Why does a fire detection panel need a backup power supply?
The backup power supply of a fire detection panel takes over as soon as a mains power failure occurs, so that monitoring is never interrupted. It relies on batteries installed in the panel cabinet or in a dedicated enclosure, permanently charged by the mains. Without this power supply, a fire that breaks out during a power cut would simply not be detected. This is why NBN S21-100-1, the Belgian standard governing the design and installation of fire detection systems, imposes precise requirements on how long this power supply must last, both in standby and in triggered alarm situations. This point is verified during commissioning and checked periodically according to NBN S21-100-2.
What minimum durations does NBN S21-100-1 impose?
The standard distinguishes two phases: the standby duration on battery in case of mains failure, and the alarm operation duration that immediately follows. In the most common configuration, the required standby autonomy is 24 hours when the repair or replacement delay of the faulty power supply can reasonably be guaranteed within this period. When this intervention delay cannot be assured (isolated site, absence of reactive maintenance contract), the required standby autonomy increases to 72 hours. In both cases, the panel must then be able to power all audible and visual general alarm devices for at least 30 minutes, with the battery already stressed by standby. This cumulative period (standby then alarm) is the reference calculation for sizing.
How to size batteries according to building category?
Sizing batteries for a fire alarm panel is calculated from the actual consumption of the installation in standby and alarm modes, multiplied by the required durations, then increased by an aging margin. This is not a single fixed figure: each installation has its own consumption depending on the number of detectors, loops, sirens and remote devices connected to the panel. The building category, defined by the Royal Decree of 7 July 1994 (low-rise, medium-rise or high-rise building), indirectly influences this calculation because it determines the number of detection zones, the possible presence of linked smoke extraction and the overall complexity of the system, hence its total electrical consumption. A high-rise building, with more loops and repeater panels, requires batteries with greater capacity than a low-rise building equipped with a simple panel.
| Calculation element | What it determines |
|---|---|
| Standby consumption (mA) | Battery capacity to last 24h or 72h |
| General alarm consumption (mA) | Battery capacity to last an additional 30 minutes |
| Number of loops and remote devices | Total installation consumption |
| Repair delay guaranteed by maintenance contract | Choice between 24h or 72h autonomy |
What happens if the actual battery autonomy is insufficient?
Insufficient autonomy means that the fire detection panel may stop operating before a maintenance intervention occurs, exposing the building to a total absence of detection during this window. This is a fault that does not always appear visually: the panel can operate normally on mains power for months while its batteries, aged or undersized, would no longer meet the regulatory period in case of power failure. Sealed lead-acid batteries used in these installations gradually lose their capacity over time and charge/discharge cycles, a phenomenon accelerated by ambient heat in the technical room. This is precisely what NBN S21-100-2 requires to be checked during periodic inspections: testing actual autonomy, measuring voltage under load, and preventive battery replacement according to their age, generally before they reach their end-of-life limit announced by the manufacturer.
How to concretely verify the autonomy of a backup power supply during maintenance?
Verifying the autonomy of a backup power supply consists of simulating a mains failure and measuring whether the batteries actually last the required standby duration then the alarm duration, without voltage drop below the panel's operating threshold. The technician performing the periodic inspection according to NBN S21-100-2 cuts the panel's mains power supply, triggers a general alarm at the end of the test, and records the battery voltage at different intervals. If the voltage drops too quickly or falls below the minimum threshold before the end of the theoretical cycle, the batteries must be replaced without waiting for the usual calendar deadline. This operation is recorded in the building's safety register, a document that the prevention advisor and the emergency zone (hulpverleningszone) can consult during their visits. Poorly anticipated battery replacement is one of the most frequent causes of failure of an otherwise correctly installed panel.
Who decides on 24-hour or 72-hour autonomy for a given building?
The choice between 24-hour or 72-hour standby autonomy depends on the actual ability to have a maintenance technician intervene within the shortest delay, a decision made at the installation design stage. Concretely, if the operator has a maintenance contract guaranteeing intervention in case of power supply failure within 24 hours, 24-hour autonomy is sufficient in accordance with NBN S21-100-1. In isolated sites, with reduced hours, or when no short-delay contractual commitment exists, 72-hour autonomy is required to cover a complete weekend or a period without on-site presence. This choice appears in the installation's technical file prepared by the installer, a document that the prevention advisor should be able to present to the emergency zone during an inspection.