The metal shielding of the insulated cable differs from the metal shielding of the paper (or oil-filled) cable, where the metal shielding is done by the lead sheath in the oil paper (or filling) cable, so that the shielding layer is actually truncated and one is consistent with the calculated value. In the insulated cable, in order to reduce the heavy view, do not use lead sheath (only 1 kV or higher electric Chek grade cable), only the copper belt in a certain way around the outer semi-conductive shielding layer, this manufacturing to the shielding layer calculation has brought a lot of trouble.

First, the role of shielding layer

1. Make the electric field direction in the same direction as the insulation radius

For medium and low voltage cables, three-phase, although cable, can still be considered as three single-phase cable installation and testing, and between phases, there is no electrical connection.

2. Bear the unbalanced current

As a centerline, there is also a capacitance current under normal conditions, and when the cable fails, the copper belt acts as a short-circuit fault current loop.

3. Prevent axial surface discharge

Cable in a good grounding environment, because the semi-conductive shielding layer has a certain resistance, in the cable axial may cause the potential distribution is not uniform and cause the cable to discharge along the surface, when there is a bad grounding in the middle of the shaft, distributed capacitance current in the two points to form a high potential zone, the closer to the ends of the voltage drop U, At both ends, A and B form high field strength, causing discharge fire phenomenon. When the current is small, metal shielding can be used to eliminate. However, for single-core cable, because there is still too high induction potential, even if there is metal shielding, can not be eliminated, must be used to compensate for the cross-transposition method, the specific will be discussed in detail at a later time.

Second, the structural form of metal shielding

One or two layers of annealed steel with spiral planks around the package to form a cylindrical concentric conductor. Multi-root wire winding, Dagger copper belt spiral gap winding package, one can increase the short-circuit capacity of the most, around the opposite can offset the L-m sense.

Third, the calculation of Shielding layer section

When a three-phase short circuit or two-phase short-circuit fault occurs in the power system, no short-circuit current flows through the metal shielding layer Short-circuit current flows through the metal shielding layer only when a single-phase short-circuit failure occurs in the cable or when a two-phase grounding short-circuit failure occurs. From the operating experience, cable accidents, especially XLPE insulated cable accidents, interphase short-circuit occurs very little, mostly after germination grounding failure, due to high temperature caused by the failure of the phase question. And the interphase short circuit all occurs in the cable attachment, such as the interphase distance is not enough, the installation is improper, the material performance is not good and so on. In general, a phase Q short circuit occurs within a few seconds of a single-phase failure. This current flowing through the shielding layer may be large at an early stage, but the current in the metal screen will decrease after the interphase short circuit occurs. Secondly, such as China's neutral point direct grounding system (to high-voltage cable), in the metal shielding layer "!, the flow of short-circuit current and cable conductive line core flow of the same short-circuit current. In the neutral point non-direct grounding system (medium and low voltage distribution system area), except for special cases (double grounding), short-circuit current will not be too large, in this regard, GB 12706-91 has stipulated that the metal shielding layer nominal section of 16,25,35,50 mm², can be selected according to the fault current capacity, However, it should be guaranteed that the maximum temperature does not exceed 300 ℃ when the short-circuit current flows through the metal shielding layer.

电缆金属屏蔽概述

绝缘电缆的金属屏蔽与油纸(或充油)电缆的金属屏蔽有所不同，在油纸(或充油)电缆中，金属屏蔽是由铅护套来完成的，因此，屏蔽层实际截而一与计算值一致。而在绝缘电缆中，为减轻重景，不采用铅护套(只有1 10 kV或更高电仄等级电缆采用)，只将铜带按一定方式绕包在外半导电屏蔽层上，这种制造给屏蔽层计算带来了很多麻烦。

一、屏蔽层作用

1.使电场方向与绝缘半径方向相同

对于中低压电缆，三相虽然成缆，但仍可看做三个单相电缆安装和试验，相与相之间，没有电的联系。

2.承担不平衡电流

作为中心线，正常情况下还有电容电流，当电缆发生故障时，铜带作为短路故障电流回路。

3.防止轴向表面放电

电缆在良好接地环境中，由于半导电屏蔽层有一定电阻，在电缆轴向可能引起电位分布不均匀而造成电缆沿面放电，当轴中间有一段接地不良时，分布电容电流在两接地点形成高电位区，越靠近两端产生的电压降△U越大，其表面单位电阻R0相同，在两端点，A和B处形成高场强，引起放电打火现象。当电流不大时，采用金属屏蔽即能消除。但对于单芯电缆，由于还存在过高的感应电位，即使有金属屏蔽，也无法消除，必须采用交叉换位方法来补偿，具体将在以后作详细讨论。

二、金属屏蔽的结构形式

一层或两层退火钢带螺旋搭盖绕包，形成一个圆柱同心导体。多根细钢丝绕包、外川铜带螺旋问隙绕包，一可增加短路容最，绕向相反时可抵消l匕感。

三、屏蔽层截面的计算

当电力系统发生三相短路或两相短路故障时，无短路电流流过金属屏蔽层。只有当电缆发生单相短路故障或系统发生两相接地短路故障时，才有短路电流流过金属屏蔽层。从运行经验看，电缆事故，特别是XLPE绝缘电缆事故，相间短路发生得很少，大都是在萌发接地故障后，由于高温引发相问发生故障。而相间短路都发生在电缆附件中，例如相间距离不够、安装不当、材料性能不好等。一般在发生单相故障几秒钟内即发生相问短路。这种流过屏蔽层的电流，在初期可能很大，但在相间短路发生后，金属屏藏层中的电流就会减小。其次，如我国中性点直接接地系统(对高压电缆)，在金属屏蔽层「!，流过的短路电流与电缆导电线芯流过的短路电流相同。在中性点非直接接地系统中(中低压配电系统地区)，除特殊情况外(双接地)，短路电流不会太大，对此，GB 12706-91已规定金属屏蔽层标称截面为16,25,35,50 mm²时，可根据故障电流容量选取，但应保证短路电流流过金属屏蔽层时，最高温度不超过300℃。