Bathroom fan wiring

[Funzbo]

I've got an overrun timer fan in my bathroom controlled from the lighting pull-switch.

The builders who installed it (before my time here) put the (non switched) Fused Unit in the (stud) wall above the bath (ca. 50 cm from the shower head).

At present if the fuse is removed, the lights work, but the fan doesn't.

I would like to move the fused unit to outside the bathroom, as recommended in the regs (required?) and replace it with a switched fuse unit. I want to be able to override the fan using the switch, but still be able to use the lights.

All the diagrams I have seen for overrun fan wiring show the FSU in the supply BEFORE the light and fan.

It doesn't seem logical to have a double-pole FSU going to the fan if there's a switched live that may still be live even though the FSU is open circuit.

How should I wire it to get what I want.

Part-P question: since I'm moving wiring OUT of the bathroom I'm presumably in the clear?

Thanks

Funzbo

[Lobster]

I asked a similar q here a while ago: here's a good response I got:

Search the newsgroup archives at - this topic comes up all the time!

David

[Lurch]

On Sun, 30 Jan 2005 12:46:18 +0000, Funzbo strung together this:

I'd be inclined to fit a triple pole fan isolator instead of a switched spur. This may involve some wiring changes, but the chances are there is a 3 core and earth that runs through the spur so just re-route those through the TP switch.

We're not particularly Part P friendly here, you'll be fine.

[John Rumm]

Ohhh nice...

You can have the FCU in the bathroom if you want, preferably in zone 3 though. In addition to the FCU you will need a THREE pole isolator (hence a FSU is not going to help you in this case)

That is fair enough... however the isolator should be between the fan and all sources of power including the switched live.

Its not... there are three requirements for switching and fusing. Functional switching, which is provided by the light switch and the timer in the fan. Isolation, which will need to be provided by a purpose made three pole isolator (these are available in conventional single pattress switch form and also as a ceiling mount pull switch), and fusing, which will need to be provided in the main power connection to the lighting circuit.

Personally I would place the three pole isolator in the circuit first, with a plain FCU (i.e. non switched) FCU after it close to the fan position. That way you can isolate the FCU as well as the fan should you need to change the fuse.

Probably not, since you will need to alter wiring in the bathroom to effect the change... your call ;-)

[Funzbo]

Thanks All

Ahh, 3 pole isolator. That was what was what I was groping for.

So, is this what you mean?

Supply -> FCU -> Lights + Pullswitch -> 3 pole isolator (ie override for fan) -> Fan

In that earlier post from Stefek (

), he suggests that, in this scheme, the FCU is redundant since the lighting circuit MCB provides that protection.

Or is an FCU a requirement. It would be nice to lose the FCU from above the shower and just have the isolator switch on the wall outside the bathroom.

Thanks

Funzbo

[Mike Clarke]

Nasty and dangerous.

Even worse. An overrun fan needs 2 live feeds, one via the light switch to trigger it and another permanent live to power the overrun time. The FCU will only control one of these, if it's in the switched live the fan will continue to run for the overrun time if you pull the fuse when the fan is running with the light on. If it's in the permanent live I would expect the fan to stop immediately you pull the fuse, or continue to run until you turn the light off when it would stop immediately without any overrun. Either way parts of the fan will still be live when the fuse is removed. In view of the proximity of the bath and shower, take precautions that the FCU is dry and you are not exposed to any water if you try this.

Assuming the fan was correctly wired to take it's power from just the lighting circuit you shouldn't need a FCU, just a 3 pole isolator switch as others have suggested. This will ensure that the fan can be completely isolated for maintenance.

A conventional circuit with a 3 pole isolator will enable you to disable the fan and just use the lights, you won't be able run the fan without the lights (apart from the overrun time after the lights go out) without adding a bit more complication.

You should only need a FCU if you're powering the fan (and therefore the light) from a higher rated circuit like the ring circuit.

Neither logical nor safe.

[Stefek Zaba]

And here he is, saying it again ;-) The F in the FCU isn't doing anything useful, and as you said yourself we need to isolate *both* lives and the N for fan maintenance, so it's one pole short of useful (unlike Ealing, which has at least enough of us... but I digress, obscurely.)

