SOA

The RRFSO Article 13 states that for fire detection, the Responsible Person must:

1. Where necessary (whether due to the features of the premises, the activity carried on there, any hazard present or any other relevant circumstances) in order to safeguard the safety of relevant persons, the responsible person must ensure that the premises are, to the extent that it is appropriate, equipped with appropriate fire detectors and alarms;
2. For the purposes of paragraph (1) what is appropriate is to be determined having regard to the dimensions and use of the premises, the equipment contained on the premises, the physical and chemical properties of the substances likely to be present and the maximum number of persons who may be present at any one time.

Fire Detection and Warning Checklist

• Can the means of detection ensure a fire is discovered quickly enough for the alarm to be raised in time for occupants to escape safely?
• Are the detectors of the right type and in the appropriate locations?
• Can the means of warning be heard and understood by everyone throughout the premises?
• Are there provisions for people where the alarm cannot be heard?
• If the fire detection and warning system is electrically powered, does it have a back-up power supply?

Detection Zone

The size and position of the detection zones will therefore tend to be dependent on the shape of the buildings, but will also depend on what the building is used for and to some extent the number of people the building is expected to contain at any one time. BS 5839-1 has some specific recommendations with respect to detection zones:

• Unless the total floor area is less than 300m2, zones should be restricted to single floors;
• If in the same fire compartment, voids of a room may be included in the same zone;
• Zones should not be larger than 2000m² except for manual systems in single storey open plan buildings, such as a warehouse, where up to 10000m² is allowed;
• Fire detectors in an enclosed stairwell, lift shaft, etc should be considered as a separate zone;
• The search distance within a zone should be less than 60m (all possible entrance points must be considered). The search distance only relates to the distance from entering a zone to being able to determine the location of the fire, it is not necessary to travel to the fire; and
• Zones should not cross fire compartments, a fire compartment can contain several zones but a zone should not contain more than one fire compartment.

Alarm Zone

For complex buildings where it is necessary to operate alarm devices differently, i.e. phased evacuation, then the building should be divided into alarm zones such that all of the alarm devices in one alarm zone operate in the same way. BS5839-1 contains recommendations for alarm zones:

• The boundaries of all alarm zones should comprise fire-resisting construction;
• Signal overlap between alarm zones should not cause confusion; and
• The same alarm and alert signals should be used throughout a building;

Selection of Appropriate Automatic Detectors

BS5839-1 lists eight categories of cover (see Table 1), depending on what is required. The category system is a simple short hand method of informing all parties of the system objectives.

Optical smoke detectors are suitable for most applications giving the fastest response to slow burning fires - the most common start to fire events. Ionisation detectors have a quicker response to fast burning fires but an inferior response to slow smouldering fires. Ionisation detectors are also less acceptable from an environmental point of view due to the radioactive material that they contain.

BS5839 section 21.1.8 (d) recommends the use of optical detectors to provide coverage for escape routes due to their superior ability to detect optically dense smoke that would easily obstruct the use of escape routes.

Opto-heat detectors have been developed to mimic the response of ionisation detectors to fast burning clean fires yet maintain the advantage of photoelectric detectors when detecting smouldering fires and allow a higher alarm threshold within the EN54-7 specification under normal conditions thus providing a greater rejection of false alarms.

Heat detectors should be used in environments where the ambient conditions might cause false alarms if smoke detection were to be used, for example where there is a high level of dust, fumes, steam or smoke under normal conditions. There are three available types of conventional heat detector, a fixed high temperature (triggers at 92°C), a medium fixed temperature (triggers at 77°C) and a rate of rise heat detector.

The rate of rise type is the most sensitive type of heat detector, particularly in areas where the ambient temperature can reach low levels. In order to avoid false alarms rate of rise detectors should not be used in areas subject to frequent temperature swings, such as in kitchens, boiler rooms and warehouses with large doors to open air.

Positioning of Automatic Detectors

The general rules for spacing between any point in a protected area and the detector nearest to that point should not exceed 7.5m for a smoke detector and 5.3m for a heat detector. However, to ensure that coverage is provided into the corners of rooms and no gap is left at the junction point of multiple detectors; spacing’s have to be reduced. Thus, for a complete coverage of square layouts, spacing’s between detectors and walls should be reduced to 5m for a smoke detector and 3.5m for a heat detector, as in Figure 1.

For corridors less than 2m wide only the centre line need be considered therefore it is not necessary to reduce detector spacing’s in order to provide complete coverage. Therefore for smoke detectors spacing becomes 7.5m from a wall and 15.0m between detectors. For heat detectors the spacing becomes 5.3m to a wall and 10.6m between detectors.

The above data is based on flat level ceilings; for pitched ceilings a detector should be mounted near (600mm for smoke and 150mm for heat) to the apex but spacing can be increased by 1% for each 1° of slope up to 25%. Under all normal circumstances point type fire detectors should be mounted on the ceiling - this ensures that the height restrictions are met together with the Table 2.

Manual Call Points

Manual call points should be located on escape routes, at all exits to free air and at all exits from each level of multi-storey buildings.

For general applications, call points should be located such that nobody need to travel more than 45m to reach the nearest call point. If users have limited mobility and these users are likely to be the first to operate the alarm or the area is high risk the distance should be reduced to 25m.

Call points should be located near to specific hazards (e.g. flammable liquid store) and at 1.4m (+/- 0.2m) from the floor in well lit easily accessible positions. Lower mounting heights might be needed to accommodate users in wheel chairs.

References

1. Regulatory Reform (Fire Safety) Order 2005, SI 2005/1541. The Stationery Office, 2005.
2. BS 5839-1:2002 Fire detection and fire alarm systems for buildings. Code of practice for system design, installation, commissioning and maintenance.
3. The Building Regulations 2000: Approved Document B fire safety. The Stationery Office.
4. Fire Safety Guides. Department for Communities and Local Government Publications, 2006.