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　PLC控制系統(tǒng)的接地設(shè)計(jì) 
      導(dǎo)讀： 設(shè)備、控制系統(tǒng)良好的接地，不僅是保證人身安全所需的“電擊防護(hù)”措施，而且也是抑制干擾、減小電磁干擾、提高系統(tǒng)可靠性的重要手段，在設(shè)計(jì)、施工階段必須予以重視。 

　　設(shè)備、控制系統(tǒng)良好的接地，不僅是保證人身安全所需的“電擊防護(hù)”措施，而且也是抑制干擾、減小電磁干擾、提高系統(tǒng)可靠性的重要手段，在設(shè)計(jì)、施工階段必須予以重視。 

　　PLC控制系統(tǒng)對接地的一般要求如下： 

　?、傧到y(tǒng)接地必須良好，對于PLC控制系統(tǒng)，接地電阻應(yīng)小于4Q。 

　?、诮拥鼐€必須有足夠大的線徑，獨(dú)立安裝的PLC基本單元，應(yīng)使用截面積在2.5nim2以上的黃/綠線與系統(tǒng)保護(hù)接地線(PE)連接。 

　?、勰K化結(jié)構(gòu)的PLC，各模塊與機(jī)架間一般可以通過模塊本身的接地連接端，使得各模塊與嘰架間保持良好的接地，但機(jī)架與系統(tǒng)保護(hù)地之間應(yīng)保證接地良好，應(yīng)使用截面積在2.5mm2以上的黃/綠線與系統(tǒng)保護(hù)接地線(PE)連接。 

　?、芟到y(tǒng)中的其他控制裝置(如驅(qū)動(dòng)器、變頻器等)的接地必須同樣符合規(guī)范，并獨(dú)立接地。 

　　按照DINEN標(biāo)準(zhǔn)規(guī)定，各控制裝置的接地線的線徑如表7-4.1所示。表中“通過固定的連接”是指控制裝置通過導(dǎo)電基座與良好接地的電氣柜(元件安裝板)進(jìn)行接地連接時(shí)的要求。 

　?、菹到y(tǒng)中的各類屏蔽電纜的屏蔽層、金屬軟管、走線槽(管)、分線盒等均必須保證接地良好。  

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 DCS系統(tǒng)接地是為了保證當(dāng)進(jìn)入DCS系統(tǒng)的信號、供電電源或DCS系統(tǒng)設(shè)備本身出現(xiàn)問題時(shí)，有效的接地系統(tǒng)能承受過載電流并可以迅速將過載電流導(dǎo)入大地。接地系統(tǒng)能夠?yàn)镈CS提供屏蔽層，消除電子噪聲干擾，并為整個(gè)控制系統(tǒng)提供公共信號參考點(diǎn)（即參考零電位）。

 

當(dāng)接地系統(tǒng)發(fā)生問題時(shí)(接地電阻過大，多點(diǎn)接地，接地線斷線或接地線與高電壓、大電流設(shè)備相接觸等)，會(huì)造成人員的觸電傷害及設(shè)備的損壞，據(jù)了解，有些DCS系統(tǒng)經(jīng)常“死機(jī)” (或不明原因的“死機(jī)”)，大多是因?yàn)榻拥叵到y(tǒng)不良或存在問題所引起的。因此，完善、可靠、正確的接地，是DCS系統(tǒng)能夠安全、可靠和良好運(yùn)行的關(guān)鍵。

二、DCS接地分類

在一般情況下，DCS控制系統(tǒng)需要兩種接地：保護(hù)地和工作地（邏輯地、屏蔽地等）。對于裝有安全柵防爆措施的系統(tǒng)如化工行業(yè)所用的系統(tǒng)，還要求有本安地。

2.1保護(hù)地（CG，Cabinet Grounding）

是為了防止設(shè)備外殼的靜電荷積累、避免造成人身傷害而采取的保護(hù)措施。DCS系統(tǒng)所有的操作員機(jī)柜、現(xiàn)場控制站機(jī)柜、打印機(jī)、端子柜等均應(yīng)接保護(hù)地。保護(hù)地應(yīng)接至廠區(qū)電氣專業(yè)接地網(wǎng)，接地電阻小于4Ω。

2.2邏輯地

也叫機(jī)器邏輯地、主機(jī)電源地，是計(jì)算機(jī)內(nèi)部的邏輯電平負(fù)端公共地，也是+5V等的電源輸出地。如CPU的正負(fù)5伏、正負(fù)12伏的負(fù)端。需要接入公共接地極。

2.3屏蔽地（AG，Analog Grounding）

也叫模擬地，它可以把現(xiàn)場信號傳輸時(shí)所受到的干擾屏蔽掉，以提高信號精度。DCS系統(tǒng)中信號電纜的屏蔽層應(yīng)做屏蔽接地。線纜屏蔽層必須一端接地，防止形成閉合回路干擾。鎧裝電纜的金屬鎧不應(yīng)作為屏蔽保護(hù)接地，必須是銅絲網(wǎng)或鍍鋁屏蔽層接地。接入公共接地極。

 

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