High-voltage insulation is the electrical separation of conductors operating above 1 kV AC or 1.5 kV DC from each other and from earth, using air, gas, oil or solid insulating material. The threshold comes from the terminology standard IEC 60050 (Wikipedia, Hochspannung). In the German grid, high voltage reaches 380 kV, and the project partners of a KIT dissertation estimate that the extra-high-voltage network alone carries more than two million porcelain long-rod insulators. Inside the equipment, papers, pressboard, mica and films carry the voltage, and they do so at a surprisingly low field strength: a 6/10 kV cable with a 3.4 mm XLPE wall runs at an average of around 1.8 kV/mm (Helukabel, N2XSEY data sheet). This overview sorts out voltage levels, insulating media and insulators, shows how insulation is built inside transformers, machines, cables and switchgear, and states where GOBA supplies flexible insulating materials and die-cut insulation and molded parts, and where it does not.
- The standard sets the high-voltage threshold at 1 kV AC, and above it grid operators distinguish medium voltage at 10 to 30 kV, high voltage at 110 kV and extra-high voltage at 220 and 380 kV.
- High-voltage insulation operates at a few kV/mm in service, an XLPE medium-voltage cable at about 1.8 kV/mm, even though its materials withstand 27 to more than 150 kV/mm in a short-time test.
- Overhead lines are insulated by air and by porcelain, glass or silicone composite insulators, transformers by oil and cellulose, machines by mica, cables by XLPE and switchgear by SF6, vacuum or cast resin.
- Most high-voltage insulation fails gradually, worn down over years by partial discharge, moisture and tracking.
High voltage starts at 1 kV: the voltage levels at a glance
Anyone looking up medium or high voltage finds three different boundaries, and each is correct in its own context. The standard draws the line at 1 kV AC. The power quality standard EN 50160 calls everything above 36 kV high voltage and everything above 150 kV extra-high voltage (Wikipedia, Hochspannung). Grid operators and energy glossaries talk about network levels instead and mean the voltages actually in use: 1 to 35 kV, 60 to 150 kV, 220 to 380 kV (Enpal, Spannungsebene). For the insulation, what counts is the actual voltage at the component.
| Level | Voltage | Typical system voltage | Usual insulation |
|---|---|---|---|
| Low voltage | up to 1 kV AC or 1.5 kV DC | 230/400 V | films, papers, plastics, clearances and creepage distances to IEC 60664 |
| High voltage in vehicles | above 30 V AC or 60 V DC up to 1 kV AC or 1.5 kV DC | vehicle battery voltage | as low voltage, with a focus on touch protection and partial discharge |
| Medium voltage | above 1 kV up to 36 kV | 10, 20, 30 kV | glass or ceramic insulators, cast resin, XLPE cables, oil and pressboard in distribution transformers |
| High voltage | above 36 kV up to 150 kV | 110 kV | suspended long-rod or cap-and-pin insulators, oil-cellulose in power transformers, SF6 |
| Extra-high voltage | above 150 kV | 220 and 380 kV, DC transmission | strings of long-rod insulators, two in parallel at 380 kV, gas-insulated substations |
The vehicle row causes most of the confusion. In an electric car, anything above 30 V AC or 60 V DC is called high voltage, yet by the standard it remains low voltage. The boundary is a legal and practical convention; physically, nothing changes at it (Wikipedia, Hochspannungstechnik). Battery, inverter and busbar in an 800 V vehicle are therefore covered in the article on automotive high-voltage insulation. This page stays in power engineering above 1 kV. The standards change at this boundary as well: below it, IEC 60664 dimensions the insulation, above it IEC 60071 (Wikipedia, Isolationskoordination).
