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A Guide to BS5839

BS5839 REQUIREMENTS

Most commercial and industrial premises require a fire detection system to protect life and property. National and local legislation cover a variety of buildings and their safety requirements, therefore it is always advisable to consult the local Fire Prevention Officer about specific premises. The following guide is intended as a reference only, and BS 5839 Part 1 2002 should be consulted for all fire system design requirements.

SYSTEM GUIDELINES  - BS5839 Part 1 : 1988 

SYSTEM CLASSIFICATIONS

Type P systems are automatically activated fire detection systems designed to protect property.
These systems are then sub-divided into:

Type P1 Fire system installed throughout building.

Type P2 Fire system only installed in a defined part of the building.

Type L systems are automatically activated fire detection systems designed to protect life. These systems are then sub-divided into:

Type L1 Fire system installed throughout building.

Type L2 Fire system only installed in a defined part of the building.

Type L3 Fire system only installed for the protection of escape routes.

Type M systems encompass all manually activated fire systems.

SYSTEM ZONING

To quickly and accurately identify the fire source, the building should be divided into zones. Each zone should be accessible from the main circulation routes leading from the location of the control panel. If the total floor area of the building does not exceed 300m', it need only have one zone, no matter how many floors it has. Otherwise, each floor should be treated as an individual zone. The total floor area for anyone zone should not exceed 2000m'.

A searcher should not have to travel more than 30m into a zone for visual indication of the fire's position. The use of remote indicators outside doors may allow a zone area to be increased.

A fire compartment is an area bordered by a fire resisting structure with a resistance of at least 30 minutes. If a stairwell or a similar structure extended beyond one floor but was in one fire compartment, the stairwell should be a separate zone. If the zone covered more than one fire compartment, the zone boundaries should follow the compartment boundaries.

If a building were split into several occupancies, no zone should ever be split between two occupancies.

If a detector were removed from the zone circuit this must not isolate a break glass call point on the same circuit.

CONTROL PANELS

The type of control panel will depend upon the size of the building and the extent of the automatic protection provided. The panel is chosen according to the number of zone and sounder circuits, ancillary control relays, battery standby time and any other individual customer requirements.

The central control panel must be positioned in a low fire risk area that provides the Fire Brigade with easy access in cases of emergency. The area must be well lit, therefore provision of emergency lighting may be necessary. A plan of the building and the zone locations must be kept beside the control panel. 

DETECTORS

Choose the correct detector type. With more and more flame retardant treatments used in building materials and furniture, photoelectric (optical) smoke detectors tend to be the most popular choice.

Optical smoke detectors are best suited to the detection of slow burning, smouldering fires caused by materials such as plastic, PVC or foam. Optical detectors react best to the large smoke particles produced by such fires and help to avoid false alarms from cooking fumes etc. Ionisation detectors are the preferred choice for the detection of the small smoke particles produced by flaming or fast burning fires, e.g. fires involving liquid fuels. They may be quicker to respond in the room of the fire's origin. Heat detectors (fixed heat or rate of rise heat) will provide adequate protection in areas unsuitable for smoke detectors.

Always ensure detectors are evenly spaced and allow for any obstructions, such as beams and partitions. Always consider the environmental conditions - there is a higher risk of false alarms if smoke detectors are placed in areas of high air movement, or where steam, vapour or exhaust fumes are likely to be present.

Smoke and heat detectors are placed at the highest points of the enclosed areas, as this is where smoke and heat from fires is most concentrated.
The heat sensitive element of a heat detector should be sited between 25mm and 150mm below the ceiling or roof. The smoke detector's sensor element should be sited between 25mm and 600mm below the ceiling or roof. If the area has a pitched or north light roof, smoke detectors should be installed in each apex.

Under flat horizontal ceilings and corridors more than 5m wide, the maximum horizontal distance between any point in the area and the nearest detector should be 5.3m for point Heat Detectors, covering a maximum area of 50m2 and 7.5m for point Smoke Detectors, to cover a maximum area of 100m2.
 
 

MANUAL CALL POINTS 

The break glass call point allows personnel to raise the alarm in the event of a fire. Manual call points should be located on exit routes, including exits to the open air, and on stairwell landings. They should be mounted 1.4 metres above the floor in easily accessible, conspicuous positions. Building occupants should not have to travel more than 30 metres within a building to reach a manual call point. This maximum distance may need to be reduced if the occupants are elderly or infirm.

In general, all call points within a building should be of the same type and operation. Exceptions to this rule would include weatherproof call points or those designed for use in hazardous areas.

If a call point is to be used on a 240V AC supply the provision of earth continuity between external circuits is required in order to comply with paragraph 4.1.7 of BS 5839, Part 2.

SOUNDERS 
 

Alarm sounders are used to alert and evacuate occupants. They are normally bells or electronic sounders and all audible warning devices in the same system must have a similar sound. This distinguishes them from other audible alarms.

The sound should not permanently damage hearing but it should be a minimum of 65dBA or 5dBA above any background noise likely to exceed 30 seconds. Should excessive noise be present, a visual indication of the alarm condition should also be provided. If the alarm is to wake sleeping occupants, such as in a hotel, the sound level should be a minimum of 75dBA at the bedhead. Sound level frequencies should normally be kept within the range of 500 to 1,000Hz

No matter how small the system, a minimum of two sounders are required and the correct sound levels must be maintained in all parts of the building.

The sound level is reduced by approximately 20dBA by a door and by approximately 30dBA by a fire door.

Multiple small sounders produce a better sound distribution than a few large ones. In some areas, a large number of quieter sounders may be preferable to a few very loud sounders to prevent noise levels becoming too loud. Due to individual site characteristics, sound level checks should be carried out to ensure minimum sound levels are obtained.
 

STAND BY POWER SUPPLY

Automatically charged batteries often supply standby power. Automotive type batteries must not be used. The supply must be able to operate the alarm for 30 minutes after a certain minimum duration. The minimum duration varies with type of system and occupancy.

For property protection, if the mains failure is immediately recognised, a standby duration of 24 hours is required, if not the required duration is 24 hours longer than the building may remain unoccupied, For life protection, a standby duration of 24 hours after the fault is detected is required.

CABLES AND INTERCONNECTIONS 

All permissable fire system cables are described in BS5839, Part 1. Fireproof cables are preferred for fire alarm installations. However, as a minimum they must be utilised for any circuits that are required to operate after Jetection of a fire e.g. sounder circuits, power supplies and signalling interconnections. Fireproof cables require fireproof supporting clips.

Detection circuits may be wired in pvc/pvc e.g. twin and earth in line with guidelines provided in BS5839, Part 1.
Conductors carrying fire alarm power or signals should be segregated from conductors used for other systems.

Where possible all cable joints should be made within one of the system components. If a joint is unavoidable then it should be enclosed in a suitable reserved junction box labelled "Fire Alarm", Joints and terminations should only be carried out by competent persons.

Connections to the mains should be via an isolating protective device e.g. an isolating switch fuse, the cover of the device should be red and should be labelled "Fire Alarm: Do Not Switch Off",

Cables other than M.I.C.C. should be provided with mechanical protection when necessary i.e. if physical damage or rodent attack is likely, or if they are less than 2.25 metres above the floor. Mechanical protection may be provided by conduit, trunking or by laying in cable tray.

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