Quantcast Nonmetallic Finishes

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2.2.4 Cable Outer Finishes. Cable outer finishes or outer coverings are used to protect the
underlying cable components from the environmental and installation conditions associated with
intended service. The choice of cable outer finishes for a particular application is based on
electrical, thermal, mechanical, and chemical considerations. Combinations of metallic and
nonmetallic finishes are usually required to provide the total protection needed for the installation
and operation. Nonmetallic Finishes.
(a) There are outer coverings (extruded jackets) either thermoplastic or vulcanized,
which may be extruded directly over insulation or over electrical shielding systems of metal
sheaths or tapes, copper braid, or semiconducting layers with copper drain wires or spiraled
copper concentric wires, or over multiconductor constructions. Commonly used materials
include: polyvinyl chloride, nitrile butadiene/polyvinyl chloride (NBR/PVC), polyethylene,
cross-linked polyethylene, polychloroprene (neoprene), chlorosulfonated polyethylene, and
polyurethane. These materials provide a high degree of moisture, chemical, and weathering
protection. They are reasonably flexible, provide some degree of electrical isolation, and are
of sufficient mechanical strength to protect the insulating and shielding components from normal
service and installation damage.
(b) A commonly used material is braided asbestos fiber. Asbestos braid is used on
cables to minimize flame propagation, smoking, and other hazardous or damaging products of
combustion which may be evolved by some extruded jacketing materials. Special industrial
applications may require synthetic or cotton fibers applied in braid form. All fiber braids
require saturants or coating and impregnating materials to provide some degree of moisture and
solvent resistance as well as abrasive and weathering resistance. Metallic Finishes. These materials are widely used when a high degree of
mechanical, chemical, or short-time thermal protection of the underlying cable components may
be required. Commonly used are interlocked galvanized steel, aluminum, or bronze armor;
extruded lead or aluminum; strip formed, welded, and corrugated steel and aluminum; and
spirally laid round or flat armor wires. The use of any of these materials will reduce flexibility
of the overall cable, but flexibility must be sacrificed to obtain the other benefits.
(a) The unprotected interlocked armor provides a high degree of mechanical protection
without significantly sacrificing flexibility. While not entirely impervious to moisture or
corrosive agents, interlocked armor does provide protection from thermal shock by acting as a
heat sink for short-time localized exposure. Where corrosion and moisture resistance are
required, in addition to mechanical protection, an overall jacket of extruded material may be
used. Commonly used interlocked armor materials are: galvanized steel, aluminum (for less
weight and general corrosion resistance), and marine bronze and other alloys (for highly
corrosive atmospheres).


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