View source for Insulator (electricity)

{{Short description|Material that does not conduct an electric current}}
{{About|electrical insulation|insulation of heat|thermal insulation}}
{{Electromagnetism|cTopic=Electrostatics}}
[[File:Insulator railways.jpg|thumb|Ceramic insulator used on electrified railways. The ridged shape increases the surface distance along which electrical leakage could occur, thereby enhancing the insulator’s effectiveness, especially in wet or polluted conditions.]]
[[File:600V CV 5.5sqmm.jpg|thumb|Three-core copper wire power cable, each core with an individual colour-coded insulating sheath, all contained within an outer protective sheath]]
An '''electrical insulator''' is a material in which [[electric current]] does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—[[semiconductor]]s and [[electrical conductor|conductors]]—conduct electric current more easily. The property that distinguishes an insulator is its [[resistivity]]; insulators have higher resistivity than semiconductors or conductors. The most common examples are [[Nonmetal (chemistry)|non-metals]].

A perfect insulator does not exist because even the materials used as insulators contain small numbers of mobile charges ([[charge carrier]]s) which can carry current. In addition, all insulators become [[electrically conductive]] when a sufficiently large voltage is applied that the electric field tears [[electron]]s away from the atoms. This is known as [[electrical breakdown]], and the voltage at which it occurs is called the [[breakdown voltage]] of an insulator. Some materials such as [[glass]], [[Electrical insulation paper|paper]] and [[polytetrafluoroethylene|PTFE]], which have high [[resistivity]], are very good electrical insulators. 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 [[Wiring|electrical wiring]] and [[Electrical cable|cables]]. Examples include rubber-like [[polymer]]s and most [[plastic]]s which can be [[thermosetting polymer|thermoset]] or [[thermoplastic]] in nature.

Insulators are used in electrical equipment to support and separate electrical [[Electrical conductor|conductors]] without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called ''insulation''. The term ''insulator'' is also used more specifically to refer to insulating supports used to attach [[electric power distribution]] or [[Electric power transmission|transmission]] lines to [[utility pole]]s and [[transmission tower]]s. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.

==Physics of conduction in solids==
Electrical insulation is the absence of [[electrical conduction]]. Electronic [[Electronic band structure|band theory]] (a branch of physics) explains that electric charge flows when quantum states of matter are available into which electrons can be excited. 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. If no such states are available, the material is an insulator.

Most insulators have 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 gives electrons enough energy to be excited into this band. Once this voltage is exceeded, electrical breakdown occurs, and the material ceases being an insulator, passing charge. This is usually accompanied by physical or chemical changes that permanently degrade the material and its insulating properties.

When the electric field applied across an insulating substance exceeds in any location the threshold breakdown field for that substance, the insulator suddenly becomes a conductor, causing a large increase in current, an [[electric arc]] through the substance. Electrical breakdown occurs when the [[electric field]] in the material is strong enough to accelerate free [[charge carrier]]s (electrons and ions, which are always present at low concentrations) to a high enough velocity to knock electrons from atoms when they strike them, [[ion]]izing the atoms. These freed electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in a [[chain reaction]]. Rapidly the insulator becomes filled with mobile charge carriers, and its [[Electrical resistance and conductance|resistance]] drops to a low level. In a solid, the breakdown voltage is proportional to the [[band gap]] energy. 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. However, if the region of air breakdown extends to another conductor at a different voltage it creates a conductive path between them, and a large current flows through the air, creating an ''[[electric arc]]''. Even a vacuum can suffer a sort of breakdown, but in this case the breakdown or [[vacuum arc]] involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself.

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.<ref name="Kakani2005">{{cite book|author=S. L. Kakani|title=Electronics Theory and Applications|url=https://books.google.com/books?id=XrSI2C9NlDIC&pg=PA7|date=1 January 2005|publisher=New Age International|isbn=978-81-224-1536-0|pages=7}}</ref><ref name="Waygood2013">{{cite book|first=Adrian|last=Waygood|title=An Introduction to Electrical Science|url=https://books.google.com/books?id=8qHGRTC7h-MC&pg=PT41|date=19 June 2013|publisher=[[Routledge]]|isbn=978-1-135-07113-4|pages=41}}</ref>

