On 01/06/2019 23:16, ss wrote:
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.

I would be tempted to make a guess knowing there's a 50% likelihood of
it being right!