A Tale of Two Codes

SHUTTERSTOCK / A_STOCKPHOTO
SHUTTERSTOCK / A_STOCKPHOTO

Installing a code-compliant fire alarm system requires you to follow more than one code. Each code relating to cable has specific requirements based on the system performance desired by the owner. Both NFPA 70, National Electrical Code and NFPA 72, National Fire Alarm and Signaling Code, contain must-follow requirements.

Section 23.4.2 of NFPA 72-2019 provides cabling and circuit information that must be used in the design and installation of building fire alarm systems. Those systems are there for protecting life and property.

First, determine the circuit designations that will meet the owner’s performance requirements. The code does not require one class of circuit over another. That decision is generally left up to the designer. In some rare cases, the local jurisdiction or the state will mandate a certain class of circuit be used for all fire alarm systems. The circuit designations will apply to all initiating device circuits, notification appliances and signaling line circuits. The class of circuit depends on the circuit’s capability to continue operating during specified fault conditions as described later in the code.

Additionally, the code states “where the power to a device is supplied over a separate circuit from the signaling line circuit or initiating device circuit, the operation of the power circuit shall meet the performance requirements of the initiating device circuit (IDC) or signaling line circuit (SLC),” unless different performance requirements are established during the design evaluation approved by the authority having jurisdiction.

The code’s intent with this requirement is to prevent situations where the signaling line circuit to a device is required to be one class of operation, while the power circuits—running in the same raceways and subject to the same threats—are wired to a lower class of operation.

The most prevalent type of circuits used today are signaling line circuits, and they are used to communicate between FACUs, annunciators and controllers while also serving as the primary method to monitor initiating devices and controlling output devices. The extent and coverage of notification appliance circuits (NACs) are also limited by the power required to operate the devices. SLCs, unlike IDCs and NACs, have few limitations, and it is now common that a single SLC can monitor and control more than 250 devices. Of course, a competent designer will not put all their eggs in one basket, as a single short or open could take down the complete circuit. You would not want a catastrophic failure of a fire alarm and life safety system due to a single open or short on an SLC, which negates the desire for an acceptable minimum level of performance and reliability for the system.

Annex A of NFPA 72-2019 suggests that “designers should carefully consider the potential that a single SLC short or open caused by a fire or inadvertent damage to the SLC could disable an entire SLC prior to the activation of an alarm condition along with the subsequent alarm signaling and emergency control functions.”

With the older, conventional-style fire alarm systems, a single open, ground or short fault on one circuit could not affect the performance of other IDCs, NACs and emergency control circuits. Annex A of the 2019 edition of the National Fire Alarm and Signaling Code provides guidance to limit potential catastrophic failure to one zone, such as the functionality of an IDC or NAC. The guidance is to limit the potential catastrophic failure to one zone, similar to how traditional IDCs and NACs have been and are now required to perform.

The annex also shows where the application of fault-circuit isolators can be used to accomplish the same functionality as described above to provide diagrams for the different classes of circuits. Chapter 12 provides the descriptions of classes A, B, C, D, E, X and N. To be clear, each of these circuit classifications has different performance, operating characteristics and reliability. And as stated earlier, the code does not require a specific class of circuit. You will typically use a Class B circuit for most applications, but it is important to determine the level of reliability needed before installing any cable.

NFPA 70 also has requirements for fire alarm circuit identification and the appropriate way to remove or label abandoned cables. Understanding the requirements of both of these important codes will lead to a reliably installed fire alarm system.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker and expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. He is a vice president with Jensen Hughes at the Warwick, R.I., office and can be...

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