The reason that fire detectors are mounted at a maximum distance from a wall or between detectors is to cater for the time for smoke and/or heat to move across a ceiling to engulf a fire detector.
The radius of coverage of 7.5 m for a smoke detector is quoted as this provides a coverage of 112 m2 or rounded off to 100 m2. However as this is circular coverage, when laid out on a plan one has to overlap these circles to ensure full coverage of the whole ceiling resulting in reducing the horizontal distance between detectors to 10.6 m and 5.3 m from adjacent walls.
When installing smoke detectors, it is these distances that have to be used 10 m between detectors and 5 m from the wall.
These distances must not be used for heat detectors as their coverage of an area is much less. The coverage of a heat detector is quoted as 50 m2. The radius of coverage is 5.3 m but when catering for the overlap for full coverage this is reduced to 3.8 m and 7.5 m spacing.
When installing heat detectors, one must ensure 7.5 m between detectors and 3.8 m from any adjacent wall.
The above distances apply to the different types of detectors. For aspiration and beam detection use the smoke detector measurements and for linear heat detectors and heat probes use the heat detector measurements.
When installing muti-criteria detectors with both elements active one must use the heat detector spacing.
By Keith Norgate – FSIB Fire Systems