Devices with external transformer separate analysis unit and transformer.Modular residual current circuit-breakers of series DMRCD has a number of usable total current transformers and therefore also a number of conductor cross-sections or rated currents. The bar display provides a view of the current residual current. The alarm is triggered at a fixed response threshold. The current level of the residual current, and the point where the response thresholds are exceeded can be seen on a 10-way LED display on the front of the device housing. A faulty connection to the external residual current transformer is immediately indicated by the alarm LED through a flashing pattern and by the activation of the signal contacts. The adjustable response delay in the range from 0.1 s to 1 s (in increments of 100 ms) makes it possible to prevent a response to brief residual current impulses, e.g. lightning strikes and switching overvoltage. This approach allows for selectivity of devices switched in series, simplifying the localisation of faults. Two independent, potential-free changeover contacts provide the option of passing on the alarm to optional indicator panels, indicator lights, acoustic signalling devices, touch panels, PLCs, etc.