RiskXtraJune2019

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RISKXtra
Detector Selection: Do Your Homework
There can be a
number of issues that
arise from choosing
the wrong type of
detector for the area it
needs to protect from
fire. For those that
perhaps have less
experience in this
practice, it can
become confusing. Do
you use an optical
smoke detector,
ionisation or a multisensor
detector? As
Bernard Laluvein
points out, errors can
easily be avoided by
following the
standards and
directions set out in
BS 5839-1:2017
Commonly, the main problems that people
face when selecting a detector are to do
with ensuring that the detector’s suitable
for the fire risk present and, also, that it doesn’t
respond to sources of non-fire phenomena.
Doing so can give false alarms, which
negatively affects people’s trust and reliance in
the fire detection and fire alarm system (a fact
that may realise some degree of reluctance to
react when the alarm does sound).
Selection of the detector depends on a
number of factors, such as the type of building
and the location of the detector. That said,
these are not the only key factors involved in
the process.
During the design of a fire system,
consideration should be given to the fire risk(s)
and the false alarm risk(s). Fire detectors are
selected depending on the nature of the
protected area and the risks associated with it.
Often, this involves using different fire detector
technologies in order to meet the diverse needs
of both fire detection and false alarm rejection
in each scenario.
Single-sensor detectors (eg optical smoke
detectors and heat detectors) are suited to the
detection of certain fire risks. Selection choices
need to be made carefully in order to optimise
the detection of fire, but also minimise the
occurrence of false alarms. The deployment of
multi-sensor detectors means that the system
could potentially detect fires earlier and yet
avoid troublesome false alarms to a greater
extent as well.
Fire system design
The choices and decisions concerning which
detector types to use are initially made during
fire system design, but are also sometimes
revisited – and even modified – during
commissioning and/or maintenance. They’re
not always well recorded, and neither is the
rationale behind them. At a later juncture, for
example during a maintenance visit, this can
give rise to confusion. A clear understanding of
the original reasons behind the choice of
detector(s) and how they’re configured is
needed in order to verify continued suitability.
Aside from multi-sensor detectors, a visual
inspection of a fire detector can often provide
sufficient information for a maintenance
technician to allow in situ functional testing to
be undertaken (eg in the case of a smoke
detector or a heat detector).
However, in the case of multi-sensors, the
type of detection employed within it and its
settings, modes or specific configuration (which
govern its detection performance) to the risk
are not always apparent from a visual
inspection. This information may not even be
available from the CIE.
In all cases, however, there remains the need
to know the detection principles and to
understand the reason for the choice of a given
detection solution relative to the risks
perceived in the protected area.
Effects on the overall system
Certain types of detector are designed to be
very sensitive to certain products of fires. For
example, optical smoke detectors respond very
quickly to dense smoke that may reduce
visibility in escape routes. Although this can be
seen as a good thing, such detectors might also
respond to non-fire products such as vapours
or steam from kettles or showers. This will be
regarded as undesirable false alarms or, as far
as the Fire and Rescue Service is concerned,
unwanted fire signals (a cost that’s estimated
to be around the £1 million mark per annum).
To avoid false/unwanted alarms, selecting
the right detector can mean all the difference.
For example, a warehouse may be a dusty
environment due to the materials stored or
manufactured within it. Here, an optical smoke
detector is unlikely to be the best type of
solution to use since they’re designed to detect
small particles within smoke. As such, they’re
likely to produce unwanted alarms.
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