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Topic Title: Fuse protection for extraction fans
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Created On: 11 January 2012 10:39 PM
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 14 November 2012 03:00 PM
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rocknroll

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Most of the fans I have seen have a thermal fuse across the motor, it looks like a small diode in clear heatshrink so why bother.

regards

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leave nothing but footprints!"
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"Oh! The drama of it all."
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 14 November 2012 03:52 PM
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AJJewsbury

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The supplied circuit shows this going before the switch (so it fuses live and switched live) but in my case it will have to go afterwards, which presumably means I need two 3A fuses

Technically, you're effectively creating two new circuits - with a shared N -- which is prohibited by BS 7671.
- Andy.
 14 November 2012 04:00 PM
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OMS

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The supplied circuit shows this going before the switch (so it fuses live and switched live) but in my case it will have to go afterwards, which presumably means I need two 3A fuses (although arguably the switched live is a bit irrelevant and I could just fuse the live?).


Two fuses = two circuits, but you have a common neutral (borrowed neutral) so not in accordance with BS 7671 ?

I wouldn't consider the switched live an irrelevance either

Any reason you can't controll the bathroom bit of the lighting circuit via 13A fused connection unit (as the fan manufacturers drawing)

regards

OMS

Edit: have you employed a lightning fast typist, Andy -

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 14 November 2012 04:38 PM
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AJJewsbury

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have you employed a lightning fast typist, Andy

Na, pure fluke!
- Andy.
 14 November 2012 05:03 PM
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pmhetherington

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Originally posted by: OMS

The supplied circuit shows this going before the switch (so it fuses live and switched live) but in my case it will have to go afterwards, which presumably means I need two 3A fuses (although arguably the switched live is a bit irrelevant and I could just fuse the live?).


Two fuses = two circuits, but you have a common neutral (borrowed neutral) so not in accordance with BS 7671 ?


I had not appreciated that, but is it really "two circuits" if both feeds and the neutral go to the same device in the same cable via the same 3-pole isolator switch? The fuse(s) are only there to protect the device, not to provide isolation.

I wouldn't consider the switched live an irrelevance either

Any reason you can't controll the bathroom bit of the lighting circuit via 13A fused connection unit (as the fan manufacturers drawing)


It's possible, but only by either hiding the fuse in the ceiling void or putting it in an equally inaccessible location on top of the boiler. Plus I need to look at how much current the bathroom lights draw, which might be a factor. I will have a think about this, but it is not my preferred solution.
 14 November 2012 05:10 PM
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AJJewsbury

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I had not appreciated that, but is it really "two circuits" if both feeds and the neutral go to the same device in the same cable via the same 3-pole isolator switch? The fuse(s) are only there to protect the device, not to provide isolation.

The BS 7671 definition of a circuit is 'an assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s) (my emphasis). Your L & SL would be protected by different overccurrent devices (fuses), hence they're two different circuits - electrically the same as two different ways from a conventional distribution board.
- Andy.
 14 November 2012 05:17 PM
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pmhetherington

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Originally posted by: AJJewsbury

I had not appreciated that, but is it really "two circuits" if both feeds and the neutral go to the same device in the same cable via the same 3-pole isolator switch? The fuse(s) are only there to protect the device, not to provide isolation.


The BS 7671 definition of a circuit is 'an assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s) (my emphasis). Your L & SL would be protected by different overccurrent devices (fuses), hence they're two different circuits - electrically the same as two different ways from a conventional distribution board.


Fair enough. Interestingly, we do this sort of thing all the time in the rail industry and people seem to cope. A typical example would be a relay in one room fed from a supply in another, with usually lower-rated fuses at the room entry/exit points, in each leg of the circuit, which might include multiple paths on the 'live' side. The idea of the lower-rated fuses is to discriminate between internal and external faults, although whether it works in practice is another thing.

So, I remain to be convinced that it actually matters, regardless of what the regs say! Although for obvious reasons I would rather be compliant.
 14 November 2012 05:49 PM
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OMS

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I'm not sure you actually provide circuits that share common conductors in the rail industry - you may have multiple feeds and multiple points of isolation but I don't recall ever having a shared conductor that may become live despite the relevant device being isolated or CPD removed.

As Andy said, "Technically" you are creating two circuits with a borrrowed neutral - it's not a major drama in this application - other than perhaps for someone fault finding or reporting on an EICR after your days

Regards

OMS

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 14 November 2012 06:08 PM
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pmhetherington

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Originally posted by: OMS

I'm not sure you actually provide circuits that share common conductors in the rail industry - you may have multiple feeds and multiple points of isolation but I don't recall ever having a shared conductor that may become live despite the relevant device being isolated or CPD removed.

