The spurious trip level(tm) (STL) complements the SIL level. The STL level is a measurement of how often the safety function is carried out without a demand from the process. The STL level is expressed quantitatively, see Figure 1 and is expressed as the probability of fail safe (PFS). The PFS is the probability that a product, loop or system causes a spurious trip because of an internal failure of the devices used.
Like the safety integrity level the spurious trip level is defined as a probability of failure after one year. There are an unlimited number of spurious trip levels. The better the safety system performs in terms of lesser spurious trips the higher the STL level.
Figure 1 - Spurios Trip Levels(tm)
The STL level can be used by end-users, system integrators as well as product developers. For end-users it is very useful to specify besides the SIL level of a safety function also the STL level. This guarantees them that they not only have a "safe" safety system but also a safety system that does not cause unnecessary spurious trips. System integrators can calculate the SIL level as well as the STL level before they deliver the system to the end-user. When they select or buy safety products they can choose products that are fit for use in the desired STL level of that safety funcion. Product developers can design products that are fit to be used in a certain SIL level but also in a certain STL level. The STL level will be part of the data sheet of products.
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