The Siemens 3UG3522-1AC20 is a SIRIUS monitoring relay in a 22.5 mm housing, designed to detect overcurrent or undercurrent conditions on a single-pole main circuit. It monitors 0.1…10 A AC/DC and switches a single changeover contact at AC 24 V 50/60 Hz control supply. The screw terminals are non-removable, and the unit snaps onto a DIN rail.
The 0.1…10 A AC/DC monitoring range covers motor branch circuits, heater loads, and general equipment protection where you need to trip on either overcurrent or undercurrent. The single-pole main circuit and one CO contact are suited for simple alarm or shutdown logic — not for multi-phase monitoring. The 22.5 mm width leaves room for additional relays in a crowded row.