After calculating the overall derating factor, the required current rating is obtained by dividing the normal current rating by the overall derating factor. According to the requires current rating we note down the cross-section of the conductor. Suitability of this conductor depends upon the minimum size of the conductor required to withstand the short circuit current within permissible temperature rise for the given duration. Suppose the cross-sectional area obtained on the basis of derating factor is a Sq. cms. and obtained on the basis of derating factor is a Sq. cms. and that on the basis of short circuit withstand capacity be A Sq. cms. Obviously we are to select the cross-sectional area which is nearer to the greater of the two (i.e. between a and A). Now we are required to calculate the cross-sectional area of the conductor to withstand short circuit current for a given duration within permissible temperature rise. Basic assumption made here is that the heat produced due to a through short circuit current raises the temperature of the conductor and none escapes by convection or radiation in the very short time of
the fault.
By equating the heat produced and the heat stored it follows that for aluminium alloy busbar.
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