Survivability vs. Reliability
They aren’t the same thing, but your fire alarm system needs both
NFPA Journal®, May/June 2010
The goal of NFPA 72®, National Fire Alarm and Signaling Code, has always been to establish “minimum levels of performance, extent of redundancy, and quality” for a fire alarm system installation. However, the code has never defined the terms “reliability” or “survivability.”
Most practitioners define reliability as “the ability of an equipment, machine, or system to consistently perform its intended or required function or mission, on demand and without degradation or failure.” Others define it as “the probability of failure-free performance—for either a specified length of time or for an item’s useful life—under specified environmental and duty-cycle conditions.” The metrics of reliability include mean time between failures (MTBF), reliability coefficient, and quality over time.
Although NFPA 72 does not define survivability, Paragraph 23.10.2 of Chapter 23 states, in part, that “fire alarm systems used for partial evacuation and relocation shall be designed and installed such that attack by fire within an evacuation signaling zone shall not impair control and operation of the notification appliances outside the evacuation signaling zone.” The code has contained this performance description of survivability since 1985, when it first appeared in NFPA 72F, Installation, Maintenance, and Use of Emergency Voice/Alarm Communication Systems.
Each level of pathway survivability, as described in Chapter 12 of the code, offers options for the designer and installer to meet the survivability requirements. In the past, some code users assumed that any circuit enclosed in a raceway had an acceptable level of survivability. Wire or cable in a raceway certainly has mechanical protection, but a raceway on its own does not provide sufficient protection against fire.
Unfortunately, many manufacturers, designers, installers, and authorities having jurisdiction use the words reliability and survivability interchangeably. Clearly, based on the definitions cited above, the manufacturing, design, installation, and maintenance of a fire alarm system should ensure reliability. If a facility owner chooses an experienced fire alarm system designer who specifies one of the major product lines of a proven manufacturer, these two factors will likely not decrease the reliability of the system.
In contrast, a number of installation factors influence the reliability. To enhance reliability, the installer should use the correct wiring method and install the wiring in a workman-like manner, as described in the National Electrical Code®; install the equipment in strict accordance with the manufacturer’s instructions; and use personnel specifically trained to install and program fire alarm systems. To further enhance reliability, the owner should contract with fire alarm system professionals to maintain the system.
In actuality, fire alarm system reliability depends on all of these items, but none of them will affect the fire alarm system’s survivability. So how does one ensure the survivability of critical communications circuits and other circuits necessary for emergency communications systems to operate properly when the code requires it?
Designers, authorities having jurisdiction, and installers must design and install such circuits so that a fire will not disable them. Specifically, the code requires that “where the separation of the in-building fire emergency voice/alarm control equipment occurs and where the circuits are run through junction boxes, terminal cabinets or control equipment, such as system control units, power supplies and amplifiers, and where cable integrity is not maintained, these components shall, in addition to the pathway survivability required by the code, also be survivable.”
Is survivability the same as reliability? No. Reliability and survivability are not the same thing. Nevertheless, both are important to the quality of a fire alarm system.
Wayne D. Moore, P.E., FSFPE, is a principal with Hughes Associates and immediate past chair of the NFPA 72 Technical Correlating Committee.