Fire alarm devices: detectors, pull stations and signals

Fire alarm devices fall into two groups: initiating devices that start an alarm, such as detectors, pull stations and sprinkler switches, and notification appliances that warn people, such as horns, speakers and strobes. Each type suits some spaces and not others, and where it is mounted matters as much as what it is.

This guide walks through the devices you will find in most commercial and multi-unit buildings, what each one responds to and the placement mistakes that cause missed fires or false alarms. For how the devices fit into the whole system, see how a fire alarm system works.

Smoke detectors

Smoke detectors are the most common initiating device in offices, corridors, residential common areas and many other spaces. Most detectors installed today are photoelectric. Inside the detector is a small dark chamber with a light source and a light sensor set at an angle, so that in clean air the light never reaches the sensor.

Photoelectric detectors respond well to the smouldering, smoky fires that are common in buildings, such as upholstery or wiring. Ionization detectors, which sense very small particles from fast flaming fires, are now less common in commercial systems. Multi-criteria detectors combine smoke sensing with heat or other sensors and use the combination to reject nuisance sources like steam or dust.

Addressable smoke detectors also report how dirty they are. The panel can flag a detector whose chamber is drifting toward alarm because of dust, which lets you clean or replace it before it causes a false alarm.

Duct, beam and aspirating detectors

Duct smoke detectors sample air in a supply or return duct and are used to shut down fans so smoke is not spread through the building. Projected beam and open-area imaging detectors watch across large spaces such as atriums and warehouses, where ceiling-mounted detectors would be too high to work well. Aspirating detectors draw air through pipes to a very sensitive central detector and are used where early warning matters, such as data centres. See how aspirating smoke detection works and open-area beam smoke detection.

Heat detectors

Heat detectors respond to temperature, not smoke. Fixed-temperature detectors operate when the air around them reaches a set point. Rate-of-rise detectors operate when temperature climbs faster than normal, and many units combine both. They are slower than smoke detectors, but they do not react to steam, dust, cooking fumes or exhaust, so they are used in kitchens, parking garages, mechanical rooms, unheated spaces and dusty areas where a smoke detector would false alarm constantly.

Placement: where detectors actually work

Smoke from a fire rises in a plume, hits the ceiling and spreads outward in a thin layer. Detectors work when they sit in that layer. Several things push smoke away from a detector or stop it arriving.

Common placement problems include:

• Detectors too close to walls, in the dead air pocket where the ceiling meets the wall.
• Detectors beside supply diffusers or in the path of strong air movement, which dilutes or blows smoke away.
• Deep beams, joists or ceiling pockets that trap smoke on one side; spacing rules change with these features.
• High or sloped ceilings, where smoke may cool and stratify below the detector.
• Detectors left in place after a renovation added walls, so a new room has no coverage at all.

CAN/ULC-S524 and the manufacturer's instructions give spacing and location rules for each detector type. Applying them to a real ceiling, with its beams, diffusers and lights, is the core of good device layout.

Manual pull stations

A pull station lets a person who sees a fire start the alarm. They are placed near exits, so someone leaving the floor passes one on the way out, at a height and in a location people can reach. Single-action stations operate with one pull. Double-action stations need a lift or push first and are used where accidental or malicious activation is a problem. Protective covers that sound a local warning when lifted are another option in schools and public spaces, if the authority having jurisdiction accepts them.

Sprinkler switches

Where a building has sprinklers, the fire alarm watches them. A water flow switch reports an alarm when water moves in the sprinkler pipe, which usually means a head has opened. Valve supervisory switches report when a control valve is turned away from fully open. Other supervisory inputs can include low air pressure on dry systems and fire pump status. These switches are tested during both fire alarm and sprinkler inspections, so the two contractors need to coordinate.

Signals: horns, speakers and strobes

Audible signals have to be loud enough to be heard clearly over the normal background noise in every space, and in sleeping areas loud enough to wake people. The codes set minimum sound levels and how far above ambient noise the signal must be. Noisy plant rooms, rooms with closed doors and suites in residential buildings are where audibility usually falls short.

Visible signals, usually strobes, are required in many spaces so that people who are deaf or hard of hearing are alerted, and in very noisy areas. Where more than one strobe is visible from one spot, they must flash in sync to limit the risk to people with photosensitive conditions. Voice systems replace horns with speakers so the building can give instructions; see voice evacuation and two-stage alarms.

How ehZee can help

We lay out detectors, pull stations and signals as part of our fire alarm system design work, checking ceilings, air movement and noise levels on site rather than relying on a typical floor plan. Installation is by licensed and certified fire alarm trades, and the finished system is verified to CAN/ULC-S537.

Common questions