cable

(or rope), a flexible article manufactured from steel wire, filaments, yarn (hemp yarn), or vegetable, synthetic, or mineral fibers. Cables can be subdivided by their method of manufacture into twisted, nontwisted, and braided types.

Metal (steel) cables are manufactured from uncoated (bright), zinc-coated, or aluminum-coated wire of circular or shaped cross section, with tensile strength δ_v = 900–3, 500 meganewtons per sq m (MN/m²), or 90-350 kilograms-force per sq mm (kgf/ mm²). The cross section of steel cables may be round, hexagonal, rectangular, or square. Twisted round cables may have various lays—for example, a single spiral lay, which may be open, semi-sheathed, or sheathed; double lay (hawser lay), made up of three to eight strands, which may be circular or shaped (trihedral or oval); or triple lay (cable lay), made up of hawser-laid cables (strands). According to the arrangement of wires in the strand layers, cables may have linear, point, or combination contact between the wires. Depending on the lay of the strands, cables may be of the untwisting, nonuntwisting, or low-twist types (there may be 18-31 strands, with opposite directions of lay in individual layers). The direction of lay for the strands of a cable can be right-hand (designation Z) and left-hand (designation 5); the combination of directions of lay for the individual elements and for the cable as a whole can be crossed right-hand (SZ), crossed left-hand (ZS), one-way right-hand (ZZ), and one-way left-hand (SS). Twisted circular cables have a diameter of up to 100 mm. Combined twisted cables are made of hemp and steel. In this case the steel strands are covered by a layer of hemp yarn or plastic. Nontwisted cables consist of tightly packed groups of steel wires or spiral cables compressed from the outside by a spiral wrapping or by clamps. Such cables are usually assembled at the place of use; their diameter may be up to 1.5 m. The breaking load of nontwisted cables depends on their diameter and can be as high as 1,000 MN. Braided cables are produced by braiding an even number of strands (usually four), half of which have a right-hand braid and the other half a left-hand braid. Such cables have a square cross section. Flat cables consist of an even number of strands (four to twelve), with an alternating (right-hand and left-hand) lay, fastened together (stitched) with strands or pins; such cables have a rectangular cross section, and they are up to 250 mm wide.

Nonmetallic (fiber) cables for ropes are spun from long bast fibers of Russian (soft), manila, and sisal (stiff) hemp, from seed fibers of coconut and cotton, and from synthetic fibers (polypropylene, capron, nylon, perlon, and others) and asbestos fibers. Short fibers (hemp and cotton) are used in the manufacture of cord, binder twine, and other articles. Nonmetallic cables are made in stranded lays (three- and four-strand), with right-hand hawser cable lay (three-strand), as ordinary braided round cables (halyards), or as high-flexibility (nautical) cables. The diameter of fiber cables made from tarred or untarred Russian hemp, manila hemp, or sisal hemp is 6.7-111.5 mm. Cables made from stiff hemp, as compared with those made of soft hemp, have the advantage of greater strength, better wear resistance, and lower weight. Cables made from synthetic fibers exhibit high-strength properties. The strength characteristics of various kinds of cables are given in Table 1.

electrical, one or more insulated wires enclosed in hermetic sheathing. As a rule, the sheathing is covered by protective coatings. Cables are used in the transmission of electric power or signals (high-voltage power lines; power supply for industrial enterprises and transportation and municipal institutions; communications trunk lines; urban telephone networks; radio and television communications links; power supply for mobile operating machines, such as excavators and coal-cutting and peat-digging machines, and electrical equipment of ships and aircraft). The design of a cable depends essentially on the operating conditions in the cable run (underground; in water, air, or chemically active mediums; at low or high temperatures; or under conditions of high humidity).

Cables of all types have some common design components: current-carrying strands, insulation, and sheathing. Current-carrying strands are made of copper or aluminum, which, with the exception of silver, have the lowest electrical resistance (the specific resistivity p of electrolytic copper is 1.7 × 10^-8 ohm ⋅ m; that of aluminum, 2.9 × 10^-8 ohm ⋅ m). Depending on operating conditions the current-carrying strands may have various degrees of flexibility and may consist of a single wire or many wires twisted together. In power cables the current-carrying strands have standard cross sections. The choice of cross section depends on the power to be transmitted. In the USSR the most commonly used cross sections are 10, 16, 25, 35, 50, 70, 95, 120, and 150 sq mm. In communications cables the current-carrying strands have standard diameters.

