Linked mains smoke alarms

[gsport george]

Not been on here in a long time... actually been, slowly, getting on with things....

Wiring is fairly well advanced (not that well, considering it has been 3 years, but no point "rushing" these things..) and I am at the point of fitting some mains linked smoke alarms. Trouble is that the instructions say they need to be on a lighting circuit that is used regularly (so I notice if it has tripped/failed), but that it also shouldnt be on a circuit with any fourescent lights or dimmer switches... and shouldnt be within a metre of any flourescent lights.... This makes it pretty difficult if you count compact flourescents in that.... The way I had planned (and started) doing the lighting was one circuit per floor (cellar, ground, first, attic) with the outside lights on the same circuit as the ground floor. Attic and Cellar arent always used everyday, so those circuits are out, ground and first floors want to have some dimmers (probably) and almost certainly some compact flourescents. I had planned to put the alarms on the stairs and in the hall but all these locations are within a metre of CFLs...

Any ideas?

If I run a seperate loop of cable from the consumer unit will this be like a seperate circuit, even though it is on the same MCB? If not, what magical process takes place in the MCB that nullifies the effects of any equipment on other circuits?

Instead of CFL's I can obviously look at using low volt halogens or LED's, but will the transformers for these be OK? (ie. not interfere)

George

 

[arthurw3]

I guess they shouldn't be within a metre of a flourescent light because the lights can ionise the air and might trigger the alarm. Presumably the electrical junk that dimmers and flourescent lights put out could also trigger the alarm. I must admit I haven't ever taken that into account, because as you say it becomes wholly impossible to install. Perhaps you could chose a non ionising type of detector, one for instance that detects particles if you are really fussed about it. Or buy a better quality deector that has filters in the circuits to stop interference from mains borne rubbish

 

[Flyfisher]

Coincidentally, I've been thinking about linked smoke alarms. However, I was starting to home in on the mains-powered but radio-linked units. Given all the restrictions and cautions mentioned in your instructions for mains-linked, I'm now leaning even more towards the radio-linked ones - or were there other reasons why you went for mains-linked over radio-linked models?

It does seem strange to recommend installation on a lighting circuit but not one with fluorescent lamps with all the current pressures to move towards CFLs. Presumably the mains linked signalling is prone to disruption by such sources of interference and I take your point about questioning what 'magic' occurs within an MCB that would isolate different circuits from such interference. Personally I doubt there is 'magic' - MCBs are primarily over-current devices not noise filters! - so that makes the instructions rather curious.

The only comfort I can offer is to confirm that I run part of my home network over the standard mains wiring (using netgear 'homeplug' devices) and they work perfectly well, supposedly up to 85 Mbps, so I'm sure a simple alarm signal should be able to be made pretty resilient to any interference effects. So perhaps you should just try installing them and see if they work (presumably they each have a 'test' button).

 

[gsport george]

The "link" is via a seperate link cable (single conductor), which is fine for me since I am re-wiring the whole house. Obviously if I wasnt, radio linked ones would make more sense.

I am curious as to what the problem with dimmers and flourescents is, the fact that you shouldnt have them on the same circuit suggests an electrical interference effect of some sort, but the one metre rule would support arthur's idea that it is also the risk of a false alarm from ionisation..

 

[Lime]

You have to ask yourself are the fire alarms any good if they are so sensitive to interference?
Are mains linked fire alarms all the same?
It's not going to be good if all the alarms go off in the middle of the night just because you've switched the light on in the loo!

Perhaps when they talk about fluorescent lights they mean starter-operated ones that create large current spikes in the mains supply?
On a long cable run that can be quite serious and create large local voltage variations.
Starter-operated fluorescents have a magnetic field pulse that can interfere with sensitive electronics.
Also 4ft plus fluorescents have a larger surface area to radiate broad spectrum radio interference than smaller compact ones.
CFLs are electronic and don't normally create too much mains-borne noise.
CFLs can suffer flashing/flickering on and off when switched off caused by capacitance and inductance in parallel cables however.

Running a fire alarm cable back to the CU and using the lighting MCB is acceptable because the MCB is directly connected to the low impedance house supply.
Therefore the voltage variation is not so great compared to that on the end of a long run of lighting cable connected to a fluorescent.

Also, run the cable away from other mains cables to minimise interference: that includes the link.

 

[gsport george]

That sounds good Lime. I think that putting it on it's own spur of cable but from the same MCB is a good compromise. It also means that if it IS a problem I can always add another MCB next to the CU for it. Not sure how easy it will be to run the cable away from others though. Spent quite a while working out where to run cables as it is and there arent a lot of alternatives...

George

 

[DaveBrigg]

I've lost the thread a little in the technical details, but on our recent rewiring we had mains connected smoke detectors fitted. They are on their own circuit, so I suppose it could trip without being noticed, but the consumer unit is easy to see, and the alarms have a light which can be clearly spotted at night. Hopefully, if correctly wired, they are unlikely to trip anyway.
Two of the alarms are within a few centimetres of compact flourescent bulbs and we have had no problems so far, so maybe the manufacturer is being overly cautious.

 

[chuckey]

As stated, flourescents are a source of radiated radio frequency noise. Tune your tranny to a medium wave station and hold it near to a flourescent and you will find that out. Some of the radio frequencies will radiate directly from the tube and some down the mains supply. As the tube is acting as an "aerial" longer the tube, the better its ability to radiate. Hence CFLs with very small tubes will not radiate much in comparison. Also the electronic convertors in the CFL will isolate the mains feed from the "noisey" current flowing through the tube. The worrying part is the single core wire that interlinks the units, because it could be acting as an aerial and depending on the design could be feeding any received noise directly into the alarm circuits, in this case the mains neutral and or the live would be the return circuit, and so any noise pickup in these could also trigger the unit.
Its all so dependent on the design of the detectors that the best way is to try it out, before commiting to real twin and earth and holes in plaster board. Temporarily wire up the detectors and their linking wire and the biggest oldest flourescent you can find and literaly try it out to find exactly which bits are sensitive and which aren't.
Frank.

 

[Flyfisher]

Its all so dependent on the design of the detectors

I'd certainly agree with that, which was my earlier point about successfully running 85Mbps ethernet over mains cabling. Good design can overcome the noise inherent in mains wiring; relying on an absence of electrical noise is sheer folly - and inherently dangerous in a device as potentially critical as a smoke alarm.

 

[gsport george]

Well whatever happens with these I will want mains powered smoke (or heat) alarms in those locations so I might as well run the cable and keep my fingers crossed, if I have a problem with false alarms later then I can simply switch the units for different ones without significant disruption. But running a seperate spur from the MCB seems a sensible precaution in case I end up with no alternative but to put them on a dedicated circuit...

George