Dielectric strength

The dielectric strength ( usually measured in kV / mm) of an insulator is that electric field strength, which may prevail in the material at the most, without causing a voltage breakdown ( arcing ).

Background

When dimensioning of electrical assemblies and insulation must also be given to ensuring that, especially with pointed conductor geometries the surrounding air can be ionized by so-called pre-discharges and the breakdown occurs thereby more or surrounding insulation materials are permanently damaged by ultraviolet radiation. Also enclosed air spaces have a similar effect.

Furthermore have insulation materials along its surface often lower insulation strength than the surrounding air on (tracking ), which can lead to creep or creeping. A non-closed solid insulation barrier is therefore also characterized by their air and creepage distances. The creepage distances must be much longer with pollution and the influence of moisture in particular, as the sparking distance in air, increasing the creepage length saving through slits, ribs or corrugation. Possible leakage paths can be avoided by water-repellent impregnation or coating.

The dielectric strength E of an insulation material corresponds to an electric field strength, it is therefore also referred to as breakdown field strength. It is calculated from the breakdown voltage U relative to the thickness l of insulation:

Material values

The values ​​given are only approximate, since the dielectric strength of other parameters, such as the precise composition and purity of the materials and the time of exposure to the voltage dependent. In air, and other materials, it is particularly dependent on the humidity and the air pressure. When air insulation distance is called the air gap that needs compared to that resulting from the breakdown strength value be sufficiently large for safe isolation. See, however, the spark gap.

Moreover, the breakdown voltage is of many substances not proportional to the thickness, since it can occur, especially inhomogeneous field distribution in DC. Therefore, thin films have higher dielectric strength than large material thicknesses. At high-voltage foil capacitors are used for this, by applying a so-called internal series connection, in which the dielectric comprises a plurality of superposed Isolierstofflagen which are separated by non-contacted metal layers. Wherein the field distribution is homogenized.

At very low thicknesses produce even small voltages that are not sufficient for ionization, the highest field strengths. So is the working of brain cells, the field strength in the few nm thin membrane at 10,000 kV / mm, electroporation occurs only at 10 times the field strength.