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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 61508: Functional safety of electrical/electronic/programmable electronic safety-related systems, IEC, 1998〕 IEC 61511,〔IEC 61511 - Functional safety - Safety instrumented systems for the process industry sector, IEC, 2003〕 IEC 62061,〔Safety of machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems, IEC, 2005〕 ISA S84,〔ANSI/ISA-84.00.01-2004 Part 1 (IEC 61511-1 Mod) Functional Safety: Safety Instrumented Systems for the Process Industry Sector〕 EN 50204〔EN 50204 - Electrical apparatus for the detection and measurement of combustible or toxic gases or vapours or of oxygen. Requirements on the functional safety of fixed gas detection systems〕 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 end-users to end this conflict in an a way that safety solutions provide both the desired safety and the desired process availability. == STL determination == The spurious trip level represents asset loss due to an internal failure of the safety function. The more financial damage the safety function can cause due to a spurious trip the higher the STL level of the safety function should be. Each company needs to decide for themselves which level of financial loss they can or are willing to take. This actually depends on many different factors including the financial strength of the company, the insurance policy they have, the cost of process shutdown and startup, and so on. All these factors are unique to each company. The table below shows an example of how a company can calibrate its spurious trip levels. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Spurious trip level」の詳細全文を読む スポンサード リンク
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