The Siemens SIRIUS 3RH1921-1PA22 is a 4-pole auxiliary switch block with a 2 NO + 2 NC contact arrangement wired to EN 50005. It snaps onto Size S0 through S3 contactors — specifically the 3RT1 power contactor family noted in the spec — adding auxiliary signalling or interlocking without extra DIN-rail space. The screw-type terminals accept 2x (0.5...1.5 mm²) or 2x (0.75...2.5 mm²) with core-end processing, and 2x (20...16) or 2x (18...14) AWG for auxiliary wiring.
Contact ratings — what they mean at the panel
The block carries a maximum thermal current of 10 A, but the real-world switching capacity depends on the voltage you are breaking. At 24 V it switches 6 A (AC-15/DC-13 typical), at 125 V it handles 6 A, at 230 V it still manages 6 A, but at 400 V it drops to 3 A, and at 600 V it is down to 0.26 A. The AC-15 rating at 690 V is 1 A. That means for a 24 VDC PLC input circuit you have full 6 A headroom; for a 400 V control transformer interlock you need to stay under 3 A per contact.
Lifecycle reality — this is a phased-out part
The manufacturer lists the 3RH1921-1PA22 as a phased-out product. The official successor is the SIRIUS 3RH2 series auxiliary contactor/switch block.
Snap-on mounting onto the contactor front — no DIN rail required for the block itself, though the host contactor typically rails. The block measures 44 mm wide by 38 mm high by 51 mm deep. The insulation voltage is rated 690 V at pollution degree 3, with a 6 kV surge voltage resistance — adequate for 400 VAC motor control panels with proper clearance.