Safety Considerations Guide, Tricon v9.0 - Tuv-fs.com

16 Safety StandardsIEC 61508, Parts 1–7The IEC 61508 standard, “Functional Safety: Safety Related Systems,” is aninternational standard designed to address a complete SIS for the process, transit,and medical industries. The standard introduces the concept of a safety life cyclemodel (see the flowchart on page 11) to illustrate that the integrity of an SIS is notlimited to device integrity, but is also a function of design, operation, testing, andmaintenance.The standard includes 4 SILs that are indexed to a specific probability-to-fail-ondemand(PFD) (see figure on page 6). A SIL assignment is based on the requiredrisk reduction as determined by a PHA.ANSI/ISA S84.01ANSI/ISA S84.01-1996 is the United States standard for safety systems in theprocess industry. The SIL classes from IEC 61508 are used and the DIN V 19250relationships are maintained. ANSI/ISA S84.01-1996 does not include the highestSIL class, SIL 4. The S84 Committee determined that SIL 4 is applicable formedical and transit systems in which the only layer of protection is the safetyinstrumentedlayer. In contrast, the process industry can integrate many layers ofprotection in the process design. The overall risk reduction from these layers ofprotection is equal to or greater than that of other industries.Draft IEC 61511, parts 1–3The IEC 61511 standard, “Functional Safety: Safety Instrumented Systems for theProcess Industry Sector,” is an international standard designed to be used as acompanion to IEC 61508. IEC 61508 is intended primarily for manufacturers andsuppliers of devices. IEC 61511 is intended for SIS designers, integrators, andusers in the process-control industry.Tricon Safety Considerations Guide