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Comparing failure rates

02.01.2001  |  Goble, W.,  exida LLC, Sellersville, Pennsylvania


Safety integrity level (SIL) verification per ISA84.01 and IEC61508 requires that a probabilistic evaluation be done on the hardware used in a safety instrumented function (SIF). Data necessary for that evaluation included failure rates, failure mode percentages and diagnostic coverage factors (if any). These data are needed for each component in the SIF.

Those manufacturers who get products third-party certified per IEC61508 are required to publish the data or provide some other means to allow a potential user of the product to do the probabilistic evaluations. One of the first manufacturers to get a safety PLC approved to IEC61508 has created an SIL calculation tool that uses its failure rate data but does not display that data. Why such secrecy? Other manufacturers who want to supply products to the safety industry are considering publishing the data but seem hesitant. As I ask companies for this information, one concern keeps being raised. Will potential customers use the data to compare my product with a competitor?

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