By HV Cable, I meant mainly 3.3KV, 6.6kV and 11KV Cables which are used in industry applications. Infact my query was for MV Cables. Sorry for the inconvenience.
Generally the neutral will be sized to carry the SLG current capable of being produced by the system until the upstream protection device opens. Also if there is no other neutral conductor make sure that it can carry any unbalance or harmonic current that is present.
The copper tape have two functions.
(a) The copper tape will be a low impedance return path for the fault current (because of the close proximity with the conductor). The copper must be able to withstand the prospective fault current for the fault duration. The cable manufacturer will specify the cable rating for earth fault currents. I will leave the calculations to the cable manufacturer and specify the prospective earth fault current. Cables making use of copper tape have a low earth fault current rating (Typical 4kA but depends on Cable manufacrurer). In other parts of the world, individually screened conductor using wire are used instead of tape with earth fault current ratings of up to 10kA. Some users opt to earth only one side of the screen.
(b) The second function of the tape is to help conduct heat generated by the three conductors. Heat will be conducted radially outwards away from the conductor. Heat will accumulate in the centre of the cable between the three conductors and the copper tape will help in conducting this heat to the oposite side of the conductors, closer to the jacket/armouring/sheath. The thicker your tape are, the better heat transfer and the higher cable rating you will have. Only if you are designing the actual cable will it be required to calculate the actual copper tape size.
You can refer to IEC60287 for the actual calculations. The earth fault current capability calculations will be particularly difficult as you have to use a dynamic model. The steady state thermal calc's can easilly be done by hand.
If there is no neutral or parallel ground, the tape must carry fault current until the fault is cleared. This can be a significant limitation, which is why separate grounds are frequently installed. Most cable manufacturer's can provide tables of fault withtand capability for their products.
Calculations are too complex to cover in this forum. See:
"Short Circuit Performance of Metallic shields and Sheaths of Insulated Cable (Second Edition)", ICEA Publication P-45-482, 1979
Siemens Power Cables and their Application, 1990
"Rating of Conductors For Short Duration Currents", V.T. Morgan, Proc. IEE, Vol. 118, No. 314, March/April 1971.
"The Short-Circuit Rating of Thin Metal Tape Cable Shields", R.C. Mildner and others, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-87, No. 3, March 1968, pp. 749-759.
I'll have to disagree with KJvR about the second function. For single core cables, with shields grounded at both ends, increasing the thickness of the tape will decrease the resistance and increase induced shield currents and losses, causing more heating and lower current carrying capacity.
jghrist, your are correct that the screen will reduce the cable ampacity of a single core cable. My mistake. I should have stated more specifically that it increase the ampacity of three core cables as most 3.3 - 11kV cables are three core cables.
Another reason for metallic screen around MV-cables:
When you move into MV and higher voltage applications for cables, you have to use semiconductor around the cable. Semiconductor does have a relative high resistance, and due to capacitive coupling there are always small currents in the screen.
P = I²R ===> High power, even if the currents are relative small, and this burns the semiconductor. To minimize the resistance, a screen is added with the semiconductor.
The thickness of the tape is thus dependant on a function of the total resistance (semiconductor + metallic screen) and the possible induced shield current.
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