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wire insulated by an outer layer of polyethylene

Electrical insulator is a material or object that resists the flow of electric current. When a Electric potential is placed across an insulator, very little current flows. 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 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 Electronic band structure (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 Plasma (physics)s 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 image:pylon.detail.arp.750pix.jpg

High-voltage insulators used for high-voltage electric power transmission are made from glass, porcelain, or composite material. 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.

Recently, some electric utilities have begun converting to polymer Composite material materials for some types of insulators which consist of a central rod made of fibre reinforced plastic and an outer weathershed made of Silicone or EPDM rubber. Composite insulators are less costly, lighter in weight, and have excellent Hydrophobe capability. This combination makes them ideal for service in polluted areas. However, these materials do not yet have the long-term proven service life of glass and porcelain.

History 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.

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, requiring manual reseating.

Amongst 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 Denby) producing them from around 1860 and Bullers from 1868. 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.

The invention of suspension-type insulators made high-voltage power transmission possible. Pin-type insulators were unsatisfactory over about 60,000 volts.

Collecting hobby Why collect insulators?

http://en.wikipedia.org/wiki/Pin_insulator#References

Summary:

In the late 1960s and early 1970s, 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. A number of websites exist that hold these items as the primary focus, and eBay even has a separate category for insulators. One private website is the Glass Insulators Reference Site.

Collectors have defined a classification system for some of the different styles of small insulators, a price guide (last published in 2003). This is specific to collectors; it is unrecognised, irrelevant and largely unknown by manufacturers. One US-based collectors organization is the ( National Insulator Association). One magazine in the hobby is a monthly publication, Crown Jewels of the Wire, which has been published since March 1969About Crown Jewels of the Wire

Insulation of antennas In most cases a mast radiator requires an insulating mounting, therefore insulators of steatite are used. They have to withstand not only the voltage of the mast radiator to ground, which can reach values up to 400 kV at some antennas, but also the weight of the mast construction and dynamic forces. Arcing horns and lightning arresters are necessary because lightning strikes in the mast are common.

At guyed mast radiators, it is often necessary to use insulators in the guy (if they are not grounded via a coil at the anchor bases), in order to prevent undesired electrical resonances of the guys. These insulators also have to be equipped with overvoltage protection equipment. For the dimensions of the guy insulation, static charges on guys have to be considered, at high masts these can be much higher than the voltage caused by the transmitter requiring guys divided by insulators in multiple sections on the highest masts. In this case, guys which are grounded at the anchor basements via a coil - or if possible, directly - are the better choice.

Insulation in electrical apparatus The most important insulation material is air. A wide variety of solid, liquid, and gaseous insulators are also used in electrical apparatus. In smaller transformers, electrical generator, and electric motors, insulation on the wire coils consists of up to four thin layers of polymer varnish film. Film insulated magnet wire permits a manufacturer to obtain the maximum number of turns within the available space. Windings that use thicker conductors are often wrapped with supplemental fiberglass insulating tape. Windings may also be impregnated with insulating varnishes to prevent corona discharge 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. Bus bars and circuit breakers in switchgear may be insulated with glass-reinforced plastic insulation, treated to have low flame spread and to prevent tracking of current across the material.

In older apparatus made up to the early 1970s, boards made of compressed asbestos may be found; while this is an adequate insulator at power frequencies, handling or repairs to asbestos material will release dangerous fibers into the air and must be carried out with a high level of precautions. Live-front switchboards up to the early part of the 20th century were made of slate or marble.

Some high voltage equipment is designed to operate within a high pressure insulating gas such as sulfur hexafluoride.

Insulation materials that perform well at power and low frequencies may be unsatisfactory at radio frequency, due to heating from excessive dielectric dissipation.

Electrical wires may be insulated with polyethylene, crosslinked polyethylene, polyvinyl chloride, rubber-like polymers, oil impregnated paper, Teflon, silicone, or modified ethylene tetrafluoroethylene (ETFE). Larger power cables may use mineral-insulated copper-clad cable, depending on the application.

Flexible insulating materials such as Polyvinyl chloride are used to insulate the circuit and prevent human contact with a 'live' wire -- one having voltage of 600 volts or less. Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.

Class 1 and Class 2 insulation Portable and hand-held electrical appliances must be electrically insulated to protect users from electric shock.

Class 1 insulation requires that the metal body of the apparatus/equipment is solidly connected via a "grounding" wire which is earthed at the main Service Panel; but only basic insulation of the conductors is needed. This equipment is easily identified by a round pin for the grounding connection.

Class 2 insulation means that the equipment/apparatus is double insulated and is used on some appliances such as electric shavers, hair dryers and portable power tools. Double insulation requires that the devices have wiktionary:basic and supplementary insulation, each of which is sufficient to prevent Electrical shock. All internal electrically energized components are totally enclosed within insulated packaging which prevents any contact with "live" parts. They can be recognised because their leads have two pins, or on 3 pin plugs the third (Ground (electricity)) pin is made of plastic rather than metal. In the European Union, double insulated appliances all are marked with a symbol of 2 squares, one inside the other.

See also

• Arcing horns
• Electricity pylon
• Electrical substation
• Mast radiator
• Mott insulator
• Emily Davenport - used silk dress

References

• Bullers of Milton Sue Taylor, Churnet Valley Books. 2003 ISBN 1-897949-96-0

Notes

External links

• Glass Insulators Reference Site
• National Insulator Association
• Specialising in UK telegraph insulators with a railway bias
• One person's obsession with telephone pole insulators
• Transcontinental Telegraph Insulators, 1867
• www.insulatorscanada.com
• Insulator Patent Reference Library - Contains over 2,800 US patents relating to electrical insulators and related items

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