A. Porcelain
B. Glass
C. Composite Polymer
D. Any of the above
Answer: D. Any of the above
Explanation:
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.
Porcelain has a dielectric strength of about 4–10 kV/mm.
Glass has higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains.