# Dielectric strength
**Dielectric strength** is the maximum electric field that an insulating material can withstand without breaking down and becoming electrically conductive. It is conventionally defined as the field at which breakdown occurs in a parallel-plate sample under a uniform electric field, and it is expressed in units of voltage per unit thickness, most commonly kilovolts per millimeter (kV/mm) or megavolts per meter.[1](https://technav.ieee.org/topic/dielectric-breakdown/) • [2](https://onlinelibrary.wiley.com/doi/10.1002/9781119833345.ch2) The value directly determines the thickness of insulation needed in a design, which makes it a fundamental parameter in electrical engineering.[3](https://www.sciencedirect.com/topics/materials-science/electrical-breakdown)
| Key fact | Detail |
|---|---|
| Definition | Maximum electric field a material withstands without breakdown; units kV/mm or MV/m[1](https://technav.ieee.org/topic/dielectric-breakdown/) |
| Air | Approximately 3 kV/mm in a uniform field at standard atmospheric conditions[4](https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926SubpartVAppB) |
| Silicon dioxide | Roughly 5 to 10 × 106 V/cm as a gate insulator[1](https://technav.ieee.org/topic/dielectric-breakdown/) |
| XLPE power cable insulation | 20 to 40 kV/mm under ideal conditions; maximum working field 12 to 25 kV/mm[1](https://technav.ieee.org/topic/dielectric-breakdown/) • [3](https://www.sciencedirect.com/topics/materials-science/electrical-breakdown) |
| Thickness effect | Breakdown strength falls with thickness following a minus power relation, \( E_{\mathrm{BD}} = E_1 d^{-a} \)[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7764431/) |
| Intrinsic strength | Independent of specimen size and electrode material; breakdown time on the order of 10−8 to 10−7 s[3](https://www.sciencedirect.com/topics/materials-science/electrical-breakdown) |
| Standard tests | ASTM D149 (power frequencies) and IEC 60243-1 (48 to 62 Hz) for solid insulating materials[6](https://store.astm.org/d0149-20.html) • [7](https://standards.iteh.ai/catalog/standards/iec/a4e59c7a-471f-41b7-ace1-6fdd818032e4/iec-60243-1-2013) |
## What breakdown is
When the field across an insulator exceeds its dielectric strength, the electric field frees bound electrons, and the material abruptly conducts.[1](https://technav.ieee.org/topic/dielectric-breakdown/) In gases and in intrinsic breakdown of solids, free electrons gain energy from the field, collide with bound electrons, and generate an avalanche of carriers that forms a conductive channel.[1](https://technav.ieee.org/topic/dielectric-breakdown/) • [8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard) The event is sudden and, in a solid, usually destructive: the punctured material cannot hold voltage again.[9](https://www.techniques-ingenieur.fr/en/resources/article/ti301/measuring-the-characteristics-of-solid-insulating-materials-d2310/v1/dielectric-strength-of-solid-insulators-2)
**Several mechanisms** produce breakdown, and they act on different time scales. Short-term mechanisms include intrinsic breakdown, thermal breakdown, and electromechanical breakdown; long-term mechanisms include partial discharge activity, inclusions of foreign particles, electrical treeing, and water treeing.[8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard) Thermal breakdown arises from excessive dielectric heating, in which higher temperature raises losses, which heats the material further.[8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard) Electromechanical breakdown results from electrostatic pressure deforming the material and is common in plastics and crystals, especially where defects exist.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7764431/)
**Intrinsic strength** is the material's own limit, reached when defects and environmental circumstances are excluded. It depends only on temperature, not on specimen size or electrode material, and breakdown occurs in about 10−8 to 10−7 seconds, so it does not depend on the waveform or duration of the applied pulse.[3](https://www.sciencedirect.com/topics/materials-science/electrical-breakdown) Measured intrinsic strengths can reach about 1000 kV/mm, far above the levels at which materials fail in practice, because practical failure is usually initiated by a localized imperfection in the material or its test environment.[8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard)
## The thickness effect
Contrary to what a datasheet might imply, dielectric strength is not a fixed intrinsic property of a material: it depends on how thick the sample is, the shape of the electrodes, the temperature, how often the applied field alternates, and whether defects or voids are present.[1](https://technav.ieee.org/topic/dielectric-breakdown/) The dependence on thickness, called the thickness effect, is the relation between the electric breakdown strength \( E_{\mathrm{BD}} \) and the dielectric thickness \( d \).[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7764431/)
