The Greek symbol λS represents safe or spurious failure rates 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).
λS is the number of safe of spurious failures per unit time for a piece of equipment.
What does that really mean?
A spurious trip or safe failure would be a time when the process is in normal operation and the system acts as if there is a problem and goes to the safe state when it is not necessary.
Examples of spurious or safe failures could include:
Loss of air pressure in the actuator in a close on trip application.
Output fails open and immediately goes to the safe state
λS can further be broken down into subclasses: lSD and lSU, the detectable spurious trip failures and the undetected spurious trip failures, where λS = λSD + λSU.
Knowing your safe failure rate can help prevent unwanted and unneeded process shutdowns, which we all know can be not only frustrating, but costly! Often the lS get overshadowed by the famous lD and the dangerous failures, but they are important too! So even if it is tempting to only consider dangerous failures in your system, take a minute to think about possible safe failures as well.