What it is and what it does
The Siemens 3RH2122-1BD40 is a SIRIUS auxiliary contactor in the S00 frame size — the smallest footprint in the SIRIUS contactor family, sized for control-circuit switching rather than motor-load handling. It carries 2 normally-open and 2 normally-closed instantaneous contacts, rated for 10 A at 24 V AC and 10 A at 230 V AC, with derated curves at higher voltages (4.7 A at 60 V, 3 A at 110 V, 1.2 A at 220 V, 3 A at 400 V, 2 A at 500 V, 1 A at 690 V, and 0.5 A at 440 V, 0.26 A at 600 V). The DC coil draws 4 W for both pick-up and hold, so there is no inrush spike to size a power supply around — the supply sees a steady 4 W load the whole time the contactor is energized.
Panel integration and mounting
Mounts via screw or snap-on onto 35 mm standard mounting rail. The S00 body measures 45 mm wide by 57.5 mm tall by 73 mm deep — that 73 mm depth is the dimension that sticks out from the DIN rail, so check your enclosure depth clearance before committing the BOM. Clearance around the contactor: 10 mm upwards, 10 mm forwards, 10 mm downwards, and 6 mm at the side. The mounting position is flexible — ±180° rotation on a vertical surface, plus ±22.5° tilt forward or backward on that same surface. Screw terminals accept 2 × (0.5 to 1.5 mm²) solid or stranded, 2 × (0.75 to 2.5 mm²), or 2 × 4 mm².