On Sun, 16 Dec 2012 21:52:38 +0000, Fredxx wrote:

You can buy affordable clamp meters where if you can pass just the live
and neutral through the clamp together you can measure residual current
directly. Depending on supplementary bonding you can also get an
indication by measuring the current flowing in the protective conductor.
You might then be able to find the offending circuit.

That's interesting. I thought that sensitive clamp meters were fairly
expensive. I'll have a look around. Another toy for the collection if
nothing else :-)

As others have suggested, each appliance does have an allowable leakage
and combined it may be close to the RCD limit.

Yes, I suspect this is part of the problem, together with a fairly
sensitive RCD.

Moving some circuits on the non-RCD portion whilst using RCBOs on the
most likely offending circuits like the kitchen circuit may solve the
problem.

Yes, RCBO is doubtless the way to go, and they look quite affordable
now. But I don't think my CU is big enough for them, as they look much
taller than a normal MCB, and my CU is about ten years old.

Here's some photos of my CU:

http://i1021.photobucket.com/albums/...U/P1000068.jpg
http://i1021.photobucket.com/albums/...U/P1000072.jpg

Nicely located less than an inch from the ceiling, and right behind
the garage door rail.

If an RCBO will fit in that, then I'd probably take that route. I
guess I could put a couple of circuits on RCBOs and move the RCD over
so it protects less circuits. If I could do that for the cost of a
couple of RCBOs plus a busbar then I'd be a happy bunny.