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Classification of Insulating Materials

Updated on August 28, 2014
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Electrical and Automation Engineer . Specialized in LV Switchgear Design and process automation

Insulating materials are non metallic materials that does not allow electric current to pass through it at normal temperatures. There are number of insulating meterials used in Electrical apparatus and machinery. They may be organic or inorganic, uniform or heterogeneous in composition, natural or synthetic. there are may properties such as resistivity, breakdown voltage, thermal stability etc that determine the suitability of a material to be used as insulating material.

Varnish insulated copper coils

Varnish insulated copper coils are used in air cooled transformers. In oil immersed transformers paper insulated coils are used
Varnish insulated copper coils are used in air cooled transformers. In oil immersed transformers paper insulated coils are used

Properties of insulating materials

An ideal insulating material must should have the following properties.

  1. High dielectric strength at the specified temperature
  2. high resistivity
  3. low dielectric hsteresis
  4. Good thermal conductivity
  5. good moisture withstanding capacity
  6. it should be chemically inert
  7. It should be able to withstand vibration, abrasion and bending

Classification of Insulating materials


Insulating materials can be classified in two way.
1. Based on their material
2. Based on their temperature withstand capacity

1. Classification based on substances and materials.

State
Materials
Examples
Solid Insulating Materials
Fibrous materials
wood, paper and card board, insulating textiles
 
Impregnated fibrous materials
impregnated paper, varnished or impregnated textiles
 
Non resinous materials
asphalts and bitumens, waxes
 
Ceramics
porcelain, steatite, alumina, titanate, etc.
 
Glass
fused quartz or silica glass, Pyrex, fiber glass
 
Natural and synthetic rubbers
natural rubber, hard rubber, butyl rubber, neoprene, hypalon, silicon rubber
 
 
Mica and its products; Asbestos and its products
Liquid Insulating Materials
Oils
Refined hydrocarbon minerals oils, Linseed oil, silicon liquids, vegetable oils etc.
 
Varnishes
synthetic varnishes and spirit
Gaseous Insulating Materials
 
Carbon dioxide (CO2), Dry air, argon, nitrogen, etc.

Temperature withstand capacity

Class
Temperature (in degree celsius)
Y
90
A
105
E
120
B
130
F
155
H
180
C
ABOVE 180

2. Based on their temperature withstand capacity

Based on the thermal stability the insulating materials can be classified into seven classes as tabulated. These classes cover the insulating materials generally used in electrical machinery based on their thermal stabilty. Materials with temperature limits lower than Class Y are not generally used as insulation for electrical machines transformers, switch gears etc. But it can be used in equipments with low temperature limits. Such materials are not included in this classification.

Insulating paper

Class Y

This class consists of materials, or combinations of materials that could withstand a temperature of 90oC. Cotton, silk, paper, cellulose, wood etc.neither impregnated nor immersed in oil comes under this class. Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class. Materials that come under this class are not preferred much for electrical machinery and apparatus as they deteriorate rapidly and are extremely hygroscopic.

Class A

This class consists of materials, or combinations of materials that could withstand a temperature of 105oC. Cotton, silk and paper suitably impregnated with natural resins, cellulose ester or immersed in oil come under this class. Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class.

Transformer winding insulated using paper

Paper insulation is used only in oil immersed transformers. In air cooled ones varnish insulated coils are used.
Paper insulation is used only in oil immersed transformers. In air cooled ones varnish insulated coils are used.

Cotton insulated wires

Class E

This class consists of materials, or combinations of materials that could withstand a temperature of 120oC. Synthetic resin enamels, cotton and paper laminates with formaldehyde bonding etc come under the Class E. Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class.

Mica sheets used for insulation

Class B

This class consists of materials, or combinations of materials that could withstand a temperature of 130oC. Mica, glass fibers, asbestos with suitable bonding substances comes under this class. Built up Mica, glass fibers, asbestos laminates also come under this class . Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class.

Glass insulated materials

Class F

This class consists of materials, or combinations of materials that could withstand a temperature of 155oC. Mica, glass fibers, asbestos Built up Mica, glass fibers, asbestos laminates etc. with suitable bonding substances of high thermal stability come under this class. Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class.

Class H

This class consists of materials, or combinations of materials that could withstand a temperature of 180oC. Materials such as Silicon elstomer and materials like Mica, glass fibers, asbestos with suitable bonding substances such as silicones come under this class. Built up Mica, glass fibers, asbestos laminates also comes under this class . Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class.

Class C

This class consists of materials, or combinations of materials that could withstand a temperature above 180oC. Mica, ceramics, glass, quartz without binders or with silicon binders of higher thermal stability come under this class. Other materials, or combinations of materials that could withstand the specified temperature can also be included in this class. Specific materials in this class will have a temperature limit which is depends upon their physical, chemical and electrical properties.

Class
Materials
Y
Cotton, silk, paper, cellulose, wood etc.neither impregnated nor immersed in oil
A
Cotton, silk and paper suitablly impregnated with natural resins, cellulose ester or immersed in oil
E
Synthetic resin enamles, cotton and paper laminates with formalidehyde bonding etc
B
Mica, glass fibres, asbestos with suitable bonding substances built up Mica, glass fibres, asbestos laminates
F
Mica, glass fibres, asbestos Built up Mica, glass fibres, asbestos laminates etc.
H
Silicon elstomer and materials like Mica, glass fibres, asbestos with suitable bonding substances such as silicones
C
ceramics, glass, quartz without binders or with silicon biners of higher thermal stability

The classification Based on temperature withstand capacity is made for the materials that are widely used in electrical machinery and apparatus. There are thousands of such insulating materials that have good temperature withstand capability. But they are not widely used.

Note: The above classification based on temperature withstand capacity as per Indian Standard Publication No. 1272.


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    • profile image

      Mike 

      7 months ago

      Have a number of 10MVA transformers all using paper as winding insulation paper. All seem to be degrading at near constant rate. Could an improvement be seen if better insulating material(s) be used and less degradation expected??

      Thanks,

      Mike

    • profile image

      megha 

      21 months ago

      Nice answer

    • profile image

      David benny 

      2 years ago

      it's very good to understand.

      THANKS FOR THIS TOPIC:)

    • profile image

      reema patel 

      2 years ago

      very useful

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