Initiating, notification and control functions

Ravi lays a document on the table in front of Sasha: the sequence of operation for Tamarind House, a grid with every type of input down the left side and every output across the top, and dots where they meet. A smoke detector on the third floor sounds the alarm everywhere, releases the third-floor door holders, recalls the elevators to the ground floor and shuts down the air handler that serves the third floor. A waterflow switch does most of the same things. A smoke detector in an elevator lobby does something extra: it chooses which floor the elevators go to. A valve tamper switch does none of these; it sends only a supervisory signal. This lesson is an overview of the three families of functions in that grid: initiating devices that tell the panel something has happened, notification appliances that tell people, and control functions that tell other systems. Later courses take each family in depth.

Manual stations

The manual station, or pull station, is the oldest and simplest initiating device: a person who sees a fire pulls a handle or pushes a lever, and a switch inside sends an alarm. Single-action stations operate with one motion; dual-action stations need two, such as lifting a cover and then pulling, which reduces accidental and malicious operation. Stations are mounted near exits, where people pass on their way out, at a height that a person in a wheelchair can reach. Some are keyed so that only staff can operate them, in places such as psychiatric wards; and in two-stage systems a key switch on the station sends the second-stage signal. Clear plastic covers that sound a local horn when lifted are used where false pulls are a problem, such as at Willowbank School.

Automatic detectors

Lesson 2 described how smoke detectors work. Spot smoke detectors, photoelectric or multi-criteria, are the most common automatic devices in offices, corridors, apartments and hospitals. Duct smoke detectors sample the air inside a ventilation duct through a tube that crosses the duct, and detect smoke being carried by the air handler, so the system can shut the fan down before it spreads smoke through the building. Beam detectors cover large open spaces such as atriums and gymnasiums. Air-sampling systems draw air through pipes to a very sensitive detector, for spaces where early warning matters or detectors are hard to reach.

Heat detectors suit places where smoke detectors would cause nuisance alarms or would not survive: kitchens, garages, boiler rooms, dusty storage areas, attics. A fixed-temperature detector alarms when its element reaches a set temperature, commonly 135 °F for ordinary spaces and higher for warm ones. A rate-of-rise detector alarms when the temperature climbs faster than a set rate, typically about 15 °F per minute, which catches a fast fire before the room gets hot; most combine both functions. A rate-compensated detector alarms at its set temperature regardless of how slowly the air heats, without the lag of a simple fixed element. Linear heat detection, a cable that alarms along any part of its length, protects conveyors, cable trays and tunnels.

Flame detectors sense infrared or ultraviolet radiation from flames and respond within seconds, in places such as fuel storage, aircraft hangars and industrial plants. Carbon monoxide detectors sense the gas that smoldering fires produce and are also used to protect against CO from fuel-burning appliances, although that use is governed by its own rules. Sprinkler waterflow switches, which detect water moving in a sprinkler pipe, are initiating devices too, and in many buildings they are among the most important ones; Lesson 8 covers them.

Supervisory devices

Supervisory initiating devices watch fire protection equipment rather than looking for fire. A valve tamper switch on a sprinkler control valve signals when the valve is moved from fully open; a closed valve would leave the sprinklers it feeds without water. Low-air switches on dry-pipe sprinkler systems, low-temperature switches in rooms where sprinkler pipes could freeze, and switches that report a fire pump running or failing all send supervisory signals. In many buildings, duct detectors are also set to give a supervisory signal rather than a general alarm, so the fan stops but the building doesn't evacuate for smoke that may be from a burnt belt in the air handler; which arrangement is used depends on the code and the design.

Notification appliances

Notification appliances turn the panel's decision into something people perceive. Audible appliances include horns, bells, chimes and speakers; visual appliances are strobes. Combination horn-strobes and speaker-strobes are common. Low-frequency sounders, with a tone around 520 hertz, are much better at waking sleeping people, especially older people and people who have been drinking, than the higher-pitched tones of traditional horns. Tactile appliances, such as bed shakers, serve people who are deaf.

The evacuation signal uses a standard rhythm called the temporal pattern, or temporal-three: three half-second pulses, each followed by a half-second pause, then a one-and-a-half-second pause, repeated. Its purpose is recognition: the same pattern in every building means people know it is a fire alarm and not a car alarm or a forklift reversing. Speakers play the pattern as a tone before and between voice messages.

