Selection of smoke detectors is an invariably complex job and requires a detail study of various influencing features and factors. Single or multiple types of detectors may be required to be used.
A well laid out detection system can perhaps do nothing to reduce the incidence of fire but it can certainly help in lessening the resultant damage by reducing the delay between ignition and the start of effective fire fighting. Effective working of heat and smoke detectors depends on the convective movement of combustion products from the fire to the detector. Mere selection of the detectors will not be effective unless they are placed and spaced properly with respect to others.
The spacing and placing requirements for these detectors are based on the need to restrict the time taken for the movement of combustion products to to reach the detectors fitted in the ceiling in adequate concentration, within a reasonable time lag.
Factors to be consider when design such installation:
Height of ceiling, type of ceiling (i.e. flat, joist / beamed and false type), internal partition, air condition / ventilation system, electric cable tranches and cable trays, rack storages and other features like lift shifts, stare wells, utility shafts, tend to divert smoke and collect it in high unattended top areas and need the provision of detectors. Similarly, AHU room, supply and return air duct need to be paid a special attention.
i) Roof corner placement: Avoiding blind spot 4″ from side wall and ceiling jet to be consider 4″ to 12″
ii) Smooth ceiling: From wall S max, spacing (S max)
iii) Distance from any corner: 0.7 S max
iv) Spacing for unusual wall configuration: Symmetrically from the corner at a distance of 0.7 S max and the spacing of other areas should be re adjusted as per the S max applicable
v) Spacing in the corridor: Sc =√2 S max-Wc (this formula may be preferably be used for the corridor whose width is not less than 3 meters
vi) Spacing for peaked ceilings: A detector must be placed within 300 mm of the roof peak, spacing of detectors placed on the roof must be measured along the floor and not along the slope
vii) Provision for floor mounted partitions: If the distance between the top of floor mounted partitions from ceiling is more than 460 mm, the presence of partition may be ignored and if it is less than the length of the partitions, it may be treated as if it has been raised up to the roof
viii) Provision inside false roof / floor / void: If such concealed spacing is more than 800 mm deeo, the same shall be individually protected on their own merit. If the depth is less than 800 mm, the detector need not be provided
Reference to read NFPA 7296-02, IS:2189-1996