Single-stage and two-stage systems

Eastview General Hospital doesn't evacuate when its fire alarm operates. Patients in intensive care, operating rooms and the birthing unit can't simply walk out, so the hospital's fire safety plan relies on staff moving patients horizontally away from the fire, behind fire separations, while the rest of the building stays put. The fire alarm is designed to support that plan, and it starts in a different way from the alarm in an office tower. Sasha spends a week at Eastview with Gerry and the hospital's own facilities team. 'In most buildings, any alarm means everybody leaves,' the facilities manager tells her. 'Here, the first signal tells staff to act. The second one, if it comes, is the one that says the building is in real trouble.' This lesson covers single-stage and staged systems, the timers and controls that make them work, and how the panel is programmed for each.

Single-stage systems

In a single-stage system, any alarm signal, from a detector, a pull station or a waterflow switch, operates the evacuation signal throughout the building at once. Everyone hears the same temporal-three signal and everyone leaves. Single-stage systems are simple, easy to understand and easy to test, and they are the normal arrangement for offices, schools, warehouses and apartment buildings of moderate height. Tamarind House, Willowbank School and Cartwright Distribution Center all use single-stage systems. The panel's programming is straightforward: every alarm input operates every notification circuit, plus the building's control functions.

Staged and staff-first responses in the US

NFPA 72 offers two ways to put trained staff ahead of a general evacuation where the authority approves. Positive alarm sequence applies to automatic detectors: when one operates, the signal goes first to trained staff, who must acknowledge it within 15 seconds and then have up to 180 seconds to investigate and reset the system if the cause is harmless. If they don't acknowledge in time, if the investigation time runs out, or if a second automatic detector or any manual fire alarm box operates, the general alarm sounds at once. The presignal feature, used mainly in facilities with a constantly staffed control point, sends the initial signal only to staff and the general alarm requires a human action. Hospitals also use private-mode notification to staff areas under NFPA 101 and the approved fire plan.

At Eastview, positive alarm sequence is approved for the corridor and patient room smoke detectors on the inpatient floors, where nurses are always present. Manual fire alarm boxes and waterflow switches go straight to the general alarm, as NFPA 72 requires for manual boxes under positive alarm sequence, and the hospital's fire plan tells staff exactly who acknowledges and who investigates on each unit.

# Positive alarm sequence: acknowledge within 15 s, investigate up to 180 s (times in seconds)
ACK_LIMIT, INVESTIGATE = 15, 180
def run(events):
    log, acked = ["0 s: automatic detector operates -> signal to trained staff only"], None
    for t, what in events:
        if what == "acknowledge" and t <= ACK_LIMIT and acked is None:
            acked = t
            log.append(f"{t} s: staff acknowledge -> investigation time starts")
        elif what in ("second detector", "manual box"):
            log.append(f"{t} s: {what} -> general alarm immediately")
            return log
        elif what == "reset" and acked is not None and t <= INVESTIGATE:
            log.append(f"{t} s: cause found, system reset -> no general alarm")
            return log
    if acked is None:
        log.append(f"{ACK_LIMIT} s: not acknowledged -> general alarm")
    else:
        log.append(f"{INVESTIGATE} s: investigation time over -> general alarm")
    return log
for name, ev in [("Toast in a staff kitchen", [(8, "acknowledge"), (95, "reset")]),
                 ("Nobody answers", []),
                 ("Real fire", [(10, "acknowledge"), (70, "manual box")])]:
    print(name)
    for line in run(ev):
        print("  ", line)

Toast in a staff kitchen
   0 s: automatic detector operates -> signal to trained staff only
   8 s: staff acknowledge -> investigation time starts
   95 s: cause found, system reset -> no general alarm
Nobody answers
   0 s: automatic detector operates -> signal to trained staff only
   15 s: not acknowledged -> general alarm
Real fire
   0 s: automatic detector operates -> signal to trained staff only
   10 s: staff acknowledge -> investigation time starts
   70 s: manual box -> general alarm immediately

The three runs show the possible paths. When staff acknowledge and find a harmless cause, they reset the system and no general alarm sounds. When nobody acknowledges within 15 seconds, the general alarm sounds. When a person pulls a manual box during the investigation, the general alarm sounds at once, whatever the timer says.

