Reliability assessment of redundant safety systems with degradation

Abstract

Reliability of transport equipment plays a crucial role in providing safety for passengers. Safety systems of transport equipment perform safety functions with assigned safety integrity levels (SIL). If the reliability of a safety system is not sufficient, it has to be improved till the required level. This can be done by improving maintenance, enhancement of diagnostics or by applying redundancy. To conclude that reliability value is sufficient (or not), it is necessary to calculate its value before and after reliability improvement. Such calculations can be done analytically or by a simulation approach. Usually simulation approach is time consuming for a large number of simulations. Small number of simulations leads to an error in the results. Therefore analytical methods are often welcomed by both – scientists and practitioners. This thesis investigates analytical methods of reliability calculation focusing on systems with degradation. Analytical formulas of reliability calculation have limitations for systems with degradation due to non-constant failure rates (in this thesis they are modelled by Weibull distribution). These limitations have been shown in the example of a braking system of moving walks in Chapter 3: analytical methods are mainly applicable only to systems with constant failure rates especially in the case of redundant systems.