History

In 1902 George Andrew Darby, an electrical engineer of 211 Bloomsbury Street, Birmingham, England, patented the electrical Heat-Indicator and Fire Alarm. The device was a heat detector rather than a smoke detector and indicated an increase temperature in the apartment where it was fixed. The device operated by closing an electrical circuit  to sound an alarm if the temperature rose above the safe limit. The contact was made by bridging a gap with a conductor  or allowing one plate to fall on another. The connection of the two plates was caused simply by a block of butter which melted as the temperature rose. This early device subsequently gave way to more modern fire and eventually smoke alarms.

The first home smoke detectors were invented by Duane D. Pearsall in 1967.

A Final Note:

Carbon Monoxide Detectors and Smoke Alarms save lives. We as building inspectors, contractors and consumers have known this for years. But we must remain diligent in or training and education efforts.

             WHAT the Code Says: (please refer to current edition of building code in your area)

                     Smoke alarms.

Smoke Alarms shall be installed in the following locations: 1. In each sleeping room.

2. Outside each separate sleeping area in the immediate vicinity of the bedrooms.

3. On each additional story of the dwelling, including basements but not including crawl spaces and uninhabitable attics. In dwellings or dwelling units with split levels and without an intervening door between the adjacent levels, a smoke alarm installed on the upper level shall suffice for the adjacent lower level provided that the lower level is less than one full story below the upper level.

When more than one smoke alarm is required to be installed within an individual dwelling unit the alarm devices shall be interconnected in such a manner that the actuation of one alarm will activate all of the alarms in the individual unit. The alarm shall be clearly audible in all bedrooms over background noise levels with all intervening doors closed.

All smoke alarms shall be listed and installed in accordance with the provisions of this code and the household fire warning equipment provisions of NFPA 72.

Alterations, repairs and additions.

When interior alterations, repairs or additions requiring a permit occur, or when one or more sleeping rooms are added or created in existing dwellings, the individual dwelling unit shall be provided with smoke alarms located as required for new dwellings;the smoke alarms shall be interconnected and hard wired.

Exceptions:

1. Smoke alarms in existing areas shall not be required to be interconnected and hard wired where the alterations or repairs do not result in the removal of interior wall or ceiling finishes exposing the structure, unless there is an attic, crawl space, or basement available which could provide access for hard wiring and interconnection without the removal of interior finishes.

2. Repairs to the exterior surfaces of dwellings are exempt from the requirements of this section.

Power source.

In new construction, the required smoke alarms shall receive their primary power from the building wiring when such wiring is served from a commercial source, and when primary power is interrupted, shall receive power from a battery. Wiring shall be permanent and without a disconnecting switch other than those required for overcurrent protection. Smoke alarms shall be permitted to be battery operated when installed in buildings without commercial power or in buildings that undergo alterations, repairs or additions regulated by adopted codes.

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TYPES:

               Optical detector

An optical detector is a light sensor. When used as a smoke detector it includes a light source (infra-red LED), a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at right-angles to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber into the path of the light beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased input of light into the sensor sets off the alarm.

Another type of optical detector works by using a straight line infra-red beam from the sender to the receiver. When smoke enters the beam, some light is scattered which results in less light detected by the receiver. A decreased input of light into the receiver sets off the alarm.

Also seen in large rooms, such as a gymnasium or an auditorium, are projected beam detectors. A unit on the wall sends out a beam, which is either received by a receiver, or reflected back via a mirror. When the beam is less visible to the "eye" of the sensor, it sends an alarm signal to the fire alarm control panel.

Optical smoke detectors are quick in detecting slow burning, smoky fires. They are less sensitive to false alarming from cooking and bathroom steam than ionization smoke alarms.

             Ionization detector

Inside a basic ionization smoke detector.

This type of detector is cheaper than the optical detector, however it is sometimes rejected for environmental reasons. It can detect particles of smoke that are too small to be visible. It includes a tiny mass of radioactive-americium -241, which is a source of alfa-radiation. The radiation passes through an ionization chamber , which is an air-filled space between two electrodes  and permits a small, constant current to flow between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this flow of current, setting off the alarm.

Approximately 0.2 micrograms of Am-241 is used, which has a half life of 432.2 years. This means that it does not have to be replaced very often, and also makes it safer for people at home (as it is less radioactive). Alpha radiation (as opposed to beta and gamma) is used for two further reasons: alpha particles have high ionization (so sufficient air particles will be ionized for the current to flow), and they have low penetrative power - meaning they will be stopped by the plastic of the smoke detector and/or the air, reducing the risk of harm to people