Soil temperature affects high voltage cable and earthing systems. Equations for soil temperature calculations based on air temperature are provided.

The current rating of bare conductors is affected by the conductor temperature, weather parameters, heat losses due to convection and radiation, the solar heat gain and conductor resistance, the calculation of which is governed by a steady-state and a non-steady-state heat balance equation.

A report investigated 6214 cable failures over a 14 year period. The main causes were insulation breakdown, excavation, joints or switchgear failure. The results may surprise you.

The definition of emergency rating is the permissible short-term rating of a cable already loaded and at a steady-state, considering the thermal capacitances and the thermal resistances of an installed cable system. The definition of cyclic rating is the maximum current of a cable when the load is varied in a sequence of steps that are repeated cyclically. The cyclic rating differs for different sequences and different cycle periods. Both emergency and cyclic ratings deal with time-varying loads