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.
