Safety instrumented systems software system failures

For certification of the products, tools like FMEDA and Markov models are used. An FMEDA extends the FMEA techniques to include online diagnostic techniques and identify failure modes relevant to safety-instrumented system design. These PFD calculations may be carried out by commercial software tools (e.g., SILence by HIMA). In all these cases, bottom-up approaches were undertaken. Also it has been found tbat there has been a serious lack of reliable failure rate data and their variations among lab specifications, lab/field and field specifications. There are wide variations in the data obtained from these different sources. Therefore it is suggestive to use a band of data instead of discrete PFD and other failure rate data [5]. 

Demonstration that it is able to perform the required functions and that the previous use has shown there is a low enough probability that it will fail in a way which could lead to a hazardous event when used as part of the safety instrumented system, due to either random hardware failures or systematic faults in hardware or software in systems in similar operating profiles and physical environments, factors to consider ... 

Most agree also that prior use requires that a system be in place to record all field failures and failure modes at each end user site. Version records of the instrument hardware and software must be kept as significant design changes may void prior use experience. Operating conditions must be recorded and must be similar to the proposed safety application. 

---