Work out failure rate to pfd instantly with clear inputs, formula shown and shareable results.
For a simplex channel tested periodically, average probability of failure on demand is the undetected dangerous failure rate times the test interval, divided by two, because on average a fault exists for half the interval. Diagnostic coverage removes the detected portion, so both better diagnostics and shorter test intervals improve PFD proportionally.
Average PFD
PFDavg = lambda undetected x test interval hours / 2
Undetected rate
lambda undetected = dangerous failure rate x (1 - diagnostic coverage)
Preliminary estimate only. Functional safety verification must follow the applicable standard using validated failure rate data and full architectural analysis.
PFDavg = lambda undetected x test interval hours / 2. For a simplex channel tested periodically, average probability of failure on demand is the undetected dangerous failure rate times the test interval, divided by two, because on average a fault exists for half the interval.
Only up to a point. Systematic capability, architectural constraints on redundancy and proof test effectiveness all limit the achievable SIL regardless of test frequency.
This calculator takes 3 inputs: Dangerous failure rate, Proof test interval, Diagnostic coverage. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.