Smoke Detector Spacing

[Simon M]

Can anyone give me some guidance as to why smoke detectors, according to the BS, have a working radius of 7.5 meters?

I presume this figure has been in the BS for many years and wonder if there is any scientific reasoning behind it or was it plucked out of the sky.

I am looking to investigate this area as part of a project I am doing and it seem to me that as technology has increased and detectors become more sensitive we can justifiably increase the spaces and my be in future devise a rating similar to sprinkler heads and so devise a spacing in relation to ability to detect.

Hope you can help.

Simon

[Chris Houston]

I don't think a point detector actually detects smoke 7.5m away, I think the logic is that the smoke has to travel no more than 7.5m to get to the detector, the detector can detect the smoke only when the smoke particles enter the chamber or scatter the light in the detector.  No matter how sensitive they are, the smoke will still have to travel 7.5m.

[Thebeardedyorkshireman]

The figure came from the old FOC rules that the maximum floor area to be proteted by one smoke detector was 1000 sq ft. This became metric to 100 sq M. If you take a room with the dimensions of 10M X 10M and place your detector in the centre, you will find that the distance to the corner equates to approx 7.5M
Thus you are protecting 100sq M MAXIMUM and the maximum horizontal distance from a fire source is the 7.5M diagonal. Take away the walls and form the space into a portion of a larger area  and the rule stops you increasing smoke travel time, even though you may have complied with the 100 Sq M rule.
Dave

[Simon M]

Thanks for two very interesting, informative and thought provoking responses.

In response to Chris, I am looking at response times really and looking at older technology you would potentially need more smoke to get the alarm condition, so on that thinking with increase sensitivity you could extend the coverage by having detectors with greater sensitivity that those when the BS was originally specified.

Mr Yorkshire man, for God own country, when was this document originally written?

Thanks again

Simon

[Thebeardedyorkshireman]

When I stopped working on the tools and started designing in 1976 I obtained a 'dog eared' copy of the F.O.C. rules which obviously predates 76. The first BS to cover the spacing would be 5839 pt1 1980 BUT I may be wrong as we had numerous parts of 3116 kicking around which covered the technical spec for the hardwear.
Increasing sensitivity is not a panacea for increasing spacing. I would read BS 6266 for a start. As more places become airconditioned we ought to consider air movement modeling
with fans on and off particularly where the system is for life safety. We all have seen detectors which will never operate regardless of sensitivity.
Dave

[Chris Houston]

I would suggest (and this is just my suggestion, not a statement of fact) that even with the most sensitive detector possible there can be no way of compensating for the fact that the smoke must move 7.5m before it gets to the detector and on that basis it might be difficult to argue for increasing detector spacing.

I also echo the views that insufficient consideration of air movement is a problem, especially in long corridors and refridgerated areas with fast moving air.

[John Webb]

By 1990 there were two standards for smoke detectors.
BS5445 Part 7 was based on European practice and was aimed at detectors for use in commercial premises. It ranked detectors into three grades according to their response to the range of test fires. Each grade had limits on the ceiling height and distance between heads that could be used.
BS 5446 Part 1 was aimed at detectors for domestic use and was a simple 'pass/not pass' response to the range of test fires.

In 1991 when at the Fire Research Station (FRS) I did work for the Home Office on trying to compare the two standards - see 'Comparison of British Standards BS 5445 Part 7 and BS 5446 Part 1'  which is a summary document published by the HO Fire Research and Development Group and authored by Cath Reynolds. You may be able to get a copy from her at DCLG as she now runs their fire research division.
The test fires were the problem area:
(a) It was difficult to compare the two because the range of test fires was different in each standard.
(b) The smoke from most of the BS 5446 tests had problems being 'seen' by optical detectors.

The FRS also did work in the 1980s for the Home Office on smoke detector spacing in corridors to detect fire in adjacent rooms. This may help you with understanding the current spacing rules. I don't have any of the HO documents on this work; again Cath Reynolds may be able to help you.

I am a little uncertain as to the detailed requirements of the current BS on detectors and how much of this has been altered from the 1990s as a result of the above or more recent research.

[John Webb]

Further to my last posting, BS 5445 part 7 was based on EN54.

