Study of Insulators- cotton insulator, Enamel, insulating oil, mica, paper insulator, Pin type insulator, Porcelain, Properties of insulators, rubber insulator, silk, Strain insulators, Suspension type Strain type Insulators, Varnish,

1. 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.
f) Low Permittivity

2. Commonly Used insulation Materials
Cotton
Cotton is used very largely for low voltage insulations. 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 insulations. The
paper is made from manila fibres, 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.

3. Insulating Oil
Mineral oil obtained from refining petroleum possesses good insulating properties.
This oil has dielectric strength of 200KV/cm. It is used in oil as transformer, some types of
high voltage capacitors, fluorescent lamps ballast, switch gears, etc.
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.

4. 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.

5. Insulators used in overhead line
Pin type insulator
Pin type insulator is 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

6. 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.