The Siemens 3RV1901-1CA00-0FQ0 is a lateral-mounting auxiliary switch block that adds two normally-closed (NC) contacts to a SIRIUS 3RV circuit breaker.
Lateral mounting attaches the switch block to the side of the 3RV breaker frame. It fits sizes S00 to S3. The two NC contacts are mechanically linked to the breaker's switching mechanism, so they change state simultaneously with the main contacts. This makes the block suitable for signaling a trip condition to a safety circuit or for status feedback in a motor control center.