Insulators

Electrical insulator is a material or object that resists the flow of electric current. When a voltage is placed across an insulator, very little current flows. more...

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An object intended to support or separate electrical conductors without passing current through itself is called an insulator. An insulator is a material with atoms that have tightly bonded valence electrons and resist the flow of electrical current.

The term electrical insulation has the same meaning as the term dielectric.

Some materials such as silicon dioxide or teflon are very good electrical insulators. A much larger class of materials, for example rubber-like polymers and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).

Physics of conduction in solids

Electrical insulation is the absence of electrical conduction. Electronic band theory (a branch of physics) predicts that a charge will flow whenever there are states available into which the electrons in a material can be excited. This allows them to gain energy and thereby move through the conductor (usually a metal). If no such states are available, the material is an insulator.

Most (though not all, see Mott insulator) insulators are characterized by having a large band gap. This occurs because the "valence" band containing the highest energy electrons is full, and a large energy gap separates this band from the next band above it. There is always some voltage (called the breakdown voltage) that will give the electrons enough energy to be excited into this band. Once this voltage is exceeded, the material ceases being an insulator, and charge will begin to pass through it. However, dielectric breakdown is usually accompanied by physical or chemical changes that permanently degrade the material's insulating properties.

Materials which lack electron conduction must also lack other mobile charges as well. For example, if a liquid or gas contains ions, then the ions can be made to flow as an electric current, and the material is a conductor. Electrolytes and plasmas contain ions and will act as conductors whether or not electron flow is involved.

Telegraph and power transmission insulators

Suspended wires for electric power transmission are bare, except when connecting to houses, and are insulated by the surrounding air and where connected to towers, as detailed below.

Material

High-voltage 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 dirt. The design of insulators often includes deep grooves, or sheds, that provides increased arc-lengths. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Glass insulators were (and in some places still are) used to suspend electrical power lines. Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to various ceramic and, more recently, composite materials.

Read more at Wikipedia.org