The correct spacing and siting of fire detection equipment cannot be overstated, as if these devices do not do their job properly precious time could be lost in safely evacuating a building’s occupants. Rob Shepherd explores the key factors that must be considered when carrying out this essential work.
When it comes to keeping building occupants safe, few things are as important as fire detection. There’s no getting away from it – correctly installed and properly spaced fire detectors can mean the difference between life and death. Electrical contractors involved in installing fire detection devices such as smoke and heat detectors, manual call points and sounders carry a significant responsibility in ensuring they are deployed in line with the requisite standards, so that they provide maximum warning with minimal delay.
Setting the standard
Fire detection systems are governed by a framework of legislation and best practice guidelines. At the heart of this framework is BS 5839, the British Standard for fire detection and fire alarm systems for buildings. BS 5839-1 sets out recommendations for planning, design, installation, commissioning and maintenance of fire alarm systems in non-domestic premises, while for residential applications BS 5839-6 applies.
Fire detection is not a one size fits all solution, so it is important to understand the distinction between different system categories. Category L systems for life safety are subdivided into five categories – L1 through to L5 – with L1 offering the most comprehensive coverage. L1 systems are typically installed in high risk commercial premises and require automatic detection in all areas including voids and storage spaces. In contrast, L5 systems are more targeted, protecting only defined risk areas. Each category demands a unique approach to spacing and siting, tailored to the risk profile and occupancy type of the building.
Made to measure
When it comes to locating smoke and heat detectors, one common misconception is that they provide the same level of coverage. However, they don’t. Smoke detection devices must have an individual coverage of 7.5 m radius, while the figure is 5.3 m for heat detection devices. These measurements are also for standard installation conditions such as flat ceilings with normal airflow and standard room height. In irregular shaped rooms with, for example, sloped ceilings or obstructions, spacing must be reduced and electrical contractors must account for these factors during the design phase. Fire modelling tools are now available that can simulate smoke movement in complex spaces.
When more than one device is required in an area, their coverage radii must overlap to eliminate any blind spots. In corridors less than 2 m wide, overlapping is not necessary, for instance, smoke detectors may be spaced up to 15 m apart. However, if the corridor is wider than 2 m, it is classified as a room, and overlapping coverage radii must be provided.
BS 5839 states that detectors should be sited no less than 0.5 m from a wall. Any obstruction located less than 300 mm from the ceiling must be considered a wall and therefore requires a detector to be installed on each side of it. Also, ceiling obstructions like beams or ductwork that are deeper than 10 per cent of the overall ceiling height, and are less than 300 mm from the ceiling, should also be treated as walls. Buildings such as warehouses need to be particularly careful that stacking items near the ceiling does not obstruct the functionality of the detectors. When it comes to voids, detection should be provided in areas of more than 800 mm but less than 1,500 mm depth, while voids deeper than 1500mm should be treated as a room.
Sound advice
It’s not just smoke and fire detectors that need to be carefully sited. When it comes to locating sounders, there are strict minimum acceptable sound pressure levels to adhere to. A system design needs to consider frequency range, sound pressure, background noise and whether there are any closed doors. 65 dB is considered the minimum acceptable sound pressure level, however, a difference of 2-3 dB is only just perceptible to the human ear.
In areas where people are sleeping, such as hotels, the sound level needs to be 75 dB at the bedhead with all doors closed. Sound is reduced by 20 dB when going through a standard door and 30db through a fire door, so to achieve 75 dB at the bedhead with only sounders in a hotel corridor, the sounder needs to be set at 105 dB to be heard in the bedroom.
Manual call points must be clearly visible and positioned so that it is impossible to exit a storey or building without passing one. The maximum travel distance to a manual call point should not exceed 45 m, or 25 m in areas with a high fire risk. The reduced distance also applies in locations where there are likely to be many occupants with limited mobility, and it is reasonable to expect that one of these individuals may be the first to activate the fire alarm system.
Fit and don’t forget
False alarms represent another challenge and often result from poor siting decisions. Installing smoke detectors too close to kitchens, bathrooms or areas with heavy dust or fumes can result in unwanted alarms. These not only disrupt operations but can cause occupants to ignore alarms or delay evacuation. BS 5839-1 requires that false alarms be minimised through correct design, selection and installation, so electrical contractors should engage with the building’s fire strategy and conduct a thorough risk assessment before installation.
Electrical contractors play a role not just in initial installation but also in long-term servicing, ensuring systems remain compliant and effective throughout their lifespan. Over time environmental conditions may change. Offices can be reconfigured, factory machinery added or ventilation systems upgraded, and these changes can alter the movement of air or smoke, potentially making previously correctly detectors ineffective. BS 5839 states that regular inspection and testing should be carried out and offers detailed guidance on the correct maintenance intervals and procedures.
Right first time
Fire investigations have revealed that improper detector placement has, on occasion, led to significant delays in alarm activation. In the event of a fire, even a delay of a few seconds can result in loss of life, particularly in buildings with vulnerable occupants. There’s much to consider when locating fire detection equipment and the consequences of poorly spaced or incorrectly sited devices can be catastrophic, so contractors must always follow standards rigorously and fully consider all factors when making design decisions.
Contributors
Rob Shepherd (author)
Rob is Consulting Editor for InstallerELECTRIC. He is a well-respected journalist with a strong history in the electrical sector.
