Types of insulator:

Different types of insulators are
Pin insulator: This type of insulator normally used in low voltage line. It is made by porcelain and connected with cross arm by galvanized steel pin and nut. The groove, on the upper portion of the insulator, is used for housing the conductor. Up to 11 kv , single shell pin insulator is used. For voltage above 11 kv more than one shell is used. The shell which acts as an umbrella for rain, sometimes called rain shed. Pin insulator is not used for voltage above 33 kv, though construction of three or four piece insulator is possible, but the size may become bulky as well as high weight which is not economical too.

Shackle insulator: Normally this type of insulators used for low voltage side in case of section pole or terminal pole. Galvanized shackle strap is used for connecting the shackle insulator. This type of insulator used both for horizontal and vertical position. In early days shackle insulators were used as strain insulator.

Post insulator: This is a type of pin insulator with single shell. A special type of pin is used for this type of insulator. It is used for supporting bare bus bar insulator. It is made by resin, sometimes porcelain and a metal base with a metal cap. It can be used at high voltage by connecting one by one is called as insulator stack.

Stay or guy insulator: By this type insulator stay are insulated from ground. This insulator is connected above 3 meter from ground. So in case of accident, when stay wire is connected to live conductor, the possible touch area of men and animal is insulated. This insulator is made by porcelain.

Disc insulator: For voltage above 33 kv disc insulators are used. For high voltage line, disc insulators are used for terminal and section tower. This is mainly made by porcelain and used at any high voltage by connecting disc one by one with metal link. The rated voltage of single disc is 12 kv, where flashover voltage are 80 kv and 40 in dry and wet atmosphere respectively. Its puncture voltage is 120 kv and tension is 2000 to 4000 kg up to 33 kv. Tension is 9000 to 16500 at 132,220,400 kv system. Ball socket or clevis eye type insulators are used in case of disc insulator whereas ball socket is popular.

Advantage & disadvantage of disc insulator:

By connecting one by one disc, this type of insulator can be used at any high voltage. Repair is economical as only damage unit can replace.
The main disadvantage is it requires good height of tower and high length of cross arm. The weight of tower should also be high to withstand the weight of disc insulator.
The disc insulators are two types (a) Suspension type insulator, (b) Strain or tension insulator.
(a) Suspension type insulator: These types of disc insulators are used to hang the conductor below the point of support. Connected cap types and Hewlett or interlinking types are the two types of suspension type insulator. This type of insulator is cheaper than pin insulator. It is more reliable and flexible too.
(b) Strain or tension insulator: These types of insulators are used in case of dead ended or intermediate anchor tower. Connected disc here act as tension insulator in horizontal position. For long span two or more strain insulators are used in parallel. In this case two strings are joined by two yoke.

Bushing: Bushing is used to take out live conductor through earthed tank or metallic parts. It is used in transformers, reactors, generators, switchgear etc. It is made by porcelain structure and various types of insulating material like paper, insulating oil, etc. are filled. It is mainly designed for vertical upright mounting. The conductor in form of copper rod or stranded conductor passed inside the bushing.


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