I am pretty sure I am right, that a vacuum cleaner, which presents
no path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?

Luke

The only exception might be a faulty or leaky "suppression" capacitor
that is permitting a current to flow from the Live wire in it to
Earth that can trip it.

Yes, but the vacuum cleaner has no earth cable and in any case I wired
it directly to the CU. There was no earth wire involved at all.

Was this wired directly into the RCD with all the other connections to the
RCD removed (apart from the incoming L and N supply) or was it wired into
the RCD with the outgoing neutral still connected. It does make a
difference.

Adam

Thank you.

To answer your question, I left all the neutrals connected to the
neutral side of the RCD but only connected one live, which was to the
vacuum cleaner. I would take a photo so you could see exactly what I
did, but I have disconnected it now and don't want to cut the power at
this time of night to recreate it.

So, I decided to simplify matters yet further by wiring the RCD into a
socket:

(Health and safety fanatics look away now)
http://tinyurl.com/LukeRCD

And this time the RCD didn't trip, and the vacuum cleaner worked fine.
I must admit I am confused as to how leaving all those neutrals
connected could have been a contributory factor, as only 1 live was
connected. Is it therefore possible there is a leak between a Live on
the non-RCD side and Neutral on the RCD side then? If you like, I will
re-execute my original experiment tomorrow morning and post a
picture.

Luke