Dielectric Strength: Air, SF6, Oil and Vacuum in Figures

Dielectric strength is the electric field strength at which an insulating medium loses its insulating effect and a conductive channel forms. It is stated in kilovolts per millimetre and applies equally to gases, liquids, vacuum and solids. The dielectric strength of air under standard conditions, that is at 20 °C and normal pressure, is about 3 kV/mm across a 1 cm electrode gap; humidity, air pressure, temperature and above all the gap itself change this value, and at 1 mm it is already around 4.3 kV/mm (Wikipedia, Durchschlagfestigkeit). SF6 withstands more than 8 kV/mm, transformer oil 5 to 30 kV/mm, high vacuum 20 to 40 kV/mm. This article provides the table for air by gap distance, Paschen’s law in figures, the effect of altitude and humidity, the comparison of media and the clearances to IEC 60664-1 from which we derive the insulation materials for your voltage class.

Dielectric strength of air: the table by gap distance

The value of 3 kV/mm is not a law of nature; it applies to a one centimetre gap between two large, smooth electrodes. The smaller the gap, the more field strength the air tolerates, and the larger the gap, the less. The table shows the breakdown voltage in a uniform field at 20 °C and 1,013 mbar, calculated with the approximation that Wikipedia and the high-voltage lecture notes of TU Darmstadt give in agreement (Ed ≈ 24.4 kV/cm plus 6.7 kV/cm divided by the square root of the gap in cm). The first row comes from the Paschen curve.

As soon as one electrode is curved, the value collapses. Between two points the spark jumps at just 1 kV (elektronik-kompendium). A sphere-plane gap of 70 cm breaks down at 315 kV under positive DC, which is 0.45 kV/mm, and the characteristic of a rod-plane gap rises at just under 5 kV/cm (TU Darmstadt). Anyone who designs with 3 kV/mm is designing with a laboratory arrangement that does not exist inside equipment. We advise planning with 0.5 kV/mm for edges, wire ends and screw heads and leaving the rest to the standard’s table further down. The basics of electric field strength at points and edges are covered in a separate article.

Paschen’s law explains why air is stronger across small gaps

In 1889 Friedrich Paschen found that the breakdown voltage of a gas in a uniform field depends solely on the product of pressure and electrode gap. Halve the gap and double the pressure, and the breakdown voltage stays exactly the same (TU Darmstadt). The formula relates Ud to the product p·d; for air at 20 °C the ZVEI gives the constants A = 1,130 per mm and B = 27.4 kV/mm (ZVEI, partial discharge measurement guide, 2024).

Plotted as a curve, it first falls and then rises again. For air the minimum lies at 327 V and a 7.5 µm gap at normal pressure (Wikipedia, Paschen’s law), measured at 330 to 350 V at 7.3 bar·µm (TU Darmstadt). To the left of the minimum, electrons find too few molecules to ionise on their short path, so the voltage rises again. Three rules follow that hold inside every piece of equipment:

• Below around 330 V peak there is no breakdown in air, however small the gap. The ZVEI gives 300 V as the theoretical threshold for partial discharge and considers it unlikely below 500 V.
• Above this threshold every air gap in a winding becomes a discharge path. In 800 V drives the stress at the inverter is far above it, as the article on corona discharge shows.
• A 1 cm gap reaches its minimum at 0.73 mbar. A poor vacuum is therefore the worst insulator there is, and a good vacuum one of the best. For SF6 the minimum is 507 V, for argon only 94 to 265 V (TU Darmstadt).

Altitude, air density, temperature and humidity shift the value

In a uniform and weakly non-uniform field the breakdown voltage of an air gap is proportional to the relative air density. TU Darmstadt gives δ = 0.289 times pressure in mbar divided by (273 plus temperature in °C), and IEC 60060-1 prescribes exactly this correction for every breakdown measurement in air. Thinner air holds less: the ZVEI shows that a gap with a breakdown voltage of 1,000 V at sea level holds only 630 V at 4,000 m (616 hPa, 9 °C). IEC 60664-1 therefore sets 2,000 m as the standard operating altitude and multiplies clearances above it by the altitude correction factors of Table A.2 (WAGO, technical appendix):

• 3,000 m: factor 1.14
• 4,000 m: factor 1.29
• 5,000 m: factor 1.48
• 10,000 m: factor 3.02

In a uniform field and across short gaps the influence of humidity is negligible, which is why IEC 60060-1 has no humidity correction there. Only across long non-uniform gaps does the breakdown voltage rise with absolute humidity, because water molecules capture free electrons. Measurements by the Swiss SEV show this: at 150 mm point-to-point humidity changes nothing, at 600 mm the breakdown voltage rises from 191 kV to 232 kV when the air goes from dry to the standard humidity of 11 g/m³ (Bulletin SEV 1970). For insulating parts inside equipment this means humidity does not help; it does harm through surface leakage current, long before the clearance is affected.

