The 3RH1921-2CA01 is a front-side auxiliary switch block from the Siemens SIRIUS family, designed to snap onto the front of 3RT1 contactors. These are the currents the NC contact can make and break under inductive load conditions typical of contactor coils and solenoid valves.
The contact is rated for both AC and DC switching.
The switch measures 10 mm wide, 38 mm high, and 51 mm deep — a slim footprint that adds negligible width to the contactor stack. It snaps onto the front of the 3RT1 contactor without tools; the spring-loaded terminals face forward for wiring access after mounting. The front face carries IP20 protection — finger-safe but not sealed against dust or moisture.