Insulating material is a substance with very low electrical conductivity that separates live parts from each other and keeps current on its intended path. In electrical engineering this substance is called an insulating material or electrical insulation material. This article draws the line to thermal insulation, sorts the material groups, gives the typical thermal class and installation point for each, walks through selection in six steps and shows the supply formats in which GOBA has been processing these insulation materials since 1959.
- Electrical insulating material is described by dielectric strength in kilovolts per millimetre and by thermal class to IEC 60085, thermal insulation by thermal conductivity in watts per metre and kelvin.
- Seven material groups cover electrical insulation: films, papers and nonwovens, laminates, mica, solid plastics, casting and impregnating resins, and fabrics.
- Selection starts with operating temperature and ends with the manufacturing format in which the material enters the part.
- The most expensive mistake is a thermal class chosen too tightly. Around 10 kelvin of sustained overtemperature halves the service life of the insulation.
Why an insulating material carries no current and still ages
The electrons of an insulating material stay bound inside the atomic structure, so no appreciable current flows. Its specific electrical resistance is at least 1010 ohm centimetres, many orders of magnitude above copper. Physically the material works as a dielectric: in an electric field it only displaces charge instead of letting it flow. How strong that field may become before the material breaks down is given by its dielectric strength. How well it actually separates in service is measured as insulation resistance.
Those two values alone do not settle a material decision. An insulating material ages as soon as it gets warm, and it ages faster the warmer it gets. Electrical engineering therefore assigns every material a thermal class. The classes run from Y at 90 °C through classes F at 155 °C and H at 180 °C, both common in motor building, up to class C from 220 °C. Which class a material reaches is settled by a long-term test to IEC 60216, laid down in the thermal classes of IEC 60085.
Insulating material or thermal insulation: the most common mix-up
Search for insulating material and you land, depending on the source, at mineral wool or at aramid paper. One material holds back voltage, the other holds back heat. A material that separates electrically very well can conduct heat very well, and the other way round. The table below separates the two fields.
| Feature | Electrical insulating material | Thermal insulation material |
|---|---|---|
| Task | Separates voltage potentials from each other | Slows the heat flow through an assembly |
| Leading rating | Dielectric strength in kV/mm | Thermal conductivity in W/(m·K) |
| Governing standard | IEC 60085 for the thermal class, IEC 60243 for breakdown | EN ISO 10456 and construction product regulation |
| Typical thickness | 0.023 to 3.0 mm | 40 to 300 mm |
| Typical materials | Polyester film, aramid paper, mica, pressboard | Mineral wool, expanded polystyrene, polyurethane foam |
| Installation point | Stator slot, winding head, battery module, control cabinet | Wall, roof, pipework, furnace lining |
There is exactly one overlap that matters in device engineering: thermal insulation in battery systems, where a thin separating layer has to block voltage and limit heat transfer between neighbouring cells at the same time. There the specification calls for both ratings at once. Everything else belongs cleanly apart.
Thermal insulating materials: ratings and temperature ranges
Thermal insulating materials slow the heat flow instead of blocking voltage. Two values describe them: thermal conductivity λ in W/(m·K) and the permissible service temperature. The smaller λ is, the better the material insulates at a given thickness. The higher the temperature limit, the closer it may sit to the heat source. Five groups cover technical demand.
| Group | Thermal conductivity in W/(m·K) | Typical temperature range |
|---|---|---|
| Mineral wool (glass and rock wool) | 0.035 to 0.045 | glass wool to around 400 °C, rock wool to around 700 °C |
| Foams (EPS, rigid PU foam) | 0.025 to 0.040 | EPS to around 80 °C, PUR and PIR to around 130 °C |
| Calcium silicate | 0.05 to 0.07 | high-temperature boards to 1,000 °C |
| Aerogel as a nonwoven-reinforced blanket | 0.015 to 0.021 | to around 650 °C |
| Ceramic fibre (aluminium silicate) | 0.09 at 400 °C to 0.48 at 1,400 °C | classification temperature from 1,260 °C to EN 1094 |
Mineral wool, foams and calcium silicate belong on the wall, on the pipe and in the furnace. GOBA supplies none of that, and anyone insulating a facade should buy from a builders merchant. In the thermal field we supply exactly where the insulation enters a device as a thin layer: thermal separating layers and insulation blanks for battery systems and device engineering. We slit and punch them from aerogel nonwoven composite, mica and ceramic paper, cut to the contour of the cell, usually from under one to a few millimetres thick. The barriers between the cells have to do both: slow the heat transfer to the neighbouring cell and hold the potentials apart. That blank is produced in the same tooling as a slot liner, only from a different material.
