Secondary power and standby batteries: what NFPA 72 asks of them

Every fire alarm system runs on two independent power supplies. NFPA 72 Chapter 10 calls for a primary source and a secondary source that takes over when the primary fails. In most buildings the secondary source is the set of batteries in the panel cabinet, and if those batteries are dead the system has one power supply, not two.

How long the batteries have to carry the system

The secondary supply has to carry the system through a long quiet period and then still sound the alarm at the end of it. Chapter 10 sets the figures, and Chapter 24 adds to them for voice systems.

The battery calculation is what proves this. A system can pass a load test today and still fail the math.

Switching over

When primary voltage is insufficient, the secondary supply has to take over automatically within 10 seconds. Required signals cannot be lost, interrupted or delayed by more than 10 seconds because of a primary power failure. That 10 second figure is for the protected premises system. Supervising station facilities and equipment get 60 seconds, so do not apply one number to both.

Recharging after a discharge

After a full discharge, the panel has to bring the batteries back to full charge within 48 hours on its own. A charger that cannot do this is usually the reason a site keeps failing its next standby test.

Date codes and replacement

Under NFPA 72 Chapters 10 and 14, batteries are marked with the month and year they were made, in month/year format. Where the manufacturer did not mark them, the installer has to obtain the date code and mark the batteries. Batteries are then replaced against that date rather than run until they fail.

That puts the duty on the installer, so "the manufacturer did not put a date on it" is not an answer to the finding.

The equipment the signal depends on

The panel batteries are not the only secondary power that matters. NFPA 72 Chapter 26 requires transmitters, and shared equipment at the premises that the signal depends on, such as a router or modem, to carry at least 24 hours of secondary power. Shared equipment may drop to 8 hours, but only where the AHJ accepts it and a risk analysis supports it, so it is not something to assume from the installer's say so. Equipment serving only additional communications paths needs none, provided the first path already meets the communications requirements.

A panel with 24 hours of battery feeding a router on unswitched utility power is not monitored after a power cut. Follow the signal from the panel to the point where it leaves the building and ask what keeps each piece running.