First, the shielding layer has a uniform electric field and the role of reducing the surface field strength of the line core

The semi-conductive shielding layer on the surface of the wire core can have an uneven electric field on the surface of the wire core, and reduce the maximum field strength of the conductor surface increased by the effect of the wire guide. For example, when the insulation thickness of the cable kV,25mm² is 5 mm, there is no semi-conductive shielding layer, Emax two 4.24 Kv/mm, there is an internal shielding layer, Emax=3. Kv/mm. Emax drop, can reduce the thickness of the insulation layer, reduce the cable into wood, the highest field strength of the decline, so that the cable operation more safe and reliable. At the same time, the existence of shielding layer, so that designers can see the cable as a concentric cylindrical body to calculate. Reduce the error caused by theoretical calculation.

Second, the existence of shielding layer improves the starting discharge voltage of cable partial discharge and reduces the possibility of partial discharge.

As a result of the three-layer simultaneous extrusion of the cable structure, to prevent the shielding layer and insulation layer between the contact is not tight resulting in the air gap, so that the high and low voltage electrode tightly combined on the insulating surface, not easy to produce partial discharge.

Third, inhibit the growth of branches

When the metal burr on the surface of the conductor directly stabs the insulating layer, or when there are impurity particles, water gas, air gap inside the insulating layer, these will cause the tip to produce a high electric field, field emission and trigger branches. For the metal meter ICI burr, the semi-conductive shielding will effectively weaken the field strength near the burr and reduce the field emission, thus improving the discharge characteristics of the electric branch. If a substance that captures moisture can be added to a semi-conductive shielding material, it can effectively block the water introduced by the wire core into the insulating layer, thus preventing the water branches from being produced in the insulation. Here in Shenzhen hot shrinkage pipe network small knitting in the degree of tips, procurement of heat shrinkable pipe must choose a professional heat shrinkable pipe manufacturers.

Iv. Thermal Barrier Action

As we have discussed earlier, semi-conductive shielding layer has a certain thermal resistance, when the wire core temperature instantaneous increase, the cable has a semi-conductive shielding layer of thermal resistance, high temperature will not immediately impact to the insulation layer, through the thermal resistance of the temperature-reduction effect, so that the temperature rise on the insulation layer down.

Five, outer semi-conductive shielding effect

Although the potential for contact with metal shielding outside the XLPE insulated cable is low, the field strength is lower than the line core, but in operation when the cable is subjected to bending stress, the insulating surface is subjected to tension and Zhongchang, if there is a partial discharge, it will be due to the surface bending stress to produce microscopic cracks, triggering the formation of Tun branches, Or surface by partial discharge corrosion caused by new cracking and other adverse reactions, so the external shielding is indispensable. IEC stipulates that XLPE insulated cable more than 3 kV to have internal and external two layers of shielding.

电缆屏蔽层的5大作用

一、屏蔽层具有均匀电场和降低线芯表面场强的作用

线芯表面采用半导电屏蔽层可以均匀线芯表面不均匀的电场，减少因导丝效应所增加的导体表面最大场强。例如，10 kV,25mm²时电缆的绝缘厚度为5 mm，无半导电屏蔽层时，Emax二4.24 kV/ mm，有内屏蔽层时，    Emax=3 .16 kV/ mm。Emax下降，可以减少绝缘层厚度，降低电缆成木;最高场强的下降，使电缆运行更加安全可靠。同时，屏蔽层的存在，使得设计者可以将电缆看做同心圆柱体来计算.减少理论计算造成的误差。

二、屏蔽层的存在提高了电缆局部放电的起始放电电压，减少了局部放电的可能性

由于三层同时挤出的电缆结构，防止了屏蔽层和绝缘层间接触不紧密而产生的气隙，使得高低电压电极紧密结合在绝缘表面，不易产生局部放电。

三、抑制树枝生长

当导体表面金属毛刺直接刺人绝缘层时，或者在绝缘层内部存在杂质颗粒、水气、气隙时，这些将引起尖端产生高电场、场致发射而引发树枝。对于金属表ICI毛刺，半导电屏蔽将有效地减弱毛刺附近的场强，减少场致发射，从而提高耐电树枝放电特性。若在半导电屏蔽料中加入能捕捉水分的物质，就能有效地阻挡由线芯引入的水分进入绝缘层，从而防止绝缘中产生水树枝。在这里深圳热缩管网小编在度提示，采购热缩管一定要选择专业的热缩管厂家。

四、热屏障作用

前面我们已经讨论过，半导电屏蔽层有一定热阻，当线芯温度瞬时升高时，电缆有了半导电屏蔽层的热阻，高温不会立即冲击到绝缘层，通过热阻的分温作用，使绝缘层上的温升下降。

五、外半导电屏蔽作用

虽然在XLPE绝缘电缆外接触金属屏蔽的地方电位较低，场强也因此而低于线芯附近，但是在运行中电缆受弯曲应力时，绝缘表面受到张力作用而仲长，若这时存在局部放电，则会由于表面弯曲应力产生微观裂纹，引发屯树枝生成，或表面受局部放电腐蚀引起新开裂等不良反应，故外屏蔽不可缺少。IEC规定XLPE绝缘电缆3 kV以上即要有内、外两层屏蔽。