A voltage breakdown occurs when the electric field strength in a material becomes so high that its insulating effect collapses. Current then flows through the material, either as a spark or as an arc. The breakdown voltage is the voltage at which an electrical breakdown occurs in an insulator. It depends on the thickness of the material, on its purity and homogeneity and on the voltage waveform. In air it is around 30 kV for a gap of one centimetre in a uniform field (Wikipedia, Durchschlagfestigkeit), and below 330 to 350 V no breakdown in air is possible at any gap (Hinrichsen, TU Darmstadt). A 25 µm polyimide film of Kapton HN withstands 7.7 kV for a short time (Qnity, Kapton HN data sheet). This article explains the sequence and mechanisms of breakdown, the breakdown voltage of air in numbers, how it differs from flashover, partial discharge and surface leakage current, and its causes in die-cut insulation and molded parts.
Breakdown voltage in kV and dielectric strength in kV/mm are two different quantities
The breakdown voltage belongs to the component. It says at how many volts this particular film, cable or air gap fails. The dielectric strength belongs to the material and relates the voltage to the thickness, which is why it is given in kV/mm. The cable manufacturer KBE works it out: an insulating plastic with 20 kV/mm and a 2 mm wall gives a calculated breakdown voltage of 40 kV. That calculation is only roughly right, because the breakdown voltage grows more slowly than linearly with thickness, roughly with the square root of the distance (Wikipedia, Durchschlagspannung).
Manufacturer data for two materials we process every day show the difference on the same material (Qnity, Kapton HN data sheet, ASTM D149; Arclin, Nomex 410 data sheet, AC rapid rise). The breakdown voltage is calculated from thickness and short-time value in each case.
| Material and thickness | Dielectric strength | Breakdown voltage |
|---|---|---|
| Kapton HN, 12.7 µm | 315 kV/mm | 4.0 kV |
| Kapton HN, 25 µm | 303 kV/mm | 7.7 kV |
| Kapton HN, 50 µm | 240 kV/mm | 12.2 kV |
| Nomex 410, 0.05 mm | 18 kV/mm | 0.9 kV |
| Nomex 410, 0.25 mm | 33 kV/mm | 8.3 kV |
| Nomex 410, 0.76 mm | 27 kV/mm | 20.5 kV |
The film with the highest kV/mm value has the lowest breakdown voltage. Anyone designing a component needs the voltage in volts, anyone comparing materials needs the value in kV/mm. All values are short-time values from tests lasting 10 to 20 seconds. The full material table, the measurement to IEC 60243 and the effects of temperature, humidity and frequency are covered in the article on electrical dielectric strength. Which test voltage a finished device must pass according to its standard is covered in the article on dielectric withstand strength.
How an electrical breakdown develops
It starts with a single free electron. In the strong field it gains so much energy between two collisions that at the next collision it knocks another electron out of an atom. One electron becomes two, four, eight: an electron avalanche. In air at atmospheric pressure this avalanche mechanism after Townsend is sufficient up to a gap of about 1 cm; for larger gaps a weakly conducting channel, the streamer, grows out of the avalanche towards the opposite electrode at 10 to 100 cm/µs (Hinrichsen, TU Darmstadt). As soon as the channel bridges the gap, the voltage collapses to the arc voltage.
Inside the channel, heat and ionisation create a conductive plasma. Its ultraviolet radiation knocks further electrons out of the insulating material and raises the conductivity once more (Wikipedia, Spannungsdurchschlag). The whole process takes little more than 100 ns in air (Hinrichsen). What happens next is decided by the source: if it cannot sustain the current, the breakdown dies out as a spark. If it can deliver the current, it keeps burning as an arc, and that reaches around 30,000 °C (Wikipedia, Electrical breakdown).
For design work the consequence matters. Gases recover completely after a breakdown, liquids partially, solid insulating materials never (Benha University, Breakdown in Solid Dielectrics). A plastic carbonises along the channel, and the carbon conducts. A breakdown in a film, a pressboard or an aramid paper is therefore always permanent damage.
Four mechanisms lead to breakdown in solid insulating materials
High-voltage engineering classifies breakdown by the time it takes. The longer the voltage is applied, the lower the voltage at which the material fails.
