The materials which don’t allow the electric current to pass from them.

They less then 4 valance electrons.

• These are used to prevent the unnecessary flow of current .
• They play a major role in electrical system.
• They provides a high resistive paths to control flow of current
• Commonly used in transmission line and distribution lines

 

Properties of insulators

• They must mechanically strong enough to carry tension and weight of conductor.
• They must have very much high dielectric strength to withstand voltage stress of high voltage system.
• It must poses high insulation resistance to prevent leakage current to earth.
• It should be free from unwanted impurities.
• It should not porous.(air bubbles should not be present on the surface of insulators).
• They should not be effected so much by changing temperature.

 

 

Types of insulators w.r.t their material

Insulator are mainly distributed into three basic type based on their materials.

Porcelain insulators

Mostly commonly used type.

Porcelain is basically aluminium silicate.

Its surface is hard and glazed .

It should be free from porosity.

It should be free from impurity and air bubble inside material.

Glass insulators

• Annealed tough glass is used to make it

Advantages

• It has very high dielectric strength compared to porcelain
• Its resistivity very high as compared to porcelain.
• It has low coefficient of thermal expansion.(there should be no enhancement in size of conductor due to high temperature).
• Higher tensile strength as compared to porcelain.
• As transparent material, it is not heated by sunlight.
• Long service life and cheaper.

 

 

Comparison between glass in porcelain

glass	porcelain
Dielectric strength=140kv/cm	Dielectric strength= 60kv/cm
Tensile strength=35,000kg/cm2	Tensile strength=500kg/cm2

 

Polymer insulators

It is also known as composite insulators.

Basically consist of two parts

1. Glass fiber rod shaped core(to provide the mechanical support to sheds)
2. Silicon rubber made weather sheds (to protect core from outside environment)

Advantages

• Light weight
• Much flexibility then porcelain and glass.
• Cheaper then other (porcelain ,glass)
• Cleaning is easy