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April 12, 2012

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The spacing and siting of fire detection devices

In the fifth of our Fire Doctor series, David Brown answers some frequently asked questions on the spacing and positioning of detectors.

Do smoke and heat detectors provide the same level of coverage?

No they don’t. Smoke detection devices have an individual coverage of a 7.5m radius, while the figure is 5.3m for heat detection devices. Where there is a requirement for more than one device in an area, these radii must overlap to ensure that there are no blind spots. In a corridor less than 2m wide, the radii do not need to overlap, so smoke detectors, for example, can be spaced 15m apart. Where the corridor is more than 2m wide, it is considered to be a room and overlapping radii are necessary.

For smoke detectors, the individual coverage can be represented by a square measuring 10.6m x 10.6m giving a coverage of 112m² per device, which is usually approximated to 100m². With heat detectors this figure is 7.5m x 7.5m, giving an area of coverage of 56m² per device which is rounded down to 50m².

BS 5839 states that detectors should be sited no less than 0.5m from a wall. Any obstruction which is less than 300mm from the ceiling should be treated as a wall, thus requiring a detector either side of the obstruction. In buildings such as warehouses, measures must be in place to ensure that stacking items too close to the ceiling does not affect the performance of the detectors.

Do I need to site detectors in voids?

In voids of more than 800mm in depth, detection should be provided in the void. If the void is less than 1500mm in depth, all such detection should be sited in the top 10 per cent or 125mm of void depth, whichever is the greater.

If a detector is concealed in a relatively inaccessible area such as a ceiling or floor void, it is normally a requirement to provide a remote indication of its operation. In an addressable system the provision of an individual remote indicator might not be necessary, provided that the location of each fire detector is clearly indicated at the control panel and is addressed so that you can tell where it is located.

What is the minimum acceptable sound pressure level for fire alarm signals?

A system design should incorporate sounders that will achieve 65dB (or 5dB above any ambient noise lasting more than 30s) in all areas of the building with all doors shut (a difference of 2-3dB is only just perceptible to the human ear).

It is considered unnecessary to install additional fire alarm sounders in open areas if 65dB is achieved, and the sound pressure level can be reduced to 60dB in enclosed spaces such as small cellular offices and in stairways.

I’m about to install a fire detection system in a hotel – is there anything in particular I need to consider?

In areas where people are sleeping the sound level needs to be 75dB at the bed-head. As this has to be achieved with doors shut, most hotel bedrooms have a combined detector with a base sounder or a base sounder beacon.

As with any design, you will need to take into account the reduction of sound through a door. Sound is reduced by at least 20dB through a standard door and at least 30dB through a fire door. To achieve 75dB at the bed-head with only sounders in a hotel corridor, the sounder would need a minimum output of 105dB to achieve the required sound level in the bedroom. A word of advice – this method results in considerable current consumption and installing the sounder in the bedroom set at a lower sound pressure is a more efficient design.

Is it better to use a greater number of quieter sounders rather than a few very loud sounders?

In order to prevent excessive sound pressure levels that can cause disorientation or even damage to hearing, the use of a greater number of quieter sounders is always preferable to using fewer very loud sounders.

By designing a system with more sounders at reduced volume, your design may result in longer loops, and smaller batteries in the panel. In larger systems it can result in fewer loops, and it can also make a difference to the size of cable required.

How far should someone have to travel to activate a manual call point?

Manual call points need to be prominently positioned, readily distinguishable from non-fire alarm call points, and sited in a way so that it is impossible to leave a storey or a building without passing one.

No one should have to travel more than 45m to operate a manual call point and this distance is reduced to 25m in areas where there is a high level of fire hazard. This also applies where there may be a high number of occupants who have limited mobility and it can reasonably be anticipated that one of these people will be the first to operate the fire alarm system.

I’m concerned that the cables I am using for a fire detection installation in a school will get damaged. What can I do to protect them?

Standard fire rated cables have little mechanical protection so need to be safeguarded against physical damage or even rodent attack. However, when using mineral insulated copper sheathed (MICC) cables conforming to BS EN 60702 and steel wire armoured (SWA) cables conforming to BS 7846, there is no requirement for additional mechanical protection.

If you are installing “standard” fire alarm cables or “enhanced” cables other than MICC or SWA, and they are exposed below the height of 2m from the floor, you will need to make sure they are protected by the use of trunking or conduit.

Are there any applications that use standards besides BS 5839?

BS 6266 gives recommendations for the protection against fire of areas containing electronic equipment, including computers, servers, internet hosting centres, switching centres, data centres and systems for communications, design, manufacturing and distribution.

This British Standard covers medium, high and critical risk environments, also giving recommendations for the protection against fire in adjacent areas. It specifies that the effective area of a detector is 25m² or less due to the use of air handling units that change airflow and affect the ability of the detectors to perform their function.

Aspirating smoke detectors are particularly suitable for use in electronic equipment areas, as they can be programmed to be around 100 times more sensitive than point detectors.

Hochiki Europe has produced a guide to BS 5839, and a CPD presentation Understanding the Selection, Spacing and Siting of Fire Detection Devices. For more information email Lindsay Edmeades [email protected] or call +44 (0)1634 266566.

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May 8, 2017 5:27 am

please make correction from any obstruction less than 300mm to more than 300mm from ceiling should be treated as a wall

marcus poliszewski
marcus poliszewski
May 30, 2017 9:27 am

what the recommended measurement is between floor detection devices?

June 12, 2018 8:08 am

any obstruction less than 300mm from ceiling should be treated as a wall – IS CORRECT.

Mark Duncanson
Mark Duncanson
June 25, 2020 2:02 pm

Could I ask for clarification on the statement;
“Where there is a requirement for more than one device in an area, these radii must overlap to ensure that there are no blind spots”.
Whilst I fully accept the staement to be true, I have seen some sites (mainly manufacturers) that suggest the spacing between detectors is half of that set out as between points, but I have never seen anything in the BS to justify this.

October 6, 2021 12:05 am
Reply to  luthra

He is referring to floor mounted obstructions (e.g. partitions) where the top is less than 300mm from the ceiling they should be treated as a wall. Ceiling mounted obstructions deeper than 10% of the ceiling height should also be treated as a wall.