On reading the ARPN. The alternative risk priority number, IEC 60812:2018 Annex B.4.3 (informative). It exists because multiplying ordinal ranks is unsound: ARPN calibrates the three scales logarithmically — each level a fixed multiple of the one below, the same multiple for all three — and then adds them. Adding logs is multiplying the underlying quantities, so ARPN is the log of a relative risk, all 28 values from 3 to 30 are reachable, and one rank changing by one step always moves the result by exactly one. The standard’s worked example is S = O = D = 5 giving RPN 125 against ARPN 15; this tool returns those two numbers. Hover a value to see the risk ratio it implies. Two things to keep in mind: an ARPN only means something if your scale points really are a factor apart — set the calibration to match how you defined them and state which you used in the report, and note that over ten levels a ×10 multiple implies 10⁹ between the mildest and severest mode, which is rarely what a maintenance scale means. The severity rule still applies, so S = 10 bands high on its own. Where you hold real numbers for all three — OREDA failure rates, measured detection lead time — B.4.3 Note 2 says to calculate the risk from those directly rather than banding them at all. Switching method moves some rows. RPN and ARPN band identically on 854 of the 1000 combinations; the rest are the balanced middle cases RPN splits across a boundary despite identical ranks in a different order.