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

A fire alarm's backup battery is critical for ensuring that the system remains operational during a power failure. The duration of backup battery operation is typically established based on safety regulations and industry standards. The correct answer specifies that the backup battery should last for 24 hours plus an additional 5 minutes when the alarm is in an active alarm state.

This additional time is important because during a fire emergency, the alarm system must not only remain powered for general monitoring, but also be capable of issuing an alert when activated. The 24 hours portion ensures that the system can remain functional over a typical power outage duration, allowing for adequate response time from emergency services. The extra 5 minutes in alarm state gives further assurance that during critical moments when an alarm is sounding, the system will continue to function properly, thereby enhancing the safety of occupants and allowing them to respond to the emergency.

Other durations like 12 hours, 24 hours only, or 48 hours do not align with these requirements, as they either do not accommodate the additional time needed for the alarm to operate when activated or exceed the expected operational standards for most fire alarm systems.