Bathroom Extractor - switching issue

voycie · 16 Jan 2007

Hi, I have installed a SELV 12V timed bathroom extractor fan, using a DP 3A switched fused spur off the bathroom lighting circuit, mounted in the loft alongside the transformer.

This is in accordance with the manufacturer's instrcutions and it works fine.

My question concerns the switching. I want the fan to switch on when the light comes on, so I've wired it into the pullcord switch in the bathroom.

However, as my lighting circuit also uses this pullcord, the switching cable from the DP 3A spur to the transformer would end up being 'live' from the lighting circuit as well as the DP 3A spur.

So, is this a problem, since, the DP 3A Switch on the fan spur you would assume would isolate the supply, however, this isnt strictly true, since the switched live to the pullcord would also end up taking a live from the lighting circuit depending on whether its on or off.

I could avoid this issue by either having a seperate pullcord for the fan, or if there's a DP pullcord whereby the lighting and fan lives are not in contact.

Any advice? Just want to make sure its all wired up nice and safe.

sparkyspike · 16 Jan 2007

How have you wired up the 3P switch? L2 is designed to take a supply from the switch, L1 is the permanent live. So if you run a cable from the lighswitch to L2 of the 3P switch, as well as the live and neutral, it will be fully isolated when you turn off the 3P switch.

Taylortwocities · 16 Jan 2007

voycie said:
using a DP 3A switched fused spur off the bathroom lighting circuit, .

Why have you done that - not necessary at all. Are u sure the instructions said to do this?

TTC

voycie · 17 Jan 2007

Thanks for the help, As I say, the instructions from Manrose specifies the SELV timed fan should be connected via "A double pole isolating switch, having a contact separation of at least 3mm in all poles, must be used with a 3 amp fuse fitted".

SparkySpike, do you mean I should use a three pole isolating switch instead, allowing me to isolate the live, the switched live and the neutral?

The wiring diagram supplied with the fan shows the power coming into the DP fused switch, then the neutral and live running straight to the transformer and the switched live running from the DP fused switch on the live side to the bathroom pullcord, then on to the transformer.

Hope this clarifies.

bernardgreen · 17 Jan 2007

Taylortwocities said:
voycie said:

using a DP 3A switched fused spur off the bathroom lighting circuit, .

Click to expand...

Why have you done that - not necessary at all. Are u sure the instructions said to do this?

TTC

Looking at the Manrose site they do mention a DP 3A switched fused for some of their products.

If the fan is built to a 3 amp fault carrying capacity then using it on a 6 amp fused lighting circuit would need that 3 amp fuse.

What model is it ?

voycie · 17 Jan 2007

here's the instructions:
http://www.screwfix.com/sfd/i/cat/pdfs/34/p3133634.pdf

the model is the T12T Timer model, wiring diagram is T2. Back to my original point, if I connect the switched live to the bathroom pullcord, does there not become an additional 6A live through the lighting circuit when the lights are on?

philrosenberg · 17 Jan 2007

I have done this with my timer fan too.
Here's how to do it, according to the instructions that came with mine.
replace your pullcord with a double pole pullcord, use one pole for the light.

from your double pole switch live outgoing terminal run the phase conductor of twin and earth to the second pole of the pullcord, use the neutral conductor (labelled with brown tape/sleaving) to create a swiched live.
Back at the double pole switch you now have live, switched live and neutral to run to your fan in three and earth - you can connect the switched lives with choc block.
Don't forget you probably need a fcu before the douple pole switch to drop it down to 3A.

Taylortwocities said:
Why have you done that - not necessary at all. Are u sure the instructions said to do this?

The advantage of this is that you only need one fcu, so there is only one fuse to blow, whats the term you sparks use - discrimination?

bernardgreen · 17 Jan 2007

voycie said:
here's the instructions:
http://www.screwfix.com/sfd/i/cat/pdfs/34/p3133634.pdf

the model is the T12T Timer model, wiring diagram is T2. Back to my original point, if I connect the switched live to the bathroom pullcord, does there not become an additional 6A live through the lighting circuit when the lights are on?

Yes if you connect the permanent live to the lighting live via a 3 amp fuse and the switched live to the bath room light switch then when the switch is on the transformer is protected by the 6 amp lighting circuit fuse in the consumer unit. When the switch is off the transformer is protected by the 3 amp fuse in the FCU.

It depends on how critical that 3 amp fuse rating is for the safety of the installation if the transformer goes faulty. As there is no earth on the tranformer to possibly trip the RCD in a failure the only way the faulty transformer will disconnect is on current overload at 3 amps or 6 amps if the bath roomlight is on.

If 3 amp is critical maximum then I would fit a double pole pull switch and wire as per the instructions. If the transformer fault safely on a 6 amp fuse then I would wire as for a standard bathroom fan with a three pole isolator.

Bernard

Sharnbrook

voycie · 17 Jan 2007

Thanks All for the input - seems the safest way to do it, in compliance with the manufactoruers guidelines, is to have a DP pullcord, to ensure the circuit is protected solely by the 3A fuse.

I've only found single pole pullcords, anyone know where to get a DP item?

RF Lighting · 17 Jan 2007

http://www.tlc-direct.co.uk/Products/MK3151.html

voycie · 19 Jan 2007

Excellent - thanks v. much!

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Bathroom Extractor - switching issue

sparkyspike

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