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 in order to prevent break-down times of the electrical installation due to insulation faults and hence reduce the cost-factors for business and industry. Monitoring with residual current monitor (RCM) according to IEC 62020, enables the detection of residual currents due to insulation faults in the early stages. In modern electrical power supply systems specially designed residual current monitors (RCM) are able to detect residual AC and DC currents. This means that it is now 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 give warning when the critical value has been exceeded. For the information of 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 operate value. RCMs can be applied in addition to 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.