Short Circuit Current at the Load terminal

[NickParker]

Say if a MCC is rated for a short circuit capacity of 25kA, What is the short circuit at the load end terminal? this is to check the short circuit withstand check of the cables to that particular load.

I have the cable impedance for the length till the said load,

The calculation is as follows,

SC current at the load terminal = VL-L / (1.732 x (System impedance + Cable impedance)

Where,
VL-L = Line to Line voltage
System impedance = VL-L/ (1.732 x 25kA)

Is my approach correct?

 

[waross]

Your getting close.
You have to add the transformer impedance with the cable impedance vectorily.
√((transformer R + cable R)² + (transformer X + cable X)²))

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

 

[JuanBC]

Hi,

You are mixing the concepts.

25 kA is the maximum value of SC current that the MCC can withstand for a certain time (in general, one or three seconds - you should check it with the manufacturer)

The actual short-circuit current does not depend on this value, but on the system voltage and the impedance up to the point of failure (system impedance + transformer impedance (if applies) + cables impedance + etc..)

Be carefull: Assuming a 25kA fault current at the MCC can lead to an incorrect result in, for example, the calculated time at which the downstream protection acts.

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[wroggent]

What do you mean by ‘capacity’?

 

[waross]

Check the Available Short Circuit Current of the supply transformer.
It should be less than the short circuit rating of the MCC.
If the ASCC is more than the MCC rating, then calculate the ASCC with the added impedance of the cables.
I worked a project a long time ago where a minimum of 100 feet of cable was used to feed each unit substation. The added impedance of the cable lowered the ASCC to below the short circuit rating of the unit subs.
This was very much more economical than using unit substations with higher ASCC ratings.

You can use the same formula and add in the X and R of the cable feeding a load from the MCC to check the maximum ASCC at the load end of the cable.

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