Cable insulation structure is the core of the cable, but also the core of the heat shrinkable tube or heat shrinkable casing, it is determined by the manufacturing level, process level and raw material level.

At present, the thickness of insulating layer of XLPE insulated cable in the voltage grade of kV and above is unified as: 3.6/6 grade 2.5~3.2 mm; 6/6 Level 3.4 mm; 6/10 Level 3.4 mm; 8.7/10 level 4.5 mm; 21/35 level 9.3 mm; 26/35 levels 10.5 mm. This is mainly because the electric field strength in the insulation of medium and low voltage cable is not high, there is no need to be too thin on the thickness of the insulating layer, so as not to cause unnecessary confusion, but for high-voltage XLPE cable, for example, the XLPE cable insulation structure, the insulation thickness of the cable will change according to the conductor cross section.

In summary, for low-voltage small section cables, the thickness of the insulating layer is mainly determined by the thinnest insulation allowed by the process, while for low-voltage large-section cables it should be determined according to the mechanical damage (bending) and insulation inhomogeneity that may be suffered during installation and production. Because the stress of the melody increases with the increase of the cross-sectional area of the line core, the inhomogeneity increases with the increase of the line core cross section, so the thickness of the insulating layer of this kind of cable increases with the increase of the cross-sectional area of the line core, and the requirements of the electrical breakdown strength can be met when the insulating layer thickness is satisfied

Only when the operating voltage of the cable is higher than above, especially the UHV cable, the breakdown strength of the insulation gradually becomes the main factor that determines the thickness of the insulating layer. According to the principle that the maximum electric field strength in the cable insulating layer is equal to its breakdown electric field strength, the thickness of the cable insulating layer is designed, and considering the dispersion of the breakdown strength and ensuring that the cable insulation has a certain safety margin, the insulating layer thickness of the XLPE cable can be determined by the following formula:

Em>Emax=U/[rc in (R/RC)]

The breakdown strength of the e-corresponding to U is the maximum voltage of the working frequency, pulse and Operation Wave.

M-insulation margin;

R-cable insulated outer diameter;

Rc-Cable insulated inner diameter.

Insulation layer thickness is

=R Rc=rrc[exp (mUl/Erc) 11]

For high voltage power cables, the relationship curve between the frequency breakdown strength E of the insulating material and the pressurized time t can be expressed in the following formula:

Eb=E∞+A/n√t

In the type of EB-, the breakdown strength of the working frequency;

The breakdown strength of the e--voltage acting infinitely for a long time;

E∞-Life Index;

A-constant;

T One life, year.

如何确定电缆绝缘层厚度

电缆绝缘结构是电缆的核心，也是热缩管或热缩套管的核心，它是由制造水平、工艺水平和原材料水平来决定的。

目前我国对35 kV及以上电压等级XLPE绝缘电缆的绝缘层厚度统一为:3.6/6等级2.5~3 .2 mm; 6/ 6等级3 .4 mm; 6/ 10等级3 .4 mm; 8 .7/ 10等级4.5 mm; 21/ 35等级9 .3 mm; 26/ 35等级10.5 mm。这主要是因为中低压电缆绝缘中的电场强度不高，没有必要对绝缘层厚度分得太细，以免造成不必要的混乱，但对于高压XLPE电缆，例如，110 kV XLPE电缆绝缘结构，电缆的绝缘层厚度将根据导体截面变化而变。

概括来说，对于低压小截面电缆，绝缘层厚度主要由工艺允许的最薄绝缘决定，而对于低压大截面电缆则应根据在安装和生产过程中可能受到的机械损伤(弯曲)和绝缘不均匀性来决定。由于恋曲应力随线芯截面积增加而增大，不均匀性也随线芯截面增加而增大，因此这类电缆的绝缘层厚度随线芯截面积增加而加大，当满足上述要求的绝缘层厚度时，均能满足电气击穿强度所提出的要求。

只有电缆工作电压高至10 kV以上时，特别是超高压电缆，绝缘的击穿强度才逐渐成为决定绝缘层厚度的主要因素。根据电缆绝缘层内最大电场强度等于其击穿电场强度时电缆发生击穿的原理来设计电缆绝缘层厚度，同时考虑到击穿强度的分散性并保证电缆绝缘有一定安全裕度，XLPE电缆的绝缘层厚度，可由下式确定:

Em>Emax=U/[rc In( R/ rc)]

式中   E—相应于U是工频、脉冲、操作波最大电压时的击穿 强度;

m—绝缘裕度;

R—电缆绝缘外径;

rc—电缆绝缘内径。

绝缘层厚度为

△=R一rc=rrc[exp(mUl/Erc)一1]

对于高压电力电缆，绝缘材料的工频击穿强度E与加压时间t的关系曲线可用下式表示:

Eb=E∞+A/n√t

式中EB—工频击穿强度;

E-—电压作用无限长时间的击穿强度;

E∞—寿命指数;

A—常数;

t一寿命，年。