Glossary · Functional safety
Diagnostic coverage (DC)
Also known as: DC
German: Diagnosedeckungsgrad (DC)
In functional safety, diagnostic coverage (DC) is a measure of the effectiveness of diagnostics in detecting dangerous failures. It is expressed as the share of dangerous failures that the diagnostics detect.
- Functional safety
- Verification
In one sentence
Diagnostic coverage (DC) states what share of dangerous failures the diagnostics detect, a key input for PL and SIL.
Example
Monitoring the feedback contacts of two contactors that switch off a conveyor motor detects a welded contact before the next start, which gives high diagnostic coverage for that output.
How it applies
- Functional safety: ISO 13849-1 groups DC into none (below 60 %), low (60 % to below 90 %), medium (90 % to below 99 %) and high (99 % and above). It is combined with the Category and MTTFd.
- Diagnostics: DC depends on the measure, for example cross-monitoring, feedback monitoring of contactors or plausibility checks. The standard lists typical values per measure; claiming more needs justification.
- Verification: Fault injection tests show that the diagnostics actually detect the faults assumed in the DC estimate.
- Technical documentation: If detection depends on a user action, such as periodically opening a guard to test the switches, the information for use must say so. Otherwise the assumed DC is not reached in operation.
Diagnostic coverage vs. diagnostics
Diagnostics are the functions. Diagnostic coverage is the measure of how effective they are at detecting dangerous failures.