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Introduction

For safe and satisfactory operation of all electrical and electronics equipment insulator plays important role. Basically current carrying wires, surfaces need to be covered with insulating material. Let us see the structure of the material on the basis of energy band. In this type of material, the highest occupied energy band (Valence Band) is completely filled. The next higher band (Conduction Band) is quite empty.(Fig.1) The gap between these two bands is too large. When the electric field is applied across these materials, the electrons from valence band cannot reach the conduction band and conduction of electron stops. Such materials are known as insulators. Diamond is an example of this kind of material with a separation of nearly 6eV between valence band and conduction band.

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CLASSIFICATION OF INSULATING MATERIAL

The insulating materials used for various applications in electrical engineering are classified in three categories:
Insulating gases
Liquid insulating material
Solid insulating material

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Insulating gases �

A dielectric gas, or insulating gas, is in gaseous state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric gases are used as electrical insulator in high voltage applications, e.g. transformers, circuit breakers (namely sulfur hexafluoride circuit breakers), switchgear (namely high voltage switchgear), radar waveguides, etc.

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�Liquid Insulating Material �

These materials are used for dielectric purpose to eliminate the air and other gases Insulating liquids are organic liquids used as coolant. These are categorised according to temperature range where they are used. It is used in transformers, circuit breaker, bushings, cables, capacitors etc. along with solid insulants to operate with an acceptable performance. An ideal insulating liquid material must have following properties:

High dielectric strength, impulse strength and volume resistivity.
Low dielectric dissipation factor.
High or low dielectric constant.(depending upon application)
High specific heat and thermal conductivity.
Excellent chemical stability and gas absorbing properties.
Low viscosity, density, volatility and solvent power and high flash point.
Good arc quenching properties.
Non-flammable and non toxic

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Solid Insulating Material

Solid insulating materials may be natural organic substances like paper, cloth, rubber, etc. or inorganic materials like Mica, Glass, or synthetic materials like plastics.

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Properties Of Insulating Material

The suitability of an insulating material for a specific purpose use can be decided by knowing its different properties. So we have to know the exact requirement of the application and the required property hold by the insulating material. Based on uses in different applications following properties of materials are useful.

Electrical Properties: The insulating material used in electrical or electronics appliances, should be considered for following:
Insulation resistance
Dielectric constant or permittivity
Breakdown voltage or dielectric strength
Dielectric loss

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Visual Properties:
An insulating material possessing two opposite properties: transparency and thermal insulation is suitable in case of reduction of energy consumption for heating and air conditioning and electrical energy savings. This is known as visual properties. Study of appearance, color and crystalline structure are the measures of this property. Glass, Aerogel hold the required visual properties. Aerogel is used in case of highly energy efficient windows.

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Mechanical Properties:
Mechanical properties such as tensile strength, impact strength, toughness, hardness, elongation, flexibility, mechanical strength, abrasion resistance etc. are to be considered for choosing the insulating material.
3. 1. Mechanical Strength:

The insulating material should possess sufficient mechanical strength to respond mechanical stress. Mechanical strength is affected by following factors.
Temperature rise: It badly affects the mechanical strength of the insulating material.
Humidity: It is the climatic effect which affects also the mechanical strength.
Porosity: An insulating material of high porosity will absorb more moisture and thereby affects the electrical properties as well as mechanical strength.

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Thermal Properties:
Following thermal properties are considered for selecting insulating material of different applications.
Thermal stability: The insulating material must be stable (no change in physical state) within the allowed temperatures. Certain materials like wax and plastic get soft at moderate temperatures. So the mechanical property of the material is affected. Hence the operating temperature of the material is to be noted before its use.
Melting point: The insulating material should have melting point (temperature bearing capacity without being melt), above that of operating temperature

Flash point: This is an important property of insulating oils used in transformer. Flash point of a liquid insulator is that temperature at which the liquid begins to ignite.
Thermal conductivity: In electrical appliances heat is generated during operation, which should be transferred to atmosphere, to maintain the operating temperature within the limit. Hence the insulators should have very low thermal conductivity
Thermal expansions: Rapid and repeated load cycle on electrical appliances cause corresponding expansion and contraction of the insulators. In a result voids are created and affect the breakdown phenomenon. Thus two insulating material of different coefficient of thermal expansion should be wisely selected.
Heat Resistance: The insulating material used must be able to withstand the heat produced due to continuous operation and remain stable during the operation. At the same time it should not damage the other desired properties.

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Chemical Properties:
Certain chemical properties are also required to be considered for the insulating materials.
5. 1. Chemical Resistance: It is the ability of the insulating material to fight against corrosion in the presence of gases, water, acids and alkalis. For materials which are subjected to high voltage, high chemical resistance is also necessary.
5. 2. Hygroscopity: Many insulating materials are hygroscopic. Sometimes the insulation may come in direct contact with water. The porous materials are more hygroscopic than dense ones. Small amount of moisture absorbed by an insulating material affects its electrical properties drastically.
5. 3. Moisture Permeability: The tendency of an insulating material to pass moisture through them is known as moisture permeability. Moisture can penetrate through very small pores as the size of water molecule is very small. So this property is vital for selecting the protective coating, cable sheaths etc.

6. Ageing: Ageing is the long term effect of heat, chemical action and voltage application. These factors decide the natural life of insulators and hence of an electrical apparatus.

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