Insulating Materials-Properties & Types

Before selecting an insulation system for a particular application, the following properties need to be considered.

  1. Mechanical
  2. Thermal
  3. Environmental including chemical
  4. Electric
  5. Economic

1. Mechanical properties

The mechanical properties considered are the tensile strength, elongation, tensile modulus, compressive modulus, impact strength etc.

2. Thermal properties

Include temperature and time of exposure. In this, the important consideration is that at what temperature one can operate the device for its required lifetime. The insulating materials used must be stable within the allowable temperatures. Some of the thermal properties checked for certain insulating materials are melting point, flash point, volatility, thermal conductivity, thermal expansion and heat resistance.

3. Environmental properties including chemical

The environment includes the effect of air (Oxidation), effect of light, ultraviolet rays, acid and alkali fumes, and humidity.

4. Electrical properties

From the electrical function point of view, the most important is the dielectric strength of insulation. Dielectric strength is that value of voltage which causes the electrical rupture of an insulating material in practical use. Another important property is the dielectric constant, which determines the share of the electric stress which is absorbed by the material. For insulating materials, the loss tangent and resistivity (volume and surface) are also important properties.

5. Economic

Economic may or may not be an important factor for a given application.Types of insulating materials

Types of insulating materials

Basically, there are three types of insulating materials:

1. Gas including vacuum insulating materials
2. Liquid insulating materials
3. Solid insulating materials

Some important insulating materials are

i). Ceramics

ii). Transformer Oil

iii). Askarels

i). Ceramics

A ceramic material is an inorganic, non-metallic, often crystalline oxide, nitride or carbide material. Examples: Garnets, BaTiO3, Ferrites, TiO2 (Rutile), Quartz, ZnS, MgO, SiC, CdS, etc.

Properties of Ceramics

  1. These materials are hard, strong, dense and brittle.
  2. High-temperature stability
  3. Generally inorganic material (absent of Carbon, so it is called inorganic) except SiC.
  4. Generally crystalline except amorphous glass.
  5. These material are nonmetallic oxides, nitrites, and carbides.

Types of Ceramics

1. Porcelain

  • It is having ε< 12.
  • It is used in low and high voltage applications.
  • It is used in insulation at transmission and distribution of power system.

2. Steatite

  • It is having ε< 12.
  • It is used in high-frequency applications.

3. Alumina

  • It is having ε< 12.
  • It is used in High-frequency applications.
  • It is used in Circuit breaker and resistance cores.

4. Titanate

  • It is having εr > 12.
  • It is used in capacitor applications due to high dielectric constant.

Another Classification of Ceramic

1. Ceramics having ε< 12 used as an insulator. Examples: Alumina, porcelain, steatite.

2. Ceramics having ε> 12 used in capacitor application because of high dielectric constant. Examples: Titanate & Rutile.

ii). Transformer Oil

  • Transformer cores are dipped in mineral oil known as transformer oil.
  • It acts as an insulator.
  • It acts as a cooling medium.
  • Parameters determining the properties the transformer oil.

Parameters of Transformer Oil

  1. Viscosity: It is the resistance of flow, at normal condition. It should be low.
  2. Specific Resistance: This is the measure of DC resistance between two opposite sides of one cm3 block of oil.
  3. Pour Point: It is the minimum temperature at which oil just starts to flow under standard test condition.
  4. Flash Point: It is the temperature at which oil gives enough vapors to produce a flammable mixture with air.


  • The breakdown voltage of transformer oil decrease with contamination. Example: moisture.
  • To absorb moisture, absorbents added are “Silica gel” and “Alumina“.

iii). Askarels

  • They are fire resistant insulating material.
  • Two types of askarels i.e. Chlorinated benzene and Chlorinated biphenyl.
  • Nowadays they are not used in transformer and capacitor because on decomposition they produce toxic and poisonous gases.

Insulating Materials for Cables & Electronic Equipment

Electrical Machine Insulation

Insulating Materials for Power & Distribution Transformers


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