Why short circuit calculation is not required in LT cable
sizing and requiredin HT cable sizing only
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Answer / sudhanva madhukar mahamuni
Generally LT Cable size are not calculated based on Fault current basis, Due to High Fault current on LT System. For e.g. consider that 2500 kVA Transformer, %Z = 6.25%, 415 Volts at Secondary. Fault Current at Secondary system will be 55.64 kA. Now if we would have to select the Cable size based on Fault current then it would be ---
A = ----------------- Sq.mm --- For XLPE Cable Aluminum Conductor
585 sqmm, nearest standard rating of cable is 630 sqmm. Hence Commercially as well as laying point of view the same is not recommended nor as per good engineering practice.
HT Cable Sizing :
Cable sizing based on Fault current is possible In HT System, based practically and commercially point of view. Consider that 11kV system is feeding the power supply. As per IS 2026, 11kV Power system will have max 500MVA Fault Level. It means it will have 26 kA. 300sqmm cable will be sufficient to withstand 26 kA for 1 Sec.
Summary : System Design shall not be strictly as norms, it should be design considering Laying / installation, Practically and Commercially viable.
Sudhanva Madhukar Mahamuni
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Answer / xyz
Short Circuit withstand capacity is to be checked for HT
cables. For LT breaker controlled motor feeders, the cable
impedance reduces short circuit current considerable, hence,
short circuit withstand capacity is not required to be
checked for the rated fault level of the LT motors.
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