The Siemens 3RH1122-1VB40 is a SIRIUS coupling relay designed for switching auxiliary circuits — the control-side loads like contactor coils, indicator lamps, PLC inputs, and small solenoids that a main contactor or PLC output cannot drive directly. It is not a power contactor; it handles the logic-level and pilot-duty switching in a panel. Rated at 10 A in AC-12 duty (resistive load at 24 V rated value), it carries the typical auxiliary load curve. Pick-up voltage is 0.85 of rated (20.4 VDC), drop-out is 1.85 of rated (44.4 VDC). That wide drop-out margin means the relay stays pulled in through a sag down to about 20 V and only releases when the supply collapses or is positively switched off — useful on DC buses with weak regulation.
The Size S00 footprint — 45 mm wide, 57.5 mm tall, 72 mm deep — fits the standard S00 slot in a Siemens motor starter assembly or as a standalone auxiliary relay on the rail. Terminals accept solid conductors: 2x 0.5…1.5 mm², 2x 0.75…2.5 mm², or 2x 4 mm². The built-in surge suppressor uses a diode across the coil. That means polarity matters on the DC coil supply — wire it backward and the diode forward-biases, shorting the supply. Mark the polarity on the terminal label during panel build.
Switching capability and contact configuration
Two instantaneous normally-open contacts (2 NO), no delayed, lagging, or make-before-break switching. Electrical life depends on the switched current and voltage; for auxiliary loads at rated values it will still see millions of cycles.