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FIRE DETECTORS-3
MULTI CRITERIA DETECTORS
The FSIB introduces the third in a series, on types of detectors available for the many and varied applications one might face.
Multi criteria detectors
A multi criteria detector is one with two or more different fire detection elements within the same casing.
The use of multi-sensor detectors can help to reduce false alarms from common causes such as steam, dust, cooking, and aerosol sprays.
The more sophisticated the optical / heat multi-sensor, the less prone they are to common causes of false alarms, whilst their ability to detect real fires is not compromised.
Multi criteria detector types.
Multi Criteria, or multi sensors as they are sometimes called, are available in many configurations. Both addressable and conventional types are available.
The most common is the optical smoke sensor partnering a heat detector, but many other variations can be found by adding a CO element or infra-red device.
Some of these devices work independently, acting as a smoke detector or heat detector but others need a combination of smoke and heat to operate. Some provide a facility to programme the various options available including elevated levels of smoke or low levels of smoke.
Applications
Multi criteria detectors are installed where one may expect installation at places where abnormal environmental conditions exist e.g. where smoke, steam or dust occurs.
One may not be sure if a heat or smoke type fire will occur.
Sometimes one may want to be able to adjust the sensitivity of a device but still sense a real fire.
Multi criteria detectors can be used to combat troublesome alarms.
SANS 10139 recommends the use of multi criteria sensors for smouldering fires.
Typical applications would be electrical substations, laboratories etc.
How do they work?
The multi criteria detection device is capable of generating multiple alarm signals from any one of the sensors employed in the design, independently or in combination.
The sensor output signals are mathematically evaluated to determine when an alarm signal is warranted. The evaluation can be performed either at the detector or at the control unit for addressable devices.
All sensor signals will be continuously weighted and combined by the internal electronic evaluation system. If a signal combination matches the stored fire signal pattern within the detector, an alarm will automatically be triggered.
The method of combining the signals from each sensor varies between different proprietary multi sensor fire detection systems.
With some multi-sensor fire detectors, it is possible to disable an individual detection sensor. For example, with a heat and optical multi-sensor detector, the optical sensor might be disabled during the day, if the room is to be used as a smoking room.
It must be noted that some multi criteria detectors will activate with a large amount of smoke only or a large amount of heat only.
Due to the large amount of variations provided with multi sensors it is advisable to discuss your application with the product supplier.
Where should we NOT use it?
The standards do not recommend using multi sensors in electronic equipment environments due to the heat element.
Using multi sensors in corridors is also not recommended as it is smoke that is the danger therefore optical detectors will perform more efficiently.
Where must it be installed?
It must always be installed at the highest point where the heat or smoke will always accumulate.
This can be any ceiling or roof and must be installed within 150mm of the roof or ceiling due to the presence of the heat detection element.
For conventional detectors they work on a OR basis, either smoke or heat will activate the device therefore they must be within 3.8m of the walls and spaced 7.5m apart providing up to 56m2 coverage.
For multi criteria sensors that are being used for smoke detection only, one can revert to maximum 100m2 coverage and 5m spacing. i.e. 10m apart.
Have you suffered false alarms?
As multi criteria sensors are used where false alarms occur, false alarms are rare for multi sensors and unwanted alarms should be thoroughly investigated to determine the cause of the alarm.
Conclusion
A good detector for use where abnormal environments occur and are generally used in electrical substations or laboratories where diverse types of testing take place.
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FIRE DETECTORS-2
POINT TYPE HEAT DETECTORS
The FSIB introduces the second in a series on types of detectors available for the many and varied applications one might face.
Heat detectors
Heat detectors are a valuable detector in the fire detection designer’s armoury. They come in various forms including conventional, addressable, linear and heat probes. For this edition we are concentrating on point types of heat detection. Conventional and addressable operate in the same manner.
Heat detector types.
There are two main types of heat detectors on the market Rate of Rise heat detector or Fixed Temperature detector. Both being available as conventional or addressable types.
If you order a heat detector from your supplier, he will in most cases issue you with a rate of rise heat detector.
Rate of Rise Detector
Rate of rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detection earlier.
A rate of rise detector monitors a gradual build-up of heat in the protected space i.e.: 15oC / minute.
Should the temperature in the room rise at a rate of 15oC in one minute or faster the detector will
operate.
Fixed temperature Detector
Fixed heat detectors only change to the alarm state at their preset temperature.
Applications
Heat detectors are installed to monitor for high temperatures at the ceiling/roof level. They are normally used where smoky, steamy, or dusty environments can be experienced.
SANS 10139 also recommends the use of heat detectors for flaming fire where little smoke will be experienced.
Typical applications would be alcohol stores, kitchens, parking garages, electrical substations, oil stores.
How do they work?
The rate of rise detector operates by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions, the two thermistors register similar temperatures, but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance of the thermistors and causing the detector to change to the alarm state.
Addressable detectors use a single thermistor to sense the air temperature at the detector position.
As temperatures rise the fire panel monitors the change until it reaches the preset parameters and then raises the alarm.
The fixed temperature detectors use a single thermistor set at a specific temperature. When the temperature surrounding the detector reaches this level, it will raise the alarm.
There are a range of detectors monitoring for temperatures between 25o to 90oC.
Where should we not use it?
The standards do not recommend using heat detectors to protect corridors due to its late alarm compared to a smoke detector.
Heat detectors should not be installed in bedrooms, hospitals, clean rooms, IT environments or for actuating gas protection systems.
Where must it be installed?
It must always be installed at the highest point where the heat will always accumulate.
This can be any ceiling or roof and must be installed within 150mm of the roof or ceiling.
The detector can cover an area up to 56m2 but must be within 3.8m of the walls and spaced 7.5m apart.
Have you suffered false alarms?
The heat detector is less prone to false alarms than smoke detectors. If the device itself goes faulty you will receive an alarm at the fire panel.
Although false alarms are rare, heat detectors can be subject to nuisance alarms especially if positioned near ovens or furnaces.
Conclusion
A good detector for dirty. dusty or humid environments, commonly used in kitchens and garages.
As the heat detector is recognised as slow to operate, designers should only choose a heat detector if there are no other suitable alternatives.
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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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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.