Achieving Fire Protection of Electrical Life Safety Circuits

Polymer insulated fire-rated cable for fire protection provides an effective alternative to the traditional practice of specifying construction methods.
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Sponsored by Tyco Thermal Controls
Karin Tetlow

Fire testing in North America, on the other hand, involves mounting the cable on a brick wall and exposing it over a 2-hour period to an actual controlled fire where temperature increases to 1850 °F, at which point the cables are hit with the full force of a firefighter's hose. This subjects the cable to the same conditions that all fire-resistant building materials, such as fire doors, are exposed to, but with the added requirement that the cable has to maintain circuit integrity throughout the fire and after the hose stream application; the cable must continue to operate electrically throughout the test (electrical integrity is evidenced visually by a light supplied with power coming through the cable in the furnace) and after the hose stream.

The standards that have to be met vary widely around the world. As a rule, the International Electrotechnical Commission (IEC) and European fire test standards are weak and undiscerning. Their tests have been developed by cable manufacturers and have been described as "tests that are designed to pass cables rather than test them." They are generally similar to Flame Testing in that the tests involve placing cables over ribbon burners for some interval of time, sometimes with a weak water sprinkler spraying on the cable. Flame temperatures are measured and statements are made that the cable withstands 1381°F (750 °C) or 1742 °F (950 °C) for 30, 60 or 120 minutes. To the uninitiated, the tests look impressive, but they are not difficult to pass, and have little to do with a real fire. In addition, the tests allow different levels of passing grades, so that a cable may pass one grade but fail another, thus requiring the system designer to stipulate what grade of pass is acceptable for a given application -hardly a good idea when dealing with life safety.

In contrast, North American tests for electrical cables subject the cables to the same fire conditions as any other fire-rated product, such as door and wall assemblies. The difference being that while door and wall assemblies are judged on how they limit the temperature on the unexposed side, electrical cables are exposed to fire that is expected to develop in a real commercial building; that is, cables are completely exposed and must continue to operate for either one or two hours while the fire is developed through a standard fire exposure time-temperature curve that reaches 1850 °F. The results are pass/fail with no grades of pass/fail.


Conventional cable temperatures reached when exposed to fire


The North American fire test, American National Standards Institute/Underwriters Laboratories ANSI/UL 2196/Underwriters Laboratories Canada (ULC S 139) Standard Method of Fire Test for Evaluation of Integrity of Electrical Cables is gaining acceptance as the discerning electrical cable test around the world (the nearest contender, the Australian test, is equivalent in discernment, but is weak in terms of gradations of pass/fail) Cables are fire-rated (as are "wrap systems") and listed by UL as Electrical Circuit Protective Systems (UL Category FHIT or Category FHJRC in Canada). These guides provide precise detail of how cables must be installed in order to achieve their rating. Details include types of support, spacing of supports, materials allowed and so forth.

Codes are increasingly calling for 2-hour fire-rating. The National Electrical Code (NEC Article 695.6(B): Fire Pumps)and theNational Fire Protection Association Fire Alarm standard (NFPA 72: Fire Alarm) and the NEC (Article 760) cite a minimum fire rating of two hours (2h) for fire alarms. The reason is partially for evacuation of people from buildings, although this can generally be accomplished in a shorter timeframe, but more particularly for firefighting - for preserving power to allow firefighters a minimum of two hours of reliable power to aid in getting the fire under control.

"There is a steady trend towards a 2-hour requirement for cable survival in a fire, and not just in North America," states O'Connell.  "It makes sense that a fire is going to last more than one hour and that the firefighting systems should be maintained for two hours minimum. Standards that stipulate different levels of fire survivability for circuits of varying importance seem to lower the bar and allow less effective fire protection. In North America there is no need to accept less than the best for critical circuits - fire-rated cables that withstand one hour of the test without exception withstand two hours."

Fire Protection Using Construction Methods

Conduits encased in concrete

Encasing conventional wiring in concrete is a traditional method for addressing fire protection.Two inches of concrete has historically been considered adequate for one-hour fire resistance; approximately four inches are required to protect conductors for two hours.

Encasing conduits in concrete is very cost effective if proper coverage can be achieved throughout the entire circuit. However, while it is perfectly feasible to encase conduits up to 2-in diameter in a slab floor of a building (2-in concrete times two, plus 2-in conduit implies a 6-in thick slab), it is hard to guarantee uniform 2-in coverage, and larger conduits are difficult because of the thickness of the slab. If 4-in concrete is required to achieve a 2-hour rating, the slab can be very large indeed, e.g. 4-in conduit with 4-in concrete on either side implies a 12-in slab! In addition, the lack of fire rating of junction and pull boxes compromises the system. Moreover, this method, as with other construction methods, is not a listed system for fire protection of electrical conductors.

Note: Encasing conduit in concrete is also very difficult after the fact when retrofit work has to be done.

Gypsum board enclosures

Characteristics.  Gypsum, a mineral, has outstanding fire-resistant properties. When exposed to high temperatures, chemically combined water in the gypsum is gradually released, providing protection until all of its combined water is completely driven off. Its fire resistance plus the fact that gypsum panels were far less expensive than MI fire-rated cable (the only other viable non-concrete alternative for years) made gypsum board enclosures a traditional and obvious choice for the fire protection of life safety electrical circuits.

Other advantages include: weight- a 2-hour fire-rated gypsum wall weighs only nine pounds per sq ft; and thinness- a 2-hour fire-rated wall is only 3-½ -in thick. They also install more quickly and economically than "wet" shaft walls, such as masonry. Gypsum panel shaft walls are completed early in the construction process and finished later, along with other interior partitions. Moreover, gypsum panel systems have undergone extensive independent testing as fire barriers-gypsum boards of different core formulation are fire-rated from one to four hours. By adding layers of gypsum panels to each side of the studs, the fire resistance of virtually any wall can be increased

In addition, when used for fire protection of electrical cables, gypsum panels are commonly perceived as inexpensive; or even ‘free' in the sense that gypsum board used for fire-rated enclosures is usually absorbed within the overall gypsum board budget line item rather than listed as a separate line under the electrical estimate.



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Originally published in August 2008