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
- 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.