System Sensor BS5839 manual Lantern Lights, Location and Spacing of Optical Beam Detectors

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.

<10.5M

Figure 2.4.7. Detector in Stairwells

1.5M1.5M

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.

	Maximum 100m
		Maximum
Minimum		7.5m
500mm
Transmitter or			Receiver or
Transmitter/Receiver
			Reflector
		Maximum
		15m
Transmitter or			Receiver or
Transmitter/Receiver
			Reflector

Application Guide

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.

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.

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.

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