line insulators.

Here present the how the work of line insulator and types of insulators.

The insulators for overhead lines provide insulation to the power conductor from the ground.
The insulators are connected to the cross arm of the support structure and the power conductor passes through the clamp of insulator.
These insulator are mainly made of either glazed porcelain or toughened glass.
The materials used for porcelain art silica 20 % feldspar 30 % and clay 50 %.
It is desirable that porcelain to be used for insulators should be air free and non porous.
The dielectric strength of porcelain should be 15 to 17 kV for every one tenth inch thickness.
As porcelain is mechanically strong, less affected by temperature and has minimum leakage problem hence suitable for overhead transmission line.
Toughened glass is also sometimes used for insulators because it has higher dielectric strength (35 kV for one tenth inch thickness) which makes it possible to make use of single piece construction whatever be the operating voltage.
As the line insulators are exposed to atmosphere, dust and dirt deposits on the surface of the insulator which gradually increase the leakage current.
In order to avoid breakdown due to leakage current, clearance between two ends of the insulator should be sufficient.
The shortest distance between the two conducting parts along the surface of the insulating material is known as creepage distance.

Basically there are five types of insulators used for overhead transmission line.

this type of insulator is commonly employed for operating voltage up to above 25 KV.
Pin type insulator consists of a single or multiple shells adapted to be mounted on a single spindle to be fixed to the cross arm of the supporting structure.
For supporting the insulator steel bolt or pin secured to a cross arm on the transmission pole is used.
There is groove on the upper end of the insulator for housing the conductor.
The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.
Beyond operating voltage of 33 KV the pin type insulator becomes too bulky and hence uneconomical.

2. Suspension type of insulator:

The pin type of insulator are uneconomical beyond 33kV operating voltage. Also the replacement of these insulators is experisive.
For these reasons for insulating ovehead lines against high voltages, suspension insulators are used. When it is also called as string insulator when number of units are connected in series.
These insulators consists of the one or more porcelain units flexibly connected together in series by metal links in the form of a string.
Upper end of the string is secured to the cross arm of the tower.
Each disc or units is designed for 11 kv.

3. Strain type insulators:

Such insulators are used when there is a dead end of the line or there is corner or sharp curve where line is subjected to greater tension .
For low voltages up to 11kV shackle insulators is generally used But for high voltages a string of suspension insulators may become necessary.
These strings are placed in the horizontal plane rather than the vertical plane as is done in case of suspension insulators.

4.shackle insulators:

In early years, the shackle insulators were used as strain insulators.
But now a days their application is restricted only for low voltage distribution lines.
These insulattors can be used either in a vertical or horizontal position.
The shackle insulator can be fixed to the pole with bolt or to the cross arm directly.
Generally U shape clamp or D clamp is used to position the shackle insulators.
The conductor in the groove is fixed with a soft binding wire.

5. Guy / stay stain insulators:

Guy insulator are placed to prevent the lower part of the guy from becoming electrically energized by of the upper part of the guy when the conductor snaps and falls on them or conductor due to leakage.
No guy insulator shall be located less then 3.50 due to leakage.
A type of insulators are to be used for L.T. line guys.
One C type of insulator is to be used for 11 kv guys.
Two C type of insulators are to be used for 33kv.