What it is and what it does
The Siemens 3RH2122-2QB40 is a SIRIUS coupling relay — a size S00 auxiliary contactor designed to switch auxiliary circuits in a control panel. It carries 2 instantaneous NO contacts rated for 10 A at 24 V rated value, which means it can handle the typical pilot-duty loads (PLC outputs, indicator lamps, small contactor coils) without an intermediate relay. The coil pulls 2.8 W closing power on DC, so the backplane or PLC output driving it needs to source that inrush.
Mounting and panel fit
Mounts via screw or snap-on onto a 35 mm standard mounting rail (DIN rail). The 45 mm width and 73 mm depth fit the S00 footprint — common in compact control cabinets. Mounting position allows ±180° rotation on a vertical surface and ±22.5° tilt forward/backward, so you can orient it to fit tight gland plates or wireways. Clearances: 10 mm upward, 10 mm forward, 10 mm downward, 6 mm at the side.
Switching capability and surge suppression
Built-in varistor surge suppressor across the coil — no external snubber needed for inductive loads like contactor coils or solenoid valves. The contacts are rated for switching at 10 000 operations per hour (AC) and the arcing time runs 10 to 15 ms, so it keeps up with fast-cycle applications like stamping presses or conveyor indexing. At DC the on-time can be 7 to 20 ms; the varistor clips the turn-off spike, extending contact life on DC loads.