由于架空(kong)線路的(de)(de)(de)絕緣介質是空(kong)氣，當導線表(biao)面的(de)(de)(de)電(dian)(dian)(dian)場強度超(chao)過空(kong)氣的(de)(de)(de)耐壓(ya)強度時(shi)，空(kong)氣層就產(chan)(chan)生游離形成放(fang)電(dian)(dian)(dian)，空(kong)氣中帶電(dian)(dian)(dian)離子的(de)(de)(de)移(yi)動就構(gou)成電(dian)(dian)(dian)暈電(dian)(dian)(dian)流。此刻，我們可以(yi)聽到(dao)"滋滋"的(de)(de)(de)放(fang)電(dian)(dian)(dian)聲，并(bing)可聞到(dao)氧分子被(bei)游離后又結合(he)成臭氧（O3）的(de)(de)(de)氣味，在夜(ye)間可見到(dao)導線周圍(wei)發(fa)生淡(dan)藍色(se)的(de)(de)(de)熒光(guang)，這種現象稱為(wei)"電(dian)(dian)(dian)暈"。電(dian)(dian)(dian)暈不但產(chan)(chan)生有(you)功損耗，而且(qie)還對無線電(dian)(dian)(dian)及高(gao)頻通訊產(chan)(chan)生干擾。

為(wei)(wei)了減小電(dian)(dian)暈損耗，特(te)別是減少對無(wu)線(xian)電(dian)(dian)和高(gao)(gao)(gao)頻通(tong)訊的(de)干擾，應盡量(liang)設(she)法(fa)(fa)避免電(dian)(dian)暈的(de)發生。而(er)避免電(dian)(dian)暈的(de)方法(fa)(fa)是，提(ti)高(gao)(gao)(gao)線(xian)路電(dian)(dian)暈的(de)臨界電(dian)(dian)壓(ya)(ya)，使其高(gao)(gao)(gao)于線(xian)路運(yun)行電(dian)(dian)壓(ya)(ya)，為(wei)(wei)此就必(bi)須增大導(dao)線(xian)間(jian)的(de)距離和導(dao)線(xian)半(ban)(ban)徑(jing)。但若增大線(xian)間(jian)距離，必(bi)定(ding)使線(xian)路桿塔的(de)建造費(fei)用增加(jia)，而(er)對臨界電(dian)(dian)壓(ya)(ya)的(de)影響盛微。因此。增大導(dao)線(xian)半(ban)(ban)徑(jing)才是提(ti)高(gao)(gao)(gao)臨界電(dian)(dian)壓(ya)(ya)的(de)有效方法(fa)(fa)。所以在特(te)高(gao)(gao)(gao)壓(ya)(ya)輸電(dian)(dian)線(xian)路中，采(cai)用擴(kuo)徑(jing)導(dao)線(xian)或(huo)分裂導(dao)線(xian)（分裂導(dao)線(xian)的(de)等值半(ban)(ban)徑(jing)起了擴(kuo)徑(jing)的(de)作用）。