Calculation of fault limits for cables

[Dannielle]

Hi,
I have been trying to find a reference to help me with calculating the fault duties (limits) of various cables.

I want to include the effect of conductor size and sheath type and size for various operating temperatures and cable construction.

If anyone can give me a reference either on the web or in a book it would be great.

Thankyou

 

[gordonl]

The okonite site has some info on shield and conductor short circuit ampacities.

http://www.okonite.com/engineering/index.html

 

[cuky2000]

The short circuit capacity of cable (conductor and shields) is limited by their heat capacity and by the maximum temperature permitted in the cable insulation.

Thermal damage during short circuit are based in Temperature-Damage curves (I²t)seen in protection and coordination graph. Cable damage curves could be modeled by the following relations:
I=k.CM/SQRT(t)
_{Where: CM = Cross section of conductor (cmil)
.k = Constant that depend of conductor material and insulating temperature limits. For Instance:
Cu : k=0.0694 for To/Tf= 65/200oC, k=0.0779 for To/Tf=90/250oC)
k=0.0453 for To/Tf= 65/200oC, k=0.0509 for To/Tf=90/250oC)
To =Initial/continuous temp rating (Rubber 75oC or 90oC, paper=85oC)
Tf =final/continuous temp rating (Rubber 200oC or 250oC, paper=200oC)
.t= Total fault-clearing time (ex: relayed Bkr, direct tripping bkr, fuses)
Protection philosophy could be based either primary or back up time.}

SUGGESTION:
1) Check IEEE Std 242 or similar reference.
2)

http://www.kerite.com/catalog/catalogfiles/short_circuit_limitations.htm

 

[RalphChristie]

Check also this site:

http://www.aberdare.co.za/power/tech-articles.html

(Tech article 4)

Hope it is helpful
RCC