An electrical insulator is a material whose internal electric charges do not flow freely; very little electric current will flow through it under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.
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There are mainly six types of insulator used as electrical transmission and distribution line.
1. Pin type insulator.
2. Post insulator.
3. Suspension insulator.
4. Strain insulator.
5. Shackle insulator.
6. Bushing insulator.
Pin type insulator:-
As the name suggests, the pin type insulator is mounted on a pin on the cross-arm on the pole. There is a groove on the upper end of the insulator. The conductor passes through this groove and is tied to the insulator with annealed wire of the same material as the conductor. Pin type insulators are used for transmission and distribution of communications, and electric power at voltages up to 33 KV. Insulators made for operating voltages between 33 KV and 69 KV tend to be very bulky and have become uneconomical in recent years.
Post insulator:-
A type of insulator in the 1930s that is more compact than traditional pin-type insulators and which has rapidly replaced many pin-type insulators on lines up to 69 KV and in some configurations, can be made for operation at up to 115 KV.
Suspension insulator:-
For voltages greater than 33 KV, it is a usual practice to use suspension type insulators, consisting of a number of glass or porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the top end is secured to the cross-arm of the tower. The number of disc units used depends on the voltage.
Strain insulator:-
A dead end or anchor pole or tower is used where a straight section of line ends, or angles off in another direction. These poles must withstand the lateral horizontal tension of the long straight section of wire. In order to support this lateral load, strain insulators are used. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strings of cap-and-pin (suspension) insulators are used, attached to the cross arm in a horizontal direction. When the tension load in lines is exceedingly high, such as at long river spans, two or more strings are used in parallel.
Shackle insulator:-
In early days, the shackle insulators were used as strain insulators. But nowadays, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.
Bushing insulator:-
Bushing insulator are enables one or several conductors to pass through a partition such as a wall or a tank, and insulates the conductors from it.