Phase monitoring relay for AC mains
The Siemens 3UG4512-1BR20 is a SIRIUS phase monitoring relay that monitors three-phase AC systems for phase sequence and phase failure. It accepts a rated supply voltage of 160 to 690 V AC at 50 or 60 Hz, covering the common low-voltage motor supply range from 208 V to 600 V without needing a separate control transformer. Two changeover contacts rated at 3 A at 250 V AC or 400 V AC (50/60 Hz) handle the control signal to the motor contactor or PLC input. The relay is set for auto-reset, so it re-energizes the output contacts as soon as the monitored phase conditions return to normal — no manual reset needed on the line.