842-843 / 973
a CSA International company
FMEA of the (close/open)
Circuit type B
The example has demonstrated that diagnostics used for example
( PST) is a good credit for improving PFD and SFF of the final
element.
New SFF of the final element is improved, but the final element
should be assessed to
IEC 61508-2 table 3 (type B), hence the
product is SIL1, otherwise assessment to IEC 61508 is violated.
The big question is why final elements are claimed as SIL 3 if PST
is used?
The verdict accepted by end users, why, these elements approved
by recognised agencies, hence results accepted in good faith