Achieving Fire Protection of Electrical Life Safety Circuits
Fire-Rated Cables
Costs, however, are not the only issue in specifying fire-rated life safety circuits. With the trends towards increased survivability of critical circuits and availability of fire-rated cables - plus the growing vigilance regarding fire protection code compliance on the part of authorities - using only 2-hour rated electrical cable systems is the wisest course. Using fire-rated cables also precludes any concerns about construction deficiencies - an issue that continually challenges conscientious electrical inspectors.Â
Â
Their advantages include:
- Come with a built-in fire-rating
- Not workmanship-dependent
- Not subject to subsequent deterioration
- Listed by UL for the specific purpose
- Listed in UL and ULC Fire Resistance Directories
- Easy to install using standard procedures
- Ease coordination difficulties on site
- Take up little space
- Easier to inspect
Â
FIRE-RATED FIRE ALARM AND FIRE CABLES |
||||||||||
The test to qualify a cable as 2-hour fire-rated cable is the ANSI/UL 2196 (ULC S139 in Canada). This protocol utilizes the ASTM E-119 time-temperature curve, and requires that cables remain operational after exposure to temperatures up to 1850 °F (1010 °C) for two hours followed by the full force of a ï¬reï¬ghter's hose stream. Â
Mineral Insulated (MI) Power Cable: 2-hour fire rated cable listed by Underwriters Laboratories (UL) and Underwriters Laboratories of Canada (ULC) since 1980, is inherently fire-resistant due to its totally inorganic construction. Widely viewed as a superior system, cables are constructed entirely of inorganic materials. For all practical purposes it is fire-proof, non-ageing, mechanically strong of small diameter, and pliable to handle. Manufactured from copper and magnesium oxide, it features zero smoke, zero flame spread and zero fuel contribution. The construction is inherently tough, yet allows the cable to be bent and molded to ï¬t into tight spaces and can withstand mechanical abrasion without additional mechanical protection. In addition, the inorganic construction of an unjacketed MI cable allows for an environmentally clean electrical cable that does not burn, produce smoke, or contribute fuel when exposed to ï¬re conditions. Â
RHW Polymer insulatedPower Cable. 2-hour fire rated cable listed by Underwriters Laboratories (UL) and Underwriters Laboratories of Canada (ULC), this flexible cable is typically installed in conduit like any conventional power cable. The silicone insulation is a special type that 'ceramifies,  that is, it provides electrical protection under ï¬re conditions by turning into a hard ash that preserves the electrical insulation value between conductors. The outer jacket is a cross-linked polyoleï¬n (XLPO) that gives superior abrasion protection and has a low coefï¬cient of friction for easy pulling into conduits. Both insulation and jacket are made from "low smoke zero halogen" material. Â
Metal-Clad (MC) Polymer Insulated Power Cable.Manufactured to UL 1569, this copper-armored "MC" cable achieves a 2-hour ï¬re resistance rating without conduit protection required by other polymer cables. The cable construction is unusual for an MC cable in that the sheath is copper and meets the code requirements as an equipment grounding conductor in both the National Electrical Code (NEC) and the Canadian Electrical Code (this type is new in Canada, and has recently been included in CSA product standards). Â
Polymer insulated Twisted Pair Fire Alarm Cable.Fire-alarm polymeric cable for twisted pair communication wiring meets the survivability requirements of the National Fire Protection Association standards (NFPA 72). It is listed as a 2-hour ï¬re-rated Electrical Circuit Protective System when installed in ½-in minimum electrical metallic tubing (EMT) or rigid metal conduit (RMC) and is ideal for retroï¬t applications in existing conduit. This wire is available in shielded and unshielded twisted pair conï¬gurations, 18 AWG through 12 AWG. The conductors for both shielded and unshielded cables are insulated with silicone rubber and twisted together to form a single pair. In the shielded design, the pair is wrapped with a conductive tape shield and bare copper drain wire. In both designs, the assembly is covered with a low smoke, zero halogen polyoleï¬n jacket. The cable is tested with a 'utilization' voltage nominated by the manufacturer. A cable that can be used in 120V circuits should have a utilization voltage of 72 V to ground (details of each manufacturer's product capability are described in the UL listing. Â
Ground cable. Typical polymer insulated ground wire used with fire-rated cables. Electrical Circuit Protective System cable listings are not interchangeable; the components of one system may not be used on another, and only listed materials are appropriate. Thus a ground wire should be specified in the system listing. |
Â
Conclusion
Driven by incidents of high-rise fires and failures of critical life safety circuits, the subject of "fire protection" in general has come a long way, not only with regard to fire protection of electrical life safety circuits, but also to other building systems, for example, fire alarm systems, sprinklers, smoke management technologies, fire separations, flame spread of construction materials etc. Credit for this goes to improved cable technological development, hand in hand with codes and standards evolution.
Whether designed for commercial buildings, health facilities or airports, the systems specified by architects and engineers are increasingly complex when it comes to fire protection. It is the (and it is their) serious responsibility on the part of professionals to make sure all of the systems are designed and synchronized to function when needed. With the availability of new fire-rated products, plus a discriminating understanding of fire-rated construction methods, building professionals should be equipped to specify the safest systems.
Â
|
Â
Notice