How loud is loud enough

Audible signals have to be heard over the normal background noise in every part of the building. For public-mode signals, NFPA 72 requires a level at least 15 dB above the average ambient sound level, or 5 dB above the maximum sound level that lasts 60 seconds or more, whichever is greater, measured about 5 ft (1.5 m) above the floor, and no more than 110 dBA at the minimum hearing distance. Private-mode signals, meant for trained staff, need only be 10 dB above ambient. In sleeping areas, the signal must reach about 75 dBA at the pillow, or the higher of the ambient-based levels, and new installations use the 520 Hz low-frequency tone.

Sound level falls with distance from the appliance. In open space, it drops by about 6 dB each time the distance doubles, which follows from the inverse square law.

Walls and doors cut the level much more. A closed solid-core door can reduce a horn's sound by 20 dB or more, and a closed apartment door between a corridor horn and a bedroom can reduce it by 30 dB or more once the bedroom door is also closed. That is why codes now require appliances inside sleeping rooms or suites rather than relying on a horn in the corridor. The script below estimates the level at a few places at Riverlight Tower from a horn rated at 85 dBA at 10 ft.

# Estimated sound level from one horn at several locations (illustrative losses)
import math
L_ref, d_ref = 85.0, 10                  # horn rating: dBA at reference distance (ft)
spots = [  # place, distance (ft), loss through doors or walls (dB), ambient (dBA)
    ("Corridor, near the horn", 10, 0, 45),
    ("Corridor, far end", 60, 0, 45),
    ("Suite living room, door closed", 30, 22, 40),
    ("Bedroom, both doors closed", 40, 34, 35),
]
need_above = 15
for place, d, loss, amb in spots:
    level = L_ref - 20 * math.log10(d / d_ref) - loss
    target = amb + need_above if "Bedroom" not in place else 75
    ok = "meets" if level >= target else "falls short of"
    print(f"{place:32s} {level:5.1f} dBA  ({ok} {target:.0f} dBA)")

Corridor, near the horn           85.0 dBA  (meets 60 dBA)
Corridor, far end                 69.4 dBA  (meets 60 dBA)
Suite living room, door closed    53.5 dBA  (falls short of 55 dBA)
Bedroom, both doors closed        39.0 dBA  (falls short of 75 dBA)

The corridor is well covered, but the horn in the corridor cannot reach the bedroom at the level needed to wake people. Appliances inside the suites, sounding when the building's alarm sounds, are the answer that modern codes require in new residential buildings.

Strobes and visual signals

Strobes flash a bright white light about once a second. Their brightness is rated in candela, and the rating needed depends on the size of the room and where the strobe is mounted, from 15 candela for a small office to 75, 110, 135 or 185 candela for larger rooms. Wall strobes are mounted high, with the whole lens between 80 and 96 in above the floor, so that people see the flash over furniture and other people. In sleeping rooms, strobes must be bright enough to wake people, typically 110 candela or more, and higher still if mounted close to the ceiling. Where more than one strobe is visible from one place, they must flash together, which the panel or a sync module controls.

Control functions

Control functions are where the fire alarm system acts on the building. Door holders are electromagnets that hold fire doors open during normal use; on alarm, the system cuts their power and the doors close, keeping smoke and fire out of stairs and corridors. Elevator recall sends elevators to a safe floor, usually the main entrance level or an alternate floor if the fire is there, so that nobody is trapped in a car that opens onto a fire floor, and so firefighters can take control of them. Fan shutdown stops air handlers that would spread smoke, while smoke control systems run some fans on purpose to pressurize stairs or exhaust smoke. Electrically locked doors on exit routes unlock so that people can get out. Some systems release suppression agents such as clean-agent or pre-action sprinkler systems.

Control functions are usually performed through relays or control modules wired to the other system's controls, and the connection point, often called the interface, is where the fire alarm trade's work meets another trade's. The fire alarm technician tests that the output operates; the elevator mechanic, the HVAC technician or the door hardware installer confirms that their equipment responds correctly. Lesson 9 describes these interfaces, and FAT-320 covers them in depth.

The sequence of operation

The sequence of operation, sometimes called an input-output matrix, is the master document for programming and testing. It lists every type of input, every output, and what each input does. It also shows timing: a pre-signal delay, a supervisory delay, an alarm verification feature that waits a short time and checks whether a smoke detector still sees smoke before sounding the general alarm. Every one of those choices must be allowed by the code and recorded. During verification or acceptance testing, a technician operates each input and confirms that every output in its row responds, and nothing else does.