Rated short-time withstand current Icw of Circuit Breakers

[MakisMakaros]

Hi all,

I have a LV MCC with rating 3NPE ~50Hz 380V/TN-S 2850A 80kA 1s 176kA peak. The incoming protection Circuit Breaker will of course have a rated short-time withstand current Icw=80kA. My question is, can the vertical bus-bars and the outgoing groups protection Circuit Breakers have a rated short-time withstand current Icw < 80kA (for example 50 kA)?

 

[waross]

What is the Available Short Circuit Current from the supply?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MakisMakaros]

The available Short Circuit Current from the supply is 80 kA

 

[waross]

Can you show that the ASCC at the location of the downstream breaker is 80 kA or less?
The impedance of the supply conductors may reduce the ASCC or current limiting fuses may allow a lower rating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MakisMakaros]

The situation that I have is: Main incomer ACB with 80kA Icw, main Bus-bar with 80kA Icw, vertical (distribution) busbars with 50kA and MCB's, MCCB's connected to the vertical bus-bars are 50kA (some are 80kA). My question is, how is it possible that the main and the vertical busbars have different Icw rating? I understand that the rating of the outgoing groups protection will have the same Icw rating as the one of the vertical bus-bar.

 

[MakisMakaros]

I forgot to mention that no fuses are used before the Circuit Breakers, so we don't have a series or cascading protection method.

 

[waross]

That depends on the impedance of the main bus bars between the supply breaker and the first 50 kA vertical bus bars.

Bill
--------------------
"Why not the best?"
Jimmy Carter