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.

What Is a Multi Sensor Detector?

A multi-sensor detector is a fire detection device that combines two or more sensing technologies within a single unit, most commonly smoke and heat detection. These detectors analyse data from multiple inputs to determine whether conditions indicate a genuine fire event.

This approach allows systems to:

·        Improve detection accuracy

·        Reduce unwanted alarms

·        Respond more effectively to different fire types

·        Provide earlier warning in complex environments

Multi-sensor detectors are widely used in commercial buildings, healthcare environments, offices, and areas where traditional smoke detectors alone may be prone to nuisance activations.

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.

Why Multi-Sensor Detector Testing Is Different at Site

Because multi-sensor detectors use combined sensing logic, testing must confirm that each detection method functions correctly and that the overall system responds as designed.

Unlike single-sensor testing, multi-sensor verification typically involves:

• Confirming smoke response

• Confirming heat response

• Verifying combined detection logic

• Ensuring the alarm signal reaches the control panel

This makes structured testing essential to ensure both sensing elements work together reliably.

How to Test a Multi-Sensor Detector (Step-by-Step) at Site

Testing procedures vary depending on system design and detector type, but professional testing generally follows a structured process.

Typical workflow includes:-

1. Identify the detector type and installation environment

2. Confirm system readiness and isolate zones where required

3. Apply a controlled smoke stimulus to verify smoke detection response

4. Apply controlled heat stimulus to confirm thermal activation

5. Observe combined detector logic and activation timing

6. Confirm alarm signals reach the control panel correctly

7. Reset the system and record testing outcomes

Improvised testing methods or uncontrolled heat sources should never be used, as these can damage detectors and affect long-term reliability.

For guidance on single-sensor procedures, see our smoke and heat detector testing resources, which explain how each sensing element is tested individually during maintenance.

Equipment Used for Multi-Sensor Detector Testing

Testing multi-sensor detectors requires tools capable of delivering controlled stimulus and safe access to installed devices.

Professional testing commonly involves:

• Controlled smoke stimulus for optical sensing verification

• Controlled heat stimulus for thermal response testing

• Multi-stimulus testing devices for combined activation checks

• Detector testing heads for targeted stimulus delivery

• Access equipment for high-level installations

These tools allow technicians to confirm both sensing elements operate correctly while protecting detector integrity and maintaining consistent results across sites.

Engineer Tip:

For multi-sensor detector testing, integrated multi-stimulus devices are often preferred as they allow both smoke and heat stimulus to be applied from a single unit. The Testifire and Testifire XTR2 ranges are specifically designed for this purpose, enabling efficient testing without changing equipment between detection methods.

Modular systems such as the Solo range can also be used for multi-sensor testing; however, these typically require interchangeable heads and separate stimulus tools, which may increase setup time during routine maintenance and inspections.

Multi-Sensor Testing and BS 5839

In the UK, multi-sensor detector testing forms part of wider fire alarm inspection and maintenance guidance under BS 5839.

Best practice includes:

• Functional testing during routine maintenance

• Inspection by trained and competent professionals

• Verification using appropriate testing stimulus

• Documentation of inspection and servicing activity

Common Mistakes When Testing Multi-Sensor Detectors

Incorrect testing approaches can reduce system reliability or create inaccurate results.

Common issues include:

• Testing only one sensing element

• Using unapproved heat sources

• Applying excessive stimulus

• Failing to confirm control panel response

• Not documenting testing outcomes

Structured procedures and appropriate equipment help avoid these risks and support consistent performance.

Who Should Carry Out Multi-Sensor Detector Testing?

Testing is typically carried out by trained professionals responsible for fire alarm system maintenance, including:

• Fire alarm engineers

• Maintenance contractors

• Facilities management teams

• Fire safety professionals

These individuals use specialist testing procedures and equipment to ensure systems remain reliable and compliant.

How Multi-Sensor Testing Supports Fire Alarm Maintenance

Routine multi-sensor detector testing forms part of wider fire alarm servicing programmes, supporting:

• Planned preventative maintenance

• System commissioning

• Compliance inspections

• Fault identification

• Long-term system reliability

Ensuring multi-sensor detectors function correctly helps maintain consistent fire detection performance across commercial, residential, and industrial environments. This in return helps to maintain the safety of both the occupants and the buildings.