Insulating materials and their classification

An insulating material (dielectric) is a material with very high resistivity (about $10^{9}$–$10^{20}\ \Omega$m). It prevents the flow of current between conductors at different potentials, and between conductors and earthed parts.

Classification

1. By physical state

• Solid: mica, paper, pressboard, cotton, glass fibre, porcelain, rubber, PVC, polyester films, epoxy resins, varnishes.
• Liquid: mineral transformer oil, silicone oil, synthetic esters (askarels are no longer used).
• Gaseous: air, nitrogen, hydrogen (cooling of large alternators), SF₆ (switchgear).

2. By origin/structure

• Organic: paper, cotton, silk, wood, rubber, resins and plastics. Flexible and cheap, but limited to low temperatures.
• Inorganic: mica, glass, porcelain, asbestos and ceramics. They withstand high temperature but are brittle.

3. By thermal class (IS 1271 / IEC 60085)

Class Y and A are unimpregnated and impregnated organic materials. B, F and H are mica and glass with binders of increasing thermal stability. C covers pure inorganic materials.