Patrick McLaughlin
The supply of plenum-rated cable is a business, safety, and technical issue.
Few issues in the premises-cabling industry have gained as much attention over the past several years as the supply of plenum-rated cable. The process by which plenum cable is made, and particularly the materials used to achieve plenum rating, have gained much of the spotlight. Consumers have had to deal with the varying supply of fluorinated ethylene propylene (fep), a proven wire-insulation material, as well as with alternative insulation materials that have not always met electrical- or fire-performance requirements. Those issues remain today as fep`s supply continues to vary and industry vendors look for alternative ways to produce plenum-rated cable.
When the plenum-cable supply has been low, the obvious effect on cabling installers has been less product to deliver to users and, consequently, fewer job opportunities. Also, when supply has been low, cost has gone up. So in that sense, the supply is a business issue.
By definition, cable that is plenum-rated can be placed in a building`s air-handling or plenum spaces. The requirements to achieve plenum rating are stringent and concern the cable`s resistance to fire as well as the amount of smoke produced when subjected to fire. The UL-910 Steiner Tunnel Test conducted by Underwriters Laboratories (UL--Northbrook, IL) subjects cables to fires and determines whether those cables can claim plenum rating. The National Fire Protection Association (nfpa--Quincy, MA) has used the UL-910 test as the basis for nfpa-262, which is a specification for a cable`s fire performance. nfpa-262 requires that cables exhibit flame spread of no more than five feet; average optical density of no more than 0.15, which equates to 70% light transmission; and peak optical density of no more than 0.5, which equates to 33% light transmission.
In addition, a group called the National Fire Protection Research Foundation, which includes cable manufacturers, compound manufacturers, and testing organizations, has initiated a program intended to bring into concert all efforts to characterize plenum-rated cable. All this attention and effort are justifiable because, clearly, in addition to being a business issue, plenum cable is a life-safety issue.
The composition of plenum cable also emerged as a technical issue when some cable manufacturers began producing differing cable constructions that qualify as plenum-rated. The electrical performance associated with some of these constructions has come under scrutiny, and in some cases, end-users have ended up with cable that has met plenum requirements but has not lived up to data-transport expectations.
Wire-insulation material
In an unshielded twisted-pair cable, both the cable`s outer jacket and the insulation of the wire pairs impact fire performance. Cable manufacturers and end-users alike have found out--in some cases the hard way--that the material used to insulate the wire pairs also affects electrical performance. When fep was in short supply a few years ago, cable manufacturers used alternate constructions to achieve the fire performance necessary for plenum rating. Some 4-pair cables, called 2x2 cables, included two fep-insulated pairs and two pairs insulated with polyethylene. Other 4-pair cables, called 3x1 cables, included three fep-insulated pairs and one polyethylene-insulated pair.
Some users of these cables soon encountered performance problems, and testing showed that in some 3x1 and 2x2 cables, the data transmitted over the fep-insulated pairs had a significantly different rate of propagation than the data transmitted over the polyethylene-insulated pair or pairs. The term "delay skew" was soon heard throughout the industry, and 3x1 and 2x2 cables got a reputation for unacceptable electrical performance. In fact, the first addendum to the Telecommunications Industry Association`s (Arlington, VA) tia/eia-568a cabling standard specified propagation delay and delay-skew parameters for 4-pair cable. That addendum was ratified in September 1997.
The fep shortage subsided, but its memory and effects have lingered. DuPont Fluoropolymers (Wilmington, DE), which manufactures fep under the trade name Teflon, began to expand its fep manufacturing facilities. So did the other fep manufacturer, Daikin America Inc. (Orangeburg, NY), which produces fep under the trade name Neoflon.
Alternative construction
Last year, Cable Systems International (csi--Phoenix, AZ) introduced a non-fep plenum-rated Category 5 cable and this year, csi began producing a line of non-fep plenum-rated Enhanced Category 5 cable.
csi`s method of ensuring the plenum rating of its Velocsity Category 5 and Velocsity Plus Enhanced Category 5 cable have defied convention in that the cables` wire insulants exhibit good electrical properties, and csi relies on the jacketing material to provide the necessary fire performance. The insulation material is a foam/skin high-density polyethylene with antioxidant and metal-deactivator additives. "It has outstanding electrical properties," says Bob Schwartz, csi`s vice president of research and development/strategic business planning.
