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FIA Technical Briefing

by Brian Sims

When selecting a detector there can be a number of issues that arise from choosing the wrong type of detector for the area it needs to protect from fire. For those that perhaps have less experience in this area, it can become confusing. Do you use an optical smoke detector, ionisation or a multi-sensor detector? As Bernard Laluvein points out, errors can easily be avoided by following the standards set out in BS 5839-1:2017.

Commonly, the main problems that people face when selecting a detector are to do with ensuring that the detector’s suitable for the fire risk present and, also, that it doesn’t respond to sources of non-fire phenomena. Doing so can give false alarms, which negatively affect people’s trust and reliance in the fire detection and fire alarm system (a fact that, unfortunately, may mean some degree of reluctance to react when the alarm does sound).

Selection of the detector depends on a number of factors, such as the type of building and the location of the detector. That said, these are not the only key factors involved in the process.

During the design of a fire system, consideration should be given to the fire risk(s) and the false alarm risk(s). Fire detectors are selected depending on the nature of the protected area and the risks associated with it. Often, this involves using different fire detector technologies in order to meet the diverse needs of both fire detection and false alarm rejection in each scenario.

Single-sensor detectors (eg optical smoke detectors and heat detectors) are suited to the detection of certain fire risks. Selection choices need to be made carefully in order to optimise the detection of fire, but also minimise the occurrence of false alarms. The deployment of multi-sensor detectors means that the system could potentially detect fires earlier and yet avoid troublesome false alarms to a greater extent as well.

Fire system design

The choices and decisions concerning which detector types to use are initially made during fire system design, but are also sometimes revisited – and even modified – during commissioning and/or maintenance. They’re not always well recorded, and neither is the rationale behind them. At a later juncture, for example during a maintenance visit, this can give rise to confusion. A clear understanding of the original reasons behind the choice of detectors and how they’re configured is needed in order to verify continued suitability.

Aside from multi-sensor detectors, a visual inspection of a fire detector can often provide sufficient information to a maintenance technician to allow in situ functional testing to be undertaken (eg in the case of a smoke detector or a heat detector).

However, in the case of multi-sensors, the type of detection employed within it and its settings, modes or specific configuration which govern its detection performance to the risk are not always apparent from a visual inspection. This information may not even be available from the CIE.

In all cases, however, there remains the need to know the detection principles and to understand the reason for the choice of that detection relative to the risks perceived in the protected area. 

Affects on the overall system

Certain types of detector are designed to be very sensitive to certain products of fires. For example, optical smoke detectors respond very quickly to dense smoke that may reduce visibility in escape routes. Although this can be seen as a good thing, such detectors might also respond to non-fire products such as vapours or steam from kettles or showers. This will be regarded as undesirable false alarms or, as far as the Fire and Rescue Service is concerned, unwanted fire signals – a cost that’s estimated to be around £1 million per annum.

To avoid false and unwanted alarms, selecting the right detector can mean all the difference. By way of example, a warehouse may be a particularly dusty environment from the materials that are stored or manufactured within it.  In this instance, an optical smoke detector is unlikely to be the best type of detector to use since they’re designed to detect small particles within smoke. As such, they’re likely to produce unwanted alarms.

Designers must therefore consider carefully the types of false alarm risks present before selecting a detector.

Process of selection

The process for deciding the right type of detectors is a two-stage exercise. First, it’s important to analyse the fire risk and, from that, select the best detectors for detecting that type of risk.

Fire risks can be defined as a material, substance or action that increases the likelihood of an accidental fire occurring. For example, electric wiring is a potential fire risk, which when burning produces smouldering white smoke. Using multi-sensor detectors or optical detection would be the best type in this situation, though it needs to be considered against the second step.

That second step in the process is to consider which non-fire phenomena are likely to arise within the specific areas to be protected – whether this is steam, smoke, dust, high ambient air flow or rapid thermal change. The type of detector selected can then be refined to ensure the correct balance between detection and avoidance of false alarms.

However, if you’re not familiar with the types of risks and the protection that different detectors offer, this may be difficult to evaluate. Thankfully, there’s help available from the Fire Industry Association (FIA).

Assistance in the selection process

The FIA has created a document entitled ‘Fire Alarm Detector Applications and Documentation of the System’ to assist with the process of selecting the right type of detector based on false and unwanted alarm risks.

Initially created as part of the FIA’s input into the revision of BS 5839-1 (published in 2017), the organisation subsequently updated the document to ensure that detection technologies not included in the original text were also taken into account. Specifically, aspirating smoke detection, line type heat detection and beam detectors were added.

The document is designed to help people choose the correct detection technology for use in specific situations, taking into account the fire risk present as well as possible sources of false alarms. This document is intended to assist with the task of selecting and clearly recording the type, sensitivity and settings selected for all detectors (including multi-sensor detectors) relating to the perceived risks.

The guide begins with a simple, easy-to-use flowchart to help aid the selection process and solidify each step since there are other factors beyond considering the fire risks and the risks for false alarms mentioned here.

In addition, the document also includes a Detector Selection Table. It’s a template to allow the documentation of the main risks and all decisions and reasoning behind the detection choices made, either during system design or at the time of any subsequent change to the fire detection. It’s intended to assist designers, commissioning engineers and service/maintenance technicians to keep good records of their detection choice decisions and rationale.

False alarm risks

Bernard Laluvein

Bernard Laluvein

Since fire risk matters as well, only detector types best suited for a particular risk should be used to detect the potential fire. To help with the selection, the document also has a table about false alarm risks (ie steam, smoke, dust, sparks or naked flames or rapid thermal change). The table is given to assist the system designer in the process of assessing which types of detectors are most likely to give false alarms in given situations. This will enable the designer to be more aware and end up with a more informed decision.

By following the guide carefully – and using it in conjunction with the similar information given in BS 5839-1 – fire detection and alarm system designers should be able to provide buildings with a high level of fire protection.

If you wish to access the guide, it’s available to download free of charge from the FIA’s website. Visit www.fia.uk.com

Bernard Laluvein is Director of BEH Laluvein Consulting Ltd and Chairman of the Fire Industry Association’s Fire Detection and Alarms Council

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