Potential of Fault-Detection Coverage by means of On-Chip Redundancy-IEC61508: Are There Royal Roads to SIL 4?
This paper investigates potential to improve fault-detection coverage by means of on-chip redundancy. The international standard on functional safety, namely, IEC61508 Ed. 2.0 Part 2 Annex E.3 prescribes the upper bound of beta(IC) (common cause failure (CCF) ratio to all failures) is 0.25 to satisfy frequency upper bound of dangerous failure in the safety function for Sit (Safety Integrated Level) 3. On the other hand, this paper argues that the beta(IC) does not necessarily have to be less than 0.25 for Sit 3, and that the upper bound of beta(IC) can be determined depending on failure rate lambda and CCF detection coverage. In other words, the frequency upper bound of dangerous failure for SIL3 can also be satisfied with beta(IC) higher than 0.25 if the failure rate lambda is lower than 400 [fit]. Moreover, the paper shows that on-chip redundancy has potential to satisfy SEE, 4 requirement; the frequency upper bound of dangerous failure for SIL4 can be satisfied with feasible ranges of beta(IC), lambda and CCF coverage which can be realized by redundant code.
fault-detection on-chip redundancy iec61508 functional safety common cause failure