Wire and Cable Technology for LANs - LAN Wiring - page 102

PVC has dialectic properties that may make it unsuitable for wire insulation of
Category 5e cable, although it can be used in cable jackets. Polyethylene (PE) is some-
times used for Category 5e cable and may be substituted on one pair of some plenum-
rated cables. Fluorocarbon polymers include polytetrafluoroethylene (PTFE or TFE)
and fluorinated ethylene-propylene (FEP), which are not as flexible as PC, but meet the
stringent flammability testing for use in plenum spaces. These fluorocarbon polymers
are sometimes referred to by the DuPont trademark Teflon ® . Wire insulation and
sheaths may be a copolymer of TFE and FEP. Another fluorocarbon polymer that is
often used in cable jackets of plenum-rated cables is ethylene-chlorotrifluorethylene
(ECTFE or HALAR ® ). This material does not have the proper dielectric qualities to be
used as a wire insulation, but efforts are under way to formulate it for that purpose.
The increasing use of large amounts of cable for LAN wiring has resulted in the
creation of new cable guidelines that emphasize the flammability of the cable sheath
and insulation. Conventional wire insulations were found to be relatively flammable
and a hazard when placed in the air plenums. Plenums, or air ducts, carry the heated
or cooled air around buildings. Any cable that burned in a plenum could create toxic
gas and smoke that would be a danger to people in other parts of a building. In addi-
tion, the fire could actually spread through plenums. Manufacturers soon developed
cable insulations that were less flammable and could be used in plenums.
Early fire protection specifications referred to sections of the National
Electrical Code ® (the NEC ® ) or to flammability tests of Underwriters Laboratories ®
(UL ® ). Similar specifications were published by other countries. Several of the per-
tinent NEC articles are shown in Table 5.2. The NEC ® Article 800 is often referred
to for LAN cabling and telecommunications cable. The NEC ® differentiates cable
types by voltage class and by flammability. Class 2 (up to 150 volts) and Class 3 (up
to 300 volts) are both covered by Article 725. Article 725 might appear to apply to
LANs, but it actually refers the user to Article 800 for cable classified for commu-
nications use (which does include LAN cable). In addition, permitted use is graded
by three levels of flammability: general use, riser use, and plenum use.

TABLE 5.2
National Electrical Code ® ( NEC ® ) Articles for Low-Voltage Wiring

Article Types of Cable

725 Remote signaling and power limited circuits

760 Fire protection signaling systems

770 Fiber-optic cables

800 Communication cables

820 Coax cables