Smoke Detector Siting

[wainy1]

Hi All

BS 5839-6, 11.2 (i) states "Detectors should preferably be mounted on ceilings and should be located at least 300 mm horizontally from any wall or light fitting unless, in the case of light fittings, there is test evidence to prove that the proximity of the light fitting will not adversely affect the efficiency of the detector"

While BS 5939-1, 22.3 (g) states "Heat and smoke detectors should not be mounted within 500 mm of any walls, partitions or obstructions to flow of smoke and hot gases, such as structural beams and ductwork, where the obstructions are greater than 250 mm in depth. (This recommendation does not apply to detectors within rooms opening into escape routes in a Category L3 system)"

Does anybody know the reason for the inconsistency?

[SamFIRT]

Excuse me? What inconstancy? *confused* 

[John Webb]

I believe the difference is due to the two different situations covered by the two parts of the BS. 5839-6 is essentially for domestic detection where there are natural limitations on ceiling heights and room sizes, and where there are less likely to be deep beams or other obstructions. Domestic detectors also tend to be larger in diameter and in height compared to the detectors used in 5839-1 systems. The latter systems are usually used in commercial and industrial premises where there is a greater range of room sizes, ceiling heights and greater likelihood of obstructions across the ceiling including beams, ductwork and large light fittings.
So I don't think it's inconsistency - just 'different horses for different courses', as the saying goes.

[wainy1]

Excuse me? What inconstancy? *confused* 

Hi Sam FIRT

Part 6 specifies 300mm from the wall and Part 1 specifies 500mm

[wainy1]

I believe the difference is due to the two different situations covered by the two parts of the BS. 5839-6 is essentially for domestic detection where there are natural limitations on ceiling heights and room sizes, and where there are less likely to be deep beams or other obstructions. Domestic detectors also tend to be larger in diameter and in height compared to the detectors used in 5839-1 systems. The latter systems are usually used in commercial and industrial premises where there is a greater range of room sizes, ceiling heights and greater likelihood of obstructions across the ceiling including beams, ductwork and large light fittings.
So I don't think it's inconsistency - just 'different horses for different courses', as the saying goes.

Thanks John
I did consider the differences in the general layouts, my understanding has always been the minimum distance between the wall and the sensor was to prevent the sensor being located in a dead space with little air circulation leading to a delay in activation. 
The matter which has prompted my query: I am in receipt of a number of periodic inspection reports where the contractor has identified a number of locations where the detector is closer than 500mm from the walls (most of these being the individual rooms within a halls of residence where the sensor is located centrally behind the entrance door) the contractor has raised them as a code 2 deficiencies. I am in the process of prioritising improvements to the Fire Alarm system and am doubtful whether moving the detector a few centimetres will result in any significant improvements. 

[SamFIRT]

Is it possible the word ceiling is missing from the sentence in BS 5839-6, 11.2 (i) and it is talking about wall or ceiling mounted light fittings? Just a thought?