Extractor Fan Isolator Switch????

[JohnW2]

Yes, annoying, isn't it? You go over the top with the cooker circuit in your new kitchen, just in case - you run a 10mm cable on a 50 Amp MCB. You pop out with the missus to choose your nice new cooker and when it arrives the instructions tell you you should only connect to a 40 Amp, 6mm circuit. You're not ripping the 10mm out and yet you're not complying with the manufacturers instructions. By not complying with manufacturers instructions you're not complying with BS7671, so you can't sign off your EIC.
Great, aren't they - manufacturers can request any old twaddle that they like, probably without having a clue about electrics in the first place, and you can't question it or do what you think is better because of a couple of lines in BS7671.

I do wonder whether people perhaps take the requirement to comply with MIs too literally. The spirit is clear, and most people would want to comply with that spirit - but it does seem very odd that manufacturer's can 'make up their own requirements', which potentially go beyond what BS7671 would otherwise require, yet effectively acquire 'the power of BS7671'. In an extreme case, I don't think an electrician would have much of a leg to stand on if (s)he did something which was clearly wrong or dangerous, arguing that (s)he was obliged to do it in order to comply with BS7671 'becaue the manufacturer had (incorrectly) instructed it'!

Kind Regards, John

 

[JohnW2]

Clever idea. If you were simply putting that in the feed to to the fan, you might have to up that to two fuse modules, if you believed that the MIs were calling for both L and S/L feeds to be fuse-protected.

If I were wiring it, I would bring the lighting feed into the fan isolator grid fuse first. That's the permanent live to the isolator switch. From this point the live also feeds the bathroom light switch. The switched live is on the return leg and that goes to the L2 contact on the isolator switch. That way the live and switched live are both fused, with one fuse. Voila

Indeed, but that's obviously why I wrote "If you were simply putting that in the feed to to the fan...". If one is prepared to run additional cables from your grid box, then one can do things such as you suggest. In effect, you're fusing the whole lot - light + fan.

Kind Regards, John

 

[OwainDIYer]

Clever idea. If you were simply putting that in the feed to to the fan, you might have to up that to two fuse modules, if you believed that the MIs were calling for both L and S/L feeds to be fuse-protected.

Don't both L and SL feeds have to be protected by the same fuse though?

They may not explicitly say that, but it's common sense.

Using the grid switch components makes it easy to separate the fuse from the isolator and so have a fused but non-isolated light.

[code:1] +--------------------------------------------------+
| |
+---+---+ +--------+ |
| N | | N+------------------------------+ |
| light | | fan L+--------------------------+ | |
| L | | SL+----------------------+ | | |
+-------+ +--------+ | | | |
| | | | |
| | | | |
| | | | |
+---------------------+--------+ | | | |
| | |SL |L |N |N
+----+---+ | +--+---+---+-+ |
| SP | | | | |
| switch | | | 3 pole iso | |
| | | | | |
+----+---+ | +--+---+---+-+ |
| | | | | |
| +-------+ | | |
+--------------------+ +---+
+----+---+ |
| 3A | |
| fuse | |
| | |
+----+---+ |
| |
| |
L N[/code:1]

Drawings done at ASCIIFLOW.com incidentally.

 

[JohnW2]

Don't both L and SL feeds have to be protected by the same fuse though? They may not explicitly say that, but it's common sense.

Well, they don't even explicitly say that the S/L (as well as the L) has to be fused (and, as I've said, I strongly suspect that many people don't do that) - and we don't really know (but can guess) what the manufacturers want the fuse to protect. Functionally speaking, I think separate fusing would inevitably provide whatever 'protection' they felt was needed but I agree that a single fuse covering both is the obvious and common sense way to do it - the main advantage being that with two fuses, the operation of one could leave a live feed to the fan, which someone working on it might not realise - hence a theoretical danger.

Using the grid switch components makes it easy to separate the fuse from the isolator and so have a fused but non-isolated light.

Indeed, and I would reply exactly the same as I just did to TTC, namely: "If one is prepared to run additional cables from your grid box, then one can do things such as you suggest. In effect, you're fusing the whole lot - light + fan.". In terms of practical realisation of your system, I imagine that the simplest method would be to take a 3C+E from the grid box to the light switch, with T&E from there to the light (assuming that the N connection could be accommodated within the switch). I suppose that this slightly 'unusal' wiring might confuse someone subsequently working on the installation, if they were not provided with documentation.

However, going back to practicalities ... given that MIs of many fans do call for 3A fusing (although they usually don't say what of) and a 3-pole isolator, I wonder what folk actually are doing - since I very much doubt that either the 'grid switch' or 'DP pull switch' approaches which you have proposed are used much, if at all!

Kind Regards, John

 

[lordwoody]

View media item 55659
Would this be OK it is within Regs as there is a window in the bathroom.

 

[bernardgreen]

The theory appears to be that the switch live does not need to be fused discrimated to the light circut ( 3 amp fan, 6 amp lighting ) as only the permanent live connects to the motor by a low impedance. The switched live connects to a high impedance trigger inport. In theory the switched live will not have a high current even in the motor windings melt into a dead short.

Thus the lighting circuit will not be put out of action by a burnt out motor in the fan because the 3 amp fuse will ( should ) blow before the 6 amp for the lights.

The intro-duction of RCDs means condensation in the fan can put the lights out by a trip of the RCD. Hence total isolation is needed if lights are to be available when the fan is saturated with water.

It isn't only condensation, rain or snow blown into the vent by wind can saturate a ( badly installed ) fan.

 

[lordwoody]

Would it make things easier if I got the basic model without the timer?

Surely the basic model would not require the Isolator switch? just a 3A spur?

