In article .com,
Peter M wrote:
I have a mains pond pump which is connected to a Garage Consumer
unit via outdoor douple pole switch. The consumer unix is RCD
protected. When I switch the pump on everything works as it
should and the pump will run all day without any problem.
However, when I switch the pump off using the outdoor switch, the
RCD on the consumer unit trips. This is not intermittent; it
happens every time.

Ha! Seen something similar twice now. On our old
Hotpoint built-in fridge, and my pal's well pump.

If it is the same, there is a Neutral wire somewhere
in the motor that is nearly touching Earth. As the
motor runs down, the vibration is enough to cause
a transient N-E short.

--
Tony Williams.