Two for Smoking

One way electrical contractors (ECs) ensure safety is to install smoke alarms in every home they wire. Not only does code call for 120V AC smoke alarms in one- and two-family dwellings, but most local fire authorities also require them.

ECs and other low-voltage contractors should have a fundamental understanding of the differences between smoke alarms and smoke detectors. It’s also important that they understand how they work, how to care for and maintain them, and when it is time to replace them.

A smoke alarm generally is an autonomous device that contains a smoke sensor along with a sounder and power source. NFPA 72, 2007, Section 3.3.180, defines the smoke alarm as a “single or multiple-station alarm responsive to smoke.”

In practice, smoke alarms are commonly powered by house power using 120V AC. A small 9V battery is used for backup power. An internal sounding device alerts occupants when a fire has been detected. In most instances, a tandem line provides connectivity between all smoke alarms in the house. This allows a systems-like response, so when one goes into alarm, they all do.

The smoke detector is best defined by Section 3.3.43 of NFPA 72, 2007. Here, NFPA defines it as “a device suitable for connection to a circuit that has a sensor that responds to a physical stimulus, such as heat or smoke.”

Smoke detectors are used in system-type environments where power is provided by an external source, and an output connects to an initiating circuit in a fire alarm control panel where it is monitored and acted on when a fire is detected. From here, smoke detectors in a home signal a central monitoring or supervising station (see Section of NFPA 72, 2007).

Alarm and detector upkeep

Maintenance ensures sustainable detection over any fire alarm system’s lifespan. According to Section, NFPA 72, 2007, “Maintenance of household fire alarm systems shall be conducted according to manufacturer’s published instructions.”

In the case of smoke alarms and smoke detectors, there are two types of testing procedures used to ascertain proper operation: functional and calibrated. Either can be used on smoke alarms or detectors.

In a functional test, the technician introduces a known method of activation to see if the alarm/detector will trip. Often, the method used for testing involves an object that the contractor places into or near the detector itself, such as a plastic card or magnet. The unit also may feature a button that, when pressed, will trigger the alarm/detector to perform a functional test on itself.

Calibrated testing requires a special tool be applied to the alarm/detector to determine the operation quality. The result is a percentage that defines just how sensitive the unit is to airborne smoke particulates (see Chapter 10, NFPA 72, 2007).

The device either will work or not; if not, it may be time for replacement. For smoke alarms and smoke detectors, replacement should take place in accordance with the manufacturer’s instructions. In the case of smoke alarms, however, there is a specified time limit placed on their use if the manufacturer fails to mention one: “Unless otherwise recommended by the manufacturer’s published instructions, single- and multiple-station smoke alarms installed in one- and two-family dwellings shall be replaced when they fail to respond to operability tests, but shall not remain in service longer than 10 years from the date of manufacture” (Section 10.4.7 of NFPA 72, 2007).

NFPA 72 places a 10-year limit on use because past data indicates smoke alarms are not maintained as well as smoke detectors. For this reason, the Technical Committee on Testing and Maintenance of Fire Alarm Systems (SIG-TMS) decided that a 10-year limit was in order where none is stipulated by the manufacturer.

Note that this 10-year rule only pertains to smoke alarms, not smoke detectors. Smoke detectors should be replaced when they fail to work properly or cannot be cleaned, thereby ensuring they exhibit the correct sensitivity.

COLOMBO is a 35-year veteran in the security and life safety markets. He is director with and a nationally recognized trade journalist in East Canton, Ohio. Reach him at

About the Author

Allan B. Colombo

Freelance Writer
Allan Colombo is a 35-year veteran in the security and life safety markets. He is director with and a nationally recognized trade journalist in East Canton, Ohio. Reach him at

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