Cables Are Not Created Equal

Fire case histories make an argument for requiring survivability of all fire alarm system circuits. A fire occurred in the London Apartments for the elderly in Delaware, Ohio, on March 12, 1994. Manual fire alarm boxes and corridor smoke detectors were connected to the building’s fire alarm system. The wiring for the fire alarm system was installed in a surface-mounted metal raceway.

The fire reportedly started in a sofa in a common area and spread to adjacent furniture. Investigators believed that smoking materials were the most likely ignition source.

Smoke detectors near the common area detected the fire, and the fire alarm system actuated. However, the fire impinged on the metal raceway and damaged the wiring inside, silencing the audible fire alarm signal.

Because the fire alarm stopped shortly after starting, some residents believed it was caused by a system malfunction and did not evacuate. By the time these residents realized there was an actual fire, the flames had sufficient time grow and block exits. Two residents died (one from smoke inhalation, one from a heart attack) and seven were injured.

A fire discovered by a nurse at 8:30 p.m. on Jan. 21, 1979, in a Memphis, Tenn., hospital resulted in damages estimated at $450,000. The nurse operated a manual fire alarm box immediately; however, the fire department was not notified until a passing motorist called them, resulting in a 24-minute delay and the resultant damage. The fire had burned through the fire alarm system wiring before the manual fire alarm box was operated, thereby causing the failure.

The obvious question to ask is, “If the fire alarm system does not survive long enough during a fire to operate as required, then has the mission effectiveness goal been met?” Given the case histories above, the answer is, no.

The National Fire Alarm Code Technical Committee has issued requirements for survivability from attack by fire. Unfortunately, these requirements apply only to fire alarm systems where the occupants are selectively evacuated or relocated as we often do in high-rise buildings. The requirements apply to both audible (tone and voice) and visible notification appliance circuits.

The code requires survivable fire alarm systems be designed and installed such that attack by fire within an evacuation-signaling zone will not impair control and operation of the notification appliances outside the evacuation-signaling zone. Performance features provided to ensure survivability must be described and technical justification provided in the documentation submitted to the authority having jurisdiction during the system-design process.

All circuits necessary for the operation of the notification appliances must be protected until they enter the evacuation-signaling zone they serve. This requires the protection of circuits as they pass through fire areas other than the one served. The requirement has been inserted into the code to help delay possible damage to the circuits from fires in areas other than those served by the circuits. This is done to increase the likelihood circuits serving areas remote from the original fire will have the opportunity to be actuated and serve their purpose.

Any of the following methods are considered acceptable as meeting the requirements of survivability from attack by fire:

1. A two-hour rated cable or cable system, such as MI or CI cable.

2. A two-hour rated enclosure

3. Performance alternatives approved by the authority having jurisdiction

One or more of the following means might be considered acceptable to provide a level of survivability consistent with the intent of this requirement:

• Installing a fire alarm system in a building with sprinklers

• Routing notification appliance circuits separately

• Using short-circuit fault-tolerant signaling-line circuits to control evacuation signals

The requirement for notification appliances to operate in evacuation-signaling zones not attacked by fire will also require circuits and equipment common to more than one zone be designed and installed so the fire will not disable them. For instance, a signaling-line circuit used to control notification appliances in multiple evacuation zones should be properly designed and installed so that one fire would not impair the signaling-line circuit rendering the notification appliances that serve more than one evacuation-signaling zone inoperative.

It is also important to realize the protection 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 where notification-appliance circuits might originate.

Power supplies and amplifiers, including remote-power supplies and amplifiers, should be addressed as part of the design, meaning if a fire alarm voice-communications system has been designed with distributed amplification, the cable interconnecting the main and distributed amplifiers must be survivable as defined above.

Not being aware of these requirements and the different cables and cable systems available to install a code-compliant fire alarm system can be a costly mistake. EC

MOORE, a licensed fire protection engineer, frequent speaker and contributor to ELECTRICAL CONTRACTOR magazine and an expert in the field of life safety, is a co-editor of the current National Fire Alarm Code Handbook.


About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker, writer and expert in the life safety field, has been a principal member and chair of NFPA 72, Chapter 24, as well as a former principal member of NFPA 909 and NFPA 914. He is the...

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