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The paper evaluates the dependability of PLC systems based on assuming the failure modes of the system components. It is introduced a model of the service provided by a system to a single user as a sequence of pairs value-time which has to be recognized by the user. The service model is then improved in order to be applied for the case of a single service provided to multiple users. The formalism used for the failure modes assertions are ordered by the means of an implication graph, where each path represents a more relaxed ordered set of assertions regarding the system behaviour. The probability that a specific assumed failure mode proves to be true in the real system operation can be formalized by the concept of assumed failure mode coverage. Its effects on the system dependability are illustrated by a case study. It is demonstrated that more relaxed assertions made on components failure modes do not necessarily lead to an increase of the PLC system dependability.