 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.