Feasibility study and uncertainties in the validation of an existing safety-related control circuit with the ISO 13849-1:2006 design standard

In industry, machine users and people who modify or integrate equipment often have to evaluate the safety level of a circuit that they have not necessarily designed. The modifications or integrations may involve work to make an existing machine that does not comply with normative or regulatory specifications safe. However, how can a circuit performing a safety function be validated a posteriori? Is the validation exercise feasible? What are the difficulties and limitations of such a procedure? The aim of this article is to answer these questions by presenting a validation study of a safety function of an existing machine. A plastic injection molding machine is used for this exercise, as well as standard ISO 13849-1:2006. Validation consists of performing an a posteriori (post-design) estimation of the performance level of the safety function. The procedure is undertaken for two contexts of use of the molding machine: in industry, and then in the laboratory. The estimation-related calculations were done on Excel, and then on SIStema software. It is shown that, based on these contexts, the estimation produces different results. The variability in the results is explained by the assumptions made by the person doing the validation without the contribution of the knowledge of the molding machine designer.