Adding the Circuits and Pathways chapter was one of the many changes made in the 2010 edition of NFPA 72 as part of the restructuring of the National Fire Alarm Code. The primary reason for restructuring was to add a chapter for emergency communications systems that included requirements for mass notification systems.
The title of the document was changed at the time to become the National Fire Alarm and Signaling Code. Since NFPA 72 would now contain information on many different types of systems other than fire alarms, it was important to make it easier for non-fire alarm users to find applicable requirements. Much of the new chapter content came from the protected premises chapter. Chapter 12, “Circuits and Pathways,” was one of them.
Previously, there was information on classes and styles of circuits in the protected premises chapter. The new title for Chapter 12 added the term “pathways.” The styles of initiating device circuits (IDC), notification appliance circuits (NAC) and signaling 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. The term “pathway” was intended to provide any means to get information from one point to another, not just using electrical conductors. This could include radio signals, cellular signals, the internet or optical fiber cables.
With the expansion of classes to include Class A, B, C, D, E and X, this chapter described the operation for almost every means of transmitting information from point A to point B. In the 2016 edition, Class N was added to provide information on “network” circuits and pathways, similar to data communications circuits used for connecting computers in buildings.
Keep in mind that NFPA 72 does not require any particular class to be used for installation. Chapter 12 describes how they will operate if they are used. It is up to the system designer to choose the classes that will be used based on a number of considerations such as the purpose of the system, the number of devices or appliances on a single circuit or pathway, the number of occupants that will be in the building, the effects of a fault that may occur on any circuit and more. You can review the entire list of considerations in Chapter 23, paragraph 23.4.3.
Chapter 12 describes each class. Here are some examples of each you may find useful.
Class A is the same as it has always been. Commonly called a 4-wire circuit, it is actually two wires that begin and end at the control unit. All devices will continue to work with a single open or ground fault.
Class B is also the same as it has always been. It is a two-wire circuit that will only work up to an open fault. It will report a trouble for a ground fault.
Class C is one of the new ones. It includes one or more pathways where operational capability is verified by end-to-end communication, but the integrity of individual paths is not monitored. This describes the new monitoring methods very well, such as cellular, internet or mesh radio.
Class D is used for fail-safe operations such as door holder circuits. It is not monitored for integrity, but the equipment will operate as intended if disconnected.
Class E is also not monitored for integrity. The best example I can provide is that this would be used for an auxiliary power circuit. You can monitor it by installing an end-of-line relay in series with the initiating device circuit.
Class N is for network circuits. It includes two or more pathways where operational capability of the primary pathway and a redundant pathway to each device shall be verified through end-to-end communication. The wiring method is different from the other classes of circuits and pathways because there is a pathway to each individual device (end point). There is a lot of useful information on Class N in Annex A.
Class X is like the old Style 7 circuits. It exceeds the requirements for Class A circuits since you must provide isolation modules to prevent a single device from affecting the entire circuit.
In addition, the descriptions of the four pathway survivability levels are found in Chapter 12, Section 12.4. Like the classes, these only describe the levels used to provide survivability of the pathways when required by another source. 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.