Fire detectors. Factors Influencing Detector Positions and Numbers

A fire detector is a part of the fire alarm system and can trigger a transmitting device that raises the alarm in a remote control centre. There are automatic and nonautomatic fire detectors. The latter are those which can be activated manually.

Automatic fire detectors are parts of the overall fire alarm system that sense changes in specific physical and/or chemical parameters (either continuously or sequentially in set time intervals) to detect a fire within the monitored area. They must be:
- installed in sufficient numbers and be suited to the general arrangement of the area to be monitored
- selected according to the fire risk
- mounted in such a way that whatever parameter change triggers the alarm can be easily sensed by the detector.

Typical applications for different types of fire detectors
(1) Smoke detectors
These are used in rooms containing materials that would give off large volumes of smoke in the event of a fire.
- Optical smoke detectors: triggered by visible smoke.
- Ionisation smoke detectors: triggered by small amounts of smoke which have not been detected by optical means. These detectors provide earlier warning than optical smoke detectors and are suitable for houses, offices, storage and sales rooms.

(2) Flame detectors
These are activated by radiation emanating from flames and are used in rooms containing materials that burn without smoke, or produce very little.

(3) Heat detectors
These are useful for rooms in which smoke that could wrongly set off other early warning systems is generated under normal working conditions (e.g. in workshops where welding work is carried out).
- Maximum detectors: triggered when a maximum temperature is exceeded (e.g. 70°C).
- Differential detectors: triggered by a specified rise in temperature within a fixed period of time (e.g. a rise of 5°C in 1 minute).
The planning and installation of fire detection systems must be designed to suit the area to be monitored, room height and the type of ceiling and roofing.

Typical extracts from building regulations and guidelines produced by fire and insurance specialists. Fire development. If the initial phase of a fire is likely to be of a type characterised by smouldering (i.e. considerable smoke generation, very little heat and little or no flame propagation), then smoke detectors should be used. If rapid development of fire is anticipated in the initial phase (severe heat generation, strong flame propagation and smoke development), then smoke, heat and flame detectors can be used, or combinations of the various types.

Fire detection areas. The total area to be monitored must be divided into detection areas. The establishment of these detection areas should be carried out in such a way that rapid and decisive pinpointing of the source of the fire is possible. A detection area must only extend over one floor level (the exceptions to this being stairwells, ventilation and elevator shafts and tower type structures, which must have their own detection areas). A detection area must not overlap into another fire compartment and typically should not be larger than 1600m².

Fire detection systems for data processing facilities. The monitoring of electronic data processing facilities places special additional requirements on the planning and execution of fire alarm systems.

Factors influencing detector positions and numbers. (1). Room height. The greater the distance between the fire source and the ceiling, the greater the zone of evenly distributed smoke concentration will be. The ceiling height effects the suitability of the various types of smoke and fire detectors. Generally, higher ceiling sections whose area is less than 10% of the total ceiling area are not considered, so long as these sections of ceiling are not greater in area than the maximum monitoring area of a detector.

(2). Monitoring areas and distribution of the detectors. The number of fire detectors should be selected such that the recommended maximum monitoring areas for each detector are not exceeded. Some standards specify the maximum distance between detectors and the maximum distance allowed between any point on the ceiling and the nearest detector. Within certain limits there may be a departure from the ideal square grid pattern of the detectors.

(3). Arrangement of detectors on ceilings with downstanding beams. Depending on the room size, beams above a specified depth must be taken into account in the arrangement of the fire detectors. Typically, if the area of ceiling between the downstanding beams is equal to or greater than 0.6 of the permissible monitoring area of the detector, then each of these soffit areas must be fitted with detectors. If the portions of soffit area are larger than the permissible monitoring area, then the individual portions of soffit must be considered as individual rooms. If the depth of the downstanding beam is greater than 800mm, then a fire detector must be provided for each soffit area.

(4). For spaces with multi-bay type roofs. Generally in this case, each bay must be provided with a row of detectors. Heat detectors are always to be fitted directly to the ceiling. In the case of smoke detectors, the distances required between the detector and the ceiling, or the roof, depend on the structure of the ceiling or roof and on the height of the rooms to be monitored. In the case of flame detectors, the distances should be determined for each individual case.