The jacketing material is a foamed polyvinylidene fluoride (pvdf) with a smoke-suppressant additive. csi`s Velocsity cables have passed the UL-910 Steiner Tunnel test. On its approach to fire-safe cables, the company has opined in writing that "a plenum cable should not be judged on insulation fuel load alone, but more appropriately on the flame and smoke suppression characteristics of a total plenum cable construction as analyzed by the UL-910 Steiner Tunnel Plenum burn analysis....In other words, the flame and smoke performance of a cable is determined by the total design and not solely by properties of the individual components."
Furthermore, csi maintains that additional UL tests show that the Velocsity products maintain acceptable fire performance after aging and at elevated temperatures. The company also holds that the cables exhibit acceptable attenuation performance at elevated temperatures.
DuPont was contacted for this article and opted not to offer verbal comment, but the company has issued written concerns in a white paper, stating "the use of combustible materials other than the limited-combustible perfluoropolymers and other changes in cable construction are dramatically increasing the amount of fire-load contributed from plenum cables." Essentially, DuPont`s concern is that if the cable jacket is penetrated, twisted pairs that are combustible represent a fire hazard.
Ease of use
Working with csi`s plenum cable requires no more work than working with traditional plenum cable, the company claims. A cabling installer who recently installed Velocsity cables for the first time concurs. "The cables fed smoothly out of the box," says Bill Worthy, cabling supervisor with tad Enterprises (Tucson, AZ). "It didn`t bind up in the box, which some other cables have done.
"At several locations above the ceiling where we were pulling the cable, building infrastructure was already in place. There was no problem getting the Velocsity cables around those obstacles, and I pulled between five and eight cables per bundle."
As the csi example shows, the cable jacket can seriously impact fire performance and plenum rating. Polyvinyl chloride (pvc) is used as a jacketing material in both plenum and non-plenum cables. pvc can be manufactured to different degrees of fire resistance. Ausimont usa Inc. (Thorofare, NJ) manufactures a fluoropolymer, called ethylene chlorotrifluoroethylene (ectfe), under the brand name Halar. The company says Halar offers toughness and impact strength over a broad temperature range. To incorporate ectfe into the cable-manufacturing process, the material is melt-processable.
"Foamed grades of Halar were developed when the fep shortage first hit," says Fred Dohmann, Ausimont`s industry manager for wire and cable. It was introduced and got some use as an alternative to fep. "In standard Category 5 cables, the resin can be used as an fep alternative," Doleman continues. "In the proper cable design, it meets Category 5 specifications. Halar is still used in some Category 3 cables as a wire insulant."
Lately, he says, interest in ectfe has been rejuvenated--but this time, cable manufacturers are looking at it as a jacketing material, not a wire-insulating material. "As a jacketing material it has competed with pvc. It is a fluoropolymer, so it has inherently good electrical performance."
He adds that Ausimont has recently introduced another fluoropolymer, called Vatar. "Vatar is manufactured as a cable-jacketing material. It will most likely be used in cables that have fep as a wire-insulation material. Combining Vatar and fep will give excellent fire performance and enhanced electrical characteristics."
Like Daikin and DuPont, Ausimont has seen high demand for its materials and has announced plans to expand its manufacturing facilities. Ausimont currently has facilities in Thorofare, NJ, and Orange, TX. The company expects to begin production in a third U.S. plant in early 2001.
Certainly, the supply of plenum-rated cable remains an issue for much of the industry. And like a few years ago, end-users can choose from among different types of cables that pass the test for plenum rating. The extent to which each of these cable constructions gains market share depends largely upon those who specify and purchase the cables.
Underwriters Laboratories uses this type of setup to test cables for plenum rating. Shown here is cable before it has been subjected to the burn test.