Wiring Mains Smoke Alarm

[DonnaS]

Hi

I am replacing a mains smoke alarm. The new unit has Live, Neutral and Earth points.

The wires coming out of the ceiling from the old alarm are Blue, Yellow, Red and Yellow/Green.

That to me means I got one wire too many! Please help.

Thank you

 

[riveralt]

Do you have more than one smoke/heat alarm on the circuit?

The wiring suggests that this is the second/third etc interconnected smoke alarm on the circuit - with the yellow cable acting as the switch live.

All smoke alarms are slightly different in set up so check the instructions for interconnected smoke alarms and follow the wiring diagram.

 

[DonnaS]

Hi

Thank you for your help - this is the only mains smoke alarm.

The instructions are not any help - there is a place on the alarm for an "interconnecting" wire along side where the Neutral, Live and Earth wires should go.

Should I put the Yellow wire in here? Would the alarm still work OK?

Thank you in advance

 

[riveralt]

The first unit on a smoke alarm circuit is normally wired with a twin and earth cable. Subsequent ones are interconnected via three core and earth so I am surprised that, with only one alarm on the circuit, you have a three core and earth.

The person who originally fitted the alarm may only have had three core and earth available and so may only have used two of the cores.

You need to find out where the smoke alarm is getting its power from.

Is it directly connected to the Consumer unit with its own MCB or it linked to a lighting circuit.

Once the source is identified you should be able to establish which cable is acting as line and neutral - not necessarily red and black. If it is the only smoke alarm on the circuit then you should be able to snip back the unused cable and treat it as a twin and earth - then wire the alarm as per instructions.

 

[DonnaS]

Thanks for replying again.

This has gone way over my head - I thought it would be simple.

Looks like I'll need to get an electrician, hope I don't get ripped off. I'll wait to see if I get any replies to my other post before I book one.

And yes I want a mains one to go with an existing battery operated one in the house - safety first with a baby soon. Thanks

 

[DetlefSchmitz]

Two points.

If you haven't removed the old alarm then the wires will be going into terminals which are probably marked with their functions. This should really answer most problems, at least which ones are L/N and E.

Secondly, the point about the interconnecting wire is that only the same manufacturers model can be guaranteed to use the wire in the same way, so you should be really sure that there isnt another smoke alarm somewhere. (Silly thing to say, I know, as you must know that you don't have another one, but worth mentioning for others who read this.)

 

[sparkwright]

How was the yellow connected on the old smoke alarm?

 

[DonnaS]

I have no idea how the yellow wire was connected - I took it apart and binned it - wish I hadn't done that now!

 

[sparkwright]

OK.

Do you think the yellow WAS actually connected to something?

Sure there's no other smoke detectors or carbon monoxide detectors?

 

[DonnaS]

I honestly have no idea. 2 Feet away though is the light - there is no yellow wire coming out of this.

 

[sparkwright]

But can you confirm if you have any other alarms; a heat detector in the kitchen, a carbon monoxide detector, more smoke detectors perhaps in another hallway or lounge...

 

[DonnaS]

I can't confirm - I can't see any but what's to say they are under the ceiling/plaster.

 

[ColinJacobson]

One point on this topic. If having a smoke and heat alarm units in the hall and kitchen. Do they need to be connected together, or can they be both directly connected to a supply and not to each other?

Retrofitting can mean a long 4 core cable between the two of them, while directly wiring both from say the same light circuit, would be far easier and less hassle and destructive mess in a house.

 

[1john]

One point on this topic. If having a smoke and heat alarm units in the hall and kitchen. Do they need to be connected together, or can they be both directly connected to a supply and not to each other?

Retrofitting can mean a long 4 core cable between the two of them, while directly wiring both from say the same light circuit, would be far easier and less hassle and destructive mess in a house.

read BS-5839

 

[ColinJacobson]

One point on this topic. If having a smoke and heat alarm units in the hall and kitchen. Do they need to be connected together, or can they be both directly connected to a supply and not to each other?

Retrofitting can mean a long 4 core cable between the two of them, while directly wiring both from say the same light circuit, would be far easier and less hassle and destructive mess in a house.

read BS-5839

I do know that British Standards (BS) are recommendations, not law. The Building Regulations are law. Many do not understand the difference. I am not saying you do not.

The linking is to activate all sirens in all the alarms, so if one is activated they all sound?

Thinking about it, if one alarm ca be heard in all of the house, then I would say that would be fine.

What do most think about that?