Low voltage unearthed (IT) AC and DC systems are commonly applied for supply of power and control circuits in industry, transportation, medical objects etc. The main reasons for their use are high reliability and numerous advantages offered by isolating them against ground. Insulation level is a decisive factor for networks operational reliability and safety. Insufficient insulation-to-ground resistance can cause various disturbances. Though ground faults in IT systems do not make networks operation impossible, they may cause severe problems with their safe functioning. In this book the most important issues concerning normal operation and ground fault phenomena are described in concise form. Numerous methods of insulation resistance and capacitance measurement in live circuits are presented. Important other procedures of  these parameters determination based on measurement and calculation are explained and reviews of selected insulation resistance measurement devices as well as earth fault locating systems are included. For the text understanding merely basic knowledge of electrical circuits theory is required. This book is addressed to electrical engineers, technicians and students of this specialty and may also serve as an academic handbook.