Does a bathroom fan fed from a 6A lighting circuit really need a 3A fuse

[Mike Clarke]

The bearings on my old extractor fan are on the way out and I'm replacing it with a Vent-Axia VASF100T model. The manufacturer's instructions state it should be protected with a 3A fuse. I can see the need to use a FCU if connecting to a 32A ring circuit but I'm wondering if this is really necessary on a 6A circuit. I doubt if the current drawn if the fan stalls would be enough to blow even a 3A fuse and any fault resulting in a 3A overload would probably burn out something in the fan long before the fuse would blow.

I would have been happy to replace the 3 pole isolator with a fused one if it had been fed with just L+N with a switch drop back to the switch to get the switched line supply. In this case the isolator is fed with 3 core and earth from the light switch so a fused isolator could only provide fused protection for permanent L or switched L but not both.

One way to provide 3A fuse protection would be to fit a FCU upstream of the light switch outside the bathroom but I'd prefer to avoid having to cut in to the wall to do this unless really necessary.

I've tried Googling for opinions on this but the only useful comment I've found so far is from

which suggests that 3A fuses on 6A circuits are not needed. Normally I'd be wary of advice posted on diynot.com but this post was made by 'flameport' i.e. YouTube's John Ward so I'd be inclined to regard it as reliable.

So what's the general opinion, is the 3A fuse really needed on a 6A circuit or are the manufacturers just trying to cover their backs.

[Robin]

Not an answer but you could see what the total load is on the lighting circuit and then, if an option, whether a 3A MCB is available for your consumer unit.

[Mark Carver]

Is the 6 Amp protection for your lighting circuit an MCB ?  If so, I suspect it would break the circuit much faster than a 3 Amp fuse anyway ?

[Mike Clarke]

Yes, it's a 6A type B MCB.

[John Rumm]

That would much depend on the circumstance. At a 10A overload the 6A MCB could take over 8 mins to trip. A 3A BS1362 fuse would at worst be around 5 secs (or at best as quick as a couple of 100ths of a sec).

Having said that, the fuse protection is normally there for fault current protection, and at >100A both with be under 0.1 sec which will protect the wiring just fine.

[me]

The technicalities of whether it's needed or not aside isn't the real question whether the person certifying the circuit will say it needs one or not? I'm just installing my shower room fan as well so would like to know.

[Brian Gaff]

Well, this is 6a for all lights?? I'd have thought that you would need to know what the fan uses for a motor. One assumes its some kind of induction motor, so very unlikely to actually short out, and does it have a thermal fuse inside it? I remember many of them used to die due to that little devil failing.

The point is, if you keep the existing fuse, then its only likely to blow if the full load is on the circuit, and not if its the only thing, or one of very few things running in the same over current failure mode, so they are in fact saying fuse it separately to be safe. Brian

[Mark Carver]

Well, to avoid any future arguments and start sleeping at night again, fit an FCU with 3A fuse in the loft space to feed it. If the fuse blows you're going to need to go up there anyway to deal with the smouldering (or whatever fault has caused it) fan ?

[John Rumm]

Well you have two possible approaches, either you say, "this makes no logical sense" and ignore it, or you adopt the position of "if the fan makers instructions say it must be protected with a fuse of no more than

3A rating, then that is what you must do".

Remember also that an extractor fan is not a general purpose appliance designed to be powered via a flex with a plug on the end. The understanding that the fuse is present only to protect the flex does not necessary apply here. So it would be acceptable for the manufacturer to require a fuse for overload protection inside the device.

[ARW]

Something the NICEIC check for when checking out work - and if the manufacturer specifies a 3A fuse then we have to fit one.

[SH]

serious question.....

if the fan is one that has a permanent live and a switched live both from a lighting circuit, even if you put a 3A fuse in the permanent live, you still have a potentially live switched live on the fan.....

unless you put the 3A upstream of the light fitting that is providing the permanent live and switched live to the extrcator fan... That also brings in teh scenario that if there are further light fittings downstream, if that 3 A fuse then blows, the downstream lights would no longer be live, and could cause confusion to the house owner where some of the lights worked and some don't on the same lighting circuit being fed by the 6A MCB/RCBO?

[Robin]

You just wire the bathroom light(s) as a branch from the lighting circuit (or at the end of the circuit) rather than looping them.

[Andrew]

I have a manual fan. I open the window after a bath. Complies with all regulations, and now that we have had 40 years of excessive use of pesticides, I don't even get any pesky merths inside.

[ARW]

Serious answer.

If the fan is required to be turned on with the lights and the manufacturer of the fan says it needs a 3A fuse - eg a bathroom with no windows then I would normally put the 3A fuse inline with both the SL to the bathroom lights and the SL and PL to the fan and leave the downstream lights on the 6A MCB.

This would only mean that only the bathroom lights and fan are supplied by the 3A fuse.

[SH]

so effectively you'd make the bathroom light become a branch of the usual lighting circuit (which is typically arranged as a loop into rose

- loop out of rose) then?

That would mean rejigging the bathoom rose then.....?

[John Rumm]

Just loop through the FCU rather than the ceiling rose. Feed the rose form the output of the FCU.

[John Rumm]

In some cases a fan with a humidistat is easier to retrofit - it only needs a permanent live.

[John Rumm]

I have added a section to the wiki page on wiring extractor fans to include some detail on dealing with fans that need "fused down" supplies:

[ARW]

Trust me - you are not going to find anyone under the age of 30 that knows what loop in at the rose wiring is (apart from one of our lads that is brilliant).

[Mike Clarke]

That's the problem. I've yet to find any technical reason why the fan should require a 3A fused supply. The quoted power consumption of the fan is only 5 watts, at 250V that's a current of only 20mA or 40mA if we assume a poor power factor. If a serious internal fault in the fan were to develop it could draw up to 750 watts before reaching the rated capacity of the 3A fuse. With that amount of overload some critical components or tracks on the fan's circuit board would burn out and go open circuit and 'protect' the fuse.

So it would appear that the manufacturers don't consider the 3A fuse to be protecting the fan. They could only protect the fan by specifying a much lower fuse rating than 3A but there's no way this could be dune using a standard FCU. If they were really concerned about protecting against fires caused by internal faults in the fan they should incorporate a suitable low rated fusible link on the circuit board.

But although I'm confident the fuse is superfluous I eventually decided to add a FCU upstream of the light switch, largely as a box ticking exercise to avoid problems with any future EICRs.

Fortunately we don't have a loop in lighting circuit and every switch has an incoming t&e supply with line going through the switch and neutrals for the supply and the light strapped together in a terminal block. So the fan circuit picks up L, N & switched L from within the back box and it was only necessary to cut out an extra hole in the plasterboard to add a FCU into the incoming supply for the switch.