Battery backup during alarm conditions should last for the duration specified in NFPA 72; which option represents this duration?

• A. 30 minutes
• B. 1 hour
• C. 2 hours
• D. 24 hours under normal load, 4 minutes under alarm load

Answer: (D)

NFPA 72 specifies how long a fire alarm system must run on backup power when the mains fail. The battery must sustain normal operation for a full 24 hours, then be able to continue operating under alarm conditions for a defined short period. Under alarm conditions the system draws more current because signaling devices like horns and strobes activate, so the backup power must handle that higher load for a brief time. The standard duration for alarm load is 4 minutes, ensuring the system can keep signaling and occupants informed while power is restored or the system is reset. That combination—24 hours under normal load and 4 minutes under alarm load—meets NFPA 72. The other options fall short because they provide shorter standby times under normal operation and do not reflect the required alarm-load duration.