In this chapter several methods of continuous measurement of insulation resistance in AC IT systems are described. Measuring circuits with use of test direct current both with an auxiliary DC source and diode rectifiers are presented. The most commonly applied measurement method with an auxiliary rectangular voltage source is explained. Another method of continuous insulation resistance measurement is imposition of an auxiliary sinusoidal voltage of a specific frequency different from frequency
of the monitored network. Examples of both techniques implementation in modern insulation meters are presented.