Siemens 3RV2321-4NC10 SIRIUS 3RV2 motor starter protectors. Siemens 3RV2321-4NC10 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 size S0 for starter combination Rated current 28 A N-release 364 A screw terminal Standard switching capacity.