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I guess my question is probably trivial with an expected answer like 'It couldn't exist and would be dangerous', but still I keep it around.

Could you build a protection circuit that would protect a metalic case without ground?

That is to say, that would detect a low impedance/low voltage gap between the case and the live wire, and open the circuit?

It would of course only protect of the 'live in' wire touching the case, but of any fault in the 'live out' and 'device' fault to the case.

I know a GFCI could protect by detecting differences in the live/neutral wires and protect when there is some current through the body of someone touching the live case.

Wouldn't it be safer to detect such a fault before current is actually going though someone's body? Such a circuit could be cheaper, thus more accessible, especially where grounding commonly lacks.

Wiring schematic of a supposed protection circuit for a metalic case housing an electrical device

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    \$\begingroup\$ The obvious two solutions (that does not answer the question) is that do not use a metal case, and if you do, use double insulation so that it is not possible for the case to become live. It is far more safer that way with the same amount of money and effort, than using the money and effort to try detect the fault and disconnect it when it happens. And technically, it should be possible to design such a circuit, as there is no reason why it would be impossible. \$\endgroup\$
    – Justme
    Commented Feb 23 at 12:11
  • \$\begingroup\$ My question was more theoretical, and I guess you can always fin use cases that could indeed be more or less smart/relevant ! (an old metalic lamp, vintage appliances, etc.. in countries/home without ground). Question around that is often 'is better good enough to be sold ?' It would provide more protection than nothing, but not enough to be up to the norms. Thus it seems even if this is possible, there is no commercial market for reliable/easy to install failure detector. \$\endgroup\$
    – Simon Lang
    Commented Feb 23 at 12:55
  • \$\begingroup\$ Such systems are in fact used, e.g. on boats that have the metallic body isolated from neutral and live, so either live or neutral touching the body will trigger an alarm light. Also in hospital operating rooms, the live and neutral come from an isolation transformer and are monitored against the actual PE which grounds everything to same potential. You just need the same in a smaller scale, for each individual appliance. And lamps with metal body do use double insulation as you can't retrofit a GFCI or anything else as there is no room for it, unless it is an external box. \$\endgroup\$
    – Justme
    Commented Feb 23 at 13:22
  • \$\begingroup\$ That's interesting ! Do you know what would be the name of such a circuit in boats ? \$\endgroup\$
    – Simon Lang
    Commented Feb 23 at 13:45

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Wouldn't it be safer to detect such a fault before current is actually going though someone's body? Such a circuit could be cheaper, thus more accessible, especially where grounding commonly lacks.

This is why we earth the exposed (and vulnerable) metal case. It blows a fuse should there be any live contact with it. Of course you might consider that connecting the case to neutral would achieve the same thing but, if L and N got swapped (external to the apparatus) then you have an electrocution machine.

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  • \$\begingroup\$ True ! The neutral protection seems so dangerous ! I was comparing as when no true ground is available, and thus an expensive GFCI would work, but only once current start to flow through you.. \$\endgroup\$
    – Simon Lang
    Commented Feb 23 at 12:52

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