Fire alarm power supply requirements
Mains supply specifications, standby battery requirements, what happens during a power failure, and the BS 5839-1 rules that govern fire alarm power supplies.
A fire alarm system that fails during a power cut provides no protection when it may be needed most. BS 5839-1 requires a dedicated mains supply and a standby power source capable of keeping the system operational during and after a power failure. Understanding these requirements is essential for anyone responsible for a fire alarm installation.
Mains Power Requirements
BS 5839-1 requires that the fire alarm control panel is connected to a dedicated mains supply — a circuit taken directly from the main distribution board and used solely for the fire alarm system. This supply should not be shared with other electrical equipment and should not be switchable from a location accessible to general building users.
The supply must be permanently live — it should not be on a time switch, a lighting circuit, or any circuit that could be routinely switched off. The distribution board circuit should be clearly labelled as the fire alarm supply, and ideally secured with a lockable isolator to prevent accidental or unauthorised disconnection.
| Requirement | Detail |
|---|---|
| Dedicated circuit | A separate MCB or fuse at the distribution board, used only for the fire alarm system |
| Permanently live | Not switchable, not on a time switch, not controlled by a lighting or HVAC circuit |
| Clearly labelled | The distribution board circuit must be labelled to identify it as the fire alarm supply |
| Supply voltage monitoring | The panel must monitor the mains supply and generate a fault indication if supply is lost |
Standby power
Standby Battery Requirements
The standby power supply — typically a sealed lead-acid or lithium battery housed within or adjacent to the control panel — must be capable of maintaining the system in a fully operational state for a defined period following mains failure. BS 5839-1 specifies the minimum standby duration based on the type of premises:
| Premises type | Minimum standby duration | Plus full alarm duration |
|---|---|---|
| Most non-domestic premises | 24 hours in standby (normal quiescent) condition | Plus 30 minutes with all sounders operating at full output |
| Premises where 24-hour occupation is unlikely | 24 hours standby | Plus 30 minutes full alarm |
| Premises with 24-hour occupation or sleeping risk | 24 hours standby | Plus 30 minutes full alarm — ARC monitoring recommended to compensate for reduced response during power outage |
The 24-hour standby plus 30-minute alarm standard means that a building experiencing a mains failure at 5pm on a Friday will still have full fire alarm protection throughout the weekend — assuming the battery is in good condition and the system is not in alarm continuously throughout that period.
Battery maintenance
Keeping the Standby Battery Serviceable
A standby battery that has degraded below its rated capacity provides less than the required standby duration — potentially much less. Sealed lead-acid batteries degrade over time even when not heavily used, and a battery that is five or more years old may provide only a fraction of its original capacity.
BS 5839-1 requires that the standby battery is tested during maintenance visits to confirm it still meets the required standby duration. The control panel monitors battery condition and generates a fault if the battery voltage drops below the minimum during a discharge test. Most maintenance contractors carry out a battery impedance test at each service visit — this provides a quick indication of battery health without requiring a full discharge test.
Battery replacement is typically required every three to five years for sealed lead-acid batteries, though actual service life depends on the environment, the frequency of mains outages, and the load on the battery during standby.
What happens on power failure
System Behaviour During Mains Failure
When mains supply to the panel is lost, the system automatically switches to standby battery power without interruption. The panel generates a mains failure fault indication — a visible and audible alert that should be investigated and logged. The system continues to function normally on battery power, detecting fire and sounding alarms if triggered, until either mains power is restored or the battery is exhausted.
The mains failure indication is an important maintenance trigger. A mains failure that goes unnoticed and unreported means the system is running on battery reserve — potentially for days — without the building owner being aware. Regular inspection of the panel for fault indications is a basic maintenance obligation under BS 5839-1.
Common questions
Frequently Asked Questions
The control panel monitors battery condition and should generate a fault indication if the battery voltage is low. During service visits, the maintenance engineer should carry out a battery test and record the result in the log book. If your system is showing a battery fault — or if the battery has not been replaced in over five years — contact your maintenance contractor. An exhausted or degraded battery provides less than the required standby duration and represents a compliance and safety issue.
No — BS 5839-1 requires a dedicated circuit. Sharing a circuit with other equipment creates the risk that the fire alarm supply is interrupted when the other equipment’s circuit is switched off, trips, or is isolated for maintenance. A dedicated circuit also ensures that the fire alarm is not affected by electrical faults or overloads on other circuits in the building.
The wireless devices — detectors, call points, and sounders — typically run on long-life batteries. The control panel, however, still requires a mains supply and standby battery in accordance with BS 5839-1. The panel battery requirements are the same for wireless systems as for wired systems. Individual device batteries are monitored by the panel and a low battery fault is generated when a device battery needs replacement.