Electric Strength of Insulating Materials

Electric strength, in English usage mostly called dielectric strength, is the maximum electric field strength an insulating material can withstand before breakdown occurs. The unit is kilovolts per millimetre (kV/mm), and it is measured to IEC 60243 on a specimen of defined thickness. A 25 µm polyimide film Kapton HN reaches 303 kV/mm (DuPont, data sheet QE-10206), a 0.6 mm rigid PVC sheet 40 kV/mm (Kern, data sheet 2690), and air around 3 kV/mm. This article shows how the value in a data sheet is produced, sets the insulating materials from PET film to micanite side by side with their test thickness, explains why the value falls with thickness, and derives from that which thickness an insulating part made from our insulation materials really needs.

Short-term values from manufacturers' data sheets (DuPont, Mitsubishi Polyester Film, Kern, Centroplast, Moeschter Group, Müller Ahlhorn, Hesselmann), continuous recommendations for transformers from the DuPont data sheets for Nomex 410 and 418. The calculator does not replace a design calculation or a test.

Dielectric strength, breakdown voltage and withstand voltage are three different quantities

Dielectric strength E_d is a quotient: breakdown voltage U_d divided by the electrode distance d, which for solid materials is the specimen thickness (IEC 60243-1). A 100 µm polyester film breaks down at 11.75 kV according to the catalogue (Schupp, Mylar A and Hostaphan RN), which is 117 kV/mm. The voltage in kV is a property of the specimen or the part, the value in kV/mm a property of the material at that thickness. The event itself, its mechanisms and the breakdown voltage of air gaps are covered in the article on voltage breakdown.

Two neighbouring terms are routinely mixed up with it. The withstand voltage is the test voltage a finished component, cable or device has to survive in routine testing, it is stated in kV and follows from product standards. The breakdown strength as a physical quantity applies to all media, including air, SF6, oil and vacuum, where Paschen’s law rules. This article stays with the solid insulating material. How voltage and distance determine the electric field strength is covered in a separate article.

Measurement to IEC 60243: how the data sheet value is produced

The IEC 60243 series has three parts. Part 1 (DIN EN 60243-1, VDE 0303-21) covers testing at power frequencies from 48 to 62 Hz, part 2 direct voltage, part 3 impulse voltage with the 1.2/50 µs wave (DKE, DIN Media). Older data sheets quote DIN 53481 or VDE 0303 part 2, American data sheets ASTM D149 with the result in V/mil. All methods follow the same pattern:

1. The specimen is conditioned before the test at 23 °C and 50 % relative humidity, in test laboratories typically for 24 hours (Measurlabs, Hanser).
2. The specimen sits between two metal electrodes. The standard arrangement is two coaxial cylinders of 25 mm and 75 mm diameter with a 3 mm edge radius; under DIN a 20 mm sphere against a 50 mm plate (K20/P50) was common (IEC 60243-1, Hanser Werkstoff-Führer).
3. Specimens above about 2 mm and all high-voltage measurements are made in transformer oil, so that the current passes through the specimen instead of flashing over the surface (Intertek, Hanser).
4. The AC voltage rises evenly from zero, fast enough for breakdown to occur after 10 to 20 seconds (Hanser, DuPont Nomex 410). The standard also defines step tests and slow rates of rise up to 600 s (IEC 60243-1).
5. Five specimens are tested. If one value deviates more than 15 % from the median, five more are added. The data sheet states the median divided by the measured thickness (Measurlabs).

The standard itself limits what the value means. IEC 60243-1 states that the results reveal deviations caused by processing and ageing but are “not intended for use in evaluating the behaviour of insulating materials in an actual application”, and that different test modes are not directly comparable. A data sheet value is therefore a comparison value between materials, not a design value.

Dielectric strength of plastics and other insulating materials: the table

The table gives the product form and the test thickness for every value, because without them no value is comparable. All figures are short-time values at AC voltage and room temperature unless stated otherwise.

Three things stand out. First, the films sit at the top: as thin film, polyimide and PET reach five to ten times the value of a sheet made from the same polymer. Second, moisture separates the materials more sharply than chemistry does; PA 6 sits at 15 kV/mm when dry and absorbs 3 % water in standard climate (Kern). Third, the materials with the highest temperature resistance are not the ones with the highest kV/mm value: micanite withstands 500 °C and more but only around 20 to 25 kV/mm. Which of these plastics are suitable as insulating parts is ranked by temperature, CTI and price in the article on electrically insulating plastics.

