Electrical insulating materials, materials used in electrical and radio engineering devices to separate current-carrying parts having different potentials, to increase the capacity of capacitors, and also serve as a heat-conducting medium in electrical machines, devices, etc. Dielectrics are used as electrical insulating materials, which, in comparison with conductor materials have a significantly higher specific volume electrical resistance rv = 109—1020 Ohm·cm (conductors 10-6—10-4 Ohm·сm). Key characteristic of electrical insulating materials: specific volume and surface resistance rv and rs, relative dielectric constant e, temperature coefficient of dielectric capacitivity 1/e·de/dTdegrees-1, dielectric loss angle d, dielectric strength Еother (electric field strength when breakdown occurs. The dependence of these characteristics on the frequency of the electric current and voltage value is also taken into account when assessing electrical insulating materials.
Electrical insulating materials can be classified according to several criteria: state of aggregation, chemical composition, methods of production, etc. Depending on the state of aggregation, there are solid, liquid, and gaseous electrical insulating materials. Solid electrical insulating materials constitute the most extensive group and, in accordance with their physicochemical properties, structure, and production characteristics, are divided into a number of subgroups, for example, laminated plastics, papers and fabrics, varnished fabrics, mica and materials made of them, electroceramic, etc. These same materials can conditionally include varnishes, filling and impregnating compositions, which, although they are in a liquid state, are used as electrical insulating materials in a hardened state. Dielectric strength of solid electrical insulating materials (at 20 °C and electric current frequency 50 Hz) lies within the threshold from 1 Mv/m (for example, for some resin based materials) up to 120 Mv/m (for example, for polyethylene terephthalate). (For information on the use and production of solid electrical insulating materials, see Art. Electrical insulation, Insulator, Varnishes, Mica, Fiberglass, Plastic masses, Polymer compounds, Synthetic resins.) Liquid electrical insulating materials are electrical insulating oils, including petroleum, vegetable, and synthetic ones. Certain types of liquid electrical insulating materials differ from each other in viscosity and have different electrical specifications. Capacitor and cable oils possess the best electrical properties. Electrical strength of liquid electrical insulating materials at 20 °С and frequency 50 Hz are normally within the threshold of 12—25 Mv/m, for example, for transformer oils 15—20 Mv/m (also see Liquid dielectrics). There are semi-liquid electrical insulating materials – vaselines. Gaseous electrical insulating materials —air, SF6 gas (sulfur hexafluoride), freon-21 (dichlorofluoromethane). Air is a natural insulator (air gaps in electrical machines, apparatus, etc.) and has the electrical strength of about 3 Mv/m. SF6 gas and freon-21 have the electrical strength of about 7.5 MV/m and are used as electrical insulating materials mainly in cables and various electrical apparatus.
Organic and inorganic electrical insulating materials are distinguished based on chemical composition. The most common electrical insulating materials are inorganic (mica, ceramics, etc.). Natural (easy) materials and artificial (synthetic) materials are used as electrical insulating materials. Artificial electrical insulating materials can be created with a given set of necessary electrical and physicochemical properties; therefore, such electrical insulating materials are most commonly used in electrical engineering and radio engineering. In accordance with the electrical properties of the molecules of a substance, there are polar (dipole) and nonpolar (neutral) electrical insulating materials. Polar electrical insulating materials include Bakelite, Sovol, Halovax, polyvinyl chloride, and many organosilicon materials; non-polar – hydrogen, benzene, carbon tetrachloride, polystyrene, paraffin, etc. Polar electrical insulating materials are characterized by advanced dielectric permeability and slightly increased electrical conductivity and hygroscopicity. Mechanical properties are essential for solid electrical insulating materials: tensile and compressive strength, static and dynamic bending, hardness, processibillity, as well as thermal properties (heat resistance and heat stability), humidity permeability, hygroscopicity, spark resistance, etc. Heat resistance is characterized by the upper temperature range where the electrical insulating materials are able to maintain their mechanical and operation properties. Heat stability of electrical insulating materials – ability to withstand high temperatures (from 90 to 250 °C) without noticeable changes in the electrical specifications of the material. In electrical engineering, electrical insulating materials are classified into seven classes. The most heat-resistant electrical insulating materials are inorganic materials (mica, porcelain, glass without binders or with organoelement binders). The ability to withstand temperature drops is also important for fragile materials (glass, porcelain). While electrically separating the conductors, electrical insulating materials shall not interfere with the removal of heat from windings, cores, and other elements of electrical machines and installations. Therefore, heat conductivity is an important property of electrical insulating materials. Mineral fillers are added to liquid electrical insulating materials to increase the coefficient of heat conductivity. Most electrical insulating materials absorb moisture to one degree or another (hygroscopic). To increase moisture resistance porous el. materials are impregnated with oils, synthetic liquids, and compounds. Glazed porcelain, glass, etc. only can be considered absolutely moisture-resistant.