3a fuse for extractor fan.

[Joe Shmoe]

I'm replacing an extractor fan. The new one (manrose mf100t) instructions say it should be protected by a 3a fuse. The existing one was installed (not by me) with an isolator switch, but no 3a fuse.

I could install, but as there is both a Live and Switched Live going in, which do I protect with a fuse? Or do I do both?

 

[porker]

The problem is the manufacturer specifying 3A because I would normally rely on the 6A lighting circuit protection and isolate with a 3 pole isolator as normal.
Taking a feed from the lighting through a 3A fused spur and put the switch on the other side so the permanent and switched live are both protected on the one fuse would be the way I would go. I wouldn't fuse both personally.

Note - I'm not an electrician (an electrical engineer interested in domestic installations)

 

[Argus]

It is usual to fit a Switched spur-box with a 3 amp fuse: They are available with a flex exit if you need one.
The fuse always goes in the feed wire - never the neutral.
https://www.screwfix.com/c/electrical-lighting/fused-spurs/cat13020004?switchedunswitched=switched(This sort of thing.... I'm NOT endorsing this firm)

Now the bad news.
The building regulations demand that it's installed by a 'competent' person. (It's doubtful if this would be covered by the minor -works derogation in the reg)

 

[Felix]

I'm replacing an extractor fan. The new one (manrose mf100t) instructions say it should be protected by a 3a fuse. The existing one was installed (not by me) with an isolator switch, but no 3a fuse.

I could install, but as there is both a Live and Switched Live going in, which do I protect with a fuse? Or do I do both?

I take it there's a light-fitting in the room where the fan is located. In a domestic installation you would normally take a 3c+eth from the light fitting to a 3-pole isolator mounted outside the room (if it's a bathroom) and from that isolator run a 3c+eth to the fan unit. The fusing of the lighing circuit, as porker says, is via the breaker in your consumer unit. You should not be trying to put a fuse on either the live or switched live of the actual lighting circuit.

Is that 3A fuse actually on the fan unit itself - a local fuse?

 

[Felix]

I'm replacing an extractor fan. The new one (manrose mf100t) instructions say it should be protected by a 3a fuse. The existing one was installed (not by me) with an isolator switch, but no 3a fuse.

I could install, but as there is both a Live and Switched Live going in, which do I protect with a fuse? Or do I do both?

I just found this -
Click Mode 10A 3 Pole Fan Isolator Switch with 3A Fuse CMA3020
The 10A part refers to the max load of the isolator switch - so you will be well under that
If there is no local fuse on the fan unit you could use the above to fuse the circuit down to 3A.

I should add that all of this info is for reference only and is not to be construed as instructions on how to carry out any work

 

[flying haggis]

I just found this -
Click Mode 10A 3 Pole Fan Isolator Switch with 3A Fuse CMA3020
The 10A part refers to the max load of the isolator switch - so you will be well under that
If there is no local fuse on the fan unit you could use the above to fuse the circuit down to 3A.

I should add that all of this info is for reference only and is not to be construed as instructions on how to carry out any work

but which do you fuse perm or switched live

 

[Joe Shmoe]

Hi folks.

So from what I gather, the attached diagram is pretty much how the existing is wired (albeit 3 led downlights in place of that sibgle pendant.

The existing fan is a tiny little unit that is inadequate (think size of PC fan).

This would be a simple swap over were it not for the fact the new fan specified 3a protection.

I'm not sure why the existing doesn't have a fused spur. I'm guessing it didn't need it. It was installed by a qualified electrician about 15 years ago during a compete rewire.

 

[Joe Shmoe]

And the wiring instructions are as follows....

1. The extractor fan must be connected to a single phase mains network, 220 - 240VAC.

2. The installation must have an omnipolar switch with a contact separation of at least 3mm. The circuit must
be protected by either a 3 Amp fuse.

3. Earthing is not required as this extractor is a class II insulated appliance.

4. The extractor must be connected permanently to a fixed installation cable. (Flex of two conductor wires of
minimum section 1mm2
and maximum 1.5mm2
)

 

[artie]

The circuit must
be protected by either a 3 Amp fuse.

OR?

 

[Joe Shmoe]

OR?

Yes. The instructions leave that bit out! Handy.

 

[HappyHacker]

The requirement from some extractor fan manufacturers to have a 3A fuse in the circuit for timed over run fans is a pain in the BS. The fuse needs to be in both the live and switched live supply. The easiest way of doing this is to put the light on the fuse as well. This is why you often see both a fused spur and a fan isolator outside the bathroom. One switch manufacturer (Click, I think) has made a combined three pole isolator and fuse in to fit a single back box.

I check the suppliers instructions before buying/fitting new extractor fans as there are some who are happy with a 6A MCB which is normal on a UK domestic lighting circuit and then just a three pole isolator is required which is much easier to wire and fit. I can never remember which these are as I am semi retired and try not to fit any extractors. It is debatable whether a 3A fuse will always blow before a 6A MCB but in the event of a problem the electrician will get the blame if there is a problem and the manufacturers instructions have not been followed.