SF6, vacuum, oil and solids compared

Every figure has a test condition; without gap, pressure and electrode shape no value can be compared. The table therefore gives both.

SF6 is electronegative; it captures free electrons before an avalanche can form. At 2 to 3 bar it reaches the strength of insulating oil, and gas-insulated switchgear runs at around 7 bar (TU Darmstadt, TU Graz). Because of its global warming potential of 23,500 CO2 equivalents (energiewende.eu), the EU bans new SF6 switchgear between 52 and 145 kV from 2028, and the substitute is dry air at 8 bar with only 3.5 kV/mm (TU Graz). In vacuum breakdown starts as field emission at microscopic points on the electrodes, in practice at around 10 kV/mm mean field strength (Wikipedia, Spannungsdurchschlag). Strength grows only degressively with gap, which is why the vacuum interrupter remains a medium-voltage technology (TU Braunschweig).

Oil is the only medium whose dielectric strength is measured regularly in service: two spherical cap electrodes 2.5 mm apart, voltage rising at 2 kV/s, six breakdowns, mean value (Oelcheck, IEC 60156). Water and particles push the value from 70 kV down to 30 kV and below (SD Myers). The same caveat applies to solids in sharper form: a 40 µm polyester film reaches 160 kV/mm, a 1 mm polyamide sheet only 15 kV/mm. The material property in kV/mm with the large table for films, papers, laminates and pressboard is covered in the article on electric strength of solid insulating materials. The voltage in kV at which a specific part actually breaks down is covered in the article on voltage breakdown.

Measuring dielectric strength: sphere gap, oil cup, plate electrode

Each medium has its own method, and they are not interchangeable.

1. Air is measured with the sphere gap to IEC 60052: two spheres of 2 to 200 cm diameter at a defined distance, every table value referred to 20 °C and 1,013 hPa, deviating air density corrected with the factor δ to IEC 60060-1. At a 1 cm gap breakdown occurs at around 30 kV.
2. Oil is measured in the test cell to IEC 60156, with a 600 ml sample and electrodes 40 mm below the surface (Oelcheck).
3. Solids are tested to IEC 60243-1 between two cylindrical electrodes of 25 mm and 75 mm diameter with a 3 mm edge radius, at 48 to 62 Hz, usually under transformer oil so that the air at the electrode edge does not break down first. The standard itself notes that strength falls with thickness and duration of stress (IEC 60243-1:2013).
4. A finished component is not tested to breakdown; it must withstand a test voltage for one minute. That is voltage withstand strength, a test quantity in kV and not a material value.

A common objection is that the datasheet value is enough for design. It is not. The datasheet value applies only in new condition, is usually measured with a 500 V/s rate of rise, and the thermal class to IEC 60085 marks exactly the point at which a material still has half its breakdown voltage after 20,000 hours (ZVEI, Insulation Leaflet No. 1, 2024). Anyone who does not plan the margin has used it up after two and a half years at maximum temperature.

Clearances to IEC 60664-1: dielectric strength as a design dimension

In equipment up to 1,000 V AC and 1,500 V DC nobody recalculates the Paschen curve. Insulation coordination to IEC 60664-1 translates it into minimum dimensions: the overvoltage category of the equipment (I to IV) and the mains voltage give the rated impulse voltage (Table F.1), and pollution degree and field type give the minimum clearance from that (Table F.2). The creepage distance along the insulating surface follows separately from working voltage, pollution degree and material group (Table F.5, F.4 in the 2007 edition). The values apply up to 2,000 m altitude (DKE, DIN EN IEC 60664-1:2022).

Values for pollution degree 1 to 3 from Table F.2 (WAGO, technical appendix; Weidmüller, clearance and creepage design). Now it becomes visible how much margin the standard places on the physics. 0.6 mm of air holds around 3 kV in a uniform field according to Paschen’s formula, so case B sits close to the measured value. Case A demands 1.5 mm for the same 2.5 kV, which is just under 7 kV by Paschen, a factor of 2.7. This margin is the price of edges, burrs, dust and condensation, and it is still calculated without the altitude factor. The creepage distance comes on top: for 400 V at pollution degree 2 it is 2.0 mm with a group I insulating material, 2.8 mm with group II and 4.0 mm with group III (Weidmüller). How the material group follows from the CTI is covered under surface leakage current, and measuring the path itself under creepage distance.

