What the calculator is for
Diagnostic coverage (DC) is the fraction of dangerous failures that the device's own diagnostics detect. It decides how much of the dangerous failure rate becomes λDD, revealed within the diagnostic test interval, and how much stays λDU, hidden until a proof test or a demand. DC feeds the PFDavg and PFH calculations, the safe failure fraction and, under Route 2H, the minimum diagnostic coverage for type B elements.
- DC = λDD / (λDD + λDU)
- IEC 61508-2 bands: low ≥ 60 %, medium ≥ 90 %, high ≥ 99 %.
Worked example
A smart pressure transmitter's FMEDA gives λDD = 900 FIT and λDU = 100 FIT. DC = 900 / (900 + 100) = 90.0 %: medium coverage. Only one dangerous failure in ten waits for the proof test.
A solenoid valve with 150 FIT detected and 350 FIT undetected has DC = 30.0 %, below the 60 % low band. Partial-stroke testing is one way to raise it: it moves part of λDU into a regularly revealed share.
Where DC is used
DC appears in three places in a SIL verification. In the quantitative gate it splits λD into λDD, which is revealed quickly and contributes only through the repair time, and λDU, which waits for the proof test. In Route 1H it raises the safe failure fraction. In Route 2H a type B element needs at least 60 % DC for the reduced fault tolerance to apply. The same failure-rate split therefore has to be used consistently in all three.
Coverage bands
| Band | DC | Undetected share |
|---|---|---|
| None | < 60 % | > 40 % |
| Low | 60 % to < 90 % | 10 % to 40 % |
| Medium | 90 % to < 99 % | 1 % to 10 % |
| High | ≥ 99 % | ≤ 1 % |
Common mistakes
- Counting detected failures as safe. A detected dangerous failure is only safe if the diagnostic drives the safe state, or repair happens, within the process safety time.
- Mixing up DC and SFF. SFF also counts safe failures; DC looks only at dangerous ones.
- Taking credit without the diagnostic interval. Coverage only helps if the diagnostic test runs much faster than the demand rate.
- Using vendor DC under different conditions. Diagnostics often depend on wiring, configuration and the logic solver acting on the fault signal.
Basis
Definition and bands as in IEC 61508-2 (diagnostic coverage of an element or subsystem) and IEC 61508-4 (definitions). Failure rates come from FMEDA, certificates or field data; record their source.