Editing Insulator (electricity) (section)

===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.