Often when you are installing fire alarm systems, you are asked what style or class of circuitry you plan to use. The circuit designations may be required in a local or state code or may be specified by the designer of the fire alarm system. Generally, there is a requirement for specific circuit types to help ensure system reliability.
NFPA 72-2007 Section 6.4 must be used in the design and installation of protected premises fire alarm and mass notification systems for the protection of life and property. The code requires that all initiating device circuits, notification appliance circuits and signaling line circuits be designated by class, style or both, depending on the circuit’s capability to continue to operate during specified fault conditions.
This is an area of contention on many installations. There are only two classes of circuits used in fire alarm system wiring: Class A or Class B. The National Fire Alarm Code, in Section 18.104.22.168.1, defines these circuits based on their performance during non-simultaneous single-circuit fault conditions as follows:
“(1) Initiating device circuits and signaling line circuits that transmit an alarm or supervisory signal, or notification appliance circuits that allow all connected devices to operate during a single open or a non-simultaneous single ground fault on any circuit conductor, shall be designated as Class A.
“(2) Initiating device circuits and signaling line circuits that do not transmit an alarm or supervisory signal, or notification appliance circuits that do not allow connected devices to operate beyond the location of a single open on any circuit conductor, shall be designated as Class B.”
The contention over which circuit class to use arises during the value engineering discussions prior to the installation of the system. Class A circuits require specific wiring configurations that include two conductors going out to all devices or appliances and two conductors returning to the fire alarm control unit. Also all Class A circuits using physical conductors must be installed so the outgoing and return conductors from the control unit are routed separately. The outgoing and return (redundant) circuit conductors cannot be run in the same multiconductor cable, enclosure or raceway. The obvious issue here is cost, so this is the first place a contractor will look to value engineer the system wiring configuration to Class B. Class B systems use only two conductors, and the circuit ends at the last device or appliance (usually with an end-of-line resistor); therefore, it will be less expensive to install. In large systems, the savings could be considerable.
In addition to the class designation, the code defines styles of signaling line circuits (SLC) that may be used for various circuit configurations. Signaling line circuits are permitted to be designated as either Style 4, 6 or 7, depending on their ability to meet the alarm and trouble performance requirements during a single open, single ground, wire-to-wire short, simultaneous wire-to-wire short and open, simultaneous wire-to-wire short and ground, and simultaneous open and ground. These performance requirements are in Table 6.6.1 of NFPA 72-2007.
However, other than a local or state code mandate to use Class A, why would it ever be specified? The code requires designers to determine the class or style of circuit based on the path performance detailed in the code and on engineering judgment. The code defines what must be considered when determining the integrity and reliability of the interconnecting signaling circuits to be installed. The following must be considered:
1. Transmission media (wire, fiber, etc.)
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 on the fire alarm system
5. Nature of the hazard 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
NFPA 72-2007 actually requires that this information be considered when choosing the circuit class or style, and it requires that the results of this evaluation be included with the system installation documentation required in Section 4.5.
So before you rush to value engineer the circuit class or style from Class A to Class B, make sure the design intent is understood. A Class B circuit may be okay for a small convenience store but not suitable for a school fire alarm system where the reliability and performance under fault conditions may be extremely important to the life safety afforded by the fire alarm system.
The bottom line is you must understand the code requirements and the fire protection goals of the owner before making decisions affecting the fire alarm system performance and integrity.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current “National Fire Alarm Code Handbook.” Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.