When selecting detectors for a fire alarm system, you must give equal consideration to optimising fire detection and minimising the occurrence of false alarms. Most false alarms are down to poor system design and incorrect detector selection, and the risk can be significantly reduced by careful design and advances in technology.
Detectors are selected according to the nature of the area where their use is planned and any associated fire risk. Often, different technologies are used to ensure that fire is detected while false alarms are avoided. Detectors themselves are designed to identify one or more of the four characteristics of fire: smoke, heat, combustion gas – such as carbon monoxide (CO) – and infrared or ultraviolet radiation.
In some fire detection and alarm systems, an alarm signal is initiated when one of these characteristics reaches a predetermined threshold. In other situations, a signal may be initiated instead, or as well, when the rate at which this characteristic changes is indicative of a fire.
With point detectors, smoke, heat, gas or radiation are monitored at a defined spot in the protected area, whereas line detectors recognise these qualities along a specified line in that space.
Smoke detection is the most common type of technology used in automatic fire detection systems, and can take the following forms:
Heat detectors take the form of either point or line detectors,. and may be designed to respond when a fixed temperature is reached. The heat detector may also include a sensor that responds to the rate of rise of temperature. Combustion gas detectors in turn are point detectors that respond to one or more of the gases such as CO, that are produced when combustion is incomplete due to a restricted amount of oxygen.
Flame detectors detect the infrared or ultraviolet radiation emitted by flame, or both. Each kind uses radiation-sensitive cells that identify the fire directly or through built-in lenses or reflectors.
Some fire detectors contain more than one sensor, each of which responds to a different characteristic of fire. Such multi-sensor devices can be used to enhance detection of fire and reduce the risk of certain categories of false alarm. Video is another means of detection, which monitors the protected area and analyses images electronically to identify the presence of smoke or flames from changes in the camera's field of view.
The choice of fire detector types should ideally, be made early in the design stage to support the fire safety strategy. Sometimes this choice is revisited and modified as the building design evolves, or during installation and commissioning, or periodic maintenance thereafter. Such modifications are not always clearly documented, however; neither is the rationale behind their selection at the design stage or later. But it is absolutely essential that the reasoning for the selection and configuration of the detectors is clearly documented, and likewise the justification for any changes to them throughout the lifetime of a building, in order to verify continued suitability.
SOURCE: SANDLAND-TAYLOR CONSULTANCY
To help the selection and verification of detection the methodology introduced in annex E of BS 5839-1: 2017 Fire detection and fire alarm systems for buildings: Code of practice for design, installation commissioning and maintenance of systems in non-domestic premises can be used.
Another way to ensure the correct detection technology is selected is to complete a template form for each detector. A simplified version with several examples is provided in Table 1; the factors set out in the left-hand column should all be considered, so that the completed version can be used as a live document. Tables 2 and 3 on typical fire and false alarm risks can also aid completion of the selection table.
Selection table examples: commentary
1. Hotel room with en-suite shower: it is acceptable to install heat detectors in bedrooms; the objective in such a case is to warn all occupants before the integrity of the bedroom door is threatened by fire. Some hotel chains, however, prefer to have smoke detection fitted to warn them that a guest is smoking in a bedroom, as this enables them to take prompt action. Such smoke detection provision would inevitably increase the risk of false alarms, so it is not a decision to be taken lightly; however, providing a combined optical-heat detector might be a suitable compromise. The rationale for this kind of decision must be clearly documented in the selection table.
2. Disabled persons hotel room with en-suite shower: in this example, the hotel bedroom has smoke detection to give the earliest possible warning for disabled persons, and thereby afford them more time to escape in the event of a fire. Ionisation smoke detection is chosen in preference to optical technology to reduce the risk of false alarms from steam produced by the shower or a kettle.
3. Small kitchen with toaster and microwave: this is an example of a typical food preparation area provided in many workplaces. In this situation, ionisation or optical smoke detection is not recommended because of the risk of false alarms caused by the production of steam and smoke particulates. A low fixed-temperature heat detector with a setting that can pick up a sudden rate of rise would be most desirable in these circumstances.
4. Commercial-type kitchen: this particular scenario has the potential for three false alarm risks: smoke and steam production as well as rapid thermal change caused by opening of oven doors. A heat detector with the highest setting would strike an ideal balance between fire detection and false alarm reduction.
5. Escape stairs in protected stairway: maintaining the integrity of protected stairs is essential to ensure a safe means of escape. As these are fire-sterile areas, the risk of fire in a stair enclosure should be very low. Consideration should be given to the possibility of smoke entering the protected stairway. Either ionisation or optical smoke detectors would provide an early warning in this event, with the latter being the preferred choice as they would detect any smoke that might obscure people's vision and make escape more difficult.
6. Boiler room: smoke detection would not be suitable due to the presence of dust and dirt. Room temperatures are likely to be relatively high, and could fluctuate from time to time. A heat detector that has a high fixed-temperature setting would mitigate against potential false alarm risks. If temperature fluctuations are less significant, then a heat detector with combined high fixed-temperature and rate of temperature rise settings could suffice.
7. Data storage room or suite: loss of a data storage room or suite could devastate a business, and therefore rapid detection of fire is critical in such a situation. Aspirating smoke detection is up to 100 times more sensitive than traditional point smoke detectors, and can identify smoke even before it is visible to the human eye. Specialist advice should be sought for such an application, however.
8. Sawmill: in this scenario, a CO detector would be most suitable to prevent smouldering in the early stages from developing into a major life-threatening fire. In such an environment, a smoke or heat detector would only be activated once a significant fire had taken hold.
Simon Sandland-Taylor is director and owner of Sandland-Taylor Consultancy email@example.com
Related competencies include: Fire safety