"Martin Angove" wrote in message
...

I thought of another reason for not doing it - anyone who doesn't
know the system won't know how to isolate it, other than perhaps
through the 3-pole isolator which is supposed to be fitted these
days.

Another reason will become clear if you consider that the ring circuit might
be fed via an RCD and the lighting circuit not...

More fundamentally though it violates regulation 314-01-04 which says that
the wiring of final circuits must be kept electrically separate.

I can't find the beginning of this thread, but whomever it was that said:

The fan should be a spur from the upstairs ring main, not the
lighting circuit

wants locking up.

--
Andy