Any help on the following would be much appreciated. We moved into a 10
year old house earlier this year, the RCD has been fine up until last
Friday, having said that it is a 30mA rated RCD for the whole house
(groan). Since last Friday it has gone of 4-5 times a day.

Over the weekend I tried to identify which particular circuit was
causing the problem by turning off circuits (I realise this does not
stop N-E leaks on those circuits from tripping the RCD...). The RCD
tripped with both lighting circuits off and with the central heating
circuit off. Oh and it also tripped with the washing machine completely
disconnected from the supply.

There seems to be no pattern as to which device was being turned on/off
when the trip occurs. My suspicion is that something late last week
started leaking enough current to earth to sensitise the RCD and now
the system trips randomly when 'something' turns on or off.

OK, so the possible culprits:

1. Light in bedroom: We have a 2 month old baby who last week sounded
like he was not happy with the dry atmosphere in our bedroom so (don't
laugh!) I boiled a kettle in our room for a couple of minutes. (No, it
did not end up like a sauna). The RCD didn't trip at that point but I'm
wondering whether moisture got into the light fitting causing an
increase in the overall leakage... note that based on the fact that the
RCD has tripped when the lighting circuits were off, this leakage would
have to be N-E not L-E. The fitting itself is 3 x 40W halogen (I think)
bulbs with an earthed metal body...

2. Lights or sockets in the garage: The garage is attached to the house
and has a couple of sets of sockets which are run off the kitchen
socket circuit and 4 strip lights which I assume are run off the
downstairs lighting circuit. It rained like mad last Friday and over
the weekend... so I was wondering whether dampness had got in
'somewhere'... there are no obvious leaks in the garage. Note we have
had no problem with this before.

3. Emmersion heater on the hot water tank. I noticed quite a bit of
corrosion around the electric 'backup' heating element on the hot water
tank. This has it's own circuit.

So, last night I:

a. Removed the light fitting from our bedroom and took a hairdryer to
the circuitry beneath... hopefully got rid of any moisture.

b. Connected the fridge/freezer to the ring main rather than the
kitchen sockets circuit and turned off the kitchen socket circuit..
(thus 'disconnecting' the sockets in the garage and all kitchen
appliances... I wonder whether the electric igniter on the gas hob has
got 'leaky'?)

c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

So I'm wondering what I do next... I'm hoping that it was the light in
the bedroom as this would be by far the most easy to fix.. however if
this is the problem then I am suprised that it didn't immediately trip?
I'm also worried that I may not have found the cuplrit but simply
reduced the amount of extra 'valid leakage current'?

Any ideas about how to diagnose further where the problem maybe? I'm
not sure I'm not particularly happy about opening up the consumer
unit.... (otherwise I assume one approach would be to attempt to
measure the N-E/L-E impedances on the circuits).

The follow up to this is that once I've got to the bottom of this I'm
intending to improve our consumer unit. Which of the following is best
and roughly how much should I expect to pay to:

1. Change CU to split-load system and use a 100mA RCD. At least place
lights on non-RCD side!

2. Leave current CU in place but replace RCD with 100mA unit and
replace 'required' circuits with their own 30mA RCD/MSB 'thingy'

Should I also think about getting all the garage circuitry onto a
separate circuit... now that could be painful!

Finally (is anyone still reading?) I'm not to keen on replacing the RCD
etc myself... how the hell do I assess an electrician as 'OK' apart
from rejecting anyone with a Stetson hat on! :-)

Sorry for the long post but I wanted to get as much info down as
possible!

Ben