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The material used for insulation in electrical machines and apparatus are numerous (such as insulating varnishes, mineral waxes, synthetic waxes, varnish, cloth, silk, cotton, rubber, vulcanized rubber, hard rubber, Bakelite, paper, wood, mica, asbestos, glass, porcelain, marble, state, insulating oils etc.). Some of the most important insulating materials are briefly described below:

1.    Insulating Varnishes: Insulating varnishes are solutions of resinous materials (such as gums, asphalts etc,) in volatile liquids (such as gums, asphalts etc.) in volatile liquids (such as petrol, benzene, alcohol etc.). There are three type of insulating varnishes namely (i) impregnating (ii) finishing or coating and (iii) sticking or bonding :

Impregnating varnishes serve to treat porous fibrous or solid insulators used in electrical machines and apparatus.

Finishing or coating varnishes produces and hard lustrous coating anon materials which are more or less resistant to air moisture.

Sticking or boding varnishes are used to cement cloth, mica, paper etc.

2.    Bakelite: It is synthetic product produced by condensation of phenolic material switch formaldehyde. Its insulating properties are good and are used in manufacturing of all kinds of small electric fittings such as lamp holders, switch covers, terminal boards. Its relative permittivity is 4.5 to 6 and electric strength is 10 to 20 kv/mm.

3.    Paper: It is prepared form wood pulp, rags or plant fibre by a suitable chemical process. It is quite cheap and hygroscopic. It has low capacitance. high dielectric strength and high insulation resistively when dry or impregnated with oil. A small amount of moisture lowers considerably its insulation resistance. Hence before using paper as insulation it must be impregnated in insulating oil. It is used for condensers, cables, laminates, Bakelite sleeves and mica paper.

4.    Mica : It is a mineral product and of finely laminated structure. It is easily split into thin sheets, which are very flexible, tough and high resistance to heat. Its electric strength is very high (80 to 200 kv/mm). It is fire proof and does not abase moisture. Mica splitting etc. Most commonly mica splitting are bonded together by means of bonding varnish and built-up into sheets, plates and various commercial products. It is greatly used in commentators for separating its segments and in condensers as dielectric.

5.    Porcelain: It is made form china clay, fire clay, quartz and feldspar. Its electric strength is 6 to 10 kv/mm and the relative permittivity is 5 to 6.5. It is highly resistant to heat and electric ares and ahs a low moisture absorption. It is used in manufacturing of pin type and suspension or strain insulators, cleats, fuse holders, lamp holders etc.

6.    Rubber: Rubber may be natural and synthetic. Natural rubber is obtained form the milky sap of tropical trees. Synthetic rubber is produced form alcohol or oil products. Its relative permittivity is between 2 and 3 and electric strength is 30 kv/mm. It absorbs moisture slightly, 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 can not be used as insulating material.

7.    Vulcanized India Rubber (V.I.R.) : It is prepared by mixing India rubber with mineral matter such as sculpture, zinc oxide, red lead etc. V.I.R. is tough, more durable and resistive to high temperatures. It is used as insulating and covering or low and medium voltage cable conductors. The main drawback in V.I.R, is that it attacks copper, therefore, before, using V.I.R. as insulation the copper conductor is tinned.

8.    Ebonite: It is vulcanized rubber containing about 20 to 25 percent of sculpture, which is subjected to a high temperature of 1500C for long period. It is a hard substance and can be mounded into different shapes. It is available in boards,rods and tubing. Thought it is brittle still it can be worked, machined and polished. It s widely used for making containers or lead acid batteries, small instrument paneled and terminal mountings.

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