Safety assessment, reliability, and the probability-operation diagram

The probability-operation (P-O) diagram is introduced. New reliability and safety definitions are given in relation to this diagram. Specific problems in safety assessment are shown and new practices are proposed. Safety-assessment must be dealt with independently of the reliability-assessment, except for one specific case. The safety specification described in the P-O diagram covers the final results and, as such, it includes failures caused by the operational environment or even by operator error. This means that a top-down (functional, anti-causal, deductive, backward) method is more effective for safety assessment. An independent safety-assessment (especially when a top-down method has been used) shows the proper information which must be: collected by an accident investigation as well as during usual system-use, and stored in a data-bank, so that an acceptable system-safety monitoring function can be carried out. This is proposed as a deterministic method of selecting the statistical information to be kept in a data bank. An existing successful P-O diagram (which is in accordance with the safety-monitoring results) represents a good system-safety model for the trend study and for describing future safety requirements.