No, the GFCI doesn't need the safety ground. Like the device you're
describing, it measures the difference between the "hot" and "neutral"
wires, by running the pair together through a transformer. A GFCI is
supposed to trip in 1/40 of a second at 5 mA.

In our system, like yours, the "hot" and "neutral" wires should always
carry equal currents, and the safety ground shouldn't carry any.
However, the safety ground is sized the same as the current-carrying
conductors so it'll handle a fault current large enough to kick the
circuit breaker. Its primary function is to protect against something
like the "hot" wire making contact with a metal appliance frame, which
should be connected to the safety ground. When it comes to a GFCI, no
assumption is made about how much of the imbalance current is returned
via the safety ground and how much via the Earth or some other path.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
Paul Burridge wrote:

Ground Fault Circuit Interruptor.


How splendid! We call them Residual Current Circuit Breakers in England.


That may not be an accurate translation. As I understand it, the GFCI
requires a ground connection in order to operate, while the RCB does not.

The RCB functions on the difference between the outgoing 'live' current
and the return current in the neutral. In the British house wiring
system, those two currents should be *exactly* equal, and the difference
(the 'residual current') should be zero. Any difference at all means
something's wrong - ping - you're disconnected.

A typical household RCB will trip on a 60 or 100mA difference in supply
current surges up to 60A. That's pretty good common-mode rejection!

RCDs can also be cascaded, so an even more sensitive RCD can be used to
protect an outdoor mains socket or a workbench.

We used to have something like GFCIs over here, but changed to RCDs
because RCDs still work if the entire house ground connection has failed.