Fusing down of extractor fans

[uksparks]

Hi,

I have a question I would like to put to you to see who does what really.

The majority if not all manufacturers state something about the fan must be fused down to 3A...

Ok, scenario...

You have a bathroom, 6A light circuit, the light can either be switched at pull cord, light switch outside or the light fitting.

The customer says they want an extractor fan with Timer.

Option One
Do you just fit a fan in the ceiling / wall, and connect it off the light circuit straight to it and think nothing of it? This would be a permanent live supply and switched live.

Option Two
Do you fit a fan, and put a 3A fuse on the permanent live only? Leaving the switched live on 6A, also leaving the fan with the possibility of a blown fuse scenario but also having a live supply to it. You also fit a 3 pole fan isolator.

Option Three
Do you fit a fan, and put a 3A fuse on the permanent live supply to the light, then use that for the live for the fan, and the switched side of the light for the switched live etc. You also fit a 3 pole fan isolator.

Just to add to this
On top of these three options, lets say this is downstairs, no loft space etc, brick walls. You need to put a 3 pole isolator somewhere, where do you put it? Do you surface mount it in a box? Do you use a 3 pole pull switch on the ceiling? But hang on, where do you then put the 3A fuse? Do you not bother? Do you put in on the surface or ceiling?

Just wondering what others do as have been having this conversation with a couple of people. I also rang Manrose about this about putting a 3A fuse across the switched live too, they said, although preferable, it only really needs it on the permanent live.

 

[Marvo]

The justification for 'protecting' or attempting to protect a device that's drawing in the region of 0.1 Amps current with a 3 Amp fuse or even a 1 Amp fuse for that matter eludes me entirely TBH. How can you hope to provide protection by fusing at over 30 times the normal load current?

 

[Dave OCD]

That's what I was trying to say.:smile5:

 

[Andy78]

I recall it being common to see the small glass <1A fuses in fans. Am I just thinking of the commercial ones I used to deal with a lot ?

 

[Marvo]

I can understand if the manufacturers have a built-in fuse of an appropriate value and rating, it's just the fusing of the spur requirement that confuses me. Most small motors like bathroom fans are designed in such a way the locked rotor current and therefor the heat produced is restricted to safe values by having windings with a deliberately high resistance. The chance of this type of motor ever blowing a 3A or 1A fuse even under locked rotor conditions is even less than with a normal motor.

 

[Richard Burns]

I too cannot see the sens in fusing down a fan with a 3A fuse, it will provide a limited greater protection than a 6A circuit breaker on a overcurrent situation.

That said they cannot provide a fuse in the fan as it has two incoming supplies so you could only protect them individually not both together.
The 3A fuse is not lower because the minimum size of fuse that is generally available to the average person is 3A and so this is the fuse that would be used as a replacement (if not a 13A one!).
If the manufacturer has required a 3A fuse then they are able to claim non compliance with the instructions and so not honour a warranty which will save the manufacturer money, they can also design fans that can only cope with 3A overcurrent and save further money.
Oh yes and apparently it is safer, perhaps.

 

[soulman]

A 3 pole isolator is only required if there is no window within the room.

 

[Marvo]

......That said they cannot provide a fuse in the fan as it has two incoming supplies so you could only protect them individually not both together.

Surely the the fan manufacturers can't be specifying OCPD's in their installation instructions to protect the supply (supplies)?

If the manufacturer has required a 3A fuse then they are able to claim non compliance with the instructions and so not honour a warranty.....

This plausible deniability of warranty claims is the only theory I've seen that sounds likely to me.

 

[ackbarthestar]

Surely the the fan manufacturers can't be specifying OCPD's in their installation instructions to protect the supply (supplies)?


This plausible deniability of warranty claims is the only theory I've seen that sounds likely to me.

This sounds about right - probably because the manufacturers haven't built in the necessary overload protection due to the cost.

"But what i can't understand is why your not fusing down. Explain that part to me? ?"

A suitable design might include a thermister in the windings as with some more expensive motors, but then that costs money to manufacture. Its far easier to pass the responsibility onto an installer who cannot afford to pay the legal bills of any dispute.