Both delay general alarm so trained staff can investigate first — but only one of them has a code-mandated stopwatch, and a fail-safe that fires if nobody answers in time.
The companion Concept Explainer on alarm, supervisory & trouble signals covers what a signal means once the panel reports it as a general alarm. This one covers a step that can happen before that: a deliberate, engineered pause — a signal that goes first to a specific, staffed location instead of triggering building-wide notification immediately. That broader idea is called presignal operation. Positive Alarm Sequence (PAS)is often described as if it were just presignal by another name. It isn't.
PAS is one specific, tightly bounded, code-approved way to implement the presignal concept: NFPA 72 §23.8.1 caps the acknowledgment window, caps the total investigation time, restricts it to automatic-detector signals only, and — critically — forces general alarm automatically if the sequence isn't properly closed out in time. A generic presignal scheme isn't required to have any of that. That gap is exactly why an unconstrained presignal delay was historically linked to preventable delayed-evacuation deaths, and why the code doesn't simply permit "presignal" — it permits PAS.
The reason NFPA 72 lets a fire alarm system delay general notification at all is to reduce false evacuations and nuisance alarms from a single automatic detector signal, by giving trained staff at a constantly attended location a short window to investigate first. But a delay that depends entirely on a human being present, awake, and correctly reading a panel is a delay with no floor under it — if nobody answers, nothing happens automatically, and that gap is exactly where an unconstrained presignal scheme can fail. Positive Alarm Sequence closes that gap by making the failure mode itself safe: acknowledgment must happen within 15 seconds or the system escalates on its own; the entire investigation, even if acknowledged, cannot run past roughly 180 seconds before the system escalates on its own again; and a manual pull station or a second automatic device is never subject to any delay at all. PAS doesn't just add a pause before general alarm — it adds a pause with a guaranteed exit, which is the one thing a generic presignal implementation is not required to have.
They describe the same basic move — routing a signal to a specific location before triggering general building-wide notification — but that similarity stops at the concept. Presignal operation, as a general term, doesn't by itself imply any particular acknowledgment window, any particular time ceiling, or any guaranteed automatic escalation if the person monitoring the signal never responds. Historically, presignal schemes implemented without those constraints could leave a building in an indefinite pre-alarm state with no forced path to general notification — and that gap was linked to preventable delayed-evacuation deaths, which is a large part of why fire codes moved away from permitting open-ended presignal delays. Positive Alarm Sequence is NFPA 72's answer: it is apresignal implementation, not a different concept, but one hemmed in on every side that historically caused harm — a hard 15-second acknowledgment window, a roughly 180-second total ceiling regardless of acknowledgment, a restriction to automatic-detector signals only (a manual pull station is never delayed), and a mandatory automatic escalation to general alarm if the sequence isn't properly closed out. Calling any presignal-style delay "PAS" understates what PAS actually requires; calling PAS "just presignal" understates the fail-safe that makes it code-compliant in the first place.
Explains the difference between presignal operation — the general concept of routing a fire alarm signal to a specific location before general building-wide notification — and Positive Alarm Sequence (PAS), NFPA 72 §23.8.1's specific, tightly regulated way to implement that delay safely, with a mandatory 15-second acknowledgment window, a roughly 180-second total ceiling, and a mandatory automatic-escalation fail-safe that a generic presignal scheme isn't required to have.
Presignal operation describes any fire alarm arrangement in which an automatic signal is routed first to a specific location — often building staff, security, or a monitoring point — rather than triggering general, building-wide alarm notification immediately. The underlying rationale is to let someone with local knowledge investigate an automatic detector signal before evacuating an entire building, reducing false alarms and needless disruptions. As a general concept, though, presignal operation doesn't inherently specify how long that investigation window can last, whether acknowledgment is required within any particular time, or what happens automatically if nobody responds. Those specifics depend entirely on how a given system is designed — which is exactly the ambiguity that made unconstrained presignal implementations a genuine life-safety liability.
Positive Alarm Sequence, defined in NFPA 72 §23.8.1, is one specific, code-approved way to implement the presignal concept. Upon receipt of a signal from an automatic detector, the fire alarm control unit indicates the alarm at a constantly attended location — staffed and trained — without yet initiating general alarm. Personnel at that location have a maximum of 15 seconds to positively acknowledge the signal; if it isn't acknowledged in that window, the system automatically initiates general alarm regardless. If acknowledged, personnel investigate, but the entire sequence — from initial signal to a resolved outcome — is capped at roughly 180 seconds (3 minutes) total; reaching that ceiling without resolution also forces automatic general alarm. Manual pull stations and other alarm-initiating devices besides automatic detectors are never subject to this delay — they always cause immediate general alarm.
The difference matters because it's the fail-safe, not the delay itself, that makes a presignal-style scheme safe to use. An unconstrained presignal delay depends entirely on a human being present, aware, and responsive — if that assumption fails, nothing forces the system back toward general alarm, and a fire can progress uncorrected during an open-ended investigation window. That failure mode is historically linked to preventable delayed-evacuation deaths, which is why current codes don't simply authorize "presignal" as a general practice. Positive Alarm Sequence is the specific, bounded shape presignal operation is allowed to take: every requirement in PAS — the 15-second acknowledgment window, the 180-second ceiling, the automatic-detector-only restriction, the mandatory automatic escalation — exists specifically to close the gap that an unconstrained presignal delay leaves open. Treating "presignal" and "PAS" as interchangeable terms erases that fail-safe from the conversation entirely.
No. Presignal operation is the general concept — a signal routed to a specific location before general alarm. PAS is one specific, code-regulated way NFPA 72 permits implementing that concept, with a mandatory acknowledgment window, a total time ceiling, and a mandatory automatic-escalation fail-safe. Every PAS sequence is a presignal implementation, but not every presignal scheme meets PAS's requirements.
The fire alarm system automatically initiates general alarm — no further human action is required or possible to prevent it. The 15-second acknowledgment window exists specifically so that a non-response defaults to the safer outcome.
No. Positive Alarm Sequence applies only to signals from automatic fire detectors. A manual pull station, a sprinkler waterflow switch, or any other alarm-initiating device always causes immediate general alarm — none of those are eligible for the PAS delay.
Because it depended entirely on a person being present, aware, and able to respond, with no code-mandated limit on how long the building could remain in a pre-alarm state and no automatic path back to general notification if that person never responded. That open-ended failure mode was historically linked to preventable delayed-evacuation deaths, which is why fire codes moved toward requiring a bounded, fail-safe implementation like PAS rather than permitting generic presignal delays.
It must be a location staffed continuously by personnel trained in the emergency response procedures the system requires — not simply somewhere a signal happens to display. If that staffing or training requirement isn't genuinely met, the entire premise behind allowing a PAS delay (a competent, present investigator) doesn't hold, which is why the constantly-attended-location requirement is treated as a precondition for using PAS at all.
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