Besides being in standby for 24 hours during a power outage, the fire alarm system has to sound the alarms for 5 minutes if everyone has to get out of the building within 5 minutes, or 2 hours intermittently if some people aren't leaving right away.
There are two things to consider when deciding whether it's 5 minutes of evacuation, or 15 minutes of evacuation.
One consideration is what building is going to be used for (its occupancy), and the other is what kind of fire alarm system is going to be needed for the occupancy. The NFPA 101 or the International Building Code (IBC) show the occupancy. They show what kind of fire alarm system should be used.
The other consideration is the requirements needed for each type of fire alarm system. The NFPA 72 shows the requirements needed for a number of different types of fire alarm system.
Occupation of the Building - NFPA 101 or IBC
Either the NFPA 101 or the International Building Code can be used to determine occupancy type.
For battery backup, the fire alarm type shows how the building is going to be evacuated when a fire breaks out. However, the NFPA 72 doesn't show which kind of building is going to have a particular type of evacuation; for a particular type of evacuation, the NFPA 72 only shows what is needed.
In case of fire, there are three different types of evacuation that are used in buildings.
Total-Evacuation - Everyone gets out of the building immediately - This would primarily be places like small single story to three story offices, schools, and the like.
Staged-Evacuation - People in the greatest danger of fire get out immediately while everyone else waits their turn - Like in tall buildings that have narrow stairways.
Defend-In-Place - People who are in immediate danger, move somewhere else to a safe place in the building; people who can't easily move, get behind a closed fire door and wait for help; those not helping but can easily evacuate, get out anyway - This would be places like hospitals, nursing homes, and other places where people can't easily move.
The NFPA 101 or the International Building Code (IBC) are used to determine occupancy, and also what kind of fire alarm system is going to be used in each occupancy. These books classify different occupancies, like places of assembly, business, education, factory and industrial, high hazard, institutional, retail, residential, storage, etc.
Then the type of building also has to be considered, like a small single floor building versus 97 story building versus a sprawling business, retail, hotel complex. Even things like whether the building is fully sprinklered has a lot to do with what type of fire alarm system is used.
Once the occupancy is determined, then the NFPA 101 or the IBC show what type of fire alarm system should be used.
This shows whether there needs to be horns and strobes, or voice evacuation with strobes.
Fire Alarm System Type - NFPA 72
The NFPA 72 shows how a lot of different fire alarm systems need to operate. The NFPA 72, however, does not show which type of what type of fire alarm system is going to be used.
On the surface, the NFPA 72 seems to conflict itself because it shows 5 minutes of evacuation, and also shows 15 minutes of evacuation. This is not a conflict, though. The real reason the NFPA 72 shows both types of evacuation is that the NFPA 101 and the IBC (International Building Code) show both types of fire alarm system, so the NFPA 72 has to show both types.
Evacuation Time is Not Total Time
The times shown, 5 minutes or 15 minutes, is considered to be reasonable times.
For a small total evacuation building, 5 minutes is all the time reasonably needed to get everyone out. The horns and strobes need to operate the full 5 minutes, so all calculations need to be based on the 5 minutes.
The other types of evacuation, staged evacuation and defend-in-place, takes longer to get people out. The time needed to get everyone out isn't 15 minutes, the time needed to get everyone out is considered to be 2 hours.
However, reasonably, it is considered that the speakers and strobes are going to be in operation only intermittently during that 2 hours. The calculation for the backup batteries only needs to be for 15 minutes, not for 2 hours. That is based on testing. Testing of the fire alarm system has to be based on something, and 2 hours seems to be excessive; it is not a reasonable time for testing. Because the system is really only going to be used intermittently during that 2 hours, the test needs to be for 15 minutes of continuous evacuation for the entire building. This is for the audio system, and this is also for the strobes.
NFPA 72 Isn't Conflicting Itself
The apparent conflict in the NFPA 72 showing battery backup for 5 minutes of evacuation, and at the same time showing 15 minutes of evacuation isn't a conflict, it's two different types of fire alarm systems. The NFPA 101 or the IBC are needed to determine what type of fire alarm system should be used.
If the required time is not spelled out for you in the job specifications, most of the time, you can determine what kind of system is going to be used by backwards-engineering. If there are only horns and strobes, and they all turn on at once to evacuate the whole building at once, 5 minutes can usually be used for the calculations.
Voice evacuation is different. Any voice evacuation can also be used for Mass Communication. The rules may or may not have caught up with reality yet, but any use of the audio system for fire evacuation can also be used for Mass Communication. Even if the building is supposed to be total evacuation in 5 minutes, assume that for any fire alarm system that has speakers, eventually it is going to be used for Mass Communication, like the warning of severe weather, or warning that an active shooter is on the loose in the halls.
Where there are speakers used instead of fire horns for total evacuation, staged evacuation, defend-in-place evacuation, the rule of thumb indicates that you should base your battery calculations on 15 minutes because no one knows how long the system has to be used. This includes calculations for the power supplies for the strobes as well as the power supplies for the horns and audio system.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book
. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.