The basic technical parameter determining condition and quality of electrical insulation is its resistance. Insufficient level of this parameter can cause various disturbances. According to statistic data ground faults are the most frequent type of failures in AC and DC networks. Other common defects include line-to-line faults, breaks (broken wires), voltage loss, or its abnormal deflections. Ground faults in IT systems do not make networks operation impossible, however, they may cause severe problems with their safe functioning. Therefore, special attention should be paid to these abnormal conditions, possible threats created by them, and ways for their detection and elimination. It has been proved that many insulation breakdown cases do not happen suddenly but are the final stage of a long degradation process. This fact is a strong argument for conducting continuous supervision over insulation condition to ensure timely detection of possible problems. The main causes of electrical insulation deterioration are aging, mechanical and thermal stresses, overvoltages, humidity, chemical factors, oil, radiation, etc. Continuous monitoring is an indispensable tool for preventive maintenance, which allows to avoid possible faults caused by insulation condition deterioration. Information about current insulation level helps users to achieve high reliability and safety of electrical systems. AC and DC IT systems, as isolated against ground under normal operation, allow—in distinction from TN and TT systems—to fulfill continuous insulation monitoring. As a result, substantial qualities offered by these systems can be