What this auxiliary switch block is and where it fits
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.
Key ratings and what they mean for fit
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. That 3 A at 110 V is the figure that matters for most control-circuit applications — it tells you this block can handle typical PLC or relay coil loads without derating. The insulation voltage is 690 V AC at pollution degree 3, and the surge voltage resistance is 6 kV, so it's rated for industrial panel environments where transient spikes are common. Contact reliability is specified at 1 faulty switching per 100 million operations at 17 V, 1 mA — that's the dry-circuit performance you need when switching low-energy signals from a PLC or sensor. Mechanical life is 10 million switching cycles typical, which outlasts most contactors it will ride on.
Mounting and wiring
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). IP20 on the front means it's protected against finger contact but not washdown — standard for inside a control cabinet.
Lifecycle and sourcing reality
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.