By Rick Florio, CET
Life-Safety Wiring in High-risesEngineering
Critical life safety systems in high rise buildings must function during a fire. That is why the integrity of the wiring during a fire is paramount. The fire fighters need to be able to use designated...
Critical life safety systems in high rise buildings must function during a fire. That is why the integrity of the wiring during a fire is paramount. The fire fighters need to be able to use designated elevators, and both firefighters and occupants must rely on voice communication and fire alarm circuits to be informed of the fire conditions. Other life safety circuits such as emergency generators, fire pumps and smoke management systems are equally crucial.
There are several methods of providing fire protection to the electrical conductors for life safety electrical circuits. Mineral insulated (MI) 2-hour fire rated cable has been listed by Underwriters Laboratories Canada and UL since 1980. It is inherently fire resistant due to its totally inorganic construction and is widely viewed as a superior cable system for fire rated applications. In recent years, technology has enabled polymer-insulated cables also to pass a 2-hour UL/ULC fire test and become part of the solution.
Figure 1 depicts the life safety circuits (drawn in red) that require electrical conductors to be fire protected in high-rise buildings. The applications served by these circuits are:
* emergency power supply
* firefighters’ elevator
* fire pumps
* fire alarm
* voice communication and fire fighters’ handsets
* emergency power for lighting
* pressurization fans and smoke dampers
* smoke venting fans
Standards for safe cables
Electrical cables listed by the ULC S 139/UL 2196 “Standard Method of Fire Test for Evaluation of Integrity of Electrical Cables” are an ideal choice for the above circuits. These standards require cables to undergo a number of tests before they are approved.
The standard ULC S 139 describes the procedures as follows: “The intent of the fire test is to determine the integrity of electrical cables which are evaluated for their ability to maintain ‘circuit integrity’ at a specified voltage potential relative to ground and other conductors. Leakage current per unit length of cable shall also be quantified.”
Other provisions in these standards require that: “Cables are exposed to the standard time and temperature fire curve ASTM E-119 and then the application of a hose stream. During the fire test, cables shall be continuously energized at a minimum voltage or at their rated voltage while measurements are made of insulation resistance.
“Following the fire test, the assembly is subjected to a hose stream test after which the cable shall be tested for ‘circuit integrity’.
“Insulation resistance measurements shall also be taken to quantify leakage current.”
One or two hours of separation?
The National Building Code of Canada (NBC) 2005 requires fire protection for conductors when combustible materials are present within a service room or space. The code recommends providing a 1-hour or 2-hour fire separation for the life safety electrical conductors and the combustible materials, or using 2-hour listed cables. The actual excerpt in the NBC is 184.108.40.206(1) and it states “…not less than 1 hour” for the conductors/material separation.
Yet under sections 220.127.116.11, 18.104.22.168, 22.214.171.124, the code refers to a requirement for emergency power of 2 hours. This “grey area” is open to interpretation by the design engineer. Should he or she provide 1 hour of fire protection for the electrical conductors, when required also to provide emergency power for two hours? A prudent designer would specify a listed 2-hour rated cable system or a 2-hour fire separation.
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In Canada, one jurisdiction that has clearly addressed the fire protection of conductors is the city of Vancouver under its building code. Appendix A of that code actually defines the appropriate methods: ULC 2-hour listed cable systems, or construction methods protecting conventional cables for two hours.
Using “construction methods”
The majority of consulting firms in Canada usually specify 2-hour ULC listed cables for new construction or retrofit projects. However, not all do so. Some of the firms that specialize in new high-rise residential construction have a tendency to allow “construction methods” as alternatives to specified listed cables.
Construction methods are as follows. Note, however that these are NOT listed systems:
* Conduit encased in concrete (protection of conventional wiring). In these types of systems, 2 inches of concrete is considered adequate for 1-hour fire resistance. Approximately 4 inches of concrete is required to protect conductors for 2 hours. These systems are very cost effective if proper coverage can be achieved throughout the entire circuit. However it is easier to fire rate conduits up to 1 1/2 inches. It is harder to guarantee 2 to 4 inches. Also, non fire-rated junction and pull boxes compromise the system.
* Gypsum enclosures. These types of system provide protection for conventional wiring, as long as similar methods for providing fire protection for the metal ducts are used. However, they can be difficult to build and are “trade sensitive,” i.e. subject to poor workmanship. They also might suffer from wear and tear. Subsequent work carried out in the vicinity can easily compromise the system. Another disadvantage is that they require extensive amounts of space.
Note again, that neither of the above is tested or listed as a method of fire protecting conventional wiring for 2 hours.
High-rise buildings in Canada have become far more sophisticated in recent years along with the evolution of codes and standards. Consequently the fire and life safety systems specified by engineers are increasingly complex. In today’s world, it is a serious responsibility to design and synchronize critical life safety systems that will function when needed.
Rick Florio, CET, is the Canadian sales manager with Tyco Thermal Controls in Toronto. He is active in several associations and committees related to the fire safety industry.