On Sun, 28 Jul 2019 12:57:02 +0100, Harry Bloomfield, Esq. wrote:

After serious thinking Jeff Layman wrote :
Do you happen to know of a good summary of neutral-earth shorts? I've done a
bit of searching and am even more confused! For example "... I would look for
a neutral/earth fault. Not only can these can cause an RCD to operate, but it
is also possible for a neutral/earth fault to defeat an RCD and prevent it
from operating." (at
https://community.screwfix.com/threads/rcd-not-working-then-ring-main-neutrals-connected.187153/#post-1501878)

Others have responded to the question of double/single pole MCB.

What has to be understood is that RCD's are triggered to open on a
fault, by an imbalance between current of live versus the neutral
conductor.

Most MCB's are single pole, isolating just the live. If there is a
voltage differential between the N & E (there is usually several volts
difference), even if the MCB is open, with a N&E leakage, or short/ or
fault - current can still flow down that neutral, through the RCD and
cause it to trip.

Which is why so many people have such difficulty finding the causes of
an RCD constantly tripping. Simply turning MCB's off or unplugging
alone, may not allow an RCD to be reset.

That's the advantage of miniature RCBOs - break L & N and, so far as I can
tell, fitin the same space as an MCB (BICBW).
--
Peter.
The gods will stay away
whilst religions hold sway