In certain capacitors, shorts between electrodes formed due to dielectric breakdown can disappear when the applied electric field is reduced.<ref>{{cite journal|last1=Klein|first1=N.|last2=Gafni|first2=H.|title=The maximum dielectric strength of thin silicon oxide films.|journal=IEEE Trans. Electron Devices|date=1966|volume=13|issue=2|page=281|doi=10.1109/T-ED.1966.15681|bibcode=1966ITED...13..281K}}</ref><ref>{{cite journal|last1=Inuishi|first1=Y.|last2=Powers|first2=D.A.|title=Electric breakdown and conduction through Mylar films.|journal=J. Appl. Phys.|date=1957|volume=58|issue=9|doi=10.1063/1.1722899|bibcode=1957JAP....28.1017I|pages=1017–1022}}</ref><ref>{{cite journal|last1=Belkin|first1=A.|last2=et.|first2=al.|title=Recovery of Alumina Nanocapacitors after High Voltage Breakdown|journal=Scientific Reports|date=2017|volume=7|issue=1|page=932|doi=10.1038/s41598-017-01007-9|bibcode=2017NatSR...7..932B|pmc=5430567|pmid=28428625}}</ref>{{Relevance inline|date=October 2017}}

==Uses==
A flexible coating of an insulator is often applied to electric wire and cable; this assembly is called ''insulated wire''. Wires sometimes don't use an insulating coating, just air, when a solid (e.g. plastic) coating may be impractical. Wires that touch each other produce cross connections, [[short circuit]]s, and fire hazards. In [[coaxial cable]] the center conductor must be supported precisely in the middle of the hollow shield to prevent electro-magnetic wave reflections. Wires that expose high voltages can cause human shock and [[electric shock|electrocution]] hazards.

Most insulated wire and cable products have maximum ratings for voltage and conductor temperature. 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 systems, [[printed circuit board]]s are made from epoxy plastic and fibreglass. The nonconductive boards support layers of copper foil conductors. In electronic devices, the tiny and delicate active components are embedded within nonconductive [[epoxy]] or [[Phenolic resin|phenolic]] plastics, or within baked glass or ceramic coatings.

In [[semiconductor device|microelectronic components]] such as [[transistor]]s and [[integrated circuit|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. Oxidised silicon is [[quartz]], i.e. [[silicon dioxide]], the primary component of glass.

In [[high voltage]] systems containing [[transformer]]s and [[capacitors]], liquid insulator oil is the typical method used for preventing arcs. The oil replaces air in spaces that must support significant voltage without [[electrical breakdown]]. Other high voltage system insulation materials include ceramic or glass wire holders, gas, vacuum, and simply placing wires far enough apart to use air as insulation.

==Insulation in electrical apparatus==
[[File:MICCCable.jpg|thumb|PVC-sheathed [[mineral-insulated copper-clad cable]] with two conducting cores]]

The most important insulation material is air. A variety of solid, liquid, and [[dielectric gas|gaseous]] insulators are also used in electrical apparatus. In smaller [[transformer]]s, [[electrical generator|generators]], and [[electric motor]]s, 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|electrical corona]] and reduce magnetically induced wire vibration. Large power transformer windings are still mostly insulated with [[Electrical insulation paper|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. [[Busbar]]s and [[circuit breaker]]s 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 can release dangerous fibers into the air and must be carried out cautiously. Wire insulated with felted asbestos was used in high-temperature and rugged applications from the 1920s. Wire of this type was sold by [[General Electric]] under the trade name "Deltabeston."<ref>{{cite book| author = Bernhard, Frank|first2=Frank H.|last2=Bernhard| title = EMF Electrical Year Book| url = https://books.google.com/books?id=pf5MAAAAMAAJ&pg=PA822| year = 1921| publisher = Electrical Trade Pub. Co.| page = 822 }}</ref>

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 (either through [[electron beam processing]] or chemical crosslinking), [[polyvinyl chloride|PVC]], [[Kapton]], rubber-like polymers, oil impregnated paper, [[Polytetrafluoroethylene|Teflon]], silicone, or modified ethylene tetrafluoroethylene ([[ETFE]]). Larger [[power cable]]s may use [[mineral-insulated copper-clad cable|compressed inorganic powder]], depending on the application.

Flexible insulating materials such as [[Polyvinyl chloride|PVC (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.

In electrical apparatus such as motors, generators, and transformers, various [[insulation system]]s are used, classified by their maximum recommended working temperature to achieve acceptable operating life. Materials range from upgraded types of paper to inorganic compounds.

===Class I and Class II insulation===
{{Main|Appliance classes}}
All portable or hand-held electrical devices are insulated to protect their user from harmful shock.

