Insulating Materials:- The various insulating materials used in cables are as follows:
Silk and Cotton:– These types of insulation materials are used on conductors which are used for low voltage purposes. These insulated wires are usually used for instrument and motor winding.
Polyvinyl Chloride (PVC):- It is a synthetic compound material and comes as a white odorless, tasteless, chemically inert, non-inflammable, and insoluble powder. It is combined chemically with a plastic compound and is wed over the conductor as an insulation cover. PVC has the good dielectric strength and a dielectric constant of 5. Its maximum continuous temperature rating is 75° C. It is inert to oxygen and almost chemically stable, so it is preferred over VIR cable &. PVC insulated cables are usually employed for medium and low voltage domestic, industrial lights, and power installations.
Vulcanized India Rubber (VIR) it is useful for low-voltage power distribution systems only, because of its small size. It is prepared by mixing India rubber with mineral matter such as sulfur, zinc oxide, red lead, etc. It has a reasonable value of dielectric strength (15 kV/mm). Its use is limited because of its low melting point, low chemical resistance capability and short span of life. The advantage of using vulcanized India rubber is that when it is used for a cable, the cable becomes stronger and more durable. It can withstand high temperatures and remain more elastic than pure rubber. The drawback of using this material for insulation is that it attacks copper. Hence, before using VIR as insulation, the copper conductor must be tinned well.
Impregnated Paper:- This is used as an insulator in case of power cables because it has low capacitance, high dielectric strength (30 kV/mm), and is economical. The paper is manufactured with wood pulp, rags or plant fiber by a suitable chemical process. It has high resistance due to high resistivity under dry condition. The drawback of this cable is that it absorbs a small amount of moisture only, which reduces the insulation resistance. Therefore, a paper insulated cable always requires some sort of protective covering and it is impregnated in insulating oil before use.
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