Reliability Assessment of Safety-Critical Systems

Safety-critical systems are systems introduced to prevent, or mitigate the consequences of hazardous events.  Many of these systems are implemented by electrical, electronic, and/or programmable electronic technologies, with interaction to mechanical systems and systems for communication and human interaface, and are sometimes referred to as E/E/PE safety-related systems. The complexity of such systems can often be extensive, and it is of vital importance that the systems are designed and operated so that the specified performance is achieved. The requirements are often split into three main categories:

  • Functional safety requirements, formulating what the systems shall do to ensure correct performance during demand and fault conditions
  • Safety integrity requirements, formulating how often or with what probability the correct performance must be ensured
  • Survivability requriements, giving additional requirements for the system to survive under demand conditions, considering the exposure intensity and exposure time.

Several international standards have been introduced to frame design and operation of E/E/PE safety-related systems. Among these standards are the IEC 61508 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. The standards have introduced their own sector specific names for the E/E/PE safety-related systems, such as:

  • Safety instrumented systems (SIS) (process sector)
  • Safety-related electrical control system (SRECS) (machinery systems),
  • Instrumentation and control (I&C) (nuclear sector),
  • safety-related railway signalling system (railway sector)

The RAMS group has been working with reliability assessment of E/E/PE safety-related systems 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".