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# Thermal effects of short circuit currents according to IEC standards

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## Thermal effects of short circuit currents according to IEC standards

(OP)
Hi all!
Couple of questions about the subject.
Low voltage: Relatively new IEC 61439 standard defines rated short time withstand current Icw. It is the current which equipment or assembly can withstand for period of time given by manufacturer. Icw shall be equal to or higher than the prospective RMS value of the short circuit current Icp. It is explicitly stated that Icw is RMS value of AC component.
Medium/high voltage: There is a definition of Ik within IEC 60694 standard, practically the same as Icw, but without restriction to AC component, at least I didn't find any.
I am trying to make it clear which current within old (but precise and academic related) standards for calculation corresponds to particular values defined above. In other words, after short circuit calculation for given application, how to prove that some equipment or assembly may be safely installed regarding thermal effects of short circuit.

1. By my understanding of given definitions, Icp is closest to definition of initial symmetrical s.c. current Ik" given within IEC 60909. If so, then Icw/Ik shall be equal to or higher than calculated value of Ik", but then what is the purpose of duration of Icw/Ik, since Ik" doesn't depend on time?

2. There is a thermal equivalent short time current Ith defined within IEC 60865, as a value which is taking into account a thermal effect of DC component as well, and it depends on fault duration time, as it is physically logical. But what is the purpose of calculation of Ith if there isn't any standard value defined for equipment it may be compared with?

3. How can someone show (using IEC terms) that decreasing of fault duration by protection setting change will decrease thermal stresses as well and that same equipment can cope with s.c. for different protection setting, while it may fail for initial setting?

Back in old days, Ith and its duration for equipment or Joule integral was used, and everything was much more clear. Now I'm quite confused with all these values, but they are of course given as equipment data by manufacturers so I must comply. Please help.

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