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For a European-style city power supply (240 V AC 50 Hz) attempting to accommodate appliances from another country (USA, 110 V AC 60 Hz), how should a "Nippon" ungrounded transformer be wired up?

The supply voltage for the neighborhood is 240 V AC. The ungrounded 1500 W Nippon transformer steps that down to 110V but leaves the frequency unchanged. Most 110V appliances (heaters, etc) plugged into the 110V side work fine. However, the voltage on each leg of the 110 side ("hot" and "return") are both 55V AC relative to Earth ground. And when I try to connect "active" power regulating loads to the 110V outlet, my the GFIC breaker on my 220V power source eventually trips, whether I've grounded the power supply and/or transformer case to earth ground or not. The troublesome appliance is a 110VAC to 12V DC power supply, battery charger and solar charger system. A lower power transformer from Australia (500 W) does not cause this problem.

Any ideas on what I'm doing wrong with the wiring of the new transformer?

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The output appears to be ungrounded or center grounded. It could be a transformer intended for outdoor tools in UK. If there is also a safety ground 3 wire plug, it could be coupling some of the harmonic current from the switch mode active power supply (which should be flowing equally on hot and neutral) on the safety ground. That would be unbalanced current and trip the GFCI. The smaller transformer could be wired with normal grounding or it may have a plastic case. Adding filtering to the mains input could fix the GFCI issue.

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  • The transformer output plug is indeed 2-prong (ungrounded). Seller suggested I connect the earth ground separately myself. Yes, the active power supply is likely grounded. The smaller transformer has a metal case and 3-prong output but is labeled as an "isolation transformer." Will research filtering options and try them.
    – hobs
    Commented Dec 22, 2011 at 13:40
  • For reasons of safety, an added ground should involve replacing the cord and plug with a 3 conductor cord and the proper grounded plug (fused for those in UK). If there becomes a power fault to ground (transformer case for example), a ground wire separated from the power wires will have a higher loop inductance, and slow down the response of the circuit breaker.
    – Skaperen
    Commented Dec 23, 2011 at 22:29
  • thanks. I'll use a 3-prong to 2-prong adapter and bridge the gap between the input and output grounds with as short a wire as possible or use the case itself.
    – hobs
    Commented Dec 24, 2011 at 5:52
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First, "Nippon" is not a useful identifier of that transformer, since "Nippon" means literally "Japan" and nothing more.

UK mandates a very peculiar power standard at construction sites. It is 110V, with the equipment safety ground intentionally placed at the 55V center point so that the electrocution hazard is reduced to 55V instead of 240V. I have no idea if that has anything to do with USA appliances. The paradox is the best USA power tools are 240V instead of 120 (since more power is available), yet the best UK power tools are 110V (since that's what the pros use).

This "construction site" transformer is wired somewhat differently from the "Use USA/Japan appliances in the UK" conversion transformer, which are not a true transformer, but merely bucking/boosting. It might be possible to rewire the transformer to have ground at 0V instead of 55V.

Your GFCI trip could be as simple as you actually have a ground fault in that appliance. Generally an appliance should connect in no way whatsoever with the ground, which is nothing but a safety shield. So a healthy appliance shouldn't care whether the ground is biased at 0V or at 55V.

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