The 3RH1921-1CA10-ZW98 is a SIRIUS auxiliary switch designed to snap onto the front of S0-size contactors and contactor relays. It adds one N/O mirror contact that tracks the main contactor's position, which matters when you need to prove the main poles have opened — the mirror contact per IEC 60947-4-1 is mechanically linked so it cannot indicate closed when the main contacts are welded.
Rated 10 A at AC-12 (resistive loads) and 6 A at 230 V, but the real selection number is the AC-15 rating at the control voltage you're switching. At 24 V it handles 6 A; at 110 V it's 1 A; at 400 V it's 3 A. The AC-15 duty covers solenoid and contactor coils — the typical load on an auxiliary contact in a motor starter panel. If you're switching a 24 VDC PLC output signal, note the DC ratings: 10 A at 24 V, dropping to 0.5 A at 440 V. The 6 kV surge voltage resistance means it holds the unit on the grid through switching transients without flashover across the open contact gap.
Snaps onto the front of the S0 contactor — no tools, no DIN rail space consumed beyond the contactor's own footprint. The auxiliary switch itself is 10 mm wide, 51 mm deep, 38 mm high. That 10 mm width is the key dimension for panel fill: if you're stacking multiple auxiliaries on a single contactor, each one adds 10 mm to the overhang. Wire termination accepts two conductors per terminal: 2x (0.5 to 1.5 mm²) solid or finely stranded with ferrules, or 2x (0.75 to 2.5 mm²). For AWG, that's 2x (20 to 16) and 2x (18 to 14).