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IEC switchboard rating

IEC switchboard rating

IEC switchboard rating

(OP)
Hello! This is my first post on this very interesting forum which I have been reading for many months. I am not sure if this topic should have been opened in "Electrical Enclosures and Electronics Packaging" section, but I guess it's OK here too.

According to IEC it is usual to give a switchboard a short time withstand rating for a certain time, i.e. 50 kA, 1 s. The second rating is Ipk, in this case 105 kA. These ratings certainly apply to busbar systems, but which is the right way to choose the circuit breakers? For breakers it is known that Icu must be higher than the prospective short circuit (named Icp in the new IEC 61439-0), and Icm must be higher than Ip (peak short circuit current).

It is also stated in IEC that "short time withstand current" is the current which the "assembly" must withstand for 1 second if the upstream protection doesn't interrupt. Some of my colleagues think that breaking capacity of the breakers on the supply (downstream) side of the switchboard must be equal or higher than Icw of the switchboard. Sometimes we are required to design switchboards with Icw/Ipk much higher than the actual short circuit level, so the breakers would also be much oversized for the breaking and making capacity. I believe Icu should only be higher than maximum prospective s.c. current, and can be lower than Icw. After all, the breaker is here in order to protect, and it shouldn't allow the s.c. current to flow for 1 second. And some breakers such as MCCB and MCB (Schneider, ABB) don't even have the Icw rating, and we shouldn't confuse Icw with Icu. We are talking about marine LV switchboards where the longest time delay for s.c. protection is 0.5 s for generators, so no short circuit should last longer than that.

Can anyone clarify the selection of breakers with respect to switchboard's Icw? Thanks.

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