Anyone planning insulation for a plant, a pipeline or a furnace will find material choice and layer thickness under insulating plant and pipework. Anyone after a single rating will find the λ value with tables for metals, insulation materials and electrical insulation. For a part that carries both tasks the design order holds: first determine the temperature at the hottest point, then the thickness, and the branded grade last.
Which material groups count as insulating material?
Electrical insulation works with seven material groups. They differ by base substance and manufacturing format, and each group covers a typical temperature range. The thermal classes in the table give the usual range of the group, not the extreme value of a single branded grade.
| Material group | Typical materials | Typical thermal class | Typical use |
|---|---|---|---|
| Films | Polyester film (PET), polyimide film, PTFE film | B (130 °C) to C (220 °C) | Slot liners, phase separation, wound capacitors |
| Papers, pressboard and nonwovens | Aramid paper, pressboard, polyester nonwoven, kraft and crepe paper | A (105 °C) to H (180 °C) | Layer insulation, resin carrier layer, transformer building |
| Laminates | DMD, NMN, NKN, film and nonwoven composites | B (130 °C) to H (180 °C) | Slot insulation in series motors, slot wedges |
| Mica | Mica composite, mica tape, mica paper | C (220 °C and above) | High-voltage windings, generators, heating elements |
| Solid plastics | Thermoplastics, thermosets, laminated fabric | E (120 °C) to H (180 °C) | Terminal carriers, support bodies, connector insulation parts |
| Casting and impregnating resins | Epoxy, unsaturated polyester resin, silicone resin | F (155 °C) to C (220 °C) | Potting of coils, impregnation of finished windings |
| Fabrics | Glass fabric, aramid fabric, resin-coated fabric | F (155 °C) to C (220 °C) | Banding, reinforcing layer in laminates and moulded parts |
The first four groups arrive as web material on the reel and are slit, punched or formed. These are the sheet insulation materials, and they make up the largest share of demand in motor and transformer building. Solid plastics come from injection moulding or machining, resins are processed as a liquid and cure inside the part. Anyone looking for a semi-finished product is looking at the first four groups. Anyone looking for a part machined from solid stock is looking at group five. Which properties the individual polymers bring in either case is set out for 17 materials in the article on electrically insulating plastics.
Within the films, polyester film leads demand, sold under the brand names Mylar, Hostaphan or DyFilm. It covers class B and so is enough for most standard motors. When it gets hotter, polyimide takes over in class C, usually traded as Kapton. Among the papers, Nomex 410 is the reference for class H. These three materials explain most of the bills of material that reach us from design offices.
Which ratings decide suitability?
A data sheet for electrical insulation materials runs to twenty lines in no time. Six of them decide the selection.
- Dielectric strength in kV/mm, measured to IEC 60243. It sets how thin the material may be at a given operating voltage.
- Insulation resistance in megaohms or gigaohms, the service value tested on the finished device.
- Thermal class to IEC 60085, the permissible continuous operating temperature of the material.
- The CTI value for tracking resistance, relevant wherever moisture or dust reaches the surface and a leakage current can form.
- Dielectric dissipation factor tan delta, which governs self-heating of the insulation at high frequency and under inverter drive.
- Flammability to UL 94, a release condition in many specifications.
Dielectric strength spreads the materials far apart. Polyimide reaches over 200 kV/mm, mica 100 to 200 kV/mm, PTFE around 60 kV/mm, polyester film 15 to 20 kV/mm depending on grade. Air manages about 3 kV/mm under standard conditions. That spread shows why a thinner film of the right material often beats a thicker one of the wrong material: it saves space in the slot and raises the copper fill factor.
How do you select the right insulating material?
The order of the checks matters more than their number. Anyone who cuts it too fine at step one cannot recover the mistake at step six.
- Operating temperature and thermal class. Determine the real temperature at the hottest point in the part, not the ambient temperature, and choose the class with headroom above it.
