# Dielectric strength

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:

${\ displaystyle E = {{U} \ over {d}}}$

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 : ${\ textstyle 1 \, \ mathrm {mm}

${\ displaystyle U _ {\ mathrm {L}} \ approx 2422 \, \ mathrm {kV} \ cdot {\ frac {d} {1 \, \ mathrm {m}}} {\ frac {p} {1 {, } 013 \, \ mathrm {bar}}} {\ frac {293 \, \ mathrm {K}} {T}} + 60 {,} 8 \, \ mathrm {kV} \ cdot {\ sqrt {{\ frac {d} {1 \, \ mathrm {m}}} {\ frac {p} {1 {,} 013 \, \ mathrm {bar}}} {\ frac {293 \, \ mathrm {K}} {T }}}}}$

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

1. Handbook fiber reinforced plastics: Basics processing applications . Springer-Verlag, 2010, ISBN 978-3-8348-0881-3 , pp. 575 ( limited preview in Google Book Search [accessed April 9, 2017]).
2. Hansgeorg Hofmann, Jürgen Spindler: Materials in electrical engineering: Fundamentals - structure - properties - testing - application - technology . Carl Hanser Verlag GmbH & Company KG, 2013, ISBN 978-3-446-43748-7 , p. 223 ( limited preview in Google Book Search [accessed April 9, 2017]).
3. Wilfried Plaßmann, Detlef Schulz: Handbook of electrical engineering: Basics and applications for electrical engineers . Springer-Verlag, 2016, ISBN 978-3-658-07049-6 , pp. 295 ( limited preview in Google Book Search [accessed April 9, 2017]).
4. FM Bruce: Calibration of uniform-field spark-gaps for high-voltage measurement at power frequencies . In: Journal of the Institution of Electrical Engineers - Part II: Power Engineering . tape 94 , no. 38 , p. 138–149 , doi : 10.1049 / ji-2.1947.0052 ( crossref.org [accessed September 14, 2017]).
5. a b Jane Lehr, Pralhad Ron: Electrical Breakdown in Gases . In: Foundations of Pulsed Power Technology . John Wiley & Sons, Inc., 2017, ISBN 978-1-118-88650-2 , pp. 369-438 , doi : 10.1002 / 9781118886502.ch8 .
6. Leo Gurwitsch: Scientific principles of petroleum processing . Springer-Verlag, 2013, ISBN 978-3-642-47512-2 , pp. 139 ( limited preview in Google Book Search [accessed April 5, 2017]).
7. ^ Hans-Jürgen Bargel, Günter Schulze: Material science . Springer-Verlag, 2013, ISBN 978-3-642-17717-0 ( google.com [accessed June 22, 2016]).
8. Joachim Heintze: Textbook on Experimental Physics Volume 3: Electricity and Magnetism . Springer-Verlag, 2016, ISBN 978-3-662-48451-7 ( limited preview in Google book search [accessed November 18, 2016]).
9. ^ H. Behnken, F. Breisig, A. Fraenckel, A. Güntherschulze, F. Kiebitz: Electrical engineering . Springer-Verlag, 2013, ISBN 978-3-642-50945-2 ( google.com ).
10. Werner Müller, Stephan Frings, Frank Möhrlen: Animal and Human Physiology: An Introduction . Springer-Verlag, 2015, ISBN 978-3-662-43942-5 , pp. 358 ( limited preview in Google Book Search [accessed April 5, 2017]).
11. ^ Paul Lynch, MR Davey: Electrical Manipulation of Cells . Springer Science & Business Media, 2012, ISBN 978-1-4613-1159-1 , pp. 16 ( limited preview in Google Book Search [accessed April 5, 2017]).
12. ^ The Physis Factbook - An encyclopedia of scientific essays. Retrieved September 14, 2017 (English).
13. H. Vogel: Problems from the Physics: Exercises and solutions for the 17th edition of Gerthsen · Vogel PHYSIK . Springer-Verlag, 2013, ISBN 978-3-642-78189-6 ( limited preview in Google Book Search [accessed April 5, 2017]).
14. ^ Marcus Lehnhardt, Bernd Hartmann, Bert Reichert: Burn Surgery . Springer-Verlag, 2016, ISBN 978-3-642-54444-6 ( limited preview in Google book search [accessed April 5, 2017]).
