How a fire alarm system works

A building fire alarm system watches for signs of fire, decides what each signal means, warns the people inside and, if it is monitored, calls for help. It also watches itself, so a cut wire or a dead battery shows up as a trouble signal instead of a silent failure on the day it is needed.

The parts of a system

Every commercial fire alarm system, from a small strip mall to a hospital, is built from the same groups of parts.

• Initiating devices start a signal. They include smoke and heat detectors, manual pull stations, duct smoke detectors, and switches on the sprinkler system that report water flow or a closed valve.
• The control unit, often called the panel, receives those signals, decides what kind they are and runs the programmed response. Control units are built and listed to CAN/ULC-S527.
• Notification appliances warn people: horns, bells, speakers and strobes.
• Annunciators show firefighters and staff where the alarm came from, usually at the main entrance.
• Power supplies run the system from the building's normal power, with standby batteries that keep it working for a set period after a power failure and then still sound the alarm.
• Outputs and interfaces send the alarm to a monitoring station and operate other building systems, such as fans, door holders and elevators.

Our guide to fire alarm devices covers detectors, pull stations and signals in more detail.

Three kinds of signal

A fire alarm panel does not treat every input the same way. It sorts each one into one of three types, and the response is different for each. Understanding the difference helps staff react correctly when the panel beeps at 2 a.m.

Alarm

An alarm means the system believes there is a fire: a detector has tripped, someone has pulled a station, or water is flowing in the sprinkler pipes. The panel sounds the signals, lights the annunciator, sends the alarm to the monitoring station and operates its interfaces. In a two-stage system the first signal is an alert to staff, with a timed window before general evacuation; see voice evacuation and two-stage alarms.

Supervisory

A supervisory signal means a fire protection feature is not in its normal state. The classic example is a sprinkler control valve that has been partly closed. Nothing is on fire, but if a fire started the sprinklers might not work. Supervisory signals sound at the panel and go to the monitoring station, and they need someone to find the cause quickly.

Trouble

A trouble signal means the alarm system itself has a fault: a broken wire, a ground fault, a missing device, a failed battery or a loss of normal power. The building is not on fire, but part of the system may not work. Trouble signals are easy to ignore when they come and go, and that habit is one of the main reasons systems fail inspections.

How the system watches its own wiring

The panel constantly checks the circuits that run to its devices. On older conventional systems it does this by passing a small current through each circuit and an end-of-line device at the far end; if the current stops, the circuit is open and the panel reports trouble. Addressable systems talk to every device in turn and report any device that stops answering. Either way, the aim is the same: a fault should be found when it happens, not during a fire.

How the wiring is arranged decides what a single break does. Designers call the two common arrangements Class A and Class B.

Class B is common and acceptable for many circuits. Class A, or wiring with isolators on addressable loops, is used where losing a run of devices would matter more, such as risers serving many floors. The building code, CAN/ULC-S524 and the design decide which applies where.

Getting the signal out of the building

Many buildings must have their fire alarm monitored off site, and many owners choose monitoring where it is not required. When the panel goes into alarm, a communicator sends the signal over a phone line, cellular link or internet connection to a monitoring station. Operators there confirm what kind of signal it is and call the fire department and the building's contacts. Supervisory and trouble signals are sent too, so someone is told about a closed valve or a lost phone line even when the building is empty.

Monitoring stations for fire alarm signals are commonly listed to CAN/ULC-S561, and many jurisdictions require that. Our guide to fire alarm monitoring explains the options.

A typical alarm, minute by minute

Picture a single-stage system in a four-storey office building when a smoke detector on the third floor trips.

1. The detector reports to the panel, which goes into alarm and records the device and time.
2. Horns and strobes sound throughout the building, and the annunciator at the entrance shows the third-floor location.
3. The panel shuts down or switches the air handlers as programmed, releases magnetic door holders so fire doors close, and recalls the elevators.
4. The communicator sends the alarm to the monitoring station, which calls fire dispatch.
5. Occupants leave by the exits. Staff with fire safety plan duties meet the fire department and report what they know.
6. Only the fire department decides when the building is safe. The panel is then reset, and the cause is recorded and fixed, even when it turns out to be burnt toast.

The standards behind it

In Canada, the National Building Code and the provincial building codes based on it set when a building needs a fire alarm system and what kind. CAN/ULC-S524 covers how the system is installed and CAN/ULC-S527 covers control units. When a system is new or modified, it is verified to CAN/ULC-S537: every device and function is tested and recorded. After that, the fire code requires ongoing inspection and testing to CAN/ULC-S536, with some checks daily or monthly by the owner's staff and a full annual inspection by a qualified technician.

Provinces adopt different editions of these documents at different times, and some cities have their own requirements, so always check the editions in force where your building is.

How ehZee can help

We design fire alarm systems for new buildings, renovations and panel replacements, with drawings sealed by a licensed professional engineer where required and installation by licensed and certified trades. See our fire alarm system design service, or our fire inspections, verification and monitoring page for existing systems.

Common questions