Electrical Properties of Dielectric Materials
The term "Dielectric strength" of a dielectric material means the maximum electric field strength that it can withstand intrinsically without breaking down i.e.; without knowing about failure of its insulating properties. The hypothetical dielectric strength of a material is an intrinsic property of the bulk material and is not dependent on the configuration of the material or the electrodes by which the field is applied. At breakdown, the electric field release bound electrons, converting the material into a conductor. The field strength at which breakdown takes place in a given case is dependent on the respective geometries of the dielectric (insulator) and the electrodes with which the electric field applied; as well as the rate of increase of applied electric field. The dielectric strength is otherwise called as the breakdown voltage. All electrical apparatus are intended to work at particular voltage. If this operating voltage is slowly increased, then at some voltage, the dielectric used in the apparatus gives away. Therefore, the voltage below which the dielectric material remains stable in its property, but above which it result in the destruction of insulating properties is called "Breakdown voltage". In other words dielectric strength is defined as the property of an insulator which enables it to withstand continuous electric stress or the maximum electrical stress which it will successfully withstand. It is expressed in terms of KV/mm or kV/cm under specified conditions.
Factor Affect acting Dielectric structure:
(1)Dielectric strengths decrease with rise of temperature in case of air. In case of liquid insulators the effect with the type of oil and its viscosity.
(2)Humidity generally decreases the value of dielectric strength.