Current rating standards 69-30 Part IV
Calculation of cyclic and emergency current ratings for cables laid direct in the ground or in ducts for cables up to and including 19/33 kV.
This report, which is part 4 of the series, sets out a method for the calculation of cyclic rating factors and emergency loading for cables having rated voltages up to 19/33kV laid direct in the ground or in ducts. This method assumes that the soil surrounding the cable or duct is uniform and moisture migration does not take place during the load cycle.
The method set out in this report requires a knowledge of the variation of load for six hours prior to the time of maximum conductor temperature of the cable and an average value for times before that. Where the shape of the load variation makes it difficult to intuitively assess the time of mazimum conductor temperature it may be necessary to calculate the cyclic rating factor for several six hour periods and select the lowest factor. A guide to assist with identifying the time for the highest temperature is given in section 1.4 of this report,.
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