the sockets and matching plugs do have ground points installed in them. My question is: could I fix this problem by tying that loose green ground wire to the transformer case and replace that denuded duplex with one with a proper ground socket installed? The socket ground would go to the case as well. Or would that cause a short? 67 volts is in my view, nothing to sneeze at. Also, why would they install ground wires in plugs and sockets and then not attach them to anything?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The normal way in N.A. when isolation is caused by a transformer as in going from 240v to 120v is to re-reference (connect) one side of the 120v secondary either back to the service provided ground conductor or to a qualified ground such as a metallic water supply pipe, or ground rod etc.
    Then use this terminal to provide a separate GND and N. conductor.
    Max.
     
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