Interrupting rating (RMS Symmetrical)

[Guinevere]

Hi all,
found these doubt, hopefully someone can help me with these?

1. in MCC, for the bus bracing (RMS Symmetrical), if the requirement needs 22 kA, can 42 kA works for this and why?

2. Still in MCC, if combination starter unit has : (a)unit interrupting rating and (b) ambient complression over-load to be 22 kA RMS Sym, can 42 or 65 kA work for this?

my thought is that for the bus, it is okay if the design exceed 22 kA, but for starter unit...i don't know...

thanks before...

 

[rbulsara]

Rated SCCR (AIC) rating for an equipment or assembly needs to be equal or more than available short circuit current.

Think of the rated SCCR as "strength" of the equipment to withstand electro-mechanical forces( for a short period of time until the OCPD opens). So as long as the rated strength is equal or greater than required, you are good.

 

[waross]

The rating of the equipment must exceed the available fault current of the system.
You should not use 25 kA tared equipment on a system supplying 42 kA.
You may use 42 kA equipment in place of 25 kA equipment on a system with 25 kA or less available fault current.

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

 

[Zogzog]

"You may use 42 kA equipment in place of 25 kA equipment on a system with 25 kA or less available fault current."

Right, but it will cost you more money.

 

[jraef]

In North America, nobody I am aware of makes MCCs rated for less than 42kA bracing, and 42kA SCCR on buckets is the minimum norm. But the way the SCCR rating works on MCCs is that the lowest rated cubicle becomes the rating of the entire MCC. So if you have even one cubicle that is rated 25kA SCCR for example, the entire lineup becomes rated for 25kA regardless of what the other cubicles were rated at. And if you have even one unrated starter or controller, the entire lineup becomes de-rated to the 5kA SCCR rating for untested equipment. So you have to be careful.


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