A review of the experimental literature finds that the minus power relation
\[ E_{\mathrm{BD}} = E_1 d^{-a} \]
is supported by a large body of results, where \( E_1 \) is a constant and \( a \) is a power exponent.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7764431/) Thicker samples break down at lower fields because the probability of encountering a defect in the larger volume under test increases with thickness.[10](https://www.osti.gov/servlets/purl/1344474) The exponent \( a \) is not arbitrary: in the Weibull statistical picture it is approximately the relative standard error of the \( E_{\mathrm{BD}} \) distribution, and mechanistically different exponents point to different breakdown processes, with \( a = 1 \) associated with a Schottky-effect mechanism, \( a = 1/2 \) with a tunnel-effect mechanism, and \( a \le 1/4 \) with an avalanche process.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7764431/) • [11](https://www.mdpi.com/2079-4991/10/12/2473)
Because of this nonlinearity, testing samples of different thickness yields different values, and results on specimens of different thicknesses are not directly comparable; the common norm for reporting is a 1 mm specimen.[12](https://www.makeitfrom.com/glossary/Dielectric-Strength-Breakdown-Potential) • [7](https://standards.iteh.ai/catalog/standards/iec/a4e59c7a-471f-41b7-ace1-6fdd818032e4/iec-60243-1-2013)
## Statistics of breakdown
Breakdown is stochastic, depending critically on the statistical variation of the composition and structure of the material, so breakdown data are routinely analyzed with the two-parameter [Weibull distribution](https://www.edgepedia.org/weibull-distribution), in which the field at 63.2 percent cumulative failure probability defines the characteristic strength and the shape parameter \( m \) describes the spread.[8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard) • [11](https://www.mdpi.com/2079-4991/10/12/2473) A larger \( m \) means a more concentrated distribution, and better dielectric quality gives a larger \( m \).[11](https://www.mdpi.com/2079-4991/10/12/2473) In thin oxide films used as gate insulators, breakdown occurs when stress-generated defects build up in the oxide bulk until a percolation path joins the gate and substrate, and time-to-breakdown statistics follow Weibull or generalized (q-Weibull) forms when oxide thickness fluctuates.[13](https://research.ibm.com/publications/dielectric-breakdown-mechanisms-in-gate-oxides) • [14](https://arxiv.org/abs/cond-mat/0602515)
## Typical values and working fields
Measured strengths span orders of magnitude across materials. Representative values at 20 °C include air at \( 3 \times 10^{6} \) V/m, Pyrex glass and nylon at \( 14 \times 10^{6} \) V/m, polystyrene and Bakelite at \( 24 \times 10^{6} \) V/m, and Teflon at \( 60 \times 10^{6} \) V/m.[15](https://openstax.org/books/college-physics-2e/pages/19-5-capacitors-and-dielectrics) For polymers generally, published dielectric strengths range from about 1 to 1000 MV/m depending on the material.[16](https://www.sciencedirect.com/topics/engineering/dielectric-strength)
The field a designer may actually use is far below the measured strength. Maximum working fields in typical applications are 12 to 25 kV/mm for cross-linked polyethylene in power cables, about 2 kV/mm for filled epoxy in switchgear, and up to 1000 kV/mm for polypropylene in DC thin-film capacitors, where the film is extremely thin, clean, and operated at low temperature rise.[3](https://www.sciencedirect.com/topics/materials-science/electrical-breakdown) The gap between test strength and working field reflects aging, defects, moisture, and the statistical nature of failure over a component's life.[8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard)
## Measurement
Dielectric strength is measured destructively: a voltage is applied between electrodes on a specimen and raised until breakdown, and the strength is the breakdown voltage divided by the specimen thickness.[9](https://www.techniques-ingenieur.fr/en/resources/article/ti301/measuring-the-characteristics-of-solid-insulating-materials-d2310/v1/dielectric-strength-of-solid-insulators-2) • [17](https://www.universaltestmachine.com/uploads/ASTM%20D1000%20overview.pdf) The dominant standards are ASTM D149 for solid insulating materials at commercial power frequencies and IEC 60243-1, which covers short-time electric strength at 48 to 62 Hz; ASTM D3426 covers impulse (simulated-lightning) waves, and D3755 covers direct current.[6](https://store.astm.org/d0149-20.html) • [7](https://standards.iteh.ai/catalog/standards/iec/a4e59c7a-471f-41b7-ace1-6fdd818032e4/iec-60243-1-2013) • [12](https://www.makeitfrom.com/glossary/Dielectric-Strength-Breakdown-Potential)
