Editing Insulator (electricity) (section)

===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>