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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Cable insulation. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric installation work Cable insulation must have dielectric strength that excludes the possibility of electrical breakdown at the voltage for which the cable is designed. Paper, plastic and rubber insulation is used to isolate cable cores from each other and from outer metal sheaths. Paper impregnated insulation cable core has good electrical characteristics, long service life, relatively high allowable temperature and low cost, therefore, it is most widely used. The disadvantages include hygroscopicity, which necessitates careful manufacture and complete tightness of sheaths and cable sleeves. From multilayer reinforced cable paper based on sulfate cellulose grade KMP-120, insulation is made for power cables with voltage up to 35 kV. It is possible to produce insulation from two-layer paper of grades K-080, K-120, K-170 or multilayer paper - KM-120, KM-140 and KM-170. The thickness of the paper is respectively 80, 120, 140 and 170 microns. The cores are wrapped with paper unimpregnated tapes. The most common winding is with a gap, which allows the cable to be bent within certain limits without the danger of damaging the paper insulation. In order to avoid deterioration of the electrical characteristics of the insulation, the gaps between the turns of adjacent tapes located on top (vertically) should not match. When applying a large number of tapes, it is not possible to avoid gap coincidences, therefore the number of coincidences is normalized. No more than three coincidences of paper tapes and core-core insulation or core-sheath (screen) in 6 kV cables are allowed, no more than four for 10 kV cables, no more than six for 35 kV cables. Paper insulation should be applied tightly, without folds and wrinkles, the presence of which leads to the formation of voids, air inclusions that reduce the reliability of cables. The thickness of the insulating layer on power cables is standardized by GOST and depends on the rated voltage and cross section of the cable cores. To increase the electrical strength on the belt insulation of cables with a voltage of 6 and 10 kV, a screen of electrically conductive paper is applied to the cores and over the insulation of cables with a voltage of 20 and 35 kV. In multi-core cables, the upper insulation tapes of the cores have a digital designation or a distinctive color. With a digital designation, the number 1 is applied to the upper tape of the first core, the second - 2, the third - 3, the fourth - 4. With a distinctive color, number 1 corresponds to white or yellow, number 2 - blue or green, number 3 - red or crimson, number 4 - brown or black colors. The insulated cores of multi-core cables are twisted, filling the gaps between them with insulating materials until a round shape is obtained. On twisted insulated cores, belt insulation is applied with paper tapes of a certain thickness. The paper insulation of the cables is first dried, then impregnated with oil-rosin compositions: MP-1 for cables with a voltage of 1-10 kV and MP-2 - 20-35 kV. Impregnation achieves an increase in the electrical strength of paper insulation. plastic insulation used for power cables. It is made from polyethylene or polyvinyl chloride (PVC). Polyethylene has good mechanical properties in a wide temperature range, resistance to acids, alkalis, moisture and high electrical insulating characteristics. Depending on the method of obtaining polyethylene, low-density and high-density polyethylene are distinguished. High-density polyethylene has a higher melting point and mechanical strength compared to low-density polyethylene. LDPE softens at around 105°C, high density at 140°C. The introduction of organic peroxides into polyethylene and subsequent vulcanization significantly increase its melting point and resistance to cracking. Vulcanizing polyethylene deforms slightly at 150°C. To obtain self-extinguishing polyethylene, special additives are introduced. For electrically conductive screens of cables with polyethylene insulation, polyisobutylene, acetylene black and stearic acid are added to polyethylene. The solid product of polymerization - polyvinyl chloride - does not spread combustion. To increase the elasticity and frost resistance of PVC, plasticizers are added to it - kaolin, talc, calcium carbonate, coloring additives are introduced to obtain colored PVC. PVC ages under the influence of temperature, solar radiation, etc. due to the volatilization of the plasticizer (there is a decrease in elasticity and cold resistance). Rubber insulation consists of a mixture of rubber (natural or synthetic), filler, softener, vulcanization accelerator, antioxidant, dye, etc. Rubber RTI-1, which contains 35% rubber, is used to insulate cables. The advantages of rubber insulation are flexibility and almost complete non-hygroscopicity. Disadvantages - higher cost and low operating temperature of the core (65 °C) compared to other types of insulation, which reduces the load capacity of the cable. Over time, insulating rubbers show a significant decrease in elasticity and a change in other physical and mechanical properties. The aging of rubber insulation occurs under the influence of various factors and is mainly the result of oxidative degradation (destruction) of the rubber contained in the rubber. In order to protect the core insulation from exposure to light, moisture, various chemicals, as well as to protect it from mechanical damage, cables are provided with sheaths. The best materials for the manufacture of cable sheaths in terms of tightness and moisture resistance, flexibility and heat resistance are metals - lead and aluminum. Cables with non-absorbent (plastic or rubber) insulation