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以接地形式TN-C-S系統為例加以說明，圖11.4.20為常用的TN-C-S系統，在電源進線處PEN線分成PE線和N線（N線從此處開始與PE線絕緣），設有重復接地，不安裝總等電位聯結，如果設備發生接地故障，忽略接地故障點的阻抗，R_Ａ與R_Ｂ串聯后再與Z _PEN并聯，R_Ａ+R_Ｂ〉〉Z_PEN；人體阻抗Z_h與鞋襪和地板電阻Rp串聯后再與Z_PE并聯，Z_h+R_{p〉〉ZPE，}接地故障電流犐 I_d流經相線和PE線、PEN線，返回變(bian)壓器低(di)壓繞組， 即

從(cong)圖(tu)11. 4.20可知，做了總(zong)等(deng)電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)聯結(jie)(jie)后，在(zai)(zai)總(zong)等(deng)電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)聯結(jie)(jie)區(qu)內(nei)，作為總(zong)等(deng)電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)聯結(jie)(jie)組成部(bu)分(fen)的(de)建(jian)筑物(wu)基礎鋼筋、金屬(shu)結(jie)(jie)構件、金屬(shu)管(guan)道、金屬(shu)電(dian)(dian)(dian)(dian)(dian)纜橋架、電(dian)(dian)(dian)(dian)(dian)纜金屬(shu)護套、敷設電(dian)(dian)(dian)(dian)(dian)纜或導(dao)(dao)線金屬(shu)管(guan)等(deng)自然(ran)接(jie)地(di)體(ti)，接(jie)地(di)電(dian)(dian)(dian)(dian)(dian)阻(zu)值較小，已起(qi)到重(zhong)(zhong)復(fu)接(jie)地(di)的(de)作用。IEC標(biao)準沒(mei)(mei)有(you)規定必須(xu)為重(zhong)(zhong)復(fu)接(jie)地(di)做人(ren)工接(jie)地(di)體(ti)， 也沒(mei)(mei)有(you)明確(que)規定重(zhong)(zhong)復(fu)接(jie)地(di)的(de)電(dian)(dian)(dian)(dian)(dian)阻(zu)值。電(dian)(dian)(dian)(dian)(dian)源線路中PEN線上的(de)電(dian)(dian)(dian)(dian)(dian)壓降雖不(bu)在(zai)(zai)建(jian)筑物(wu)內(nei)產生接(jie)觸(chu)電(dian)(dian)(dian)(dian)(dian)壓， 但它能使(shi)接(jie)地(di)母排對地(di)電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)升(sheng)高。由(you)于在(zai)(zai)總(zong)等(deng)電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)聯結(jie)(jie)范圍內(nei)電(dian)(dian)(dian)(dian)(dian)氣(qi)裝置外(wai)露可導(dao)(dao)電(dian)(dian)(dian)(dian)(dian)部(bu)分(fen)和裝置外(wai)可導(dao)(dao)電(dian)(dian)(dian)(dian)(dian)部(bu)分(fen)都(dou)和接(jie)地(di)母排相連通，其電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)都(dou)同(tong)樣升(sheng)高而基本處于同(tong)一電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)上， 人(ren)體(ti)接(jie)觸(chu)這些導(dao)(dao)電(dian)(dian)(dian)(dian)(dian)部(bu)分(fen)時， 沒(mei)(mei)有(you)接(jie)觸(chu)不(bu)同(tong)電(dian)(dian)(dian)(dian)(dian)位(wei)(wei)， 自然(ran)不(bu)存在(zai)(zai)電(dian)(dian)(dian)(dian)(dian)擊(ji)危(wei)險(xian)的(de)。

