Towards an Operational Safety Standard for Software: Modelling IEC 61508 Part 3

Safety standards are an important means for developing safety-critical (software) systems. Usually they provide a set of requirements and recommendations in order to support implementing safe software, such as the prominent IEC 61508 functional safety standard. Meeting them leads to safer software and reduces the likelihood for harms on people and environment. But for both, the development team and assessors, the application of the standard in practice is difficult and often leads to problems due to unclear requirements and (missing) acceptance criteria. For this purpose a quality model for the safety standard helps applying the standard objectively and provides guidance for the software development team. Concrete measures and rules of code, architecture and documentation analysis tools help to operationalize the standard that is covered by the quality model. Using documentation analysis tools helps supporting the project lead for determining the quality of the input and output artifacts of the lifecycle phases. The application of the operational quality model for continuous quality assessments of software projects may contribute to emerging a safety culture in the organization as the developers become more safety aware. In a first analysis we found out that over 50 percent of the analyzed tables in the annexes of IEC 61508 part 3 can be measured 'largely' by means of such automatic measurement tools. Using tools is important for the continuous quality assessment. However, we still need to analyze the rest of the standard's software part and apply it to projects in order to validate this approach. In conclusion, the focus of this paper is to show the need for an operational quality model for objectively applying the safety standard and for continuously giving guidance for stakeholders in the development process for safety-critical software.