Functional
Testing of Multisensor detectors
The test fires that are used to assess
ionisation and optical smoke detectors were developed in the 1980s.
The
test fires used to assess smoke detectors
Current
standards use the same methodology for identifying the most challenging
conditions under which to test smoke detectors. Four test fires are used to
assess smoke detector performance – these are
TF2: smouldering wood,
TF3:
smouldering cotton,
TF4: flaming plastics and
TF5: flaming n-heptane.
The average
smoke profiles produced from the four test fires are shown in Figure 1. The
y-axis (m) represents the optical density (measured in dB/m) and indicates the
larger particles which are generated in greater quantities during smouldering
fires. The x-axis (y) is a dimensionless quantity that reflects the amount of
ionisation taking place and represents the number of smaller particles which
are generated in greater quantities during flaming fires.
Test Methodology
Twelve
approved smoke detectors and smoke alarm devices from undisclosed manufacturers
were used for the fire tests; eight of these were installed on the ceiling and
four on an adjacent wall. The detectors comprised of eight domestic smoke alarm
devices (four ionisations and four opticals) and four commercial smoke
detectors (two ionisations and two opticals).
To define the
end point of the tests, guidance was taken from current standards, which
specify end of test limits for smouldering and flaming fires that are m=2 dB/m
or y=6 respectively.
Test
fires and detector responses
Twenty-nine
test fires were conducted, including four test fires specified in current fire
detection standards. Of these eleven were smouldering fires, sixteen were
flaming fires and two started off smouldering and went on to become flaming
fires. The fuels used included unleaded petrol, medium density fibreboard
(MDF), PVC cable, flame retardant polyurethane foam, sunflower oil, newspaper,
polyester, nylon, ABS, polystyrene, polycarbonate and polyethylene.
A
smouldering fire test
All of the detectors were
periodically replaced, as exposure to the smoke from a number of tests could
cause contamination in the smoke chambers that could potentially affect their
response.
A flaming fire test
Of the
twenty-nine test fires conducted one produced too little smoke and could not be
reproduced however five produced too little smoke and were repeated with
greater quantities of fuel. For the twenty-three complete tests, sixteen
fell within the m/y limits specified in current standards. From these tests
there were six no responses and 270 responses which represents positive
responses 97.8% of the time. The six no responses are attributed to the
inconsistent responses of one particular type of detector and suspected
contamination for the remaining ones.
Even though no
statistical data was gathered by repeating tests, the results do provide
evidence of the response characteristics for the types of detectors (optical or
ionisation) to a variety of smoke types produced from smouldering and flaming
fires.
Conclusions and further work
The test fires
TF2-TF5 do cover most general purpose applications as a real fire is unlikely
to involve only a single type of material. As more materials with different smoke
characteristics are involved in the fire the likelihood of detection increases.
However, it
should be noted that smouldering fires can continue for a long time with only
one material being involved, potentially leading to the production of toxic
gases in fatal concentrations. An example is bedding in contact with a heat
source such as a lit cigarette. In this case an ionisation detector may not
respond and therefore should not be sited in locations where such a scenario is
possible. In contrast a flaming fire in a building will eventually produce
sufficient heat that will radiate onto other materials and lead to the
production of smouldering smoke particles to which the optical detectors are
expected to respond.
This research
demonstrated that commercial and domestic approved ionisation and optical smoke
detectors respond to a broad range of fires within and beyond the fire test
limits of existing standards. The fire tests specified in current standards are
considered to be appropriate and are sufficiently wide in terms of distribution
of smoke characteristics. This demonstrates that the fire tests specified in
these test standards are still applicable today and, despite the changes in the
use of materials over the decades, approved smoke detectors have very wide
smoke response capabilities.
Both
ionisation and optical smoke detectors are attuned to detecting certain types
of fires. In order to ensure that the most appropriate type of device is
installed, guidance on the use of ionisation and optical smoke detectors should
be sought from relevant codes of practice.
The increasing use of multisensor
detectors in fire detection and fire alarm systems has lead to some discussion
as to how they should be tested in the field.
The recommendations detailed below
should be considered as the minimum for properly testing these complex devices:
1.
Multisensor
fire detectors should be physically tested by a method that confirms that
products of combustion in the vicinity of the detector can reach the sensors
and that the detector responds appropriately. A test method purely reliant on
an electronic and / or mechanical means is not sufficient to comply with this
requirement.
2.
Due
to the complex nature of multisensor fire detectors, they should also be tested
in accordance with the manufacturer's instructions.
3.
Where
the detector or system design allows each sensor on which a fire detection
decision depends (e.g. smoke, heat, CO) to be physically tested independently,
then these sensors should be physically tested independently.
4. Alternatively, individual sensors may
be physically tested together if the detection system design allows
simultaneous stimuli and individual sensor responses to be verified either
individually or collectively.
5. Only where the detector or system
design is such that individual sensors cannot be physically tested
individually, for example certain types of conventional multisensor detectors,
the primary sensor alone should be tested.
6.
The
response to each test should be at least confirmed by the CIE.
7.
All
tests and their results should be recorded.