Special Applications: Partial, selective and other coverage requirements in NFPA 72

iStock / Franck-Boston / Shutterstock / Tom Stocker / Kavee29
iStock / Franck-Boston / Shutterstock / Tom Stocker / Kavee29
Published On
Mar 15, 2021

When I hear “NFPA 72 requires smoke or heat detectors to be installed throughout a space,” I immediately know the fire alarm system installer has not read the entirety of the code, nor do they understand the code process.

NFPA 72, the National Fire Alarm and Signaling Code, does not require any fire alarm system to be installed in a building. That requirement comes from the building, fire or life safety code. Once a system is planned for a building, then the applicable requirements for installation and application found in the code must be followed.

As stated in the opening scope section, “NFPA 72 covers the application, installation, location, performance, inspection, testing, and maintenance of fire alarm systems, supervising station alarm systems, public emergency alarm reporting systems, fire and carbon monoxide detection and warning equipment, and emergency communications systems, and their components.”

This statement is backed up by Section 1.2.4 and its associated material in Annex A. NFPA 72-2019, Section 1.2.4, states, “This Code shall not be interpreted to require a level of protection that is greater than that which would otherwise be required by the applicable building or fire code.”

NFPA 72 Chapter 10 contains the requirement for smoke detectors to be installed at the fire alarm control unit (FACU), notification appliance circuit power extenders and supervising station transmitting equipment used to provide fire notification at the protected premises. Those fire-control units that notify occupants and first responders of a fire are the ones to be protected.

The term “control unit” does not include equipment such as annunciators and addressable devices. Additionally, the smoke detectors are only used to protect the control units and not the entire room or area where the FACU is located. A new statement was added to the 2019 edition of the code stating that, “Smoke or heat detector(s) shall not be required to be installed at the location of dedicated function(s) fire alarm control unit(s) that are not required to provide local or supervising station notification signals.”

Another often misunderstood code issue is that of partial, selective and complete coverage detection.

As you already understand, the building code requirements for fire alarm systems only compel partial coverage. This is often misunderstood by owners who think when you tell them you are providing code-compliant fire alarm system installations that what you are providing is all the coverage they need and that the system will provide early warning of any fire in their facility.

This misunderstanding leads to many litigation arguments about how soon a detector should respond to a fire in their building. Obviously, if there are no detectors in the room or area of fire origin, the fire could exceed the design objectives before being identified by remotely located detectors.

In addition to partial coverage, the code allows selective coverage. The intent of selective coverage is only to address a specific hazard. This type of coverage meets the definition of nonrequired coverage, but where a specific area or specific hazard is to be protected, all points within that area should be within 0.7 times the adjusted detector spacing for spot-type detectors as required by the code. An example of a specific risk being protected from is that the smoke detector is required by the code to be within 21 feet of an elevator, where elevator recall is required.

Fire detection by itself is not fire protection, and there may be instances where you have to advise the owner that, in order to meet their fire protection goals, they need to install an automatic sprinkler system. Nonrequired coverage is defined as “detection installed for reasons of achieving specific fire safety objectives, but not required by any laws, codes, or standards.” Nonrequired coverage must meet all of the requirements of NFPA 72, with the exception of the prescriptive spacing criteria of Chapter 17. Additionally, the code states that “Where nonrequired detectors are installed for achieving specific fire safety objectives, additional detectors not necessary to achieve the objectives shall not be required.”

There are numerous special applications of detection. One of the more common is smoke detection used for air duct applications. The important installation requirement to know is that the passive and active air sampling system must be listed for air duct applications and that the inlet and exhaust sections of pipe that are installed inside the air duct are air-tight and must exhaust the sampled air in accordance with the manufacturer’s published instructions and be installed in such a way as to obtain a representative sample of the air stream.

Air duct systems are protected with passive and active air sampling smoke detectors to prevent recirculation of smoke from the fire area to the rest of the building. To do this, a detector approved for air duct use is installed on the supply side of air-handling systems as required by NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems.

Air duct detectors are not allowed to be used as a substitute for open area smoke detection and are not considered early warning smoke detection. “Passive air sampling” refers to the typical air duct smoke detectors with rigid sampling tubes. The air passes over the tubes and the holes in the tubes “sample” the air flow. If there is no air flow—in other words, if the HVAC system is off—then no sampling takes place and detection is unlikely to occur. When air is flowing, the tubes are only sampling approximately 0.5% of the air. This is the obvious reason that these detectors are not considered early warning smoke detectors.

Dampers and filters

The requirement to install duct-type smoke detectors in air ducts is found in NFPA 90A. Typically, it is necessary to manage smoke flow in buildings, and duct smoke detectors are generally used to shut down HVAC systems, close dampers or initiate smoke management.

As stated in Annex A of NFPA 72-2019, “Where duct detectors are used to initiate the operation of smoke dampers, they should be located so that the detector is between the last inlet or outlet upstream of the damper and the first inlet or outlet downstream of the damper.”

Filters affect the performance of duct smoke detectors. Because there is also a history of filter fires, the location of the detector relative to the filter and the source of smoke must be considered during the design process. Also, when the duct smoke detectors are installed in the supply side of the duct system, they will detect smoke that enters the system through the HVAC unit’s fresh air intake. The duct smoke detectors installed on the supply side of the air duct system cannot be expected to serve the purpose of providing detection for the system’s return side.

Active air sampling smoke detection is often used for protection of cabinets containing electrical equipment. In these applications, the air sampling ports are in the main airflow at the exhaust vents, downstream of the airflow distribution path, or in accordance with the manufacturer’s published instructions.

Duct smoke detection is just one of the many special applications of detectors that can be found in NFPA 72. When you encounter any of these special applications, always make sure that you read the associated Annex A material, which provides installation guidance and references that will help to ensure your design and installation are code-compliant.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker, writer and expert in the life safety field, has been a principal member and chair of NFPA 72, Chapter 24, as well as a former principal member of NFPA 909 and NFPA 914. He is the...

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.