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- Written by: Keith Norgate
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FIRE DETECTORS - 1
OPTICAL SMOKE DETECTORS
The FSIB introduce a series on the several types of detectors available for the many and varied applications one might face.
Applications
The optical detector is installed to sense smoke and is used in entertainment areas, theatres, commercial buildings, exhibition centres, museums, hospitals, prisons, shops, warehouses, IT environments etc. in fact in all areas that are clean and free of dust and steam.
SANS 10139 recommends the use of optical smoke detectors for smouldering electrical, wood and furnishing fires as well as burning paper fires.
How does it work?
An optical detector consists of a pulsing infra-red LED light source and an infrared receiver obscured by means of baffle plates in a light proof housing which allows free ingress of smoke.
When smoke enters the detector chamber infrared light is reflected off the smoke particles and if seen by the receiver for two consecutive pulses it causes the detector to switch on. This is known as the light scatter principle.
Some manufacturers have diverse options to this, but the principles are all similar.
Where should we not use it?
One should avoid installing optical smoke detectors in very dusty areas, areas where steam may occur, wet or damp areas, where gasses occur, very high or low temperatures can be expected, or invisible smoke conditions may occur.
These areas include where cooking takes place, fumigation of food stores, freezers, areas where they wash down, parking garages with exhaust fumes, alcohol stores etc.
Where can we install it?
It must always be installed at the highest point where the smoke will always accumulate.
This can be any ceiling or roof and must be installed within 600mm of the roof or ceiling. It can be used inside ceiling voids and on the underside of raised floors.
The detector can cover an area up to 100m2 but must be within 5m of the walls and spaced 10m apart.
Where should we NOT install optical smoke detectors?
These detectors cannot be installed at a height in excess of 10.5m.
Must be kept clear of air conditioning outlets, obstructions such as ceiling beams and at least 300mm clear of storage racks.
Have you suffered false alarms?
If the optical smoke detector is to blame check for stray smoke conditions, air conditioning re-gassing, burning toast, fumigation processes, water ingress, steam from showers or boiling kettles.
Do not cover the detector or isolate it in software, replace it with a new one and monitor for a similar occurrence, if it alarms again investigate the environment or change it to an alternate type of detector.
Conclusion
A good general-purpose detector that can be used in many different applications.
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- Written by: Keith Norgate
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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.
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Changes were made in the British Standard in 2017 regarding positioning of fire beam detectors. These changes have been generally accepted in South Africa and are listed in the updated SANS 10139 shortly to be released by SABS.
Here we attempt to provide clarity on the positioning of fire beam detectors.
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- Written by: Keith Norgate
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KEEPING YOUR HAND ON FIRE DETECTION
Seen on a recent site, must have forgotten the SANS rule of 500 mm clearance around all fire detectors!
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One of the biggest failings we find in fire detection system installations during our inspection process is the lack of fire detection in the stairwells.
Even though fire detection designers/installers classify their systems as an L category system, fire detectors are missing from the stairwell.