Results
- PFDavg
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- Risk reduction factor (1 / PFDavg)
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- SIL band (low demand)
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How it works
Choose the voting architecture, enter the dangerous undetected failure rate λDU (in FIT, failures per 10⁹ hours), the proof-test interval and the common-cause factor β. The tool calculates PFDavg with the simplified equations and the corresponding SIL band for low-demand mode.
Formula
PFDavg(1oo1) = λDU · T / 2 · (1oo2) = ((1−β)λDU)² T² / 3 + β λDU T / 2 · (2oo3) = ((1−β)λDU)² T² + β λDU T / 2
Assumptions and limits
A teaching estimate, not a SIL verification. It uses the simplified equations for one component group and ignores diagnostics, repair time, partial-stroke tests, imperfect proof tests and the final element's own data. A real SIL claim also needs architectural constraints, systematic capability and a lifecycle assessment under IEC 61508 and IEC 61511.
λDU is per identical channel, not for the whole safety function. Channels have synchronized, complete proof tests; one year is 8,760 hours. β must be the effective common-cause fraction for the selected architecture. This tool limits λDU × T to 0.1 and assesses only the selected component group, not the complete safety loop.
For education and quick estimates only. Check the result against the applicable standards and your project data before using it in design, operation or safety decisions.
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