BS 7769-1.2:1994

Electric cables. Calculation of the current rating. Current rating equations (100% load factor) and calculation of losses. Sheath eddy current loss factors for two circuits in flat formation | Status: Confirmed, Current

1 Scope This section of IEC 287‑1 provides a method for calculating the eddy current losses in the metallic sheaths of single-core cables arranged as a three-phase double circuit in flat formation. The sheaths are bonded at one point or are cross-bonded so that there are no significant sheath circulating currents. Where metallic sheaths are bonded at both ends there are significant circulating currents which result in a lower current-carrying capacity. A method of calculating circulating current losses for double circuits is under consideration. The method provides coefficients which are applied as corrections to the loss factors for the sheaths of one isolated three-phase circuit. These corrections are negligible for cables where the parameter m is less than about 0,1 (m = ω/107Rs), which corresponds to a sheath longitudinal resistance higher than 314 µΩ/m at 50 Hz. Consequently the method should be used for most sizes of aluminium-sheathed cables, but is not required for lead-sheathed cables unless they are unusually large: The coefficients are provided in tabular form and have been computed from fundamental formulae for sheath losses, the evaluation of which calls for expertise in computer programming which might not be readily available in general commercial situations. The development of simplified formulae for some of the tabulated coefficients is under consideration. Losses for cables in a single circuit will be covered in IEC 287‑1‑1 (under consideration).