Siemens 3RV2321-4AC20 SIRIUS 3RV2 motor starter protectors. Siemens 3RV2321-4AC20 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 16 A N-release 208 A Spring-type terminal Standard switching capacity.