A Simplified Procedure for Safety Requirements Derivation

The paper develops a procedure for analysis of PLC-controlled system risk due to component failure and for derivation of safety integrity requirements for components, focusing on software requirements. The approach allows fully integrated treatment of random and systematic failure. It can be applied at different levels of design detail and at different stages of the system development lifecycle. The procedure does not address how to assess failure rates, but provides a foundation for integrating PLC software assessment into system riskassessment and for making trade-offs in design.