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Safety and SIS · Glossary

What is Diagnostic Coverage?

Symbol: DC%Safety and SIS

Diagnostic coverage is the fraction of dangerous failures that automatic diagnostics detect.

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Diagnostic Coverage Explained

Detected dangerous failures can be alarmed and repaired, or trip the process, so they barely add to PFDavg. Undetected ones remain hidden until the proof test.

Smart transmitters with NAMUR NE43 fault currents and comparison between redundant sensors improve diagnostic coverage.

Diagnostic Coverage Formula

DC = λDD ÷ (λDD + λDU)
λDDdangerous detected rate
λDUdangerous undetected rate
Example: λDD = 600, λDU = 100 FIT: DC ≈ 86 %.
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Calculators for Diagnostic Coverage

CalculatorSafe Failure Fraction CalculatorFree online calculator with formula and worked example.CalculatorPFDavg CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Diagnostic Coverage?

Diagnostic coverage is the fraction of dangerous failures that automatic diagnostics detect.

Do diagnostics count if nobody acts on them?

Only if the detected fault leads to a safe state or a repair within the assumed time.

Is diagnostic coverage the same as proof test coverage?

No. Diagnostics run automatically online; proof test coverage refers to periodic manual testing.

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Related Terms

Safety termSafe Failure FractionSafe failure fraction is the share of all failures that are either safe or dangerous but detected.Safety termFailure RateFailure rate is the number of failures per unit time of a device; in safety it is split into safe and…Safety termProof Test CoverageProof test coverage is the fraction of dangerous undetected failures that a proof test actually reveals.

More Safety and SIS Terms

Safety termHardware Fault ToleranceHardware fault tolerance is the number of hardware faults a subsystem can have while still performing its…Safety termVoting ArchitectureVoting architecture describes how many of N redundant channels must act to trip, such as 1oo2 or 2oo3.Safety termCommon Cause FailureCommon cause failure is a single cause that makes redundant channels fail together, such as a shared…Safety termProof Test IntervalProof test interval is the time between periodic tests that reveal dangerous undetected failures in a…Safety termSpurious TripA spurious trip is an unwanted shutdown caused by a safety system fault rather than a real process hazard.Safety termMean Time Between FailuresMean time between failures is the average operating time between failures of a repairable item.

All safety and sis terms · Full glossary A to Z

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