當電(dian)源側（變壓(ya)(ya)器或(huo)發電(dian)機）或(huo)者負(fu)載側為(wei)星(xing)形接(jie)(jie)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)時(shi)，三(san)(san)相(xiang)(xiang)(xiang)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈的(de)(de)首端（或(huo)尾端）連(lian)(lian)接(jie)(jie)在一起(qi)的(de)(de)共同(tong)接(jie)(jie)點(dian)稱(cheng)(cheng)為(wei)中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)，簡稱(cheng)(cheng)中(zhong)(zhong)(zhong)點(dian)。中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)分電(dian)源中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)和負(fu)載中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)。由中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)引出(chu)的(de)(de)導線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)稱(cheng)(cheng)為(wei)中(zhong)(zhong)(zhong)性(xing)(xing)(xing)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)，簡稱(cheng)(cheng)中(zhong)(zhong)(zhong)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)。如(ru)果中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)與(yu)接(jie)(jie)地(di)(di)(di)裝置直接(jie)(jie)連(lian)(lian)接(jie)(jie)而取得(de)大地(di)(di)(di)的(de)(de)參考零(ling)(ling)(ling)電(dian)位，則該(gai)中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)稱(cheng)(cheng)為(wei)零(ling)(ling)(ling)點(dian)，從(cong)零(ling)(ling)(ling)點(dian)引出(chu)的(de)(de)導線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)稱(cheng)(cheng)為(wei)零(ling)(ling)(ling)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)。通常220伏(fu)單相(xiang)(xiang)(xiang)回(hui)路(lu)(lu)(lu)兩根(gen)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)中(zhong)(zhong)(zhong)的(de)(de)一根(gen)稱(cheng)(cheng)為(wei)"相(xiang)(xiang)(xiang)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"或(huo)"火線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"，而另一根(gen)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)稱(cheng)(cheng)為(wei)"零(ling)(ling)(ling)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"或(huo)"地(di)(di)(di)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"。"火線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"與(yu)"地(di)(di)(di)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"的(de)(de)稱(cheng)(cheng)法，只是實用中(zhong)(zhong)(zhong)的(de)(de)一種俗稱(cheng)(cheng)，特別是"地(di)(di)(di)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"的(de)(de)稱(cheng)(cheng)法不(bu)確(que)切(qie)。嚴(yan)格地(di)(di)(di)說，應該(gai)是：如(ru)果該(gai)回(hui)路(lu)(lu)(lu)電(dian)源側（三(san)(san)相(xiang)(xiang)(xiang)配電(dian)變壓(ya)(ya)器中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)）接(jie)(jie)地(di)(di)(di)，則稱(cheng)(cheng)"零(ling)(ling)(ling)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"；若不(bu)接(jie)(jie)地(di)(di)(di)，則應稱(cheng)(cheng)"中(zhong)(zhong)(zhong)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"，以(yi)免與(yu)接(jie)(jie)地(di)(di)(di)裝置中(zhong)(zhong)(zhong)的(de)(de)"地(di)(di)(di)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)"相(xiang)(xiang)(xiang)混。當為(wei)三(san)(san)相(xiang)(xiang)(xiang)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)路(lu)(lu)(lu)時(shi)，除了三(san)(san)根(gen)相(xiang)(xiang)(xiang)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)外(wai)，還可從(cong)中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)引出(chu)一根(gen)導線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)，即中(zhong)(zhong)(zhong)性(xing)(xing)(xing)點(dian)，從(cong)而構成三(san)(san)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)四線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)制(zhi)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)路(lu)(lu)(lu)。這種線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)路(lu)(lu)(lu)中(zhong)(zhong)(zhong)相(xiang)(xiang)(xiang)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)之間的(de)(de)電(dian)壓(ya)(ya)，稱(cheng)(cheng)為(wei)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)電(dian)壓(ya)(ya)，相(xiang)(xiang)(xiang)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)與(yu)零(ling)(ling)(ling)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)(xian)之間的(de)(de)電(dian)壓(ya)(ya)稱(cheng)(cheng)為(wei)相(xiang)(xiang)(xiang)電(dian)壓(ya)(ya)。

中(zhong)性(xing)(xing)點(dian)是否接地(di)，亦稱為中(zhong)性(xing)(xing)點(dian)制(zhi)度。中(zhong)性(xing)(xing)點(dian)制(zhi)度可(ke)以(yi)大致分(fen)(fen)為兩大類(lei)，即中(zhong)性(xing)(xing)點(dian)接地(di)系(xi)統(tong)與(yu)中(zhong)性(xing)(xing)點(dian)絕緣系(xi)統(tong)。而按(an)照國際電工委員會（IEC）的規(gui)定，將(jiang)低壓配電系(xi)統(tong)分(fen)(fen)為IT、TT、TN三種，其中(zhong)TN系(xi)統(tong)又分(fen)(fen)為TN－C、TN－S、TN－C－S三類(lei)。

