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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Technical Advice => Topic started by: Wiz on October 09, 2008, 01:30:21 PM
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In the drawing of a roof space shown below, where should a smoke detector be sited (with regard to BS recommendations)?
Is it position A, B, C, or D?
Please explain why which position was chosen.
Please note that the distance from the top of the space to the dotted line just below position B is 600mm (this dimension hasn't come out in the drawing, for some reason!)
(http://i290.photobucket.com/albums/ll263/pxenon/BS.jpg)
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Ok... to open the flood gates.....!!!
Personally I would have said A, B or C preferably B
My reasoning...
A could be dead space - stratification etc.
C I think would react slower as I'd expect smoke to travel easier up the slope side than the vertical...
B doesn't appear to be way off the 500mm rule out from the wall and so would be a minor variation and smoke moving away from the vertical has no choice other than to travel up the diagonal passed B or fill the "inverted trough" enough to reach it
D may still be acceptable if noted as a variation from the 600mm rule but to me this seems the worst place
............Next.......!!
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I assume this is a cross section not a plan...
I think A B & C may require a very thin sparky with long bendy arms. D looks more viable to me.
I don't have the standard to hand so I'll leave that to others.
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A = poor placement, doubt that it would ever work
B = Probably most compliant with BS
C = Would probably give best detection due to the circulating effect of the air
D = Although not covering the top 600mm due to the shape of the roof space it would again detect smoke as it circulates
I have a feeling the answer will be all of the above though but if I had to choose it would be C, at least you could get to it with a pair of ladders to maintain it..
SHame you didnt make it into a poll Wiz, that way you would have had independant answers and give us a chance to say why once the poll had ended
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Wiz
B for earliest warning
D for ease of maintenance
depends on what you are protecting because I think there may be a longer delay with D
If its a normal office or similar go with D
David R explains it best
davo
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I think A B & C may require a very thin sparky with long bendy arms. D looks more viable to me.
Or maybe one that's eaten a few less pies..........:D !!!
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If the space was wider I'd say A but as I reckon it's too small even for a detector from a Wendy house then it's B.
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Location A would comply with 22.3 f assuming that the void is unventilated and not greater than 1.5M.
B,C&D orientation could allow dirt and dust to fall into the chamber.
The only location accesable with a smoke pole ( if you can't reach) is A.
Installation can be achieved via pre wiring base and detector and mounting via uni-strut. 500mm rule does not apply under note 6.
Dave
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A = poor placement, doubt that it would ever work
B = Probably most compliant with BS
C = Would probably give best detection due to the circulating effect of the air
D = Although not covering the top 600mm due to the shape of the roof space it would again detect smoke as it circulates
I have a feeling the answer will be all of the above though but if I had to choose it would be C, at least you could get to it with a pair of ladders to maintain it..
SHame you didnt make it into a poll Wiz, that way you would have had independant answers and give us a chance to say why once the poll had ended
D if you use if you can apply the horizontal calculations as per FIA
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A = poor placement, doubt that it would ever work
B = Probably most compliant with BS
C = Would probably give best detection due to the circulating effect of the air
D = Although not covering the top 600mm due to the shape of the roof space it would again detect smoke as it circulates
I have a feeling the answer will be all of the above though but if I had to choose it would be C, at least you could get to it with a pair of ladders to maintain it..
SHame you didnt make it into a poll Wiz, that way you would have had independant answers and give us a chance to say why once the poll had ended
D if you use if you can apply the horizontal calculations as per FIA
Surely the horizontal calcs only apply where the vertical distance is no greater than 600mm (where you calculate as normal for 7.5m radius).For higher then you can increase your detector spacing.
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Location A would comply with 22.3 f assuming that the void is unventilated and not greater than 1.5M.
B,C&D orientation could allow dirt and dust to fall into the chamber.
The only location accesable with a smoke pole ( if you can't reach) is A.
Installation can be achieved via pre wiring base and detector and mounting via uni-strut. 500mm rule does not apply under note 6.
Dave
I would say A for least/no dust and C for most.A,B and C all have their sensing element close to the apex so they tick the boxes there.
D would be installed as additional detection or listed as a variation.
