Throwing a bridge between risk assessment and functional safety

Risk assessment must be established for reasonable operation of functional safety. So far, the relationships between the risk frequency, i.e., hazardous event rate, and the safety integrity of safety-related systems have not been clear for the general demand modes of operation. Then, the present paper describes the new formulations of hazardous event rate (HER) as well as the risk reduction ratio (RRR) for the general demand modes of operation, and proposes the method how to determine safety integrity levels of SRS using HER, RRR, table 2 and 3 in the standard of IEC 61508, functional safety of electrical/electronic/programmable electronic safety-related systems.