TT方(fang)式供電系(xi)統(tong)是指將電氣設備的金屬外殼(ke)直接接地的保護系(xi)統(tong)，稱為保護接地系(xi)統(tong)，也稱TT系(xi)統(tong)。 

TN-C系統是用工作零線兼(jian)作保護(hu)線，可(ke)以稱(cheng)作保護(hu)中性線，可(ke)用PEN表示。 

TN-S供(gong)電(dian)系統(tong)是把工作零線N和專用(yong)保護線PE嚴格分開的供(gong)電(dian)系統(tong)，TN-S 供(gong)電(dian)系統(tong)的特點如(ru)圖。

由以上比較我們還可以得出電網中性(xing)(xing)點(dian)不同運行方式下的(de)安(an)全(quan)措(cuo)施(shi)，即中性(xing)(xing)點(dian)的(de)絕緣運行方式和中性(xing)(xing)點(dian)的(de)直接(jie)接(jie)地運行方式。

中性點絕緣運行方(fang)式(shi)下(xia)應做到(dao)：

①所有用(yong)(yong)電設備都(dou)必須(xu)采用(yong)(yong)保護接地，而(er)不允(yun)許(xu)采用(yong)(yong)保護接零；

②中性(xing)(xing)線(xian)(xian)的機械強(qiang)度應與相(xiang)線(xian)(xian)相(xiang)同，中性(xing)(xing)線(xian)(xian)不允許斷開；

③中性(xing)線(xian)電(dian)流(liu)不應(ying)超(chao)過變壓器二次線(xian)圈額定電(dian)源的(de)25%，三(san)相負荷電(dian)流(liu)不應(ying)相差太(tai)大(da)，以(yi)免影響三(san)相電(dian)壓的(de)平衡；

 ④杜絕(jue)中性線直接(jie)接(jie)地，低壓配(pei)電(dian)盤必須設置三(san)相絕(jue)緣監察(cha)裝置，以便及(ji)時發現和排除低壓電(dian)網中的接(jie)地故障(zhang)；⑤配(pei)電(dian)變壓器二(er)次側應加(jia)裝4只避雷(lei)器，以防止雷(lei)電(dian)過電(dian)壓。

中性點直接(jie)(jie)接(jie)(jie)地(di)運行方(fang)式下應做到：

①所有用(yong)電(dian)設備在(zai)正常情況下不帶電(dian)的金屬(shu)部分，都必須采(cai)用(yong)保護接(jie)零或(huo)保護接(jie)地(di)；

②在三相四(si)線制(zhi)的(de)同(tong)一低壓配電系統中，保(bao)(bao)護(hu)(hu)(hu)接(jie)零(ling)(ling)和保(bao)(bao)護(hu)(hu)(hu)接(jie)地不(bu)能混用(yong)，即一部分(fen)采用(yong)保(bao)(bao)護(hu)(hu)(hu)接(jie)零(ling)(ling)，而另(ling)一部分(fen)采用(yong)保(bao)(bao)護(hu)(hu)(hu)接(jie)地，但若在同(tong)一臺設(she)備上同(tong)時采用(yong)保(bao)(bao)護(hu)(hu)(hu)接(jie)零(ling)(ling)和保(bao)(bao)護(hu)(hu)(hu)接(jie)地則(ze)是允許(xu)的(de)，因(yin)為其(qi)安全效果更好；

③要求中(zhong)(zhong)性線必須(xu)重(zhong)復接地(di)(di)，因為(wei)在中(zhong)(zhong)性線斷開的情(qing)況(kuang)下(xia)，接零(ling)設備外殼上都帶有220V的對地(di)(di)電(dian)壓，這是絕不允許的。

供電線路N線重復接地：

中性線(零線)和(he)地線的(de)區(qu)別。

在工頻低壓電路中(zhong)，簡單講他們有結(jie)構和原理上的區別(bie)。

1.結構的區別：

零線（N）：從變壓器中性點接地后引出主干線。

地線（PE）：從變壓器中性點接地后引出主干線，根據標準，每間(jian)隔20-30米重復(fu)接地。

2.原理的區別(bie)：

零線(xian)(xian)（N）：主要應用于工(gong)作回路，零線(xian)(xian)所產生的(de)(de)電(dian)壓(ya)等于線(xian)(xian)阻乘以工(gong)作回路的(de)(de)電(dian)流。由于長距離的(de)(de)傳輸，零線(xian)(xian)產生的(de)(de)電(dian)壓(ya)就(jiu)不可(ke)忽(hu)視，作為保護人身(shen)安全(quan)的(de)(de)措施就(jiu)變得不可(ke)靠。

