WITHOUT GIVING IT MUCH THOUGHT, most of us rely on an Ethernet connection to a computer network for much of our daily work. With the proliferation of such networks, it makes sense to explore their use for the transmission and interconnectivity of fire alarm systems.
The technical committees for NFPA 72®, National Fire Alarm and Signaling Code, have struggled with issues relating to the use of such networks for several code-revision cycles, and the discussions that took place during the 2013 code cycle seemed to generate more questions than answers. Essentially, many committee members tried to measure the performance of the Ethernet and networks by using the current code requirements as a basis. This resulted in a small step forward in the development of requirements for the “shared pathway” designation in Chapter 12 of the 2013 edition of NFPA 72.
The requirements provide a list of “levels” of shared pathways and forbid the use of some levels for code-required functions. As stated in Annex A, for example, “In a Shared Pathway Level 3, life safety equipment is not shared with equipment of non–life safety systems.” However, the code does not define “shared pathways.”
These designations do not really go far enough to help code users judge the use of Ethernet or network connections. The Correlating Committee for Signaling Systems for the Protection of Life and Property felt the need to establish a task group to resolve the issues.
The correlating committee gave the Task Group on the Use of Networks in Fire Alarm and Emergency Communication Systems the primary responsibility for developing language for use in Chapters 10, 12, 23, 24, and 26 of NFPA 72 that will address the use of Ethernet, local area networks, and wide area networks in fire alarm systems, fire emergency voice and alarm communications systems, and mass notification systems.
The task group submitted the result of its work as a proposal for consideration by the affected chapters in the code. The proposal for both code language and annex material includes the development of a new class of wiring, Class N, with performance characteristics that include “(1)… 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 a combination of faults on one pathway shall not impact any other pathway. (4) Conditions that affect the operation of the primary pathway(s) and redundant pathway(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.”
In addition, a proposed change in Chapter 23 would state that “Class N pathways shall be required to use Shared Pathway Level 3.” The proposed annex addresses additional details of Class N pathways.
These and other proposed changes would help bring the fire alarm and emergency communications industry closer to using interconnectivity already used by most other microprocessor-based systems. Maintaining life safety objectives and reliability of operation remain key to the task group’s efforts, and it will be interesting to see what the technical committees accept as final changes.
Wayne D. Moore, P.E., FSFPE, is a principal with Hughes Associates.