Correct, and nor will my proposed bathroom fan circuit, or at least that is what I was trying to say.

As Andy said, "Technically" you are creating two circuits with a borrrowed neutral - it's not a major drama in this application - other than perhaps for someone fault finding or reporting on an EICR after your days


OK. So maybe I shouldn't technically do this, but if the view seems to be that this is just a technicality... going back to the original question, I still need a smaller fuse holder which fits the space!
 14 November 2012 06:13 PM
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OMS

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How much room do you have for the fan isolator - could you fit a dual box with 4 plate screws designed for two single plate accessories - put the isolator on one "half" and a gridswitch yoke with 13A fuse holders in the other half

regards

OMS

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 14 November 2012 06:16 PM
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pmhetherington

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Originally posted by: OMS

How much room do you have for the fan isolator - could you fit a dual box with 4 plate screws designed for two single plate accessories - put the isolator on one "half" and a gridswitch yoke with 13A fuse holders in the other half

Will check tonight. Thanks.
 14 November 2012 07:29 PM
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primo

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I noticed at Elex that Click are making a range of fan isolators incorporating a fuse on a single plate. Didn't look closely but probably worth a look.
 14 November 2012 08:21 PM
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Grumpy

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In order to get my loft conversion signed off building control insisted that I install a smoke alarm outside my downstairs toilet, by the front door as it contained a "source of ignition" ie an extractor fan. Pah! Whoever heard of an extractor fan catching fire I grumbled. Two days later I was called to a nursing home after a serious fire in one of the en-suites caused by a fluffed up fan. My handy man contact there now has an additional task of de-fluffing fans, he's thrilled!
 27 November 2012 02:12 PM
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pmhetherington

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Originally posted by: OMS

How much room do you have for the fan isolator - could you fit a dual box with 4 plate screws designed for two single plate accessories - put the isolator on one "half" and a gridswitch yoke with 13A fuse holders in the other half


Apologies for the delayed reply to this. There is room for a double box (i.e. the size of a double socket) - just about - but not room for a box designed for two single plate accessories.

If there is a way of fixing the fan isolator in one half of a double box then we might have a chance of this idea working. Otherwise the fuses will have to be on a single box, tucked out of the way via some conduit, which will make them virtually inaccessible...

Phil
 27 November 2012 02:20 PM
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OMS

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They'll be the same size box I suspect

Try here for an example.

3 pole fan isolator on one side and a two gang gridswitch yoke with fuse holders and a two gang grid plate on the other.

Regards

OMS

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 27 November 2012 02:29 PM
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pmhetherington

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Originally posted by: OMS

They'll be the same size box I suspect

Afraid not. This version fits, the one you linked to doesn't:
http://cpc.farnell.com/pro-ele...ng-backbox/dp/PL13064
 27 November 2012 02:49 PM
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OMS

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Ok - so height wise it'll be the same - I'mguessing about 25mm to 35 mm difference in width is too great for the space you have.

Back to plan A then - break the supply to the room and rewire it via a 13A fused spur that protcts everything.

Simple, compliant and ready for modification when nextyou rewire.

failing that - put the box on the ceiling, not the wall ?

Regards

OMS

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 27 November 2012 03:02 PM
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pmhetherington

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The wider box would partially block a vent. It could be done, but it will look awful.

Don't plan on putting the bathroom on a separate spur, too awkward as the spur would have to go in the ceiling void because of where the other wiring is, or I'd have to drill through a ceiling joist to get the spur into the cupboard, which seems like a bad idea.

Ceiling is a removeable panel (for access to the boiler flue) so can't do that either.

I did say it was awkward!

It seems like the best I can come up with is to put two single boxes connected via a bit of trunking, one either side of the vent, and hope that the fuses never blow (as access would be very awkward). Your gridswitch yolk with two fuseholders idea should work like that, though it does mean that one of the cables will have to be stripped right back since the Neutral will not go through one of the boxes (as there would be nowhere to terminate it - presumably?).
 27 November 2012 03:07 PM
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AJJewsbury

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(as there would be nowhere to terminate it - presumably?)

Floating wago or even 'choc block' connector would be usual.
- Andy.
 27 November 2012 03:29 PM
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OMS

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OK - you know what you have to deal with, I'm just guessing.

As i said, it's your desire to provide two fuses that appears to be the problem - I'd just stick a 13a fused spur outside the bathroom door at high level.

Failing that, don't worry about fusing the fan - if it goes kaput then a warranty claim will takes ages anyway - just buy another fan !!

Regards

OMS

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Failure is always an option
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