地線（PE）：不用于工作(zuo)回路，只作(zuo)為保護線。利用大(da)地的(de)絕(jue)對(dui)"0"電(dian)壓，當(dang)設備外殼發(fa)生(sheng)漏電(dian)，電(dian)流(liu)會迅速流(liu)入(ru)大(da)地，即使(shi)發(fa)生(sheng)PE線有開路的(de)情況(kuang)，也會從附近(jin)的(de)接地體流(liu)入(ru)大(da)地。

其實地(di)線不止保護(hu)接地(di)一種，下面介紹地(di)線：

地線是接(jie)(jie)地裝置的簡(jian)稱，地線又分為工作接(jie)(jie)地和安(an)全(quan)性接(jie)(jie)地，其中安(an)全(quan)性接(jie)(jie)地又可分為保護接(jie)(jie)地、防(fang)雷擊接(jie)(jie)地和防(fang)電(dian)磁輻射(she)接(jie)(jie)地。

1.工作接(jie)地(di)(di) 是(shi)用(yong)它完成(cheng)回(hui)路使設備達到性能要求的(de)接(jie)地(di)(di)線(xian)(xian)。如六、七十年代農村(cun)家(jia)家(jia)戶戶使用(yong)的(de)廣(guang)播有(you)一(yi)根地(di)(di)線(xian)(xian)，而且接(jie)地(di)(di)處要經常用(yong)水淋濕。工作接(jie)地(di)(di)是(shi)把(ba)金屬導體銅塊埋在土壤里，再(zai)把(ba)它的(de)一(yi)點用(yong)導線(xian)(xian)引(yin)出(chu)地(di)(di)面，這就建成(cheng)了接(jie)地(di)(di)系統(tong)，地(di)(di)線(xian)(xian)要求接(jie)地(di)(di)電阻≤4Ω。

2.保(bao)護接(jie)地 為防止人們在(zai)使用家(jia)電(dian)(dian)(dian)(dian)(dian)及辦公(gong)等電(dian)(dian)(dian)(dian)(dian)子設(she)備時發生(sheng)觸電(dian)(dian)(dian)(dian)(dian)事故而采取(qu)的(de)一(yi)種保(bao)護措施。家(jia)用電(dian)(dian)(dian)(dian)(dian)器和(he)辦公(gong)設(she)備的(de)金(jin)屬外(wai)殼都設(she)有接(jie)地線(xian)，如其絕(jue)緣損壞外(wai)殼帶電(dian)(dian)(dian)(dian)(dian)，則電(dian)(dian)(dian)(dian)(dian)流(liu)沿著安(an)裝的(de)接(jie)地線(xian)泄入大(da)地，以達到(dao)安(an)全的(de)目的(de)，否則會給人身安(an)全造成危害。用電(dian)(dian)(dian)(dian)(dian)規(gui)(gui)程(cheng)規(gui)(gui)定(ding)(ding)保(bao)護接(jie)地電(dian)(dian)(dian)(dian)(dian)阻應≤4Ω，而人體(ti)(ti)的(de)電(dian)(dian)(dian)(dian)(dian)阻一(yi)般大(da)于2000Ω，根據歐姆定(ding)(ding)律，絕(jue)緣損壞時通過人體(ti)(ti)的(de)電(dian)(dian)(dian)(dian)(dian)流(liu)僅為總電(dian)(dian)(dian)(dian)(dian)流(liu)的(de)1/500，從(cong)而起到(dao)保(bao)護作用。（電(dian)(dian)(dian)(dian)(dian)壓(ya)越高(gao)，人體(ti)(ti)電(dian)(dian)(dian)(dian)(dian)阻越小，也就是說，在(zai)大(da)電(dian)(dian)(dian)(dian)(dian)壓(ya)的(de)情況(kuang)下，很(hen)有可能(neng)你成了地線(xian)，電(dian)(dian)(dian)(dian)(dian)流(liu)回從(cong)你的(de)身體(ti)(ti)上瀉下）

3.防雷(lei)(lei)擊(ji)接地 為防止(zhi)在雷(lei)(lei)雨季節，高(gao)大建(jian)筑物，各類通信系(xi)統(tong)以及架于建(jian)筑物上的各種天線和(he)其它(ta)一些設施被雷(lei)(lei)擊(ji)，需加裝避雷(lei)(lei)針，然(ran)后(hou)用導線將其引到安裝的防雷(lei)(lei)擊(ji)接地系(xi)統(tong)。

4.防電磁輻射(she)接(jie)地 在一些(xie)重要(yao)部門為防止電磁干擾，對電子(zi)設(she)備加(jia)裝屏蔽(bi)(bi)網，安裝的屏蔽(bi)(bi)網要(yao)接(jie)入相應的接(jie)地系統，并要(yao)求接(jie)地電阻≤4Ω。