What it is and where it fits
The Siemens SIRIUS 3RH1122-1KF40-2AA0 is a size S00 auxiliary contactor — a compact switching block meant to extend the control-circuit contacts on a larger contactor or to build out a standalone signal-switching function in a panel. It carries a 110 V DC coil with built-in varistor suppression, so you don't need to add an external snubber on the coil wiring. The 2 NO instantaneous contacts are rated 10 A in AC-12 duty (resistive loads at up to 690 V), which covers most PLC output and relay interface applications you'd throw at it in a control cabinet. Mounting is screw or snap-on onto a DIN rail — the 45 mm width and 57.5 mm height fit the standard S00 slot alongside contactors like the 3RT10 series. The IP20 front protection means it's fine inside a closed panel but not for washdown or outdoor exposure.
Key ratings and what they mean for the build
The 10 A AC-12 rating is the headline figure — that's the maximum continuous current through the auxiliary contacts when switching resistive loads. At 110 V DC the contacts are rated 1 A, and at 230 V AC they handle 6 A. The 690 V AC rating drops to 1 A, so if you're switching a 690 V control transformer secondary, derate accordingly. These numbers are the contact limits; the coil draw is only 11 W closing and 4 W holding, so the control transformer sizing is negligible. The built-in varistor clamps the coil voltage spike when de-energizing — that's a reliability plus for PLC outputs driving the coil directly. No external diode or RC snubber needed across the coil terminals. Operating temperature range is -25 to +60 °C, with storage down to -55 °C. Shock resistance is 10g / 5 ms and 5g / 10 ms, which is standard for industrial panel gear — no special handling needed for shipping or vibration. Terminals accept 2x 0.5 to 1.5 mm², 2x 0.75 to 2.5 mm², or 2x 4 mm² solid — covers most control-circuit wire sizes you'd run in a panel. Stripping length is standard; spring-cage or screw terminals depending on the variant, but this order code comes with screw terminals.