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Fire Safety for Facilities Management Personnel – Fire Detection & Fire Alarms – Part 200

May 9, 2023 11:29 am

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 199, LWF discussed the types of fire detection and fire alarm systems. In part 200, we discuss analogue addressable fire alarm systems.

An analogue addressable fire alarm system has detectors which perform as sensors and transmit a signal level relating to the value of the heat, smoke or flame (depending upon the type of detector) in that moment. The interpretation of the signal is undertaken by the control and indicating equipment (CIE) as the detectors contain no circuitry to do so themselves. In the most basic analogue systems, the CIE has four indicative states for a detector – normal, fault, pre-warning and fire.

The fault signal is given when the signal from a detector is very low, which indicates a lack of sensitivity. The pre-warning would be triggered when a signal level had risen above normal, but had not reached the level indicative of fire.

In some systems, the thresholds are able to compensate for changes in the environment and pollution of the smoke detectors. The CIE may be able to analyse other data before determining the signal means there is a fire. For example, the rate of rise of the signal, or mathematical analysis of various parameters of the signal.

The most basic analogue systems were designed this way because, in the past, any element of processing power involved a sizeable processor and thus, a central point in which to process. These days, a large amount of processing power can be available in a very small package and the application of fixed thresholds and an element of signal processing can be undertaken by each detector head. The detector head then only needs to transmit specific conditions, such as the four states described above. Such processing detector heads are multi-state detectors.

The pre-warning state is particularly useful in avoiding false alarms in analogue and multi-state systems. A pre-warning indication will not trigger a fire alarm, but rather a warning at the CIE so that further investigation of the area can take place and potentially, a fire can be avoided.

In part 201 of this series, LWF will continue to talk about analogue fire detection and alarm systems. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.


Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 35 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.


While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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