For the outer semiconductive shielding layer of the cable, it is generally believed that since the electric field intensity on the outer surface of the insulation is relatively low, its effect is not significant. However, this is not the case. According to IEC regulations, cables with a voltage rating of 6kV and above must have both an inner and outer semiconductive shielding layer. This is because when the cable is bent during operation, the surface of the cable insulation layer is subjected to tension and elongates. If there is local discharge at this time, submicroscopic cracks will be generated due to the bending stress on the surface, leading to the initiation of electrical tree branches, or new cracks will be caused by the corrosion of the surface by local discharge, thereby triggering new branches. Therefore, it is considered that the outer semiconductive shielding layer is also indispensable. However, depending on the voltage rating, the structure and method of the shielding can be changed. 

Another function of the outer semiconductive shielding layer of 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 the metal shielding layer is, during its operation and construction, due to bending deformation and temperature changes, etc., there will still be circular flat air gaps between the metal shielding layer and the insulating layer. These air gaps have a significant adverse effect on the electric field. They first cause air gap discharge until the insulation is broken down. 

The so-called "shielding" in the cable structure is essentially a measure to improve the distribution of the electric field. The cable conductor is formed by twisting multiple conductors together. There are often air gaps between the conductor and the insulation layer, and the surface of the conductor is not smooth, which can cause the concentration of the electric field. Adding a layer of semiconductive material shielding layer on the surface of the conductor can make it electrically potential to the shielded conductor and have good contact with the insulation layer, thereby avoiding local discharge between the conductor and the insulation layer. This layer of shielding is the inner shielding layer. Similarly, at the contact point between the insulation surface and the sheath, there may also be gaps, which are factors causing local discharge. Therefore, a layer of semiconductive material shielding layer should be added on the surface of the insulation layer, which has good contact with the shielded insulation layer and is electrically potential to the metal sheath, thereby avoiding local discharge between the insulation layer and the sheath. This layer of shielding is the outer shielding layer. For extruded insulated cables without a metal sheath, in addition to the semiconductive shielding layer, a metal shielding layer wrapped with copper strips or wires should also be added. The function of this metal shielding layer is to conduct the capacitive current during normal operation; when a short circuit occurs in the system, it serves as a channel for the short-circuit current and also plays the role of shielding the electric field.