In article ,
"Andrew Mawson" writes:
Been having lots of trips on my main RCD, mainly in the small hours of
the night. I've isolated each individual circuit, and checked with a
megger to earth for leakage and there is zilch. On the last couple of
nights I've manually tripped all the MCBs in the box before trying to
reset the RCD to eliminate external issues and it is very reluctant to
hold in, taking several attempts before it holds.

I suspect the RCD itself is knackered, and I've a replacement on order
(Wylex 30mA/100A) but is this a common failure mode for RCD's?

Not common, but not completely unheard of.
Has it been operated lots of times? Could be the mechanics are worn out,
which can happen with MCBs too.

The other possibility is that there is an earth leakage in the CU.

Most common failure IME is a failure to trip when it should.

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