The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 160

November 27, 2023 12:10 pm

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 159, LWF began to consider the siting and spacing of detectors for a fire alarm system. In part 160, we continue to discuss the placement of detectors as per BS 5839-1 and NFPA 72.

When installing a fire alarm system, the placement of detectors is of the utmost importance to ensure overall effectiveness in case of a fire. In some cases, the natural passage of smoke and hot gases upwards may be complicated by an obstruction, such as a ceiling beam. Where this is the case, BS 5839-1 states that allowances should be made:

• Where there is an obstruction with a depth greater than 10% of the height of the ceiling, it should be treated as if it were a wall.
• Detectors should be avoided within 500 mm of a wall or obstruction.

Some buildings/spaces may have perforated ceilings which allow the smoke and hot gases to pass through to the space above. Where at least 40% of the ceiling is ‘open’ and the perforations are evenly distributed, measuring at least 10 mm, detection may be provided above the perforated ceiling. The ceiling thickness, however, should be less than 3x the perforation diameter.

Detectors should not be placed within 1 metre of a ventilation supply point.

NFPA 72 differs in its guidance about the placement of detectors. It states that point-type smoke detectors should not be installed closer than 100 mm (4 inches) from the edge of the ceiling. Where the detector is to be placed on a side wall, they should be sited between 100 mm and 300 mm from the ceiling.

It is further suggested that a spacing of 9.1 m can be used as a guideline for smoke detector spacing, however, this should be confirmed with the specific detector manufacturer.

The guidance is not specific when relating to heat detection, and suggests referring the engineer to the manufacturers for information. It does also state that certain conditions should be met, however, including that detectors should be no more than half the manufacturer’s listed spacing from walls or partitions and that all points of the ceiling should have a detector with 0.7x the listed spacing.

The type of ceiling also affects the guidance given in NFPA 72, with different spacing recommendations based on multipliers of the listed spacing.

Overall, it is felt that BS 5839-1 offers more clear guidance and illustrates issues with diagrams.

In part 161 of LWF’s series on fire engineering, we will discuss beam detectors and flame detectors. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.