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Drying-out of soil with multiple systems

The method described in the IEC 60287 standard to calculate buried cables where partial drying-out of the soil occurs is limited to a single isolated cable or circuit only. Cableizer introduced a calculation method for multiple systems according to a method by VDE.

Posted 2015-11-02
Categories: New feature, Theory

The newest edition of the IEC 60287-1-1 from 2014 has still a limitation when it comes to calculate buried cables where partial drying-out of the soil occurs. The method shall be applied to a single isolated cable or circuit only, laid at conventional depths. The method is based on a simple two-zone approximate physical model of the soil where the zone adjacent to the cable is dried out whilst the other zone retains the site's thermal resistivity. This method was described in Electra No 104, 1966. This paper addressed the situation when more than one cable is present and demonstrated the method on a single multi-core cable and a single circuit of three single core cables in flat formation.

Mr. Franz Winkler wrote a comprehensive report about the current-carrying capacity of buried cables under consideration of drying-out of soil and daily load-cycles. This report was published by the VDE in 1978. The report uses the same two-zone method to calculate the effect of partially dried-out soil. It combines it with the two-zone method used to calculate the current-carrying capacity with a load-loss factor smaller than 1.0. The report also describes the calculation of the isothermal curves for multiple cables, not limited to one system.

Cableizer implemented the same formulas so the user can now enter a separate load-factor for each system plus consider drying-out of the soil at the same time. Following limitations apply:

  • The soil behavior is identical around all cables. As a consequence, a backfill area is not possible together with drying-out of the soil. However, Cableizer allows the combination of a backfill area with load-factors.
  • The cables must be laid in an adequate depth for the calculation method to be correct. A minimum laying depth of 60 cm for m.v. and 80 cm for h.v. cables can be considered sufficient. In shallow laying depths, the effect of non-isothermal ground temperature must be taken into account - Cableizer is working to implement a new calculation method which allows correct calculations up to a laying depth of 20 cm.
  • Two and more circuits require sufficient spacing in between. The IEC mentions the necessary spacing being under consideration, but no papers were found which address this issue further. We recommend that the isothermal curves at the critical temperature for drying-out of the soil do not cross each other for multiple circuits when calculation is done without drying-out and for each circuit separately. A spacing of 50 cm is usually sufficient.

Example

The following example was calculated with one system in trefoil (left side) and a second system flat spaced (right side). Please note for example the 40°C isoline.

without drying-out of soilwith drying-out of soil
Approximate temperature field without drying-out of soil
Fig. 1 Approximate temperature field without drying-out of soil		 Approximate temperature field with drying-out of soil
Fig. 2 Approximate temperature field with drying-out of soil
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