smoke alarm and ceiling rose

[magico]

hi,
i`m gonna connect my mains smoke alarm up to the ceiling rose
i`ve interconneted the 3 alarms using 3C+E. so its twin and earth to ceiling rose from the alarm.
the ceiling rose already has 4 cables in,is it ok to add another?
cheers

 

[breezer]

you can try

 

[magico]

i can get the wire in physically.the live and nuetral will obviously be sharing same hole as another L+N
just wondering whether there is a limit to how many is allowed.
and is it safe

 

[breezer]

yes its safe.

the question that is to be worried about is overloading, which you wont be

 

[jj4091]

yes its safe.

the question that is to be worried about is overloading, which you wont be

There is a bit of a safety issue, but not from an electrical point of view, in as much as if you are running the alarms off the lighting circuit instead of an independant supply & the lighting supply fails ( if mcb trips when a lamp blows) then your detectors become dis-armed.

 

[sm1thson]

( if mcb trips when a lamp blows) then your detectors become dis-armed.

arguably better than the mcb tripping and no one knowing! (its generally not long before you realize your light are out), either way shouldnt it be battery backed? (and again arguably less likely to be left with no batteries and the mcb off in on the lights)

 

[bernardgreen]

Difficult one this.

A light circuit could fail in the fire, it could even be the cause of it, and the MCB opens when the early stage of the fire melts the cable before there is enough smoke below the ceiling to set the alarm off.

So battery backing is essential, you could have a fire in the middle of an area wide power cut.

Personally I would have the alarms on their own MCB with a battery backed emergency ( auto illuminating ) light(s) on that circuit. If the power fails or the MCB trips these come on as both indication the power to the alarms has failed and as illumination if you need to evacuate the house in a fire with no normal lights working.

Bernard
Sharnbrook

 

[Spark123]

Do mains smoke detectors come without a battery backup? What if there is a fire which burns through the supply cable to the smoke detectors?

 

[bernardgreen]

Do mains smoke detectors come without a battery backup? What if there is a fire which burns through the supply cable to the smoke detectors?

Yes, there are some but I recall they are designed to be used with "fireproof" cabling. The batteries were seen to be a hazard in some industrial locations. ( don't ask, it was a long time ago )

But many (if not most) domestic smoke alarms have dead batteries in them.

 

[Pensdown]

So battery backing is essential, you could have a fire in the middle of an area wide power cut.

Bernard, that is a very valid point.

During night power cuts most people still use candles which greatly increases the risk of fire so I agree that battery backup is essential whether they are fed via the lighting or not.

I also prefer them on there own circuit even though there is a very slight risk that breaker may trip. Most of the mains sensors that I've seen have a green mains on LED so the risk of the tripped breaker going un-detected is also low.

 

[Spark123]

Do mains smoke detectors come without a battery backup? What if there is a fire which burns through the supply cable to the smoke detectors?

Yes, there are some but I recall they are designed to be used with "fireproof" cabling. The batteries were seen to be a hazard in some industrial locations. ( don't ask, it was a long time ago )

But many (if not most) domestic smoke alarms have dead batteries in them.

Do dead batteries not cause mains alarms to chirp?
---------
We used the 24v ones in monitored fire alarm systems in the past where the batteries are housed remotely in the control panel, the sounder circuits being seperate from the detectors. I seem to recall in years gone by (but don't quote me on it!!) that in monitored systems only the sounder circuits need to be in fireproof cable.

 

[breezer]

Most of the mains sensors that I've seen have a green mains on LED so the risk of the tripped breaker going un-detected is also low.

and what makes you think joe blogs is going to look?
and what will they do when they see no green light?

Domesti smoke detectors should be wired to the nearest light.If they are non their own mcb and they go off because of burnt toast, what will joe blogs do? switch off the mcb, now that was a good idea wasnt it? But if wired on the lighitng cct they will soon realise whats wrong and turn it back on again

 

[EddieCurrent]

Hello all
I think that the regs allow both an individual circuit , or into a regulary used lighting circuit.
However, the guy that used to do my testing would not accept them on there own circuit for the reason Breezer gives. ( folk can hush them when the toast is burning and then forget, or not bother even, to turn the circuit back on).
Even though I pointed out the regs etc to him, he would not issue certificates if they were wired independently. (stating another one of his own 'new rules').
We don't work together anymore.

 

[Pensdown]

Joe blogs will soon realise that because the alarms are battery backed turning off the MCB won't make the slightest bit of difference.

For the average DIYer lighting circuits are complicated enough without feeding smoke alarms from them as well. It's just as possible that joe blogs will replace a light fitting and inadvertently disconnect the smoke alarm.

If they are going to be wired on the lighting circuit personally I would run a separate labeled cable from the MCB.

There are pro's and cons both ways which I suppose is why the regs allow them to be wired either way.

 

[kai]

You will probably find it a bit difficult to squeeze an extra cable into a cramped ceiling rose connection - the lid probably will not close properly!

Using a junction box will possibly be better - the 30amp ones have lots of room and nice big terminals to make it easier to work in than a 6amp one.

Having the smoke alarms on the same RCD side as the freezer supply has an advantage of letting you know if the RCD feeding the freezer has tripped - the green light will extinguish - One detector in the front hall, and one outside the bedrooms - green power light shining.