Functional Testing of Multisensor detectors

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.