How long should a fire alarm's backup battery last during a power failure?

The correct answer is that a fire alarm's backup battery should last for 24 hours plus an additional 5 minutes in an alarm state. This requirement ensures that the alarm system remains operational during a power failure, allowing adequate time for response in an emergency situation.

Having a backup battery that supports a 24-hour period provides a reasonable buffer for short power outages, ensuring that the fire alarm system can continue to operate and alert occupants if a fire occurs during that time. The additional 5 minutes in an alarm state is crucial because, during an actual fire emergency, the alarm may need to sound for an extended period to ensure that it effectively alerts all occupants to evacuate.

This provision reflects the importance of reliability in life safety systems. It's essential for fire alarms to not only function during standard power scenarios but also to maintain operability during critical failures to provide a timely warning to individuals, thus enhancing the overall safety of the premises.