What is a smoke alarm?

A smoke detector is a device that senses smoke, usually as an indicator of fire. These alarms work differently on the commercial scale & household level. In commercial level security devices issue a signal to a fire alarm control panel as part of a fire alarm system, whereas household smoke detectors generally issue a local audible or visual alarm from the detector itself.

Normally, the smoke detectors are housed in plastic enclosures, which are shaped like a disk, & are about 150 millimeters (which is 6 inches) in diameter & 5 millimeters (which is 1 inch)  thick, but their shape & size vary.

Smoke can be detected in two ways. It can be detected either optically (photoelectric) or by physical process (ionization). Detectors may have both abilities to detect a cloud of smoke. In commercial, industrial & residential buildings, smoke detectors are usually powered by a central fire alarm system, that is powered by the building power with a battery backup. While domestic smoke detectors range from separate battery-powered units to several inter-linked mains-powered units with battery backup. With these inter-linked units, if any unit detects smoke, all trigger even if household power has gone out.

Why smoke alarms

Smoke alarms are very useful devices because the risk of dying in a home fire is cut in half in homes with working smoke alarms. According to the US National Fire Protection Association reports, there were only 0.53 deaths per 100 fires in homes with working smoke alarms as compared to 1.18 deaths in homes without alarms. This survey was taken from 2009 to 2013.


In the year 2004 ‘NIST’ report concluded that ‘both types of smoke alarms, either the ionization type or the photoelectric type, consistently provided time for occupants to escape from most residential fires’, where ionization type alarms provide somewhat better response to flaming fires than photoelectric alarms. Which is 57 to 62 seconds faster response. On the other hand, photoelectric alarms provide considerably faster response to smoldering fires than ionization type alarms which is 47 to 53 minutes faster response.

Remember, regular base cleaning can also prevent false alarms caused by the build-up of dust & insects, particularly on optical type alarms as they are more sensitive to these factors. For this, a vacuum cleaner can be used to clean domestic smoke detectors to remove detrimental dust.

Installation & placement

In  US most state & local laws regarding the required number & placement of smoke detectors are based upon standards established in NFPA 72, National Fire Alarm & Signaling Code. Laws governing the installation of smoke detectors differ depending on the locality. Nevertheless, some rules & guidelines for existing homes are comparatively consistent throughout the developed world. For instance, Canada & Australia require a building to have a working smoke detector on each & every level. The US NFPA code cited in the previous paragraph requires smoke detectors on every habitable level & within the vicinity of all the bedrooms. Habitable levels include attics that are tall enough to allow access. Most of the other countries have comparable requirements.

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