# Inter-connected mains smoke alarms



## Noel (7 Aug 2006)

Hi, anybody know what I need to inter-connect 3 or 4 mains powered smoke alarms? 1.5 3 core & earth? What's recommended? Is ok to feed them off a lighting circuit or do they _need_ a dedicated circuit?

TIA

Noel


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## Adam (7 Aug 2006)

You could try the electricians forum on screwfix. I've just bought The electricians Guide http://www.tlc-direct.co.uk/Main_Index/ ... index.html and its superb for these type of questions. Ideally you want your smoke alarms on a seperate spur, which is not 30mA RCD protected (although your lights probably aren't anyway). See page 17 of the Part P documentation below which shows a diagram of best practice. It clearly shows a sperate spur for the smoke detectors, on the non-RCD side of the split CU.

You could try here:

http://www.planningportal.gov.uk/upload ... P_2006.pdf


Adam


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## RogerS (7 Aug 2006)

Adam...good links. 

Just to be a bit pedantic, the Part P diagram does not say 'best practice' but 'what you might commonly come across'... 

I am struggling to find the logic for a separate circuit to smoke detectors..especially those being retrofitted...the thought of having to run a cable/cables back to the consumer unit...especially if, like me, you hate seeing cables on the surface.

Not having a smoke alarm I don;t know what happens when the power fails and is then reconnected? Do you have to manually go round and turn them off? That's about the only reason that I can see...ie not having to faff about when the lightbulb failing trips the circuit breaker. 

Thinks a bit more...that could be the reason...if you had a lightbulb on and it tripped the breaker while you were asleep then your smoke alarms would be without power.

What's wrong with battery ones?


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## Taffy Turner (7 Aug 2006)

Noel,

I would have thought that they would need to be on a seperate circuit for the reason that Roger mentions - a bulb blowing and tripping the circuit breaker would disable your alarms (or at least make them behave as stand-alone battery powered ones).


Similarly, you really should come off the non-RCD side of the distribution board, as RCDs are prone to false tripping in thunderstorms etc - by coming of the non-RCD side, you avoid this issue.

Regards

Gary


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## dedee (7 Aug 2006)

Noel,
from the IEE on fire alarms

Andy


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## LyNx (7 Aug 2006)

Mine are on it's own circuit.


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## Adam (7 Aug 2006)

dedee":3jhyvd3d said:


> Noel,
> from the IEE on fire alarms
> 
> Andy



Thats a nice link. Thanks.

Adam


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## Noel (7 Aug 2006)

Thanks all for your prompt relies. I think I should be ok running the alarms off a lighting circuit which is used on a daily basis. It's mainly for my loft conversion (in it 3rd year I'm told....) but may as well take the opportunity to install mains alarms on the ground floor as well. The alarms I'm installing are Grade D with 4 year (so it says on the box) Lithium battery back-up. Think they should be ok. Thanks Taffy and Roger.
Thanks also Dedee and Adam for the links. Will get the book mentioned too.
So that leaves me with what to inter-connect them with so that when one is activated they all go off (I presume the battery back-up will still be able to activate the others in the event of power failure or trip) Cannot find any info on that. Anybody know if 1.5 3 core & E is suitable?

Rgds

Noel


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## RogerS (7 Aug 2006)

Noel

It depends on the voltage level of the switching signal. If low voltage then you could get away with telephone single core cable...very thin and so not as noticeable or as expensive as twin and earth. Something like this or this

Any URL links to the destructions that came with your alarms?


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## Noel (7 Aug 2006)

Roger, haven't gone into town to collect them yet so haven't a clue what make they are. A cursory look/surf over the weekend failed to gives any clues with on-line instructions. But will try and find out if the switching is low and/or 240 volt.

Cheers

Noel


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## Noel (5 Sep 2006)

Well, finally ready to fit these smoke alarms and I'm still in the dark about inter-connecting them. I've no worries about powering them and if the lighting circuit does trip they will remain quiet and they also have a battery back-up. they're Kidde Fyrnetics 123/9HI units. Here's the instructions for wiring the units togethers:
http://www.kiddefyrnetics.co.uk/utcfs/T ... 05,00.html
Why is 3 core and earth recommended? As the earth isn't recommended as the inter-connect line T & E is no good (or does it make any difference?) Or use a single line for the inter-connect?
I rang Kidde technical and asked why a 4 core cable was needed. Answer? _We can't tell you_. Well you printed the installation sheet!. _Sorry still can't tell you_. Why? _It's what we've been told to use_.
Well thanks for all your help, not.
Can't find anything on the IEE site. Help required please.

TIA
Noel, who may be missing the obvious...


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## OLD (5 Sep 2006)

I do not understand the reference to 4 core the diagram shows 3 cores in use looks like when a alarm is triggered a potential appears on the alarm terminal and by looping this through to all alarms causes them to sound also, clearing the alarm will stop the sounders.
The earth need not be used . Best set up for feed would be separate or lighting feed not protected by rcd a mcb only as in a split load distribution situation minimising any false tripping.


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## RogerS (5 Sep 2006)

I think that they are making the assumption that these are being fitted with their own dedicated mains supply cable. This mains supply cable goes into the first one and then loops out to feed the next one then loops out to feed the next one and so on. There is, of course, no reason why the alarms could not be powered in a radial fashion...as is in your case IIRC since you're feeding them from the lighting circuit??

