Insulation, Electrical

insulation intended to prevent formation of an electrical contact between parts of electrical-engineering equipment that are at differing potentials. Electrical insulation can be described in terms of electric strength, volume and surface resistance, dielectric loss, corona resistance, resistance to heat and cold, and mechanical strength.

The choice of dielectrics for electrical insulation depends on the operating conditions. For example, heat resistance is of decisive importance for the insulation of electric machines (generators and motors); in this case the insulation usually consists of mica. For the insulation of overhead power transmission lines, resistance to moisture and mechanical strength are of paramount importance; therefore, porcelain and glass are most suitable. In radio engineering equipment, electrical insulation usually consists of materials with minimum dielectric loss and maximum volume and surface resistance. Combination electrical insulation, which consists of mineral oil and of oil-impregnated cellulose (paper, cardboard, or pressboard), is used for transformers, electric capacitors, and cables.

The dimensions of insulation components are determined by the operating voltage of the installation and by the long-term strength of the electrical insulation for a specified period of service. If overvoltages (short-duration rises of voltage) can occur in an installation, the design and dimensions of the electrical insulation are also determined by the magnitude of possible over-voltages and by the short-term electrical strength.