Markov models for 2004 architecture for safety related systems

An advanced safety architecture is the 2 out of 4-system (2004). In order to trigger the safety function at least two of the four channels must work correctly. It is said: "A 2004-system is 2-failure safe". In order to classify the quality of a system we calculate different parameters. In the report equations are indicated for PFD for normal and common-cause-failures. Also the Markov-model for a 2004-architecture is introduced. We can calculate the MTTF (Mean Time To Failure) of this architecture with this Markov-model. The results are a high availability and a high reliability.