Bathroom Extractor Fans and FCU's

riveralt · 28 Nov 2010

For some reason, not that I am complaining, I have had a number of jobs recently that involve the fitting of timer extractor fans linked to the lights in bathrooms.

I have noticed that manufacturers instructions are stating that the fan must be able to be isolated for maintenance - no problem with a 3 pole isolator but also the circuit must be protected by an FCU with 3amp fuse. And where they don't mention the FCU, like Vent Axia, they imply the same by stating that the circuit must be protected by a 3amp fuse.

Fitting a switched FCU as a direct replacement for the lighting switch would be ideal, but as we know without a neutral at the switch this becomes a problem, especially when chasing new cable to the switch is prohibited.

Without this option the only way I can think of designing such a circuit is by interupting the lighting circuit with a jb taking the line/neutral and cpc which feeds through a FCU onto another jb from which the cable to the switch - switch live to the lights and three core through a three pole isolator to the extractor are linked.

Am I over thinking this one or Is there another way?

sparkwright · 28 Nov 2010

I suppose one solution is to have an unswitched 3 amp fused spur interupting the permanent live and neutral supply cable that feeds all the bathroom lighting, including the fan and it's 3 pole switch. No need for junction boxes.

If the fused spur was switched, this would do the same job as the 3 pole fan switch, as it would effectively isolate the live, neutral and also the switched live. The only problem with not having a 3 pole switch before the fan is that you wouldn't be able to work on the fan in the dark - because the bathroom lighting would be isolated.

PrinceofDarkness · 28 Nov 2010

The MI's are often misinterpreted in these situations. Most MI's dont take into account that the fan might be connected to a 6A lighting circuit (as they mostly are). Their product might be connected to a larger circuit for all they know, hence the instruction for a 3A FCU.

The plain fact is that neither a 3A fuse nor a 6A MCB will protect a domestic bathroom fan itself, anyway. They are rated typically, at 20W, and such is their high restance windings that even when stalled their LRC never rises above their FLC. It's often the case that such fans suffer bearing seizure and come to a standstill - but it never results in MCB/fuse tripping (unlike larger induction motors).

There's no need for further fusing-down for such a fan on a standard lighting circuit.

So what if the manufacturers' instructions aren't complied with - eight Pounds worth of warranty claim down the drain? Big deal!

Lucia.

bernardgreen · 28 Nov 2010

PrinceofDarkness said:
The plain fact is that neither a 3A fuse nor a 6A MCB will protect a domestic bathroom fan itself

The advantage of the 3 amp fuse is that catastrophic failure in the fan will ( hopefully ) take out only the 3 amp and not the 6 amp lighting fuse / MCB leaving the lights working.

And 750 watts of catastrophic failure is better than 1,500 watts of catastrophic failure.

ban-all-sheds · 28 Nov 2010

Wiki: Please see if the answer to your query is already covered by the Wiki

See the 2nd diagram here: //www.diynot.com/wiki/electrics:fan

riveralt · 28 Nov 2010

ban-all-sheds said:
Wiki: Please see if the answer to your query is already covered by the Wiki

See the 2nd diagram here: //www.diynot.com/wiki/electrics:fan[/QUOTE]

Yes BAS I have wired dozens of fans and the diagram you have referred to is missing the issue I was raising. My comment was about the now common manufacturers instruction that as well as putting a three pole isolator in the circuit they also want a 3Amp FCU in the circuit or make the MCB a 3Amp which means much the same thing. Which adds considerably to the wiring costs especially, as in this case, if you cannot convert the wall switch to a switched FCU.
The second and third diagrams do not cover thiat particular scenario.

PrinceofDarkness said:

There's no need for further fusing-down for such a fan on a standard lighting circuit.

So what if the manufacturers' instructions aren't complied with - eight Pounds worth of warranty claim down the drain? Big deal!
Lucia.

Click to expand...

The Timer fans I fit tend to cost about 5 times that value - so its a question of the extra cost of the wiring (paid by the customer) or do not follow the manufacturers instructions and the fan fails (costs paid by me).

ban-all-sheds · 28 Nov 2010

riveralt said:
Yes BAS I have wired dozens of fans and the diagram you have referred to is missing the issue I was raising. My comment was about the now common manufacturers instruction that as well as putting a three pole isolator in the circuit they also want a 3Amp FCU in the circuit or make the MCB a 3Amp which means much the same thing. Which adds considerably to the wiring costs especially, as in this case, if you cannot convert the wall switch to a switched FCU.
The second and third diagrams do not cover thiat particular scenario.

