The EUW 4 supervises three-phase networks on undervoltage. The supervision isn‘t carried out only for the mean average value of the voltage but for every phase separately. By this means an asymmetry of the phases or even phase failures become recognized. The switching threshold is, depending on design variant either fixed or by a frontal potentiometer adjustable. In addition the neutral wire is supervised on wire breakage. At measuring voltages above the switching point the output relay is pulled in with 2 change-over contacts and the operating light (green LED) shines. When one or several phase voltages sink below the switching point, the relay drops off and the operating light goes out. The inserted hysteresis prevents the permanent condition change by varying measuring voltage around the switching point.