The application of the “safety chain” consisting of proaction, prevention, preparation, repression/mitigation, recovery and learning will be studied in this paper. The chain appears to be a multi layer system, that is at least as safe as the safest layer. It will additionally be observed that the effectiveness of resources spent in prevention is most probably higher than on repression, because repression becomes only effective after the disaster has occurred and at least the economic damage has become a fact. Several examples of multi layer safety systems will be analysed in this paper. Mathematical methods of risk analysis and probabilistic reasoning are essential in the design and the understanding modern safety systems. Verbal reasoning alone is insufficient.