Procedure & Guidance Fire Alarm Systems

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Procedure & Guidance

Fire Alarm Systems


- NFG002

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This Note for Inspecting Officers provides guidance on the standards that are considered appropriate in respect to fire alarm systems.
The Note aims to set general principles and to provide the reader with information as to which approved document or technical standard is considered by this Authority to be appropriate.
Officers are expected to use their judgement to ensure that any fire alarm and detection system they require, or recommend, is reliable for the level of fire risk involved. A clear distinction should be drawn between systems intended for life safety and those designed solely for property protection. 
For existing premises in West Yorkshire, Inspecting Officers will adopt the standards for fire alarm systems advocated in the guidance notes issued by the DCLG, i.e.
Entry Level Guide - A short guide to making your premises safe from fire

Guide 1 Offices and shops

Guide 2 Factories and warehouses
Guide 3 Sleeping accommodation

Additional guidance for domestic flats is available in the Local Government Group publication Fire safety in purpose-built blocks of flats.
Guide 4 Residential care premises

NASHiCS and CFOA felt that additional guidance was required for use with some areas of the government’s guide No4. The additional guidance is available on both the CFOA and NASHiCS websites so officers can download and print copies at their convenience. This guidance covers a number of topics including: evacuation of a protected zone in two and a half minutes, the issue of residents who are unable to be evacuated and staff remaining with them, the use of external fire escapes, the travel distance to evacuation routes and the use of by-pass routes.
Guide 5 Educational premises
Guide 6 Small and medium places of assembly
Guide 7 Large places of assembly
Guide 8 Theatres and cinemas
Guide 9 Outdoor events
Guide 10 Healthcare premises
Guide 11 Transport premises and facilities
Guide 12 Animal premises & facilities
Means of escape for disabled people (Supplementary guide)

For new systems, and for certain more complex premises, other guidance documents will be used as appropriate. For example:

British Standards BS 5839: Part 1 "Code of practice for system design, installation and servicing"
BS 5839: Part 6 "Code of practice for the design and installation of fire detection and alarm systems in dwellings"
BS 58391: Part 8 "Code of practice for the design, installation and servicing of voice alarm systems",

Although BS 5839: Part 6 is primarily concerned with the installation of fire alarm systems for a range of domestic properties, the recommended standards may be suitable and appropriate for some small workplaces

Smoke Detectors and Smoke Alarms (Ionisation V Optical)
There have been a number of research projects recently that have examined the effectiveness of smoke detectors. As specifiers of automatic fire alarm systems, we should be aware of the advantages and disadvantages between the main two types of smoke detectors available on the market (i.e. Optical and Ionisation).
Optical (photoelectric) smoke detectors operate by detecting the scattering, or absorption, of light by smoke particles inside a detection chamber. In ionisation detectors, an electric current flowing through an ionisation chamber is reduced when smoke particles enter it.
Ionisation chamber smoke detectors are particularly sensitive to smoke containing small particles such as are produced in rapidly burning flaming fires. However, they may be less sensitive to the larger particles found in optically dense smoke, which may be produced by smouldering materials. Ionisation chamber detectors may be more appropriate type for installation in rooms, such as the living room or dining room, where a fast-burning fire may present a greater danger to occupants than a smouldering fire.
Optical smoke detectors are sensitive to the larger, optically active particles found in slow burning or smouldering fires (a most important consideration taking account of the introduction of many flame-retardant materials currently used in fabrics, furnishings and building products). Further general advantages of optical smoke detectors include their stability when subjected to a wide range of physical and climatic variations. In areas where air movement, temperature change, humidity and changes in air pressure are likely to occur, optical smoke detectors can be expected to remain more stable whilst reacting to the majority of fires
Optical detectors are also more sensitive to smoke that has travelled some distance from the seat of the fire, during which time smoke particles have cooled and coalesced to form larger particles, making them more effective on escape routes in systems designed for life protection.
Carbon Monoxide Detectors
Carbon Monoxide (CO) is a toxic gas produced by fires and responsible for a high proportion of fire fatalities. CO is odourless, colourless and tasteless making it almost impossible for people to be aware of its presence. Exposure to relatively low levels for just one or two minutes can lead to permanent brain damage or death. Lower levels of CO can cause disorientation and collapse during physical exertion thus hampering the ability of victims to escape.
For many years it has been known that CO detectors can be used as a means of providing early warning of fire. Especially in slow developing and smouldering fires where CO is produced significantly before traditional detectable smoke (aerosols and particulates).
Because CO is a gas and more mobile than smoke, detector positioning relative to the seat of a fire is less critical, giving a higher likelihood of early detection. Smoke movement is constrained by convection currents created by the fire, whereas CO, being a gas, is much more mobile than smoke and also moves by diffusion. This provides an advantage for the CO fire detector allowing a high tolerance on where the detectors can be located.
CO detectors will detect fire from outside the closed door of a cupboard, bedroom or cabin before spreading outside the room. This helps to detect fires whilst the means of escape is still clear.

CO detectors will continue to detect even if incorrectly mounted on a wall or under floor.

CO detectors will operate where thermal barriers inhibit smoke movements such as hot roof spaces in atria and metal roofed buildings.
CO detectors will operate on beamed ceilings, which present obstructions to possible smoke flow.
Carbon Monoxide fills a space evenly and the CO fire detector is far less sensitive to the point where the fire started.
Applications include: -
Warehouses & large covered areas.

Large foyers, halls & reception areas

Cinemas, theatres, concert halls

Sports halls & large waiting areas

Hotel rooms and changing rooms where steam and water mist can cause problems.

Production facilities where large quantities of dust are produced.

Production facilities where chemical vapours cause traditional detectors to false alarm.

Kitchens and restaurants where burnt toast and similar causes of false alarm are prevalent. (In these circumstances the alarm threshold will only be reached once the toast is actually burning).

Farm buildings with high levels of dust and other airborne materials.
The effective life of the detector may only be 5 years.
Some fires generate little or no CO and as such are not suitable for protection via the Carbon Monoxide fire detector. Such fires include the early stages of electrical fires and burning cables where aspirating systems are more suitable.
CO fire detectors would not detect other non-carbon fires such as pure metal fires.
CO fire detectors are also unsuitable for protection of areas where fast burning chemical fires represent the main hazard. In this case ion-chamber or flame detectors are more suitable.
Duct probe units in heating and air conditioning ducts are best suited to use with optical fire detectors. The use of CO fire detectors and ionisation detectors is not recommended under normal circumstances for duct air sampling applications.
CO fire detectors will only operate in situations where combustion takes place; it will not detect smoke and fumes alone.
After considering the above guidance, and subject to a site-specific risk assessment, Inspecting Officers may accept/recommend CO detectors (Only types third party approved) as an alternative to heat and smoke detectors inside rooms.
Optical smoke detectors should be recommended for circulation areas such a corridors and stairways, unless the need to prevent false alarms becomes essential. The siting of the detectors should be in accordance with the recommendations of BS 5839: Parts 1 or 6.

1 Note/ BS 5839 Part 8 expands the recommendations given in clause 9.12 of BS 5839: Part 1: 1988. It should be read in conjunction with BS 5839: Part 1; BS 6259; and BS 7443

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