A generic approach is proposed in this work that automatically constructs suitable calculation models from a herefore invented formal description language. Two positive effects arise from that: firstly, all inherently and explicitly contained complexity of Safety Instrumented Systems is identified once and condensed into a set of transformation formulas generating the actual calculation models. Secondly, providing a flexible and extensive description language encourages smart and quick engineering processes, enables for non-standard solutions, and relieves safety engineers from the challenge of actually deriving explicit calculation models by hand. A suitable new type of discrete time multiphase markov model is chosen as mathematical basis, and comes along with the required solver algorithms for retrieving the desired unavailability characteristics: the probability of failure on demand (PFD), and the probability of fail-safe (PFS), i.e. an operational unavailability as economical indicator.