Editing Overhead power line (section)

==Insulators==
[[File:Power line with ceramic insulators.jpg|thumb|Medium-voltage power lines with ceramic insulators in California]]
[[File:Pylon.detail.arp.750pix.jpg|thumb|Modular suspension insulators are used for high-voltage lines.]]
[[Electrical insulation#High-voltage insulators|Insulators]] must support the conductors and withstand both the normal operating voltage and surges due to switching and [[lightning]].  Insulators are broadly classified as either pin-type,  which support the conductor above the structure, or suspension type, where the conductor hangs below the structure. The invention of the [[strain insulator]] was a critical factor in allowing higher voltages to be used.

At the end of the 19th century, the limited electrical strength of [[telegraph]]-style [[pin insulator]]s limited the voltage to no more than 69,000 [[volt]]s. Up to about 33 kV (69 kV in North America) both types are commonly used.<ref name=Fink78/> At higher voltages only suspension-type insulators are common for overhead conductors.

Insulators are usually made of wet-process [[porcelain]] or [[toughened glass]], with increasing use of glass-reinforced polymer insulators. However, with rising voltage levels, polymer insulators ([[silicone rubber]] based) are seeing increasing usage.<ref>{{cite web |url=http://www.ngk-locke.com/polymer.html |work=NGK-Locke |archive-url=https://archive.today/20120905000404/http://www.ngk-locke.com/polymer.html |archive-date=2012-09-05 |url-status=dead |title=Polymer insulator manufacturer}}</ref> China has already developed polymer insulators having a highest system voltage of 1100 kV and India is currently developing a 1200 kV (highest system voltage) line which will initially be charged with 400 kV to be upgraded to a 1200 kV line.<ref>{{cite web|url= https://www.worldenergynews.com/news/abb-energizes-transformer-record-mln-volts-651995 |title=ABB Energizes Transformer At Record 1.2 Mln Volts|work=World Energy News|accessdate=7 October 2016}}</ref>

Suspension insulators are made of multiple units, with the number of unit insulator disks increasing at higher voltages.  The number of disks is chosen based on line voltage, lightning withstand  requirement, altitude, and environmental factors such as fog, pollution, or salt spray. In cases where these conditions are suboptimal, longer insulators must be used. Longer insulators with longer creepage distance for leakage current, are required in these cases. Strain insulators must be strong enough mechanically to support the full weight of the span of conductor, as well as loads due to ice accumulation, and wind.<ref>{{Cite web |url=http://www.arp-hivoltageinsulators.com/suspension-insulators.html |title=Advanced Rubber Products - Suspension Insulators |access-date=2013-09-17 |archive-date=2022-03-18 |archive-url=https://web.archive.org/web/20220318041111/http://www.arp-hivoltageinsulators.com/suspension-insulators.html |url-status=dead }}</ref>

Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few milliamperes) passes through the insulator.  This warms the surface slightly and reduces the effect of fog and dirt accumulation.  The semiconducting glaze also ensures a more even distribution of voltage along the length of the chain of insulator units.

Polymer insulators by nature have hydrophobic characteristics providing for improved wet performance. Also, studies have shown that the specific creepage distance required in polymer insulators is much lower than that required in porcelain or glass. Additionally, the mass of polymer insulators (especially in higher voltages) is approximately 50% to 30% less than that of a comparative porcelain or glass string. Better pollution and wet performance is leading to the increased use of such insulators.

Insulators for very high voltages, exceeding 200 kV, may have [[grading ring]]s installed at their terminals.  This improves the electric field distribution around the insulator and makes it more resistant to flash-over during voltage surges.