Anyway, going back to their assumption....since you are looping a mains cable from one alarm to the next it makes sense to use 3 core mains cable (this is most usually used in the context of two way switching) and then use the 'spare' cable as your interconnect cable between the alarms. You could use single core insulated cable to run as the interconnect cable between the alarms instead and again it does not have to loop from one to the other as per their diagram. The interconnects could all come back to a common point..

Hope that that makes sense?


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## Shadowfax (5 Sep 2006)

Noel 
What you are installing is an LD2 Grade D fire alarm system. You are not using smoke detectors, they are self-contained alarms.
The L is for life risk. The D is for domestic and the type 2 means the alarms must have inbuilt battery back-up and must be interconnected. The BS allows for the alarms to be connected to lighting circuits and 1.5mm 3 core + earth is sufficient.
The reason you can't get an answer about using the earth conductor is that it is simply bad practice to use it because it is not insulated in its own sheath and has a single purpose. Using an earth in this way could be dangerous.
If the power fails the alarms will remain working for ages.
Are you including a heat alarm in the kitchen?
Cheers.

SF


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## Noel (5 Sep 2006)

Thanks for all your replies but, surprisingly, still not got my head around things.
My initial querry was why use 3 core & E? I fully understand that the earth should only be used for earth, fine. The earth is redundant from the point of view of powering the alarms. So is there no such thing as 3 core without earth? (or am I being stupid here?). It was the four wires in a cable that threw me, not so much the earth part of it.
So, it should be ok to use 1.5 twin and a separate single line (lighting switch should do?) for the interconnect aspect as I have a few rolls of this.
Although don't mind getting 3 core without earth if there is such a thing.
Sorry to be a PIA..

Noel
PS, a heat sensor alarm is on the list for the kitchen.


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## Jake (5 Sep 2006)

3 core plus earth is just easily available because it is used for two and three-way switching.

A single and a T&E will work just as well.

Are you powering them all from the same circuit?


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## RogerS (5 Sep 2006)

Noel":2bvxyor7 said:


> So is there no such thing as 3 core without earth? (or am I being stupid here?).



Possibly but not relevant to your situation!


> So, it should be ok to use 1.5 twin and a separate single line (lighting switch should do?) for the interconnect aspect as I have a few rolls of this.



Yes..I assume you mean lighting cable...not switch?



> Although don't mind getting 3 core without earth if there is such a thing.



Probably doesn't exist...a red herring!



> Sorry to be a PIA..


No...you're not

:lol:


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## Noel (6 Sep 2006)

Thanks.
Jake, 2 units on the upstairs circuit and 2 on the downstairs circuit. Think that that should be ok. BTW what would be the limit be for one circuit? Bearing in mind we're talking very small loads.

Noel


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## Jake (6 Sep 2006)

I was just wondering about connecting the two different circuits via the switch line. Nothing other than caution, because I have no idea how isolated the switching is from the live and neutral circuits, but I'd hesitate. 

If you have to run a single up/down for the switch anyway, I'd run the power up/down as well so it is all off one circuit.

An electrican might tell you I'm talking out of somewhere personal.


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## Noel (6 Sep 2006)

Jake, that concerned me as well. My initial plan was to run them all from one circuit so there'd be no conflict with the switch line and to be honest it would be the easier route to take. I use a sparky to do all the critical stuff like final connection to the board. I just do the donkey work and the easy stuff. Think I'll just use the single circuit. Be alot easier. I'll check with the sparky.
We don't have Part P overhere so there's isn't much hassle but I'd rather be careful all the same.

Noel


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## RogerS (6 Sep 2006)

Noel..I would guess that the load is miniscule so go ahead and feed them all from the same circuit....but if you are running a mains cable to each unit then adopt the manufacturers recommendation and use 3 core plus earth ...that way you get your interconnection.


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## Les Mahon (6 Sep 2006)

Noel,

I stuck very similar units in my place last week. I just used 2 core 1.5 to power them and a seperate single core 1.5 as the interconnect. They test fine - thankfully I haven't had any smoke to see if they "Reaaly" work yet!

My choice of cabeling was based exclusivly on the rolls of cable I had knocking around. I took the power from the lighting circuit as IIRC the load is tiny - I haven't the figures to hand, but 2 of these units wouldn't hold a candle to putting another light on the circuit was the logic I remeber.

Les


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## Noel (6 Sep 2006)

Les, I'm doing exactly that. 

Cheers

Noel


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## Jake (6 Sep 2006)

The info isn't very clear, but it seems that they may draw 80mA each at max (when sounding, presumably) - that's 19 watts by my reckoning. So four from a circuit would be 80 watts. 

How many lights are on the circuit that you are tapping off from? A 6amp MCB allows 1300 watts max, with each light fitting counted as 100watts (or actual lamp wattage if greater than 100watts). So if you have 12 or less, you should be fine. IANAE, etc, - and this is UK practice.


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## soapy (6 Sep 2006)

Hi Noel, If you have a 10A circuit you should use 1.5 cable, if it's a 6A circuit you can get away with 1mm cable. Using 3c & earth will give you a much neater installation. When I've fitted these on larger jobs the client usually stipulate that they are fed from the lighting cct to stop tenants turning the fire alarms off. If you use a single core cable make sure it's insulated & sheathed unless you intend to install it in conduit.
Regards
Soapy


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