The Siemens SIRIUS 3UG3012-1AP50 is a 45 mm three-phase voltage monitoring relay that detects phase asymmetry between 5 % and 20 % on a 400 V, 50/60 Hz supply. It draws about 1 W from the measured circuit — no separate control supply needed. The IP20 enclosure and snap-on DIN-rail mount mean it lives inside a panel, not out on the machine frame.
The 5–20 % asymmetry adjustment range is the core spec: it sets the trip point for unbalanced voltage conditions that overheat motors and shorten winding life. On a 400 V line, 5 % asymmetry means a 20 V difference between phases — catch that early and you save the motor. The relay monitors all three phases (3-pole measurement) and operates on the line voltage itself, so no auxiliary 24 VDC is needed.