NFPA 72 code sections · Signal Types & Monitoring
Positive alarm sequence: 15 seconds to acknowledge, 180 to investigate
In attended buildings the AHJ may permit positive alarm sequence. When a detector activates, trained staff have 15 seconds to acknowledge the signal at the panel; if they don't, the full alarm sounds. If they do, they get up to 180 seconds to investigate and reset the system. If they fail to reset in time, a second device activates, or a manual box is pulled, the alarm sounds immediately. The sequence requires written AHJ approval and trained personnel on site whenever it is enabled.
NFPA 72 reference
Chapter 23 — Protected Premises Fire Alarm Systems
23.8.1.3 Positive Alarm Sequence (acknowledge within 15 s; investigation up to 180 s; immediate alarm on second device or manual box)
References use NFPA 72 2019/2022 numbering — section numbers can shift between editions, so confirm in the edition your AHJ has adopted.
On the job
A high-rise lobby uses positive alarm sequence. A detector trips at 2 a.m. and the lone night guard, away from the desk, misses the 15-second acknowledge window — the building alarms, which is the safe outcome. The owner later asks to extend the acknowledge time to five minutes; the code does not allow it, and the AHJ declines.
Quick check
1. Under positive alarm sequence, staff must acknowledge the first signal within:
2. The maximum investigation period after acknowledgement is:
3. During the investigation period, a pull station activation:
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