Use of visual propositional calculus to derive safety critical functions

It is proposed that system safety requirements can be decomposed into a number of sufficient and necessary safety critical functions. The approach is applied to an example of a high voltage working procedure. First a given solution is expressed in the form of a finite state machine. Then the concept of interacting Safety Sub-systems is introduced to avoid having to verify the safety of every state. The main focus of this paper is then to decompose one of these Safety Sub-systems into a set of necessary and sufficient safety critical functions using a visual form of propositional calculus. These safety critical functions can then be used as a check-list to examine the completeness of different implementations, often involving high voltage exclusion region barriers, doors, controllable power supplies, interlock keys and human procedures. This analysis approach is particularly appropriate for two application domains: access security and arming chain safety.