Guidance given by the IEE and a manufacturer, differs a little from your sensible approach to fire alarm supplies
Your opinion (which differs from Bs 5839-6) quote
It is NOT good practice to wire smoke detection into lighting circuits at all, frankly.
The prefered method of the IEE as in the on-site guide where all circuits are protected by rcds,"there is advantage to wire them off a regularly used local lighting circuit "
On site guide page 66 Note
We, as always, need to take care to be clear what is personal preference,so that the choices remain clear on these muddy issues
Yes, however, the industry views this differently. Advice notes issued by FIA have contradicted the advice given by the IEE on site guidance previously too.
It is, as you say, muddy, hence why I pointed out my hope that the next amendment of BS 5839 would clarify, once and for all this issue.
There are a greater number of reasons why it is NOT advantageous to wire even smoke detection from a local lighting feed. Chief among those being that battery life is almost always quoted under "optimum" conditions, which often bear little relation to "real life" scenarios, and the more than chance possibility that a feed intended to supply a smoke detector directly will end up spurred into any other number of devices, or that lighting loads will change, as fittings do, possibly rendering the circuit faulty, or overloaded, increasing the risk to properly working detection.
Another point which was used to assist the view of a seperate supply for these type of alarms was
quote
If a lighting circuit goes, the back up batteries, often only alkaline, in a smoke alarm will be called into play. Once an alkaline battery is called into play, it will lose its charge fairly quickly. This in turn, may cause the detector NOT to operate when needed.
This is the advise taken off the aico site in this regard
Battery life expectancy
quote
This will depend on a number of factors that will differ slightly
according to the type of alarm in use.
The 140 series alarms are supplied with an Alkaline battery which,
dependent on conditions, can provide up to 4 years standby supply,
up to 2 years without mains power.
Indeed - Aico use Alkaline cells only in the 140 series detectors, and in standard bases. However, the "get out" is the key phrase "dependent upon conditions" - with experience, and having fitted well over a thousand Aico detectors of one sort or another, I can say with some conviction, that if anyone can show me a four year old alkaline battery, which has been used over that period, I will show you a smoke detector which has not (a) been tested, and (b) which in all probability does not work.
Quite contrary to what Aico claim, we, in common with many other companies in the industry, change alkaline batteries annually. The specific reason for this is that a proper test of this detection involves disconnection from the mains supply and testing on battery. Once that's done, battery is on the way out.
Characteristically, an alkaline cell will lose charge fairly quickly once current has been drawn from it. It is a primary reason why alkaline technology is only used rarely now, for low likelihood back up - in clock radios, for example, and in "cheap" smoke detectors....
FWIW, Aico, good as their products are, are still manufacturers, and not regulation writers.....and advice solely from one manufacturer's site should be taken, shall we say, carefully.
The 150 series and the 160 series alarms are fitted with rechargeable
Lithium power cells which are continuously charged by the alarm
circuitry. They have a life expectancy greater than that of the Smoke
Alarm, i.e 10 years or more,which eliminates the need for
replacement.
I mentioned alkaline cells or batteries as distinct from lithium batteries, which are designed to be integrated into circuits which are likely to require their use from time to time. It is, in fact, a key reason that Aico moved from using alkaline batteries in their detectors to lithium.
It is also worth mentioning that we won't fit 140 series detection, specifically because it uses alkaline back up rather than lithium.
That advise appears to contradict one of your personal reasons why a local lighting circuit should not be used
I make these comments as an electrican who has limited skills in alarm systems and installation
The comments I have made are based on guidances given to us where doubt may exist
I welcome your comments on the points made above
Regards Des
And valid your points are too, Des - it is ALWAYS good to pull up the advice you find and question everything. Whilst what you quote is indeed the case, and probably the advice many rely upon, I also speak, as I say, from experience of this kit every single day in life - and other fire detection kit too.
One of the points I tried to highlight, was the apparent conflict between BS 5839 and BS 7671 in places.
Safety, in terms of BS 7671, is "if it's an issue, shut it down. Fast". Which is sound advice in the main, as the risk from shock, or exposure to burns, etc. from electricity is very real, and very definite.
However, FIRE safety, is a different matter, and in this case, shutting the juice off as quickly as possible isn't always right, though electrical safety is also important (from the risk of ignition of fire as well as personnel safety).
As I mentioned earlier, the very best thing, potentially, would be to make it mandatory for fire safety circuits to be fed in isolation from any other supply, and clearly marked indelibly, in such a way as to be easily identified by eye.
I also maintain that twin and earth PVC cable should not be used for life safety circuits, whether supply, detection, or warning.
We are moving away from the era where smoke detectors were either mains only, or battery only, thankfully - but there is still a distance to travel here.
As far as it goes, the Onsite Guide is quite limited in the advice it gives in respect of fire detection - and the note you refer to, on P66, is unqualified advice - by which I mean, there is NO reason given as to WHY it is advantageous, in the note writer's opinion - it is a comment which has been challenged many, many times.
But that said, the whole "grading" of part 6 has been challenged many times too - why, for example, is there no need for fire resistant cable, or segregation at one level, yet there is at the next? Because your house is smaller, does it deserve a lesser level of protection? I don't think so.
You are right, however, that I offered advice based as much on experience, as on anything quoted by manufacturers, or admittedly, an unqualified comment in the OSG - at least, unqualified in the sense that no reasons are given for the note.
That, sadly, is still so much of the nature of both BS 5839 and BS 7671 - it is in many cases far too interpretive and subjective, rather than definite, and objective.
Different angles of approach perhaps - from fire safety, and from electrical compliance alone.
It remains, for now, I think, a question that will go on and on, all the time there is such differentiation between the lower three, and the higher three grades in part 6.
For the record, my "personal" opinion - remains - all fire detection devices should be wired using fire resistant cable, in a segregated, or easily identified manner, and fed separately, preferably from as direct a feed as possible (i.e. no RCD or RCBO)- precisely for reasons of integrity of the system. In all honesty, I would far rather see a system in place for fire systems of all grades and categories which limited the provision and distance of low voltage in the system at all - e.g. some kind of transformer local to the main supply, and the entire system running on extra low voltage at all points - 24VDC being the most common.
Thanks, though for your input - it is good, I feel to have discussion and alternative views on these matters.
To the original post
here is a link to some useful information about househod smoke alarms
Aico Ltd[/QUOTE]
Good stuff fellas