- Purely electrical breakdown is the fastest. The field tears electrons out of the lattice or out of defects at grain boundaries and foreign atoms, and the avalanche runs through the material (IEH Hannover, Isolierstoffe practical). It takes around 10 to the power of minus 8 seconds (Benha University) and occurs under lightning and switching overvoltages.
- Thermal breakdown sets in when the heat supplied to the insulating material is permanently greater than the heat removed (IEH Hannover). Dielectric losses generate heat, heat increases the losses, until the material is thermally destroyed. Under AC it is reached earlier than under DC, because the losses rise with frequency (Benha University).
- Electromechanical breakdown affects soft films. The attraction between the charges on the two electrodes compresses the insulating material, thinner means higher field strength, and once the material is compressed to 60 % of its original thickness the system tips over (highvoltageisoco).
- Erosion breakdown is the slowest and, in practice, the most common. Partial discharges in voids eat through the material over years to decades until a channel bridges the gap (University of Hannover, dissertation on epoxy resin insulation systems). It follows the life law E to the power of N times t equals constant, and as a rule of thumb 10 K more temperature halves the life of the insulation system (elektro.net).
The mechanisms rarely occur alone. The test laboratory CRW Engineering calls it an interplay: a void starts the erosion, the erosion raises the losses, the losses heat the material, and in the end it breaks down electrically. How partial discharge ignites in a void and how it is measured is covered in the article on partial discharge, the discharge at points and edges in air in the article on corona discharge.
Breakdown voltage of air in numbers
For air, forums and reference works quote values from 0.1 to 3.3 kV/mm, and all of them are right somewhere. The voltage depends on the gap, on the shape of the electrodes and on the pressure. In a uniform field between two large, smooth electrodes at 20 °C and atmospheric pressure, the approximation Ud = 24.36 kV/cm times the gap plus 6.72 kV times the square root of the gap in centimetres applies, valid from 1 to 10 cm (Hinrichsen, TU Darmstadt). From this, from the Paschen minimum (Hinrichsen), from the values for 1 mm and 1 cm (Wikipedia, Durchschlagfestigkeit), the value for points (Elektronik-Kompendium) and the rule of thumb for spark gaps (Wikipedia) the table follows.
| Arrangement in air | Breakdown voltage | Average field strength |
|---|---|---|
| Paschen minimum, gap of about 7 µm | 330 to 350 V | around 45 kV/mm |
| 1 mm, uniform field | around 4.3 kV | 4.3 kV/mm |
| 1 cm, uniform field | 30 to 31 kV | 3.0 kV/mm |
| 2 cm, uniform field | around 58 kV | 2.9 kV/mm |
| 5 cm, uniform field | around 137 kV | 2.7 kV/mm |
| 10 cm, uniform field | around 265 kV | 2.6 kV/mm |
| two points, a few millimetres | from around 1 kV | highly non-uniform |
| rule of thumb for real spark gaps | 1 kV per mm | 1 kV/mm |
Two things stand out. The average field strength falls with the gap, so air is stronger per millimetre at 1 mm than at 10 cm. And points lower the voltage to a third or less, because the field concentrates at the point and pre-discharges ignite there (Hinrichsen). The rule of thumb of 1 kV per millimetre is therefore a safety margin for arrangements with edges, humidity and dust, and no physical constant. Below the Paschen minimum there is no breakdown in air, however small the gap. Pressure, altitude and the comparison with SF6, oil and vacuum are covered in the article on the dielectric strength of air and other media.
Breakdown, flashover, partial discharge and surface leakage current: four terms, four events
The standards separate the terms by where the discharge occurs and by the damage. The trade journal Elektropraktiker summarises DIN VDE 0303 and DIN EN 60664-1 as follows: breakdown is the complete electrical discharge in a solid insulating material and leads to permanent bridging of the insulation. Flashover is the complete discharge in a gas, a liquid or along the surface of a solid insulating material, with temporary or permanent loss of insulation.
| Event | Location | Gap bridged | Damage to the insulating body |
|---|---|---|---|
| Breakdown | through the solid insulating material | completely | always, carbonised channel |
| Flashover | through air, oil or along the surface | completely | usually none, tracking with contamination |
| Partial discharge | void, point, interface | only partly | erosion over years, then breakdown |
| Surface leakage current | surface with moisture and dirt | as current, without discharge | tracking, later flashover |
A flashover on an insulator in the rain is over once it dries. A breakdown through a slot liner never is. That is why designers size clearances so that a flashover comes before a breakdown. The surface leakage current that flows on a contaminated surface without any flash has its own article, as does the creepage distance used to keep it in check.
