On Sat, 15 Dec 2012 19:06:34 +0000, Caecilius
wrote:


I guess I could megger the fixed wiring, but it seems like clutching
at straws.

Not quite, neutral to earth faults in fixed wiring are not that
uncommon and quite often go unnoticed for years. One of the most
common causes is a nail through a cable which shorts neutral to earth.
Some years ago I helped a friend with such a fault. It was discovered
when plugging in a drum type extension lead (with nothing attached to
it). Plugging it in to some sockets would cause the RCD to trip.
Plugging in to others (on the same ring or an alternative) wouldn't.
The extension lead was perfectly OK when tested. An alternative
extension lead did the same thing - but on different sockets!

Both leads were fine if his wife was putting clothes into the airing
cupboard (noticed only by chance rather than skilled detective work).
It turned out that the upstairs ring had a floorboard nail neatly
through it on the landing near the airing cupboard. When standing
near it it moved just enough to break the E-N link.