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The amount of derating depends upon the shapes of the conductors
(the “sharp points” issue), the insulator thickness, and the insulator
homogeneity.
Generally, you derate the dielectric strength as the material gets
thicker. For many materials you can find an equation that describes
the relationship between thickness and dielectric strength.

Figure 12.1 - 400/220KV GIS switchgear

Insulation materials
Here are 7 most common and known materials used in
electronics assemblies as high voltage insulation, and descriptions
of issues that require special attention.
1. Air

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