Spurious Trip Level (STL) is defined as a discrete level for specifying the spurious trip requirements of safety functions to be allocated to safety systems. An STL of 1 means that this safety function has the highest level of spurious trips. The higher the STL level the lower the number of spurious trips caused by the safety system. There is no limit to the number of spurious trip levels.
Safety functions and systems are installed to protect people, the environment and for asset protection. A safety function should only activate when a dangerous situation occurs. A safety function that activates without the presence of a dangerous situation (e.g., due to an internal failure) causes economic loss. The spurious trip level concept represents the probability that safety function causes a spurious (unscheduled) trip.
The STL is a metric that is used to specify the performance level of a safety function in terms of the spurious trips it potentially causes. Typical safety systems that benefit from an STL level are defined in standards like IEC 61508 IEC 61511, IEC 62061, ISA S84, EN 50204 and so on. An STL provides end-users of safety functions with a measurable attribute that helps them define the desired availability of their safety functions. An STL can be specified for a complete safety loop or for individual devices.
For end-users there is always a potential conflict between the cost of safety solutions and the loss of profitability caused by spurious trips of these safety solutions. The STL concept helps the endusers to end this conflict in an a way that safety solutions provide both the desired safety and the desired process availability.
Read more about Spurious Trip Level: STL Determination, STL Levels, STL Vs SIL, See Also
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