It can be seen from the structure of the cross-linked polyethylene cable that there is a layer of outer semiconductor and copper shielding outside the main insulation layer of the cable. If this layer of outer semiconductor layer and copper shielding does not exist in the cable, then the core and core of the three-core cable Will an insulation breakdown occur during the period?
In the three-core cable terminal head, there must be a small section of the cable whose outer semiconductor and copper shielding layer have been stripped off, so is this small section of cable a weak link?
Can you overcome this problem by stripping less of the semiconductor and copper shielding layer (preserving the longer outer semiconductor and copper shielding layer as much as possible)? What is the disadvantage of keeping the longer outer semiconductor and copper shielding layer? The so-called “shielding” in the cable structure is essentially a measure to improve the electric field distribution. The cable conductor is formed by twisting multiple wires. An air gap is easily formed between it and the insulating layer. The surface of the conductor is not smooth, which will cause electric field concentration.Add a shielding layer of semi-conductive material 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 partial discharge between the conductor and the insulating layer. This layer of shielding is the inner shielding layer; Similarly, there may also be gaps at the contact between the insulating surface and the sheath, which is a factor causing partial discharge. Therefore, a shielding layer of semi-conductive material is added to the surface of the insulating layer. It has good contact with the shielded insulating layer and is in contact with the metal shield. The sheath is equipotential, so as to avoid partial discharge between the insulating layer and the sheath. This layer of shielding is the outer shielding layer; for extruded insulated cables without metal sheaths, in addition to the semi-conductive shielding layer, copper tape must be added Or the metal shielding layer wrapped with copper wire, the role of this metal shielding layer is to pass capacitive current during normal operation; when the system is short-circuited, it acts as a channel for short-circuit current and also plays a role in shielding the electric field.
It can be seen that if this outer semiconductor layer and copper shield do not exist in the ABC cable, the possibility of insulation breakdown between the core and the core of the three-core cable is very high. The main purpose of stripping off a small section of the shielding layer when making cable terminations or connectors is to ensure the creepage distance of the high voltage to the ground. The stress on the shielding fracture is very concentrated, which is a weak link! Appropriate measures must be taken for stress treatment. (Use stress cone or stress tube, etc.)
The length of the shielding layer is stripped to ensure the creepage distance; the basis is to enhance the creepage resistance of the insulating surface. Excessive stripping of the shielding layer will increase the difficulty of construction, and it is completely unnecessary to increase the cost of cable accessories.