Here is a list of ten main insulating materials used to manufacture electric cables.
Material # 1. Rubber:
Rubber may be natural and synthetic. Natural rubber is obtained from the milky sap of tropical trees. Synthetic rubber is produced from alcohol or oil products. Its relative permittivity is between 2 and 3 and dielectric strength is 30 kV/mm. Though it possesses high insulating qualities but it absorbs moisture readily, softens when heated to a temperature of 60° or 70° C, swells under the action of mineral oils and ages when exposed to light. Hence pure rubber cannot be used as insulating material.
Material # 2. Vulcanized India Rubber (VIR):
It is prepared by mixing India rubber with mineral matter such as sulphur, zinc oxide, red lead etc. Though vulcanization process improves the non-hygroscopic and other properties like strength, durability, resilience, elasticity etc.; but it slightly reduces its specific insulation resistance. It has dielectric strength of about 10-20 kV/mm and dielectric constant of about 2.5.
The main drawback of VIR is that owing to sulphur content it attacks copper and therefore in cables using VIR insulation the copper conductor is tinned before providing this insulation. Sometimes a layer of pure rubber is also given on the conductor to protect it from sulphur.
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As compared to impregnated paper, the performance of VIR is poor because VIR cables are expensive, less durable and have poor mechanical properties such as electrostatic capacity, elasticity and tenacity. VIR is used as insulating covering for low and medium voltage cables. Such type of cables is normally covered with cotton tape and jute braiding soaked in the compound in order to protect the cables against ingress of moisture.
Material # 3. Impregnated Paper:
It is prepared from wood pulp rags or plant fibres by a suitable chemical process. It is quite cheap, has low capacitance, high dielectric strength (30 kV/mm), high insulation resistivity (of the order 107 Ω-m), high thermal conductivity, better thermal withstand capability and high durability.
The only disadvantage is that it is hygroscopic, and even if it is impregnated with compound, it absorbs moisture, which lowers its insulation resistance. That is why paper insulated cables are always provided with some sort of protective covering and are never left unsealed. Its ends should be temporarily covered with wax or tar, even if it is required to be left unused on site while being laid.
The maximum safe temperature of paper insulated cable is about 95° C. The compound usually employed for impregnation purposes is a heavy-grade mineral oil mixed with resin, the actual proportions of oil and resin depending on the particular manufacturer.
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For providing insulation over the conductor, the paper tape is lapped on to the conductor until the required thickness is obtained; it is then dried and impregnated with insulating compound. The insulating compounds are required to have, besides high electrical insulation properties, a high viscosity at operating temperatures (50-80° C), a low viscosity at impregnation temperatures of 50-120° C, a smooth change in viscosity with temperature and very low coefficient of thermal expansion.
The impregnation of paper insulation increases its dielectric strength from about 3-5 to 10-80 kV/mm. The dielectric constant of impregnated paper insulation is about 3.5.
Paper insulated cables are used for conveying large blocks of power, in transmission and distribution, and particularly for distribution at low voltage in congested areas where the joints are to be provided only at the terminal apparatus or where the joints are rare, owing to cheapness and durability over VIR cables.
Material # 4. Polyvinyl Chloride:
This is a synthetic compound. For obtaining this material as a cable dielectric or sheathing it is processed with certain materials known as plasticizer and its type will depend upon the use of the finished product i.e. PVC. It is inert to oxygen and almost inert to oils and too many alkalies and acids and therefore its use is preferred over VIR in extreme environments such as in cement factory or chemical factory.
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PVC has dielectric strength of 17 kV/mm and dielectric constant of 5. Maximum continuous temperature rating is 75° C. The mechanical properties (i.e., elasticity and recovery from stretching) of PVC are not as good as those of rubber. PVC insulated cables are usually employed for low and medium voltage domestic and industrial light and power installations.
Material # 5. Polythene:
Polythene is a straight chain polymer derived from ethylene. Its electrical properties are very good—low permittivity (dielectric constant), high insulation resistivity and very low power factor at all frequencies. The thermal dissipation properties are better than those of impregnated paper and the impulse strength compares favourably with an impregnated paper-insulated cable. The maximum operating temperature under short-circuits is 100° C. It has limited use in power cables. However, a 3-core 11 kV self-supporting aerial cable, which can be employed in rural distribution, has been developed.
Material # 6. Cross-Linked Polythene:
Low density polythene, when vulcanized under controlled conditions, results in cross-linking of carbon atoms and the compound produced is a new material having extremely high melting point with light weight, small dimensions, low dielectric constant and good mechanical strength. By using chemical process it has been made technically possible to cross-link polythene in conventional equipment for the manufacture of rubber. This is why the product is said to be “vulcanized” ‘or cross-linked’ polythene.
Cross-linked polythene (XLPE) cables have high maximum continuous temperature rating of 90° with dielectric strength of about 20 kV/mm. Because of high temperature withstand capability and very low water absorption; such cables can be buried directly in soil. These cables have been found very suitable for all voltages up to 33 kV.
Material # 7. Varnished Cambric (or Empire Tape):
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This is a cotton cloth impregnated and coated with varnish. It has very smooth surface. The cambric is lapped in the form of a tap on to the conductor and its surfaces are coated with petroleum jelly compound to give easy sliding of the surfaces. Such cables require protective covering like lead sheath because this insulating material is hygroscopic. Its dielectric strength is 4 kV/mm and its dielectric constant is 2.5 to 3.8. Such cables do not require sealing.
Material # 8. Gutta-Percha:
It is similar to rubber but it becomes soft at about 65°C. It is non-hygroscopic but cannot withstand even medium voltages. It is mostly used for submarine cables for telephone and telegraph purposes.
Material # 9. Silk and Cotton:
This is used in low voltage cables. The conductor may have a single layer or double layer covering depending upon the requirements of service. Silk or cotton covered wires are usually used for instrument and motor winding.
Material # 10. Enamel Insulation:
Enamelled wires are also used for instruments and motor winding. The wires are cheaper than silk and cotton covered wires and therefore for low voltage machines and instruments enamelled wires are used. Enamel insulation is liable to crack.