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What is an Automatic Fire Alarm?

By Ken Black
Updated May 16, 2024
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An automatic fire alarm is a device that monitors local conditions, usually an interior room of a building, for the presence of changes that may indicate unwanted fire. Once the alarm detects a possible problem, it may then sound an alarm for those in the immediate area, automatically alert emergency services, or do both. Automatic fire alarms are required for most buildings and rental properties in most jurisdictions. Basic models are available for less than $10 US Dollars.

The detection system works somewhat differently on automatic fire alarm systems, depending on the model. Most basic models work by detecting the smoke through a method known as ionization. More sophisticated systems may use a photoelectric system in which light is the primary method of determining when smoke may be present in the air. Neither system is totally accurate, but both do an adequate job of detecting changes in the local atmosphere that may or may not be unwanted smoke.

In the case of the ionization system, the automatic fire alarm contains a small amount of a radioactive substance known as americium-241. This substance is used to create an electrical current within the detector. Smoke disrupts this electrical current, and when the automatic fire alarm senses that the current has been disrupted, it sounds an alarm.

With a photoelectric system, light enters the automatic fire alarm through a tube, and does not come into contact with a photo detector. If light is present, then that light is dispersed enough that it hits that detector. When that happens, the alarm begins to alert those in the area of a possible smoke situation. If individuals are not in the room at the time the smoke starts, or are asleep, this may provide them with enough warning to move to safety before the fire gets too large.

Depending on the type of automatic fire alarm, the system may go one step beyond alerting those in the general area. It may also alert a private monitoring company, or may be linked directly with an emergency communications center. In such cases, if the alarm sounds, emergency services are notified immediately and will respond with or without a person reporting a problem. Generally, a fire department is aware that it is responding to an automatic fire alarm, rather than a human report.

As with all emergency equipment, making sure the automatic fire alarm is in good working condition is essential to proper operation. For home use, the International Association of Fire Chiefs recommends changing batteries annually in units not connected to a permanent power source, and the organization runs a campaign encouraging homeowners to change the batteries when clocks are switched off of daylight savings time in the fall. Fire alarms should also be tested once a month to ensure they still work properly.

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