Insulator (electricity)

A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at normally used voltages, and thus are employed as insulation for electrical wiring and cables.

This allows electrons to gain energy and thereby move through a conductor, such as a metal, if an electric potential difference is applied to the material.

Electrical breakdown occurs when the electric field in the material is strong enough to accelerate free charge carriers (electrons and ions, which are always present at low concentrations) to a high enough velocity to knock electrons from atoms when they strike them, ionizing the atoms.

These freed electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in a chain reaction.

When corona discharge occurs, the air in a region around a high-voltage conductor can break down and ionise without a catastrophic increase in current.

In addition, all insulators become conductors at very high temperatures as the thermal energy of the valence electrons is sufficient to put them in the conduction band.

[1][2] In certain capacitors, shorts between electrodes formed due to dielectric breakdown can disappear when the applied electric field is reduced.[3][4][5][relevant?]

In coaxial cable the center conductor must be supported precisely in the middle of the hollow shield to prevent electro-magnetic wave reflections.

The product may not have an ampacity (current-carrying capacity) rating, since this is dependent on the surrounding environment (e.g. ambient temperature).

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.

In high voltage systems containing transformers and capacitors, liquid insulator oil is the typical method used for preventing arcs.

In smaller transformers, generators, and electric motors, insulation on the wire coils consists of up to four thin layers of polymer varnish film.

Windings may also be impregnated with insulating varnishes to prevent electrical corona and reduce magnetically induced wire vibration.

Large power transformer windings are still mostly insulated with paper, wood, varnish, and mineral oil; although these materials have been used for more than 100 years, they still provide a good balance of economy and adequate performance.

Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.

In electrical apparatus such as motors, generators, and transformers, various insulation systems are used, classified by their maximum recommended working temperature to achieve acceptable operating life.

All internal electrically energized components are totally enclosed within an insulated body that prevents any contact with "live" parts.

[7] Conductors for overhead high-voltage electric power transmission are bare, and are insulated by the surrounding air.

Insulators used for high-voltage power transmission are made from glass, porcelain or composite polymer materials.

Porcelain insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed water.

[8] Glass has a higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains.

Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers.

The wires are suspended from a 'string' of identical disc-shaped insulators that attach to each other with metal clevis pin or ball-and-socket links.

Each unit is constructed of a ceramic or glass disc with a metal cap and pin cemented to opposite sides.

They are designed to reduce the electric field at the point where the insulator is attached to the line, to prevent corona discharge, which results in power losses.

The first electrical systems to make use of insulators were telegraph lines; direct attachment of wires to wooden poles was found to give very poor results, especially during damp weather.

This construction has the advantage that the ceramic is under compression rather than tension, so it can withstand greater load, and that if the insulator breaks, the cable ends are still linked.