A holistic approach to developing safety-related systems in compliance to the IEC61508

Abstract

In this report, several aspects related to the design of safety instrumented systems (SIS) have been presented and discussed. Some challenges related to the methods used to demonstrate functional safety were highlighted, especially when adopting the risk-based approach proposed by the IEC61508 (2010). Two issues were identified when using the standard approach for designing purposes. Firstly, the methods used do not highlight features of risk such as uncertainty and knowledge. This is particularly alarming given that risk acceptance criteria (RAC) and quantitative risk assessments (QRAs) are used to set the necessary performance requirements of the system and further referred throughout the process to verify if the requirements are met. If the risk is misrepresented, the final product can be a system that is eighter too complex or too simple to provide the necessary risk reduction. Secondly, the standard lacks a detailed approach to follow from a product development perspective. Based on these issues, this work presents a discussion that tries to shed some light on how to overcome them during the design of SIS. Furthermore, depending on the technology adopted the standard approach might not be suitable to perform safety demonstration. Hence, new safety philosophies might be needed to support the development of systems that violate the traditional requirements.

Key words: IEC61508, SIL, design, uncertainty, knowledge, QRA, RAC