Air, SF6, oil and solids: the four media of high-voltage insulation
Every high-voltage insulation system combines a medium that fills the space with solid parts that support conductors and bridge distances. The media differ in two properties that matter more to the designer than any other: how much field strength they hold and whether they recover after a breakdown.
| Medium | Dielectric strength | After breakdown | Typical use |
|---|---|---|---|
| Air at atmospheric pressure | around 3 kV/mm | insulates again immediately | overhead lines, outdoor substations |
| SF6 | three to four times air, 5 to 10 bar in service | insulates again | gas-insulated switchgear from 6 to 800 kV |
| Transformer oil, dried | 5 to 30 kV/mm | usually lasting damage | transformers, together with paper and pressboard |
| Porcelain | 20 kV/mm | permanently destroyed | insulators, bushings |
| Mica | up to 60 kV/mm at 1 mm, above 150 kV/mm as thin flakes | permanently destroyed | windings of high-voltage machines |
Sources: Glasgow Caledonian University, HV Technology lecture (air), Wikipedia on gas-insulated switchgear (SF6), Wikipedia on dielectric strength (oil, porcelain and mica at 1 mm) and a thesis on mica tape insulation (Cuvillier, mica flakes). How air varies with gap, pressure and altitude, and how SF6 and vacuum compare in detail, is covered in the article on the dielectric strength of air and other media.
Oil and cellulose work as a pair in a transformer. Almost all the water in the system sits in the paper and pressboard, because cellulose, with a permittivity of 5.1, is far more polar than mineral oil at 2.2. Damp cellulose breaks down by hydrolysis, and that is what limits service life. Ester oils dissolve around 2,700 ppm of water at 20 °C against 44 ppm for mineral oil, so they draw moisture out of the paper, and they only ignite above 250 °C (Wikipedia, Transformatorenöl).
SF6 is on its way out. The gas has a global warming potential of 24,300, so one kilogram has the effect of 24 tonnes of CO2 (Wikipedia, Schwefelhexafluorid). The EU F-gas regulation bans putting new medium-voltage switchgear up to 24 kV with F-gases into service from 1 January 2026, and up to 52 kV from 2030. For high-voltage switchgear from 52 to 145 kV and up to 50 kA short-circuit current, the ban on gases with a GWP of 1 or more applies from 2028, and above that from 2032 (Art. 13(9), Regulation (EU) 2024/573). Vacuum, air, new gas mixtures and solid insulation take the place of SF6.
High-voltage insulators: porcelain, glass and silicone composite
High-voltage insulators keep the bare conductor at a distance from the earthed tower and carry its weight at the same time. In German-speaking countries, they have mostly been alumina porcelain long-rod insulators since 1953. Toughened glass cap-and-pin insulators are common mainly outside this region, and silicone composite insulators have been added in growing numbers since the end of the 20th century (KIT). The values come from the German and English Wikipedia. For porcelain, one gives 20 kV/mm and the other 4 to 10 kV/mm, which shows how strongly such values depend on thickness and test setup:
| Material | Dielectric strength | Strength | Weakness |
|---|---|---|---|
| Porcelain | 4 to 20 kV/mm | standard in German-speaking grids since 1953 | heavier than composite, loses mechanical strength over decades |
| Glass | 10 to 40 kV/mm | higher dielectric strength than porcelain | attracts condensation |
| GRP rod with silicone sheds | 20 to 40 kV/mm (GRP) | light, water-repellent, does not shatter | silicone ages under corona |
| Epoxy cast resin | above 10 kV/mm | freely mouldable, post and indoor insulators | needs void-free casting for a low partial discharge level |
Length follows voltage. A string of cap-and-pin insulators needs six units at 115 kV, twenty-four at 400 kV and sixty at 765 kV, with each cap rated for 10 to 12 kV operating voltage (Wikipedia, Insulator (electricity)). A silicone long-rod insulator for 550 kV is up to 5,603 mm long and withstands a lightning impulse of 1,550 kV (Siemens, 3FL5 and 3FL6 data sheet). The sheds lengthen the creepage path on the surface; at least 20 to 25 mm per kV is recommended, more near the coast and in industrial areas. When wet, the flashover voltage drops by more than half. Silicone partly makes up for this: when discharges destroy its water-repellent surface, it recovers within hours to days, as long as there is no continuous corona (Cuvillier, behaviour of cyclically dewed silicone surfaces).
All designs follow the same rule: an insulator should flash over before it punctures. A flashover through the air heals, a puncture through the body destroys it. Creepage distance and tracking are explained in the articles on creepage distance and leakage current. GOBA does not make overhead-line insulators, bushings or cast-resin post insulators; for those, an insulator manufacturer is the right contact. Insulators as components in general, from the difference to a conductor to the insulating part inside a motor, are covered in the article on electrical insulators.
