Introduction to Fire Safety Management

Figure 9.74 Proprietary device for protecting a manual
fi re alarm call point from accidental operation
Automatic – fi re may be detected automatically in a
variety of ways and apparatus designed for the purpose
needs to be sensitive to at least one of the particular
phenomena that are associated with fi re, i.e. the presence
of heat, smoke or fl ame.
Different detection methods may be incorporated
into the same system so that the most suitable detection
method can be chosen for any location. In addition to
their method of operation automatic detectors can be
classifi ed as being either point detector (i.e. equipment
that detects a fi re at a particular location or area detectors
that are capable of detecting a fi re over a wide area).
Some point detectors are designed to have the dual
function of both detection of fi re and sounding an alarm.
Traditionally these types of detectors have had a mainly
domestic application but are increasingly being used for
workplace fi re alarm systems.
Smoke detectors – the most common form of
detection is the point smoke detector. There are two
types of detection unit which are:
➤ Ionisation units. These detect all types of smoke
that contain small particles, but they respond most
rapidly to smoke caused by fi res such as a chip
pan fi re (very sensitive). The detector in these units
consists of a small amount of radioactive material
that detects any invisible smoke particles that are
fl oating in the air. When particles in the smoke are
detected, the alarm is sent to the panel.
➤ Optical units. These detect all types of smoke, but
are ideal for smoke with larger particles, such as
burning furniture. In this unit regular pulses of ultraviolet
light are sent to a detector; if the light is prevented
from reaching the detector, the alarm will be
transmitted to the panel.
Fire protection in buildings
Figure 9.75 An ionisation smoke detector
Figure 9.76 A typical heat detector
Heat detectors – these detectors operate by sensing
heat in the environment. They operate in two distinct
ways. Some are designed to operate at a given temperature
whereas others detect the rapid ‘rate of rise’ of temperature
that is associated with a developing fi re. They
are often used in a kitchen area, where smoke detectors
would continually actuate because of the normal processes
in a kitchen area.
Beam smoke detectors
Beam smoke detectors are designed to detect the
obscuration caused by smoke over a wide area, typically
atria and warehouses. They usually consist of a combined
transmitter and receiver unit with a remote refl ective elem -
ent, or a separate transmitter/receiver unit. They are normally
positioned high on walls at either side of a monitored
area. In this way the system can detect smoke from
fi res that might occur over a large area. Typically, an infrared
beam is transmitted along this length. In the event of
smoke passing through the beam, the receiver measures
the resultant attenuation.
Aspirating smoke detectors
Aspirating smoke detection systems draw the air from a
monitored area via pipework to a remote detection unit
where the air is sampled for the presence of smoke.
The sampling pipework usually contains predetermined
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