- Voltage load and dielectric strength. Operating voltage, test voltage and safety factor together give the minimum thickness in the chosen material.
- Mechanical load. Tensile strength, tear propagation resistance and edge stability matter while pulling into the slot, while winding, and in service under vibration.
- Media resistance. Impregnating resin, transformer oil, coolant, cleaning agents and moisture attack materials differently. Pressboard swells in moisture, polyester film takes up practically no water.
- Processability and manufacturing format. Settle early whether the part will be slit, punched, folded or hot-formed. A material that cannot be folded cleanly is out for a moulded part, however good its electrical values are.
- Availability and price. Only once the five technical points are settled is it worth looking at lead time, minimum quantity and price per square metre.
The most expensive selection mistake is a thermal class chosen too tightly
In practice an insulating material rarely fails through breakdown. It fails through temperature. Thermal life assessment to IEC 60216 works with a simple rule of thumb: around 10 kelvin of sustained overtemperature halves the service life of the insulation. A motor fitted with class B instead of class F and running 15 kelvin above its design point in the field loses roughly two thirds of its insulation life. The insulation embrittles, the slot liner cracks at the next temperature cycle, and failure arrives years before the planned service.
We therefore advise one class of headroom wherever the design allows it. Moving from class B to class F usually costs a low double-digit percentage on the material price, on a part that adds up to cents inside the motor. A winding failure in the field costs disassembly, a rewind and plant downtime. In series production that calculation almost always favours the higher class.
The second most expensive mistake follows from it: choosing the material without looking at the manufacturing format. Fix the material first and ask only afterwards how the part will be made, and you find out during prototyping that the laminate will not fold or the film frays when punched. Then selection starts over, this time under deadline pressure.
In which formats does GOBA supply insulating material?
The material decides the function, the supply format decides how much work is left for the customer. GOBA processes electrical insulation materials in four stages, from the reel to the fitted moulded part.
- Slit reels are produced by roll slitting with a shear cut, in widths from 5 to 1,600 mm at a width tolerance of plus minus 0.1 mm, wound onto cores with 25, 55, 76 or 152 mm inner diameter.
- Flat sheets for customers who process further in house or need small quantities without tooling.
- Punched parts from material thicknesses of 0.023 to 3.0 mm, produced with steel rule dies or solid steel tooling, in dimensions up to 1,000 x 2,000 mm. That is the route through stamping and forming.
- Moulded parts brought from the flat into the third dimension by folding, embossing, bending and flanging, with heat applied for complex geometries. They are fitted into the customer core as insulation and moulded parts.
The grades processed come from the European producers: Hostaphan from Mitsubishi, Mylar from DuPont Teijin Films, Nomex and Kapton from DuPont, Triflexil from Pucaro, Trivoltherm and Evitherm from Krempel, DyTerm and DyFilm from Coveme, Viledon from Freudenberg. For Hostaphan thick films GOBA is the largest converter in Europe. All these grades are UL listed, and production runs to ISO 9001 and ISO 14001.
One detail from the shop floor appears on no data sheet: when slitting thin films the knife shaft is run up slowly, otherwise the edge frays. With aramid paper the opposite applies, where too slow a cutting speed carries dust into the reel. That experience decides edge quality, and edge quality decides whether the part pulls cleanly into the slot during assembly.
Would a standard catalogue material not do?
For many applications it would. A polyester film in class B, 0.25 mm thick, covers a large share of standard motors, and specifying a special laminate for that job buys nothing. We say so to customers whenever the requirement allows it.
It gets tight in three places: at high continuous temperatures from class H upwards, under inverter drive with steep voltage fronts and a risk of partial discharge, and in tight installation space, where every tenth of a millimetre of insulation costs copper cross section. There the right material clearly beats the catalogue item, and there it pays to talk before the drawing rather than after the first sample.
GOBA Takeaway
Insulating material in electrical engineering is settled along a chain of temperature, voltage, media and manufacturing format. Work through that chain in order and a handful of materials will carry you a long way: polyester film for class B, aramid paper and laminates for classes F and H, polyimide and mica above them. For reel stock and standard formats the route runs through our material overview, for parts to drawing through custom products. Send us your drawing with the required thermal class and we will settle material and manufacturing format in one pass.
