On Sat, 05 Apr 2008 18:51:54 GMT, spamlet wrote:

Also light circuit RCD always trips when a light bulb goes,

Lighting RCD? or do you mean the MCB? An MCB tripping when a bulb blows is
fairly common for the reasons already posted.

but main also trips if the light's earth is touched when removing lamp
holders even when the lighting RCD is 'off'?

I don't quite follow what you mean. What do you mean by "the light's
earth"? The metal body that the pins of a bayonet bulb twist and latch
into or one of the power connections?

An MCB is a single pole device, the neutral is still connected to the
circuit. Shorting neutral to earth can cause an RCD to trip as not all the
current going to the load in the live is coming back through the RCD on
the neutral, some is going via the short to earth. This sort of indicates
a possible problem with the supplies neutral/earth bonding or with the
earth conection.

But if you have a lot of devices with SMPSU's there could be a lot of
"normal" earth leakage which will have the affect of making the RCD appear
over sensitive. How many, PCs, printers, monitors, dvd and/or video
machines, digi boxen, TVs etc are plugged in? They don't have to be
powered on, just plugging in puts the filter components across the mains.

Do the RCDs wear out?

Yes or more correctly go out of tolerance.

How can they be tested?

Yes.

--
Cheers
Dave.