Zones, floors and staged evacuation

Staging can also be about where, not just when. In a high-rise building such as Riverlight Tower, the evacuation strategy may sound the alarm on the fire floor, the floor above and the floor below first, and an alert or a different message on the other floors, so the stairs aren't overwhelmed and firefighters can work. This kind of staging is usually done with voice communication, which FAT-310 covers, but it relies on the same panel features: notification circuits divided by floor, programming that knows which floor each device is on, and controls for firefighters to extend the alarm to more floors or the whole building.

Whatever the staging, the panel's programming must handle the case where devices on more than one floor operate. A fire that spreads up a shaft, or smoke that travels through a corridor, can put detectors on several floors into alarm. The sequence of operations must say clearly what happens then, usually extending the alarm to every affected floor and the floors around them, and the acceptance test must include it.

Alarm verification versus staging

Two-stage systems and positive alarm sequence are sometimes confused with alarm verification, but they solve different problems. Alarm verification, described in FAT-120, is an automatic check on a single smoke detector: the panel resets the detector and confirms the signal before acting, which filters out momentary nuisance signals. It involves no people. Staging, in contrast, is about who hears the first signal and when people are called to act. A building can use both: verification on the smoke detectors to filter brief nuisance signals, and a staged response for confirmed signals. The programming must make sure the total delay before an alarm is within what the code allows, and verification is never applied to manual stations or waterflow switches.

Controls, keys and records

Staged systems depend on people doing the right thing at the right time, so the controls must be clear and the records complete. Acknowledge, general alarm and silence controls are labeled at the panel and every annunciator. Keys for second-stage stations are kept where the fire safety plan says, and the plan lists who is trained to use them. The panel's event history records each signal, each acknowledgment and each escalation with the time, so that after any event the hospital can see how long staff took to respond. Eastview's fire safety committee reviews these records after every alarm, and slow acknowledgments are followed up with training.

Because positive alarm sequence and presignal features depend on staff, the authority having jurisdiction must approve them and the fire department must know how they work. Captain Helen Strand of the fire marshal's office reviews Eastview's sequence each year with the hospital, and any change to the timing or the escalation controls goes through her office before it is programmed.

Programming and testing a staged system

In the panel, a staged response is built from timers, logic and output groups. The program defines which inputs start the first stage, which outputs the first stage operates, the timer that escalates automatically, the inputs and controls that escalate immediately, and what the second stage operates. It also defines what acknowledge does, whether it can be done from annunciators as well as the panel, and how the system returns to normal. Testing covers every path: the automatic escalation with no acknowledgment, the acknowledged and investigated path, immediate escalation from each second-stage control, and the case of a second device operating during the first stage. Each path is timed with a stopwatch and recorded.

Training and drills for staged systems

A staged system is only as good as the people who answer it. Staff on every shift need to know what the first signal sounds like, where the nearest acknowledge control is, who investigates, what they look for, how to escalate, and what to do if they find a fire. The fire safety plan sets this out, and fire drills, held at the frequency the fire code requires, test it with real people and a stopwatch. At Eastview, drills are run on every unit and every shift through the year, and the panel's event history from each drill is printed and reviewed with the unit's staff, so a slow acknowledgment becomes a lesson rather than a surprise.

Drills also test the system's programming in a way an acceptance test can't, because they show how the staging behaves in a building full of people. Staff sometimes find that an acknowledge control is behind a locked door at night, that the alert signal can't be heard in a noisy area, or that the escalation key is kept somewhere nobody can reach in time. Each finding goes back to the designer and the authority, because fixing it may mean a programming change, a new annunciator or a change to the plan. Bramwell's technicians attend Eastview's large drills so they can see these problems first-hand.

When a staged response is not allowed

Staging is never used to delay the response to a fire someone has already seen. That is why manual fire alarm boxes and waterflow switches go straight to the general alarm under positive alarm sequence, and why the acknowledge and investigation limits exist: the delay can last only as long as trained staff are actively responding. Buildings without staff present at all hours, or whose fire plans can't support an investigation, use single-stage systems. Where a building's use changes, for example when a hospital wing becomes offices, the design must be reviewed, because a staged response may no longer be appropriate.

Finally, any staged sequence is written so that the fire department can always override it. Arriving firefighters may want the whole building evacuated at once, whatever the first stage was doing, so the general alarm control at the main annunciator works at any time, in any stage, with one action. The test plan includes pressing it in the middle of each stage to prove it.