As we have already known that, every pole or tower is earthed, we also need some insulation between the live conductor and earth. Insulators are doing the same, i.e. isolate the live conductor from support as well as earth.

Properties of insulator:
(i) High resistance, so that it can resist the current flow between support and conductor.
(ii) High mechanical strength to withstand conductor weight and wind pressure.
(iii) High dielectric strength.
(iv) High ratio of puncture strength to flashover.
(v) Materials should be non porous.
(vi) Materials should be free from impurities.

Insulator materials:

The materials used for insulator are (a) porcelain, (b) glass, (c) synthetic resin.

(a) Porcelain: This is the most common ceramic material used for insulator. To make the porcelain insulating material, aluminium silicate is mixed with plastic kaolin, feldspar and quartz. For using as insulator, porcelain should be free from holes, cracks, porosity etc. For minimize dust and moisture, porcelain should be glazed and heat treatment also done. Its dielectric strength is 60kv/cm.

(b) Glass insulator: Now a days glass is also a common material used for insulator instead of porcelain. It is mainly used for EHV A.C. and D.C. systems. Annealed tough glass is used for insulating purposes where glass is toughened by heat treatment. Due to its transparency crack, defects within it are easily noticeable. It is cheap, has high dielectric strength, high tensile strength, high resistivity as compared to porcelain, whereas it is less strong and high moisture condensation. Its dielectric strength is 140 kv/cm.

(c) Synthetic Resin: It is widely used in indoor system. Compound of silicon rubber resin is the material of this type of insulator. It has high strength and lower weight. It is cheaper also. Its high leakage current and low longevity placed it in application like limited type of bushing.

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