Concerning modern detection, particularly those detectors combining two or more methods of detection, it might be worth looking at:
"Signatures of smouldering/pyrolizing fires for multi-element detector evaluation", by T Cleary, W Grosshandler, M Nyden and W Rinkinen, NIST, USA, published in Interflam 96, pages 497-506, Interscience Communications Ltd., 1996. ISBN 0 9516320 9 4

[kurnal]

John Thanks for this very interesting background information. The 5446 tests focussed on a flaming fire test source which opticals struggle to detect.  Does this  indicate why ionisation detectors became the mass produced first choice and why opticals still are much harder to find in the DIY sheds?
Or am I taking your point too far?

For many years I had concerns that most houses had only ionisation detectors and people were not being roused if they had a smouldering fire such as a tumble drier or sofa fire in the night. Fortunately over the last couple of yers the guidance has changed to recommend an appropriate mix- but all this could have happened years earlier if the tests had been carried out along the lines of BS5445?

[John Webb]

A number of optical detectors failed the tests of BS5446, whereas the ionisation detectors did not. With some optical detectors it was possible to 'tap in' to the sensor and look at how its output changed with increasing smoke; it did not alter very quickly.

The detectors were also placed in a recirculating wind-tunnel with a set velocity 'breeze' passing past and a rising level of smoke. Their outputs changing closely with the rise in smoke level measured just outside the detectors, indicating smoke was passing into the detectors with only a short delay.

It was concluded that
(a) The optical detectors needed a minimum air speed past the detector to get the smoke inside with reasonable rapidity.
(b) The smouldering test fires of BS5446, on the whole, produced only low velocities past the detector head in the test chamber and were unable to force smoke through the labyrinthine entry that optical detectors have.

Ideally we should have taken measurements of air velocity in small domestic fires, together with temperature and smoke measurements, and compared these with the test fires. I am convinced that such measurements would have shown that the smouldering test fires in particular were not representative of 'real' fires.

However, by the 1990s FRS had dropped work on detectors and related standards, and I was unable to pursue the matter further.

[Thebeardedyorkshireman]

I believe that the main push for domestic Ions in the early days was the high current consumption of opticals. Thus the performance of ions to the fire tests john has refered to , their relative ease of manufacture and suitability for operation from a 9V battery ensured their sucess.

[wee brian]

The reason Ion detectors were/are so popular was because they could amke them so cheaply. Opticals have dropped in price over the years so now there isn't much in it.

I think there have been some situations where "wrong type of smoke" has been a contributory factor to some incodents but in most domestic fires you have a mixture of stuff burning inneficiently. Both types of technology are good enough.

Opticals are less prone to some false/unwanted alarms

[Gel]

Little price difference in detectors, but different matter with smoke alarms whereby Opticals are in region of 40% more.

B&Q sell one, amongst huge range of Kidde alarms.
Opticals increasingly the choice of social housing specifiers, because of less nuisance alarms
from steam sources.
However, they do generally require more regular cleaning to prevent dust build up.
There are some battery operated combined Ion/Optical alarms in UK, with UL approval.

Another reason for choice of Opticals is fact that is some EC directive incumbent on public specifiers re Radioactive products (ie Ions) which says if there is an alternative, that should be used.

And there is WEEE round the corner; though not applicable on these products yet, it may well become so.
For a product selling @ £3.99 the cost to the maufacturer of recovery/recycling
may well be uneconomic to continue selling Ions.

I gather some detector manufacturers have already taken decision.

And of couse only UK & Eire still permit use of Ion alarms.

Optical usage in US smoke alarm market much smaller because of er their environmental
stance.

[Simon M]

To everyone

Many thanks for all your comments and some very interesting thoughts.  

Can some one point me in the direction of the FOC as well as telling me what FOC stand for.

I take the concers of perople that increasing the distance may not be a viable result but I do think that it can be investigated once all the historical data has been reviewed.  This can then be balance against the increase in technology and only then make judgement.

To finish off I would say this is only research and as we all know most research doesn't go very far.

Thanks again for your help.

Simon

[Chris Houston]

The Fire Offices' Commitee.

It doesn't exist anymore, it was replaced by the Loss Prevention Certification Board (LPCB).

The LPCB is part of BRE Certification Limited.

The LPCB write standards for, test and approve Fire and Security products and systems.  A lot of what they do is driver by specifiers (such as insurance companies) needing universal standards.

See: www.brecertification.co.uk and www.redbooklive.com for a list of their approved products and systems.

Also see, InFiReS: http://www.infires.co.uk