Published on May 3, 2021.

In Compliance | NFPA 72

Ensuring the fire alarm system remains reliable with a secondary power supply


As we witnessed this winter in Texas, and as we’ve seen in many other instances, extreme weather and increased usage of the power grid can result in a loss of primary power to a building in the form of power outages or rolling blackouts. In order to remain operational during the loss of primary power, fire alarm systems are provided with a secondary source of power. NFPA 72®, National Fire Alarm and Signaling Code®, outlines how the secondary power must be designed and how it must be inspected, tested, and maintained to ensure it is operational when needed the most.

The most common forms of secondary power supplies are batteries or an emergency generator. Secondary power supplies are designed to provide enough capacity to power the entire system for 24 hours on standby and then operate the system for at least 5 minutes under emergency conditions (15 minutes for emergency voice/alarm communication systems). If a generator is used for secondary power, batteries are still required, but only need to provide capacity for four hours—enough time to get the generator operational if there is an issue. To ensure that the secondary power supply is always available, the fire alarm system itself is able to monitor for the presence of voltage as well as the battery charging system, and will annunciate a trouble signal if there is an issue with the power supply or charging system.

Although the fire alarm system can monitor some aspects of the secondary power supply, there is some inspection, testing, and maintenance (ITM) that needs to be completed to ensure that the secondary power supply is reliable. Batteries need to be inspected semiannually to confirm that the connections are tight and there is no corrosion on the connections. During inspection, the batteries need to be checked for damage such as cracks in the case, bulges, or leaking. The batteries also need to be marked with the month and year of manufacture, not the date of installation—this information is important for tracking the age of the batteries. If a battery’s age exceeds the manufacturer’s replacement date, the battery needs to be replaced.

The batteries and charger need to be tested semiannually. Tests include measuring the temperature to ensure that the battery is not 18° F (10° C) above ambient temperature; measuring the voltage to ensure the battery and charger are still operational; measuring the voltage at each cell of the battery to confirm each cell is greater than 13.26 volts; and measuring the internal ohmic value of each battery and comparing it to previous tests to ensure that the battery does not have 30 percent or more conductance or 40 percent or more resistance or impedance than previous tests, or that it is outside the manufacturer’s acceptable ranges.

Every three years the batteries need to either be replaced, or a load test needs to be conducted. Load tests are conducted by putting a known load (which can be obtained from the battery manufacturer) on the battery for a given time. The battery is discharged until it reaches its end voltage. Based on the known load and the time taken to discharge, you can then calculate the capacity of the battery and apply any adjustments for temperature. The battery must be replaced if the capacity is less than 80 percent of its rated capacity.

All of these ITM requirements focus on the batteries themselves, but there are additional tests that need to be completed to make sure the entire system will operate under secondary power. First, if the system is supplied by an emergency generator, power will need to be transferred to the generator monthly to ensure that the transfer switch and generator will be able to supply the fire alarm. Additionally, all primary power to the system needs to be disconnected annually so the required standby and alarm current to the system can be measured and compared to the available battery capacity. It is important to remember that these batteries need to be able to provide the 24-hour standby and 5 (or 15) minutes of alarm, or 4 hours of standby if there is also an emergency generator. Finally, the system needs to be operated under secondary power in alarm for at least 5 or 15 minutes depending on the system type.

We rely on the fire alarm system as a key component of the fire and life safety of our buildings, and we rely on the secondary power supply to keep the fire alarm system operational in the event of a power outage. It is important that you understand and follow the ITM requirements in NFPA 72 to ensure that the fire alarm system remains reliable with the secondary power supply.

SHAWN MAHONEY is an engineer at NFPA. NFPA members and AHJs can use the Technical Questions tab to post queries on NFPA 72 at Top photograph: Getty Images