Why the value in kV/mm falls with thickness

IEC 60243-1 states it plainly: the electric strength of most materials decreases as the specimen thickness increases and as the time of voltage application increases. The Hanser Werkstoff-Führer phrases it as a prohibition: the value determined for one specimen thickness “by no means” applies to other thicknesses. A thick specimen contains more inclusions and voids where the field is locally enhanced, and it dissipates the loss heat less well. The catalogue values of a PET film show the consequence:

The voltage rises with thickness, but more slowly than the thickness itself (Schupp, Mylar A and Hostaphan RN). Kapton HN shows the same picture: five times the thickness, from 25 to 125 µm, halves the value from 303 to 154 kV/mm (DuPont). As a rule of thumb, the breakdown voltage of a film grows roughly with the square root of its thickness. Anyone who scales the often quoted 180 kV/mm linearly to a 350 µm film expects 63 kV and gets 20 kV.

Aramid paper behaves the other way round. Nomex 410 reaches only 17 kV/mm at 0.05 mm, but 32 to 33 kV/mm at 0.18 to 0.51 mm and 27 kV/mm again at 0.76 mm (DuPont). The thinnest papers are more porous and have a less closed surface, so each micrometre carries less there. For design, both curves lead to the same rule: read or measure the breakdown voltage at the service thickness, never extrapolate from a kV/mm value of a different thickness.

Temperature, humidity, frequency, ageing and voltage type shift the value

The standard lists nine influences, from thickness and moisture via frequency and rate of rise to electrode geometry and surrounding medium (IEC 60243-1). Manufacturer data show how large the effects are:

• Humidity lowers the value measurably but moderately for dense films. Kapton HN falls from 339 kV/mm at 0 % relative humidity via 303 kV/mm at 50 % to 268 kV/mm at 100 %, which corresponds to 2.8 % water in the film (DuPont). Nomex 410 goes from 33.5 to 30.7 kV/mm between oven dry and 96 % humidity. Polyamide, by contrast, loses three orders of magnitude in volume resistivity (Kern).
• Temperature lowers the value because conductivity and chain mobility increase. For Nomex 410 DuPont calls the effect minor up to 200 °C; for thermoplastics the insulating effect ends with softening, rigid PVC at 60 °C, PE-HD at 90 °C, PP at 100 °C (Kern).
• Frequency acts through dielectric losses. Up to 10 kHz the dielectric constant of Nomex 410 hardly changes (DuPont); at high frequencies and steep inverter edges the dissipation factor heats the material and lowers the thermal breakdown limit. More in the article on dielectric losses.
• Direct voltage is withstood considerably better than alternating voltage: Hostaphan RN reaches 510 kV/mm with DC at 36 µm against 290 kV/mm at 50 Hz, and 420 against 150 kV/mm at 190 µm (Müller Ahlhorn, FI 13011). Impulse voltage 1.2/50 µs is higher still, Nomex 410 at 0.25 mm reaching 63 instead of 32 kV/mm (DuPont).
• Time and ageing are the biggest lever. The short-time value is produced in 10 to 20 seconds. DuPont rates the life of Nomex 410 by how long the paper still holds 12 kV/mm at a given temperature, and recommends staying below 1.6 kV/mm for continuous stress in transformers to avoid partial discharges. That is one twentieth of the data sheet value.
• Partial discharges erode organic insulating materials far below their dielectric strength. Why Kapton HN fails after 200 hours at 20 kV/mm while the corona-resistant grade CR lasts more than 100,000 hours is explained in the article on corona discharge, the measurement in the article on partial discharge.

Reading a data sheet correctly: five questions for every kV/mm value

For rigid PVC, reputable sources give 20 kV/mm (Hug Technik to DIN 53481), 32 kV/mm (Maagtechnic), 39 kV/mm with sphere against sphere in transformer oil (KB Kunststoffdreherei) and 40 kV/mm at 0.6 mm test thickness (Kern, TMK, GEHR). None of these values is wrong, they answer different questions. Hanser therefore demands that conditioning, electrode arrangement and test conditions be stated with every value. We check five points before we take a value into a design:

1. Which test thickness? A value without thickness is useless for films.
2. Which electrodes and which medium? Cylinders 25/75 mm in oil give different numbers from sphere against plate in air, and air limits thin specimens through flashover at the edge.
3. Which voltage type and which rate of rise? AC rapid rise, DC and impulse differ by a factor of 2 to 3 for the same film.
4. Which climate? 23 °C and 50 % humidity is the normal case; values at 90 °C in oil apply to transformers, not to the control cabinet.
5. Typical value or minimum value? DuPont explicitly states for Nomex 410 that the table contains typical values, “not recommended for design purposes”. Minimum values are found in the specification, not in the data sheet.

