Electrical insulation

''This page refers to electrical insulation. For thermal insulation see thermal insulation, and for sound insulation see sound proofing.

An insulator is a material or object that prevents the flow of electrical charges, thereby preventing the flow of an electric current.

The term electrical insulator has the same meaning as the term dielectric, but the two terms are often used in different contexts. Conductors and semiconductors, which permit the flow of charge are the opposite of electrical insulators. Very pure semiconductors are insulators at low temperatures unless doped with impurity atoms that release extra charges to help carry the current. A few materials (such as silicon dioxide) are almost ideal electrical insulators, a property that is invaluable in flash memory technology. Teflon is another almost ideal insulator, making it a valuable material for long term charge storage in electrets. A much larger class of materials, for example rubber 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 electrical insulation

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) 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.

High-voltage insulators

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. Alumina insulators are used where high mechanical strength is a criterion. Glass insulators were, and in some places are still used to suspend electrical power lines. Most insulator manufacturers stopped making glass insulators in the late 1960's, switching to various ceramic and, more recently, composite materials.

Recently, many electric utilities have begun converting to composite insulators which consist of a central rod made of fibre reinforced plastic and an outer weathershed made of silicone rubber or EPDM. Composite insulators are less costly, lighter weight, and they have excellent hydrophobic capability. This combination makes them ideal for service in polluted areas.

The first glass insulators used en masse had an unthreaded pinhole. These pieces of glass were positioned on a tapered wooden pin, vertically extending upwards from the pole's crossarm (commonly only two insulators to a pole and maybe one on top of the pole itself). Natural contraction and expansion of the wires tied to these "threadless insulators" resulted in insulators unseating from their pins, which required manual re-seating by a designated person.

Amongst the first to the first to produce ceramic insulators were companies in the United Kingdom, with Stiff and Doulton using stoneware from the mid 1840s, Joseph Bourne (later renamed Dendy) producing them from around 1860 and Bullers from 1868 Utility patent number. 48,906 was granted to Louis A. Cauvet on July 25, 1865 for a process to produce insulators with a threaded pinhole. To this day, pin-type insulators still have threaded pinholes.

Low-voltage insulating materials

Flexible insulating materials such as PVC (polyvinyl chloride) or neoprene are used to reduce the possibility of a person coming into contact with a 'live' wire.

Class I and Class 2 insulation

Class one insulation requires that the metal body of the equipment is solidly earthed but only basic insulation of the conductors is needed.

Some appliances such as electric shavers and hair dryers are doubly insulated to protect the user. They can be recognised because their leads have two pins, or on 3 pin plugs the third (earth) pin is made of plastic rather than metal. In the EU, double insulated appliances all are marked with a symbol of 2 squares, one inside the other.

Class 2 insulation means that the equipment is double insulated. Double insulation requires that the devices have basic and supplementary insulation, each of which is sufficient to prevent electric shock. All internal electrically "live" components are totally enclosed within insulated packaging which prevents any contact with live parts.

Collecting insulators

In the late 1960's and early 1970's glass insulators were being removed from telephone poles as advances in technology made them obsolete. As linemen were taking down the old lines, they started to notice the multitude of bright colors, company names, variety of shapes, and important historical position held by insulators in the expansion of communication technology. Presently the insulator collecting hobby has thousands of people around the world. Many collectors live in the U.S. and Canada. A number of websites exist which hold these items as the primary focus, and eBay even has a separate category for insulators. Quite probably, the largest, and most informative private website in the hobby is the Glass Insulators Reference Site.

Collectors have defined a classification system for the different styles, a price guide (last published in 2003), and a national organization (the National Insulator Association). The main magazine in the hobby is a monthly publication, Crown Jewels of the Wire, which has been published since 1969.

References

Bullers of Milton Sue Taylor Churnet Valley Books. 2003 ISBN 1897949960

External links

Insulators | Dielectrics

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