Insulators are insulating bodies used to support conductors. They are mostly applied for insulating external live conductors in overhead transmission and distribution lines, busbars in power plants and substations, and various electrical equipment. They are generally composed of insulating parts (such as glass or ceramic) and metal parts (such as steel feet, steel caps, flanges, etc.) bonded with adhesive or mechanically clamped. The basic properties of insulators include electrical, mechanical, and thermal properties. In addition, they also have properties such as environmental resistance and aging resistance. When the voltage level of an insulator increases, its size and weight also increase accordingly, but the electrical and mechanical properties do not increase proportionally, and the thermal shock resistance decreases.

(1) Electrical properties: The destructive discharge that occurs along the insulation surface is called flashover, and flashover characteristics are the main electrical properties of insulators. For different voltage levels, the withstand voltage requirements for insulators vary. The indicators include power frequency dry and wet withstand voltage, lightning impulse withstand voltage, lightning impulse chopped wave withstand voltage, switching impulse withstand voltage, etc. To avoid breakdown during operation, the breakdown voltage of an insulator is higher than its flashover voltage. In factory tests, breakdown-type porcelain insulators generally undergo a spark test, i.e., applying high voltage to cause frequent sparks on the insulation surface for a certain period to check whether breakdown occurs. Some insulators also require corona tests, radio interference tests, partial discharge tests, and dielectric loss tests. For insulators used at high altitudes, the electrical strength decreases due to lower air density; therefore, their withstand voltage, when converted to standard atmospheric conditions, should be correspondingly increased. The flashover voltage of contaminated insulators under wet conditions is significantly lower than their dry and wet flashover voltages. Therefore, in polluted areas, insulation must be reinforced or anti-pollution-type insulators must be used, with a creepage distance ratio (ratio of creepage distance to rated voltage) higher than that of normal types. Compared with AC insulators, DC insulators have poorer electric field distribution, and are also subject to contamination particle attraction and electrolysis, resulting in lower flashover voltage. They generally require special structural design and a larger creepage distance.

(2) Mechanical properties: During operation, insulators are often subjected to the gravity and tension of conductors, wind force, ice weight, the insulator's own weight, conductor vibration, mechanical forces from equipment operation, short-circuit electromagnetic forces, earthquakes, and other mechanical forces. Relevant standards impose strict requirements on mechanical performance.

(3) Thermal properties: Outdoor insulators are required to have the ability to withstand rapid temperature changes. For example, porcelain insulators must withstand several thermal cycles without cracking. For insulating bushings through which current flows, the temperature rise of their components and insulating parts, as well as the permissible short-time current value, must comply with the requirements of relevant standards.