Ian White, G3SEK wrote:

. . .

An interesting thing happens when you switch off to isolate part of the
house wiring, and then cut the "dead" three-wire cable. (We call it
"flat twin and earth". It has PVC-insulated hot and neutral wires with a
bare ground wire in between, all in a flattish grey or white PVC sheath.
Is this what you call "Romex"?)

Yes, that's "Romex". The official designation is type NM (for
Non-Metallic). 12 gauge wire, for 20 amps maximum, is "12-2 with ground"
and 14 gauge, for 15 amps, is "14-2 with ground". Those are the most
common sizes for residential lighting and outlet circuits.

If you cut the hot and ground wires
together, nothing happens; but if you cut the neutral and ground wires
together, the RCD trips. That's a puzzle the first time it happens, but
it's because that cable isn't quite as "dead" as you think. the
isolating switches interrupt only the hot wire, so the neutral and earth
are still being shared with everything else that's switched on in the
house.

Does that happen in the USA, or do you have two-pole isolating switches
for individual sub-circuits?

I don't quite follow where you're cutting the wires or where the RCD is
located. Here, you can get a GFCI as either a circuit breaker you
install in the service box to protect a whole circuit, or incorporated
in a duplex outlet. Both are easily identifiable by the conspicuous test
button. The breaker differs from an ordinary breaker in that the neutral
wire is also routed through it.

Circuit breakers interrupt only the "hot" wire. It's been a long time
since I studied the code (NEC - National Electrical Code), but I'm sure
that switching the neutral is still a no-no. Neutral wires are all
brought back to the service box, where they're all connected to the
central power feed wire. The safety grounds are brought back to the
service box separately, where they're connected to the same point as the
neutrals. Neutral and safety ground are connected together only at the
service box, nowhere else.

This isn't totally idle curiosity. If there's an electrical problem
while we're staying with the in-laws in Atlanta, I'm the one who's
supposed to know about these things...

I'd at least get a home-handyman book on electrical wiring, and a real
code book if you'll be doing anything other than pretty simple wiring.
The rules are quite complex, dictating things like how close to a
junction box wires have to be supported, how they can be routed, what
size junction box is required for various combinations of
entering/exiting wires, etc. In Oregon, wiring can legally be done only
by the homeowner or a licensed electrician, but each state and many
localities have their own rules. I think all, or just about all, the
states now at least have agreed to use the NEC. (You're probably
technically forbidden to touch the wiring, depending on Georgia and
Atlanta regulations.) The presence of non-code wiring could result in
non-payment of insurance if a home were to have an electrical fire.

I wired the addition to our home, which involved adding seven circuits
including outside outlets and lights. I also replaced and upgraded the
service box. Those jobs were done with electrical work permits, and both
were inspected and passed. So I have done a bit of this before. But it
was a lot of years ago now, the code evolves, and my memory fades.

Roy Lewallen, W7EL