Fault Current Monitoring with the method of residual current measurement in electrical installations is a new method of preventive maintenance. Often suitable devices are applied to detect insulation faults early on, to prevent break-down times of the electrical installation caused by insulation faults and hence to reduce the cost-factors for business and industry. Monitoring with residual current monitor (RCM) according to IEC 62020 allows residual currents arising from insulation faults to be detected in the early stages. In modern electrical power supply systems, specially designed residual current monitors (RCM) can detect residual AC and DC currents, making it possible to measure insulation faults with fractions of direct voltage in AC systems as well as insulation faults in pure DC systems and to indicate and to warn once the critical value has been exceeded. To inform the operator and user of electrical installations, residual current monitors (RCM) are often equipped with LED-displays, which indicate the respective residual current mostly in relation to the operating value. RCMs can be applied concurrently with other known protective devices. This book describes applications and methods of measuring with residual current monitors (RCMs) in AC and DC systems. This book also contains many references to International Standards IEC 62020, IEC 61140, IEC 60364-4-41 and more.