I was recently asked how the PFDavg value in an exida FMEDA report was calculated. Those PFDavg values are calculated using the exSILentiaTM program. The person who posed the question was using the equations from Part 6 of IEC 61508 and got a different value. Why?

exSILentia does a detailed calculation using discrete time Markov models and considers all realistic variables including:

• All dangerous failure rates
• Proof test intervals
• Bypass time
• Proof test coverage
• Maintenance capability
• Other application parameters

This is all done semi-automatically making it very easy to do professional verification calculation.

The equations in Part 6 of IEC 61508 are just simplified equations that consider only a subset of the important variables. I would never use those equations for actual SIF verification calculations. I have seen them produce results that are an order of magnitude optimistic!

Safety is serious and competency is required. Those who are doing this work really must understand how to do the calculations.