On Sun, 13 May 2018 06:14:26 -0700 (PDT)
me wrote:

On Sunday, 13 May 2018 14:04:40 UTC+1, wrote:

You can use a current sensing relay on the mains supply to the
shower.

http://wiki.diyfaq.org.uk/index.php?...tivated_switch

Owain

I was an electronics engineer but my days of being bothered to cobble
together electronic circuits are well gone. The principal is what I
was after though. I just wish the shower manufacturer would build
something into the shower itself. All it would need is an extra
output on the shower power switch and a bit of 1mm T&E could be run
from it to the fan...job done.


You could join into the output side of the switch with some suitably
sized cable, take that to a fused spur and then power the fan off that.

I've always thought that an auxiliary contact on the shower switch would
be the easiest solution.