Power Units

Fire Alarm Systems — Types of Fire Alarm SystemsSystem DesignControl of Indicating PanelsDetector HeadsManual Call PointsAlarm SoundersPower UnitsDiversion RelaysSelf Contained Fire AlarmsWiring & InstallationInspection & ServicingStatutory Regulations

Two power supplies are required i.e. mains and battery and these are normally built into the Fire Alarm Control Panel. Standby batteries must allow the system to operate without mains for 24 hours longer than the building is likely to be unoccupied and then support the sounders for an additional half hour. If the mains supply is supported by an emergency generator then six hours standby plus half an hour alarm load is sufficient. All modern Fire Alarm Systems are 24 volts. On the medium and larger sized Fire Alarm Systems, the standby batteries will often not fit within the Control Panel. Where standby batteries are contained within a separate housing, then this housing must be as close as possible to the main Fire Alarm Control Panel. If the power supply or battery housing is located more than 10 metres from the main Fire Alarm Control Panel then serious volt drop problems can arise. Standby batteries are invariably of the sealed lead acid variety. Use of Nickel Cadmium Batteries is not cost effective and automotive batteries must not be fitted.

Points to consider

System voltage
Battery charger output
Battery capacity
Indication of battery and/or mains supply failure
Secondary battery exclusive to fire protection system
Where system voltage exceeds extra-low voltage compliance
Diversion Relays and Supervisory Buzzers

No British Standard exists specifically for this item and indicating panels. A diversion relay permits the audible signal until the system is reset.

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