Insulation class is a measure of the ability of insulation materials to withstand electric stress. It is expressed by a letter and a number, such as class B or F. Class B insulation materials have higher dielectric strength than Class F, which means they can withstand higher voltage and current. The higher the number, the better the insulation material's ability to resist voltage and current. For example, Class H has an even higher dielectric strength than Class F, making it suitable for use in high-voltage applications.