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25 kA, 31,5 kA and 40 kA are typical ratings of switchgear with respect to maximum short-circuit current capabilities (typically denoted \$I''_k\$)

I can understand 40 and 25 as they are nice numbers - but 31.5 is downright ugly. How has that number become an standard ? Does anyone know the history or background here?

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  • \$\begingroup\$ It's about sqrt(1000) rounded reasonably nicely. \$\endgroup\$
    – user16324
    Commented May 31, 2021 at 12:52
  • \$\begingroup\$ It is also -11 ln(2) + 15 π - 4 sqrt(3) - 5 sqrt(2) + 6. Rounded much more nicely. \$\endgroup\$
    – fluxmodel
    Commented Jun 1, 2021 at 13:22

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It's part of a power series. To a close approximation, 25 x 1,25 = 31,5. Then 31,5 x 1,25 = 40. The next few values are 50 and 63, which also appear a lot in component specifications.

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