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Ah, Dave,

But the electrical system in the US is grounded. What you are referring to is the ability for the GFCI to work without the presence of a ground conductor attached to the GFCI. The electrical system? in the building is a grounded system, defined as neutral connected to gound at one point for reference, even though a green or bare ground wire is not present at the receptacle.

Remember, for current to flow there needs to be a complete circuit. Once an isolation transformer is in a circuit, there is no longer a current path between sides.

You think this is hard to get a mental picture, Some rural places used to use a distribution system of 15 to 30 killoVolts three phase, one phase GROUNDED! This was for lightning protection.

On the opposite side we have Navy ships with neither conductor connected to ground. No ground reference, just monitors for if a wire becomes grounded. Gives time to fix the wire without a circuit going down when it is needed....? And no electrocution hazard until the first wire becomes grounded, hence the monitors. The GFCI won't work on this circuit either, just like after a GFCI/RCD.

Frank DuVal