The Greek symbol  λD represents dangerous failure rates in functional safety, usually expressed in the unit of measurement of FITs, and can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

λD is the number of dangerous failures per unit time for a piece of equipment. This 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. 

Examples of dangerous failures could include: 

• A valve getting stuck in the open position in a closed-on-trip application

• Solenoid signal not alerting the actuator

• A valve stem not able to move the ball from being sheared

• Pressure sensor not sensing high pressure

• A flame detector not sensing the flame

• A PLC not transmitting a signal

• Major Internal leaks

• A Transmitter shorting

 λD can be broken down into subclasses:  λDD and  λDU; the detectable dangerous failures, and the undetected dangerous failures, where  λD =  λDD +  λDU.