Methods of reliability assessment of heterogeneous redundant systems

Reliability assessment is required for the determination of a safety integrity level (SIL) of safety systems in accordance to the functional safety approach. Functional safety standards suggest formulas for calculating PFD/PFH which numerical values are used for establishing correspondence to the SIL. However these formulas cannot be used for heterogeneous redundant systems with a combination of mechanical, electronic/electrical components and constant and non-constant failure rates. In this paper we present an overview of reliability assessment methods that are able to cope with these features of heterogeneous redundant systems, show their advantages, drawbacks and limitations in application.