Dielectric strength

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Insulating oil in a breakdown test

The dielectric strength (usually expressed in kV / mm ) of an insulating material is that of electric field strength which must prevail in it at most, without causing a dielectric breakdown ( arc or spark occurs).

Their value depends on various factors and is therefore not a material constant.

background

The dielectric strength of an insulating material is the electrical field strength E at which the material breaks down. It is accordingly also referred to as the breakdown field strength. It is calculated from the breakdown voltage U related to the thickness d of the insulation:

The dielectric strength that can be achieved in practice is significantly influenced by the shape of the field. The conductor geometries and inhomogeneities in the insulating material have the greatest influence on this. This also results in the effect that thin foils have a significantly higher dielectric strength than thick barriers.

Even enclosed air spaces have an effect that reduces the permanent dielectric strength in the case of alternating voltage. The cause is so-called pre - discharges , which ionize the air and the surrounding insulating material is permanently damaged by ultraviolet radiation .

Insulating materials often even have lower insulation strengths along their surface than the surrounding air ( creepage resistance ), which can lead to creeping or sliding discharges . An insufficiently large solid insulation barrier can therefore also be characterized by its air and creepage distances, in particular if the insulation material has a high dielectric strength. There is no connection between the tracking resistance and the dielectric strength. The required creepage distances are often 100 times longer than the material thickness required for insulation. The water absorption capacity of the material has an influence on the tracking resistance and also on the dielectric strength.

Dielectric strength of air

The breakdown voltage U L in the unit kV of air can in many cases be approximated for direct voltage in the range with the following empirical equation derived from Paschen's law :

With the air pressure p in the unit bar , the temperature T in Kelvin and the thickness d in meters. For a thickness of 1 cm, for example, at normal pressure and 20 ° C., a breakdown voltage of 30.3 kV results , that is, a breakdown strength of 3 kV / mm.

Material values

The procedure for determining the dielectric strength is defined in the IEC 60243 series of standards. It specifies test conditions for the various material classes and applications (Part 1: AC , Part 2: DC , Part 3: Impulse voltage ). Usually a series of similar specimens is tested and then the median of the individual values ​​is given. Such values ​​are, however, only guidelines, as the dielectric strength of other parameters, such as the exact composition and purity of the materials, type of electrical current, the time at which the voltage is applied (speed of the increase in the electrical field) as well as the size and shape depends on the electrodes used. If a high field strength acts on the insulator over a longer period of time, its conductivity increases due to heating and a decrease in dielectric strength can be determined. In the case of gases such as air and other materials, it depends in particular on the air humidity and the air pressure and therefore varies greatly depending on the type of prevailing gases and under non-constant conditions. In addition, the dielectric strength decreases with increasing temperature and increasing frequency . In the case of air insulation, the distance is called the air gap, which for reliable insulation must be sufficiently large compared to the value resulting from the dielectric strength. However, see also spark gap .

In addition, the breakdown voltage of many substances is not proportional to the thickness, since an inhomogeneous field distribution can occur, especially with direct voltage. Therefore thin foils have higher dielectric strengths than large material thicknesses. In the case of high-voltage film capacitors, this is exploited by using what is known as an internal series connection, in which the dielectric consists of several layers of insulating material arranged one above the other, separated from one another by non-contacted metal layers. This homogenizes the field distribution.

In the case of very small thicknesses, even low voltages that are insufficient for ionization generate the highest field strengths. For example, the 5 nm thick plasma membrane of neurons in the resting potential has a field strength of 200,000 volts / cm. Electroporation (collapse of the double lipid layer ) only occurs at field strengths in the range from 300 kV / cm to 700 kV / cm.

Dielectric strength of selected materials (20 ° C)
material Dielectric strength
[kV / mm] 		Physical state
dry air ( normal pressure , DC ) 3 gaseous
Air (assuming long throw distances) 0.1 gaseous
Air effective (without peak) 0.35 gaseous
Helium (relative to nitrogen) 0.15 gaseous
porcelain 20th firmly
Hard-paste porcelain 30-35 firmly
Sulfur hexafluoride > 8 gaseous
Glass (textile glass) > 8 firmly
enamel 20-30 firmly
Quartz glass 25-40 firmly
Borosilicate glass 30th firmly
Distilled water 65-70 liquid
Aluminum oxide (pure) 17th firmly
Polycarbonate (PC) 30th firmly
Polyester (glass fiber reinforced) 12-50 firmly
Polyethylene terephthalate (PET) 20-25 firmly
Polymethyl methacrylate (acrylic / plexiglass) 30th firmly
Polyoxymethylene (POM) 40 firmly
FR4 (glass fiber reinforced plastic) 13 firmly
Polypropylene (PP) 52 firmly
Polystyrene (PS) 20-55 firmly
FR2 (hard paper) > 5

short term: 19.7

firmly
Transformer oil (carefully dried) 05-30 liquid
Polyvinyl chloride 30th firmly
Polytetrafluoroethylene (PTFE) 18-105 firmly
Acrylonitrile-butadiene-styrene - copolymer (ABS) 24-40 firmly
Polyoxymethylene > 20 firmly
Neoprene 15.7-26.7 firmly
mica up to 60 firmly
High vacuum 20–40 depending on the shape
of the electrodes 		-
diamond 2000 firmly

Individual evidence

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  40. ↑ Acrylonitrile-butadiene-styrene copolymers (ABS) data sheet. (PDF) Retrieved April 9, 2017 .
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