do not need a metal sheath, so they are usually made in a plastic or rubber sheath. The thickness of the sheath is normalized and depends on the material from which it is made, the diameter of the cable and operating conditions. Lead sheaths are made from lead grade C-3 (pure lead not less than 99,95%). Lead is one of the very heavy metals (density 11340 kg/m327,4). Melting point - XNUMX ° C. Lead has low mechanical strength and significant fluidity, which must be taken into account when laying cables vertically in a bare lead sheath. As the temperature rises, the fluidity of lead increases. The normal electrochemical potential of lead is -0,13 V, so it has low chemical activity and high corrosion resistance. The disadvantage of lead sheaths is their low resistance to vibration loads, especially at elevated temperatures. An increase in vibration resistance and mechanical strength is achieved by introducing antimony additives into lead. The lead sheath of cables without protective covers is made of lead-antimony alloys of grades SSuM, SSuMT. Lead sheaths shall be free from marks, scratches and dents that would take them outside the minimum thickness tolerances. Aluminum shells are made by extrusion from A-5 aluminum with a purity of at least 99,97%. Aluminum density - 2700 kg/m, tensile strength - 39,3-49,1 MPa. Aluminum sheaths are 2-2,5 times stronger and 4 times lighter than lead sheaths, have increased resistance to vibration loads and have high shielding properties. The disadvantages of aluminum sheaths are the great technological difficulties of applying them to the cable and low resistance to electrochemical corrosion, which is explained by the high normal negative potential of aluminum (-1,67 V). Corrosion is reduced to the displacement of hydrogen ions from the medium with which aluminum comes into contact and the transition of aluminum itself in the form of ions into solution. Therefore, cables with aluminum sheaths are protected with particularly rot-resistant covers that do not allow moisture to pass to the sheath. Plastic shells are made of hose PVC compound or polyethylene. Plastic sheaths combine lightness, flexibility and vibration resistance, but water vapor gradually diffuses through the plastic, which leads to a drop in the insulation resistance of the cables. Therefore, they are used in cables with non-hygroscopic insulation made of polyethylene, PVC, etc. The hose compound differs from the insulating compound by the selection of plasticizers and stabilizers, which provide greater resistance to light aging. For cable sheaths, PVC compound grade 0-40 is used. Cable sheaths made of PVC-compound at temperatures below the permissible become rigid and can be destroyed upon impact. The good mechanical strength of PVC-compound makes it possible to widely use sheathed cables without protective covers. It does not spread combustion, it is moisture and oil resistant, resistant to electrical and chemical corrosion. Cables in such a sheath are easy to manufacture and easy to install. Polyethylene sheaths of cables are characterized by high physical and chemical properties, low moisture permeability and resistance to electrical and chemical corrosion. Rubber shells are made of oil-resistant rubber RSHN-2, flame retardant. Rubber shells are highly resistant to tensile, impact and torsional loads. As rubber fillers, carbon black (soot) is used, which protects it from the action of solar radiation. Protective covers consist of cushion, armor and outer cover and are designed to protect cables from mechanical damage and corrosion. The letter "G" is added to the designation of the cable brand that does not have a protective cover. Cable pads are concentric layers of fibrous materials and bitumen composition or bitumen over the sheath and are intended to protect the sheaths of the cable from damage by tapes or wires of the armor and protect it from corrosion and do not have a designation. A reinforced pillow with an additional winding with two plastic tapes, which provides protection against corrosion and stray currents, is marked with the letter "l". To improve corrosion resistance, the pillow is made with two layers of plastic tapes and is marked with a number and the letter - "2 l". In order to increase the corrosion and moisture resistance of the cushion, a layer of extruded polyethylene or PVC compound is applied over PVC-compound tapes (and other equivalent material). In the marking, this type of pillow is denoted by the letters "p" (polyethylene) and "v" (PVC plastic compound). Protective covers without a pillow are marked with the letter "b". The minimum thickness of the pillow depends on the design, cable diameter and is 1,5-3,4 mm. Armor serves to protect cables from mechanical damage. For cables that are not subjected to tensile forces during operation, tape armor is used, which consists of two steel tapes with a thickness of 0,3 to 0,8 mm (depending on the diameter of the cable along the sheath) and is applied so that the upper tape covers the gaps between turns of the lower tape. For cables that are subjected to tensile forces, armor made of galvanized steel flat or round wires is used. The thickness of the armor made of galvanized steel flat wires is 1,5-1,7 mm, the diameter of the round wires is 4-6 mm. Outer cover, which includes a layer of bituminous composition or bitumen, impregnated yarn and coatings that protect the coils of the cable from sticking, does not have a designation in the marking. The cover with a non-combustible element in the cable marking has the letter "H". With a pressed-out polyethylene protective hose, the covers are designated "Shp", and with a PVC hose - "Shv". The minimum thickness of the outer cover depends on the cable diameter and is 1,9-3 mm. Author: Bannikov E.A.
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