Fire Alarm Power Supplies and Standby Batteries
A fire alarm system is only useful if it has power when it is needed, and the one time it is guaranteed to be needed is during a fire, which is exactly when the mains supply might fail. That is why every fire alarm system runs on two sources of power: the normal mains, and standby batteries that take over automatically. The power supply that manages both sits at the heart of the system's resilience. This guide explains how it works and what to check.
The essential point: mains powers and charges the system, standby batteries carry it through a mains failure, and the power supply manages, monitors and reports on both.
Who this is for
This is for fire alarm engineers working with control panels and power supplies. The experience level assumed is competent engineer. It explains the principles and the checks; the specific power-supply requirements, battery durations and calculations for a system come from the current BS 5839-1, the BS EN 54-4 product standard and the manufacturer's documentation.
Two sources of power
A fire alarm system must keep operating even when the mains supply is lost, including during a fire that may damage the building's electrical installation. So it is powered normally from the mains and backed up by standby batteries that take over automatically on mains failure. The batteries support the system for a standby period defined by the design, with enough capacity remaining to drive the alarm load at the end of it. In normal conditions the mains powers the system and keeps the batteries charged; the batteries are the safety net that makes the system dependable when the supply is not.
The power supply and its monitoring
The power supply, or PSU, converts the mains to the low voltage the system uses, charges and monitors the standby batteries, and provides the monitored supply that keeps the panel and devices working. Fire alarm power supplies are built to the relevant product standard, BS EN 54-4, and are monitored so that a power or battery problem is detected and reported rather than left to fail silently. That monitoring is a defining feature: the system is expected to know the state of its own power, so a mains failure, a charger fault or a failing battery shows up as a reported condition.
Sizing the standby batteries
Standby batteries are not one-size-fits-all. They are sized from the system's quiescent (standby) and alarm currents and the standby and alarm durations the design requires, so that the batteries can carry the system through a mains failure and still raise the alarm at the end. Getting this right is a genuine calculation, not a rule of thumb, and it is exactly what the battery calculator is for. The durations are a design decision made against BS 5839-1 and the building's needs, and batteries are replaced on a planned basis as they age, because their capacity falls over time.
Common power and battery faults
In the field, power and battery faults are among the most common conditions engineers meet. Standby batteries at or beyond end of life are a leading cause — capacity falls with age until the system reports it. Others include a mains supply that has been switched off or lost, charger or PSU faults, and degraded battery connections. Because the panel monitors power and battery condition, these usually present as reported faults; the job is to confirm the battery condition and the mains supply and to replace ageing batteries before they fail. This sits within the duty to keep the system maintained under the Regulatory Reform (Fire Safety) Order 2005 and the servicing practice in BS 5839-1.
When not to rely on this alone
When not to use this article: do not use it to size batteries or specify a power supply for a real system. Those follow from the current BS 5839-1, BS EN 54-4 and the manufacturer's documentation, applied by a competent person. This guide explains the principles and the checks.
Professional disclaimer
This is an educational resource for competent engineers. It does not replace the current British Standards, manufacturer documentation or professional judgement. Confirm power-supply and battery requirements against the current BS 5839-1, BS EN 54-4 and the manufacturer's data.
Related reading
Read this with fire alarm testing and maintenance, the free battery calculator and cabling: standard vs enhanced.