The Siemens SIRIUS 3RH1921-2KA20 is a 2 N/O auxiliary switch designed to snap onto S0-size contactors and contactor relays. It uses spring-loaded terminals (cage clamp) and carries IP20 finger protection on the front — standard for panel-mounted control gear where accidental contact is a concern.
The contacts are rated for 10 A at AC-12 (resistive load) and 6 A at 24 V, 2 A at 48/60 V, 1 A at 110 V, 0.9 A at 125 V, 0.3 A at 220/250 V, and 3 A at 400 V. The AC-12 rating (10 A) is the headline number for general-purpose resistive switching; the lower DC and higher-voltage AC figures reflect real arc-interruption limits. For inductive loads (AC-15, DC-13), derate accordingly — the mirror-contact compliance (IEC 60947-4-1) means this switch is suitable for feedback circuits in safety-related contactor applications when paired with a 3RT1 contactor.
Mechanical endurance and contact reliability
Rated for 10 million mechanical operating cycles typical.
Panel integration — snap-on and wiring
Snap-on mounting for S0 contactors; a second unit can be mounted laterally. Accepts 2 x (0.5…2.5 mm²) solid/stranded or 2 x (20…14 AWG) without ferrules, and 2 x (0.5…1.5 mm²) finely stranded with core-end processing. Dimensions: 71 mm deep, 10 mm wide, 72.5 mm high — the 10 mm width is the critical panel-space number when stacking multiple auxiliaries.