What it is and what it does
The Siemens SIRIUS 3RH1921-1HA13 is a 4-pole auxiliary switch block with 1 normally open and 3 normally closed instantaneous contacts, designed for snap-on mounting onto size S0 through S3 motor contactors and contactor relays. Rated for a maximum of 10 A, it handles AC-15 pilot duty loads up to 690 V with a surge voltage resistance of 6 kV, making it suitable for control circuits in motor starter assemblies. The screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) with core end processing, and the front face carries IP20 protection — standard for enclosed panel mounting.
Key ratings and what they mean for fit
The 10 A maximum is the thermal current rating (Ith); the actual switching capacity depends on voltage and duty. At 24 V DC the contacts are rated for 10 A, at 110 V DC for 3 A, and at 220 V DC for 1.2 A — these are the numbers that govern whether the block can switch a given DC solenoid or indicator load without welding. For AC control circuits, the AC-15 rating at 690 V is 1 A, which covers pilot-duty switching of contactor coils and relays in 400 V and 690 V panels. Contact reliability is specified at 1 faulty switching per 100 million operations at 17 V and 1 mA, so the block is dependable for low-energy PLC input signals as well as power-level switching. The mechanical service life is 10 million switching cycles typical, matching the contactor it mounts on for coordinated maintenance intervals.
Where it fits in the panel
Snap-on mounting onto the contactor's front face — no tools needed, and it occupies the standard EN 50012 footprint for size S0, S2, and S3 frames. Dimensions are 44 mm wide, 38 mm high, and 51 mm deep, so it adds minimal depth to the contactor assembly inside the enclosure. Operating temperature range is -25 to +60 °C, with storage from -55 to +80 °C, covering most industrial panel environments without derating.
Lifecycle and sourcing reality
This part is a phased-out product. The official successor is the SIRIUS 3RH2 series auxiliary switch block, which offers the same contact configuration and mounting footprint. For new designs or BOM freeze replacements, the 3RH2 series is the recommended drop-in; confirm the exact order code for your contactor frame size when transitioning.