In service, insulation runs at a fraction of its data sheet value
The short-time value on a data sheet comes from a test lasting ten to twenty seconds, as DuPont states for Nomex 994. High-voltage insulation has to last for decades, with voids, moisture, heat and overvoltages. Designers therefore size it to the field strength at which no partial discharge ignites, and that lies far below the short-time value. Four sources show the gap:
| Insulation | Field strength in service | Test or short-time value | Source |
|---|---|---|---|
| XLPE cable 6/10 kV, 3.4 mm wall | average of around 1.8 kV/mm (6 kV to earth) | 4.4 kV/mm at 15 kV test voltage | Helukabel, own calculation |
| Aramid pressboard Nomex 994 | recommended maximum of 1.6 kV/mm continuous in dry-type transformers | 27 kV/mm short-time and 70 kV/mm impulse, measured in oil at 3.2 mm | DuPont, data sheet H-52750 |
| Mica main insulation of a high-voltage machine | below about 5 kV/mm at the conductor edges, with inner corona protection | mica above 150 kV/mm | Cuvillier |
| Solid insulation in general | mean value rarely above 15 kV/mm | 100 kV/mm and more | Glasgow Caledonian University |
DuPont states the reason in the data sheet: continuous stress should not exceed 1.6 kV/mm, to minimise the risk of partial discharge. A partial discharge in an air pocket eats its way through organic material over the years, as the article on partial discharge describes. For you as a designer or buyer, this gives a clear rule: compare materials by their short-time value, size the thickness by the continuous value. The material table in kV/mm and the test to IEC 60243 are in the article on electrical dielectric strength.
High-voltage insulation inside the equipment
Inside transformers, machines and switchgear there is no room for metre-long air gaps. Thin solid insulating materials do the work instead, and each type of equipment has its own build-up.
Transformers: oil, paper and pressboard
In an oil-filled transformer, pressboard barriers divide the oil gaps between the phases and to the tank into narrow ducts. Angle rings, chimney segments, spacers and cylinders made of the same material follow the field around the winding ends (Pucaro, components). Weidmann developed high-density transformerboard in the late 1920s from pure sulphate cellulose without resin or binder, and by the 1930s it had replaced almost all insulating parts in the transformer (Wikipedia, Electrical insulation paper). For higher temperatures there is aramid pressboard: Nomex 994 comes in 14 thicknesses from 1.0 to 9.6 mm and absorbs 8 to 10 % oil (DuPont). How the transformer works and which types exist is explained in the article on the electrical transformer, the cellulose material in the article on pressboard.
High-voltage machines: mica around every conductor
High-voltage motors run at several kilovolts, and CHEManager refers to motors up to 11 kV (risks with high-voltage motors). Von Roll grades its insulation systems by rated voltage into below 6 kV, 6 to 13.8 kV and above 13.8 kV. A VPI winding, meaning one impregnated with resin under vacuum, is built from the inside out like this (FH Joanneum, Sumereder and Trattnig, after Von Roll):
- Copper conductor with conductor insulation of enamel, glass yarn or thin mica tape.
- Inner corona protection, which smooths the field at the conductor edges.
- Main insulation made of several layers of mica tape.
- Outer corona protection in the slot and stress-grading tape at the slot exit.
- Phase separator and spacer between the coils.
- Slot wedge, which holds the coil in the slot.
Mica is at the centre because this layered silicate contains no carbon chain that partial discharge could break down. For slot wedges and slot insulation, Von Roll lists G-11 glass laminate and epoxy laminates, and the mica laminate Samicanite for moulded parts. How the tape is built and how resin-rich and VPI differ is explained in the article on mica tape, rigid mica sheets in the article on micanite.
Cables: XLPE between two semiconducting layers
A medium- or high-voltage cable carries its cross-linked polyethylene (XLPE) insulation between an inner and an outer semiconducting layer. Both smooth the field and prevent air gaps at the conductor surface. XLPE is standard up to 36 kV and, in a suitable design, reaches 500 kV, as far as the older oil-filled cable with impregnated paper (Wikipedia, Hochspannungskabel). Helukabel extrudes both semiconducting layers and the insulation in one pass and calls this a partial-discharge-free design. GOBA does not make cables; the layer structure is explained in the article on cable insulation.
