Introduction

When ISA84.01 (Ref. ^[1]) was first released in 1996, several made the comment, "No one has good failure rate data." This led some to believe that the whole idea behind probabilistic failure calculations was impractical.

In the early years of the functional safety standards, industry failure databases could provide failure data information. While this failure data was not product specific or application specific, it helped designers recognize problems in their designs. One such problem was the "weak link" design (Ref. ^[2]). Such a design included a high quality SIL3 safety PLC that was connected to a switch and a solenoid. Many of the engineers thought they had a SIL3 design until they did the safety verification calculations. Such a design may not even meet SIL1 depending on proof test effectiveness and manual proof test time interval!

Even with approximate data, the methods began to show how designers could achieve higher levels of safety while optimizing costs. The safety verification calculations required by the new functional safety standards have shown designers how to design much more balanced designs. The calculations have shown many how to do a better job. But, failure rate and failure mode data for random failures on the chosen equipment is required.