We have a piece of equipment that runs on 230VAC being shipped to the USA (110VAC). We are shipping it with an isolation transformer (5kW) The input is E,N,L and the output is neutral and live (isolated). What do we do with the earth connection on the equipment?

(a) Leave earth unconnected

(b) Connect it to 230VAC neutral at the transformer

(c) Connect it to 110VAC earth (ie the building's earth)

Additional comment: There is likely to be local mains driven equipment connected to our box eg computers

  • 1
    \$\begingroup\$ Are there local regulations for connection of safety ground you can follow. Common sense would dictate it's grounded to the safety ground of any other nearby mains-powered equipment, however regulations don't always follow common sense. The fact that it's powered from an isolation transformer might make one think it's double insulated, but perhaps other live conductors could contact the chassis, and without safety grounding they could do so silently without blowing a fuse. \$\endgroup\$
    – Neil_UK
    Commented Jan 30, 2020 at 11:17
  • \$\begingroup\$ @Neil_UK I am not familiar with US regs, which is why I hope someone who is will comment. I think the answer is to connect the input (real) earth to the equipment earth ie option (c) \$\endgroup\$ Commented Jan 30, 2020 at 11:22
  • \$\begingroup\$ Is the isolation a requirement? \$\endgroup\$
    – Jeroen3
    Commented Jan 30, 2020 at 11:42
  • 1
    \$\begingroup\$ It was a rhetorical question really. If you're shipping stuff to the US, and 5kW sounds like it's a serious piece of kit, then get familiar with the electrical regulations for where it's going. \$\endgroup\$
    – Neil_UK
    Commented Jan 30, 2020 at 12:17
  • 1
    \$\begingroup\$ 5kw off 110V is close to 50A. You are probably better off supplying it with both legs, namely a centre tapped 220V supply, or at least allowing that option. (Large domestic appliances run that way so it's not unusual). GFCI is American for RCD, a good idea too. \$\endgroup\$
    – user16324
    Commented Jan 30, 2020 at 12:28

2 Answers 2


I'd advise you to ship with a US style GFCI (Ground fault circuit interrupter).

enter image description here

Above picture from here.

This is because any building GFCI will not be able to detect potentially dangerous live leakage currents to earth that occur on the isolated side of the isolation transformer.

So, you would wire as follows: -

  • Normal mains from board to
  • Isolation transformer input
  • Isolation transformer output to
  • GFCI then
  • GFCI to equipment

The "equipment's" earth wire should connect to the earth from the main switch/fuse board (the same as normal equipment directly connected to the mains AC).


there are many source cited you can't put neutral with ground together because it will create unbalance current since the 1phase connection bot Live and Neutral have same rating of current.

or you can try to read this (this is good if you need isolation transformer connected for your device to work on) https://www.powerinspired.com/isolation-transformer-need-know/


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