Electrical insulation
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- 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 which contains no free electrons to permit the flow of electricity. When a voltage is placed across an insulator, no charge flows.
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[edit] Explanation
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 contain movable charges are the opposite of electrical insulators. Very pure semiconductors are insulators at low temperatures unless doped with impurity atoms that release extra charges which can flow in a 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).
[edit] 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) 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.
[edit] 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, and are covered with a smooth glaze to shed dirt. 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 1960's, switching to various ceramic and, more recently, composite materials.
Recently, some electric utilities have begun converting to composite for some types of 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. However, these materials do not yet have the long-term proven service life of glass and porcelain.
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 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.
The invention of suspension-type insulators made high-voltage power transmission possible. Pin-type insulators were unsatisfactory over about 40,000 volts.
[edit] Insulation of mast radiators
In most cases a mast radiator construction 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 mast radiators, 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.
[edit] Insulation in electrical apparatus
The most important insulation material is air, but a wide variety of solid insulators are used in electrical apparatus. In small transformers and electric motors with only a low voltage between turns, often the insulation on the coil wires is only a couple of layers of varnish. Large power transformer windings are still mostly insulated with paper, wood, and 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 old apparatus made up to the early 1970's, boards made of compressed asbestos may be used; 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 marble. Electrical power cables may be insulated with polyethylene, PVC, rubber (natural or synthetic), impregnated paper, teflon, or even compressed inorganic powder, depending on the application and voltage class.
Insulation materials that perform well at power and low frequencies may be unsatisfactory at radio frequency, due to excess dielectric dissipation.
[edit] Low-voltage insulating materials
Flexible insulating materials such as PVC (polyvinyl chloride) or neoprene are used to insulate the circuit and prevent human contact with a 'live' wire. Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.
[edit] Class 1 and Class 2 insulation
Class 1 insulation requires that the metal body of the equipment is solidly earthed but only basic insulation of the conductors is needed.
Class 2 insulation means that the equipment is double insulated and is used on some appliances such as electric shavers and hair dryers. 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. 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.
[edit] Collecting insulators
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). The main magazine in the hobby is a monthly publication, Crown Jewels of the Wire, which has been published since 1969.
[edit] See also
- Arcing horns
- Pylon
- Electrical substation
- Mast radiator
- Mott insulator
- Emily Davenport - used silk dress
[edit] References
Bullers of Milton Sue Taylor Churnet Valley Books. 2003 ISBN 1-897949-96-0
[edit] 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.comcs:Elektrický izolant
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