Wiring mains smoke alarm with interconnect

[Lobster]

I'm installing a pair of smoke alarms; identical mains models with battery backup (Screwfix ref 10347), one upstairs, one downstairs. I'm going to wire them into the lighting circuits, of which I have separate ones upstairs and down. So far so good, but is there any reason why I shouldn't add the interconnect wire if using this configuration? Or should I run both alarms off the same lighting circuit, and add appropriate warning labels?

Apart from the obvious distinction between upstairs and down, I also like the idea of having them on separate circuits; although there's battery backup, it must be less likely that both circuits will trip or fail in a fire than just one.

(I realise the optimum would be to have a separate circuit altogether for the alarms, but that would cause too much grief at this stage...!)

David

[Andrew Gabriel]

They must run off the same circuit. Different circuits must be isolated from each other.

That's what the battery is for, and you'll notice any outage of a lighting circuit quite quickly.

Actually I think that's a bad idea because you won't notice failure of the circuit. ISTR the regs require a dedicated circuit if there is no battery backup, which always seems very silly to me (although this only applies where you are required to have smoke detectors).

[Lobster]

Damnit - hoped I wasn't going to hear that!

OK, so I need to run a cable comprising live, neutral, and 'interconnect' between the two units. Thinking how best to do this... given that the units have no earth, would it be reasonable to use the (redundant) earth wire of a standard twin core and earth cable for the interconnect, provided the junction boxes were clearly labeled at both ends? If not, how should the wiring be done to comply with regs? The units have a short white interconnect wire, but don't know where I'd get a reel of that!

Thanks David

[Bob Eager]

I'm just about to embark on this, with six alarms.

I'm putting them on their own circuit, back to the consumer unit. They are fed with the usual flat twin and earth, and interconnected with flat triple and earth (the stuff you use for two way lighting circuits). I guess I could use the earth wire, but I'd rather it were insulated.

[stefek.zaba]

Not remotely reasonable: the earth wire (CPC = Circuit Protective Conductor if you want to get all pedantick about it) isn't insulated, and any reasonable person working on the wiring knows and expects it will be part of the earth continuity arrangements. Putting it to any other use is a really poor idea - sorry! You can get lengths of triple+E from your friendly local electrickle trade counter, or even at Been&Queued or similar, and he's your man for the job. It's still kind to label your use of the three cores, but a widely-used convention would be to use the red for the perm. live, blue for the neutral, and yellow for the switched-live/ interconnect.

HTH - Stefek

[Bill]

In message , Bob Eager writes

Much better idea to use 3 core and earth, as I understand it only plumbers use the "earth" wire as a signalling line.

In case you are worried about not noticing that the dedicated circuit has tripped here is a thought for you. When I rewired my present house I fitted mains, with battery back up, detectors in each room and emergency lights on the same feed at the top of the stairs, in the main bedroom and one by the consumer unit. So if the circuit trips it is obvious by the lights coming on.

>

[Bob Eager]

Yes, it was never a serious possibility for me.

Good idea!

[Lobster]

OK that's me told then! Just thought it might be OK if labeled at each end... basically when I was going to power the two alarms from adjacent light fightings on each floor (as per original query), I'd already put a length of twin and earth in place, intending for one of the conductors to serve as interconnect leaving two redundant. So I'll yank it out and replce with triple and earth instead.

Thanks as ever for all the replies.

David

[Lurch]

On 24 Jun 2004 00:05:14 -0700, [e-mail] (Lobster) strung together this:

Good boy, I can carry on breathing properly again!

[usenet]

Surely only 'obvious' if it's dark, in bright sunlight lights in the house can be almost totally unnoticeable.

[RichardS]

Good move.

This very same situation occurred with a friend who was renovating a C17 house. Building regs were involved in the changes, and one of the requirements was that interlinked mains powered smoke alarms were installed in various places.

One of the jobs that he undertook himself was to install the smoke alarms.... guess what he used to connect the interconnect, guess what the building regs inspection failed on, and guess what had to be ripped out and replaced with triple and earth....

At the end of the job you can now sit back and be smug in the confidence that you've wired it in accordance with the letter of the regs, which can only be a good thing.

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[Bob Eager]

Since the alarms are battery backed, it's sufficient to make it noticeable when it next gets dark - I guess. Or have an alarm set to go off on mains failure on that circuit.

[Bill]

In message , Lobster writes

Good move. The point you have to consider is someone in the future cutting into your cable, for what ever reason, they would expect the earth wire to be earth and it could prove a bit of a shock (no pun intended) if it isn't.

[Christian McArdle]

Plus, the sheath is insufficient insulation for a live conductor, both in the cable generally, and especially at the terminations, where there wouldn't even be a sheath.

Christian.