Q: We have been specifying our Category 5 plenum cable by manufacturer and product number for some time now and it was always all-FEP (fluorinated ethylene propylene). But without any notification, the manufacturer changed the composition of the product to a 3+1 cable. Do any manufacturers still make plenum-rated Category 5 cabling with FEP insulation on all 4 pairs of conductors? How does the cost of this product compare with the 2+2 and 3+1 cables?
Henry S. Radwanski
University of Pennsylvania
A: Yes. Even though the 4+0 construction may not be specified in their catalogs, most manufacturers make the 4+0 cable. With regard to cost, the University of Texas at Austin has been buying low-bid and installing the 4+0 cable all along. Cabling manufacturers that have not made the 3+1 or 2+2 cables are very competitive bidders.
The Telecommunications Industry Association (TIA) UTP cable task group has been very active and expects to issue a delay-skew specification. For those readers not familiar with recent developments in the TR41.8.1 working group, in a recent press release, the TIA stated that all Category 5 cables may not support all 100-megabit-per-second data applications (see "TIA issues warning about Category 5 cable," June 1996, page 67). The reason is excessive delay skew, which is not a specified test parameter for UTP cable in the TIA-568A standard.
Delay skew is the difference of nominal velocity of propagation (NVP) between individual pairs in a link. Velocity of propagation is the speed at which a signal can be transmitted over a medium. We are most familiar with velocity of propagation stated as a percentage of the speed of light (NVP = 72.5%); however, it can also be stated as time-to-distance, for example: 500 nanoseconds per 100 meters (500 ns/100m).
The continuing shortage of FEP and a 40% to 50% annual growth in the demand for Category 5 plenum cable present a challenge for cable manufacturers. Traditionally, FEP has been used to insulate paired conductors in Category 5 plenum cable because it was perceived that only FEP would comply with the electrical performance requirements specified in the ANSI/TIA/EIA-568A standard and the plenum requirements for communications cable in the National Electrical Code. With only two FEP suppliers--Daikin America (Orangeburg, NY) and Dupont (Wilmington, DE)--the supply-and-demand problem continues, causing manufacturers and distributors to scramble for product.
Ingenuity produced several solutions. One solution is to use FEP for three of the four pairs of conductor in 4-pair cable and polyolefin for the fourth pair. This solution would save the manufacturer 25% of its FEP allocation. Polyolefin was not used to insulate all of the pairs because that construction would not pass the Underwriters` Laboratory (UL) flame test. Another solution is to use FEP for two of the four pairs of conductors in 4-pair cable and modified polyolefin for the other two.
While these solutions result in a product that can pass the UL flame test and meet existing Category 5 requirements, some will not support high-speed data applications.