Wired as per the 2nd diagram the SFCU does isolate all 3 poles of the fan supply, so it meets the manufacturers requirements.

And if you put a 3A fuse in it that meets their other requirement, and does not put the light on the same fuse as the fan and does not stop the light from working if the fan has to be isolated.

It does not require the wall switch to be converted to a SFCU, it just requires it to be converted to a DP switch, and if it's a pull-switch the swap is a POP.

The SFCU does not have to be wall mounted - it could be flushed into the bathroom ceiling if required.

If those address all of the concerns that the person who gave my post a thumbs down would you please remove the -ve mark now?

If there's still something else you aren't happy with would you please explain what it is?

PrinceofDarkness said:
There's no need for further fusing-down for such a fan on a standard lighting circuit.

So what if the manufacturers' instructions aren't complied with - eight Pounds worth of warranty claim down the drain? Big deal!
Lucia.

134.1.1

PoD must have had some special EICs printed with a full A4 page sized box for departures given the readiness with which he says that any regulations he happens not to like, or are too inconvenient for him, can just be ignored...

joinerjohn · 28 Nov 2010

Would it be acceptable to mount 2 fcu's side by side? One for the perm live and one for the switched live? That's the only way to positively disconnect the fan. Or perhaps an fcu on the perm live but then a DP switch for both the perm live and switched live?
Seems the manufacturers just have not thought this through for timed overrun fans.
There again, I'm probably talking b******s.

sparkwright · 28 Nov 2010

I prefer not to use double pole switches as wall mounted switches, as they don't always quite match the other light switches.

Is there any reason why you shouldn't use a double pole switch to switch two lives when the terminals are marked L and N? Not a problem that I know of.

I still think the best way is a fused spur controlling the whole of the bathroom lighting, with a 3 pole fan switch isolating just the fan.

ban-all-sheds · 28 Nov 2010

joinerjohn said:
Would it be acceptable to mount 2 fcu's side by side? One for the perm live and one for the switched live?

Yes.

That's the only way to positively disconnect the fan.

No it isn't - look at the diagram in the Wiki.

Or perhaps an fcu on the perm live but then a DP switch for both the perm live and switched live?

If you switch the permanent live it's no longer permanent, is it, and then the overrun wouldn't work.

Seems the manufacturers just have not thought this through for timed overrun fans.

You might think they do this sort of thing to give them a way to wriggle out of warranty claims, but I couldn't possibly comment.

DetlefSchmitz · 28 Nov 2010

ban-all-sheds said:
joinerjohn said:

Would it be acceptable to mount 2 fcu's side by side? One for the perm live and one for the switched live?

Click to expand...

Yes.

Dunno. Not exactly a single point of isolation.

ban-all-sheds · 28 Nov 2010

2 FCUs to meet the maker's fusing requirement are not a substitute for, nor do they preclude the use of, a 3P isolator after them to provide the isolation required.

ban-all-sheds · 29 Nov 2010

ban-all-sheds said:
If those address all of the concerns that the person who gave my post a thumbs down would you please remove the -ve mark now?

If there's still something else you aren't happy with would you please explain what it is?

Apparently no retractions or explanations will be forthcoming.

What a surprise.

Note to anyone following this topic now, or reading it in the future:

There's nothing wrong with this: //www.diynot.com/forums/viewtopic.php?p=1814134#1814134 - it got a thumbs down because there's a pathetic immature idiot at loose here who thinks it is funny to give posts negative ratings even though he can't actually find anything wrong with them.

riveralt · 29 Nov 2010

Having given some further thought to this issue - is there anything wrong with replacing a wall switch (just with live, switched live and earth) with a switched single pole 3amp FCU and just using the two live terminals? In otherwords just replacing one switch with another that happens to also have a means of isolation in it.
Together with a three pole isolator this would appear to meet the manufactuer's instructions and reduce the amount of cable and time needed to complete the installation.

ban-all-sheds · 29 Nov 2010

riveralt said:
Together with a three pole isolator this would appear to meet the manufactuer's instructions.

Please post a diagram showing where the 3A fuse is in the permanent live supply to the fan.

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Bathroom Extractor Fans and FCU's

PrinceofDarkness

joinerjohn

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