Mains Smoke Alarm

[thankyousam]

I'm adding a couple of mains fed smoke alarms to my lighting circuit. The first one is simple enough as there is a run of cable above the location in the hallway. I'm intending to "loop in" the smoke alarm as you would with a ceiling rose - allbeit without a switch ;)

I'm not sure how to build the second alarm (upstairs) into the circuit. Can I simply spur it off the downstairs smoke alarm using three+earth (for the interconnect)? Or do I need to "loop in" the second alarm as well? I've tried to illustrate both. I can't see why the first method would be a problem, just wanted to check.

Spurred: ( = T&E, ||| 3&E Interconnect)

Rose ==== Smoke Alarm 1 ==== Rose ||| ||| Smoke Alarm 2

Full Loop In: ( = T&E, =~= 3&E Interconnect)

Rose ==== Smoke Alarm 1 =~=~= Smoke Alarm 2 ==== Rose

[Owain]

Yes

No, and you MUST NOT do so if the downstairs and upstairs are on different lighting circuits.

Owain

[rrh]

You can just spur it off the first alarm.

[Steve]

That's a bit dodgy innit? If your lighting is set up as one circuit to cover the whole house, it may be OK but if the downstairs lights are on one fuse and the upstairs lights a different fuse, someone could pull the fuse for the upstairs and expect the smoke alarm to be safe to work on when it's not.

Steve.

[robgraham]

I must admit that my instinct would always be to supply any sort of alarm in a seperate manner to anything else. Disaster scenarios are always "well thats's unlikely !" - but that's why you do a risk analysis.

Let's say a fault develops in the lights circuit the OP is talking about - it might generate adequate smoke for the alarm to sound before the fuse blows, but what if it was the other way round - OK the fuse has protected the system but the heat generated might by then have ignited something else which will now not be detected by the alarm - because it / has / no power !!

I woudl suggest if you can that you supply them both off a seperate circuit from the CU. And if you want to be really secure, you have a couple of battery ones as well and exercise a bit of discipline in changing the batteries regularly. Rob

[rrh]

Can't lay my hands on the regs just now but they insist that interconnected alarms are wired to one circuit only. Mains-powered alarms without battery backup ("Grade E") should have their own dedicated (and preferably non-RCD-protected) circuit from the consumer unit; mains-powered alarms with battery backup ("Grade D") may be wired either that way or to a regularly used local lighting circuit that is separately electrically protected.

[Owain]

Smoke alarms which are supplied from lighting circuits have inbuilt battery or capacitor back-up power for several hours.

Ones that are supplied from a separate circuit don't have to have battery back-up; this, and the liklihood that loss of power on a separate circuit is less likely to be noticed, makes exclusively-powered alarms *less* reliable, IMHO.

The usual problems of batteries dying or being removed applies.

Owain

[Owain]

Not if the CU is properly labelled "downstairs lights and all smokes"

Anyway, anyone working on anything electrical should be testing for dead rather than relying on a fuse/MCB

Owain

[Bob Eager]

I fitted battery-backed ones on a separate circuit, but included a very visible (halfway up the stairs) non-maintained emergency light on the same circuit (thus making it 'not exclusively powered', I'll admit..)

[Bill]

In message , Owain writes

They can have.

This is easy to overcome.

Just a thought, I have mains powered, battery backed up, smoke detectors in most rooms of the house and they are fed from one supply and also inter linked so that if one goes then they all sound, plus an isolating switch so that it is possible to turn off all bar the one that activated. The other thing I have done is to fit some emergency light units to the same circuit, this has the advantage of indicating if the circuit has tripped for any reason.

[Owain]

A centralised 'hush' switch is very useful if the installation is prone to nuisance alarms.

Generally a workable solution with the proviso that if the *lighting* circuit trips, the emergency lights won't come on.

Owain

[ARWadsworth]

How long the batteries will last without ac power varies. Try 4 years not several hours.eg.

For what it is worth IMHO smoke alarms should be connected to the power for the TV in most houses as any disruption in that supply would be noticed first.

Adam

[Christian McArdle]

They shouldn't expect any such thing, as to connect them up in that manner would be against the regulations, which requires all smoke alarms to be interconnected and on the same circuit (either a regularly used lighting circuit, or a dedicated alarm circuit).

If they are expecting smoke alarms to be distributed across multiple alarm circuits, then they aren't competent to work on an electrical installation. They are also not competent to work on electrical installations if they think that flicking an SP MCB is enough to consider a circuit to be deenergised, let alone isolated.

Christian.

[Christian McArdle]

That is bad design. The alarm system should be designed not to have nuisance alarms, as they seriously damage the usefulness of the system. My system has gone off just once since it was installed 3 years ago. This was when the toaster set on fire. If my smoke alarm system went off, I would be immediately convinced that there was a fire. Compare with a poorly designed system with inappropriately chosen locations and sensor types that goes off every time someone has the full English and is completely ignored by residents.

Christian.

[The Natural Philosopher]

Up till two days ago I would have said the same...4 a.m. Alarms go off. I paddle round the house in the altogether prodding at each one. No sign of smoke or fire. After about 5 minutes they stopped..weird..

Wasn't sure how to ID which one was the one that set the others a going..

[Mark]

I agree that Nuisance alarms seriously damage their usefulness. However I have never managed to find alarms that do not have false alarms. About a year ago we had some mains smoke alarms professionally fitted (I have checked they have been fitted correctly) and they still go off occasionally when we cook or when I do some DIY.

The false alarms have gradually increased in frequency since they were fitted. And we do vacuum them out regularly.

Mark

[Christian McArdle]

What type of alarms are on the same floor as the kitchen (ionisation/optical/thermal) and where are they installed?

Christian.

[Mark]

I'll need to check on the type. There is one installed on the ground floor in a hallway near the bottom of the stairs. It's about 2m from the Kitchen door.

Mark.

[Christian McArdle]

Do all rooms on the ground floor lead from the hall? What rooms are there? How large is the hall?

Christian.

[Mark]

On the ground floor the Lounge and Kitchen lead off the hall. The Hall is about 3m x 2m approx.

Mark