High Tension Cables

The conductor size in the cables for any installation is also governed by its ability to carry short circuit current of system. For L.V Distribution cable may be selected on the basis of continuous load current. But in case of MV /HV distribution it is always safer to select the cable on the basis of ability of conductor to carry expected short circuit current. Short circuit current rating of cable should in line with short circuit capacity of demanding apparatus such as circuit breakers, Transformer & Reactor etc. beside its capacity to carry desired load current . Short circuits rating of cables each size are given in relevant tables & have been calculated on the basis of IEC -949 & IEC -986 & on the following assumption

A) Temperature of conductor just prior to short circuit

i) With XLPE Insulation – 90 Deg. C
ii) With PVC Insulation – 70 Deg. C

B) Maximum permissible conductor temperature during short circuit

i) With XLPE Insulation -250 Deg. S
ii) With PVC Insulation – 160 Deg. C

C) Volumetric specified heat of the conductor

i) With Aluminum conductor – 2.5 x 10 -3 J/Deg. C/MM3
ii) With Copper conductor – 3.45 x 10 -3 J/Deg. C/MM3

D) Reciprocal of temperature co –efficient of resistance of resistance at 9 Deg. C

i) With Aluminum conductor -228
ii) With copper conductor – 234.5.

Short Circuit current rating at different duration may be calculated as
Ish ( for t duration ) = Ish ( for 1 sec .) /t
Ish for 1 Sec .Duration is given in relevant tables in KA
t = Time duration required tobe calculated of short circuit in sec.