Smoke Alarm User Manual

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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
2.4.2. Ceiling Height
Smoke or heat detectors can only detect fires once a certain
amount of smoke or heat has reached the sensor. As the
height of a ceiling increases, the time taken for smoke or heat
to reach a sensor will increase, and it will become diluted
with clean, cool air. As a result, maximum ceiling heights are
limited as indicated in table 2.4.1 below.
Table 2.4.1: Maximum ceiling height for different types of
detector
Often, a boundary layer can form close to the ceiling, which is
free of smoke and remains cool. To avoid this, and maximise
the probability of detection, smoke detectors should normally
be mounted with their smoke entry 25mm-600mm below the
ceiling, and heat detectors should be mounted with their heat
element 25mm-150mm below the ceiling. Detector design
normally ensures that the minimum requirement is met,
but care needs to be taken if the detectors are to be stood
away from the roof, for example mounting on an open lattice
suspended ceiling.
Another problem that should be considered is the possibility
of stratification of the air in a room into hot and cold layers,
causing the smoke or heat to stop at the boundaries. This
particularly affects high rooms or atria, where beam detectors
are often used. Stratification is very difficult to predict,
and can vary, even within the same room as environmental
conditions change.
2.4.3. Ceiling Obstructions
Ceiling obstructions such as beams greater than 10% of the
ceiling height should be treated as a wall, and will thus divide
a room. Detectors should not be mounted within 500mm of
such an obstruction.
If the depth an obstruction such as a beam is less than 10% of
the height of the ceiling, but greater than 250mm deep, then
detectors should not be mounted any closer than 500mm to
the obstruction.
Where an obstruction such as a beam or a light fitting is less
than 250mm in depth, detectors should not be mounted any
closer to the obstruction than twice its depth (see figure 2.4.3
below)
Where a ceiling comprises a series of small cells, for example
a honeycomb ceiling, or a series of closely spaced beams, for
example floor of ceiling joists, the recommended spacing and
siting of detectors changes further, dependant on the ceiling
height and the depth and spacing of the beams. Reference
should be made to relevant standards for details (in the UK
BS5839 Part 1: 2002, 22.3.k Tables 1 and 2).
2.4. LOCATION AND SPACING OF AUTOMATIC FIRE
DETECTORS
It is important to consult applicable local and national
standards when choosing the spacing and location of fire
detectors. The following information is intended only as
a guide to the location and spacing of detectors. There is
currently no European standard available; hence this guide is
based on BS5839 part 1, 2002.
2.4.1. Location and Spacing of Point Fire Detectors on
Flat Ceilings
On a flat ceiling with no obstructions, the radius of protection
of fire detectors is 7.5m for a smoke detector and 5.3m for a
heat detector, and detectors should be mounted a minimum
of 0.5m from a wall. Some analogue multi-criteria detectors
have a heat sensor only function, switched by the control
panel, typically used to reduce the possibility of false alarms
during daytime when a building is occupied, reverting to multi-
sensor operation at night time. If this type of operation is
employed, the radius of protection for a heat sensor should be
used. Figure 2.4.1 gives a simple spacing plan based on these
figures, however it should be noted that this might not be the
most efficient layout for a given site; for example in larger
areas, it is also possible to use a staggered layout, see figure
2.4.2, which may reduce the number of detectors required. In
practice, the layout of the room must be considered to obtain
the most efficient detector layout.
Figure 2.4.1: Simple spacing plans for smoke and heat
detectors
Figure 2.4.2: Alternate smoke detector spacing plan for
protecting large areas
Standard Smoke Detector Spacing
Standard Heat Detector
Spacing
10.5m
5.3m
7.5m
3.7m
5.3m
7.5m
60 °
60 °
11.25m
13m
Detector type Maximum ceiling height
Point smoke detector conforming
to EN54–7
10.5m
Heat detector conforming to
EN54–5 Class A1 (threshold 58°C) 9m
High temperature heat detector
conforming to EN54–5 Class B
(threshold 78°C)
6m
Optical beam detectors 25m