Circuit Integrity Cable Re-examined
Editor's note: The following comments by September circuit integrity cable author Dean Wilson are regarding a video produced by mineral insulated cable manufacturer Pyrotenax warning of potential fire and smoke hazards associated with ceramifiable silicone cable. The video was brought up in a letter to the editor in the December 2001 issue, and again last month.
With regard to my article “Circuit Security” [Sept. 2001, p. 55], and the question it evoked regarding the safety of circuit integrity cable in the context of the video released by Pyrotenax at last year’s  NFPA conference, it should first be noted that circuit integrity (CI) fire-alarm cable was never shown in the video and, presumably, is not addressed by the video’s allegations.
Regarding the video itself, when exposed to the heat of a fire, all thermoplastic insulation materials and artificial or man-made rubber insulation materials will either burn or smolder and produce products of combustion. These products may ignite if the following conditions occur:
Products mix in a percentage of products with air that falls within the flammable/combustible range of the materials; and
When in the presence of a source of ignition that has sufficient joules of energy.
Thus, it should not be a surprise that the ceramifiable silicone rubber insulation material used in the manufacture of UL-listed Type MC and Type RHH cables—that successfully pass the UL 2196 fire test and achieve listing as a fire-rated cable—will produce ignitable products when subjected to the heat of a fire.
The fact remains that in the history of fires in industrial and commercial buildings, the effects of this small quantity of ignitable product due to the decomposition of the cable insulation has proven inconsequential to the extent of the fire. The fuel that burns in such fires has such an enormously large quantity in comparison to the product given off by the decomposition of cable, that this small amount of product offers virtually no increase in hazard.
In describing the ceramification process, one CI manufacturer offers the following:
“…during the ceramification process three byproducts are released as the transition from silicone to ceramic occurs at temperatures exceeding 800°F. The first is a non-conductive water solution. In the area exposed to high temperatures, the water solution will exit as steam. In the cooler areas this material will condense and exit as a non-conductive solution. Since the ceramic insulation is impervious to liquids and gases, no detrimental effects to cable performance will occur.
“The second byproduct is silica. This can be seen exiting the end of the metallic sheathed cable as white smoke particles, which will collect on nearby surfaces. Silica is an inorganic material that is commonly used to produce silicon chips for the computer industry as well as fiber-optic cables. It is inert, non-corrosive, non-toxic, non-flammable and is an excellent insulator.
“The third byproduct is a generic series of silicone liquids comprised of simple molecules containing silicon, oxygen, carbon and hydrogen. These silicone liquids all have the same molecular structure and proportion of chemical elements, but with different molecular weights. Only the lowest molecular weight silicone liquid byproduct becomes a gas at room temperature. These can be observed exiting the cable as liquid droplets highly diluted by water … If these silicone liquids are burned, only carbon dioxide, water and silica are formed. The combustion products are also non-conductive, non-corrosive and non-toxic in the low quantities given off.”
The whole reason why this new cable technology offers users an advantage comes from the relative ease of installation compared with previous wiring methods that could achieve circuit survivability during a fire.
The hazards in the video exist with all cables using ordinary insulation materials. Rather than a case of “an accident waiting to happen,” as the video suggests, these new cables offer users a cost-effective way of providing circuit integrity. By using cables that are as easy to install—and which offer no greater hazard than ordinary cables—users gain a significant increase in safety by assuring the integrity of the connected circuits during a fire.
DEAN K. WILSON, P.E., C.F.P.S., Hughes Associates, Inc., Erie, PA