1kV low-voltage overhead cables do not require an inner semiconductive shielding layer due to the very low electric field strength on the conductor surface. However, when the overhead cable is close to the grounding electrode, the electric field strength on the conductor surface of a 10kV overhead cable is 2-3kV/mm, while that of a 35kV overhead cable can reach as high as 5kV/mm, necessitating the use of an inner semiconductive shielding layer.
Because the conductor and insulation layers in a cable are not firmly bonded, they can separate under temperature changes. In rainy weather, the separated parts become filled with moisture, reducing their dielectric strength. Dry air has a dielectric strength of up to 3kV/mm, while humid air only has 0.5-1kV/mm. Thus, under normal operating conditions, a corona discharge forms between the conductor surface and the insulation gaps, causing corona discharge losses and easily damaging the insulation medium. Therefore, in countries worldwide, overhead cables of 10kV and above employ an inner semiconductive shielding layer on the conductor.
Single-core overhead cables do not require an external semiconductive shielding layer, but three-core laminated overhead cables of 10kV and above must use an external semiconductive shielding layer. This is because the external electric field of a three-core laminated overhead cable is uneven, and without an external semiconductive shielding layer, a strong corona discharge will occur when the air is humid.