The Greek symbol λDD is the detectable dangerous failure rate in functional safety expressed in the unit of measurement of FITs which can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).
λDD is the number of dangerous detected failures per unit time for a piece of equipment.
What does that really mean?
A dangerous failure would be a time when the failure would prevent the Safety Instrumented Function (SIF) from performing its intended job, and the SIF cannot achieve the safe state if needed. So, a dangerous detected failure would be a time when the failure that would prevent the safety instrumented function from performing its intended job is noticeable by diagnostics and alerted. Until the SIF is fixed, it cannot achieve the safe state if needed; however, operators are aware of the degradation.
Examples of dangerous failures could include (depending on the diagnostics):
Pressure sensor not sensing high pressure
Solenoid signal not alerting the actuator
λDD is a subclass of λD, the total dangerous failures, where λD = λDD + λDU. ( λDD is the dangerous detected failures).