Jeff Layman wrote:
On 28/07/19 10:43, Harry Bloomfield wrote:
After serious thinking Brian Gaff wrote :
If its a machine that still retains a mechanical switch or rotating knob,
often returning it to the off then plugging it in can release the door, but
if it does trip even then, IE its not a motor fault, then I feel you need a
person who knows the tricks of the trade.

Sorry Brian, that is wrong...

If the fault is an N to E short or leakage, then even turned off, it
can still cause an RCD to trip.

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)

I would have thought that a modern MCB would be double-pole and break
live and neutral (although I accept that the CU the OP was referring to
might have had an old one. Were they ever single pole?). Or were you
referring to shared neutrals which might still have caused the RCD to
trip even if the live was disconnected via the MCB? Or is there another
explanation?


Current (no pun intended) MCBs for domestic CUs are single pole.

RCDs are double pole.

Even the latest CUs have a common neutral busbar ( or two if a split load
unit) - which wouldnt work if you had double pole MCBs.

The RCD(s) cut the supplies to the live and neutral busbars or wires in
some cases.