What Is Concealed behind the Hazardous Failure Rate of a System?

Quantitative safety assessment of safety-related control systems consists of many steps, with the creation of a valid mathematical model being the one of most important one. Assumption on constant failure rate of a system leads to numerous approximations that are made in the process of model creation. These approximations need to be coherent with the philosophy of safety assessment and must be discarded if they could artificially increase the safety level of the modelled system. Nevertheless, if probability of the hazardous failure and hazardous failure rate as safety measures are compared, currently used approximations allow to achieve inaccurate or even invalid results of the safety assessment. The scope of the paper covers exact and approximate approach to the safety assessment of a two-channel redundant system structure comprised of dissimilar channels. Outcomes and conclusions of the paper can be immediately applied in many forms of the quantitative safety evaluation.