The total failure rate of safety-related hardware is 1000 FIT, of which single-point faults and residual faults (faults that remain undetected by safety mechanisms) total 5 FIT. What is the single-point fault metric (SPFM)?

Answer: A. 99.5%

SPFM is calculated as 1 minus the "sum of the single-point and residual fault failure rates" divided by the "total safety-related failure rate." In this example, 1 − 5 / 1000 = 1 − 0.005 = 0.995, or 99.5%. This meets the ASIL D guideline of 99% or higher. 95% is the value obtained by overestimating the proportion by an order of magnitude, and 99.95% by underestimating it by an order of magnitude. To raise SPFM, add safety mechanisms with high diagnostic coverage to reduce faults that remain undetected.