Siemens 3RV2121-4NA10 SIRIUS 3RV2 motor starter protectors. Siemens 3RV2121-4NA10 is the exact printed commercial reference for a motor protection circuit breaker in the SIRIUS 3RV Motor Protection range. Siemens' official product description is: circuit breaker frame size S0 for motor protection, Class 10 with overload relay function thermal release 23...28 A short-circuit release 364 A screw terminal standard switching capacity.