The conductor size in a Cable for an installation is also governed by its ability to carry short circuit current of the system. Short circuit ratings are based on the assumption that the duration of short circuit is so small and apparently there is no transmission of heat, produced during short circuit through the insulation and the whole of it is absorbed by the conductor.
The short circuit current rating (r.m.s. values) of XLPE / PVC insulated cables with aluminium and copper conductor for one second duration are given as under. These ratings have been calculated on the following assumptions.

 

  UNIT ALUMINIUM COPPER
Temperature of conductor just prior to short circuit      
  • With general purpose insulation
 0C 70 70
  • With heat resisting insulation
 0C 85 85
  • With XLPE insulation
 0C 90 90
Max. permissible temperature of conductor during short circuit      
  • With PVC insulation
 0C 160 160
  • With XLPE insulation
 0C 250 250
Volumetric specific heat of conductor at 200C J/0Cmm3 2.5 x 103 3.45 x 103
Receptivity of conductor at 200C ohm-mm 28.264 x 106 17.241 x 106
Reciprocal of temperature co-efficient of resistance of conductor at 00C 0C 228 234.5

With the above assumptions the short circuit rating "Ish" is given by the following formula.
 

Where
K = Constant (depends upon the type of insulation and conductor material)
A = Nominal cross-sectional area of conductor in mm2
t = Duration of short circuit in seconds

 

SHORT CIRCUIT RATING OF 1100 VOLTS GRADE PVC INSULATED
HEAVY DUTY CABLES ( FOR DURATION OF ONE SECOND )

SHORT CIRCUIT RATING FOR XLPE INSULATED
CABLES ( FOR DURATION OF ONE SECOND )

REACTANCE OF CABLES AT 50 Hz

Approx. Reactance of Cables in Ohms / Km at 50 Hz
 

 

CAPACITANCE OF CABLES

Capacitance of Cables in Micro Farads / Km