I still say taking into account the distance implied that A would be too narrow for installation at the very top - hold on,why not drop it down onto a bit of unistrut etc. so that it is horizontal and within the 600mm,eh?A sort of A (Mkll) with it mounted around the height of B or C.
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Buzz, DavidR, Davo, Graeme, GregC, TBY & WeeB, thanks for your answers to my question.
I have been most impressed with the quality of the answers and the thought that has gone into them.
I don't know of any definitive answer to my question and that was why I was interested in your opinions.
I believe that that we would all look to BS5839 recommendations for an answer to this and any similar question. There may be other practicalities that could be taken into account, but at the end of the day I'm sure that most people would prefer to comply with BS recommendations and have the 'safety net' of those recommendations justifying any desicion we make.
The problem is that, in my opinion, BS does not provide a definitive answer.
These are my thoughts on my own question in respect of BS recommendations (all of which you have mentioned/alluded to in your answers)
a) Smoke detectors should be mounted within the top 600mm of the space.
b) Fire detectors should be mounted at least 500mm from a wall (except where the space is less than 1000mm when it should be mounted in the middle of the space)
In my question it is impossible to meet both the 600/500mm recommendations.
These are my thoughts on the matters raised in your answers:
In my opinion most BS recommendations point at position A being the most suitable although it is not 500mm from the wall. I understand the potential practicalities of servicing the device although I know of no BS recommendation that takes this problem into consideration for easement of recommendations. I'm impressed with the comment about stratification, but fire alarm system design and installation engineers are not normally sufficiently trained to understand the implications of this and what you need to know to calculate it's possibility and how to compensate for it. I also believe that the layer of hot gasses from a fire that keeps smoke away from the very highest points in a space, would really affect this position if the area is small. My cross-section drawing obviously cannot show the full area and if this was a cross-section of an areaa that was, say, only as deep as it is wide, then it would provide a small 'funnel' typearea that the hot gasses would soon fill and prevent any smoke from reaching the detector. Obviously the suggestion of mounting the detector in 'mid-air' using something like a bit of uni-strut is a good idea that would reduce the problems of the detector being installed 'too high'.
In my opinion position B is second best because it also complies with the top 600mm recommendation but we do have the problem of detector orientation and the 500mm distance from wall recommendation.
In my opinion position C complies with the top 600mm recommendation but we again have the problem of detector orientation (even more pronounced) and the 500mm deistance from wall recommendation. I know that BS allows smoke detectors affixed to a wall in very strict circumstances. But these apply where a door to an exit route is below the detector. Unless we could apply the easement of this recommendation to our application (i.e if there was such a door and it was an L3 system etc. etc.) then this position is the least suitable of A, B & C.
In my opinion position D is the least acceptable. Whilst we are now 500mm away from the wall we are outside the top 600mm. In my opinion the 600mm recommendation must be more important than the 500mm. This opinion is based on the fact that there is are easements of the 500mm allowed (as mentioned in the paragraphs above) in BS, but never an easement of the 600mm under any circumstances, so could it not be considered the more important of the two recommendations?
Graeme mentioned the horizontal distance easements as something to be considered. Buzz replied that this only related to detectors in the top 600mm anyway. I would add that this easment only appears to affect the maximum detector spacing distances where sloped roofs are part of the considerations and BS makes no mention of it easing either the 600mm or 500mm recomendations in any way.
I believe that the problem highlighted in my original question is one that needs to be addressed by the BS committee.
I have my own opinions on the subject; It appears to me that the whole concept of the requirement for placing smoke detectors within 600mm of the highest point must relate to a calculation of the volume of smoke needed to fill the maximum space covered by a single detector that could be positioned as low as 600mm of the highest point i.e. if this was in a square room with sides of each approx. 14m long, the calculation of volume should be something like 14m x 14m x 0.6m = 117.6m3. Using this figure of 117.6m3 then surely, if a different space was, say, 7m x 7m then the detector could be placed up to 2.4m below the highest point to react just as quickly to the same volume of smoke, or if a different space was only 3.5m x 3.5m then the detector could be placed up to 4.8m below the highest point to react just as quickly to the same volume of smoke.
