What are the main selection and design criteria for cables used in underground distribution?

Underground power cables must give a service life of at least 20-25 years whilst carrying the expected load and overload currents without overheating under worst case ambient and ventilation conditions, simultaneously withstanding mechanical pulling stresses during installation, and radial stresses during their operating life. These must also be as compact as possible, since multiple utilities vie for the same constrained underground space in congested urban areas - copper cables are more suitable being around 40% smaller in cross-section than aluminium cables.

There are three critical design criteria involved – ampacity, sheath thickness and pulling tension.

Cable ampacity is determined according to IEC 60287 together with the method of installation as documented in IEC 60364-5-52. This is the steady state current at which the equilibrium (heating rate = cooling rate) temperature of the conductor reaches the maximum permissible for the type of insulation e.g. 90°C for XLPE. The cable heating rate principally depends on its construction (conductor resistivity, loss factors of the insulation /sheath / armor, no. of cores), and marginally on the installation (type of sheath bonding, mutual heating effects). The cooling rate depends on the difference between the conductor and outside ambient temperatures, and the collective thermal resistance of all elements in-between (insulation, sheath, armor, jacket, soil, earth, backfill, concrete) and varies widely with the type of installation.

A cable buried directly in the ground cools faster and has a much higher ampacity than an identical one laid through ducting and encasement.  A 3-core cable has a lower ampacity than a 1-core cable due to the mutual heating effect. Cables in vertical configuration have lower ampacities than those in horizontal formation for the same reason. A higher ambient temperature, and a deeper installation lowers the ampacity as well.

The cable sheath is important for mechanical protection during installation and operation and the identification of a suitable sheath thickness is essential to prevent damage. The sheath thickness is calculated according to IEC 60502-1 and IEC 60502-2.

Cables must have a higher tensile strength than the maximum pulling tension expected during installation in order to escape any damage. The pulling tension depends on a number of parameters -  the cable weight per unit length, the maximum length to be pulled, the gradient expected to be encountered, the bends in the route and the friction between the cable and the soil / conduit as well as with other adjacent cables for which selection of the appropriate cable configuration is important.  Precise calculations are needed to dimension conduits so as to prevent jamming , and to limit bending. Copper cables are better suited from this perspective due to a higher tensile strength to weight ratio, higher flexibility and a smaller bending radius.

Reference: Power Cables Handbook, Volumes 1&2, Conversion of Overhead Lines to Underground Cables, Copyright@2014, International Copper Association South East Asia Ltd. And Provincial Electricity Authority, Thailand