What is a general alarm?
The general alarm is the audible (and sometimes visual) signal intended for all occupants of a building, which triggers their total and immediate evacuation. It is the final signal of the fire detection system: once issued, it must leave no ambiguity about the action to be taken. In the logic of standard NBN S21-100-1, which governs the design and installation of fire detection systems in Belgium, this signal can be triggered automatically by the detection control panel as soon as a detector confirms an alarm, or after an intermediate phase of human verification. The choice between direct triggering and delayed triggering depends on the building's configuration, its occupancy (healthcare facility, office, warehouse) and the risk analysis validated during the system's design.
What is a restricted alarm?
The restricted alarm (sometimes referred to as restricted general alarm) is an intermediate signal that does not alert the entire building but only designated personnel: the security post, reception, or a trained person permanently present. This signal allows this person to go to the location and verify the reality of the incident before the general alarm is sent to all occupants. This step, called verification, avoids total evacuations triggered by false alarms (dust, steam, technical fault). It is only permitted if the building has qualified and available human presence permanently during occupancy hours, a condition verified during system design according to NBN S21-100-1.
How does the time delay between detection and general alarm work?
The time delay is the organized period, regulated by the standard, between the detection of a fire start and the automatic triggering of the general alarm if no action has confirmed or invalidated the incident. Concretely: a detector activates, the control panel sends a restricted alarm to designated personnel, who have a limited time to go to the zone and intervene on the control panel (acknowledge, confirm, or restart the process). If no action is recorded within the set timeframe, the control panel itself triggers the general alarm, without waiting for additional human decision. This time delay is never left to the operator's sole discretion: it must be justified in the system's design documentation and remain compatible with the admissible evacuation times for the building concerned.
Who decides on triggering the general alarm?
Triggering the general alarm is either an automatic process of the control panel, or a voluntary action by the safety manager or designated person present at the security post during the verification phase. This person, trained in operating the detection control panel and evacuation procedures, must be able to quickly assess the situation: presence of smoke, flames, abnormal behavior of technical installations. Their role is not to delay evacuation through excessive caution, but to avoid unjustified general evacuation while retaining the ability to trigger the alarm manually at any time, even before the time delay expires, as soon as reasonable doubt about the reality of the incident disappears.
What maximum delay is tolerated before total evacuation?
The maximum time delay is not a single figure set identically for all buildings: it is determined project by project, based on the building's height as defined by the Royal Decree of 7 July 1994 (low, medium or high building), its occupancy and the effective presence of qualified personnel. This delay must remain compatible with the estimated evacuation time of occupants and with the intervention times of the competent emergency zone (hulpverleningszone). It is validated during the detection system's design, documented in the installation's technical file, and verified during periodic inspections required by NBN S21-100-2. No time delay can be applied without this written justification.
In which buildings is time delay authorized?
Time delay with restricted alarm is only possible in buildings with permanent and qualified human presence during occupancy periods: hospitals and healthcare facilities with on-duty staff, large tertiary complexes with occupied security posts, industrial sites with first response teams. In a building without continuous human supervision (empty office in the evening, closed shop at night), the general alarm must trigger automatically and immediately upon detection confirmation, without an intermediate phase. It is the risk analysis carried out upstream, in consultation with the prevention advisor and often with the opinion of the emergency zone (hulpverleningszone), that determines whether the conditions for time delay are met for a given building.
What role does the emergency zone play in this organization?
The emergency zone (hulpverleningszone) intervenes upstream, when analyzing the building's safety file, to give its opinion on the relevance of a time delay and associated evacuation procedures. It verifies that the chosen organization (staff presence, training, timeframes) is consistent with the building's characteristics and the emergency services' intervention capabilities. It may also be required to verify, during visits, that the operator complies with commitments made in the initial file: effective presence of designated personnel at scheduled times, up-to-date procedures, regular training of persons responsible for verification. These checks are part of the broader framework of periodic inspections required by NBN S21-100-2 for fire detection system maintenance.