Test conditions must be specified because they change the result. ASTM D149 defines three voltage-application methods: a short-time test, a step-by-step test, and a slow rate-of-rise test; the short-time method is the most common for quality control, while the longer methods usually give lower results and are more meaningful when comparing materials.[6](https://store.astm.org/d0149-20.html) Electric strength also varies with temperature and moisture content, so conditioning before the test follows the material specification.[7](https://standards.iteh.ai/catalog/standards/iec/a4e59c7a-471f-41b7-ace1-6fdd818032e4/iec-60243-1-2013) The surrounding medium matters as well: a medium of lower permittivity than the specimen enhances the field at the sample-electrode interface and lowers the measured value, so a medium of equal or higher permittivity is recommended.[8](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard) Because the test is destructive and even an overpotential below breakdown can injure insulation, repeated application of test voltage to the same specimen is not recommended in component testing.[18](https://nepp.nasa.gov/DocUploads/1F6AB74B-4517-4AD0-A34813268E75B8EB/MIL-STD-202.pdf)
## References
1. [Dielectric breakdown, IEEE Technology Navigator](https://technav.ieee.org/topic/dielectric-breakdown/)
2. [Electric Fields and Electrical Breakdown, in Practical Partial Discharge Measurement on Electrical Equipment (Wiley, 2023)](https://onlinelibrary.wiley.com/doi/10.1002/9781119833345.ch2)
3. [Electrical Breakdown, ScienceDirect Topics (Materials Science)](https://www.sciencedirect.com/topics/materials-science/electrical-breakdown)
4. [OSHA 1926 Subpart V Appendix B, Working on Exposed Energized Parts](https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926SubpartVAppB)
5. [L. Zhao and C. L. Liu (2020). Review and Mechanism of the Thickness Effect of Solid Dielectrics. Nanomaterials 10(12):2473.](https://pmc.ncbi.nlm.nih.gov/articles/PMC7764431/)
6. [ASTM D149-20, Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies](https://store.astm.org/d0149-20.html)
7. [IEC 60243-1:2013, Electric strength of insulating materials – Test methods – Part 1](https://standards.iteh.ai/catalog/standards/iec/a4e59c7a-471f-41b7-ace1-6fdd818032e4/iec-60243-1-2013)
8. [Electrical Breakdown Testing of Polymeric Materials for PV Modules, NIST/Atlas seminar](https://www.nist.gov/document/electrical-breakdown-testing-polymeric-materials-intended-use-pv-modules-iec-standard)
9. [Dielectric strength of solid insulators, Techniques de l'Ingénieur](https://www.techniques-ingenieur.fr/en/resources/article/ti301/measuring-the-characteristics-of-solid-insulating-materials-d2310/v1/dielectric-strength-of-solid-insulators-2)
10. [SAND2016-6113J, Large Area Dielectric Breakdown, OSTI](https://www.osti.gov/servlets/purl/1344474)
11. [L. Zhao and C. L. Liu (2020). Review and Mechanism of the Thickness Effect of Solid Dielectrics, MDPI version](https://www.mdpi.com/2079-4991/10/12/2473)
12. [Dielectric Strength (Breakdown Potential), MakeItFrom glossary](https://www.makeitfrom.com/glossary/Dielectric-Strength-Breakdown-Potential)
13. [S. Lombardo, J. H. Stathis et al. (2005). Dielectric breakdown mechanisms in gate oxides, Journal of Applied Physics](https://research.ibm.com/publications/dielectric-breakdown-mechanisms-in-gate-oxides)
14. [An Improved Description of the Dielectric Breakdown in Oxides Based on a Generalized Weibull Distribution, arXiv](https://arxiv.org/abs/cond-mat/0602515)
15. [19.5 Capacitors and Dielectrics, College Physics 2e, OpenStax](https://openstax.org/books/college-physics-2e/pages/19-5-capacitors-and-dielectrics)
16. [Dielectric Strength, ScienceDirect Topics (Engineering)](https://www.sciencedirect.com/topics/engineering/dielectric-strength)
17. [ASTM D1000 overview, Universal Test Machine](https://www.universaltestmachine.com/uploads/ASTM%20D1000%20overview.pdf)
18. [MIL-STD-202G Method 301, Dielectric Withstanding Voltage, NASA NEPP](https://nepp.nasa.gov/DocUploads/1F6AB74B-4517-4AD0-A34813268E75B8EB/MIL-STD-202.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Electromagnetism › Electric and magnetic fields › Field constants and interface conditions › Constitutive relations as field definitions*
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