What are the three main types of point detectors?
The three main families of point detectors used in Belgium are the optical smoke detector, the thermal detector and the multi-criteria detector (also called multi-sensor). Each is based on a different physical principle to identify the start of a fire, and each has its areas of relevance and limitations. The choice should never be automatic or identical for an entire building: it depends on the nature of the combustible material present, the probable mode of fire development and the ambient conditions of the room (dust, vapour, air currents). It is this intersection between technology and actual risk that must guide the design of the installation, well before any question of cost or equipment availability.
| Technology | Physical principle | Detects well | Sensitive to false positives |
|---|---|---|---|
| Optical smoke | Light scattering by smoke particles | Smouldering fire, cold smoke | Dust, vapour, insects |
| Thermal | Temperature rise or rate of rise | Rapidly developing fire, open flames | Low sensitivity to false alarms |
| Multi-criteria | Combination of several sensors (optical, thermal, sometimes chemical) | Wide range of fire types | Reduces false positives by signal correlation |
How does an optical smoke detector work?
An optical smoke detector works by light scattering: a diode emits an infrared beam into a measuring chamber, and smoke particles entering this chamber deflect the light towards a photoreceptor. When the amount of scattered light exceeds a threshold, the alarm is triggered. This principle makes it particularly effective at detecting smouldering fires, i.e. slow combustion that releases a lot of smoke before producing significant heat (mattresses, cables, furniture in slow combustion). The term cold smoke is often used to describe this type of emission, because the ambient temperature remains low while smoke is already filling the room. In return, this type of detector is sensitive to dust, water vapour and insects, which makes it unsuitable for certain workshops, industrial kitchens or car parks without specific treatment.
How does a thermal detector work?
A thermal detector responds either to an absolute temperature being exceeded or to a rate of temperature rise (rate-of-rise). It does not detect smoke as such, but the heat produced by the fire. This technology is suitable for rooms where a fire would develop rapidly with open flames and high heat production, such as certain storage areas for flammable liquids or rapidly combustible materials. It is also the only reasonable option in environments where the air naturally contains particles or vapour permanently: welding workshops, industrial kitchens, garages, dusty rooms. An optical detector would trigger repeated false alarms there. The counterpart of this robustness to false positives is later detection of a smouldering fire: heat takes time to reach the trigger threshold when combustion remains slow.
What is a multi-criteria detector and when should it be chosen?
A multi-criteria detector combines several physical principles, typically optical and thermal, sometimes supplemented by a chemical sensor, within a single housing. The algorithm correlates the signals from each sensor to confirm the start of a fire before triggering the alarm, which reduces the risk of false alarms while maintaining good sensitivity to different types of fires. This type of detector is relevant in rooms where the nature of the fire risk is not homogeneous or changes over time (offices with paper archives and electrical equipment, rooms in healthcare facilities, common circulation areas in apartment buildings). It is also sought after in environments sensitive to false triggering, where an unjustified evacuation has a significant organisational cost. The multi-criteria detector does not, however, eliminate the need for room-by-room risk analysis: it does not replace thoughtful choice, it broadens it.
Which detector to choose according to the type of room risk?
The choice of detector must start from the actual risk identified in each room, not from a general preference for one technology. For a smouldering fire with cold smoke (bedding, furniture, cabling in slow combustion), the optical smoke detector remains the reference because it detects smoke before heat rises. For a dusty room or one laden with vapours (workshop, industrial kitchen, garage, technical room), the thermal detector avoids false triggers linked to the environment. For a room with mixed or uncertain risk, or high sensitivity to false positives (patient room, meeting room, archive), the multi-criteria detector offers the best compromise between sensitivity and reliability. None of these three families constitutes a universal solution: it is the combination of several technologies in the same building, room by room, that characterises a well-designed installation.
What does NBN S 21-100 say about the choice of detector type?
The Belgian standard NBN S 21-100-1 governs the design and installation of fire detection systems, and requires that the choice of detector type be adapted to the nature of the risk present in each room rather than applied uniformly to an entire building. NBN S 21-100-2 then governs the maintenance and periodic inspection of the installation, once it is in service. In practical terms, this means that a design office or installer must document, room by room, the reasons for the technological choice made: presence of dust, vapour, type of combustible, ceiling height, air currents. This analysis determines both the reliability of detection and the frequency of maintenance interventions; a poorly chosen detector generates more false alarms and therefore more disruption and technical visits.
What selection errors should be avoided?
The most common error is to install the same type of detector throughout a building without taking into account the actual uses of each room, which exposes either to repeated false alarms or to detection that is too late. Here are the main points of vigilance:
- Installing an optical detector in a dusty workshop or garage without appropriate treatment, which multiplies false triggers.
- Installing a thermal detector alone in a storage room for slow-combustion materials, which delays detection of a smouldering fire.
- Ignoring changes in room use (transformation of an office into an archive, a workshop into storage) without reviewing the type of detector in place.
- Neglecting BOSEC certification of the installed equipment, which guarantees its compliance with requirements applicable in Belgium.
A prevention adviser or qualified installer must revalidate these choices with each significant change in room use, and not simply rely on a plan fixed at the time of construction.