Fire alarm systems, notification and emergency planning

Detecting a fire is only useful if it leads to action. The fire alarm system turns a detector signal into warnings for occupants, messages to the fire department, and commands to other building systems: closing dampers, releasing doors, shutting down air handlers, recalling elevators and, in rooms with gaseous suppression, starting the release sequence. People then have to act on those warnings in the right way, which depends on a written fire safety plan, trained staff and regular drills. This lesson covers the parts of a fire alarm system, how occupants are notified in noisy data halls, how alarms reach the fire department, how alarm sequences are coordinated with suppression and other systems, and the emergency planning that makes the whole arrangement work.

Parts of a fire alarm system

At the center of every fire alarm system is the fire alarm control unit (often called the fire alarm panel). It monitors all connected devices, decides what each signal means, operates notification appliances and outputs, and records events. Modern systems are addressable: each device on a signaling loop has its own address, so the panel knows exactly which detector or manual station has operated, and analog detectors report their measured value so the panel can apply alarm thresholds and drift compensation. Large campuses network several panels together and display events at a central location and at an annunciator near the fire department entrance.

Connected devices fall into a few groups:

• Initiating devices start an alarm: smoke, heat and aspirating detectors (Lesson 3), manual pull stations, and sprinkler waterflow switches that detect water moving in the pipes.
• Supervisory devices report abnormal conditions in fire protection equipment that could prevent it from working: a closed sprinkler control valve, low air pressure in a pre-action system, low temperature in a valve room, or a fire pump running.
• Notification appliances warn occupants: horns, bells, speakers and visible strobes.
• Control outputs operate other systems: door holders, damper actuators, fan shutdown, elevator recall, clean agent release panels and the transmitter to the monitoring station.

The panel distinguishes three kinds of signal. An alarm signal indicates a fire or a condition requiring immediate action. A supervisory signal indicates that part of the fire protection system needs attention. A trouble signal indicates a fault in the alarm system itself, such as an open circuit, a ground fault or a failed battery, that could stop it working. Each must be responded to, but with different urgency.

In the US, fire alarm control units are listed to UL 864 and systems are installed, inspected and tested to NFPA 72, the National Fire Alarm and Signaling Code, as required by the IBC and IFC. Most systems are single-stage, sounding an evacuation signal throughout the building or zone when any alarm is initiated; positive alarm sequence, allowed by NFPA 72 in buildings with trained staff, gives staff a short period to investigate an automatic detector alarm before general evacuation. The standard evacuation signal is the three-pulse temporal pattern.

Notifying people in noisy spaces

Data halls are loud. Cooling fans in servers and air handlers commonly produce 70 to 85 dBA in the aisles, and the hot aisles of high-density rows can exceed this. Alarm signals must be clearly audible above this background noise everywhere people work, without being so loud that they cause hearing damage or confusion.

NFPA 72 requires public-mode audible signals to be at least 15 dB above the average ambient sound level, with a maximum of 110 dBA at the minimum hearing distance. Where the average ambient sound level exceeds 105 dBA, visible notification appliances are required in place of relying on audibility; in practice, visible appliances are also required by accessibility rules in most public and common-use areas.

Sound level falls with distance from a horn or speaker. In open space, a point source loses about 6 dB each time the distance doubles, so a horn rated at 99 dBA at 3 m produces about 93 dBA at 6 m and 87 dBA at 12 m. Racks, containment panels and walls reduce the level further in their shadow, while reflections in a closed room partly offset the loss. Designers place appliances in each aisle or contained pod and verify audibility by measurement after installation.

import math

L3 = 99.0            # horn rating, dBA at 3 m
ambient = 80.0       # average ambient noise in the data hall, dBA
need_ca = max(ambient + 10, 65)     # NBC: 10 dBA above ambient, at least 65 dBA
need_us = ambient + 15              # NFPA 72 public mode: 15 dB above average ambient

def level(r):
    # point source in open space: -6 dB per doubling of distance (20 log10)
    return L3 - 20 * math.log10(max(r, 3.0) / 3.0)

def combine(*levels):
    # sound levels add on an energy basis
    return 10 * math.log10(sum(10 ** (x / 10) for x in levels))

spacing = 24.0       # two horns 24 m apart along an aisle
print(f"required: Canada {need_ca:.0f} dBA, US {need_us:.0f} dBA")
print(" x (m)  one horn  two horns   Canada   US")
for x in (3, 6, 9, 12, 15, 18, 21):
    one = level(x)
    two = combine(level(x), level(spacing - x))
    ok_ca = "ok" if round(two, 1) >= need_ca else "low"
    ok_us = "ok" if round(two, 1) >= need_us else "low"
    print(f"{x:5.0f}  {one:7.1f}  {two:9.1f}   {ok_ca:>6}  {ok_us:>4}")
# How far can one horn reach before falling below each requirement?
for label, need in (("Canada", need_ca), ("US", need_us)):
    r = 3.0 * 10 ** ((L3 - need) / 20)
    print(f"one horn meets the {label} requirement out to about {r:.1f} m")

required: Canada 90 dBA, US 95 dBA
 x (m)  one horn  two horns   Canada   US
    3     99.0       99.1       ok    ok
    6     93.0       93.4       ok   low
    9     89.5       90.8       ok   low
   12     87.0       90.0       ok   low
   15     85.0       90.8       ok   low
   18     83.4       93.4       ok   low
   21     82.1       99.1       ok    ok
one horn meets the Canada requirement out to about 8.5 m
one horn meets the US requirement out to about 4.8 m

