Risk estimation studies in the context of a machine control function

Nowadays the safety requirements of a machine are increasingly associated with programmable electronic control systems. Risk estimation is an essential part of risk analysis in a machine development process, because the categorisation and allocation of safety requirements is based on this. It is important to know how the risk estimation is performed, since wrong safety integrity level (SIL) or performance level (PL) selection may lead to multiple costs of the safety-related part of the control system. On the other hand, wrong PL or SIL selection can weaken the safety of the system. In this article, a risk estimation process of a safety-related control function is presented. Different groups carried out three case studies including risk estimation for the same safety-related control function of a machine. The results of the risk estimations of the groups differ from each other. The possible reasons for the variations are discussed.