Structural analysis is a powerful tool for early determination of
detectability/isolability possibilities. It is shown how different
levels of knowledge about faults can be incorporated in a structural
fault-isolability analysis and how they result in different
isolability properties. The results are evaluated on the DAMADICS
valve benchmark model. It is also shown how to determine which faults
in the benchmark that need further modeling to get desired isolability
properties of the diagnosis system.