| If they're in proper working condition, there shouldn't be any. This is also
| Wayne's point, at least in the US, the manufacturers are responsible for
| building their stuff to the latest safety standards, so they will work
| reliably with gfci protection
I recently bought an old house with 120V fused-neutral (well, they had
been sort of bypassed) service and a mixture of old BX (no ground ribbon)
and various vintages of NM wiring. The service lines blew down right
before I bought it and the section with the service entrance was going
to be replaced anyway, so the contractor ordered a temporary service
instead of trying to get the antique reconnected.
I wanted power in the house and the new panels wouldn't be in for a few
weeks so I installed an isolated neutral bar in the old fuse box, separated
ground/neutral, and plugged the whole house into a GFCI outlet on the
temporary drop. I had to fix one ground/neutral short in new (bad, DIY)
wiring, but apart from that I was able to run the whole house (including
shallow well pump) from that GFCI outlet. Just to be sure I wasn't fooling
myself I used my plug-in GFCI tester on some outlets in the house and it
did indeed trip the GFCI.
All things considered I thought this was a pretty good indication that
minimal leakage requirement are pretty easy to meet. That said, at home
I do get nuisance trips from lightning and there doesn't seem to be much
to do about this. I also used to get trips when the gaskets in cheap(?)
floodlight holders got wet, but I made teflon replacements that don't
have this problem.