Beginning with the 2010 edition of NFPA 72, National Fire Alarm and Signaling 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 signaling line circuits. The term “pathways” was added because “circuit” has an electrical connotation—basically copper conductors carrying current.
Pathways can be any means of communication between two points. NFPA 72 technical committees are doing a great job of using Annex A to better describe the requirements in the code chapters, so be sure to take advantage of that.
Here is a brief review of each class with examples of pathways that meet the requirements. Then we will discuss the newest addition, Class N (for networks), that was added in the 2016 NFPA 72. Please note that this new Chapter 12 does not require any particular class. It describes the operation of each class for proper installation. A requirement for any particular class would be made by other laws, codes or standards or by design documentation.
Class A is the same as it was before. It is a redundant pathway in which all devices will continue to operate with a single open or ground fault. The fault (open or ground) will annunciate a trouble signal. This type of pathway provides a higher level of assurance that the fire alarm system will operate when needed.
Class B also is the same as it was before. It is a two-wire pathway in which devices will only continue to operate up to an open-circuit fault. It will also continue to operate with a single ground fault and will annunciate a trouble signal when an open or ground occurs.
Class C is new and includes one or more pathways that verify its operation through end-to-end communications. Although this is very similar to the way a signaling line circuit operates, this class is intended more for wired or wireless pathways connected to a LAN, WAN, the internet or connections between a digital alarm communicator transmitter and the public switched telephone network. A loss of the “handshake” from a device on the pathway will result in a trouble signal.
Class D also is new. It is a fail-safe circuit and not supervised, and it will perform its intended function when it loses its power source. Otherwise, an open circuit occurs. A good example of a Class D pathway is a door holder circuit.
Class E is a new pathway that is not supervised. An example of Class E is a power circuit on a fire alarm control panel. The way this type of pathway can be supervised is by using an end-of-line relay with its contacts connected in series with the initiating device circuit or signaling line circuit (SLC). When power is lost, the relay contacts open, and a trouble signal would be reported.
The last new class introduced in 2010 is Class X. This is a pathway like the old Style 7, which exceeds Class A requirements. Isolation modules would be added to keep one device from making the entire pathway inoperative.
Class N was added in the 2016 edition. This pathway is intended for connections made using ethernet. Class N pathways are similar to Class C in that a loss of end-to-end communications will result in a trouble signal. One significant difference is that, while many Class C pathways use an SLC with all devices connected on a single pair of wires, Class N uses a “home run” methodology where a single ethernet circuit is connected to a device, referred to as an end point. It can also be used between the control unit and a switch that would be used to connect to multiple end points. Redundant pathways are required for most runs.
Significantly, monitoring for ground faults is not required, since a single ground does not affect the operation of an ethernet cable. The annex material does a great job in explaining in detail how Class N pathways are supposed to operate, and it presents a number of figures showing how Class N pathways are configured. I highly recommend you read that section.
Now that NFPA 72 has seven classes for circuits and pathways, every connection to a fire alarm panel will fit into one of the categories. Designers now have to specify which class should be used for each application, and installers will have to become more familiar with each of the classes to install them correctly.