Editing Insulator (electricity) (section)

==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, manufactured for AT&T in the period from c. 1890 to [[World War I]]; it is secured to its support structure with a screw-like metal or wood pin matching the threading in the hollow internal space. The transmission wire is tied into the groove around the insulator just below the dome.]]
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]]
An insulator that protects a full-length of bottom-contact [[Third rail#Safety|third rail]].
{{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|The [[Brookfield Glass Company]] gained widespread recognition for their prolific production of CD145 insulators, commonly known as "Beehive" insulators, owing to their superior craftsmanship and extensive distribution.]]
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>