Glass is a better electrical insulator than thermal insulator, is also called glass electrical insulators. It insulates electrically because the atoms and molecules have no unpaired electrons in their outermost orbitals. Atoms with full orbitals are very stable. It takes a lot of voltage to pull electrons off the atoms of glass. Glass disc suspension insulators are made from glass with high performance and one of main glass electrical insulators.

Glass electrical insulators are not a very good thermal insulator because it is a solid. If you make it into fiberglass, then it is better because it traps air in all the little spaces between the fibers. It's hard for heat to travel from one pocket of trapped air to another. It's easy for heat to travel through a solid object (glass or other object) because all the atoms touch each other so they pass the heat energy easily from one atom to the next.

In the electrical field, we make it into fiberglass; the rod of composite suspension insulator is made of fiberglass.  

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