A Fire Alarm System is an active fire protection system that detects fire or the effects of fire, and as a result provides one or more of the following: notifies the occupants, notifies persons in the surrounding area, summons the fire service, and controls all the fire alarm components in a building. Fire alarm systems can include alarm initiating devices, alarm notification appliances, control units, fire safety control devices, annunciators, power supplies, and wiring.

Fundamental Design

A fire alarm system is composed of components which can be classified into the following categories.

Initiating Devices – these devices either sense the effects of a fire, or are manually activated by personnel, resulting in a signal to the fire alarm panel. Examples are heat detectors, smoke detectors, manual pull stations or call points, and sprinkler pressure or flow switches.

Main Fire Alarm Control (panel) – this is the central brain of the overall system, which coordinates the signals and resultant actions of the system.

Fire Alarm Control Unit (Panel) – any panel providing required functions, and has inputs and outputs. NAC Power booster panels are the most common example.

Power Supply – because one of the fire alarm system objectives is life safety, fire alarm system power supplies are redundant, and reliable when compared to electronic or electrical systems of similar complexities (e.g., HVAC control systems). Primary supply- Commercial light and power. Back-up/secondary supply – Usually sealed, lead-acid batteries. NAC power supplies for additional notification appliances beyond the original capability of the FACP. Generators are permitted under strict rules.

Notification appliances – these devices provide stimuli for initiating emergency action and provide information to users, emergency response personnel, and occupants. Examples are bell, siren, horn, speaker, light, or text display that provides audible, tactile, or visible outputs.

Signaling Line Circuits (SLC)– the wiring which carries data information.

Supervisory Signals – detecting devices and signaling to indicate a condition in fire protection systems which is not normal and could prevent the fire protection system from functioning as intended in the event of a fire. An example is a closed valve which controls the water supply to a fire sprinkler system. This does not indicate the failure of a component or subsystem of the fire alarm system.

Trouble Signal – signaling to indicate a wiring fault. Sometimes specific components or features of the fire alarm system, of which could prevent the fire alarm or fire suppression system from functioning as intended. An example is a disconnected wire at a heat detector.

Remote Annunciation – A usually alpha-numeric display (may be graphic) that indicates where in the building the alarm originated. It may also indicate the type of device. Used by emergency personnel for locating the fire quickly. Sometimes these will contain some control functions such as alarm silence and alarm reset. Must be key or keypad controlled.


Fire alarm systems have devices connected to them to detect the fire/smoke or to alert the occupants of an emergency. Below is a list of common devices found on a fire alarm.

Manual Pull Stations/Manual Call Points – Devices to allow people to manually activate the fire alarm. Usually located near exits. Also called “manual pull boxes”. Other countries have different devices as standard, for example the UK uses ‘break-glasses’, when people break a small pane of glass to activate the alarm.

Smoke Detectors – Spot type: Photoelectric and Ionization; Line type: Projected Beam Smoke Detector; Air-Sampling type: Cloud Chamber

Water Flow Switches – Detect when water is flowing through the fire sprinkler system

Rate-of-Rise and Thermostat (heat) Detectors – Detect heat changes

Valve Supervisory Switch – Indicates that a fire sprinkler system valve that is required to be open, is now closed (off-normal).

Carbon Monoxide Detectors – Detects poisonous carbon monoxide gas and usually only connected to household fire alarm systems. Very rarely seen in commercial systems.


Notification Appliance – A fire alarm system component used to notify people of the need for action in connection with the activation of a fire alarm device. Notification appliances may employ audible, visible, tactile, textual or even olfactory stimuli to alert the occupants of a fire or other emergency. Audible or Visible signals are the most common and may utilize speakers to deliver live or pre-recorded instructions to the occupants. In the United States fire alarm evacuation signals are required to use a standardized interrupted four count temporal pattern to avoid confusion with other signals using similar sounding appliances.

Magnetic Door Holders – Wall or floor mounted electromagnets controlled by a fire alarm system or directly connected smoke detectors that hold open spring-loaded self-closing smoke tight doors by magnetic attraction to a door mounted iron armature. When energized these magnets hold open the door to allow free movement in building corridor. Upon detection of smoke or other fire conditions power is removed from the electromagnet, and the spring-loaded door is allowed to return to its closed position thus limiting the transmission of smoke from one space to another through the door opening.

Emergency Voice Alarm Communications (EVAC) Systems

An ‘Emergency Voice Alarm Communication’ system is part of a fire alarm system that uses high reliability speakers to notify the occupants of the need for action in connection with a fire or other emergency. These speakers are special types of fire alarm Notification Appliances. ‘Emergency Voice Alarm Communication’ systems are employed in large facilities where general undirected evacuation is considered impracticable or undesirable. The Audible Textual signals from the speakers are used to direct the occupant’s response during a fire or other emergency. The system may be controlled from one or more locations within the building known as Fire Wardens Stations, or from a single location designated as the building Fire Command Center. Speakers are automatically activated by the fire alarm system in a fire event, and following a pre-alert tone, selected groups of speakers may transmit one or more prerecorded messages directing the occupants to safety. These messages may be repeated in one or more languages. Trained personnel activating and speaking into a dedicated microphone can suppress the replay of automated messages in order to initiate or relay real time voice instructions.