Cable crossings-derating considerations. I. Derivation of derating equations

Power Cable Rating Calculations - A Historical Perspective [History]

“…A few months later, Anders and Dorison extended the earlier papers [68], [69] and proposed a new formula for the derating factor of cables crossing external heat sources, which considers the longitudinal heat flow in the cable conductor and metallic screen [74]. The same year, Brakelmann et al [75], [76] developed an algorithm to calculate the current-carrying capability and temperature of the hottest conductor in cable bundles.…”

“…This book covers most of the previous findings reported on the subject, providing an extensive literature review and a comprehensive and continually illustrated presentation of the existing computation methods, such as those of the IEC Standard 287 and the NeherMcGrath paper [38]. A valuable extension to the previous method for the thermal rating of cable systems in their environments is later addressed by the authors in [68] and [69]. In these works, they adopt a numerical approach to study the temperature and ampacity of buried cables crossing external heat sources and propose a formula for the calculation of a derating factor that allows for the consideration of these effects.…”

Derating Factor for Cable Crossings With Consideration of Longitudinal Heat Flow in Cable Screen

“…For perpendicular and oblique crossings, these interference temperatures are usually ignored for distribution circuits, whereas for transmission cables, corrective actions in physical installation condition are sometimes taken. As it was shown in [1] and [2], the derating in the range of 40% may be required when the cable circuit crosses external heat sources.…”

“…In the developments presented in [1] the longitudinal heat flux in the conductor was considered whereas the same flux in the metallic screen was ignored. The purpose of this paper is to fill this gap by introducing a mathematical algorithm required to take into account the longitudinal heat flux in the metallic screen.…”

“…1. The temperature rise in the conductor of cable 2 caused by the heat rate generated by the heat source 1 can be obtained by applying Kennelly's principle, as (1) where thermal resistivity of the soil (K m W); temperature rise at the location of the conductor of cable 2 caused by the heat generated by the heat source 1, uninfluenced (index " ") by any effect of cable 2 (K). To find the temperature at any point of the rated cable taking into account conductor longitudinal heat flux and the interfering temperature given by (1), we will discretize the length of the rated circuit into intervals, each of length .…”