Soil Thermal Resistivity for Underground Cables

A property of the soil surrounding a directly buried cable that indicates how well or poorly the heat generated by the cable can be conducted away through the surrounding earth, expressed in K·m/W; a high soil thermal resistivity (for example in dry sand or loosely backfilled ground) impedes heat dissipation and therefore significantly reduces the permissible current-carrying capacity of the cable compared with the same cable in moist, well-compacted clay. Calculations per NEN-EN 60287 explicitly account for this effect alongside the usual correction factors for grouping, laying depth and ambient temperature, and when there is uncertainty about soil composition — for example dry-out-prone sandy ground during a hot summer — an overly optimistic assumption about soil thermal resistivity can lead to a cable that becomes structurally overheated in practice. For large or continuously heavily loaded routes (such as medium-voltage cables to wind farms or data centres), a specific on-site soil thermal resistivity measurement is therefore sometimes carried out instead of relying on table values.