In addition to the existing IEC 60287 method, partial soil drying-out can now also been calculated based on the VDE standard 0276-1000, a method that is especially suitable for cyclic loads. It is also know as 'Heinhold' method due to its publication in the famous book by L. Heinhold and R. Stubbe. The method is based on the brilliant work from Dipl.-Ing. Franz Winkler from Siemens AG.
Posted 2021-04-06
Categories: New feature
The soil around an object may partially dry out due to heat losses, thus decrasing the thermal conductivity of the surrounding soil. It is important to correctly account for this change in thermal resistivity of dry soil, because soil drying-out can considerably increase the temperature of a cable system. In Cableizer, partial soil drying-out can be considered in the module for buried systems. In the 'General' tab, switch from the 'directly buried' method to 'with drying-out of soil'. In the 'Soil' tab, you can then select between the drying-out methods described below.
Both IEC and VDE methods define a critical soil temperature $\theta_x$ on the boundary between dry and moist zones. Depending on whether the outer surface temperature $\theta_o$ of an object exceeds $\theta_x$ or not, partial soil drying-out will be considered or not. If soil drying-out occurs, both methods apply a critical soil temperature rise $\Delta\theta_x$ and a ratio of thermal resistivity between dry and moist soil $v_4$ to the calculation.
The critical soil temperature rise $\Delta\theta_x$ is the temperature rise of the boundary between the dry and moist zones above the ambient temperature of the soil. Both methods are based on a simple two-zone approximate physical model of the soil where the zone adjacent to the system is dried out whilst the other zone retains the site's thermal resistivity, the zone boundary being an isotherm. These methods are considered to be appropriate for applications where soil behaviour is considered in simple terms only.
Both methods are of binary nature, i.e. either soil drying-out occurs or it does not. Soil drying-out is not considered to happen gradually, and it is not taken into account how much the outer surface temperature $\theta_o$ exceeds the critical soil temperature $\theta_x$. In Cableizer, the permissible current rating is at first always calculated under assumptions of no partial soil drying-out. Then, soil drying-out is applied to all systems where $\theta_o$ > $\theta_x$. This step is repeated until there are no new systems that will be subject to soil drying-out. The following two sections describe the differences between the two methods and how they are implemented.
The IEC method defines $\theta_x$ as a user input, based on which $\Delta\theta_x$ is calculated as the difference to the ambient temperature $\theta_a$:
$$\Delta\theta_x = \theta_x - \theta_a$$The IEC method should be applied to a single isolated cable or system only, laid at conventional depths. Installations of more than one system should consider sufficient spacing between systems in order to ensure their thermal independence, but this is not formally covered in IEC 60287-1-1 Ed. 2.1.
The VDE method defines $\Delta\theta_{x0}$ as a user input, which is the temperature rise of the boundary between the dry and moist zones above the ambient temperature of the soil for continous load ($LF$ = 1). In the VDE 0276-1000 standard, this value is fixed to 15 °C, while it can be adjusted within reasonable limits in Cableizer. Based on $\Delta\theta_{x0}$, we can calculate $\Delta\theta_x$ and $\theta_x$:
$$\Delta\theta_x = \Delta\theta_{x0} + \frac{100(1-LF)}{3}$$ $$\theta_x = \theta_a + \Delta\theta_x$$Compared to the IEC method where soil drying-out is generally set to 50 °C, the VDE method assumes soil drying-out to start at temperatures of as low as 15 °C above ambient temperature. This may result in conservative current ratings / pessimistic temperature estimations, which is the reason that Cableizer allows you to adjust $\Delta\theta_{x0}$.
The VDE method is also especially suitable for cyclic loads since the critical soil temperature rise $\Delta\theta_x$ is increasing with a decreasing load factor $LF$. As such, systems with different cyclic loads will have different critical soil temperatures $\theta_x$. But as for the IEC method, installations of more than one system should consider sufficient spacing in order to ensure their thermal independence.
Soil drying-out according to VDE 0276-1000 was introduced on customer request. If there is anythig that we could do to enhance your Cableizer usage experience, please let us know!
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