In article ,
John Rumm writes:
chris French wrote:

I'm thinking that maybe my RCD isn't doing it's job properly? (We have a
TT earth system with just one main incoming RCD - about as old looking
as the CU) The test button works ok though.

Are you sure it is a RCD and not an older "voltage operated" ELCB?

If that's the case, then the leak could be via a water main or
similar to earth (you won't get 40A or anything like through a
VOELCB). Either this water main (or whatever) is not connected
to the house earthing (and has virtually full mains voltage on
it), or it is connected to the house earthing but has an
overlapping resistance area with the VOELCB earth rod, preventing
the VOELCB from working.

Another option is the VOELCB has become disconnected from the
house earthing, either due to an internal fault or deliberate
action (possibly to prevent tripping in the past).

Probably other options too, but those two spring to mind as
matching the symptoms described.

Beware, as you may have live metalwork around the house.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]