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Topic Title: Ith in IEC 60909
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Created On: 20 October 2010 09:32 AM
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 20 October 2010 09:32 AM
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Joined: 05 October 2010


I have been using different electrical design software for short circuit calculations following standard IEC 60909.

When it comes to the calculation of Ith (at 1 second), I always get Ith values greater than Ik.

Looking into the standard, I see that the equation for Ith calculation is:
Ith=Ik''*sqrt(m+n). The standard says that when Ik''/Ik=1 then n=1 and gives an equation for the case Ik''/Ik>=1,25. It seems that electrical design software has implemented n=1 for Ik''/Ik<1,25.

My question is: what to do when 1<Ik''/Ik<1,25?

When choosing switchgear, Ith has to be compared with the Ithr of the switchgear, so it is important to accurately calculate Ith, not to overdesign.

IET » Energy » Ith in IEC 60909

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