The LWF Blog

Facilities Management – Types of Fire Detection & Fire Alarm System – Part 15

September 14, 2016 11:41 am

In our recent blog series for those who work in Facilities Management and who have an interest in or responsibility for fire safety, we have been looking at fire detection in buildings and the different types of detector. In part 14 of this series, we began to discuss the two different types of fire detection and fire alarm system – conventional (or non-addressable) and addressable.


These two types differ in the method that the detectors and call points communicate with the Control and Indicating Equipment (CIE).


When a fire occurs, a conventional system is unable to identify the precise detector which has detected the fire and can only indicate a zone to the CIE. As this could theoretically make pin-pointing the location of a fire difficult, the size of a zone is restricted and the details laid out in BS 5839:1.


An addressable system is able to provide the location of the detector which was triggered at the CIE. The CIE will also display the zone in which the detector sits, however these zones are not circuit-based, but simply a part of the software.

With addressable systems, the size of a detection zone is also limited by BS 5839:1, but a circuit can serve many different zones and it is possible to link more devices on a single pair of conductors than would be possible with a conventional system.


An addressable system can be further broken down into three categories – two state, multi-state and analogue. A two-state detector works in the same way as a conventional system detector, with the difference being that the CIE is able to isolate and identify the signal by detector. However, a two-state addressable system is rarely seen in practice, with the majority falling into the analogue category.


An analogue detector is not designed to analyse findings, but simply to be a sensor which transmits a continual signal level which corresponds to the value of the phenomenon they are monitoring, e.g. heat, smoke or flame. The CIE interprets the data received and with that can give information to the user. In a simple analogue system, the CIE might report a detector to have one of four states – normal, fault, pre-warning and fire.


A normal state indicates that the detectors are functioning within normal parameters, the fault signal would trigger if the signal level became low, which indicates a lack of sensitivity requiring attention or maintenance. A pre-warning signal would be given if levels indicated that smoke/heat/flame was above normal level but not yet classified as a fire (giving an opportunity to investigate to the operator) or the final option is fire, which indicates of course that a fire is in progress.


The CIE may also be able to ‘make decisions’ based upon other processing functions or algorithms, such as the rate of increase or analysis of other parameters.


In our next blog, we will continue looking at conventional and analogue systems and how they work. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact Peter Gyere in the first instance on 0208 668 8663.


Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.


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