Live and switch live from different circuits

[studentspark]

Just hypothetical at the moment, but may be real soon!

Bathroom fan needs to be installed

Will obviously have switch live from the lights, but not necessarily permanent live.

If no P/L at lights I could get a P/L from boiler circuit which is in a cupboard in the bathroom.
This would mean a live conductor from two different circuits, to power the fan.
But since the fan has one point of isolation, this should be OK?

Im I correct.

This would also require two different methods of RCD protection

RCBO for the lighting circuit for S/L (No RCD at moment)
And RCD fused spur fro P/L from boiler circuit

Any thoughts

Thanks

 

[Risteard]

Prohibited.

 

[sparkwright]

Can't be done.

Can you pick up the perm live from the bathroom light switch?

Does the fan need to be a timer model/come on with the light?

You could have a manual fan fed from another circuit (same live and neutral).

You could have a timer fan that isn't activated by the light. You could have the timer fan activated by IT'S OWN SEPARATE pull switch.

Has the bathroom got a window?

 

[studentspark]

OK

Knew it didn't feel right. Fan would have isolation but the isolator would have two supplies.

Need fan over run, so switching is the thing.
Anybody installed pir sensor fans
Humidistat I'm not so sure of

Fan will own pull chord would not be an ideal choice as could get left off.
Bathroom has a mould problem

Thanks

 

[sparkwright]

Any way you can get perm live to the light?

What is above the bathroom ceiling? And what type of wiring is in the bathroom?

 

[studentspark]

Don't know at the moment.
They could well be a P/L at the lights but have not looked yet, just thinking it through, and what to do depending on what I find.

Just looking at PIR fans. That might do the trick, only thing is it will come on during the day if you just pop in for a waz.

Thanks

 

[sparkwright]

Presumably if you've access to the light, you've access to the pull switch (if there is one).

 

[studentspark]

Light switch outside bathroom, hence my doubts about P/L

 

[JohnW2]

Just hypothetical at the moment...

Since it's hypothetical, there would be one (slightly complicated) hypothetical solution ... you could feed the fan with the P/L and N from the boiler circuit (via an FCU if the fan manufacturer demanded it) and then use the S/L and N from the light to operate a small relay, the ('normally open') contacts of which were used to connect the boiler circuit P/L to the trigger input (S/L) of the fan.

Kind Regards, John

 

[333rocky333]

just considering doing so is reckless.
Theres enough posts on here regarding borrowed neutrals and the consequences

and how would you fit 2 Rcds to a fan using 1 neutral

 

[JohnW2]

just considering doing so is reckless.
Theres enough posts on here regarding borrowed neutrals and the consequences

I presume that wasn't directed at my hypothetical solution, since the whole purpose of the relay would be to avoid any 'borrowing' (of neutrals or lives).

As for the OPs proposal, there is no way that I would support or defend it. However, the reality is that it would probably not, in practice, result in any 'borrowed neutral' (or 'borrowed live') problems, provided that the P/L and N were both taken from the boiler circuit and only the S/L from the lighting circuit. The nature of the 'trigger' input of fans is such that they will usually only draw 1mA or so (probably less than plugging in a computer!) - so that, unless there was already 'leakage current' on one of the circuits which was approaching 30mA, no RCD would operate. Assuming that one had a '3-pole isolator' for the fan, another issue is that the input side of that switch would have feeds from two circuits - which, at the least, would call for a warning label. However, I agree that it's a plain nasty approach, and I probably wouldn't even do it in my own house.

Kind Regards, John

 

[333rocky333]

no john, your always precise

 

[studentspark]

You can get overtaken with how to install an electrical appliance with as little disruption to the property as possible, because it always seems that people decorate first then decide they need something added.

That starts you thinking of all manner of ways of doing things to minimise installation damage, which can lead you into questionable territory.


Thanks for your good guidance.

 

[JohnW2]

You can get overtaken with how to install an electrical appliance with as little disruption to the property as possible, because it always seems that people decorate first then decide they need something added. That starts you thinking of all manner of ways of doing things to minimise installation damage, which can lead you into questionable territory.

Indeed - and if you go to far down that road and subscribe to current fashions, you could end up switching/controlling everything wirelessly!

Kind Regards, John