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.