Definition:
Full Variability Language (FVL) is a IEC 61508 and IEC 61511 programming category that include language that offers maximum freedom in control logic but introduces more risks related to systematic errors. Think of the standard PLC programming process. This should be used only if the standardized pre-configured safety function blocks are not sufficient.
A SIS process using FVL must then proceed via IEC 61508-3, which is very extensive. Because of this, FVL has been used less and less. Perhaps only on the largest of SIS. Using a limited variability language (LVL) is the preferred route for most facilities.
FVL typically uses the programming languages structured text (ST), instruction list (IL), and higher level languages like C and Pascal. But there can be variants.
Key Points:
- Requires stringent verification and validation methods via IEC 61508
- Least common of the three IEC programming categories
- Programming language choice can vary just like in LVL.
Example:
A facility has 300+ SIFs. Due to the complexity of some of the SIFs, it was determined that the existing safety function blocks in their PLC/DCS software are not sufficient. Thus they decided to go with an FVL approach and accept the extra cost of IEC 61508-3 certification.
See Also: FPL, LVL, application program
Cited Source:
- IEC 61511-1:2016, Clause 3.2.75
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