Materials used for insulation
Rubber:
Rubber may be obtained from the milky sap of tropical trees or it may be produced from oil products. It has relative permittivity varying between 2 and 3, dielectric strength is about 30 kV/mm and resistivity of insulation is 1017Ù cm. Although pure rubber has reasonably high insulating properties, it suffers from some major drawbacks viz., readily absorbs moisture, the maximum safe temperature is low (about 38ºC), is soft and liable to damage due to rough handling, and ages when exposed to light. Therefore, pure rubber cannot be used as an insulating material.
Vulcanized India rubber (V.I.R.):
It is prepared by mixing pure rubber with mineral matter such as zinc oxide, red lead, etc., and 3 to 5% of sulfur. The compound so formed is rolled into thin sheets and cut into strips. The rubber compound is then applied to the conductor and is heated to a temperature of about 150ºC. The whole process is called vulcanization and the product obtained is known as vulcanized Indian rubber.
Vulcanized Indian rubber has greater mechanical strength, durability, and wear-resistant property than pure rubber. Its main drawback is that sulfur reacts very quickly with copper and for this reason, cables using VIR insulation have a tinned copper conductor. The VIR insulation is generally used for low and moderate-voltage cables.
It consists of chemically pulped paper made from wood chippings and impregnated with some compound such as paraffinic or naphthenic material. This type of insulation has almost superseded rubber insulation. It is because it has the advantages of low cost, low capacitance, high dielectric strength, and high insulation resistance. The only disadvantage is that paper is hygroscopic and even if it is impregnated with a suitable compound, it absorbs moisture and thus lowers the insulation resistance of the cable. For this reason, paper-insulated cables are always provided with some protective covering and are never left unsealed. If it is required to be left unused on the site during lying, its ends are temporarily covered with wax or tar. Since paper-insulated cables have the tendency to absorb moisture, they are used where the cable route has a few joints. For instance, they can be profitably used for distribution at low voltages in congested areas where the joints are generally provided only at the terminal apparatus. However, for smaller installations, where the lengths are small and joints are required at a number of places, VIR cables will be cheaper and more durable than paper-insulated cables.
It is a cotton cloth impregnated and coated with varnish. This type of insulation is also known as empire tape. The cambric is lapped onto the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent. As the varnished cambric is hygroscopic, therefore, such cables are always provided with a metallic sheath. Its dielectric strength is about 4 kV/mm and its permittivity is 2.5 to 3.8.
This insulating material is a synthetic compound. It is obtained from the polymerization of acetylene and is in the form of white powder. For obtaining this material as cable insulation, it is compounded with certain materials known as plasticizers which are liquids with high boiling points. The plasticizer forms a gell and renders the material plastic over the desired range of temperature. Polyvinyl chloride has high insulation resistance, good dielectric strength, and mechanical toughness over a wide range of temperatures. It is inert to oxygen and almost inert to many alkalis and acids. Therefore, this type of insulation is preferred over VIR in extreme environmental conditions such as in cement factories or chemical factories. As the mechanical properties (i.e., elasticity, etc.) of PVC are not so good as those of rubber, therefore, PVC insulated cables are generally used for low and medium domestic lights and power installations.
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