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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?

fluxmodel
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1 Answers1

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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.

Simon B
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