Douglas Krantz - Technical Writer - Describing How It Works
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What do We Include in Battery Calculations?

Batteries keep the fire alarm system fully working during a power blackout. Rule-of-Thumb - - if anything is going to use power at all during a power blackout, it has become part of the battery use calculations.

Included in the battery calculations are all devices on the SLC, NAC, Powered Relays, Powered Door Holders, Anything Else.

Greetings Douglas,

When calculating addressable fire alarm system load (current drain) during main power outage, do the loops, and field devices connected to the loops, have an impact on the current drain? Does one calculate only the drain related to the components of the fire alarm control panel?

Thank you, R B

When deciding what to include in the battery calculations, we look at the reasons for making these calculations in the first place. A fire alarm system is a life safety system; if the system doesn't work, people can get hurt - or worse. The times that the fire alarm system is supposed to work are "Always", including the time during a power outage.

The battery calculations are for you, the designer, the installer, or the technician. They show you that the system will still work even if there's a power blackout. The calculations are also for the fire marshal, UL, the insurance representative, and even the building owner; the calculations show them that you took the effort to make sure the system works during a power outage.

Stand-by Time

Reasonably, the National Fire Protection Association (NFPA) has decided that when there is a power outage, the fire alarm system should still operate for 24 hours, and should there be a fire at the end of the 24 hours, the fire alarm system needs to be able to evacuate the building.

That's where we come up with the 24 hours.

Anything that drains the batteries during that 24 hours has to be considered because otherwise, the life of the batteries is shorter than 24 hours.
  • The components in the panel use the batteries; the components have to be included
  • The devices on the loops use the batteries; the devices have to be included
  • If non-fire alarm relays use the batteries; the non-fire alarm relays have to be included
  • If door holders use the batteries; the door holders have to be included
  • If anything else uses the batteries; that has to be included

Alarm Time

Then we get to the alarm time.

Five Minutes - During a power outage, if a fire alarm system that has only horns and strobes, and also is used only to evacuate the building in less than five minutes, using its batteries, the fire alarm system has to sound the alarms for only five minutes.

That's where we come up with the five minutes of alarm time.

Two Hours or 15 Minutes - During the power outage, fire alarm systems that use audio messages (which may say other things like get to a safe place inside the building), or can take longer than five minutes to evacuate the building, have to be active during the alarm for two hours.

The NFPA is reasonable on this, also. Because they consider that the alarms will only be sounding intermittently during the two hours of alarm, they say that the batteries have to keep the whole building sounding in full alarm for only 15 minutes.

That's where we come up with 15 minutes during the two hours of alarm.

Include Everything

What it boils down to is if something is able to get its power to operate off the batteries, that device has to be included in the calculations.

It doesn't matter if it's an "Addressable System" or a "Conventional System", the calculations are for a life safety fire alarm system.

Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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