2010 NFPA 72 - Circuits and Pathways

New in your NFPA 72-2010 in Chapter 12 on Circuits and Pathways, previously found in Chapter 6 of earlier editions. Once again, circuit designations have changed, but this is mainly due to new wiring methods such as fiber optics, Ethernet (CAT5), and wireless. Also added are circuit designations for door holder circuits, DACTs, IP communicators and even door locks.

The new wiring style categories are Class X, C, D and Class E. The Classic Class B and Class A circuits still remain. Old circuit designations like Class Y and Z, and Style 4, 6 and 7 have gone away.

Class A
Class A is circuit wiring that with a single open or single ground, all devices on that circuit still operate. This Class A circuit is for IDC (Initiating Device Circuit) (conventional initiation), SLC (Signaling Line Circuit) (Addressable, Analog Addressable, or digital communications circuit) or NAC (Notification Appliance Circuit) (horns, strobes, speakers).

Class B
Class B is circuit wiring that with a single ground, all devices on that circuit still operate. With a single open, all devices that are between the panel and the open still operate; all other devices past the open are inoperable. The Class B designation is for IDC, SLC and NAC Circuits.

Class X
Class X is circuit wiring that with a single open or single ground and a single wire to wire short, all devices on that circuit still operate. This Class X circuit is for SLC for Addressable, Analog Addressable, or digital communications circuits, such as network connectivity. When Isolators are used on the Analog Addressable Circuit, network communications, this prevents the wire to wire short from affecting all the devices on a circuit.

Class C
The Class C reference is new and is intended to describe technologies that supervise the communication pathway by polling or continuous communication “handshaking” similar to DACT and IP communicators. This includes the following:

• Fire control unit or supervisory station connections to a wired LAN, WAN, or Internet
• Fire control unit or supervisory station connections to a wireless LAN, WAN, or Internet
• Fire control unit or supervisory station connections to wireless (proprietary communications)
• Fire control unit digital alarm communication transmitter or supervisory station digital alarm communication receiver connections to the public switched telephone network

Class D
The Class D reference is intended to describe pathways that are not supervised but have a fail-safe operation that performs the intended function when the connection is lost. Examples of such pathways include the following:

• Power to door holders where interruption of the power results in the door closing
• Power to locking hardware that release upon an open circuit or fire alarm operation

Class E
The Class E reference is new and is intended to describe pathways, which do not require supervision as described in 10.17, Monitoring Integrity. A type of circuit that would be of this nature could be a door lock not wired failsafe.

A Tentative Interim Amendment (TIA)* was added to the Circuits and Pathways section in regard to circuit protection—specifically, lightning protection for conductors leaving a building structure and entering another. Lightning protection is required unless any of the following apply:

• Circuits in large metropolitan areas where buildings are close together and sufficiently high to intercept lightning
• Inter-building cable runs of 140 ft (42 m) or less, directly buried or in underground conduit, where a continuous metallic cable shield or a continuous metallic conduit containing the cable is connected to each building grounding electrode system
• Areas having an average of five or fewer thunderstorm days per year and earth resistivity of less than 100 ohm-meters. Such areas are found along the Pacific coast [70:800.90(A), FPN No. 2]

It is important to protect the fire alarm system from lightning. One of the key requirements related to transient protection is NFPA 70, National Electrical Code, Section 760.32, which covers installation requirements. Part of those installation requirements are the grounding and bonding rules contained in Part IV of Article 800. Connections to the building grounding electrode system should be made where the circuits enter and exit a building. To minimize potential damage from induced transients, the circuits entering and exiting a building should connect to the grounding electrode system and transient protection equipment nearest the point of entry, before being intermingled with other circuits.

NEC Section 760.32 provides references for fire alarm circuits extending beyond one building. The requirements for the installation of power-limited circuits and communications circuits are covered by Parts II, III, and IV of Article 800, Communications Circuits. The methods and equipment used for providing transient protection of circuits addressed by Article 800 are not necessarily suitable for voltages expected on all fire alarm circuits.

*A Tentative Interim Amendment is a section that is added to the code that can’t wait for the next edition. At the ROP portion of the next code cycle, the TIA is provided as a proposal. Excerpts; NFPA 72-2010