For the outer semi conductive shielding layer of cable, it is generally considered that the effect is not significant due to the low electric field intensity on the outer surface of insulation. In fact, it is not the case. According to IEC regulations, cables of 6kV and above should have inner and outer semi conductive shielding layers. When the cable is bent in operation, the surface of the insulation layer of the cable will be stretched by tension. If there is partial discharge at this time, the surface bending stress will produce submicroscopic cracks, which will lead to the initiation of electrical branches, or the surface will be corroded by partial discharge, which will cause new cracks and new branches. Therefore, it is considered that the outer semi conductive shielding layer is also indispensable, but the structure and mode of shielding can be changed with different voltage levels.

　　Another function of the semi conductive shielding layer outside the cable is:

　　Eliminate the air gap between the insulating layer and the outer metal shielding layer. No matter how perfect the processing technology of metal shielding layer is, the bending deformation, cold and hot effect in its operation and construction will produce annular flat air gap between the metal shielding layer and the insulating layer. It has a great effect on the deterioration of the electric field, which first leads to the air gap discharge, until the insulation breakdown.

　　The so-called "shielding" in the cable structure is actually a measure to improve the electric field distribution. The cable conductor is made up of several wires, which is easy to form an air gap between it and the insulation layer, and the conductor surface is not smooth, which will cause the electric field concentration. A shielding layer of semi conductive material is added on the surface of the conductor, which is equipotential with the shielded conductor and in good contact with the insulating layer, so as to avoid the occurrence of partial discharge between the conductor and the insulating layer; Similarly, there may be a gap between the insulating surface and the sheath, which is the factor causing partial discharge. Therefore, a shielding layer of semi conductive material is added on the surface of the insulating layer, which has good contact with the shielded insulating layer and equipotential with the metal sheath, so as to avoid partial discharge between the insulating layer and the sheath. This shielding layer is the outer shielding layer; For extruded insulated cables without metal sheath, in addition to the semi-conductive shielding layer, the metal shielding layer wrapped with copper tape or wire should also be added. The metal shielding layer can pass the capacitive current during normal operation; when the system is short circuited, it can act as the channel of short-circuit current and shield the electric field at the same time.