The so-called "shield" on the cable structure is essentially a measure to improve the electric field distribution. The cable conductor is formed by stranding a plurality of wires, and an air gap is easily formed between the cable conductor and the insulating layer, and the surface of the conductor is not smooth, which causes electric field concentration.
A layer of semiconducting material is applied to the surface of the conductor, which is equipotential to the shielded conductor and in good contact with the insulating layer to avoid partial discharge between the conductor and the insulating layer. This layer of shielding, also known as the inner shield. There may also be gaps at the contact between the insulating surface and the sheath. When the cable is bent, the insulating surface of the oil-paper cable is liable to cause cracks, which are factors that cause partial discharge. A layer of semiconductive material is applied to the surface of the insulating layer, which has good contact with the shielded insulating layer and is equipotential with the metal sheath to avoid partial discharge between the insulating layer and the sheath.
The cable shielding layer can also play a certain role of grounding protection. If the cable core is damaged, the leaked current can flow through the shielding layer, such as the grounding net, to play a role of safety protection.