Reliability of computer based safety systems

Safety systems are installed in various industries to prevent undesirable events and to minimize the adverse effects of such events if they occur. Because of their nature, safety systems must be highly reliable. From a safety point of view, the probability of responding to a shutdown demand must be high (have a high safety availability). For economic reasons, the production shutdowns caused by safety equipment faults (spurious trips) must be kept to a minimum. The authors focus on computer-based safety shutdown systems. The main components of such a system are: sensors (gas, fire, pressure level etc.), signal transmission/logic units (programmable logic controllers, PLC´s) and actuators (valves etc.). When analyzing the reliability of fault-tolerant systems, redundancy may take several forms. Most attention however, is given towards hardware redundancy which is the physical replication of hardware for the purpose of detecting and tolerating faults and towards software redundancy like addition of small programs to perform periodic tests