Materials Cable sheathing

Lead—copper alloys are the primary material used in the continuous extmsion of cable coverings for the electrical power cable industry in the United States. Other alloys, containing tin and arsenic as well as copper, have also been developed for cable sheathing in the United States to provide higher fatigue strength. 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

Fig. 13-1 Resistance load R of cable sheathing as a function of cable diameter d and sheathing material.

About 75 years ago, the main material for cable sheathing was lead, protected from corrosion by asphalt-impregnated jute coating. Even then corrosion problems prevailed. Later on, extruded or drawn aluminum sheaths were in use, soon to compete with polythene. A large number of failures in the 1960s caused by water in the cable led to the development of sophisticated methods of sheathing involving coated aluminum and clad metals of copper adjacent to a number of steels (45). 

Most inside plant cables do not use buffer tubes, but may instead use subunits to organize the fibers within an overall outer cable sheath. A subunit serves many of the same purposes as a buffer tube except that it does not contain water blocking materials and it is usually made from materials that are chosen primarily for their fire-resistance properties. In some of these unitized designs the subunit itself may be structurally equivalent to a complete indoor cable. This allows the individual subunits to be broken away from the rest of the cable and routed independently, in order to deliver one or more fibers to where they are needed within a building. Such cables are usually called break-out, or fan-out, cables. The latter generally refers to cable in which each subunit contains only one fiber. 

Other materials may be incorporated into the overall cable sheath, such as shields, moisture barriers, and identification tapes. These materials see fairly limited use because of the minimal benefit/cost return they provide. Specific cable designs may also employ sheath constructions consisting of multiple jackets and layers of armor to improve the cable survivability in extremely harsh environments. 

The metal also finds appHcations in other alloys. Cable sheaths are made of lead that contains antimony, and lead shot for ammunition is not made of pure lead but of lead alloyed with 1-8% antimony. The American Isaac Babbitt (1798-1862) invented the bearing alloy named babbitt metal. It is an alloy of copper, antimony and tin, characterized by good antifriction properties, and used as the bearing material for axles and crankshafts. Britannia metal is another tin alloy with about 2% copper and 5-10% antimony. It is a silvery white alloy of tin, similar in appearance to pewter but harder. It is used for the manufacture of tableware. 

Poly(chloroethene), also known as PVC (polyvinyl chloride), is very widely used in all forms of construction materials, packaging, electrical cable sheathing and so on. It is one of the world s most important plastics. Its widespread use is, however, somewhat controversial as its synthesis is associated with some toxic by-products known as dioxins which must be very carefully contained. Dioxins (Chapter 22) are linked to reproductive disorders and a variety of... 

It is not always appreciated that some cable sheathing materials are adversely affected by the ultraviolet radiation in sunlight, and to counter this effect a black compound is added see Regulation 522-11-01. Unfortunately, this increases the absorption of infrared radiation and this solar heating can increase the cable temperature considerably, so outdoor cables should either be screened from sunlight or derated to allow for an increase in ambient temperature of about 20°C. 

Disordered carbon-black-polymer composites have many common uses in modem technology. These uses include inks, automobile tires, reinforced plastics, wire and cable sheaths, antistatic shielding, resettable fuses, and self-regulating heaters. As an immediate example, the inked letters on this page consist of a disordered carbon-black-polymer composite bound to the surface of the paper. Despite these widespread applications, many of the important physical properties of these composites are not well understood. There is a long history of experimental and theoretical work on disordered carbon-black-polymer composites [7]. One of the most exciting recent advances in the field has been the application of the scaling theory of percolation [2] to these composites. This is based on the many similarities between the percolative transition in a disordered conductor-insulator composite and the thermodynamic phase transition common in many materials. 

Polyvinyl chloride is widely used in extruded form for wire insulation and cable sheathing. Although alternatives have been sought, it remains the predominant material for this application. Irradiated, crosslinked grades are now in use where greater toughness and cut-through resistance are required. 

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