Power engineering. Underground cables are installed in the ground for electricity distribution. They are surrounded by cable sheaths which protects against mechanical or chemical damages. Underground cables of up to 110 kV are laid in compact three-conductor bundles about 1 m in depth. A greater distance between the conductors is necessary for underground cables with voltages above 110 kV due to heat dissipation. Therefore these underground cables are laid at a depth of 2 - 3 m. Similar to power lines the section width of underground cables for 380 kV is 50-70 m. Underground cables can be operated either with three-phase alternating current or direct current (HVDC, often sea cables).