The Siemens 3RH1921-2FJ22 is a SIRIUS auxiliary switch block designed to snap onto 3RT1 contactor relays and power contactors (sizes S0 through S12). It provides 2 normally-open and 2 normally-closed auxiliary contacts — the NO contacts are encapsulated, the NC contacts are not — giving you feedback or interlock signals from the contactor without wiring a separate relay.
The auxiliary contacts are rated for switching currents that vary with voltage: 0.3 A at 24 V and 60 V, 3 A at 110 V, and 1 A at 220 V. Mechanical life is 10 million switching cycles typical, which outlasts most contactors it will ride on.
Snap-on mounting onto the contactor — no tools needed, no DIN rail required. The block is 44 mm wide, 51 mm deep, and 38 mm high, so it adds minimal footprint to the contactor assembly. Screw-type terminals accept 2x (0.5 to 2.5 mm²) solid or stranded without ferrules, or 2x (0.5 to 1.5 mm²) with ferrules. AWG equivalents are 2x (20 to 14).
This part is flagged as a phased-out product. The official successor is the SIRIUS 3RH2 series. If your BOM is frozen on the 3RH1921-2FJ22, you can still source it through independent distribution — we quote and procure to order against an RFQ. For new designs, the 3RH2 series is the recommended drop-in replacement; verify the contact configuration and mounting compatibility with your 3RT1 contactor before committing the BOM line.