Smoke Alarm User Manual

15
Note: This document is based on the recommendations of BS5839 Part 1: 2002. It is intended only as a guide to the application of fire detection systems.
Reference must be made to relevant national and local standards.
Application Guide
2.4.7. Stairwells and Lift Shafts
Internal stairwells and lift shafts and other vertical service
ducts through a building provide a clear path for smoke to
pass between floors of a building as if they were chimneys.
It is therefore important to protect these, preferably using
smoke detectors.
All vertical shafts through a building must be protected by
a smoke or heat detector at the top of the shaft, and by a
detector within 1.5m of each opening onto the shaft.
In internal stairways, a detector should be mounted on each
main landing (Figure 2.4.7). In addition, if the detectors on
the landings are separated by more than 10.5m, intermediate
detectors should be mounted on the underside of the stairs.
Detectors should also be fitted into any room opening directly
onto a stairway other than a WC cubicle.
Figure 2.4.7. Detector in Stairwells
Figure 2.4.8. Protection of Vertical Shafts
2.4.8. Voids and False Ceilings
Detectors need not normally be installed in voids less than
800mm deep, unless on the basis of a fire risk assessment it
is thought that fire or smoke could spread extensively through
the voids before detection, or unless the fire risk in the void
is such as to warrant protection. Use of heat and smoke
detectors in voids greater than 800mm high is dependant on
the protection category, and fire risk assessment.
Where they are installed into voids, a detector's sensing
element should be mounted either in the top 10% or the top
125mm of the void space whichever is greater. Although it
can be difficult to install detectors the correct way up in void
spaces, care should be taken as incorrect orientation of a
detector can lead to increased ingress of dirt and dust, leading
to reduced maintenance intervals, and possible nuisance
alarms.
Detectors above a false ceiling may be used to protect the
area below it, if the false ceiling is perforated uniformly across
the complete area of the ceiling, with the holes making up
over 40% of the ceiling surface area, having a minimum size
of 10mm and the false ceiling having a thickness of less than
three times the dimensions of the perforations.
In all other cases, the areas above and below a false ceiling
should be treated as separate, and thus should be protected
separately with detectors below the ceiling, and if necessary
in the void above the ceiling.
2.4.9. Lantern Lights
A detector should be mounted in any lantern light used
for ventilation or having a height exceeding 800mm. The
temperature in lantern lights can change rapidly owing to
heating by sunlight, which means that rate-of-rise heat
detectors should not be used and heat detectors should be
protected from direct sunlight.
2.4.10. Location and Spacing of Optical Beam Detectors
Generally, for an optical beam detector mounted within
600mm of a ceiling, the fire detection coverage is up to 7.5m
either side of the beam (Figure 2.4.9). The beam of the
detector should not be closer than 500mm to any obstruction.
Similar recommendations to above apply to the application
of beam detectors with sloped ceilings, voids, false ceilings,
walls and partitions and ceiling obstructions.
Figure 2.4.9: Standard Beam Detector Layout
Where it is likely that people will be present in an area
protected by beam detectors, the detectors must be mounted
at a minimum height of 2.7m, and consideration must also be
given to the possibility of other temporary obstructions to the
beam such as forklift trucks.
For further information on the use and mounting of beam
detectors, see System Sensor Europe's Guide to Projected
Beam Detectors.
< 10.5M
1.5M
1.5M
Maximum
7.5m
Maximu
m
15m
Transmitter or
Transmitter/Receive
r
Receiver or
Reflector
Receiver or
Reflector
Transmitter or
Transmitter/Receive
r
Maximum 100m
Minimu
m
500mm