Wiring extractor fan from the shower pull-switch - yes or no?

Why do you use the shower isolator in normal use?

You can do it with a fused spur from the shower isolator switch it you want. You should label it at the consumer unit to avoid surprises for later maintainers. Also since you would be removing all power from the fan when you turn the switch off, you would lose any ability to have a an automatically timed run on after the shower.

If you wanted to power and trigger the fan from the lighting circuit, but also trigger it from the shower circuit, then you get into the realms of needing contactors/relays etc to keep the circuits electrically isolated from each other.

or even wire the fan into the shower itself, so picking up the permanent live before the controls and the switched live after the controls?

Thus removing the risk of confusion if wired into the the shower isolation switch?

S.

Could he wire it from the supply side of the shower switch & avoid that?

The permanent live for overrun I mean...

@John Rumm, not sure I understand what you're saying. What I intended was to run some three core and earth from the shower pull switch so that I'd have a permanent live and neutral and a switched live all going to the fan via a fused fan isolating switch. So the fan will still run on when the shower pull switch is turned off after using the shower. None of that would go near the consumer unit of course. Does that sound ok?

Most people would opt for a fan with a humidistat.

I wouldnt they fail all the time...

It's how mine is wired here. Except to the light, and not the shower

What "controls" are you talking about?

Exactly what the op doesn't want then...

You could although you would need to fuse that as well, since many fan manufacturers spec a 3A supply fuse.

Yup, that would be fine - your OP made it sound like you just wanted to pickup just from the switched live from the shower isolator.

The only slight practical difficulty you may have is getting a good termination on the smaller fan cable at the same time as the (sometimes significantly) larger shower supply cable, bearing in mind shower connections need to be 'kin tight!

You will also need to check that whatever cable you use to connect the fan is offered adequate fault current protection by the shower MCB. This is not likely to be a problem with say a B32 MCB and 1.5mm^2 3&E. But you start getting more marginal on larger MCBs and shorter cable lengths (as the prospective fault current rises). Having the fuse close to the main shower isolator helps reduce the risks.

Presumably it'll need fusing wherever it's fed from then?

What's the neatest way to do that?

What about if I feed the permanent fan supply from a light fitting as normal with 1.5mm twin and earth cable and then run a 1mm twin and earth cable from the shower pull switch just for the switched live feed? That way the fan goes on with the shower pull switch but is not fed permanently by the shower circuit. Not sure where to fuse though if it is ok?

Your fan should not be fed from more than one circuit.

If you want it to come on when the shower is on, you would need a relay, preferably a changeover one (SPDT) that won't put the bathroom light on at the same time as the fan.

Have you considered of a PIR sensor to detect someone in the bathroom? Might be additionally useful for when someone does a smelly No 2.

Really *really* bad idea - that would be powering one appliance from two separate circuits (which might not even share a RCD!)

(not to mention feeding a lighting circuit wired in 1 or 1.5mm^2 T&E from combined nominal current rating on a pair of MCBs that could be over 50A)

If you want to do something like that, then you would be better with an arrangement like:

Yup, played that game with a fan setup combining humidistat along with delayed light activated operation with timed run on: