Circuit Classifications Vs. Styles
Stuart Gilbert, president of West Hempstead, N.Y.-based Superior Protection Services, wrote me to say that, as outlined in chapter six (“Protected Premises Fire Alarm System”) of NFPA 72, 2007 Edition, the Class A circuit classification is associated with the Style 6 type.
Gilbert is correct, however, the new 2010 Edition no longer refers to circuit style when classifying performance under 6.5 and 6.6. The class designation has been expanded from Class A and B to that of A, B, C, D, E and X. The 2010 Edition does, however, contain the same initiating device circuit (IDC), signaling line circuit (SLC) and notification appliance circuit (NAC) tables in Annex A, Chapter 12, for reference only. In a word, it appears that the new NFPA 72 has made circuit style references obsolete.
Chapter 10 provides an in-depth view of what you do not have to monitor insofar as integrity goes. It’s Chapter 12 that outlines what you need to know about circuit classifications. Circuit Class all but disappeared in the 1990s with the introduction of circuit styles. For good reason, I’m sure NFPA 72 now refers to Pathway Classifications.
Following are brief descriptions of the expanded collection of Pathway Class designations that the Technical Correlating Committee on Signaling Systems for the Protection of Life and Property (SIG-AAC) released in the new code:
Class A — This type of pathway, be it metallic or fiber-optic cable, must include a redundant path. It must maintain its operational capability beyond a single open circuit and any condition that affects the operation of the pathway should be annunciated per code.
Class B — This pathway type does not include a redundant path. Operation stops at a single open circuit and all conditions that can affect its interoperability must be annunciated in the appropriate manner.
Class C — This type of pathway may include more than one pathway, such as T-tapping offers, but end-to-end communication is monitored so when communication stops, a supervisory condition is annunciated in the appropriate manner.
Class D — This particular pathway classification involves fail-safe operations, as in a closed-relay circuit. NFPA 72, 2010 does not require annunciation when a circuit fault occurs, but rather the intended function associated with the fail-safe circuit should be invoked. A good example of this is an access egress door where a closed-relay circuit is made to open when the relay coil either loses power from the alarm system motherboard or the coil in the relay fails.
Class E — This pathway classification does not require the alarm system to monitor Pathway integrity.
Class X — Like a Class A pathway, Class X must provide a redundant pathway and any conditions that can affect the circuit must be annunciated. But unlike Class A, this one must provide operations beyond both a single open or single short circuit. For the sake of comparison, this is equivalent to the older Class A, Style 7 designation.
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