A have heard of a local scout hut often used for sleeping accommodation that suffered severe damage due to an overheating extractor fan but I never managed to find out if it needed or had a 3A fuse in circuit.

 

[Felix]

but which do you fuse perm or switched live

In that configuration (single fuse) it would be on the phase conductor (permanent live). The switch line already has a form of isolator - the switch for the lighting in that room (This will be protected by the circuit fuse at the consumer unit). If you are working on the fan the 3-pole isolator should be off and if you're working on the actual lighing circuit itself you will isolate the circuit at the consumer unit. Under 'NO' circumstances should you be working on a live circuit - 'dems da rools'

 

[Joe Shmoe]

Thanks for all the replies.

I think ill add a 3amp fuse spurring from the main lighting circuit and then both the LED downlights and the fan will be after this. I don't believe having 3 downlights on a 3a fuse will cause an issue.

Seems my question has popped up many times before on Electrician forums and they don't appear to be unanimous in what to do. Some say don't bother with a fuse at all and some say fans that require protecting by a 3a fuse are poor quality.

 

[mindthatwhatouch]

This is two separate issues. That of Isolation and Protection.

1. The regs have a requirement for the fan to have an isolator for means of isolation for maintenance. (We used to be happy to just turn off all the lighting circuit but things have changed)

2. The fan manufacturer is asking for a 3 amp fuse for protection, as they have no idea what size MCB you have on the consumer unit.

The combined Click mode unit posted above will do the job. Or a fused spur for the lights and then the isolator to the fan.
The former being a better solution, then the lights are not on a separate fuse.

Or do the same as the vast majority of installations and just install the isolator!!! Never come across anybody who seems to consider it an issue.

 

[Jetset]

Thanks for all the replies.

I think ill add a 3amp fuse spurring from the main lighting circuit and then both the LED downlights and the fan will be after this. I don't believe having 3 downlights on a 3a fuse will cause an issue.

Seems my question has popped up many times before on Electrician forums and they don't appear to be unanimous in what to do. Some say don't bother with a fuse at all and some say fans that require protecting by a 3a fuse are poor quality.

Hey Joe. Well done on the research. I did the same thing in my bathroom after doing the same research.

You’re right in that the only way to get a fused spur on that fan is to fuse the lights down in that room too.

That’s only achieved by spurring off the lighting circuit to your fused spur then feeding to the light(s) and also to isolater.

Put the fused spur & isolater side-by-side for neatness and so that it’s obvious that’s what the fused spur is for.

Don’t forget to change the 13A fuse in spur to a 3A one!

(I’m not a qualified spark but I’ve done this in two properties with my electrician coming along later to install a new consumer unit.)

 

[Felix]

Thanks for all the replies.

I think ill add a 3amp fuse spurring from the main lighting circuit and then both the LED downlights and the fan will be after this. I don't believe having 3 downlights on a 3a fuse will cause an issue.

Seems my question has popped up many times before on Electrician forums and they don't appear to be unanimous in what to do. Some say don't bother with a fuse at all and some say fans that require protecting by a 3a fuse are poor quality.

I have never ever seen that done before. What you are proposing is not wrong, but unnecessary work. I'm guessing here, but I think the reason the switch line is non-fused is that it only triggers some sort of relay in the fan and allows the permanent live to run the fan - and when the light is turned off the permanent live stays 'connected' for the duration set by the timer (which is why you have a permanent and switched live) and the switch line no longer carries any voltage. As a result by using an isolator with a 3A fuse is fine as it's providing protection whilst your fan is running. If you are set on putting a fused spur in-line with your lighting circuit I would get an unswitched one - and remember - the fused spur does not supply the isolator in any way shape or form......

 

[Felix]

Put the fused spur & isolater side-by-side for neatness and so that it’s obvious that’s what the fused spur is for.

I think this is a bit confusing - the spur does not feed the isolator - the spur is fed by the lighting circuit and the load side goes to the first light in the room (thereby maintaining the lighting circuit). The isolator is fed via a 3-core from the light fitting and then goes to the fan.....

 

[Felix]

I have never ever seen that done before. What you are proposing is not wrong, but unnecessary work. I'm guessing here, but I think the reason the switch line is non-fused is that it only triggers some sort of relay in the fan and allows the permanent live to run the fan - and when the light is turned off the permanent live stays 'connected' for the duration set by the timer (which is why you have a permanent and switched live) and the switch line no longer carries any voltage. As a result by using an isolator with a 3A fuse is fine as it's providing protection whilst your fan is running. If you are set on putting a fused spur in-line with your lighting circuit I would get an unswitched one - and remember - the fused spur does not supply the isolator in any way shape or form......

To maintain the lighting circuit as supplied from the consumer unit you will have to take both cables (loop in and loop out) from the light fitting to the supply side of the spur and the load side of the spur back to the light fitting - this will make that room completely controlled by the spur. You do not have to do anything with the switch line.