15. Kögler / Cimolino: Standard rules of use: Electric current in use . ecomed-Storck GmbH, 2014, ISBN 978-3-609-69719-2 ( limited preview in Google book search [accessed April 5, 2017]).
16. a b William M. Haynes, David R., Lide, Thomas J. Bruno: CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. 2016-2017, 97th ed. Boca Raton FL, ISBN 1-4987-5428-7 .
17. Product information porcelain C 110. (PDF) p. 1 , accessed on April 9, 2017 .
18. Liviu Constantinescu-Simon: Handbook Electrical Energy Technology: Basics · Applications . Springer-Verlag, 2013, ISBN 978-3-322-85061-4 , pp. 113 ( limited preview in Google Book Search [accessed April 9, 2017]).
19. O. Zinke, H. Since: Resistors, capacitors, coils and their materials . Springer-Verlag, 2013, ISBN 978-3-642-50981-0 ( google.at ).
20. AVK Industry Association Reinforced Ku: Handbook fiber composite plastics / composites: Basics, processing, applications . Springer-Verlag, 2014, ISBN 978-3-658-02755-1 ( google.at ).
21. physical properties of the email. (PDF) p. 3 , accessed on April 9, 2017 .
22. Material specification for quartz glass ilmasil PI. (PDF) p. 3 , accessed on April 9, 2017 .
23. Borosilicate float glass from Schott. (PDF) (No longer available online.) P. 27 , archived from the original on April 9, 2017 ; Retrieved April 9, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
24. ^ William M. Haynes: CRC Handbook of Chemistry and Physics . Taylor & Francis, ISBN 978-1-4398-2077-3
25. Al2O3 aluminum oxide, technical high-performance ceramics. Retrieved April 9, 2017 .
26. PC polycarbonate - technical data sheet. (PDF) p. 1 , accessed on April 9, 2017 .
27. Glass fiber reinforced GRP profiles made of polyester. (PDF) p. 2 , accessed on April 9, 2017 .
28. Material characteristics PET (polyethylene terephthalate). (PDF) Grünberg Kunststoffe GmbH, p. 1 , accessed on April 9, 2017 .
29. PET - polyethylene terephthalate (Mylar®). Reichelt Chemietechnik, accessed on April 9, 2017 .
30. Plates made of PMMA - acrylic glass - plexiglass. (PDF) p. 2 , accessed on April 9, 2017 .
31. Material data sheet POM. Liedtke Kunststofftechnik, accessed on March 20, 2018 .
32. Glass fiber hard tissue HGW2372.1 (FR4-HF). (PDF) p. 1 , accessed on April 9, 2017 .
33. polypropylene. In: Material Archive. Retrieved April 9, 2017 .
34. Helmut Ohlinger: Polystyrene: First part: Manufacturing process and properties of the products . Springer-Verlag, 2013, ISBN 978-3-642-87890-9 ( google.at ).
35. Rotek hard paper HP 2061 (Pertinax). (PDF) Retrieved April 9, 2017 .
36. Data sheet RTP PP FR2. In: Material Data Center. M-Base Engineering + Software GmbH, accessed on April 9, 2017 .
37. ^ Egon Döring: Material science of electrical engineering . Springer-Verlag, 2013, ISBN 978-3-663-13879-2 ( google.at ).
38. senodur® PVC glass - technical properties. Retrieved April 9, 2017 .
39. (PTFE) polytetrafluoroethylene data sheet. (PDF) Retrieved April 9, 2017 .
40. Acrylonitrile-butadiene-styrene copolymers (ABS) data sheet. (PDF) Retrieved April 9, 2017 .
41. Electrical dielectric strength of materials. (PDF) Retrieved April 9, 2017 .
42. ^ CRC Handbook of Chemistry and Physics
43. Willy Pockrandt: Mechanical technology for machine technicians: Chipless forming . Springer-Verlag, 2013, ISBN 978-3-642-99131-8 ( google.at ).
44. ^ S. Giere, M. Kurrat, U. Schumann: HV dielectric strength of shielding electrodes in vacuum circuit-breakers ( Memento from March 1, 2012 in the Internet Archive ) (PDF)
45. Electronic properties of diamond . el.angstrom.uu.se. Retrieved August 10, 2013.