Cable insulation consists of a one-piece, laminated or frame-and-air dielectric separating the current-carrying strands from one another and from the sheath. In multistrand cables the twisted, insulated strands are covered with additional insulation (belted); as a rule, its material is the same as that of the primary insulation. The belted insulation serves as a binding that imparts a circular shape to the cable. Insulation materials must have high electrical resistance and provide the electrical strength needed for particular operating conditions, while maintaining minimum insulation thickness; dielectric losses (tan **) must be low; and the dielectric constant (e) must be minimized. The materials must have high resistance to aging. For certain operating conditions additional requirements may apply to insulation, such as incombustibility and increased flexibility and wet strength. The heat resistance of insulation—that is, the ability to withstand high temperatures without a significant reduction in operational reliability—is of particular importance, since an increase in the upper limit of operating temperatures makes possible a reduction in overall dimensions and weight of the cable. Materials most widely used as insulation are cable paper and telephone paper, rubbers based on natural and synthetic latex, and plastics (polyethylene in various modifications, polyvinyl chloride, and polystyrene). Mineral oils, oil-rosin compounds, and certain inert gases under pressure can also be used as components of insulating materials.

Sheaths in the form of continuous tubes covering the current-carrying strands protect the strands from mechanical damage and the effects of moisture, light, and chemicals. For cables with easily moistened (hygroscopic) insulation the preferred sheath materials are lead or aluminum (materials with a near-zero diffusion constant). Lead sheaths are easily molded at relatively low temperatures (180°-220°C). In spite of several disadvantages—high density (11.4 g/cm³), danger to workers’ health, and low vibration resistance and mechanical strength—lead sheaths are widely used in the manufacture of cables. Aluminum is more promising for this purpose, since its strength is 2.0-2.5 times that of lead, it is lighter by a factor of 3.3, its vibration resistance is higher, and it is in better supply than lead. However, the molding of aluminum requires more complicated equipment, because plastic deformation of aluminum requires considerable force even at temperatures of 450°-500°C. To increase flexibility, aluminum sheaths for large-diameter cables are corrugated. Cables with continuous plastic insulation usually have sheaths consisting of various polyvinyl chlorides and polyethylene pigmented with 1-2 percent carbon black (the moisture permeability of polyvinyl chlorides is 10 times higher than that of polyethylene). Cables with rubber insulation usually have a sheath made of materials with a synthetic latex base, which imparts to the insulation oil and gasoline resistance, noncom-bustibility, and high cold resistance, flexibility, and mechanical strength.

To protect cable sheaths against mechanical damage and corrosion, protective coverings, most of which contain an armored layer (armor), are applied to the sheaths. In most cases the armor consists of two steel strips 0.3-0.8 mm thick, sometimes galvanized or asphalt-covered. The strips protect the cable from damage during underground laying, as well as within buildings and in channels, blocks, and tunnels. To protect a cable from the effect of considerable tension forces, it is enclosed in an armor made from round (less frequently flat) galvanized steel wires with a diameter of 1.4-6.0 mm (such protection is mandatory for cables laid on the bottom of bodies of water and in boreholes). Soft covers or fillers consisting of several layers of asphalt-impregnated paper strips or of cable braid (jute) are placed above and below the armor. Cables laid in highly corrosive environments or in the ground where stray currents are present, as well as all cables with an aluminum sheath (independent of operating conditions), are protected by reinforced cover layers that contain layers of plastic, either in strips or continuous. Cables laid in mines or in buildings exposed to fire hazard are protected by noncombustible coverings, such as glass braid or coal-tar pitch. Protection of cables against minor mechanical damage is provided by armor shielding made from galvanized steel wires with a diameter up to 0.3 mm or by braiding with fibrous materials impregnated with nondecaying substances.

The USSR produces more than 1,000 types of cables, whose brands, types, usage, design, and characteristics are given in corresponding standards. A detailed classification of cables by groups, taking into account common technological processes, has been adopted for manufacturing planning and organization and is the basis for specialization of cable-producing factories

and workshops. Cables usually have a letter designation, with an indication of the number and cross section or diameter of the current-carrying strands (see Table 1). For some cables the value of the most important characteristic is also given (operating voltage or rated characteristic impedance), or a characteristic design feature is indicated (type and number of coaxial pairs, double or quadruple twist, and so on). The letters usually denote the metal of the conductor, sheath, and insulation, as well as the presence and type of protective covering and armor and frequently the area of usage (control, shipboard, signaling and block signaling, or installation work). For example, ASK 3 × 95-6 is a power cable that has three aluminum strands with a cross section of 95 sq mm enclosed in a lead sheath and an armor made of circular steel wires, with a reinforced protective covering on the outside, for a rated voltage of 6 kilovolts (kV); TPVBG 100 × 2 × 0.5 is a telephone cable with polyethylene insulation in a polyvinyl chloride sheath, armored with 100 pairs of steel strips with corrosion-proof coating; the diameter of the copper strands is 0.5 mm.

Data on the cables that are most widely used in various areas of technology are given in Table 1, along with an indication of the main brands for each type and a description of design characteristics, main parameters, cable-laying conditions, operating conditions, and main areas of use.