Fuses for an extractor fan

[ban-all-sheds]

Thanks for all the info, chaps - I was so consumed with sorting out a way to fuse the fan supplies that I didn't think about the run of cable coming from the shower supply to the fuses.

The shower isolator and the proposed SELV source for the fan are all outside zone 3, so presumably the fuses could also be there - i.e. contiguous with the shower isolator. This would mean a very short run (10-20cm) of cable to the fuses, and well protected, as it would be going from the inside of 1 box to another that was right next to it.

What do the regs say about insulation colours? I could use 3C+E as suggested by spark, but that would mean 1.5mm cable. If I use 2C+E, or singles, I could have 4mm cable - safer from an electrical viewpoint, but only black/red, (unless I can find some yellow sleeving).

The chances of anything going wrong with the short bit of cable twixt shower switch and fuse are pretty small, but I'd feel happier with 4mm protected by a 40A breaker rather than 1.5mm.


As for why I want to have the fan controlled by the shower switch, rather than the light, it's because I want to ensure the fan operates when the shower is on. My bathroom has plenty of natural light, so there is no guarantee that anyone using it will turn the light on, leading to the possibility (almost certainty in the case of the Inattentive 15-Year Old Dork, aka my son) that the shower will be run without the fan. I agree that it's more likely that a shower switch would be left on rather than lights (extremely likely in the case of IFYOD), but I'd rather have the fan running when not needed than vice-versa....

 

[spark]

You need to sleeve yellow cables with a red sleeve not the other way round; there is never an instance (other than 3 phase) where you would need to sleeve a cable with yellow. You can't use singles unless you're going to provide them with some mechanical protection. As for the fan supply, just tell him he must put the lights on for a shower and I would still strongly recommend that you use the lighting circuit. Would it really be the end of the world if he had a shower without the fan running? Don't give yourself a false sense of security by using 4mm; either it is installed as per my earlier post or it isn't. If it is then 1.5mm will be adequate if it's not then don't even think about doing it. By doing what you propose, you will be taking an unnecessary risk. Why not fit a PIR to control the fan? That way it will operate without anyone having to do anything!

 

[ban-all-sheds]

Mmm. Point(s) taken, and will be very seriously considered.

If I use the light circuit, the corollary of not having the fan come on when the shower does is having it come on completely unnecessarily when someone turns on the light. Or, with PIR, just walks into the room.

It all sounds so trivial when spelled out, but it's what I want to achieve, and I don't like to be beaten like this - I'm determined to find a way to get the functionality I want without compromising safety.

If I can find a fuseholder that will take 6mm cable then I'll be OK. The MEM grid system fuse blocks will accept 3 x 2.5mm, so should take a single 6mm - I've asked them.

Do I have to use something which accepts BS1362 fuses, or would any type of fuse, or rewirable holder be OK?

Or a small enclosure like this

with a couple of 3A MCBs? If I used a slightly larger one, I could fit a switch module in it which would act as a fan isolator. Location would either be in the loft, or inside a cupboard in the bathroom, (outside zone 3), if this is allowed.

One thing puzzles me:

"Don't give yourself a false sense of security by using 4mm; either it is installed as per my earlier post or it isn't. If it is then 1.5mm will be adequate if it's not then don't even think about doing it. "

Your earlier post didn't say "absolutely not, no way", it said "well if you really must then it is just about OK provided that...", and suggested a method using 1.5mm cable - all I was proposing was the same scheme but with 4mm. I don't think that's a false sense of security, as it considerably narrows the gap between the point at which the conductors get too hot and the MCB trips. It doesn't reduce the gap to zero, but the capacity of a 4mm cable is a lot closer to 6mm than is 1.5.

But a reduced risk is still a risk, and I should find a way to avoid it.

If I have to use the lighting circuit I will, but I've not given up yet