2007 Volume 46 Issue 1 Pages 16-23
Recently computer―based systems have been widely applied for achievement of safety functions. This general trend forced IEC to compile IEC 61508 as a standard related to functional safety of electrical／electronic／programmable electronic safety ―related systems, i.e., E／E／PE SRS (SRS). Currently JIS includes the translated standard of IEC 61508, JIS C 0508. These standards require to estimate the risk reduction achieved by safety functions of SRSs. It has been believed that self―diagnosis functions of SRSs are able to reduce the risk resulting from failures or faults of SRSs to a negligible level. However, the risk will not be negligible for the cases where a demand occurs on the maintenance of SRS in the low demand mode of operation or where SRS fails to function on a demand in the high demand or continuous mode of operation. This paper firstly models the causation of hazardous events of the overall system in which the SRS with self―diagnosis function is installed and formulates the hazardous event rate based on the model. The SRS in overall system is assumed to implement a safety function in a dynamic demand state. Moreover, new demand modes of operation and estimations of hazardous event rates are proposed for allocation of SILs to SRSs.
