On Mon, 19 Aug 2013 15:09:29 +0100 (BST), "Dave Liquorice"
wrote:
IT/electronic kit tends to leak rather a lot to earth because of the
filters on the mains input to keep all the nasty RF interference off
the mains cabling. IIRC it's not recomended to have more than 10 such
appliances on a single RCD.

Switching transients may well create enough earth leakage current to
push an RCBO that is already near its leakage limit past the leakage
trip point.

That's what I suspect I may be seeing. Perhaps the UPS contains MOVs
which snub spikes to earth, so noise on the supply causes earth
leakage which trips the RCD.