Smoke detectors (alarms) are installed in single-family dwellings to provide early warning of a fire. Their installation is required by building codes in many municipalities and by many fire departments. NFPA 72, the National Fire Alarm Code, provides guidance for the proper selection, installation, operation, testing and maintenance of smoke detector systems. It is, therefore, imperative that we understand the operation of fire alarm equipment, including its installation instructions to accomplish this very important task of helping to safeguard a family from the ravages of fire.
There are two basic methods of providing fire protection in dwellings. The first is the household fire warning system panel, commonly called the fire alarm panel. They are installed as a complete system with notification appliances (horns, speakers and strobe lights) and initiating devices (heat detectors, smoke detectors and pull stations).
Fire alarm panel systems are usually installed in larger dwellings or where a more sophisticated fire alarm system is desired. Residential-grade panels are specifically listed for household use and should not be used in commercial or industrial installations. The local fire department and the municipal inspection authority must be notified of the installation of the fire alarm system and a permit obtained, where applicable.
The second basic method of fire protection in a dwelling involves the installation of single- or multiple-station alarm devices (smoke or heat alarms) not incorporated into a fire alarm system. A single-station alarm device is an assembly that incorporates the detection device, the control equipment and an alarm notification device into one unit. Single-station alarm devices are normally installed as stand-alone devices and do not have provisions for interconnection to other single-station devices.
A multiple-station alarm device is a single-station device designed to be interconnected to one or more additional alarm devices so that the actuation of one device will cause all other interconnected devices to sound a similar alarm.
Single- or multiple-station alarm devices are operated from a power supply located within the device, such as a battery; from a power source located at the point of connection for the alarm device (a 120-volt branch circuit); or from both a 120-volt branch circuit with battery backup. The normal power source connection for these alarm devices is a nonswitched branch circuit located in the protected area.
NFPA 72 requires smoke detectors (alarms) to be installed in all sleeping rooms of the dwelling (except in existing dwellings not previously wired for smoke detection). Smoke detectors (alarms) are also required to be outside of each separate sleeping area, such as in the hallway, and on each story of a multistory dwelling. For example, if the bedrooms are on the second floor of a dwelling, a smoke detector (alarm) must be on the first floor at or near the base of the stairs leading up to the bedroom area.
Most municipalities require multiple smoke alarms in a dwelling to be interconnected and manufacturers will limit the number of devices that can be connected in their installation instructions. NFPA 72 limits the number of interconnections to no more than 18 initiating devices, of which no more than 12 can be smoke alarms.
Interconnected devices require three conductors plus an equipment grounding conductor: a black hot conductor, a white neutral conductor, a red or orange interconnection conductor plus the green or bare equipment grounding conductor. The interconnection conductor causes the connected smoke alarms to signal an alarm when one device senses smoke, thus providing early warning of smoke. The total length of the interconnecting conductor cannot exceed 1,000 feet.
Since the branch circuit supplying the smoke alarms also powers the interconnecting conductor, Section 300.3(B) requires all four conductors in the smoke alarm circuit to be run in the same cable or raceway.
Smoke alarm manufacturers require all interconnected devices to be powered from the same branch circuit. Section 210.12(B) of 2002 National Electrical Code (NEC) now requires the entire branch circuit supplying outlets in a dwelling unit bedroom to be protected by an arc fault circuit interrupter (AFCI) device. Since 120-volt smoke alarm devices are considered to be outlets and at least one smoke alarm is required in each bedroom, the branch circuit supplying the smoke alarms must be protected by an AFCI device. If a separate circuit were run to the smoke alarms, then an additional arc fault device would be required.
Careful design of the smoke detection circuits in dwellings will provide the best possible fire protection and will comply with the revised requirements in the 2002 NEC. EC
ODE is staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at email@example.com.