Class I insulation requires that the metal body and other exposed metal parts of the device be connected to earth via a ''grounding wire'' that is [[earth (electricity)|earth]]ed at the main service panel—but only needs basic insulation on the conductors. This equipment needs an extra pin on the [[power plug]] for the grounding connection.

Class II insulation means that the device is ''double insulated''. This is used on some appliances such as electric shavers, hair dryers and portable power tools. Double insulation requires that the devices have both basic and supplementary insulation, each of which is sufficient to prevent [[electric shock]]. All internal electrically energized components are totally enclosed within an insulated body that prevents any contact with "live" parts. In the [[European Union|EU]], double insulated appliances all are marked with a symbol of two squares, one inside the other.<ref>{{cite news |title= Understanding IEC Appliance Insulation Classes: I, II and III |url= http://www.fiduspower.com/news/understanding-iec-appliance-insulation-classes-i-ii-and-iii |work= Fidus Power |date= 6 July 2018 |access-date= 16 October 2018 |archive-date= 17 February 2020 |archive-url= https://web.archive.org/web/20200217041731/http://www.fiduspower.com/news/understanding-iec-appliance-insulation-classes-i-ii-and-iii |url-status= dead }}</ref>

<!-- above is materials and genera; - below is about insulator objects as used in different fields -->
{{split | electrical insulation | electrical insulator|date=June 2021}}

==Telegraph and power transmission insulators==
[[File:Open-wire glass insulator for telephone transmission lines 20221225 kbrose.jpg|thumb|200px|Pin-type glass insulator for long-distance [[open wire|open-wire]] transmission for telephone communication]]
Conductors for overhead high-voltage [[electric power transmission]] are bare, and are insulated by the surrounding air. Conductors for lower voltages in [[Electric power distribution|distribution]] may have some insulation but are often bare as well. Insulating supports are required at the points where they are supported by [[utility pole]]s or [[transmission tower]]s. Insulators are also required where wire enters buildings or electrical devices, such as [[transformer]]s or [[circuit breaker]]s, for insulation from the case. Often these are [[bushing (electrical)|bushing]]s, which are hollow insulators with the conductor inside them.

===Materials===
Insulators used for high-voltage power transmission are made from [[glass]], [[porcelain]] or [[composite material|composite polymer materials]]. Porcelain insulators are made from [[clay]], [[quartz]] or [[alumina]] and [[feldspar]], and are covered with a smooth glaze to shed water. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain has a dielectric strength of about 4–10&nbsp;kV/mm.<ref>{{cite web
  | title = Electrical Porcelain Insulators
  | work = Product spec sheet
  | publisher = Universal Clay Products, Ltd.
  | url = http://www.ucp.net/pdf/Electrical%20Catalogue.pdf
  | archive-url = https://web.archive.org/web/20090220025021/http://www.ucp.net/pdf/Electrical%20Catalogue.pdf
 | access-date = 2008-10-19| archive-date = 2009-02-20
 }}</ref> 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.<ref name="Cotton">{{cite book
  | last = Cotton
  | first = H.
  | title = The Transmission and Distribution of Electrical Energy
  | publisher = English Univ. Press
  | year = 1958
  | location = London
  }} copied on [http://www.myinsulators.com/acw/bookref/insulator/ Insulator Usage, A.C. Walker's Insulator Information] page</ref> Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to ceramic materials.

Some electric utilities use polymer [[Composite material|composite]] materials for some types of insulators. These are typically composed of a central rod made of [[fibre reinforced plastic]] and an outer weathershed made of [[Silicone|silicone rubber]] or ethylene propylene diene monomer rubber ([[EPDM rubber|EPDM]]). Composite insulators are less costly, lighter in weight, and have excellent [[Hydrophobe|hydrophobic]] properties. This combination makes them ideal for service in polluted areas.<ref>{{cite journal |last1=Hu |first1=Yi |last2=Liu |first2=Kai |title=Transmission lines detection technology |journal=Inspection and Monitoring Technologies of Transmission Lines with Remote Sensing |date=2017 |pages=205–279 |doi=10.1016/B978-0-12-812644-8.00004-7 |isbn=978-0-12-812644-8 |quote=Composite insulators can take wind and rain and have good self-cleaning performance under wind and rain, so need checking for pollution only once every 4–5 years, and requiring less time for the repair and power interruption.}}</ref> However, these materials do not yet have the long-term proven service life of glass and porcelain.
<gallery mode="packed" heights="160px">
File:Power line with ceramic insulators.jpg|Power lines supported by ceramic pin-type insulators in [[California]], USA
File:Ceramic electric insulator.jpg|upright|left|10 kV ceramic insulator, showing sheds
</gallery>

