FCU for Extractor fan

[Pat H]

Adding an extractor fan off of a 6A lighting circuit.
Installation instructions specify a local isolator (as I would anyway) so 3 pole isolator will be used.
Switching L SL and N however should I have an FCU upstream of the live?
It would be 6A fused as specified which seems pointless the RCBO will trip before the fuse.
Also if you fuse the live feed the Switched live to the fan is still connected even with the fuse pulled.
I think the 3 pole isolator achieves all I need to re regs and the whole circuit is protected by a 6A RCBO.

 

[The Ghost]

Normally I would agree, however there is a case (somewhere on this forum) where same thinking was used and the person did not fuse the fan to 3a as per Manufacturers instructions and a fire resulted. Of course the contractor suffered disastrous reverses as a result! And today in fact like you I am going to do exactly the same as you with a bit of an Ohh-err moment thinking about that. Will follow manufacturers instructions or "give consideration" to. But how do you imagine a two pole fuse works? And if one goes on PL and not on SL what then...

 

[Rpa07]

I agree with V on that one - search through the related and one post contained the function of having a fuse within the three pole isolator. Covers you then in both instances. I thought I'd saved it to screenshot but I've got a lot of screenshots and not enough time.

 

[SparkyChick]

For the case you've given, I generally add a SFCU with a 3A fuse and run the whole bathroom lighting off it to save the complexity of worrying about fusing the PL and SL separately, and provide a 3 pole isolator regardless of what the instructions may say.

Whilst some fans don't require the 3A fuse down, I think it's better to be safe than sorry especially as you never know who may come along after you and just replace the fan.

 

[TheCodboy]

Excel 3 Pole Fused Fan Isolator Switch - White | QVS Electrical Wholesalers - http://www.qvsdirect.com/excel-3-pole-fused-fan-isolator-switch-white Seen someone talking about this on another forum before.

 

[Pat H]

The fuse element in the combined unit is internal separate so the fuse still only covers L or SL not both.
Having read the instructions with Fan no fuse is mentiined only a dual pole isolator. And that should be a 3 pole for my unit.
I like the idea of fusing the whole bathroom circuit but then don't we fall foul of the reg for diversity of circuits?
Fan shorts and the lights go off...
I know the SL is only a trigger feed and not a load so my money is on FCU on tge L.

 

[westward10]

There was recently another thread on this and RB did a diagram to show how the single fuse in the TP can protect both lines.
I go with SparkyChick though, install unswitched FCU with 3A fuse to protect both the light and the fan and fit a TP isolator just for the fan.
Division of circuits is irrelevant.

 

[Des 56]

The "fusing the live and the switched live" concern never causes me any problem because I never have a extractor fan triggered by the light,I always use a switched fused spur for isolation and fusing combined,along with the fans own means for triggering(pull cord or remote switch) it satisfies all any fan requires

My opinion has always been that triggering the fan using the light is p poor design
The lighting requirement and the ventilation requirement in all Bathrooms are sometimes two very separate functions

 

[Octopus]

When I choose tthe fan, I make sure they don't specify a 3A fuse!

Most 6A MCB's would trip quicker than a BS 3A fuse!

 

[Taylortwocities]

I like the idea of fusing the whole bathroom circuit but then don't we fall foul of the reg for diversity of circuits?

Your lights are on the common lighting circuit in any case!

IF you decide to have an FCU, IMO this needs to be in the line feed BEFORE the bathroom light, that way you fuse both the permanent live as well as the switched live.

However, the whole thing is nuts. Lets assume there is a 6A MCB protecting the whole lighting circuit., Now you introduce a BS1361 3A fuse just for the fan and light. HAve a look at the discrimination graphs and tell me which device trips first in the event of
1. Overload (unlikely)
2. High current short circuit.

Boo Murdock got there first…

 

[Pat H]

Well of course the MCB would (or RCBO in this case)
So what's the point? Aren't we adding more elements into the equation?

 

[Taylortwocities]

Aren't we adding more elements into the equation?

Yes. The FCU. It is not needed for a fan connected to a 6A MCB protected lighting circuit.
Period.

 

[SparkyChick]

Here's the post from @Richard Burns for reference.

http://www.electriciansforums.co.uk/threads/extractor-fans-in-bathroom.108900/#post-1171394

It's a neat solution, but probably requires more changes depending on the existing installation.

In terms of leaving the SL unprotected, I thought long and hard about that and whilst I think the biggest risk of fire is from the motor stalling or winding failure and the associated heat, if there is a timer/pir/humidistat involved there is a control circuit and we have no way of predicting the failure modes this may present. It's entirely conceivable that a component could fail and result in a SL/N short, so I've tended to go belt and braces.

To address your point @Des 56 . I generally concur, but the customer wants what the customer wants and sometimes no amount of 'have you thought about doing it this way' brings them to your way of thinking... have yet to try physical 'persuasion' with a heavy object, maybe one day

@Pat H, @Taylortwocities... yes, we are adding stuff and no it's probably not strictly required in terms of circuit protection, but reg. 134.1.1 includes this "The installation of electrical equipment shall take account of manufacturers' instructions". If it's required by the instructions, it must be there.

 

[SparkyChick]

I should have added to that, anything else is personal preference. I prefer belt and braces so I generally fuse down for the fan/bathroom lights, depending on the fan control mechanism and if it has a trigger input, I would add a 3p isolator regardless of whether the manufacturers require it or not because IIRC it's stipulated elsewhere in the regs.

 

[Taylortwocities]

In my response I have taken account of the MIs, and decided they do not apply.
So I comply with BS7671.

Take a look at the weedy tracks on the PCB for the average fan, there's your fuse, right there!

 

[SparkyChick]

Take a look at the weedy tracks on the PCB for the average fan, there's your fuse, right there!

True, but in designing PCBs I've sometimes been surprised how much little tracks can take before they burn out... well, you have to test these things sometimes

 

[westward10]

Your lights are on the common lighting circuit in any case!

IF you decide to have an FCU, IMO this needs to be in the line feed BEFORE the bathroom light, that way you fuse both the permanent live as well as the switched live.

However, the whole thing is nuts. Lets assume there is a 6A MCB protecting the whole lighting circuit., Now you introduce a BS1361 3A fuse just for the fan and light. HAve a look at the discrimination graphs and tell me which device trips first in the event of
1. Overload (unlikely)
2. High current short circuit.

Boo Murdock got there first…

That'll be a BS1362 fuse

 

[telectrix]

i'm with des. i'm not a fan of triggering the fan with the light ( pun intended, so there ). separate switch for fan every time. in our gaff, it's a 4 way grid outside shower room... shower room lights, fan, foyer light, soffit lights (PIR override).

 

[Taylortwocities]

That'll be a BS1362 fuse

Bloomin' spel chker.

 

[Pat H]

The fan is a humidistat fan and very quiet. So no real issue with it coming on with the light.
An outside switch isn't always the most convenient if you discover a need for sudden ventilation...