Safety-Critical Systems

– A pillar in RAMS group research


Safety-critical systems are introduced into a system to prevent, or stop the development of an accident. Safety-critical functions carried out by these systems may range from simple physical barriers to very complex systems including a lot of software. The safety-critical functions can be of technical nature, but the interaction with  administrative controls and human actions may need to be included to fully understand how the performance is affected. Several international standards have been introduced related to design and evaluation of safety-critical functions. Among these standards are the IEC61508 standard, and its application specific versions/related standards, such as IEC 61511 for the process industry, IEC 62061 for machinery (ISO 13849), ISO 26262 for automotives, EN 50126/EN 50129/ IEC 50128 for railway (IEC versions are IEC 62278, IEC 62425, and IEC 62269), and IEC 61513 for nuclear.

On these web-pages we will often refer to safety-critical systems as safety instrumented systems (SIS) - due to the simplicity of the term "SIS", but it may be noted that this term is most commonly used in the process industry for systems that involve electrical/electronic/programmable electronic technology. In the search for literature on safety-critical systems, you may need to search for safety-related electrical/electronic/programmable electronic (E/E/PE) systems (IEC 61508 term), safety-related electrical control system (SRECS) (IEC 62061 term), instrumentation and control (I&C) (IEC 61513 term), safety-related railway signalling system (IEC 62425 term), plus all possible names describing what the system does (e.g., emergency shutdown system, process shutdown system, and so on).

The RAMS group has been working with safety-critical functions and safety instrumented systems (SIS) since the mid 1980s, and we have developed analytical tools and software for reliability assessment of safety-critical functions. The complexity of safety-critical functions is, however, rapidly increasing and more sophisticated methods and tools for reliability, safety and security assessment are therefore required.

Research activities in the RAMS group include:

  • Contribute to the further development of the PDS method
  • Development of methods for reliability assessment that support the adaption of the IEC 61508 standard, such as for:
    • Low-demand SIS
    • High-demand SIS
    • Partial and imperfect testing
    • Inclusion of common cause failures (CCFs)
    • Spurious activation of SIS
    • Reliability assessment of specific systems, like blow-out preventers, process shutdown systems, emergency shutdown systems, high integrity pressure protection systems, railway signaling systems, and subsea control and safety systems.
  • Balancing production availability (regularity) and safety
  • Safety and reliability of subsea facilities

Some examples of research by the RAMS group are:


Mary Ann Lundteigen is a member of the IEC 61511 technical committee. IEC 61511 is the process sector specific implementation of IEC 61508, and adopted for use in many countries including Norway.


ROSS logo

The RAMS group has recently published a new book in Reliability of Safety-Critical Systems. The book is based on research carried out in the group, and the book is used actively in our education. 

PDS forum

SINTEF Safety Research has in collaboration with members of the RAMS group developed a method for quantifying the reliability/availability of SIS, called the PDS methodPDS is the Norwegian acronym for "reliability of computer-based safety systems".