===Design===
[[File:Fotothek df n-15 0000283 Facharbeiter für Sintererzeugnisse.jpg|thumb|High voltage ceramic bushing during manufacture, before [[Ceramic glaze|glazing]] (1977) ]]

The electrical [[Breakdown voltage|breakdown]] of an insulator due to excessive voltage can occur in one of two ways:
* A ''puncture arc'' is a breakdown and conduction of the material of the insulator, causing an [[electric arc]] through the interior of the insulator. The heat resulting from the arc usually damages the insulator irreparably. ''Puncture voltage'' is the voltage across the insulator (when installed in its normal manner) that causes a puncture arc.
* A ''flashover arc'' is a breakdown and conduction of the air around or along the surface of the insulator, causing an arc along the outside of the insulator. Insulators are usually designed to withstand flashover without damage. ''Flashover voltage'' is the voltage that causes a flash-over arc.
Most high voltage insulators are designed with a lower flashover voltage than puncture voltage, so they flash over before they puncture, to avoid damage.

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 flashover voltage can be reduced by more than 50% when the insulator is wet. High voltage insulators for outdoor use are shaped to maximise the length of the leakage path along the surface from one end to the other, called the creepage length, to minimise these leakage currents.<ref>{{cite web
  | last = Holtzhausen
  | first = J.P.
  | title = High Voltage Insulators
  | publisher = IDC Technologies
  | url = http://www.idc-online.com/technical_references/pdfs/electrical_engineering/highvoltage.pdf
  | access-date = 2008-10-17
  | archive-url = https://web.archive.org/web/20140514000839/http://www.idc-online.com/technical_references/pdfs/electrical_engineering/highvoltage.pdf
  | archive-date = 2014-05-14
  | url-status = dead
  }}</ref> To accomplish this the surface is moulded into a series of corrugations or concentric disc shapes. These usually include one or more ''sheds''; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in wet weather. Minimum creepage distances are 20–25&nbsp;mm/kV, but must be increased in high pollution or airborne sea-salt areas.

===Types===
[[File:3phceramicins.jpg|thumb|upright=0.8|A three-phase insulator used on distribution lines, typically 13.8&nbsp;kV phase to phase. The lines are held in a diamond pattern, multiple insulators used between poles.]]
Insulators are characterized in several common classes:
* [[Pin insulator]] - The pin-type insulator is mounted on a pin affixed on the cross-arm of the pole. The insulator has a groove near the top just below the crown. The conductor passes through this groove and is tied to the insulator with [[Annealing (metallurgy)|annealed]] wire of the same material as the conductor. Pin-type insulators are used for transmission and distribution of communication signals, and electric power at voltages up to 33&nbsp;kV. Insulators made for operating voltages between 33&nbsp;kV and 69&nbsp;kV tend to be bulky and have become uneconomical.
* Post insulator - A type of insulator in the 1930s that is more compact than traditional pin-type insulators and which has rapidly replaced many pin-type insulators on lines up to 69&nbsp;kV and in some configurations, can be made for operation at up to 115&nbsp;kV.
* Suspension insulator - For voltages greater than 33&nbsp;kV, it is a usual practice to use suspension type insulators, consisting of a number of glass or porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the top end is secured to the cross-arm of the tower. The number of disc units used depends on the voltage.
* [[Strain insulator]] - A ''dead end'' or ''anchor'' pole or tower is used where a straight section of line ends, or angles off in another direction. These poles must withstand the lateral (horizontal) tension of the long straight section of wire. To support this lateral load, strain insulators are used. For low voltage lines (less than 11&nbsp;kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strings of cap-and-pin (suspension) insulators are used, attached to the [[Crossarm (utility pole)|crossarm]] in a horizontal direction. When the tension load in lines is exceedingly high, such as at long river spans, two or more strings are used in parallel.
* Shackle insulator - In early days, the shackle insulators were used as strain insulators. But nowaday, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.
* [[Bushing (electrical)|Bushing]] - enables one or several conductors to pass through a partition such as a wall or a tank, and insulates the conductors from it.<ref>IEC 60137:2003. 'Insulated bushings for alternating voltages above 1,000 V.' IEC, 2003.</ref>
* Line post insulator
* Station post insulator
* Cut-out

===Sheath insulator===
[[File:Third rail vienna 1.jpg|thumb|right|Bottom-contact third rail in a sheath insulator]]
Sheath insulators are insulators that protect a full-length of bottom-contact [[Third rail#Safety|third rail]]. Sheath insulators must protect the full length to prevent accidental conduction or contact. Sheath insulators also protect the third rail physically.

