Electrical insulating materials and their classification. Fibrous electrical insulating materials

Some materials used in electrical appliances and power supply schemes have dielectric properties, that is, they have a large current resistance. This ability allows them not to pass current, and therefore they are used to create insulation of live parts. Electrical insulating materials are not only designed to separate current-carrying parts, but also to create protection against the dangerous effects of electric current. For example, the power cords of electrical appliances are covered with insulation.

Electrical insulating materials and their application

Electrical insulating materials are widely used in industry, radio and instrument making, the development of electrical networks. The normal operation of an electrical appliance or the safety of a power supply scheme largely depends on the dielectrics used. Some parameters of the material intended for electrical insulation determine its quality and capabilities.

The use of insulation materials is subject to safety rules. The integrity of insulation is the key to safe operation with electric current. It is very dangerous to use devices with damaged insulation. Even a small electric current can have an effect on the human body.

Properties of dielectrics

Electrical insulating materials must have certain properties in order to perform their functions. The main difference between dielectrics and conductors is the large value of the specific volume resistance (109-1020 ohm-cm). The electrical conductivity of conductors is 15 times greater than in dielectrics. This is due to the fact that insulators by their nature have several times less free ions and electrons, which provide current conductivity of the material. But with the heating of the material, they become larger, which increases the current conductivity.

There are active and passive properties of dielectrics. For passive properties the most important are the passive properties. The permittivity of the material should be as small as possible. This allows the insulator not to introduce parasitic capacitances into the circuit. For a material that is used as the dielectric of a capacitor, the dielectric constant must, on the contrary, be as large as possible.

Isolation parameters

The main parameters of electrical insulation include electrical strength, specific electrical resistance, relative dielectric constant, angle of dielectric losses. When assessing the electrical insulation properties of the material, the dependence of the listed characteristics on the values of the electric current and voltage is also taken into account.

Electrical insulating products and materials have a greater value of electrical strength in comparison with conductors and semiconductors. Also important for the dielectric is the stability of the specific values when heated, increasing the voltage and other changes.

Classification of dielectric materials

Depending on the power of the current passing through the conductor, different types of insulation are used, which differ in their capabilities.

What are the parameters for dividing electrical insulating materials? Classification of dielectrics is based on their aggregate state (solid, liquid and gaseous) and origin (organic: natural and synthetic, inorganic: natural and artificial). The most common type of solid dielectrics, which can be seen on the cords of household appliances or any other electrical appliances.

Solid and liquid dielectrics, in turn, are divided into subgroups. Solid dielectrics include lacquers, laminates and various types of mica. Waxes, oils and liquefied gases are liquid electrical insulating materials. Special gaseous dielectrics are used much less often. This type also includes a natural electrical insulator - air. Its use is caused not only by the characteristics of air, which make it an excellent dielectric, but also its economy. The use of air as insulation does not require additional material costs.

Solid dielectrics

Solid electrical insulating materials are the widest class of dielectrics that are used in different fields. They have different chemical properties, and the value of the permittivity varies from 1 to 50,000.

Solid dielectrics are divided into nonpolar, polar and ferroelectrics. Their main differences are in the mechanisms of polarization. This class of insulation has such properties as chemical resistance, tracking, dendrites. Chemical resistance is expressed in the ability to withstand the influence of various aggressive environments (acid, alkali, etc.). Trigoregostoykost determines the ability to resist the effects of the electric arc, and dendritic resistance - the formation of dendrites.

Solid dielectrics are used in various fields of energy. For example, ceramic electrical insulating materials are most often used as linear and bushing insulators in substations. As insulation of electrical devices use paper, polymers, glass-cloth. For machines and apparatuses, varnishes, cardboard, compound are most often used.

For application in various operating conditions, insulation is given some special properties by combining different materials: heat resistance, moisture resistance, radiation resistance and frost resistance. Heat-resistant insulators are able to withstand temperatures up to 700 ° C, they include glasses and materials based on them, organosilites and some polymers. Moisture-proof and tropic-resistant material is fluoroplastic, which is non-hygroscopic and hydrophobic.

