Insulators are commonly used as a flexible coating on electric wire and cable. Since air is an insulator, in principle no other
substance is needed to keep power where it should be. High-voltage power lines commonly use just air, since a solid (e.g.,
plastic) coating is impractical. However, wires which touch each other will produce cross connections, short circuits, and fire hazards. In coaxial cable the center conductor must be supported exactly in the middle of the hollow shield in order to prevent EM wave reflections. Finally, wires which expose voltages higher than 60V can cause human shock and electrocution hazards. Insulating coatings help to prevent all of these problems.
Some wires have a mechanical covering which has no voltage rating;e.g.: service-drop, welding, doorbell, thermostat. An insulated wire or cable has a voltage rating and a maximum conductor temperature rating. It may not have an ampacity (current-carrying capacity) rating, since this is dependent upon the surrounding environment (e.g. ambient temperature).
In electronic systems, printed circuit boards are made from epoxy plastic and fibreglass. The nonconductive boards support layers of copper foil conductors. In electronic devices, the tiny and delicate active components are embedded within nonconductive epoxy or phenolic plastics, or within baked glass or ceramic coatings.
In microelectronic components such as transistors and ICs, the silicon material is normally a conductor because of doping, but it can easily be selectively transformed into a good insulator by the application of heat and oxygen. Oxidized silicon is quartz, i.e. silicon dioxide, the primary component of glass.
In high voltage systems containing transformers and capacitors, liquid insulator oil is the typical method used for preventing arcs. The oil replaces the air in any spaces which must support significant voltage without electrical breakdown. Other methods of insulating high voltage systems are ceramic or glass wire holders, gas, vacuum, and simply placing the wires with a large separation, using the air as insulation.
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