Sheath insulators are commonly made out of epoxy, due to its longevity and low conductivity. Ceramic is another choice, and is often used in addition to plastic.
{{expand section|date=April 2021}}
{{Clear}}

===Suspension insulators===
{| class="wikitable floatright"
|+Typical number of disc insulator units for standard line voltages<ref>{{cite book| last = Diesendorf| first = W.| title = Insulation Coordination in High Voltage Power Systems| year = 1974| publisher = Butterworth & Co.| location = UK| isbn = 0-408-70464-0 }} reprinted on [http://www.myinsulators.com/acw/bookref/overvoltage/index.html#dies ''Overvoltage and flashovers'', A. C. Walker's Insulator Information website]</ref>
|-
! Line voltage<br />(kV) !! Discs
|-
| 34.5 || 3
|-
| 69  || 4
|-
| 115  || 6
|-
| 138  || 8
|-
| 161  || 11
|-
| 230  || 14
|-
| 287  || 15
|-
| 345  || 18
|-
| 360  || 23
|-
| 400  || 24
|-
| 500  || 34
|-
| 600  || 44
|-
| 750  || 59
|-
| 765  || 60
|}

Pin-type insulators are unsuitable for voltages greater than about 69&nbsp;kV line-to-line.   Higher voltage [[transmission line]]s usually use modular suspension insulator designs. 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. The advantage of this design is that insulator strings with different [[breakdown voltage]]s, for use with different line voltages, can be constructed by using different numbers of the basic units.  String insulators can be made for any practical transmission voltage by adding insulator elements to the string.<ref name=STDHBK>Donald G. Fink, H. Wayne Beaty (ed).,''Standard Handbook for Electrical Engineers, 11th Edition'', McGraw-Hill, 1978, {{ISBN|0-07-020974-X}}, pages 14-153, 14-154</ref>  Also, if one of the insulator units in the string breaks, it can be replaced without discarding the entire string.

Each unit is constructed of a ceramic or glass disc with a metal cap and pin cemented to opposite sides. To make defective units obvious, glass units are designed so that an overvoltage causes a puncture arc through the glass instead of a flashover. The glass is heat-treated so it shatters, making the damaged unit visible. However the mechanical strength of the unit is unchanged, so the insulator string stays together.

Standard suspension disc insulator units are {{convert|25|cm|in}} in diameter and {{convert|15|cm|in|0|abbr=on}} long, can support a load of {{convert|80-120|kN|lk=on}}, have a dry flashover voltage of about 72&nbsp;kV, and are rated at an operating voltage of 10–12&nbsp;kV.<ref name="Grigsby">{{cite book| last = Grigsby| first = Leonard L.| title = The Electric Power Engineering Handbook| url = https://books.google.com/books?id=wiv1tuMDbTEC&pg=PA1346| year = 2001| publisher = [[CRC Press]]| location = USA| isbn = 0-8493-8578-4 }}</ref> However, the flashover voltage of a string is less than the sum of its component discs, because the electric field is not distributed evenly across the string but is strongest at the disc nearest to the conductor, which flashes over first. Metal ''[[grading ring]]s'' are sometimes added around the disc at the high voltage end, to reduce the electric field across that disc and improve flashover voltage.

In very high voltage lines the insulator may be surrounded by [[corona ring]]s.<ref>{{cite book| last = Bakshi| first = M| title = Electrical Power Transmission and Distribution| url = https://books.google.com/books?id=REww2ZF2RwwC| year = 2007| publisher = Technical Publications| isbn = 978-81-8431-271-3 }}</ref> These typically consist of [[torus]]es of aluminium (most commonly) or copper tubing attached to the line. 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.
<gallery mode="packed" heights="160px">
File:pylon.detail.arp.750pix.jpg|Suspension insulator string (the vertical string of discs) on a 275&nbsp;kV suspension pylon
File:LIC U70.jpg|Suspended glass disc insulator unit used in suspension insulator strings for high voltage transmission lines
</gallery>

===History===
[[File:CD145.jpg|thumb|A "Beehive" insulators from the [[Brookfield Glass Company]]]]
The first electrical systems to make use of insulators were [[telegraph line]]s; direct attachment of wires to wooden poles was found to give very poor results, especially during damp weather.

