Frequently Asked Question

The calculation of PFD shows a discrepancy between the exSILentia and ISA formulas. Would you be kind enough to share the formula used in exSILentia so we may compare and note what are the differences and evaluate ISA formula accordingly.
Last Updated a year ago

In exSILentia we do not use approximation formulas, but rather time dependent, fully defined, Markov Models to calculate PFDavg. The formulas like the one you show, and the examples in IEC 61508 are approximation formulas that tend to leave out important parameters that can have a drastic impact on your calculated results, see
http://www.exida.com/Resources/Whitepapers/the-key-variables-needed-for-pfdavg-calculation for example. To better understand the modeling approach, we would recommend
http://www.exida.com/Books/Control-Systems-Safety-Evaluation-and-Reliability-3rd-Edition or http://www.exida.com/Books/Safety-Instrumented-Systems-Verification-Book.

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