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Breaking capacities of circuit breaker

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    Breaking capacities of circuit breaker

    Hello guys,

    I'm pretty new in electrical engineering, just finish my schools and now I need some help from practice. I have one distribution board with incoming circuit breaker. Because the short circuit value on the busbars of D.B. is 23kA I defined incoming CB (F0) as 25kA (breaking capacites). At the end of the feeders the short circuit is (for example): 16, 17 and 22kA, as it's showed in the picture from attachment. Should my CBs F1,F2,F3 have breaking capacites od 25kA due short circuit on the busbar, or 20kA for F2 and F3 CB is enoguh?
    Click image for larger version. 

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    Hope somebody can help me.
    Thanks!

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    Quote Originally Posted by Ivan01 View Post
    Hello guys,

    I'm pretty new in electrical engineering, just finish my schools and now I need some help from practice. I have one distribution board with incoming circuit breaker. Because the short circuit value on the busbars of D.B. is 23kA I defined incoming CB (F0) as 25kA (breaking capacites). At the end of the feeders the short circuit is (for example): 16, 17 and 22kA, as it's showed in the picture from attachment. Should my CBs F1,F2,F3 have breaking capacites od 25kA due short circuit on the busbar, or 20kA for F2 and F3 CB is enoguh?
    Click image for larger version. 

Name:	Isc.jpg 
Views:	1 
Size:	114.9 KB 
ID:	427
    Hope somebody can help me.
    Thanks!
    The distribution Breakers F1, F2, F3 should have 25kA rating, because they should able to handle a short circuit in the load side of the breaker from their lugs.

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    Quote Originally Posted by alexeic View Post
    The distribution Breakers F1, F2, F3 should have 25kA rating, because they should able to handle a short circuit in the load side of the breaker from their lugs.
    Thanks for your reply. But, on the load side of the F3 CB the short circuit current is only 16kA, so why 20kA isn't enough? That is a problem which I can't understand. Thx.

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    Quote Originally Posted by Ivan01 View Post
    Thanks for your reply. But, on the load side of the F3 CB the short circuit current is only 16kA, so why 20kA isn't enough? That is a problem which I can't understand. Thx.
    The 16kA is at the end of the feeder, load 3, but the breaker must protect all the cable from the beginning to the end. The SC in load side lugs of F3 is 23kA.
    Breaker's SC ratings of feeder could be smaller with serie coordination, but is very risky and has to do deeper analysis of the installation y following some rules. More of the people play safe and chose the breaker's SC rating the same o bigger of the DB.

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    Quote Originally Posted by alexeic View Post
    The SC in load side lugs of F3 is 23kA.
    I tried to draw this situation on paper from attachhment. This is sitation which I don't understand. You said: The SC on the load side lugs of F3 is 23kA, but I don't undestand how, because the load side lugs are on the bottom side of F3 CB (Or I'm wrong?) and te SC there is 16kA?
    Click image for larger version. 

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