By Arindam Bhadra
Circuits and Pathways in NFPA 72 In Greece, over 2500 years ago, near the small town of Marathon, there was a battle. After the battle, one of the winning Athenians ran all the way from Marathon to Athens carrying the news. He ran along a path. Nowadays, the message could be carried by a person running along a narrow mountain road, a verbal telephone call, a news story over the microwave towers, a data signal carried over fiber optics, Etc.; so may choices. The NFPA would consider all of these to be communication paths. The paths are no longer just copper wires, but wireless radio waves are also used, Ethernet data cables are also used, and fiber optic cables are also used in fire alarm systems. Because of all the different types of communication paths for fire alarm systems being used nowadays, the NFPA is addressing them all differently than they did in the past. Beginning with the 2010 edition of NFPA 72, National Fire Alarm and Signalling Code, all of the "styles" of circuits were deleted, and four "classes" were added. A new chapter, Circuits and Pathways, was added during the reorganization of this code. In the past, the styles only described the operation during an alarm or fault condition for initiating device circuits, notification appliance circuits and signalling line circuits. NFPA 72, National Fire Alarm and Signalling Code, defines the performance of fire alarm circuit and pathways in Chapter 12. While the terms "circuit" and "pathway" are often used interchangeably, they are different. The styles of initiating device circuits (IDC), notification appliance circuits (NAC) and signalling line circuits (SLC) were eliminated, and four new classes were added. The intent was to add information about all circuits and pathways, not just IDCs, NACs and SLCs. A circuit is 36
defined in Chapter 3 as "either means of providing power or a connection path between locations", while a pathway is "any circuit, conductor, optic fiber, radio carrier or other means connecting two or more location." Basically, a circuit is copper, while a pathway can be copper or any other type of connection. There are three things considered by the NFPA with the carrying of the signals on the fire alarm system paths: 1. Supervision - The method of self-checking for faults - the end-of-line resistor continuity check and handshaking using data signals are two common methods 2. Redundancy - The continued operation of the whole fire alarm system, or a second path to carry signals around a problem is redundancy - an open wire or wire-to-wire short are some of the problems addressed with redundancy 3. Protection from Damage - Conduit is one common method used to protect the path Pathway performance is defined in Section 12.3, using Class A, B, C, D, E, N, or X based on the circuit performance. NFPA 72 does not determine what class is used on a particular pathway, but rather defines the performance of the pathway. The specification, designer, AHJ determine what class of pathway is used. NFPA 72, Chapter 24, "Emergency Communications Systems," Section 24.3.14 is the only place I have found any requirements for survivability. Survivability for fire alarm pathways is typically only required for systems employing partial evacuation or relocation. Since building and fire codes require systems to be installed in accordance with NFPA 72, they do not have specific provisions for survivability. The Chapter 24 technical committee is proposing important changes to the 2022 edition of the code. A pathway classification describes more than that. When describing a Class, the NFPA is concerned with is Reliability, Fixability, and Survivability. Reliability - The NFPA wants to make sure the fire alarm system continues to work in the long run. March 2021 - www.visionmediahitech.com
