This piece was originally published in the May 2016 issue of ei, the magazine of the electroindustry.
By Denise L. Pappas, Executive Director, Technical Standards, Valcom Inc., and Gordon Bailey, Director, Engineering Services, Valcom Inc.
Changes to the National Fire Protection Association (NFPA) code on fire alarms reflect a movement in the fire alarm world that acknowledges the evolving role of technology. In 2010, NFPA changed the name of NFPA 72 from National Fire Alarm Code to National Fire Alarm and Signaling Code. The change reflects significant changes and expansion in the 2010 edition.
NFPA technical committees reordered chapters of NFPA 72 into logical sections that allow for future expansion and added chapter 24, “Emergency Communication Systems,” to detail what is required when a mass notification system is able to override an active fire alarm signal. While permitted, this override capability can only be used after a complete risk assessment of the site has been completed.
This change is seen in this section of NFPA 72, 2010 edition:
22.214.171.124.2. When the fire alarm system has been activated, and mass notification has been given priority over the fire alarm system, a distinctive audible and visible indication shall be provided at the building fire alarm control unit.
As the need for mass notification grew along with building system convergence, NFPA committees struggled with how to include Ethernet as a viable pathway option. Some committee members made attempts but failed during the 2013 code-making process. The majority had concerns with reliability and survivability of this type of pathway.
After 2013, NFPA formed the Correlating Committee Task Group to develop a way to use Ethernet. This group included members from large-scale public infrastructure, fire alarm companies, engineering consultants, mass notification system manufacturers, end users, Building Industry Consulting Service International (BICSI), and UL. As a result of their efforts, the new pathway designation of Class N was introduced into the 2016 Code.
Pathfinding Class N
What constitutes a Class N pathway? Class N is different from the other class pathway designations in a number of ways. The old pathway designations (Class A, B, and X) presumed a pair of wires looped to multiple devices in parallel, a two-wire circuit.
According to the 2016 edition of NFPA 72, a Class N pathway is as defined below.
12.3.6 Class N. A pathway shall be designated as Class N when it performs as follows:
(1) 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.
Exception: When only one device is served, only one pathway shall be required.
(2) A loss of intended communications between endpoints shall be annunciated as a trouble signal.
(3) A single open, ground, short, or combination of faults on one pathway shall not affect any other pathway.
(4) Conditions that affect the operation of the primary pathway(s) and redundant pathways(s) shall be annunciated as a trouble signal when the system’s minimal operational requirements cannot be met.
(5) Primary and redundant pathways shall not be permitted to share traffic over the same physical segment.
The Class N pathway opens up the opportunity to safely and reliably use modem network architectures for control units and devices, such as smoke detectors, strobes, pull stations, speakers, audio amplifiers, and digital signage. It offers a way to converge mass notification systems with fire alarm systems to form an emergency communication system.
Choosing the Right Technology
NFPA 72 defines the requirements for Class N and allows designers to choose the technology and techniques required to comply with those requirements. A basic Class N application is depicted in figure 1.
The main control unit on the left could be a fire alarm control unit, autonomous control unit (for mass notification systems), or emergency communications control unit (see NFPA 72, chapter 24.) In this figure, there is a note to reference the supervision exception in the code for the 20-foot maximum inside a protected enclosure or raceway. This means that where the control unit is in the same room (and protected by conduit, raceway, or enclosure) as the network switch, it does not require a redundant network pathway to it. Redundant pathways are shown between switch 1 and switch 2 but only one path is shown between switch 2 and the endpoint devices. This illustrates that when a section of Class N pathway services or controls only one endpoint device, a redundant pathway is not required.
Network infrastructure, i.e., components that make up a Class N network, are not defined as “devices” by NFPA. They are considered transport equipment (e.g., switches, routers, and hubs) and do not require the specific supervision required for Class N endpoint devices. General supervision is provided, as they are part of the end-to-end supervision to the Class N endpoint devices. Backup power is required for all Class N network transport equipment used for life safety and, of course, for Class N devices.
As stated, a redundant pathway is not required where a Class N endpoint device encompasses only one device or appliance. On the other hand, if an endpoint services more than one device or appliance downstream, it is required to have a redundant pathway. The exception to this rule is illustrated in figure 2 by the dotted line. Redundant pathways are not required inside an enclosure or raceway within 20 feet in the same room.
Other considerations for Class N pathways are physical separation of redundant pathways and pathway survivability levels. Class A and X are required to have their redundant paths supervised. Class N does not require physical separation of redundant pathways, but local codes, design specifications, or the authority having jurisdiction (AHJ) may require it. Pathway survivability levels are defined in NFPA 72, section 12.4. Requirements are typically agreed upon by the design professional and the AHJ for the specific project.
As the convergence of technology continues, Class N pathway designations will evolve over the next several code cycles. Always consult your local AHJ and system designer when implementing any fire alarm system or mass notification system/emergency communication system.
Ms. Pappas serves on NFPA 72, 101, 5000 Technical Committees; NFPA 99 Correlating Committee, BICSI ESS Committee; BICSI Wireless Standard Subcommittee Chair; and ICC Technical Committee. She is a published author and JMT-certified speaker.
Mr. Bailey has spent more than 30 years in the telecommunications industry, designing and installing overhead paging and sound systems.