Insulators

Insulators are substance which does not readily conduct electricity or Insulators are those substances which offer such a huge resistance. Di-electric strength is the term used for comparing insulation property of an insulator.
 Properties
An ideal insulator has the following properties
a)      High Resistivity
Insulators are those substances which offer such a huge resistance, that they allow practically no electricity.
b)      Di-electric strength
All insulator conduct electricity if subjected to apply extreme voltage. The voltage at which the material lost his insulating property is known as di-electric strength or breakdown voltage.
c)      Mechanical strength
The insulating material must be mechanically strong enough to carry tension and weight of conductor
d)     Ability to withstand moisture
Their must not be any entrance on the surface of electrical insulator so that the moisture or gases can enter it.
e)      Withstand at high temperature
There physical as well as electrical properties must be less affected by changing temperature.


Commonly Used insulation Materials

Cotton
            Cotton is used very largely for low voltage insulation's. Cotton is much easier to handle and also serves mechanical support for other insulating materials. 
Silk
Silk is an excellent insulator. It is quite expensive and it is used in very small instruments.
Paper
            Electrical insulation paper is paper type that is used for electrical insulation's. The paper is made from manila fibers, wood pulp and cotton range.
Rubber
            Rubber is a good insulator; it is used outside of wires that are used to conduct electricity. Rubber provides protection from the dangers of electrical charges being carried through the metal conductor. The mechanical properties of rubber can improved by addition of sulphur (vulcanizing). 

 Mica
            Mica is a mineral consisting chiefly of a double silicate of aluminium and potassium. It is extremely stable when exposed to moisture and extreme temperature while maintaining superior electrical properties as an insulator.

Insulating Oil
            Mineral oil obtained from refining petroleum possesses good insulating properties. This oil has dielectric strength of 200 KV/cm. It is used in oil as transformer, some types of high voltage capacitors, fluorescent lamps ballast, switch gears, etc. It is also used for cooling purpose.
Varnish
            Varnish is an insulating material used to give protection against moisture and chemical corrosion environment

Enamel
            Enamel is applied to the wires and strips, which are smaller in the cross-section. Thin enamel can give dielectric strength 600 KV/cm.

Porcelain
Porcelain has very good electrical and mechanical properties as an insulator. It is used on overhead lines for bus-bar, switch-gear support and terminal bushing for transformer. It is made by chiefly of china clay, quartz and feldspar. Porcelain has a dielectric strength of 4-10 kV/mm.


Bakelite
It is also known as polyoxybenzylmethylenglycolanhydride. It is a thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. It is used in radio and telephone casing, for making switches, etc.

Insulators used in overhead line
Pin type insulator
            Pin type insulator are used for the transmission of lower voltages (up to 11 kV). A pin type insulator is attached above the cross arm. The transmission line conductor passes through the groove on head of insulator  
                    

Suspension type Insulators
            They are cost effective for higher voltage transmission. A suspension type insulator is suspended below the cross arm. It consists of number of disc connected in series with the help of a metal link. They are three types
1)      Cemented cap suspension insulator
2)      Interlinking/Hewlett suspension insulator
3)      Link/Core suspension insulator
Strain type insulators
            They are horizontally placed suspension insulator. It is used to handle mechanical stress and take pressure off a conductor at the end.


Shackle type insulator
            The shackle insulators are used on sharp curves, end poles and in section poles. A shackle insulator is used for low voltage distribution lines

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