What it is and what it does
The Siemens 3RH2122-2LF40 is a SIRIUS coupling relay for switching auxiliary circuits — a compact, size S00 device that mounts on 35 mm DIN rail via screw or snap-on fastening. It carries 2 normally-open instantaneous contacts and is rated for 10 A at both 24 V and 230 V, making it a straightforward interface between a control signal and a higher-power auxiliary load.
Key ratings and what they mean for fit
The 10 A rating at 24 V and 230 V covers most pilot-duty and control-transformer loads in a panel. The contactor size S00 (73 mm deep, 45 mm wide, 70 mm high) is the standard footprint for this class of auxiliary relay — it fits alongside S00 contactors in the same DIN-rail zone without crowding. The mounting position allows ±180° rotation on a vertical surface and ±22.5° tilt, which helps when routing wires in a dense cabinet. Coil closing power is 2.8 W at DC, a reasonable draw for a 24 VDC control loop. The built-in varistor surge suppressor handles back-EMF from the coil, so you do not need an external flyback diode on the PLC output card.
Where it goes in the panel
Snap it onto a 35 mm DIN rail in any control cabinet — no tools needed for installation. The screw terminals accept 2x 0.5 to 4 mm² solid or stranded wire. Clearance: 10 mm upwards, 10 mm forwards, 10 mm downwards, 6 mm to the side for arc flash and wiring access. The size S00 profile keeps it shallow enough for shallow enclosures.