The LWF Blog
Fire Engineering Design and Risk Assessment – Siting of fire Alarm Detectors – Part 18September 27, 2016 2:48 pm
In this Fire Engineering Design and Risk Assessment blog series for Architects and those involved in building planning, we have been looking at the siting of fire alarm detectors in relation to ceiling height, room or passage width and any obstructions.
In the part 17, we discussed what distance should be between each detector, distance between ceiling and detector (and floor and detector) and how pitched roofs and beams can impact upon placement.
Today, we look at ceiling height limits and why they are necessary. In a space with a very high ceiling height, the placement of fire alarm detectors at ceiling height could result in delayed activation of the detector in case of fire. Put simply, the fire could be better established in the time it would take for the smoke/heat layer to reach the detector and this would reduce the warning time given when the alarm is triggered.
In some areas, such as a room where only a small area (no more than 10%) of the ceiling is above the general height limit, this may be compensated for by use of point type heat detectors which can be used up to a height of 10.5m, or by point type smoke detectors if the height does not exceed 12.5m.
The delay in warning time can sometimes be recouped in cases where the alarm system is connected directly to the Fire Service, as this reduces the time between detection and the start of firefighting. However, as policies of the Fire Service can vary and change, it may be that a manual confirmation of the fire is necessary before they will be called to action.
However, if a rapid response is possible, this extends the ceiling height limits in those cases where it is by no more than 10% of the overall ceiling area to 15m for point type heat detectors and 19m for point type smoke detectors.
NFPA 72 in the U.S. has a slightly different view to British Standards. Their guidance states that point type smoke detectors should not be installed any closer than 4” from the edge of a ceiling and if mounted on a side wall, should be between 4” and 12” from the ceiling. They also say that detectors should be spaced at 30 feet apart, as a guideline. These spacing guidelines cannot be set in stone as it is dependent upon the guidelines given by specific detector manufacturers.
For point type heat detectors, much depends on manufacturer specifications, but should be no more than half the manufacturers listed spacing from walls or partitions and at every point of the ceiling there should be a detector with 0.7 times the listed spacing.
Further guidance for the planning of installation of fire alarms and detectors with high ceiling heights, to UK standards can be obtained from BS 5839:1.
In next week’s blog, we will look at beam 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.