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

Challenges of specifying gypsum panel enclosures

Since the introduction of polymer fire-rated cables as another alternative, many in the building industry are reassessing the apparent obvious advantages of gypsum board enclosures.

There are several significant issues that come to the forefront when assessing gypsum board enclosures. One is the complexity of designing and constructing fire-resistant walls and shafts that are effective in producing the fire rating required as a fire barrier.

A second is the performance of gypsum enclosures given the fact that while they have been tested for their resistance to fire, they have never been tested or listed for their protection of electrical conductors. A third concerns the construction/inspection issues of gypsum enclosures.

 

2-hour fire separation using vertical shaftway and horizontal run
construction methods, left; and fire-rated cable, right.

 

Designing gypsum board enclosures.  Familiarity and knowledge of fire ratings of different gypsum panel types is critical as panel types vary in fire resistance. Since wall and shaftway systems involve a number of products, a knowledge of assembly details is also critical in meeting performance criteria. Such details include: size of framing members; number of fasteners; type of joint compound and finish; and fire-stopping materials to address penetrations that breach the panel wall.

It cannot be stressed enough that the integrity of a gypsum enclosure must be evaluated as an assembled system. The selection of individual products with appropriate fire ratings will not compensate for deficiencies in the size of studs or number of fasteners, for example. In addition, it is essential that the assembly built in the field is representative of the one tested. Given these concerns, it is not at all surprising to learn that many industry experts consider fire-resistant assembly design and construction to be among the most complicated issues facing today's architects and specifiers.

 

2-hour rated assembly horizontal metalduct enclosure (fire tested both sides).
While meeting test temperature requirements, this assembly is not tested
or listed for protecting conventional wiring for 2 hours.

 

In order to explore where and why gypsum board enclosures are used for protection of critical electrical conductors, and to elicit in-the-field experiences of using them, one cable manufacturer surveyed contractors, design/build firms, architects and engineers across the country. It found that fire-rated vertical shaftways were commonly used in high-rise buildings for protecting life safety cables and that gypsum walls were used for separating "normal supply" electrical cables from emergency and life safety cables. "Common practice" and "not being aware of alternatives," were the most common reasons for specifying gypsum enclosures.

Answers relating to the experiences of using construction methods were varied and can be summarized: 
2-hour fire-rated enclosures (are):

  • Difficult to make
  • Workmanship-dependent
  • Subject to subsequent deterioration
  • Not listed for electrical cable protection
  • May not be dedicated, as called for by code
  • Contribute to difficulties in on-site trades coordination
  • Take up valuable space
  • Not as cost-effective as they are perceived to be

Performance of gypsum enclosures: fire protection code issues. A closer look at code and testing requirements raises the question of what is being tested. Although gypsum assemblies are tested, the testing does not determine if an electrical conductor behind a gypsum wall would survive. The test only stipulates a maximum temperature at the back (or inside) of the wall, which, it turns out, exceeds many cables' capability.

For example, UL/ULC wall and partition system test criteria states:

"…Surface temperatures on the unexposed side of the test specimen are measured at a minimum of nine locations…" "… Average temperature of the unexposed surface is not permitted to increase more than 250 °F above ambient nor is any individual thermocouple permitted to rise more than 325 °F above ambient…"

If ambient temperature is assumed to be 70 °F, the average temperature of the unexposed surface should be 325 °F with a maximum temperature of 395 °F. According to results of a fire test conducted by Warnock Hershey International, an independent fire-testing laboratory, unprotected conventional wiring failed in less than three minutes when the temperature reached 450 °F.

Conventional wiring is simply not designed to withstand temperature levels of 450 °F. Most conventional wiring is rated 194 °F (90 °C); and in some cases a temperature as low as 248 °F (120 °C) can cause failure in PVC.  Other insulations are better than that, but they could well fail under the type of temperatures found in testing gypsum enclosures.

Performance of gypsum enclosures: construction/inspection issues. As pointed out earlier, gypsum shaft assemblies are highly complex and difficult to make properly. They are not listed for electrical cable protection. They are very trade sensitive and are subject to subsequent damage. They also take up space and create construction coordinating problems.

Construction detail issues that can impact enclosure performance include:

  • Substitution of a different gypsum panel than the one specified
  • Whether panels are applied horizontally or vertically, joints should be staggered so that a joint will not occur on both sides of the same stud; on multilayered walls, each gypsum layer should be offset from the layer beneath
  • Framing - type and spacing that does not correspond to enclosure assembly tests
  • Fasteners - type and spacing that does not correspond to enclosure assembly tests
  • Penetrations - deficiencies in noting size, type and number of breaches in the wall (e.g., from conduit or pipe) and addressing them with fire-stopping materials
  • How and where a partition intersects with a floor or ceiling assembly that does not correspond to design in enclosure assembly tests
  • Location of conductors, which must be inside a shaft created by gypsum or on the opposite side of a wall, rather than within the wall cavity space
  • Electrical conductors cross expansion joints, which must be fire-stopped and are subject to expansion and contraction
  • If splices are made, an access panel has to be provided in the enclosure
  • Not dedicating the gypsum enclosure to life safety circuits

One of the many lessons learned in the tragic high-rise fire in Philadelphia in 1991, which resulted from major failures in nearly all fire protection systems, was the importance of having truly independent emergency electrical systems. Primary and secondary electrical risers had been installed in a common enclosure, which led to their almost simultaneous failure when fire penetrated voids in the wall above the ceiling of an electrical closet. NEC Article 700, Section 700-12(d) of the then-existing National Electrical Code recognized separate feeders as a means of supplying emergency power, but those services to be "widely separated electrically and physically...to prevent the possibility of simultaneous interruption of supply." The revised code requires dedicated enclosures for life safety circuits.

 

 

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Originally published in August 2008

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