Our recommendation for design: take case A even when the geometry looks uniform, because the first stamped part with a burr makes it non-uniform. Above 2,000 m apply Table A.2. And plan for ageing, because EN 61558 explicitly requires enlarged clearances and creepage distances for transformers against ageing-related breakdown (ZVEI, Insulation Leaflet No. 1).

Where the clearance does not fit the space, the insulating part takes over

Inside an electric motor the 3 mm from the table do not exist. In a hairpin winding the slot opening is only about 0.4 mm wider than the wire, and in the end winding the conductors of different phases sit close together at 800 V DC-link voltage (article on hairpin windings). At such points solid insulation replaces the air, and the standard tests it with an impulse voltage instead of a distance. Aramid paper reaches 18 to 40 kV/mm depending on thickness (article on stator insulation). A phase separator of 0.25 mm Nomex therefore carries more than 4 kV, while the same 0.25 mm of air carries around 1.6 kV by Paschen’s formula. At the same time the part extends the creepage distance to the lamination stack by its full height.

The edge of the insulating part is an electrical property here: a burr or a frayed cut edge acts like the point in the table above. That is why we manufacture slot liners, phase separators, slot closures and covers with steel rule dies or solid steel tools in thicknesses from 0.023 to 3.0 mm and up to 1,000 x 2,000 mm, from Nomex, Kapton, Mylar, Hostaphan, pressboard and NMN or DMD laminates. Slit rolls for slot insulation are supplied in widths from 5 to 1,600 mm with plus or minus 0.1 mm, and typical slot liners are 0.18 to 0.35 mm thick. For drives in electromobility we match material, thickness and edge quality to the DC-link voltage. Send us your drawing, voltage class and operating altitude, and you will receive a proposal from our insulation and molded parts with the matching characteristic value.

FAQ on dielectric strength

How high is the dielectric strength of air at 1 mm, 1 cm and 1 m?

In a uniform field at 20 °C and normal pressure, 1 mm of air holds around 4.3 kV, 1 cm around 30 kV and 10 cm around 260 kV, that is 4.3, 3.0 and 2.6 kV/mm. Beyond a metre and with curved electrodes the value falls to 0.1 to 0.5 kV/mm, because breakdown then proceeds via streamers and leaders.

What does Paschen’s law state?

Paschen’s law states that the breakdown voltage of a gas depends only on the product of pressure and electrode gap. The curve has a minimum: for air it lies at 327 V and a 7.5 µm gap at normal pressure. Below this voltage air never breaks down, however small the gap.

How does the dielectric strength of air change with altitude?

It falls in proportion to air density. An air gap with a breakdown voltage of 1,000 V at sea level holds only around 630 V at 4,000 m. IEC 60664-1 therefore requires larger clearances above 2,000 m, by a factor of 1.29 at 4,000 m, 1.48 at 5,000 m and 3.02 at 10,000 m.

Why does SF6 insulate better than air?

SF6 is electronegative and captures free electrons before they can trigger an avalanche. Its dielectric strength is around 8.8 kV/mm per bar, about three times that of air, and at 2 to 3 bar it reaches the strength of insulating oil. Because of its global warming potential of 23,500 CO2 equivalents, the power industry is increasingly replacing SF6 with dry air at 8 bar.

What clearance does a device on a 230 V mains need?

According to IEC 60664-1 that depends on the overvoltage category. A household appliance of category II has a rated impulse voltage of 2.5 kV and needs a 1.5 mm clearance in a non-uniform field, a permanently installed device of category III with 4 kV needs 3.0 mm, and equipment at the supply origin with 6 kV needs 5.5 mm.

What is the difference between dielectric strength, breakdown voltage and withstand voltage?

Dielectric strength is a field strength in kV/mm and describes the medium or the material. Breakdown voltage is the voltage in kV at which a specific part with its thickness actually breaks down. Withstand voltage is a test quantity: the voltage a component must survive for one minute in the test without breakdown. A material with 20 kV/mm has a breakdown voltage of 40 kV at 2 mm thickness, and it is tested far below that.