The history of classes and styles of circuits is interesting. Back in 1987, a proposal was accepted to start using Styles of Circuits instead of the Class A and B we were all used to. Well, at least some thought so. Eliminating the classes was very confusing to the industry, so in 1993, Class A and B were put back in the National Fire Alarm Code—along with the styles.
Everything stayed the same for 14 years, but in the 2007 edition of NFPA 72, another change eliminated all the styles except three of them for signaling line circuits. When NFPA 72 became the National Fire Alarm and Signaling Code in 2010, a new Chapter 12, “Circuits and Pathways,” was created as part of the code restructuring. The remaining three styles were removed and the classes expanded to six.
I serve on the Protected Premises Technical Committee, which is responsible for Chapter 12. I remember being opposed to this change at the time. In my opinion, changing the code this dramatically and adding more classes was going to cause more harm than good. I have always worked to make the codes easier to understand, to get rid of unenforceable language and reduce the number of misinterpretations. I always believed that, by making it simpler, we would have better enforcement, and that would be good for the industry.
I have since changed my mind. In my opinion, it is much better now. Before, the class designations only covered some circuits: initiating device circuits, signaling line circuits and notification appliance circuits. The classes did not apply to power circuits, fire safety function circuits or the monitoring paths. Now they do. The code no longer just refers to circuits, since we now also have other means of transmitting information, like the Internet and wireless transmission. So, the chapter was titled “Circuits and Pathways.”
According to Chapter 3, the definition of path, or pathway, is, “Any conductor, optic fiber, radio carrier, or other means for transmitting fire alarm system information between two or more locations.” All the NFPA Technical Committees have improved the use of Annex A to provide better explanatory information than in the past.
Class A and B still contain the same requirements they always have, but they are described better now than in past editions. Class C “includes one or more pathways where operational capability is verified via end-to-end communication, but the integrity of individual paths is not monitored,” and “a loss of end-to-end communication is annunciated.”
The annex describes a Class C quite well: “A.12.3.3 The Class C reference is new and is intended to describe technologies that supervise the communication pathway by polling or continuous communication ‘handshaking’ such as the following:
“(1) Fire alarm control unit or supervising station connections to a wired LAN, WAN, or Internet
“(2) Fire alarm control unit or supervising station connections to a wireless LAN, WAN, and Internet
“(3) Fire alarm control unit or supervising station connections to a wireless (proprietary communications)
“(4) Fire alarm control unit digital alarm communicator transmitter or supervising station digital alarm communicator receiver connections to the public switched telephone network
“Individual pathway segments are not required to be monitored. Supervision is accomplished by end-to-end communications.”
A Class D circuit is described in 12.3.4 as a “pathway shall be designated as Class D when it has failsafe operation, where no fault is annunciated, but the intended operation is performed in the event of a pathway failure.” A good example of this type of circuit would be a door-holder circuit.
A Class E circuit is not monitored for integrity, such as a power circuit not using an end-of-line relay for supervision.
Last but not least, we have Class X. The Class X reference is new and describes pathways previously described as Class A Style 7 for signaling line circuits. Style 7 was used when a circuit exceeded Class A requirements by using isolation components to keep one device from affecting the entire circuit. Chapter 12 describes a Class X circuit quite well. It is simply a circuit that will continue to operate past a single open, short, combination of the two, or a ground fault and will annunciate a trouble condition if any of those conditions occur.
So spend a little time with Chapter 12, and understand the requirements. Designers also need to understand these requirements, since they will now have to specify which class is to be used for each circuit or pathway.