2010 Volume 49 Issue 2 Pages 94-103
The international standards of functional safety for electric, electronic and programmable electronic safety-related systems（ SRS）, IEC 61508/61511, were published through 1998 to 2003. They have been applied to various kinds of industries. In general, programmable electronic safety systems can realize highly sophisticated safety functions. However, the framework of IEC 61508/61511 include neither SRS nor Safety Instrumented System（ SIS） that have multiple sub-safety functions. There is many SIS for chemical or nuclear power plants, which have such multiple sub-safety functions as parameter safety control and safe trip functions. This paper, firstly, presents a state transition model for a hazardous event caused by both the demand on SIS and the failure of SIS in a typical process for chemical or nuclear power plants. Next, the hazardous event rate is formulated by analyzing the state transition model probabilistically. Then, some discussions are made on several points of the approach for estimation of the hazardous event rate provided by the standards. It is concluded that the approach of current IEC 61508/IEC 61511 is not always appropriate for the functional safety assessment of SRS/SIS with multiple sub-safety functions and the method presented by this paper can substitute the approach.
