Page "Insulator (electricity)" Paragraph 20

Insulators used for high-voltage power transmission are made from glass, porcelain or 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 kV / mm.

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.

Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to ceramic materials.