Insulating materials are used in electric devices to keep current from flowing where it is not desired. They are simply materials with a sufficiently high resistance (or sufficiently low conductance), also known as dielectric materials. Typically, plastics or ceramics are used. When an insulator is functional, its resistance is infinite, or the conductance zero, so that zero current flows through it.
Keywords: Electrical insulation, Insulating materials, Dielectric materials, High-voltage insulation, Ceramic insulators, Plastic wire insulation, Insulation breakdown voltage, Insulator resistance, Arc prevention in insulators, Industrial insulation, Voltage protection devices, Heat-resistant insulation, Transmission line insulators, High-voltage electrical safety
Any insulator has a specific voltage regime within which it can be expected to perform. If the voltage difference between two sides of the insulator becomes too large, its insulating properties may break down due to microscopic changes in the material, where it actually becomes conducting. Generally, the thicker the insulator, the higher the voltage difference it can sustain. However, temperature can also be important; for example, plastic wire insulation may melt if the wire becomes too hot.
The insulators often seen on high-voltage equipment consist of strings of ceramic bells, holding the energized wires away from other components (e.g., transmission towers or transformers). The shape of these bells serves to inhibit the formation of arcs along their surface. The number of bells is roughly proportional to the voltage level, though it also depends on climate. For example, the presence of salt water droplets in coastal air encourages ionization and therefore requires more insulation to prevent arcing.
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