Circuit Survivability – Not a New Thing
By: Jack Poole, P.E., FSFPE
Circuit survivability is not something new that has been developed in recent editions of NFPA 72. Survivability of emergency voice /alarm communication systems was addressed in the first edition of the National Fire Alarm Code in 1993, which is when the 1989 edition of NFPA 71, Standard for the Installation, Maintenance, and Use of Signaling Systems for Central Station Service; the 1990 edition of NFPA 72, Standard for the Installation, Maintenance, and Use of Protective Signaling Systems; the 1990 edition of NFPA 72E, Standard on Automatic Fire Detectors; the 1989 edition of NFPA 72G, Guide for the Installation, Maintenance, and Use of Notification Appliances for Protective Signaling Systems; the 1988 edition of NFPA 72H, Guide for Testing Procedures for Local, Auxiliary, Remote Station, and Proprietary Protective Signaling Systems; and the 1989 edition of NFPA 74, Standard for the Installation, Maintenance, and Use of Household Fire Warning Equipment was consolidated. Sections 3-2.4 and 3-12.3 of NFPA 72 (1993) clearly explained that the system was to be designed and installed such that a fire in an evacuation zone, causing loss of communications to that zone, shall not result in the loss of communications in any other zone. If the emergency voice/alarm communication system used relocation or partial evacuation (by zone or floor), then circuit survivability was required.
The 1996 edition of NFPA 72 was the first edition to incorporate additional specific protection requirements for circuits from the attack of fire. Section 3-12.4.3 provided two options for this additional protection: 1) installing the wiring in metal conduit or metal raceway in a 2-hour fire rated enclosure; or 2) enclosing the wire in a 2-hour fire rated cable assembly and installing the cable in metal conduit or metal raceway. The 1999 edition, follows the same basic concept; however, it permitted three options for circuit protection in Section 3-220.127.116.11.4: 1) a 2-hour rated cable assembly; 2) a 2-hour rated shaft or enclosure; and 3) a 2-hour rated stairwell in a fully sprinklered building. The 2002 edition, Section 6.9.4 changed the third option to permit performance alternatives approved by the authority having jurisdiction (AHJ), but this section was also moved under the Emergency Voice/Alarm Communications section of the code. The 2007 edition allowed five methods of maintaining circuit survivability: 1) a 2-hour fire rated circuit integrity (CI) cable; 2) a 2-hour fire rated cable system (electrical circuit protective system); 3) a 2-hour fire rated enclosure; 4) performance alternatives approved by the AHJ; and 5) buildings fully protected by an automatic sprinkler system installed in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems, and with the interconnecting wiring or cables used for the operation of notification appliances installed in metal raceways and in accordance with Article 760 of NFPA 70.
The 2010 edition of NFPA 72, had the most significant change as it related to circuit or pathway survivability. The 2010 edition took a different approach and identified different levels of survivability, and the use of the equipment drove the required level of survivability. The code provided a definition for “Pathway Survivability” and provided four pathway survivability level designations. Here are the different levels of pathway survivability:
- Pathway Survivability Level 0: This level does not impose any additional criteria for survivability beyond the requirements of NFPA 70: National Electrical Code that would be applicable to the installation of the type of circuit.
- Pathway Survivability Level 1: In buildings having full sprinkler protection in accordance with NFPA 13: Standard for the Installation of Sprinkler Systems, circuits run in metal raceways meet this level of survivability.
- Pathway Survivability Level 2: This level of survivability requires either pathways that are protected by 2-hour fire-rated CI or fire resistive cable, 2-hour fire rated cable system (electrical circuit protective systems), 2-hour fire rated enclosure or protected area, or performance alternatives approved by the AHJ.
- Pathway Survivability Level 3: This level of survivability requires compliance with the criteria of Level 2, and be in a building having full sprinkler protection in accordance with NFPA 13.
Engineers, designers, installing contractors and AHJs often misunderstand the term “survivable” or “survivability." They presume it relates to the reliability of the equipment or the manner the installer has chosen to use either a Class A wiring scheme or has placed the circuits in metal raceway. Pathway Survivability is defined in Section 3.3.175 as the ability of any conductor, optic fiber, radio carrier, or other means for transmitting system information to remain operational during fire conditions. In summary, survivability as intended by NFPA 72 is intended to mean that the fire alarm circuit will properly perform and remain operational, even when under attack by fire.
The approach taken in the 2010 edition of NFPA 72 is the same approach utilized in the 2013 and 2016 editions of NFPA 72.
By designing a system using a Class A wiring configuration or installing wiring in metal raceway will not alone make the circuit or pathway survivable. Utilizing a Class A wiring configuration will make the system more reliable, and the metal conduit or metal raceway may protect the wiring from mechanical damage, but it does not mean the wiring will remain operational during fire conditions.
It is important to understand that survivability only applies to specific notification circuits and other control circuits for equipment that must continue operating during a fire condition. Survivability requirements do not apply to detection circuits and do not normally apply to signaling line circuits—unless the signaling line circuit controls notification appliance circuits or other equipment that must continue to operate during a fire condition. The survivability requirement intends to meet the performance goal of ensuring continuity of system operations where the designated critical circuits pass through fire areas other than the one they serve. Survivability will delay possible damage to the circuits from fires in areas other than those served by the circuits. This will increase the likelihood that circuits serving areas remote from the original fire will have the opportunity to operate as intended without compromise from that fire.
Chapter 23: Protected Premises Fire Alarm Systems requires a voice system used for relocation or partial evacuation of occupants be designed so that a fire within a single notification zone does not impair the ability of the system to operate in other notification zones. The code does not identify a specific pathway survivability level in this case.
Chapter 24: Emergency Communication Systems (ECS) specifies pathway survivability levels under certain circumstances. When ECS are used for partial evacuation or relocation, Chapter 24, Section 18.104.22.168.1 requires the pathways to be designed to meet Level 2 or Level 3 survivability. Chapter 24 criteria for other types of systems are noted here:
- Pathways for mass notification systems (MNS) are allowed to be Level 0 or higher based on a risk analysis.
- Two-way ECS are required to be designed as survivability Level 2 or 3.
- Area of refuge two-way communication systems are required to be designed as Survivability Level 2 or 3.
Furthermore, the survivability requirement would also apply to a signaling line circuit that extends from a master fire alarm control unit to another remote fire alarm control unit from which notification appliance circuits might originate.
Jack Poole, P.E., FSFPE, is with Poole Fire Protection