Isolation resistant to radiation is used in devices with atomic elements. It includes inorganic films, certain types of polymers, glass-textolite and mica-based materials. Frost-proof insulation is considered that does not lose its properties at temperatures up to -90 ° C. Special requirements are imposed on insulation intended for appliances operating in space or vacuum conditions. For these purposes, vacuum-dense materials are used, which include special ceramics.

Liquid dielectrics

Liquid electrical insulating materials are often used in electrical machines and apparatus. In the transformer, the role of insulation is played by oil. Liquid dielectrics also include liquefied gases, unsaturated vaseline and paraffin oils, polyorganosiloxanes, distilled water (purified of salts and impurities).

The main characteristics of liquid dielectrics are dielectric permeability, electrical strength and electrical conductivity. Also, the electrical parameters of dielectrics largely depend on the degree of their purification. Solid impurities can increase the electrical conductivity of liquids due to the expansion of free ions and electrons. Purification of liquids by distillation, ion exchange, etc. Leads to an increase in the strength of the electrical strength of the material, thereby reducing its electrical conductivity.

Liquid dielectrics are divided into three groups:

• Oil oils;
• vegetable oils;
• Synthetic fluids.

The most commonly used oil oils, such as transformer, cable and condenser. Synthetic fluids (organosilicon and organofluorine compounds) are also used in instrumentation. For example, organosilicon compounds are frost-resistant and hygroscopic, therefore they are used as an insulator in small transformers, but their cost is higher than the price of petroleum oils.

Vegetable oils are practically not used as insulating materials in electrical insulation technology. These include castor, linseed, hemp and tung oil. These materials are weakly polar dielectrics and are used mainly for impregnation of paper capacitors and as a film-forming substance in electrically insulating varnishes, paints, enamels.

Gaseous dielectrics

The most common gaseous dielectrics are air, nitrogen, hydrogen and SF6 gas. Electrical insulating gases are divided into natural and artificial. The natural applies to air, which is used as insulation between live parts of power lines and electric machines. As an insulator air has disadvantages, which makes it impossible to use it in sealed devices. Due to the presence of a high concentration of oxygen, air is an oxidizing agent, and low electrical strength of air appears in non-uniform fields.

In power transformers and high-voltage cables, nitrogen is used as insulation. Hydrogen, in addition to electrical insulating material, is also forced cooling, so it is often used in electrical machines. In sealed installations, SF6 gas is most often used. Filling with SF6 gas makes the device explosion-proof. It is used in high-voltage switches due to its arc-suppression properties.

Organic dielectrics

Organic dielectric materials are divided into natural and synthetic. Natural organic dielectrics are currently used extremely rarely, as production of synthetic ones is increasingly expanding, thereby reducing their cost.

To natural organic dielectrics include cellulose, rubber, paraffin and vegetable oils (castor oil). Most of the synthetic organic dielectrics are various plastics and elastomers, often used in electrical household appliances and other equipment.

Inorganic dielectrics

Inorganic dielectric materials are divided into natural and artificial. The most common of natural materials is mica, which has chemical and thermal resistance. Phlogopite and muscovite are also used for electrical insulation.

To artificial inorganic dielectrics include glass and materials based on it, as well as porcelain and ceramics. Depending on the field of application, an artificial dielectric can be given special properties. For example, for fence insulators use feldspar ceramics, which has a high tangent of dielectric losses.

Fibrous electrical insulating materials

Fibrous materials are often used for insulation in electrical apparatus and machines. They include materials of plant origin (rubber, cellulose, fabrics), synthetic textiles (nylon, kapron), as well as materials from polystyrene, polyamide, etc.

Organic fibrous materials have high hygroscopicity, so they are rarely used without special impregnation.

Recently, instead of organic materials, synthetic fibrous insulation is used, which have a higher level of heat resistance. These include glass fiber and asbestos. The glass fiber is impregnated with various varnishes and resins to increase its hydrophobic properties. Asbestos fiber has a low mechanical strength, so often it adds cotton fiber.