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大量电缆成组敷设时会降低电缆的载流量吗?


发布时间:

2022-10-09

大量电缆成组敷设时,由于相互间的加热作用,降低了电缆的载流量。规格大的电缆有时候需要考虑用两根或多根较小规格的并联电缆来代替,因为大截面电缆会由于集肤效应和邻近效应使得单位截面的载流量减少。

大量电缆成组敷设时,由于相互间的加热作用,降低了电缆的载流量。规格大的电缆有时候需要考虑用两根或多根较小规格的并联电缆来代替,因为大截面电缆会由于集肤效应和邻近效应使得单位截面的载流量减少。另一方面,大截面电缆的表面积对横截面积的比值减小使得大电缆散热能力差。若多根电缆并联使用时,应考虑各个电缆的相对位置,以降低电缆载流量的不均匀分布效应。

对敷设在地下管道中的电缆,当使用负荷系数时,应考虑管组及其周围土壤的平均热损失的热容量。地下部分的温度随平均热损失的变化而变化,因而可允许较高的短时负荷系数是平均负荷对尖峰负荷的比值,通常以昼夜平均负荷为基准进行测量。而尖峰负荷一般是指24h内出现的、0.5~1h期间的最大负荷的平均值。对于直埋电缆,其平均表面温度可根据土壤条件限制在0~60℃之间, 以防止土壤水分的散失和电缆热击穿。

当电缆靠近其他带负荷的电缆或热源时,或者当周围环境温度超过规定电缆载流量的环境温度时,必须降低电缆的额定载流量。电缆装置的正常环境温度是指电缆不带负荷时安装电缆处的温度。为了恰当地确定某一给定负荷所需 要的电缆规格,应该透彻地了解这个温度。例如,在空气中与其他电缆隔开敷设的电缆,其环境温度是指该电缆带负荷以前的温度。对于空气中的电缆,还要假定电缆周围有足够的空间散发电缆产生的热量,并且不会提高整个房间的温度。如果规定了上述正确条件,那么,下述的环境条件就可用来计算电缆的载流量。

城市高、中压配电线路有下列情况应采用电缆线路。城市繁华地区、重要地段、主要道路,以及城市规划和市容环境有特殊要求的地区;技术上难以解决的严重腐蚀地段;重点风景旅游区的区段;易受盐污或热带风暴侵袭的沿海主要城市的重要供电区段;其他为电网结构和运行安全需要的地段。

城市低压配电线路有下列情况应采用电缆线路负荷密度高的城市中心地区;建筑面积较大的新建居民住宅小区及高层建筑小区;依据规划不宜通过架空线路的街道或地区及进出线拥挤地区;经过技术经济比较采用电县团级线路比较合适的其他情况。对于应该采用电缆线路而地下不具备条件时,可采用绝缘电缆架空敷设。

户内对低压电缆来说,国家电气法规上的载流量表是以环境温度30 ℃为基础的。但是,大部分地区夏天的月份,至少对建筑物的某些部分40℃为宜。在确定电缆载流量时必须考虑附近对电缆最不利的热源。电缆局部过热的情况可能是由蒸汽管道或靠近电缆的热源所引起的,也可能是由于电缆穿过锅炉房或其他高温的场所所致。为了避免这类问题,可能需要改线。

户外对于安装在遮荫处的电缆,其最高环境温度一般取40℃, 而对于安装在阳光下的电缆,最高环境温度一般取50℃。在使用这些环境温度时,假定最大负荷正好是在规定的环境温度时出现。在一天的最热时间里,或者在阳光晒得最厉害的时候,有些回路并不是在满负荷运行。在这样的条件下,采用环境温度40℃对于户外电缆从安全方面来说,是比较合理的。

地下在一个国家的不同地区,用于地下电缆的环境温度是有变化的。我国北方地区,环境温度常取20℃,对中部地区,则常用25℃;而对最南端和西南端,环境温度可能要取30℃。这些环境温度的地理界线是不可能精确划定的。在确定电缆的载流量时,电缆周围介质的热性能是重要的参数。埋设电缆或电缆管块的土壤种类,对电缆载流量有着重大的影响。多孔疏松土壤,例如砾石和灰渣回填土,通常要比砂土或粘土有较高的温度和较低的载流量。

因此,在计算电缆规格以前,应该知道土壤的种类及土壤热阻率。土壤的含水量对电缆的载流量也有重要的影响。在干燥地区,为了补偿由于缺少水分而使热阻增加,必须降低电缆的额定载流量,或者采取其他的预防措施。另一方面,在经常潮湿的地下或受潮水影响的地区,电缆可以通过比正常电流大的电流。

直埋敷设于冻土地区时,宜埋入冻土层以下,当无法深埋时可在土壤排水性好的干燥冻土层或回填上中埋设,也可采取其他措施。直埋敷设的电缆,严禁位于地下管道的正上方或下方。用隔板分隔为0.25m;用电缆穿管时可为0.1m; 特殊情况可酌减。直埋敷设于非冻土地区时,电缆埋置深度对于电缆外皮至地下构筑物基础,不得小于0.3m。

直埋敷设的电缆与铁路、公路或街道交叉时,应穿保护管,且保护范围超出路基、街道路面两边以及排水沟边0.5m以上。直埋敷设的电缆引入构筑物,在贯穿墙孔处应设置保护管,且对管口实施阻水堵塞。直埋敷设电缆的接头配置,接头与邻近电缆的净距,不得小于0.25m。 并列电缆的接头位置宜相互错开,且不小于0.5m的净距。斜坡地形处的接头安置,应呈水平状。对重要回路的电缆接头,宜在其两侧约100mm开始的局部段, 按留有备用量方式敷设电缆。直埋敷设电缆在采取特殊换土回填时,回填土的土质应对电缆外护套无腐蚀性。

关键词:

电缆成组敷设


这里是占位文字

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