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 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.
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 braidc. Asbestos braid