short circuit rating for the main disconnect switch

[tarekbadr]

Hopefully someone can help me out on this:

A panel , consists of 3 branch circuit breakers , 300A(36KA), 200A(25KA)AND 40A(25KA) , IF WE USE DISCONNECT SWITCH FOR MAIN OF THIS PANEL , WHAT IS THE SHORT CIRCUIT LEVEL FOR IT

Regards

 

[e57]

A lot more info required...

 

[tarekbadr]

what is required more , like what?

 

[Fulthrotl]

what is required more , like what?

are you referring to arc fault calculations?

impedance of the poco's supply transformer, feeder size and length,
type of connected load (large motor loads increase the value)
among other things....

however, people lurking in the safety forum are guru's of flash-bang.

 

[tarekbadr]

thx ,
I mean for all molded case disconnect switch for schnider electric or ABB , with max. 7KA S.C. Level , is that ok or we must use mloded case circuit breaker with 36KA Isc.

OR WHEN WE USE DISCONNECT SWITCH as a main for a panel , SC level doesnt matter.?

 

[Jraef]

You still haven't really indicated that you know what you are asking about, which makes it a bit difficult to answer.

Assuming you are concerned with the SCCR rating of an overall system; Typically a non-fused disconnect has a very low rating, i.e. 5-10kA, so if you used one in a panel system with those other components, you will drag the overall SCCR rating down to that level. But a fused disconnect will typically be rated at what the fuses are rated for. However the proper way to do this is to look at the AVAILALE fault current and then the LET THROUGH of the fuses to ensure that nothing gets through that exceeds the lowest rated device, in your case 25kA.

 

[don_resqcapt19]

... However the proper way to do this is to look at the AVAILALE fault current and then the LET THROUGH of the fuses to ensure that nothing gets through that exceeds the lowest rated device, in your case 25kA.

That would be an engineered series rated system and that is not permitted by the NEC rules, with the exception of existing installations. Series rated systems must be a tested and listed combintation.

 

[jim dungar]

The AIC ratings of the exsting devices have no impact on what rating is required for the main switch. You must know how much short circuit amps (SCA) are available at the location where you want to put your main.

After you know the SCA at your main, you can calculate the SCA at your exisiting breakers and then check if they are rated correctly.

 

[tarekbadr]

Thank you for all , the over all SCC for the panel is 36KA , SO any branch circuit breaker must be rated 36KA , but we can use switch ( with low SCC) at main .

That is right?