A wide variety of finishes are used; they are referred to as
jackets, sheaths, armors, and braids. These coverings are required primarily
because of the physical or chemical characteristics of the particular
insulation involved and the required mechanical protection. Finishes can be
divided into two categories:
(1) metallic finishes and (2) nonmetallic finishes.
Metallic Finishes
Metallic armor should be applied where a high degree of
mechanical protection is required along with protection from rodents, termites,
and the like. All metallic sheaths are subject to electrolytic damage. Metallic
finishes are subdivided into the following:
1. Lead sheaths: One of the earliest types of
metallic sheaths still in use.
2. Flat-band armor: Consists of jute bedding, two
helical tape wraps, and a protective jute covering over the tapes. The tape may
be either galvanized or plain steel.
3. Interlocked armor: Consists of galvanized steel,
aluminum, or bronze strip (0.750 in. wide and 0.020–0.030 in. thick) over the
cable in such a way as to provide excellent protection.
4. Aluminum-sheathed cable: A recently introduced
cable that offers advantages such as lightweight, resistance to fatigue, good
corrosion resistance, and positive moisture barrier.
5. Wire armor: Available in two types, round and
basket-weave or braided wire. Round wire armor offers extremely strong cable
and has high tensile strength for vertical applications. Braided or
basket-weave wire armor consists of a braid of metal wire woven directly over
the cable as an outer covering where additional mechanical strength is
required.
Nonmetallic finishes
Most of the nonmetallic finishes include PVC, PE, neoprene,
hyplon, and EPR.
1. PVC: This covering (i.e., finish) offers excellent
moisture-resistance characteristics, but does not provide mechanical
protection.
2. PE: It has excellent resistance to water, ozone,
and oxidation. It is resistant to gasoline, solvents, and flames.
3. Neoprene: It is commonly recommended where service
conditions are usually abrasive and extreme. By itself, it is not flame
retardant.
4. Hyplon: It possesses similar properties as
neoprene, but also has better thermal stability and resistance to ozone and
oxidation.
5. EPR: It exhibits excellent weathering properties
and is resistant to ozone. It has good chemical and mechanical properties, but
is not inherently flame retardant.
6. Braids: Generally, present-day trends are away
from the use of nonmetallic braid coverings. Braids may be of the following
types:
a. Heat- and moisture-resistant cotton braid
b. Flame-resistant cotton braid
c. Asbestos braid
No comments:
Post a Comment