The use of IEC 61508  and IEC 61511  has increased rapidly in the past several years. Along with the adoption of the standards has come an increase in the need for accurate reliability data for devices used in Safety Instrumented Systems (SIS), both electronic and mechanical. While the methodology of determining failure rates for electronic equipment is fairly well accepted and applied, the same can not be said for mechanical equipment. Several methods are currently being utilized for generating failure rates for mechanical components. These methods vary in their approach and often lead to dramatically different failure rates which can lead to significant differences when calculating the reliability of a safety instrumented function (SIF). Some methods can result in dangerously optimistic failure rate numbers.
This paper reviews the methods utilized to determine mechanical reliability for components utilized in safety systems and provides a recommendation for the most appropriate methodology.