Modelling border-line tolerated conditions of use (BTCU) and associated risks

For the design of most technical systems a desirable safe field of use is calculated from systems technical constraints, and expectations of human capacities and limitations. Performance incursions outside the safe field are then limited by means of hard-protections, instructions, education, and regulations. However, once in service, the socio-technical conditions of work create conditions for performance to migrate and stabilise outside the expected safe field of use. The stabilisation of migration results from a compromise between global performance improvement, individual additional advantages, and apparent risk control. This paper proposes a double modelling approach to such migrations, first in terms of a cognitive model of the production of migrations, and second in terms of a mathematical safety analysis of severity and consequences. Both approaches lead to the emergence of methodologies in order to take BTCU into account during design. Conclusions highlight the impossibility of avoiding such in service migrations of use, and advocate for an early consideration of potential migrations in order to improve the robustness of safety analysis techniques. The field example chosen for demonstration is the design and use of a rotary press.

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