I appreciate my examples above might be pretty extreme and that calculations for spaces with sloped sides will be more complicated but surely the concept is right? Maybe the BS could say something like where the space containing the detector has a volume of less than, say 80m3, then the detector can be positioned up to, say 1.4m from the highest point of that space and thereby provide the greater liklihood of also being able to mount the detector also at least 500mm from a wall (using unistrut?)
I would appreciate more comment / opinions on all of the above.
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But Wiz, it is possible to meet a 600/500mm recommendation. It is impossible to meet a 600/500mm requirement.
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But nearlythere, It is impossible for me to work out if your reply is cryptic, sarcastic or humorous!
You would have to elaborate on your 20 word response to my 200,000 word post, for me to try to have any chance of understanding which it is!
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Location A would comply with 22.3 f assuming that the void is unventilated and not greater than 1.5M. .....
Dave, we are not specifically talking about a void, just a room with a funny-shaped roof. Even if we were, can you clarify the 1.5M bit of your answer, please.
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But nearlythere, It is impossible for me to work out if your reply is cryptic, sarcastic or humorous!
You would have to elaborate on your 20 word response to my 200,000 word post, for me to try to have any chance of me understanding which it is!
Its humorous Wiz and its Friday. Have a nice weekend.
PS. 200,000 words? Are you being sarcastic or what?
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....Its humorous Wiz and its Friday....
Well that's a relief - I thought you'd spotted something I'd missed!
PS. 200,000 words? Are you being sarcastic or what?
I'd like to say that I just can't count, but actually I'm prone to wild exaggerations!
Have a nice weekend yourself. If your are in the Banter Bar over the weekend I'll tell you the story about my role in the winning of the World Cup in 1966. ;)
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....Its humorous Wiz and its Friday....
Well that's a relief - I thought you'd spotted something I'd missed!
PS. 200,000 words? Are you being sarcastic or what?
I'd like to say that I just can't count, but actually I'm prone to wild exaggerations!
Have a nice weekend yourself. If your are in the Banter Bar over the weekend I'll tell you the story about my role in the winning of the World Cup in 1966. ;)
Yes. Nice goal I must say. Pity it went in the wrong net.
Cheers.
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But Wiz, it is possible to meet a 600/500mm recommendation. It is impossible to meet a 600/500mm requirement.
Madam Im Adam
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But Wiz, it is possible to meet a 600/500mm recommendation. It is impossible to meet a 600/500mm requirement.
Madam Im Adam
Sex at noon taxes
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Wiz
In your initial question you refered to a ''roof space'' , not a room, in a roof with sloping sides or a room with
an unusual roof configuration . A roof space that is not a room is a void, and the !.5 M reference is from BS 5838 pt 1 2002 + A2 :2008 ref as given. for unventilated voids up to 1.5M height.
Dave
ps in practice it does not matter where you put the heads, they will work!!
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Wiz
In your initial question you refered to a ''roof space'' , not a room, in a roof with sloping sides or a room with
an unusual roof configuration . A roof space that is not a room is a void, and the !.5 M reference is from BS 5838 pt 1 2002 + A2 :2008 ref as given. for unventilated voids up to 1.5M height.
Dave
ps in practice it does not matter where you put the heads, they will work!!
Thanks for this reply Dave.
firstly I'm assuming that you meant to type 5839, of course
My original post was meant to refer to a room with an unusual ceiling. It would have been better if I used the word ceiling but even BS sometimes uses the word roof when talking about detector placement on a ceiling! See Figure 6 of BS 5839 pt 1 2002 for example.
Terminology can confuse many issues! Sorry for the confusion.
I am not yet in possession of the 2008 version, so I can't yet clarify what you meant by your mention of 1.5M. Could you be so kind as to give me a fuller explanation of this recommendation so that I understand.
Cheers
Wiz
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ps in practice it does not matter where you put the heads, they will work!!
But just in case it didn't work, and someone accused me of choosing the wrong position. I might like to be able to justify my desicion as being based on a BS recommendation, rather than saying Thebeardedyorkshireman said it would be o.k. :). As far as I can see, the BS recommendations don't cover the situation in question.
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Wiz
Ping me your Email and I will scan the new rule in, rather than type it into the forum.
Dave