Switchgear: gas, vacuum and cast resin
Gas-insulated switchgear needs far less space than air-insulated switchgear and has almost entirely replaced it indoors. One gas without SF6 is ABB's fluoroketone mixture AirPlus, which first went into service in 2015 at the Zurich municipal utility ewz in switchgear for 170 kV and 24 kV (Wikipedia, Gasisolierte Schaltanlage). Cast-resin post insulators carry 12 to 40.5 kV in indoor installations (Wikipedia, Isolator).
Why high-voltage insulation fails
A single overvoltage rarely destroys well-designed insulation. Failure usually comes after years, and it starts at a small spot:
- Partial discharges in voids, gas bubbles and at burrs erode organic insulation from inside and drive branching channels through the material, known as treeing. It was first described as a failure cause in buried polymer cables in 1969 (Wikipedia, Electrical treeing).
- In polyethylene exposed to water, fine water-filled structures called water trees grow. They are usually not critical until an electrical tree starts from them.
- On dirty, damp surfaces, leakage current flows and dries out bands, and discharges ignite at these bands, burning tracks and bridging sheds.
- Moisture in paper and pressboard splits the cellulose by hydrolysis and lowers the dielectric strength of the oil.
- At edges, points and wire ends, corona ignites, produces ozone and attacks nearby plastics.
In solid insulating parts, almost every one of these chains starts at an edge or an air inclusion. A burr on a die-cut part, a frayed cut edge or delamination in a laminate concentrates the field exactly where the first discharge ignites. How corona forms is covered in the article on corona discharge, the mechanisms from spark to erosion in the article on voltage breakdown.
Which test voltage high-voltage insulation has to pass
In the examples below, the power-frequency test voltage is one and a half to two and a half times the rated voltage, and the lightning impulse lies far above that. Testing covers two kinds of stress: power-frequency AC for one minute and the lightning impulse, a voltage pulse with a 1.2 µs rise and a 50 µs time to half value.
| Equipment | Rated voltage | Test | Source |
|---|---|---|---|
| Medium-voltage switchgear | 12 kV | 28 kV AC, 75 kV lightning impulse | Siemens, 8DJH tender specification |
| XLPE cable | 6/10 kV | 15 kV test voltage | Helukabel |
| Machine winding | 6.6 kV | 14.2 kV by the rule 2 U plus 1 kV, at least 1.5 kV | OVE, after IEC 60034-1 |
| Silicone long-rod insulator | 550 kV | 1,550 kV lightning impulse, 1,050 kV switching impulse | Siemens, 3FL5 and 3FL6 |
The OVE guideline is frank about the purpose of testing machines: coils whose strength lies only just above the rated voltage are deliberately broken down during the test and replaced. The intact insulation is not harmed. Which standards apply to household appliances, motors and capacitors, and how a high-voltage test runs, is covered in the article on dielectric withstand.
What GOBA supplies for high-voltage equipment
For high-voltage equipment, GOBA supplies the flexible insulating materials and the parts made from them. From our insulation materials we supply insulating papers such as pressboard, aramid paper (Nomex as standard) and the laminates DMD, NMN and NKN, plus polyimide film (Kapton as standard), mica tape in all variants on request, micanite as sheet or roll, and GRP and fabric-based laminate (HGW) sheets on request. Formats range from roll and sheet to blanks, die-cut and formed parts, in thicknesses from 7 µm to 3 mm depending on the material. In stamping and forming we handle formats up to 1,000 x 2,000 mm.
An obvious objection: high voltage is a job for the large system suppliers, and a cutting and die-cutting shop is too small for it. The electrical design stays with you, and we process the same branded materials, such as Nomex and Kapton, that appear in the insulation systems. We die-cut thicknesses from 0.023 to 3.0 mm, produce from a single piece and deliver stocked material within two weeks at most. That makes a prototype or a spare-parts run for a single machine worthwhile. For a quote we need:
- A drawing with tolerances.
- Material, thickness and quantity.
- For rolls, also width, length, core diameter and core material.
- Operating voltage, thermal class and installation position if you would like a material recommendation.
For generators and transformers in wind and hydro power plants, the page on renewable energy shows where our materials are used. Questions about material and thickness are answered by our technical consulting team.
