On Mon, 19 Aug 2013 16:51:55 +0100, Caecilius wrote:

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.

MOVs breakdown at quite high voltages so a bit of "noise" won't
trigger them.

I'm thinking of a interuption and possible phase change when the UPS
switches from it's internal frequency back to mains causing the
capacitors in the filters to charge/discharge all at once thus
dumping or pulling current via the earth. An RCD is only looking for
an imbalance between L and N it doesn't care if that imbalance
current leaking to earth or current leaking from earth...

--
Cheers
Dave.