TN系統內因絕緣損(sun)壞發生(sheng)接(jie)地(di)故(gu)障(zhang)后有三(san)種可能情況：一是(shi)(shi)(shi)(shi)故(gu)障(zhang)點相(xiang)接(jie)觸(chu)(chu)的(de)兩(liang)金(jin)屬(shu)部(bu)(bu)分(fen)因數百以至數千安的(de)電(dian)(dian)(dian)(dian)(dian)流通過，熔(rong)(rong)化(hua)成(cheng)團而脫(tuo)離接(jie)觸(chu)(chu)，接(jie)地(di)故(gu)障(zhang)自然消(xiao)失；二是(shi)(shi)(shi)(shi)兩(liang)金(jin)屬(shu)部(bu)(bu)分(fen)熔(rong)(rong)化(hua)成(cheng)團脫(tuo)離接(jie)觸(chu)(chu)后引燃電(dian)(dian)(dian)(dian)(dian)弧，形(xing)成(cheng)大故(gu)障(zhang)點阻抗的(de)電(dian)(dian)(dian)(dian)(dian)弧性接(jie)地(di)故(gu)障(zhang)，由于(yu)(yu)相(xiang)當(dang)大一部(bu)(bu)分(fen)的(de)線(xian)路電(dian)(dian)(dian)(dian)(dian)壓(ya)降(jiang)落在電(dian)(dian)(dian)(dian)(dian)弧上，接(jie)觸(chu)(chu)電(dian)(dian)(dian)(dian)(dian)壓(ya)相(xiang)對減少(shao)，它的(de)后果大多(duo)是(shi)(shi)(shi)(shi)火災而非人(ren)身電(dian)(dian)(dian)(dian)(dian)擊；三(san)是(shi)(shi)(shi)(shi)兩(liang)金(jin)屬(shu)部(bu)(bu)分(fen)熔(rong)(rong)化(hua)后互相(xiang)焊牢，使故(gu)障(zhang)繼續(xu)存在，其故(gu)障(zhang)點阻抗可忽略不計(ji)，其后果大多(duo)是(shi)(shi)(shi)(shi)人(ren)身電(dian)(dian)(dian)(dian)(dian)擊，這就是(shi)(shi)(shi)(shi)接(jie)地(di)故(gu)障(zhang)。正是(shi)(shi)(shi)(shi)由于(yu)(yu)接(jie)地(di)故(gu)障(zhang)電(dian)(dian)(dian)(dian)(dian)壓(ya)存在，沿(yan)PEN線(xian)或PE線(xian)竄入的(de)危險(xian)故(gu)障(zhang)電(dian)(dian)(dian)(dian)(dian)壓(ya)易引起的(de)電(dian)(dian)(dian)(dian)(dian)擊。

（1） 保護(hu)裝置未動作(zuo)而引(yin)(yin)(yin)起(qi)的(de)(de)(de)接(jie)地(di)(di)(di)故障電(dian)壓(ya)(ya)。（2） PEN線(xian)(xian)折(zhe)(zhe)斷(duan)而引(yin)(yin)(yin)起(qi)的(de)(de)(de)接(jie)地(di)(di)(di)故障電(dian)壓(ya)(ya)。（3） 當電(dian)源(yuan)干線(xian)(xian)中(zhong)(zhong)的(de)(de)(de)PEN線(xian)(xian)折(zhe)(zhe)斷(duan)時(shi)（俗稱斷(duan)零(ling)），由于(yu)三相負荷不均衡，負荷側中(zhong)(zhong)性(xing)點漂移，也能使(shi)PEN線(xian)(xian)和設備外殼對地(di)(di)(di)帶電(dian)位。如(ru)果建筑物內(nei)有總(zong)等電(dian)位聯(lian)(lian)結(jie)，使(shi)外露(lu)可導電(dian)部分都處于(yu)該(gai)電(dian)位，同樣也可消除(chu)由此引(yin)(yin)(yin)起(qi)的(de)(de)(de)電(dian)擊危(wei)險。（4） 高(gao)壓(ya)(ya)系(xi)統(tong)中(zhong)(zhong)性(xing)點不接(jie)地(di)(di)(di)或(huo)(huo)經消弧線(xian)(xian)圈接(jie)地(di)(di)(di)發(fa)生(sheng)(sheng)接(jie)地(di)(di)(di)故障引(yin)(yin)(yin)起(qi)的(de)(de)(de)低(di)壓(ya)(ya)系(xi)統(tong)接(jie)地(di)(di)(di)故障電(dian)壓(ya)(ya)。（5） 高(gao)壓(ya)(ya)系(xi)統(tong)中(zhong)(zhong)性(xing)點經低(di)電(dian)阻接(jie)地(di)(di)(di)系(xi)統(tong)發(fa)生(sheng)(sheng)接(jie)地(di)(di)(di)故障引(yin)(yin)(yin)起(qi)的(de)(de)(de)低(di)壓(ya)(ya)系(xi)統(tong)接(jie)地(di)(di)(di)故障電(dian)壓(ya)(ya)。雖(sui)然(ran)(ran)PEN線(xian)(xian)或(huo)(huo)PE線(xian)(xian)上(shang)存在危(wei)險故障電(dian)壓(ya)(ya)，但由于(yu)PEN線(xian)(xian)或(huo)(huo)PE線(xian)(xian)在建筑物內(nei)均已等電(dian)位聯(lian)(lian)結(jie)，在等電(dian)位聯(lian)(lian)結(jie)范圍內(nei)人(ren)體同時(shi)可觸及(ji)的(de)(de)(de)電(dian)氣(qi)裝置內(nei)、外露(lu)可導電(dian)部分基本上(shang)處于(yu)同一電(dian)位， 火災及(ji)人(ren)身電(dian)擊自(zi)然(ran)(ran)不會產生(sheng)(sheng)。