Material selection: how we size the thickness of an insulating part

The most common objection from design engineering is: then we simply take the material with the highest kV/mm value. That almost always leads to polyimide and almost always to an unnecessarily expensive solution. What counts is the breakdown voltage of the part at its thickness, divided by a safety factor, and that has to match the voltage in service, the temperature and the humidity at the installation site. A slot liner made of 0.25 mm Nomex 410 withstands around 8 kV in the short-time test (32 kV/mm, DuPont). For continuous stress DuPont recommends less than 1.6 kV/mm, which is 400 V across this layer. On a 400 V mains supply that is enough; on an 800 V inverter with reflected voltage peaks up to twice the DC-link voltage it calls for a thicker layer, a laminate or a partial-discharge-resistant material.

We proceed in this order: first clarify the voltage class and the voltage waveform, then the thermal class to IEC 60085, and only then the material and its thickness. For thermal class B up to 130 °C, PET film is the economical choice in 95 out of 100 cases, and we convert it as polyester film from 12 to 350 µm. For classes F and H, Nomex and laminates such as NMN follow, for class C from 220 °C polyimide from 25 µm. The cut edge belongs in the calculation: at a burr or a frayed edge the field strength rises locally, and that is where partial discharge starts, whatever the catalogue value says. We slit rolls from 5 to 1,600 mm wide to plus or minus 0.1 mm and die-cut insulation and molded parts from 0.023 to 3.0 mm thick with steel rule dies or solid steel tools. Send us your drawing, voltage and thermal class, and we will propose material and thickness.

FAQ on dielectric strength

Which plastics have the highest dielectric strength?

As thin film, polyimide and polyester lead: Kapton HN reaches 303 kV/mm at 25 µm, Hostaphan RN 290 kV/mm at 36 µm, polypropylene film 200 kV/mm at 40 µm. As a 1 mm sheet, PE-HD with 53 kV/mm and PP with 50 kV/mm are ahead, rigid PVC at 40 kV/mm, PC at around 29 kV/mm, PTFE at 20 kV/mm on a 3.2 mm specimen and PA 6 at 15 kV/mm. The ranking only holds at equal thickness, which is why every reputable table states the test thickness.

What is the dielectric strength of PVC?

Rigid PVC (PVC-U) has 20 to 40 kV/mm depending on test conditions: 40 kV/mm at 0.6 mm test thickness to IEC 60243-1 (Kern, TMK, GEHR), 39 kV/mm with sphere electrodes in transformer oil, 32 kV/mm at Maagtechnic and 20 kV/mm in older tables to DIN 53481 without a thickness. As a 40 µm film PVC reaches 150 kV/mm (PSM wiki). For design, the value at the service thickness counts, and rigid PVC can only be used continuously up to 60 °C.

From what value is dielectric strength considered high?

Values above 20 kV/mm on a 1 mm specimen are considered high, which PE, PP, rigid PVC, PC and the mica materials achieve. Thin films are an order of magnitude above that, polyimide and PET at 150 to more than 300 kV/mm. At the lower end, phenolic paper laminate, pressboard in air and vulcanised fibre with 5 to 15 kV/mm bound the field, and air sits at around 3 kV/mm.

What is the dielectric strength of mica?

A pure mica sheet reaches around 118 kV/mm (Wikipedia), a material made from mica considerably less: micanite sheets with 90 % mica and 10 % silicone or epoxy binder sit above 20 to 25 kV/mm to IEC 60243 (Hesselmann, ISO-Tech), mica tape on glass silk above 1.5 kV per 0.15 mm layer. The advantage of mica lies in temperature: the dielectric strength stays constant up to 380 °C and only halves at 700 °C (Final Materials).

Why does the data sheet give a different value from the encyclopaedia table?

Because dielectric strength is not a material constant. The value depends on test thickness, electrodes, test medium, voltage type, rate of rise and climate, and encyclopaedia tables usually state none of them. A data sheet to IEC 60243-1 gives the test thickness and the method, which is why it is the better source. Compare only values with the same thickness and the same method.

How does dielectric strength differ between direct and alternating voltage?

Under direct voltage an insulating material withstands considerably more, because there is no repolarisation and no partial discharge in every half cycle. Hostaphan RN reaches 510 kV/mm with DC at 36 µm and 290 kV/mm at 50 Hz (Müller Ahlhorn). Impulse voltage to IEC 60243-3 is higher still, Nomex 410 at 0.25 mm reaching 63 instead of 32 kV/mm. Data sheets therefore always state the voltage type, usually AC rapid rise.