Guide to Fire Alarm Basics – Power Supplies

A fire alarm system is a crucial part of the overall fire protection and life safety strategy of a building. A fire alarm system serves many functions and the differences between the functions can be a bit confusing, so I created a visual guide to fire alarm basics. The objective of this blog series is to discuss some of the major components and functions of a fire alarm system. For an overview of the entire system take a look at my Guide to Fire Alarm Basics Blog. This blog will take a deeper dive into fire alarm power supplies.

It is important for a fire alarm system to be provided with reliable power so it can operate during any emergency. There are a few different options when it comes to choosing a reliable power supply, as well as some calculations that are necessary to ensure that the fire alarm system is provided with sufficient backup power.

Power Supply OPtions  

There are a few different options out there when it comes to providing a reliable power source. They include providing an additional power source in addition to the primary power such as batteries or an emergency generator so there is backup power if primary power is lost or providing power through a single source such as a Stored-Energy Emergency Power Supply System (SEPSS).

Primary power to the fire alarm system can be provided by the electric utility, an engine-driven generator (this is not a standby generator, however it is a site generator meeting the requirements in NFPA 72), and Stored-Energy Emergency Power Supply System (SEPSS), or a cogeneration system.

Batteries are a common way to provide a secondary power supply, the most common type of battery is a Valve-Regulated Lead-Acid battery and they are typically located within the fire alarm control unit enclosure, or in a separate battery box located near the fire alarm control unit. Batteries need to be sized so that they can provide power to the entire fire alarm system for 24 hours in standby and 5 minutes in alarm, if the system is an emergency voice alarm communication system (EVACS), then the batteries need to provide capacity for 15 minutes in alarm in addition to the 24 hours in standby. The additional time is required to allow for a longer evacuation time as buildings with an EVACS typically utilize a partial evacuation that would require constant communication with the occupants during the evacuation.

Another common way of providing a secondary power supply for a fire alarm system is the use of an emergency generator designed, installed, and maintained in accordance with NFPA 110, Standard for Emergency and Standby Power Systems, which provides power to the fire alarm system through an automatic transfer switch. If using an emergency generator, you are still required to provide batteries as well just in case there is an issue with getting the emergency generator started. These batteries however, only need to provide a capacity for 4 hours instead of the 24 hours in standby.

Instead of providing two separate power supplies, you are permitted to provide power via a Stored-Energy Emergency Power Supply System (SEPSS) otherwise known as an Energy Storage System (ESS) or an Uninterruptible Power Supply (UPS).  The SEPSS must be configured in accordance with NFPA 111 and provide 24 hours of backup battery. The SEPSS is also fed via a compliant primary power supply such as utility power or an on site generator.

As noted above, if batteries are part of the secondary power source for a fire alarm system then they must be sized to provide capacity to run the system for 24 hours in standby and then either 5 minutes in alarm or 15 minutes in alarm for EVACS. A simple calculation for a basic fire alarm can be seen above.

1) First the total system standby current and the total system alarm current is calculated. This is done by multiplying the standby current and alarm current for each piece of equipment by the total quantity of each piece of equipment and adding them together, the result is the total AMPS required in standby and alarm. Both the standby current and the alarm current for equipment can be found from the manufacturer in the data sheet.

2) Next total standby capacity is required by multiplying the total system standby current by the required 24 hours to achieve the required standby capacity in AMP-HRS. The same is done with the alarm capacity, however, instead of 24 hours, the current is multiplied by either 5 minutes (0.083 hours) or 15 minutes (.25 hours) to achieve the required alarm capacity in AMP-HRS.

3) Finally, both the standby capacity and the alarm capacity is added together and a 25% safety factor is applied to arrive at the total required battery capacity.

Want to Learn More?

Like I noted in the beginning of this blog, if you are interested in learning more about fire alarm basics, take a look at my Fire Alarm Basics Blog. If you found this article helpful, subscribe to the NFPA Network Newsletter for monthly, personalized content related to the world of fire, electrical, and building & life safety.

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Shawn Mahoney
Technical Services Engineer

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