On Top of the Changes: Fire Alarm Power Supplies

On Top of the Changes
Published On
May 10, 2018

There have been a few changes to the requirements for fire alarm system power supplies, and more are coming. This is a good time to review the ones we have so far. Obviously, since a fire alarm system is a life safety system, it is important that it works when needed. That is why we must have both a primary and secondary supply.

However, not all of the requirements will be found in NFPA 72. For example, all fire alarm systems do not need to be connected to a generator. So, where does that requirement come from? You will find these requirements in building, fire or life safety codes. A number of places in the codes provide requirements for emergency power, such as in 907. for emergency voice/alarm communication systems: “Emergency voice/alarm communications systems shall be provided with emergency power in accordance with Section 2702. The system shall be capable of powering the required load for a duration of not less than 24 hours, as required in NFPA 72.”

Typically, you will find requirements for emergency power for facilities where there will be many people or where keeping equipment running is critical, such as in a hospital.

Once you establish the need for emergency power, look at the power supply section of NFPA 72: “ Engine-driven generators used to provide secondary power for a protected premises fire alarm system or an emergency communications system shall comply with NFPA 110 Chapter 4, requirements for a Type 10, Class 24, Level 1 system.”

This means the generator must be capable of providing power within 10 seconds and must last 24 hours without being refueled. Level 1 means it is an emergency system: “ Installation of engine-driven generators used to provide secondary power for a protected premises fire alarm system or an emergency communications system shall be in accordance with NFPA 70, Article 700.”

Although you are not required to have a backup generator all the time, you are allowed to choose to do so. Keep in mind, if you are using a generator as part of the secondary power supply, you still need to provide a minimum of four hours of battery backup. This four-hour requirement would be included in the 24 hours of secondary power required by NFPA 72. This is to help ensure you will not lose your fire alarm system due to a failure of the generator to start. Batteries allow a trouble signal to be transmitted in the event of primary power failure. Without these batteries, you could lose the entire system, and no one would know. Remember that, typically, generators do not start if someone turns a breaker off, so this is a good feature to have in the event a maintenance person accidentally turns off the wrong breaker.

A change was made in the 2013 NFPA 72 regarding the replacement of fire alarm system batteries. It used to require replacement of the batteries within five years of the manufacture date (hence the month/year marking on the batteries). Now it states to replace the batteries in accordance with the alarm equipment manufacturer’s recommendations or when the battery voltage or current falls below the manufacturer’s recommendations.

The language for testing the primary and secondary power supplies in the testing table, Table, has been changed somewhat over the last couple of cycles. In my opinion, it is more confusing now than it was. Very few people know about the primary power supply testing. It is common knowledge that we need to disconnect the primary power and test the system on secondary power only, but were you aware that you also have to do the opposite: Test the system on primary power only with the secondary power disconnected?

I found a problem with this only one time that I can recall, and it was in a large high-rise with a heavily loaded voice communication system. When the batteries were disconnected, we found some of the more remote speaker circuits would not operate. Remember, with no batteries, you can only draw as much current as the primary power input transformer allows. Batteries act as a buffer to increase available current, at least for a limited period of time.

One other significant requirement was added in the 2013 NFPA 72 for a “listed” circuit breaker lock if a circuit breaker is the disconnecting means. In the 2016 edition, “listed” was changed to “approved.”

This is a short review of the power supply requirement for most fire alarm systems. Keep in mind I do not cover all power supply options.

About the Author
Tom Hammerberg

Thomas P. Hammerberg

Life Safety Columnist

Thomas P. Hammerberg, SET, CFPS is an independent fire alarm presenter and consultant in The Villages, Fla. He can be reached at TomHammerberg@gmail.com.

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