In semiconductor technology the same voltage is called breakdown voltage of a junction. There it is an operating point rather than damage: a Zener diode works in breakdown, and the avalanche breakdown of a junction is reversible as long as the power dissipation is limited (Wikipedia, Durchschlagspannung). Diodes, the Zener effect and the avalanche effect are covered in the article on semiconductor breakdown voltage.
Damage patterns and causes in insulating parts
An insulating part almost never breaks down because the material was too weak. It breaks down because the field strength at one spot was higher than expected, or the insulation thinner than drawn. The damage pattern usually shows which case applies. A clean hole with a carbonised rim in the middle of the surface points to an electrical or thermal breakdown. A branched, tree-shaped track below the surface is treeing, the erosion breakdown after long partial discharge (Wikipedia, Electrical treeing). A dark track on the surface from the conductor to the edge is a flashover with tracking. We see five causes again and again:
- A burr or a frayed cut edge acts like a point. The field concentrates there, the first discharge ignites there, even if the surface next to it has reserves. Burrs form when films are die-cut with a cutting clearance that does not match the thickness.
- Delamination at the cut edge of a laminate opens a gap filled with air. It occurs when the blade is too blunt.
- An air inclusion in the resin or between two layers is the classic starting point of partial discharge. Inside the void the field strength is higher by the factor of the permittivity of the surrounding material, roughly three to four times for epoxy resin (Benha University), and air only holds 3 kV/mm. Casting voids are regarded as a manufacturing-related weak point (IEH Hannover).
- Moisture lowers the breakdown voltage of cellulose and paper most of all. In oil, moist fibres align in the field and form conductive bridges (IEH Hannover, fibre bridge breakdown); in pressboard the conductivity rises and thermal breakdown comes closer.
- Thermal ageing makes organic insulating materials brittle and cracked. Every crack is a new void.
The most common objection from design engineers is: then we simply take the material one step thicker. That helps less than it seems. The breakdown voltage grows only with the square root of the thickness, and the burr on the edge remains a burr. A 0.25 mm Nomex slot liner with a clean edge holds more than a 0.38 mm one with delamination. Our recommendation is therefore: geometry and edge first, then thickness.
Preventing breakdown: in design and in production
Every measure lowers the field strength at the weakest point or removes the air in which the first discharge ignites. The order is deliberate:
- Size clearances to DIN EN 60664-1. For a 230/400 V mains the rated impulse voltages are 2.5 kV (overvoltage category II), 4 kV (III) and 6 kV (IV). The minimum clearances for these are 1.5 mm, 3 mm and 5.5 mm in a non-uniform field and 0.6 mm, 1.2 mm and 2 mm in a uniform field, each up to 2,000 m altitude (Weidmüller and WAGO after IEC 60664-1, table F.2).
- Round edges and avoid points, on the conductor as on the insulating part. A slot liner gets folded-over edges, the cuffs, so that no lamination edge lies exposed against the paper.
- Design the insulation thickness with the continuous value instead of the short-time value. Arclin gives 1.6 kV/mm for Nomex 410 in continuous operation, although the data sheet shows 33 kV/mm. In between lie ageing, moisture and partial discharge.
- Eliminate voids: impregnate or pot windings, ideally under vacuum.
- Treat the edge of the insulating part as an electrical property. GOBA die-cuts slot liners, phase separators and slot closures from Nomex, Kapton, pressboard and laminates in thicknesses from 0.023 to 3.0 mm and up to 1,000 x 2,000 mm, using steel rule dies or solid steel tools, and inspects the parts for edge cracks and delamination. In stamping and forming we match cutting clearance and blade to the thickness so that film and laminate leave the tool without burrs.
- Release the finished component with a test voltage. Which voltage, for how long and with which test current is regulated by the equipment standards in the article on dielectric withstand strength.
If you know the voltage class and the installation situation, we will recommend the material and thickness from our insulation materials that holds the continuous value rather than only the short-time value from the data sheet. Send us your drawing, operating voltage and environment, and our consulting team replies within 24 hours on working days.
