MRCDs (modular residual current devices) consist of a combination of residual current transformer and evaluation unit. In connection with a separate external circuit-breaker, they allow the implementation of protective measure "Automatic switch-off of power supply" as per DIN VDE 0100-410 and IEC 60364-4-41. They are primarily useful when no RCCBs or CRBs can be used due to high load currents or mains voltages. The MRCD detects the residual current and evaluates it in terms of its level and duration. If the residual response current thresholds and the response time are exceeded, it activates a separate switch-off device that disconnects the system part for which it is responsible from the power supply. Modular residual current devices with characteristic B+ detect pulsating and smooth DC residual currents as well as AC residual currents up to 100 kHz. Modular residual current circuit-breakers from series DMRCD have a number of usable total current transformers and therefore also a number of conductor cross-sections or rated currents. They have one changeover contact and one LED each for the pre-alarm and main alarm. The bar display provides a view of both the current residual current and the set value for the pre-alarm. The devices have two response thresholds. The response thresholds for the pre-alarm can be set from 10% to 90% of the response threshold for the alarm. The set value for the pre-alarm threshold is displayed as a continuously lit LED on the bar display.