CHAPTER 8
SHEATHS, JACKETS, AND ARMORS
Lawrence J. Kelly and Carl C. Landinger
1. SHEATHS
The terms “sheaths” and “jackets” are frequently used as though they mean the
same portion of a cable. Sheath is properly the term that applies to a metallic
component over the insulation of a cable. An example is the lead sheath of a
paper insulated, leadcovered cable. See the definition of jacket in 2.0 below.
Various metals may be used as the sheath of a cable such as lead, copper,
aluminum, bronze, steel, etc. A sheath provides a barrier to moisture vapor or
water ingress into the cable insulation. It is necessary to use such a sheath Over
paper insulation, but it also has a value over extruded materials because of water
ingress.
The thickness of the metal sheath is covered by ICEA and AEIC standards and
specifications, but there are some constructions that are not mered. The
thickness is dependent on the forces that can be anticipated during the
installation and operation of the cable. Designs range from a standard tube to
ones that are longitudinally corrugated. The bending radius of the finished cable
is dependent on such configurations.
To fully utilize the metal chosen, one should consider first cost, ampacity
requirements -- especially during fault conditions, and corrosion [a-11.
2. THERMOPLASTIC JACKETS
The term jacket should be used for nonmetallic coverings on the outer portions
of a cable. They serve as electrical and mechanical protection for the underlying
cable materials.
There are many materials that may be used for cable jackets. The two broad
categories are thermoplastic and thermosetting. For each application, the
operating temperature and environment are important factors that must be
considered.
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Copyright © 1999 by Marcel Dekker, Inc.
2.1. Polyvinyl Chloride (PVC)
PVC is the most widely used nonmetallic jacketing material in the wire and
cable industry. Starting in 1935, when it first became available, the use of PVC
grew rapidly because of its low