What are the most common causes of false fire alarms?
False fire alarms come in the vast majority from four technical causes: fouling of detectors by dust, the presence of vapour or condensation, unsuitable positioning in relation to the activity of the room, and ageing of equipment not detected due to lack of maintenance. These are not device defects in most cases, but mismatches between the installed technology and the actual environment of the building.
In an industrial or tertiary building, these unwanted triggers have a real cost: unnecessary mobilisation of the emergency zone (hulpverleningszone), staff evacuation, loss of confidence of occupants in the system. NBN S 21-100-2 requires periodic maintenance and inspection of installations, precisely to identify these drifts before they result in repeated triggers. A system that multiplies false alarms is therefore not only annoying: it often reveals a lack of maintenance or a poorly adapted detector choice from the start.
Can dust and construction sites trigger a false alarm?
Yes, dust is one of the most common causes of false alarms, particularly in workshops, warehouses and during renovation work. Optical smoke detectors work by light scattering in a measurement chamber: a suspended dust particle produces a signal close to that of real smoke.
On an industrial site with production of fine particles (wood, metal, textile), or during a construction site with demolition and sanding, this type of detector is easily triggered without the slightest fire starting. The solution is not to switch off detection during work without supervision: the NBN S 21-100-1 standard requires compensatory measures in case of temporary deactivation. The installer instead favours, depending on the zones, a detector less sensitive to non-combustible particles or approved temporary mechanical protection, in consultation with the prevention advisor.
Why do vapour or humidity trigger detectors?
Water vapour and condensation trigger false alarms because suspended droplets scatter light in the same way as smoke, in conventional optical detectors. Collective kitchens, shower rooms, laundries and technical rooms with vapour production are the most exposed areas.
This phenomenon is aggravated by temperature variations that cause condensation directly inside the detector housing, especially in unheated rooms or those subject to large temperature variations (loading docks, airlocks). A standard optical detector installed in the immediate vicinity of a fryer, shower or industrial washing machine without prior consideration is almost doomed to trigger unwanted alarms. The installer analyses the actual use of the room before choosing the technology, and in these areas favours a thermal or multi-criteria detector, less sensitive to vapour, or a relocation of the detection point outside the direct axis of the vapour source.
How does fouling of detectors cause unwanted triggers?
Fouling of the optical chamber of a smoke detector decreases the actual trigger threshold over time, until it causes alarms for minimal variations in ambient air. Dust that accumulates on optical components ends up being interpreted as smoke, even in the absence of any new particles in the air.
This ageing is gradual and invisible to the eye for the operator: the detector appears to function normally until the day it triggers for a trivial event. This is exactly what NBN S 21-100-2 seeks to prevent by requiring periodic inspection of the installation, with verification of the condition of detectors and replacement of those whose fouling exceeds the admissible threshold. A detector that is too fouled is not cleaned with a cloth by the installer: it is replaced, because the optical chamber is not designed to be dismantled on site without loss of reliability.
Can poor positioning of detectors cause false alarms?
Yes, a detector poorly positioned in relation to air flows, heat sources or room equipment is a common cause of unwanted triggers, regardless of the condition of the equipment. A detector placed too close to a ventilation outlet, halogen lighting, a cooker or an open loading dock captures phenomena that have nothing to do with a fire.
NBN S 21-100-1 sets installation rules (distances from walls, obstacles, ventilation) precisely to avoid this type of malfunction. A building that has changed use since the initial installation, for example a workshop converted into an office or a storage area that has become a production area with new equipment, often ends up with detectors positioned for a use that no longer exists. The installer then carries out a survey and repositions the detection points according to the actual and current use of the room.
How does an installer reduce false alarms without desensitising detection?
An installer reduces false alarms by acting on technology, positioning and maintenance, never by lowering detection sensitivity below what NBN S 21-100-1 requires. Artificially reducing the sensitivity of a detector to avoid unwanted triggers amounts to creating a risk of non-detection of a real fire, which is unacceptable.
The correct approach follows several concrete steps:
- 1.Identify the precise cause of the trigger (analysis of system history, recording of conditions at the time of the alarm).
- 2.Adapt the technology to the room: thermal or multi-criteria detector in humid or dusty areas, aspiration detection in demanding environments.
- 3.Reposition poorly installed detectors while respecting the distances of NBN S 21-100-1.
- 4.Strengthen the periodic inspection NBN S 21-100-2 on risk areas, with early replacement of fouled detectors.
- 5.Verify that the installed equipment is BOSEC certified, guaranteeing the reliability of the product itself.
This approach targets the cause of the trigger rather than the symptom, which maintains a compliant detection level.
Should false alarms be reported to the emergency zone?
Repeated false alarms must be monitored and documented by the operator, as they are part of the elements examined during inspections and may be requested by the emergency zone (hulpverleningszone) during its visits. A register of triggers, with date, identified cause and corrective action, demonstrates that the installation is properly maintained in accordance with NBN S 21-100-2.
This monitoring is not just an administrative formality: it allows the installer to identify recurring patterns (same detector, same time, same room) and therefore to treat the cause rather than reset the system each time. An operator who records no false alarms loses this analytical capacity and is exposed to a repetition of the problem without ever understanding its real origin.