Wiring bathroom fan into light circuit

[citoyen]

I've bought a new bathroom extractor fan.

The existing fan is wired into the lighting ring main so it comes on when you switch on the bathroom light (or would do if it weren't broken).

Here's the existing wiring arrangement - grey flex is from the mains, white flex is integrated into the fan...


The replacement fan only has two poles - L and N. The old fan had a run-on timer but the new one doesn't.

This is the new fan (presumably the blank bit marked LS is where there should be a terminal for Switched Live?):


Does the new fan only need to be connected to Live (brown) and Neutral (blue/grey?) in the junction box in the first pic or should the Switched Live (black?) be connected too? Is this possible or do I need a different fan? And do I need to get a qualified Part P electrician to install/certify it?

[Feanor]

You pretty much have it.

The new fan doesn't have a run-on timer, so if you want that feature, I think you need a different fan.
Yes, I think you are correct that the PCB markings show where a Switched Live would go, but the other missing components might also be required, so I don't think it's as simple as making a connection to that PCB pad.  ( It looks like R6, D3, C2 form an RC timer, which probably drives the base of transistor TR1 to switch it off after the time has elapsed.  It looks like TR1 has been shorted out C-E so the fan will always run. )

If you are happy with the fan just going on/off with no run-on, you need to connect the fan L to the Switched L, and that might be either the Black or Brown. Needs testing.
The fan N should go to the incoming Grey- Sleeved-with-Blue (9-o'clock).   The Blue sleeve indicates the grey wire is not a phase, but has been re-purposed as Neutral.

[Kim]

If you are happy with the fan just going on/off with no run-on, you need to connect the fan L to the Switched L, and that might be either the Black or Brown. Needs testing.

I still can't get used to those colours.  Seems unnatural.

[Feanor]

If you are happy with the fan just going on/off with no run-on, you need to connect the fan L to the Switched L, and that might be either the Black or Brown. Needs testing.

I still can't get used to those colours.  Seems unnatural.

I agree. And once the wiring is covered in manky dirt, and in a dark location, they are much harder to tell apart that the old colours ( assuming normal colour vision, of course... ).

[citoyen]

The new fan doesn't have a run-on timer, so if you want that feature, I think you need a different fan.

I deliberately chose a new fan without a run-on timer, but once I got it out of the box I started to panic because there was nowhere for the third wire to go - I assumed all three would need to be connected in order to 'complete' the circuit. Having done a bit of googling, I've got as far as understanding that Switched Live is only required for a fan with a run-on timer but it's good to have it confirmed by the boffins of yacf.

I'll be honest, I really don't understand how lighting circuits work - but as long as I know enough to know which bit needs to be wired to where, I'm not too worried about that...

If you are happy with the fan just going on/off with no run-on, you need to connect the fan L to the Switched L, and that might be either the Black or Brown. Needs testing.

Ah, OK. I was assuming brown was Live and black was Switched Live...

Is testing just a case of trying them both to see which works, or will wiring up the wrong one cause it to explode or something? Is there another method of testing?

The fan N should go to the incoming Grey- Sleeved-with-Blue (9-o'clock).   The Blue sleeve indicates the grey wire is not a phase, but has been re-purposed as Neutral.

I'm not entirely sure what that means, tbh, but as long as it's functionally neutral, that's probably enough to be getting along with.

Thanks for the info & advice.

[citoyen]

I still can't get used to those colours.  Seems unnatural.

Black as live really confuses me.

[Feanor]

Wire the new fan with it's:

N to the Grey-with-Blue sleeve;
L to either the Brown or Black ( I'd try black first ).

It won't explode if you get it wrong, it'll just run continuously.
If it's right, it will go on and off with the light.

[citoyen]

Magic, thanks! 

[rogerzilla]

FWIW, you are allowed to do this job as a like-for-like replacement, which you have, but there are two reasons you couldn't wire in a new fan like this:

1. Part P building regs - it's in a bathroom so would need wiring (or signing off) by a competent person, which basically means an NICEIC electrician.

2. New bathroom fans need to be on a fused spur with an RCD (and I think it's a separate low current RCD at that).  You can't wire tham into the lighting circuit any more.

When dealing with jobs like this, a screwdriver with a test neon is really useful to see what's live and what's switched.  Do I have to say that any electrical tester MUST be tested with a known live to confirm it's working, immediately before you use it for any diagnosis...?

[Polar Bear]

Where's John the Sparky when we need him?

[john the sparky]

Hi, the greys are neutrals, and should be sleeved blue (the one at the junction box is)
The black and the brown are lives, one is switched live, one is permanent live.

If you have a pull cord or humistat you would use the permanent live, if not the switched live (so the fan switches on/off with the lights). Which is which is unknown (but I'd use brown for permanent and black for switched lives)


As said replacing the fan is ok, installing new is not. But replacing like for like does not need to be brought up to 17th edition AFAIK
Powering from the lighting is still allowed, but 17th edition regs say all circuits in a bathroom require 30mA protection (701.411.3.3) there should be isolation for the fan (I'd fit a fan isolator switch rather than a fused spur... as it will disconnect all phase wires (including the neutral)

HTH John

[john the sparky]

Where's John the Sparky when we need him?