But you wrote in your earlier posting, 'I want to be able to override the fan using the switch, but still be able to use the lights'. Not sure what you mean by 'override' here. If you want only to be able to turn the fan *off* sometimes, an on-off pullswitch in its permanent-live would do the business. If you want only to be able to turn it *on* sometimes independently of the lights, a change-over pullswitch so it takes its 'trigger' input either from the light's switched-live or from its perm-live would do the trick - but it seems unlikely to be all that useful, since you can get the fan to come on and run for the duration of its run-on timer by flicking the lights on and off. If you want full three-way control of the fan - 'perm off, comes-on-with-light, perm-on', you're either into using a single centre-off double-pole toggle-switch (*please* use one with a plastic paddle, and mount it outside the bathroom), so that the 'trigger' feeds from either perm-live ("perm-on"), nothing (the centre-off position), or the light's switched-live ("comes-on-with-light"), or accepting that the isolator's a perfectly good thing to operate for the 'perm-off' function, leaving the pullswitch changeover (or a normal 2-way plateswitch mounted outside the bathroom, or at least in Zone3 or non-zone) to select 'perm on' or 'light-controlled'.

HTH, rather than confuses - Stefek

[Funzbo]

Thanks very much - that clarifies enormously.

I'm trying to get it so that the fan can be off during the summer (when windows are open) and occasionally off in winter (eg when there are lots of people here using the bathroom for purposes that don't create much steam).

I hadn't thought of simply sticking a single pole switch in the perm live to fan.

I suppose it could be a pullswitch with a very short pull, so as not to be confused with the light.

Then again, I suppose I really should remove the present FCU from above the bath and replace it with an isolator which isolates N + *both* Lives. Logical solution would be a 3 pole switch outside the bathroom. Perhaps marked 'FAN'!

Funzbo

[Stefek Zaba]

You'll find the usual 'plateswitch'-style 3-pole isolator already says 'FAN' - or has a tasteful Euro-drawing of a 4-blade fan on it. Like this:

Screwfix catnum 51728, at

[Lurch]

On Sun, 30 Jan 2005 16:20:11 +0000, Funzbo strung together this:

Something like;

would be what you're after.

The FCU can be removed and the isolator fitted between the light and fan so it provides a functional switch for 'off' or 'on with the lights'.

[Dave]

Lurch wrote: ... snipped

... snipped

Why does it need to be triple pole? Take the feed to the fan through a standard 2 pole isolator and then wire from the fan to the switch (and light, if needed). What am I missing?

[Stefek Zaba]

A fan with a run-on timer has two lives: one is the 'trigger', usually connected to the bathroom light's switched-live, which tells it to start running and keep running while that trigger's live. The other is the permanent live, which lets the fan *keep* running for a preset/adjustable time - a few minutes, usually - to carry on clearing the air of pongs and steam after the bathroom light's been turned off again. The third pole we want to isolate is the neutral, as usual in double-pole switching. Hence a total of 3 poles - trigger, perm-L, and N

- to be Isolated.

If you run only the perm-L and N through the 'standard 2-pole isolator' you mention, then the switched-live wire from the '(light, if needed)' isn't isolated, and while you're working on the fan, A.N.Other Household Member comes and flicks the light on, livening up proceedings in an at-once unexpected yet quite predictable fashion...

[Lurch]

On Sun, 30 Jan 2005 23:14:22 +0000, Dave strung together this:

The fact that there are three cables, besides the earth, to a fan with over-run timer. Those being;

Permanent live. Switched live. Neutral.

Equals three poles that require isolating.

[Andrew Gabriel]

Might be a bit of a bugger servicing the fan if the isolation switch also disables the bathroom light.

[Dave]

... snipped

But if you feed the switched L from the permanent L on the fan the whole lot is isolated by the 2 pole switch to the permanent L (and N) ... Fans are a pain in the middle of the night so I've got 2 pull-cords, one for the main light and one for the fan + the light over the shower.

[Lurch]

On Mon, 31 Jan 2005 08:22:06 +0000, Dave strung together this:

You see, you're moving the goal posts now. A standard fan with no timer requires 2 pole isolatione, a fan with over-run timer requires 3 poles.

It doesn't matter whether you like them or not. Stop being a troll.

[Stefek Zaba]

Sorry, I didn't read your earlier post closely enough to realise you meant running both light and fan downstream of a DP isolator. That'd be OK from the safety-isolation point of view, but is borderline as far as convenience goes (both in real life and in the Regs requirement), as it means you can't use the bathroom light while servicing the fan: a particular consideration for 'boxed-in', no-window, loos.

I think we've done this topic to death now...

Stefek