spamlet was thinking very hard :
The main RCD in our consumer unit seems to be very much on a 'hair trigger'
and cuts out regularly, causing mayhem to pc users and dvd recorders!

We thought at first it was just when the heater in the washer/drier kicked
in, but closer observation shows that it trips in various combinations of
heating element devices like kettle/toaster/cooker/washer, tending most to
cut out if two of these happen to be on at once.

Also light circuit RCD always trips when a light bulb goes, but main also
trips if the light's earth is touched when removing lamp holders even when
the lighting RCD is 'off'?

The consumer unit was fitted by a competent electrician and the problem was
not immediately apparent after the fitting.

Do the RCDs wear out?
How can they be tested?

RCD's operate on the difference in current flow between what goes in on
the live and what comes out via he neutral - the difference being what
is leaking to earth. Enough leakage and they trip. Some of the leakage
can be deliberate in the form of filters. PC's, monitors, printers and
certain appliances etc. are fitted with filters on there mains input.

Sometimes the heating elements on kettles, washers and dishwashers can
deteriorate and start to leak current to ground.

All of the above can cause nuisance tripping of RCD's.

Lacking an insulation tester, the best way is to note what was running
at the time of the tripping and gradually work out the cause.

MCB's quite often trip when lamps suddenly burn out - they are very
sensitive and fast to respond to over current, unlike a fuse. As a lamp
burns out, it can strike up an internal arc which draws excess current,
thus causing the MCB to trip.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk