Proving system properties such as fail-safety is a challenge for systems engineering since industrial automation is nowadays embedding intensive on-site and remote infotronics components engineered with increasing intuitive ease-of-use techniques. Since a formal proof of the complete safe-behaviour of the resulting ad-hoc system is not possible, this paper argues that Proof Oriented Systems Engineering formal techniques should bridge the gap with Fault Tolerant Systems Engineering practical techniques in order to mathematically check the proof of fail-safety. Rationales, experiments and open issues are addressed on combining the formal B event-based method using the B proof assistant with a technical-safety modelling formalized-framework.