List four examples of insulating material and explain any two.
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Examples of insulating materials: 

i) Solid: Ceramic, Porcelain, Mica, Glass, Rubber, Resinous, Fibers 

ii) Liquid: Synthetic, Mineral etc. 

iii) Gaseous: Hydrogen, Air, Nitrogen, Sulphur-hexa-fluoride 


a)Porcelain: Porcelain is widely used material in electric fields. In mineral form it can mix with water and when it is wet it can be easily shaped. After backing it becomes water resistant and acquires mechanical strength. Porcelain is made from china clay and quartz. Its compression strength is 5000 kg/cm2 and tensile strength is 400 kg/cm2 . Specific weight is 2.3 to 2.5 gm/cm2 . It is water and heat resistant but at a very high temperature it deteriorates sharply. Its resistivity is high. Chemical effect on it negligibly small. 

Use: it is mostly used for making different types of insulators, bushings, oil C.B., disconnecting switches, Arresters, plugs, fuses, bodies and mounting plates. 


b)Glass: It is manufactured by fusing silica (sand), alkali (potash, soda), and base (lead oxide or lime). Properties of glass depend on the composition and heat treatment. Its compression strength is 6000 to 21000 kg/cm2 and tensile strength is 100 to 300 kg/cm2 . Specific weight is 2 to 8.1 gm/cm2 . Silica has high insulating properties, high heat resistant and hydraulic strength. 

Uses: As dielectric in capacitors, Light and electron tubes, filament support, Various kinds of insulating supports, antennas, bushings etc. 


c)Mica: Mica is mineral substance obtained from earth, requires no thermal and chemical process and can directly be used. Its resistance is very high and mechanical strength is also very high. It is moisture resistant, heat resistant, also has good elasticity. At high temperatures it retains its electrical and mechanical properties. 

Uses: Insulation in commentator, insulators in heating equipments, also used in stator and rotor windings of electric machines. 


d)Varnishes: These are solutions of certain materials like resins, bitumen, drying oils and some base. When thin film of varnish is applied on the solid surface it dries up and forms hard film. It has high insulating properties and low hygroscopicity. 

Uses: Insulation in electric field, transformer stampings, armature, pole stampings

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