Like this....

View media item 55665
This is the fan manual....
http://www.tlc-direct.co.uk/Technical/DataSheets/Aidelle/ADAUE_Instructions.pdf

 

[JohnW2]

Would it make things easier if I got the basic model without the timer? Surely the basic model would not require the Isolator switch? just a 3A spur? Like this.... This is the fan manual....

Having a non-timer fan certainly makes things simpler (no 'switched live' to worry about), but not quiote as simple as you suggest - in particular, the argument for needing an isolator is as strong as it is for a timer fan (however strong you think that argument is in the first place!). Those instructions call for a "3A switched isolator" - which I imagine would probably be satisfied by your proposal if you used a switched 3A FCU.

Kind Regards, John

 

[JohnW2]

The theory appears to be that the switch live does not need to be fused discrimated to the light circut ( 3 amp fan, 6 amp lighting ) as only the permanent live connects to the motor by a low impedance. The switched live connects to a high impedance trigger inport. In theory the switched live will not have a high current even in the motor windings melt into a dead short.

It may be wishful thinking to believe that you'll get reliable discrimination between a 3A fuse and 6A MCB - but, yes, one imagines that probably is the manufacturer's thinking, although most don't actually say so (that the S/L does not need to be fused). However, as I said, although extremely unlikely, it's not impossible that a low impedance fault could arise in relation to the S/L (e.g. the high-value input resistor failing S/C).

The intro-duction of RCDs means condensation in the fan can put the lights out by a trip of the RCD. Hence total isolation is needed if lights are to be available when the fan is saturated with water.

Given that probably all such fans are Class II, that would usually only be an issue if the fan was so 'saturated with water' that the water managed to establish a path to the 'loose end' of the (unused) CPC of the supply cable. Not impossible, but would usually require a lot of water to be present.

Kind Regards, John

 

[lordwoody]

This is the best advise that I think I have had so far......

The best way to wire a timer fan from a spur is to utilse two live feeds from the load side of the spur. One live goes directly to the fan, the other goes to the switch. The lighting and the fan effectively become fed from the spur. If you did it this way the temptation to switch off the fan is reduced because in doing so they would lose the light too. This method also provides 3A protection to the fan and obviates the need for a triple pole switch (assuming a dp spur L/N is used). I believe new-builds adopt this approach too. Fans are useful for removing smells as well as venting for showers so I would avoid a standard switched fan and go for the timer as originally planned

So basically what he is saying is that the Switched 3A spur would operate the light and the Timer fan and if the Tenant was to switch it off they would be in the dark.

I don't suppose anyone could do me a wiring diagram for this method could they.

 

[bernardgreen]

At first sight, that looks like a pretty Mickey Mouse lock - are appearances misleading? More to the point, can it lock in the 'on', as well as the 'off', position (which is obvioulsy what the OP wants)?

I had no feedback from the site but out of interest I contacted the manufacturer who confirm it locks in both positions.

 

[bernardgreen]

that would usually only be an issue if the fan was so 'saturated with water' that the water managed to establish a path to the 'loose end' of the (unused) CPC of the supply cable.

I was thinking along the lines of damp fan touched by person in room, leakage via metal ducting from the fan rather than via the CPC

 

[JohnW2]

that would usually only be an issue if the fan was so 'saturated with water' that the water managed to establish a path to the 'loose end' of the (unused) CPC of the supply cable.

I was thinking along the lines of damp fan touched by person in room, leakage via metal ducting from the fan rather than via the CPC

I'm not sure I fully understand your answer. I was replying to your comment:

The intro-duction of RCDs means condensation in the fan can put the lights out by a trip of the RCD. Hence total isolation is needed if lights are to be available when the fan is saturated with water.

... and neither of us had said anything about anyone touching the fan. I suppose that it's just about possible that condensation could provide a relatively low impedance path from live parts to metal ducting (although that wouldn't be at all easy) and that the metal ducting could provide a low enough path to earth to cause an RCD to operate, but I think all that would be incredibly improbable - less so that what I suggested (a path to the 'unconnected' of a CPC terminated at the fan).

Kind Regards, John

 

[lordwoody]

This is the best advise that I think I have had so far......

The best way to wire a timer fan from a spur is to utilse two live feeds from the load side of the spur. One live goes directly to the fan, the other goes to the switch. The lighting and the fan effectively become fed from the spur. If you did it this way the temptation to switch off the fan is reduced because in doing so they would lose the light too. This method also provides 3A protection to the fan and obviates the need for a triple pole switch (assuming a dp spur L/N is used). I believe new-builds adopt this approach too. Fans are useful for removing smells as well as venting for showers so I would avoid a standard switched fan and go for the timer as originally planned

So basically what he is saying is that the Switched 3A spur would operate the light and the Timer fan and if the Tenant was to switch it off they would be in the dark.

I don't suppose anyone could do me a wiring diagram for this method could they.

Oh dear its getting more complicated!
Someone has also now told me it will also have to be RCD protected!

I am not doing it myself but I am trying to find out the best way to do it without having trunking stuck everywhere (its a 1st floor flat no loft) and to try and stop my tenants turning it off.

Would one of these by OK.... http://www.toolstation.com/shop/p45636
it combines the FCU and RCD obviously I would change the Fuse to 3A.

 

[bernardgreen]

... and neither of us had said anything about anyone touching the fan

I would have thought ( or at least hoped ) that anyone talking technically about fans or any other electrical items in a bathroom would naturally consider the possibility of someone touching the fan.

I haven't heard of metal ducting being affected by voltage via damp but I have seen some very wet fans, both from condensation and rain blown in.

Should exposed ducting be bonded ? or should it be "earthed" to the CPC ?