Cabling Mysteries And Myths

Contractors often experience some confusion over the types of circuits required and permitted for fire alarm system installations. First, contractors need to recognize that they must install all circuits as required by chapters 12 and 23 of NFPA 72 2016, National Fire Alarm and Signaling Code.

However, many code users are confused about the types of required circuits. Chapter 23 covers the performance requirements of the various circuit classes permitted in a fire alarm system installation. Chapter 12 contains the actual class designations and operation descriptions.

Chapter 23 describes three circuit designations: initiating device circuits (IDC), notification appliance circuits (NAC) and signaling line circuits (SLC). These circuits must bear a designation by class, depending on the circuit’s capability to operate during specified fault conditions, as defined in Chapter 23. These specified fault conditions must result in the annunciation of a trouble signal at the protected premises within 200 seconds, as Chapter 12 requires.

Using an evaluation based on the path performance as required by governing laws, codes, standards and a site-specific engineering analysis, the code provides the method to determine which class of circuit should be used.

Specifically, Section states, “When determining the integrity and reliability of the interconnecting signaling paths (circuits) installed within the protected premises, the following influences shall be considered:

“(1) Transmission media used

“(2) Length of the circuit conductors

“(3) Total building area covered by, and the quantity of initiating devices and notification appliances connected to, a single circuit

“(4) Effect of a fault in the fire alarm system that would hinder the performance objectives of the system that protects the occupants, mission, and property of the protected premises

“(5) Nature of hazards present within the protected premises

“(6) Functional requirements of the system necessary to provide the level of protection required for the system

“(7) Size and nature of the population of the protected premises”

A system designer should include the results of the evaluation described above in the system-design documentation required by Chapter 7.

Power circuits also have requirements when the power to a device is supplied over a circuit separate from the SLC or IDC. The power circuit operation must meet the IDC or SLC performance requirements unless the evaluation process has established different performance requirements that the authority having jurisdiction has approved.

The fire alarm system must monitor IDCs, SLCs and NACs for integrity. Typically, the fire alarm system monitors the IDCs and NACs using a small amount of current flow that initiates a trouble indication whenever an open fault occurs on the circuit. A contractor must not use a wiring procedure referred to as “T-­tapping,” because the T-tapped portion of the circuit will compromise 
the monitoring.

The system monitors SLC integrity through the interrogation of all devices. This allows for T-tapping of devices or appliances installed on a Class B SLC circuit. The code does not allow T-tapping of Class A or X signaling line circuits.

The code and electrical performance characteristics limit the extent of coverage traditional IDCs provide. The quantity of circuit-powered initiating devices possible on a particular initiating device circuit limits the coverage that circuit provides. To reduce the amount of protection impaired when a circuit fails, the code also limits the number of initiating devices permitted on particular initiating device circuit. For notification appliances, the extent and coverage provided by NACs becomes inherently limited by the power required to operate the connected appliances.

As explained in Annex A for Chapter 23:
 “SLCs, unlike IDCs and NACs, have few limitations, and it is now common that a single SLC can monitor and control more than 250 devices. In addition, a single SLC can be the only pathway by which alarms are initiated, emergency control functions are controlled, and audible and visual notification appliances are activated.”

Designers should consider circuit operation during a fire and the possible loss of SLC operation. Such a scenario could disable an entire SLC prior to alarm actuation, along with the subsequent alarm signaling and emergency control functions.

As stated in Annex A, “One method for providing an acceptable level of performance and reliability of SLCs is to limit the potential catastrophic failure to one zone, in a way similar to how traditional IDCs and NACs have been and are now required to do.”

The code does not require a designer or installer to use a specific class of circuit. A state or local code may specify a required class of wiring, but, generally, the system designer determines the class of circuit using the evaluation described above. Once the designer chooses this, the requirements of Chapter 12 prevail.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist
Wayne D. Moore, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. Moore is a vice president with JENSEN HUGHES at the Warwick, R.I., office. He c...

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