The output shows that a single horn can cover only a limited area in a noisy hall, and that adding a second horn raises the level where their coverage overlaps by up to 3 dB. With 80 dBA of ambient noise, horns 24 m apart just meet a 10 dBA margin but fall well short of a 15 dB margin, which would need appliances every 10 m or so, or a lower ambient level. Raising horn output is limited by the maximum permitted level near each appliance, so in very noisy halls designers add more appliances and visible signals rather than louder ones. In a hot-aisle containment system, each contained aisle generally needs its own notification appliances, because the containment panels block sound and light from appliances outside.

Visible notification appliances (strobes) are rated by their effective light intensity in candela (cd). Spacing tables relate intensity to the area one appliance can cover; for example, NFPA 72 allows a 15 cd strobe to cover a room up to 6.1 m (20 ft) square and a 110 cd strobe a room up to about 16.5 m (54 ft) square, with large rooms covered by several appliances. Where more than two strobes are visible at once, they must flash in synchronization so that the combined flash rate does not trigger seizures in people with photosensitive epilepsy.

Voice communication and mass notification

Simple horns tell people that something is wrong, but not what to do. Voice systems replace or supplement them with recorded and live spoken messages, which allow different instructions to different zones (evacuate the battery room, stay in place elsewhere), explain the reason for an alarm, and give directions around blocked routes. Spoken messages must be intelligible as well as loud; intelligibility is commonly assessed with the speech transmission index (STI), with a target average of about 0.5 in each acoustically distinct space. Loud, reverberant data halls make intelligibility hard to achieve, and designers often use many low-power speakers close to listeners rather than a few loud ones.

In the US, the IBC requires emergency voice/alarm communication systems in high-rise buildings and certain other occupancies, designed to NFPA 72. Mass notification systems, covered by NFPA 72 Chapter 24, extend voice messaging to other emergencies (severe weather, security threats, hazardous releases) on a campus, and a risk analysis determines whether such messages may take priority over fire alarm signals.

Reaching the fire department

An alarm in an unattended or lightly staffed building is useless unless it reaches someone who will respond. Fire alarm systems in most commercial buildings therefore transmit alarm, supervisory and trouble signals to a monitoring station, which calls the fire department for alarms and the owner's contacts for other signals.

In the US, supervising stations (central station, remote station or proprietary) are covered by NFPA 72, with central station service often listed by UL. A large data center operator may run a proprietary supervising station at its own operations center, which must meet NFPA 72 requirements for staffing, equipment and retransmission of alarms to the fire department.

Communication paths are monitored: if the link to the monitoring station fails, a trouble signal is generated. Modern systems use cellular radio, internet protocol paths or both, with at least one path supervised at short intervals. Alarm verification policies, in which a monitoring operator calls the site before dispatching, are sometimes used to reduce unwanted fire department responses, but they must not delay the response to a real fire, and many jurisdictions restrict them.

Alarm sequences and system interfaces

Lesson 3 described how aspirating detection gives several levels of warning, from an early alert that prompts investigation to a fire level that triggers the building alarm. The fire alarm system implements this staged response and connects it to other systems through a defined sequence of operations, documented in a matrix that lists each input and the outputs it causes. Typical interfaces in a data center include:

• Air handling: shutting down or switching modes on smoke detection, closing smoke dampers, and in some designs keeping cooling running until suppression discharges.
• Doors: releasing hold-open devices on fire doors, and unlocking access-controlled doors on the egress path.
• Elevators: recalling cars to the designated floor and preventing them from stopping at the fire floor.
• Suppression: monitoring pre-action valves, waterflow and clean agent systems, and controlling agent release.
• Power: signaling the EPO or shutdown of affected equipment where the strategy requires it (Lesson 1).

Clean agent systems use a release sequence that guards against accidental discharge while making sure a real fire is suppressed. A common arrangement is cross-zoning (or counting): the first detector in alarm produces a local alarm and alerts staff; a second detector in a separate zone confirms the fire and starts the pre-discharge alarm and a time delay, often 30 seconds, so that people can leave; at the end of the delay the agent is released. An abort switch, which must be held in by hand (a deadman type), suspends the countdown while staff verify the situation, and a manual release station allows immediate discharge. The release panel also confirms that doors are closed and dampers shut before discharge so that the agent will hold.