I was writing a reply.... but thought I'd check the refs to be sure

[Von Broad]

If you are happy with the fan just going on/off with no run-on, you need to connect the fan L to the Switched L, and that might be either the Black or Brown. Needs testing.

I still can't get used to those colours.  Seems unnatural.

It's bloody ridiculous.
The old colour coded wiring was very explicit, and worked well. Why change it? More importantly it was visible when you're working in less than optimal lighting conditions [like wiring in a new circuit in a loft or something].
There are some laws made in the name of European convergence that are welcome, forward thinking and for the better, and others that just seem utterly nonsensical. This is one of them I'm afraid.

[john the sparky]

It's been a while now (2004) I think the 'new' colours are here to stay now 

[Kim]

Convergence is a good thing, but we could surely have picked better colours to converge on...

[mmmmartin]

Hi, the greys are neutrals, and should be sleeved blue (the one at the junction box is)
The black and the brown are lives, one is switched live, one is permanent live.
If you have a pull cord or humistat you would use the permanent live, if not the switched live (so the fan switches on/off with the lights). Which is which is unknown (but I'd use brown for permanent and black for switched lives)
As said replacing the fan is ok, installing new is not. But replacing like for like does not need to be brought up to 17th edition AFAIK
Powering from the lighting is still allowed, but 17th edition regs say all circuits in a bathroom require 30mA protection (701.411.3.3) there should be isolation for the fan (I'd fit a fan isolator switch rather than a fused spur... as it will disconnect all phase wires (including the neutral)

I'd just like to say how much I respect clever people with lots of training who understand what this means. I pay for people to do this, I'd never understand it.

[citoyen]

FWIW, you are allowed to do this job as a like-for-like replacement

As said replacing the fan is ok, installing new is not. But replacing like for like does not need to be brought up to 17th edition AFAIK
Powering from the lighting is still allowed, but 17th edition regs say all circuits in a bathroom require 30mA protection (701.411.3.3) there should be isolation for the fan (I'd fit a fan isolator switch rather than a fused spur... as it will disconnect all phase wires (including the neutral)

Thanks both. This is as I suspected but it's good to have it confirmed by people who know what they're talking about. Ideally, I would prefer to have the fan isolated but for now I am happy to just replace what I've got.

If you have a pull cord or humistat you would use the permanent live, if not the switched live (so the fan switches on/off with the lights). Which is which is unknown (but I'd use brown for permanent and black for switched lives)

No humistat or pull cord, so will go for the switched live, as Feanor suggested. Again, good to have a second opinion to confirm the earlier advice. I'll work on the assumption that black is SL and go from there.

When dealing with jobs like this, a screwdriver with a test neon is really useful to see what's live and what's switched.  Do I have to say that any electrical tester MUST be tested with a known live to confirm it's working, immediately before you use it for any diagnosis...?

Good advice, thanks. For the likes of me? Yes, this stuff does bear repeating!

I'd just like to say how much I respect clever people with lots of training who understand what this means. I pay for people to do this, I'd never understand it.

+1

As long as I know which colour wire to connect to which terminal, I'm more or less OK with not really understanding it, but I would feel a lot more comfortable about it if I could get my head around circuit diagrams.

I've also got some new light fittings to install in the kitchen - one circuit with two ceiling roses and two wall switches. I looked at it for a while then realised it is beyond me.

[Polar Bear]

Where's John the Sparky when we need him?

I was writing a reply.... but thought I'd check the refs to be sure

Top geezer!   

[citoyen]

Thanks again for the advice, all. Job, as they say, is a good'un. Light comes on, fan comes on. Light goes off, fan goes off.

And it's noticeably quieter than the one it replaced, even though it's considerably more powerful.

[rogerzilla]

I'd love a fan in my upstairs bathroom but getting a neat 100mm hole through the wall is a bigger problem than the wiring...there is a porch underneath which probably means the use of scaffolding for whoever makes the hole.  It might be possible to drill it out from the inside with a diamond core bit without spalling the external bricks -  if whoever did it was really careful not to exert much pressure for the last half inch.  I reckon it would be a £1000 job by the time the builder and sparky had done their jobs, so I'll keep opening the window 

[Kim]

Could you fit a fan in the window?

[rogerzilla]

Could you fit a fan in the window?

That's so 1970s, dahling.

[Kim]

Then get a cowboy aerial rigger to pass a coax through the fan...

[Basil]

Then get a cowboy aerial rigger to pass a coax through the fan...

Splort!

[rogerzilla]

It's quite common to see them run through wall vents, but a *working* fan?

It's not that difficult or expensive to find a long enough drill to go clean through a cavity wall!