Definition:
Diagnostic test interval (DTI) is thetime between diagnostic runs in a component with automatic diagnostics. It is a product of its diagnostics and is not something the facility can change. Modern components have near 0 sec DTI, but older ones can be 1 hour or more. For components without diagnostics, DTI would not be applicable.
A related term is Mean Detection Time (MDT) which is mathematically half of the DTI. DTI is not a term that shows up by itself in PFDavg and STR calculations. However, its value would impact the decision of what MTTR is.
- a – time to detect the failure
- b – time spent before starting the repair
- c – effective time to repair
- d – time after repair to put back in service.
- MTTR = a + b + c + d –> from the failure itself to the restoration
- MRT = b + c + d –> meaning MRT does not include time to detect.
- MTTR = a + MRT
- MDT = a –> mean detection time
- DTI ≈ 2 * MDT
Key Points:
- Is used to help decide on the appropriate value for MTTR.
- More of an established term than MDT
- It is provided by the manufacturer, generally in the Safety Manual.
Example:
A plant has a SIF with 2oo3 instruments. The SIF is SIL 2, and the instruments are modern with diagnostics. It has been confirmed that their diagnostics run every 1 second. This is used to determine MTTR.