Technical Guidance Note 22: Automatic Fire Detection & Alarm Systems to BS 5839 Part 1

Categories of System

Category  L - Systems are automatic fire detection systems intended for the protection of life.  They are further subdivided into:

1. L1 Detectors are installed throughout all areas of the building.
   
   
2. L2 Detectors are installed only in defined parts of the building. (an L2 system should normally include the coverage required of an L3 system).
   
   
3. L3 Detectors are installed only for the protection of escape routes.
   
   
4. L4 Detectors installed within those parts of the escape routes comprising circulation areas and circulation spaces, such as corridors and stairways
   
   
5. L5 Systems in which the protected area(s) and / or the location of detectors is designed to satisfy a specific fire safety objective (other than that of a Category L1, L2, L3 or L4 system)

Category M - Manual alarm system consisting of manual call points. Suffix X denotes systems intended for use in multiple use buildings.

Category P - Systems are automatic fire detection systems intended for the protection of property. They are further subdivided into:

1. P1 Detectors are installed throughout all areas of the building.
   
   
2. P2 Detectors are installed only in defined parts of the protected building.

NB Under flat ceilings, the horizontal distance between any point in a protected area and the detector nearest to that point should not exceed:

1. 7.5 metres if the nearest detector is a smoke detector
   
   
2. 5.3 metres if the nearest detector is a heat detector

Zones

1. The floor area of a single zone should not exceed 2000m².
   
   
2. The search distance should not exceed 60m.
   
   
3. If the total floor area of a building is 300m² or less, a zone may cover more than a single storey.
   
   
4. If the total floor area is greater than 300m², each zone should be restricted to a single storey (refer to BS 5839: Pt 1 for exceptions).
   
   
5. Where a zone extends beyond a single compartment, the zone boundaries should be boundaries of fire compartments, i.e. two compartments in one zone, or two zones in one compartment.
   
   
6. In systems containing only manual call points, provision should be made for information relating to the position of the fire to be passed to the person in charge.
   
   
7. For systems in buildings in multiple occupation the zoning arrangement should take into account the fact that the premises may not be completely occupied at the same time, therefore no zone should include areas in more than one occupancy.
   
   
8. In very large systems the number of zones may need to be restricted.  Zones can be compatible with the evacuation procedures.
   
   
9. In premises having phased evacuation schemes, the zoning of smoke/fire detectors should be compatible with the evacuation procedures.

High risk areas should be considered as separate zones.

Siting of Manual Call Points

Call points may be connected to the detector circuit within the zone that serves the adjacent accommodation (see clause 13.2.1 of BS 5839).  The delay between operation of a call point and the giving of the general alarm should not exceed three seconds.

Manual call points should be located on escape routes and in particular at all storey exits and all exits to the open air.  The travel distance to a call point should not exceed 45m, however when specifying the travel distance between call points consideration should be given to the type of occupancy, and the fire risk attached to each zone.

Call points should be sited against a contrasting background at a height of 1.4m above floor level.  The call point should be well illuminated, free from obstruction and should be surface mounted or only semi-recessed with the front face proud of the mounting surface by no less than 15mm.

Types of Fire Detector

• Smoke Detectors

Ionisation chamber smoke detectors: These are based on the principle that the electrical current flowing between electrodes in an ionisation chamber is reduced when smoke particles enter the chamber.  These are particularly sensitive to smoke containing small particles which are produced in rapidly burning flaming fires, but may be less sensitive to the larger particles found in optically dense smoke, which may be produced by smouldering materials.

Optical smoke detectors: These operate by detecting the scattering, or absorption of light by smoke particles.  These are sensitive to the larger, optically active particles found in optically dense smoke.

Smoke detectors cannot detect the products of combustion from clean burning liquids (such as alcohol).  Detectors should be fitted not less than 25mm or more than 150mm below a ceiling, or roof.  The general limit on ceiling heights for point smoke detectors is 10.5m, however see BS 5839 for further details.  Voids less than 800mm in height need not have independent coverage.

• Heat Detectors

There are two main types of heat detector:

1. Point detectors respond to the temperature of the gases in the immediate vicinity of a single point.
   
   
2. Line detectors respond to the temperature of gases in the vicinity of a line (this is a specialist device and not considered further in this document).

Point detectors can be categorised as follows:

1. Detectors using fixed temperature (static) elements operate when they reach a pre-selected threshold temperature (operating temperature usually set 30^oC above maximum ambient temperature).  These should be used where ambient temperatures are likely to fluctuate rapidly over short periods of time, e.g. kitchens.
   
   
2. Detectors using rate of rise temperature elements operate when the temperature rises at an abnormally fast rate.

NB Heat detectors complying with BS EN 54-5 operate in the manner described above.  Heat detectors that operate only when the rate of rise of temperature is abnormal, but not when a fixed temperature is reached, do not comply with BS EN 54-5 and ought not to be used;  they can fail to detect a slowly developing fire.

Combustion Gas Detectors

Combustion gas detectors are point-type detectors that respond to one (or more) of the gases produced by a fire.  For example, carbon monoxide is produced when incomplete combustion occurs as a result of restriction of the amount of oxygen available to support the combustion process.

Electrochemical sensors within combustion gas detectors have a finite life, after which replacement is necessary.  It is important that the user is made aware of the likely lifetime of any combustion gas detector used within a fire detection and alarm system.

Flame Detectors

These devices detect ultraviolet and/or infrared radiation (specialist use, e.g. surveillance of large open areas, or critical areas where a fire could spread very rapidly).

Multi-Sensor Fire Detectors

In a fire detection system incorporating multi-sensor detectors each multi-sensor fire detector contains more than one sensor, each of which responds to a different physical and / or chemical characteristic of fire.  The purpose of combining sensors in this way is to enhance the performance of the system in detection of fire, or its resistance to at least certain categories of false alarm, or both.

The purpose of this document is to provide the reader with basic guidance on Fire Detection and Alarm systems for buildings.  For full guidance, reference should be made to the current BS 5839: Part1.

Building Control Group
Planning Section
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Worthing
West Sussex
BN11 lLF

Tel: 01903 221344

E-mail: building.control@worthing.gov.uk

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