On Sat, 01 Jun 2019 23:49:31 +0100, Jim K.. wrote:

ss Wrote in message:
I am changing an extraction fan with timer.
The old fan has 2 red and a black wire but no indication as to what one
is live switch, how do I determine what one of the reds is live switch,
with the power switch off at mains.


Switch power on at mains, test each wire.

Insufficient advice. Doesn't allow for eliminating false positives
("ghost voltages").



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Leave first - THEN negotiate!