Short Circuit Rating of Cable

[skm]

We are required to design a 11kV system for a rated fault current of 25kA for 1 sec. Based on this requirement the minimum cross section of XLPE insulated Cu cable to be used comes out as 185 sqmm. The actual current required for feeder is 40A.

Now my question:

The permissible short circuit rating of the cable is calculated based on the initial conductor temperature of 90 deg C and final conductor temeperature of 250 deg C. Since the cable is not loaded to its rated current the initial temperature is not going to be 90 deg C as ssumed earlier and hence the actual short circuit handling capacity would be higher than calculated. Is there a method to calculate the actual temperature rise for a particular cross section? Can anyone guide me how to proceed further on this?

Thanks
SKM

Good day gives you happiness, Bad days give you experience. Count Both....

 

[jghrist]

http://www.okonite.com/engineering/short-circuit-currents.html

gives the equation for calculating required area at any initial or final temperature (in terms of circular mils - you'll have to convert). There is also a table of correction factors for 75°C and different final temperatures. Correction is 1.06 for 75°C initial and 250°C final.

You also have to consider the fault capability of the shield. See

http://www.okonite.com/engineering/shielding-currents.html

You should also question the requirement. It would be unusual for protection to allow 25 kA for a full second. If this is a client requirement, let them know how much this requirement will cost them in excess cable size.

 

[cuky2000]

See the enclose link

http://cuky2000.250free.com/Cable_SC_IEEE-Std-242.jpg

Paraphrasing Yogi Berra, engineering is 90% technical and the other half is administrative.