In performance based functional safety standards, safety function designs are verified using specified metrics. A key metric for process industry designs is called average Probability of Failure on Demand (PFDavg). After several studies of many field failure and proof test reports, several variables have been identified as key to a realistic PFDavg calculation. Most simplified equations including the informative section in IEC 61508, Part 6 do not include several key variables. It is shown that exclusion of these parameters may result in an optimistic metric calculation which may result in an unsafe design.
This paper identifies the key variables that need to be included in a PFDavg calculation and provides some simplified equations showing the impact of most variables. An example showing two sets of variables reveals an entire SIL level difference in PFDavg calculation results.