Quantitative safety analyses

The current state of the art expects quantitative evidence of safety. The calculation of the failure probability PFD or PFH of a complex system relevant to safety can hardly be performed anymore without detailed modeling. The transfer of a technical design into an RBD (Reliability Block Diagram), a Markov process, or a Petri network, and the subsequent quantitative analysis of the model requires thorough understanding of the modeling methods and probability theory.

The reliability parameters of the components used form the quantitative basis for the safety analyses: MTTF, MTBF, MTTR of the failure rate Lambda. Reliability analyses of complex electronic circuits, be it on the basis of standards (e.g. SN-29500) or from field or test data, are also no easy matter.

Along with reliability and safety, the availability and maintainability of critical systems also plays a major role.

These four areas are often summarized under the RAMS abbreviation.

Together with our partner institutes at ZHAW we can provide support in these areas with sound knowledge about statistical data evaluation, the stress analysis of electronic circuits or the load analysis of mechanical systems.

Safety and RAMS management

The acronym RAMS stands for::

Reliability
Availability
Maintainability
Safety

An additional S, which stands for  "security", is being added more and more frequently to the RAMS abbreviation.

A commonly used definition of the RAMS concepts can be found in IEC 60050 series of Standards published by the International Electrotechnical Commission (IEC):

Reliability:
“The probability that an item can perform a required function under given conditions for a given time interval (t1, t2).”

Availability:
“The ability of an item to be in a state to perform a required function under given conditions at a given instant of time or over a given time interval, assuming that the required external resources are provided.”

Maintainability:
“The probability that a given active maintenance action, for an item under given conditions of use can be carried out within a stated time interval when the maintenance is performed under stated conditions and using stated procedures and resources.”

Safety:
“Freedom from unacceptable risk (of harm).”

This definition is frequently used in other standards, such as in the rail sector in CENELEC EN 50126, but is unfortunately not adopted systematically.

Quantification of the RAMS performance indicators

The quantification of the RAMS performance indicators is based on reliability technology. Only random hardware (electronic and mechanical) failures are considered.

Quantitative orientation values for reliability, availability, maintainability, and for the probability of dangerous failures can be computed on the basis failure rates, MTTF (Mean-Time-To-Failure) or MTBF (Mean-Time-Between-Failures) data, repair times, etc.

 

 

Schematic representation of the time span between two failures which becomes the value "Mean Time between failure" (MTBF).