Abstract
The paper deals with the problem of modelling safety features of the safety Fieldbus transmission system used within safety related control systems. The basic principles of the modelling failures effect upon the safety of closed transmission system and standards used in the process of safety evaluation are summarized in the paper. The practical part is oriented to a description of a realized Markov model for determination of the random failures effect on the safety of a closed transmission system. The model reflects the safety analysis of failures effect caused by electromagnetic interference in the communication channel and random HW failures of the transmission system. In the paper the results of simulation of parameters of the transmission system are discussed, such as the probability of an undetected corrupted message.