Some materials used in electrical appliances and power supply circuits have dielectric properties, that is, they have high current resistance. This ability allows them not to pass current, and therefore they are used to create insulation of live parts. Electrical insulation materials are intended not only for the separation of live parts, but also to create protection against the dangerous effects of electric current. For example, the power cords of electrical appliances are coated with insulation.
Electrical insulation materials and their application
Electrical insulation materials are widely used in industry, radio and instrument making, and the development of electrical networks. The normal operation of an electrical appliance or the safety of a power supply circuit is largely dependent on the dielectrics used. Some parameters of the material intended for electrical insulation determine its quality and capabilities.
The use of insulating materials is subject to safety regulations. The integrity of the insulation is the key to safe operation with electric current. It is very dangerous to use appliances with damaged insulation. Even a slight electric current can affect the human body.
Dielectric Properties
Electrical insulating materials must have certain properties in order to fulfill their functions. The main difference between dielectrics and conductors is a large value of the specific volume resistance (109–1020 ohm · cm). The electrical conductivity of conductors is 15 times greater than that of dielectrics. This is due to the fact that insulators by their nature have several times less free ions and electrons, which ensure the conductivity of the material. But when heated, the material becomes larger, which contributes to an increase in conductivity.
Distinguish between active and passive properties of dielectrics. For insulating materials, passive properties are most important. The dielectric constant of the material should be as small as possible. This allows the insulator not to introduce stray capacitances into the circuit. For a material that is used as a dielectric of a capacitor, the dielectric constant should, on the contrary, be as large as possible.
Insulation options
The main parameters of electrical insulation include dielectric strength, electrical resistivity, relative dielectric constant, dielectric loss angle. When assessing the electrical insulation properties of the material, the dependence of the listed characteristics on the values of 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 specific values when heated, increased 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 of electrical insulation materials? The classification of dielectrics is based on their state of aggregation (solid, liquid and gaseous) and origin (organic: natural and synthetic, inorganic: natural and artificial). The most common type of solid dielectrics that can be seen on the cords of household appliances or any other electrical devices.
Solid and liquid dielectrics, in turn, are divided into subgroups. Solid dielectrics include varnishes, laminates and various types of mica. Waxes, oils and liquefied gases are liquid insulating materials. Special gaseous dielectrics are used much less frequently. This type also includes a natural electrical insulator - air. Its use is due not only to the characteristics of the air, which make it an excellent dielectric, but also to 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 used in various fields. They have different chemical properties, and the dielectric constant varies from 1 to 50,000.
Solid dielectrics are divided into non-polar, polar and ferroelectrics. Their main differences are polarization mechanisms. This insulation class has properties such as chemical resistance, tracking resistance, dendritic resistance. Chemical resistance is expressed in the ability to withstand the influence of various aggressive environments (acid, alkali, etc.). Traction resistance determines the ability to withstand the effects of an
electric arc, and dendritic resistance determines the formation of dendrites.
Solid dielectrics are used in various fields of energy. For example, ceramic insulating materials are most often used as linear and bushing insulators in substations. As insulation of electrical appliances use paper, polymers, fiberglass. For machines and apparatuses, varnishes, cardboard, and compounds are most often used.
For use in various operating conditions, insulation gives 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, these include glass and materials based on them, organosilites and some polymers. Moisture-resistant and tropic-resistant material is fluoroplastic, which is non-hygroscopic and hydrophobic.
Radiation-resistant insulation is used in devices with atomic elements. It includes inorganic films, some types of polymers, fiberglass and mica-based materials. Insulations that do not lose their properties at temperatures up to -90 ° C are considered frost-resistant. Particular requirements are placed on insulation intended for instruments operating in space or under vacuum conditions. For these purposes, vacuum-tight materials are used, which include special ceramics.
Liquid dielectrics
Liquid insulating materials are often used in electrical machines and apparatuses. In the transformer, oil plays the role of insulation. Liquid dielectrics also include liquefied gases, unsaturated vaseline and paraffin oils, polyorganosiloxanes, distilled water (purified from salts and impurities).
The main characteristics of liquid dielectrics are dielectric constant,
dielectric 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 growth of free ions and electrons. Purification of liquids by distillation, ion exchange, etc. leads to an increase in the electric strength of the material, thereby reducing its electrical conductivity.
Liquid dielectrics are divided into three groups:
- petroleum oils;
- vegetable oils;
- synthetic fluids.
Most commonly used are petroleum oils such as transformer, cable and condenser. Synthetic liquids (organosilicon and organofluorine compounds) are also used in apparatus engineering. 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 electrical insulating varnishes, paints, enamels.
Gaseous dielectrics
The most common gaseous dielectrics are air, nitrogen, hydrogen and SF6. Electrical insulating gases are divided into natural and artificial. Air is natural, which is used as insulation between live parts of power lines and electrical machines. As an insulator, air has drawbacks that make 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 electric strength of air is manifested in inhomogeneous fields.
In power transformers and high voltage cables, nitrogen is used as insulation. Hydrogen, in addition to electrical insulating material, also represents forced cooling, which is why it is often used in electric machines. In sealed installations, SF6 is most often used. Filling with SF6 gas makes the device explosion proof. It is used in high-voltage circuit breakers due to its arc-suppressing properties.
Organic Dielectrics
Organic dielectric materials are divided into natural and synthetic. Natural organic dielectrics are currently used extremely rarely, since the production of synthetic ones is expanding more and more, thereby reducing their cost.
Natural organic dielectrics include cellulose, rubber, paraffin and vegetable oils (castor oil). Most synthetic organic dielectrics are represented by 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. Also for electrical insulation use phlogopite and muscovite.
Artificial inorganic dielectrics include glass and materials based on it, as well as porcelain and ceramics. Depending on the application, artificial dielectric can be given special properties. For example, feldspar ceramics, which have a high dielectric loss tangent, are used for bushing insulators .
Fibrous electrical insulation materials
Fibrous materials are often used for insulation in electrical apparatus and machines. These include materials of plant origin (rubber, cellulose, fabrics), synthetic textiles (nylon, nylon), as well as materials from polystyrene, polyamide, etc.
Organic fibrous materials are highly hygroscopic, and therefore rarely used without special impregnation.
Recently, instead of organic materials, synthetic fiber insulation has been used, which has a higher level of heat resistance. These include glass fiber and asbestos. Glass fiber is impregnated with various varnishes and resins to increase its hydrophobic properties. Asbestos fiber has low mechanical strength, so cotton fiber is often added to it.