Hazardous Failure Rate of the Safety Function

Abstract

Quantitative assessment of safety function integrity against random failures is necessary assumption for railway signalling system acceptance and its implementation into operation. The railway signalling system can be modelled as continuous mode system and therefore the criterion for quantitative assessment safety integrity of safety function is hazardous failure rate. Most of commonly available software tools for evaluation of the RAMS parameters offer calculation of safety function failure probability, but don’t offer direct calculation of safety function failure rate. The paper is focused on some of problems associated with comparing the exact analytical solution and approximate calculation of safety function failure rate due to presence of random failures. This approach can be successfully applied to “manual” calculation of also complex analytical terms. The proposed method is based on the generally accepted assumption that occurrence of random failures of electronic systems corresponds to an exponential distribution law.