An Evaluation of Schneider M580 HSBY PLC Redundancy in the R744 System A Cooling Unit

The Detector Technologies group at CERN has developed a 2-stage transcritical R744 cooling system as a service for future detector cooling. This is the first system in operation at CERN where Schneider HSBY (Hot Standby) redundant PLCs are used. This cooling system provides a good opportunity to test the Schneider redundant PLC system and understand the operation, limitations and probability of failure in a con-trolled environment. The PLC redundancy is achieved by connecting Schneider M580 HSBY redundant PLCs to the system where one is the primary which operates the system and the other is in standby mode. A series of tests have been developed to understand the operation and failure modes of the PLCs by simulating different primary PLC failures and observing whether the standby PLC can seamlessly take over the system operation.