Classes of Insulation
Substances used as insulators in practical electrical work are classified into four groups, as follows:

Class A Insulation.
Class A insulation consists of (a) cotton, silk, paper, and materials similar to paper when impregnated
or immersed in an insulating liquid; (b) molded or laminated materials with cellulose filler, phenolic resins, or similar resins; (c) films or sheets of cellulose acetate or similar cellulose products; and (d) varnishes or enamel applied to conductors.

Class B Insulation.
Class B insulation consists of mica, or fiberglass, all with a binder.

Class C Insulation.
Class C insulation consists entirely of mica, porcelain, glass, quartz, or similar materials.

Class O Insulation.
Class O insulation consists of cotton, silk, paper, or similar materials that are not impregnated or immersed
in an insulating liquid.

We say that an insulation is impregnated when the air spaces within the insulation are filled up by an impregnating substance such as paraffin. An impregnating substance is itself a good insulator.

Insulation Resistance
Since no insulator is perfect, the insulation resistance of an insulated conductor may be measured in megohms. An instrument called a megger (proper name megohm meter) is used when it is desirable or necessary to measure insulation resistance.

A megger is a type of ohmmeter that measures resistance in megohms (millions of ohms); it can measure up to several thousand megohms. Any insulating substance will break down at some value of high voltage, and the insulation of commercial insulated conductors is rated (guaranteed) to withstand a certain value of voltage.

Therefore, meggers are designed to measure insulation resistance at high voltages.

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