According to NFPA 72, National Fire Alarm and Signaling Code, the secondary power supply for an in-building Mass Notification System (MNS), which encompasses emergency voice/alarm communications systems, must be capable of operating the system under quiescent (normal) load for a minimum of 24 hours. Following this standby period, the power supply must have sufficient capacity to operate the system under maximum connected load for a period of 15 minutes. This requirement ensures that critical voice instructions can be broadcast throughout an emergency, even after a prolonged primary power failure.

B. 30 minutes: This is not the minimum duration specified by NFPA 72 for this type of system; it exceeds the required time.

C. 45 minutes: This duration is incorrect and not a standard requirement found in NFPA 72 for secondary power supplies of voice alarm systems.

D. 60 minutes: While a risk analysis for a specific facility might mandate a longer duration, 60 minutes is not the prescribed minimum in the code.

1. NFPA 72, National Fire Alarm and Signaling Code, 2022 Edition. Chapter 24, "Emergency Communications Systems," Section 24.4.1.5 states: "The secondary power supply for an in-building MNS shall be capable of operating the system under quiescent load for a minimum of 24 hours and then shall be capable of operating the system under maximum load for a period of 15 minutes."

2. SFPE Handbook of Fire Protection Engineering, 5th Edition. Chapter 53, "Detection and Alarm," Section 53.5.3, "Power Supplies," notes that for voice evacuation systems, the requirement is for 24 hours of standby followed by 15 minutes of full-load operation, referencing the requirements within NFPA 72. (DOI: 10.1007/978-1-4939-2562-453)

3. Oklahoma State University, School of Fire Protection and Safety Engineering Technology, FST 4353: Advanced Fire Alarm & Detection Systems course materials. The curriculum directly cites and teaches the power supply requirements from NFPA 72, Chapter 24, confirming the 24-hour standby plus 15-minute alarm duration for emergency voice/alarm communication systems.