The Greek symbol λDU is the undetectable 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).
λDU is the number of dangerous undetected 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 undetected failure would be a time when the failure that would prevent the safety instrumented function from performing its intended job is unnoticeable by diagnostics, and the SIF cannot achieve the safe state if needed.
Examples of dangerous failures could include:
A valve stem sheared from the ball
Solenoid signal not alerting the actuator
Pressure sensor not sensing high pressure
Major internal leak
A valve getting stuck in the open position in a closed-on-trip application
A Transmitter shorting – (even if you know about it, but cannot do anything about it)
λDU is a subclass of λD, the total dangerous failures, where λD = λDD + λDU. ( λDD is the dangerous detected failures).