The first glass insulators used in large quantities 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 [[Royal Doulton|Doulton]] using [[stoneware]] from the mid-1840s, Joseph Bourne (later renamed [[Denby Pottery Company|Denby]]) producing them from around 1860 and Bullers from 1868. [[patent|Utility patent]] number [http://reference.insulators.info/patents/detail/?patent=48906&type=U 48,906] was granted to Louis A. Cauvet on 25 July 1865 for a process to produce insulators with a threaded pinhole: pin-type insulators still have threaded pinholes.

The invention of suspension-type insulators made high-voltage power transmission possible. As transmission line voltages reached and passed 60,000 volts, the insulators required become very large and heavy, with insulators made for a safety margin of 88,000 volts being about the practical limit for manufacturing and installation. Suspension insulators, on the other hand, can be connected into strings as long as required for the line's voltage.

A large variety of telephone, telegraph and power insulators have been made; some people collect them, both for their historic interest and for the aesthetic quality of many insulator designs and finishes. One collectors organisation is the US National Insulator Association, which has over 9,000 members.<ref>{{Cite web|url=http://www.nia.org/|title=Insulators : National Insulator Association Home Page|website=www.nia.org|access-date=2017-12-12}}</ref>

==Insulation of antennas==
[[File:Tamagaishi.jpg|thumb|Egg-shaped strain insulator]]
Often a [[broadcasting]] [[radio antenna]] is built as a [[mast radiator]], which means that the entire mast structure is energised with high voltage and must be insulated from the ground. [[Steatite]] mountings are used. They have to withstand not only the voltage of the mast radiator to ground, which can reach values up to 400&nbsp;kV at some antennas, but also the weight of the mast construction and dynamic forces. [[Arcing horns]] and [[lightning arrester]]s are necessary because lightning strikes to the mast are common.

[[Guy wire]]s supporting antenna masts usually have [[strain insulator]]s inserted in the cable run, to keep the high voltages on the antenna from short circuiting to ground or creating a shock hazard. Often guy cables have several insulators, placed to break up the cable into lengths that prevent unwanted electrical [[resonance]]s in the guy. These insulators are usually ceramic and cylindrical or egg-shaped (see picture). 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.

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. For 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.

[[Feedline]]s attaching antennas to radio equipment, particularly [[twin-lead]] type, often must be kept at a distance from metal structures. The insulated supports used for this purpose are called ''standoff insulators''.

==See also==
{{Portal|Electronics}}
* {{annotated link|Stephen Gray (scientist)|Stephen Gray}}
** [[Electrical conductor]]
* [[Dielectric material]]
* [[Electrical conductivity]]

==Notes==
{{Reflist}}

==References==
{{Commons category|Electric insulators}}
* {{cite book| first = Sue| last = Taylor| title = Bullers of Milton| date = May 2003| publisher = Churnet Valley Books| isbn = 978-1-897949-96-2 }}
*[http://lhotser.com/upfile/201503/Influence_of_Installation_Position_of_500kV_Grading_Ring_for_Composite_Insulator_towards_Ice_Flashover_Process.pdf  Function of Grading rings to Composite Insulator]
* Aayush Kejriwal July 12, 2024, [https://www.insulatingmats.com/which-is-the-best-electrical-insulator-rubber-glass-plastic-or-ceramics Which Is The Best Electrical Insulator- Rubber, Glass, Plastic Or Ceramics?]
* {{cite journal |last1=M. Shakiba |first1= F. |first2= S. Azizi |last2=M |first3=Zhou |last3=M. |title=A Transfer Learning-Based Method to Detect Insulator Faults of High-Voltage Transmission Lines via Aerial Images: Distinguishing Intact and Broken Insulator Images |journal= IEEE Systems, Man, and Cybernetics Magazine|volume=8 |issue=4 |pages=15–25|date=Oct 2022 |issn=2333-942X |doi=10.1109/MSMC.2022.3198027|s2cid= 252999399 }}

{{Condensed matter physics topics}}
{{Authority control}}

[[Category:Materials]]
[[Category:Electrical resistance and conductance]]