The Siemens 3RH2122-4KB40 is a SIRIUS coupling relay (product designation: coupling relay for switching auxiliary circuits) in the S00 contactor size. It is designed to switch auxiliary circuits in control panels, typically driven by a PLC or pushbutton signal to energize contactor coils, signal lamps, or small loads. Dimensions are 45 mm wide, 57.5 mm high, 73 mm deep — the 45 mm width is a standard modular increment, so it occupies one 45 mm slot on the rail.
Switching capability and contact ratings
The relay carries 2 instantaneous NO contacts. Rated switching currents are given at several voltage levels across AC and DC: 10 A at 24 V rated value, 10 A at 230 V rated value, 3 A at 110 V, 1.2 A at 220 V, 0.5 A at 440 V, 0.26 A at 600 V, and 1 A at 690 V. These are the maximum continuous currents the contacts can make and break at those voltages — for motor-circuit use, derate per the contactor's utilization category. The coil is 24 VDC with a built-in suppressor diode. Coil closing power is 2.8 W, holding power also 2.8 W — a DC coil that draws constant power, not an AC economizer circuit. The arcing time is 10–15 ms, and the relay is rated for 10,000 switching cycles per hour at AC and 7–20 ms at DC. Mechanical life is 30,000,000 operations typical.
Clearance and panel integration
These are the minimum air gaps to adjacent components or enclosure walls for proper heat dissipation and arc containment. When laying out a DIN rail, account for these clearances — the 45 mm width plus 6 mm side clearance on each side means an effective slot width of about 57 mm per relay in a multi-device row.