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Systematic Capability
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When using an element or device that has been designed in accordance with IEC 61508, the Systematic Capability
is determined from tables and measures defined in IEC 61508 parts 2 & 3 as part of the original design /
assessment.
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Hardware fault tolerance in both IEC 61508 and IEC 61511 is defined as “ability to continue to perform a required
function or operation in the presence of faults or errors”.
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i.e. hardware fault tolerance = 1, means that the function will still operate in the presence of 1 undetected fault.
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When considering Systematic Capability, Two identical elements or devices are likely to be affected by the same
systematic faults.
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So if one device fails due to a systematic fault, then a second identical device in the same environment is likely to fail at the
same time.
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Therefore two identical devices may not meet the Systematic Capability criteria required.
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i.e. two identical devices each with SC=2, if used in a 1oo2 configuration so that they will have 1 level of hardware fault
tolerance, will only meet SIL 2 requirements, even though they may meet the hardware fault tolerance and PFD / PFH
requirements for SIL 3.
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The above information is not always clear in product safety manuals.