An electrical insulator is a component made from a material with very low electrical conductivity that separates live parts from each other or from earthed structures. Anyone searching the term usually means one of two very different things: the porcelain or glass insulator on an overhead-line mast, or the insulating part inside a device that keeps winding, trace or contact apart. GOBA makes only the second group, as insulation materials and insulation parts for motors, transformers and switchgear, from materials with a dielectric strength between around 15 kV/mm (polyester film) and 200 kV/mm (polyimide). This article draws a clear line between the two groups and covers the materials and ratings that matter for the device-side version.
Why an insulator does not conduct current
The electrons of an insulating material stay tightly bound and cannot move freely as they do in a conductor. Its electrical conductivity therefore sits a factor of 100,000 to 1,000,000 below that of copper or aluminium. Why plastics hold this line and which four properties separate them is explained in the article on electrically insulating plastics. How well a material resists an applied voltage is described by its dielectric strength, in kilovolts per millimetre. It sets how thin an insulating part can be at a given voltage, and is the single most important selection value for any device-side insulator.
Overhead-line insulator or device-side insulating part: what is the difference?
Overhead-line insulators hang or stand on the mast and keep the bare conductor cable clear of the earthed crossarm, outdoors, under wind load, rain and pollution. That calls for mechanically stable, weatherproof materials: porcelain, glass or silicone-coated composite rods, designed around a long creepage path that keeps moisture and dirt on the surface away from the direct route between conductor and mast.
An insulating part inside a device solves a different task. It separates windings, slots or traces inside a closed motor, transformer or switchgear housing, with no wind, rain or UV load. What matters there is film thickness, thermal rating and dielectric strength per millimetre, not mechanical weather resistance. That is exactly why overhead-line insulators are made from ceramic or glass, while device-side insulating parts are made from polyester film, polyimide, aramid paper or mica. Anyone searching for an “electrical insulator” for a device is usually after the latter.
Which insulating parts does the electrical industry count as insulators?
In motors, generators and transformers, several clearly named components carry out the insulator function. Each has a fixed position and task in the build.
| Component | Function | Typical material |
|---|---|---|
| Slot insulation | Separates the winding from the earthed stator core in the slot | Polyester film, aramid paper, DMD laminate |
| Slot closure wedge | Closes the slot opening and mechanically retains the winding | Glass-fabric laminate, pressboard |
| Phase separator | Separates individual phase windings from each other | Aramid paper, polyester film |
| Bushing insulator | Feeds a conductor through a transformer housing or cover, insulated | Oil-paper insulation, epoxy resin, porcelain at high voltage |
| Connector insulating part | Separates contacts in plugs and terminals of electrical devices | Thermoplastic, glass-fibre reinforced polyamide |
The bushing insulator shows that the line is not always sharp: in a small distribution cabinet it is a plain plastic part from device manufacturing, in a high-voltage substation it is a porcelain body from power engineering. At GOBA, the device-side version falls into our material scope, the overhead-line insulator does not.
Which materials does GOBA use for insulating parts?
Material choice depends on the required dielectric strength, operating temperature and mechanical load. Four material groups cover most device-side insulation tasks.
| Material | Dielectric strength | Typical use |
|---|---|---|
| Polyimide (Kapton) | around 200 kV/mm | High-temperature slot insulation, hairpin windings |
| Mica | 100 to 200 kV/mm | High-voltage windings, generators |
| PTFE | around 60 kV/mm | Chemically resistant connector insulating parts |
| Polyester film (Mylar) | around 15 to 20 kV/mm | Standard slot insulation, phase separators |
For comparison, air reaches only about 3 kV/mm under normal conditions, well below any film system. The full overview with further materials is in the article on dielectric strength.
How is an insulating part selected for a device?
Three values fix an insulating part: the thermal class for continuous operating temperature, the film thickness for the required dielectric strength at the given operating voltage, and the manufacturing tolerance for fit within the available space. Choosing only the material while overlooking the thermal class risks premature ageing from overheating, even if the voltage rating is sufficient.
- Thermal class under IEC 60085, class F (155 °C) and class H (180 °C) cover most motor and transformer applications.
- Thickness in the range of 0.05 to 0.5 mm for standard slot insulation, depending on operating voltage and the required safety margin.
- Cutting tolerance under DIN ISO 2768, important for molded parts that must fit precisely into a stator slot or housing.
A thinner film saves space and material but lowers the dielectric strength per part in volts. For tightly dimensioned applications, such as hairpin stators in e-mobility, a material with higher dielectric strength per millimetre is worth the cost over simply using a thicker standard film.
GOBA Takeaway
Anyone searching for “electrical insulators” for device manufacturing needs films, papers and laminates with a defined dielectric strength, not porcelain or glass. GOBA has made exactly these insulating parts for over 60 years, from slot insulation to phase separators, in polyester film, polyimide, aramid paper and mica. For standard formats and roll goods, the route runs through insulation materials, for molded parts to drawing through custom products.
