Friday, March 06, 2015

What Is Dielectric Strength ?


 What is a dielectric strength?

In an insulator or dielectric, the valence electrons are tightly bound so that no free electrons are available to conduct current. But when a potential or voltage is applied to the material, there comes a point where these electrons may break away and the material could loose its insulating properties.

What is a breakdown voltage?

Breakdown voltage is the maximum voltage which a unit of thickness of a dielectric can withstand without being ruptured.

In one way or the other dielectric strength and breakdown voltage are used interchangeably.
The dielectric strength is measured in kV/mm or kV/cm. For example when a material has a dielectric strength of 25kV/cm, it means that the maximum voltage or potential difference which 1 cm thickness of material can withstand without breaking it. When the voltage exceeds 25kV then a large value of current will flow and it will rupture the said material.
  
Typical insulators with high dielectric strength
  • Air - 30 kV/cm
  • Paper (oiled)- 200 kV/cm
  • Paraffin - 350 kV/cm
  • mica - 500 kV/cm
  • glass - 1000 kV/cm
Uses of materials with high dielectric strength

  • The mica is used as a good insulator for electric motor windings and its stator bars.
  • For high voltage transformers and transmission line connectors, glass and porcelain are widely used.
  • Naptha or paraffin oil is used when it is necessary that the insulator should be in liquid form, like in the case of transformer and large circuit breakers.

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