Undervoltage relay, when the connection to the three phases and the neutral conductor is established, the relay is activated in the error-free case and the green power LED lights up. If the monitored nominal voltage UN in one, two or all three phases is less than the switching voltage US, the relay returns to its rest position. Optical display: Power. Green LED, error in phases L1, L2, L3. Red LED flashes, neutral conductor N missing. Green power LED flashes. Single-phase application: bridging L1-L2-L3