# Staged alarm and cross-zoned clean agent release sequence for a network room
DELAY = 30           # pre-discharge time delay, seconds

def run(events, abort_held):
    zones_in_alarm = set()
    countdown = None
    released = False
    start = None
    log = []
    for t in range(0, 400):
        for et, source, zone, level in events:
            if et != t:
                continue
            if level == "alert":
                log.append((t, f"{source} alert: notify operations centre"))
            elif level == "action":
                log.append((t, f"{source} action: dispatch staff to investigate"))
            elif level == "fire":
                first = not zones_in_alarm
                zones_in_alarm.add(zone)
                action = "building alarm, call monitoring station" if first else "zone alarm"
                log.append((t, f"{source} fire in zone {zone}: {action}"))
                if len(zones_in_alarm) >= 2 and countdown is None:
                    countdown = DELAY
                    start = t
                    log.append((t, "second zone confirms: pre-discharge alarm, close dampers, stop air handlers"))
        if countdown is not None and not released and t > start:
            holding = abort_held[0] <= t < abort_held[1]
            if holding:
                if t == abort_held[0]:
                    log.append((t, f"abort switch held: countdown paused at {countdown} s"))
            else:
                if t == abort_held[1] and abort_held[1] > abort_held[0]:
                    log.append((t, f"abort released: countdown resumes from {countdown} s"))
                countdown -= 1
                if countdown == 0:
                    released = True
                    log.append((t, "agent released: discharge alarm, do not enter"))
    for t, msg in log:
        print(f"  t = {t:3d} s  {msg}")

events = [(0, "ASD sector 2", 1, "alert"), (70, "ASD sector 2", 1, "action"),
          (150, "ASD sector 2", 1, "fire"), (190, "Spot detector 14", 2, "fire")]
print("Without abort:")
run(events, (0, 0))
print("Abort switch held from 200 s to 260 s:")
run(events, (200, 260))

Without abort:
  t =   0 s  ASD sector 2 alert: notify operations center
  t =  70 s  ASD sector 2 action: dispatch staff to investigate
  t = 150 s  ASD sector 2 fire in zone 1: building alarm, call monitoring station
  t = 190 s  Spot detector 14 fire in zone 2: zone alarm
  t = 190 s  second zone confirms: pre-discharge alarm, close dampers, stop air handlers
  t = 220 s  agent released: discharge alarm, do not enter
Abort switch held from 200 s to 260 s:
  t =   0 s  ASD sector 2 alert: notify operations center
  t =  70 s  ASD sector 2 action: dispatch staff to investigate
  t = 150 s  ASD sector 2 fire in zone 1: building alarm, call monitoring station
  t = 190 s  Spot detector 14 fire in zone 2: zone alarm
  t = 190 s  second zone confirms: pre-discharge alarm, close dampers, stop air handlers
  t = 200 s  abort switch held: countdown paused at 21 s
  t = 260 s  abort released: countdown resumes from 21 s
  t = 280 s  agent released: discharge alarm, do not enter

In this simulation, releasing the abort switch resumes the countdown where it stopped. Release panels offer several abort modes, and in some the countdown restarts from the full delay or a fixed time when the switch is released; the mode chosen must be agreed with the authority having jurisdiction and explained in staff training, because staff who expect one behavior and meet another may leave too late.

Every interface is a place where something can go wrong: a damper that does not close, a door that stays locked, a release panel that was left disabled after maintenance. That is why integrated testing of all interconnected systems together, rather than each system on its own, is required for new installations and periodically afterwards (Lesson 11).

The fire safety plan

The IFC requires emergency action plans and fire safety and evacuation plans for many occupancies, prepared in accordance with its emergency planning provisions and reviewed by the fire code official. They set out emergency procedures, evacuation routes and assembly areas, the duties of staff, procedures for accounting for people, fire hazards and their control, fire protection equipment and its maintenance, and arrangements for drills.

For a data center, the plan should address features that general templates may miss:

• Investigation procedures for early-warning alerts: who goes, what they look for, how they find the source using the aspirating detector's sector information and thermal cameras, and when to de-energize equipment.
• Clean agent rooms: pre-discharge evacuation, use of abort and manual release stations, and re-entry only after ventilation.
• Battery rooms: actions on off-gas or battery management alarms, and keeping people out of rooms where explosive gas may have accumulated (Lesson 7).
• Security interfaces: how doors and mantraps behave on alarm, and how security staff help account for everyone in the building.
• Contractors and visitors: briefing on alarm signals and assembly areas, since they may make up a large share of people on site during construction or upgrades.
• Business continuity: who decides to transfer load to another site and how customers are informed.

Fire drills test the plan with real people. Drills reveal problems that drawings do not: alarms that cannot be heard in a contained aisle, mantraps that trap people, staff who do not know where the assembly area is, or a security desk that cannot produce a list of who is on site. Lessons from each drill and from real alarms should be recorded and used to update the plan.

Working with the fire department

Firefighters arriving at a data center face an unfamiliar building with high-voltage equipment, batteries, security barriers and possibly gaseous agents. The pre-incident plan described in Lesson 1, regular site visits, a key box at the entrance, a fire alarm annunciator showing which zone is in alarm, and a knowledgeable staff member who meets the crew all reduce delay and risk. The fire safety plan should name who provides this liaison at every hour of the day, including weekends when staffing is low.