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在描述此類處方之前，了解不同的電路很有用
上述標(biāo)準(zhǔn)中描述的類型；特別是在工廠中
電路可分為：
•終電路：通常為設(shè)備供電的電路（例如
吸氣器、橋式起重機(jī)等）
•配電電路：為配電板供電的電路
連接其他終電路。
在TT系統(tǒng)中，通過以下方式實(shí)現(xiàn)對(duì)間接接觸的正確保護(hù)：
在自動(dòng)斷開電路時(shí)，必須尊重其中一個(gè)
以下條件（符合IEC 60364-4）：
通過剩余電流裝置進(jìn)行保護(hù)
假設(shè)50V為極限電壓（標(biāo)準(zhǔn)環(huán)境），以實(shí)現(xiàn)保護(hù)
對(duì)于通過剩余電流裝置進(jìn)行的間接接觸，有必要
滿足以下條件：
RA·I?n≤ 50V然后：RA≤ 50V
我
?n
1.
接地電極的電阻與保護(hù)導(dǎo)體的電阻串聯(lián)，如果
與電阻RA比較；因此，在公式中可以考慮
僅限用戶工廠接地電極的電阻。
哪里：
RA是接地電極和保護(hù)電極的總電阻（單位：歐姆）
外露導(dǎo)電部件1的導(dǎo)體
;
我
?n是剩余電流斷路器的額定剩余工作電流。
ABB |電氣設(shè)備449
4保護(hù)人類
4.4 TT系統(tǒng)
50V<U0
≤120V
s
130V<U0
≤230V
s
230V<U0
≤400V
s
U0
>400V
s
系統(tǒng)a.c.d.c.a.c.d.c.a.c.d.c.a.c.d.c。
TT 0.3注1 0.2 0.4 0.07 0.2 0.04 0.1
Uo是標(biāo)稱交流或直流線對(duì)地電壓。
從上面可以看出，接地電阻RA的值
通過使用剩余電流斷路器和
靈敏度不同，因?yàn)樯鲜龇帜柑幍碾娏髁?br />上述關(guān)系不同。事實(shí)上，通過使用剩余電流裝置
具有30mA靈敏度，接地電阻值低于
表1：終電路的大斷開時(shí)間不超過32A
關(guān)于斷開時(shí)間，該標(biāo)準(zhǔn)區(qū)分了兩種可能性；
•額定電流不超過32A的終電路：在這種情況下，有必要
上述條件的時(shí)間如表1所示（數(shù)值
指明顯高于額定剩余電流的故障電流
剩余電流斷路器通常為5·I?n
)滿足；
•額定電流超過32A的配電電路或終電路：
在這種情況下，有必要在一段時(shí)間內(nèi)滿足上述條件
不超過1秒（常規(guī)時(shí)間）。

Before describing such prescriptions, it is useful to know the different circuit types described in the above mentioned Standard; in particular, in a plant, the circuits can be divided into: • final circuit: it is a circuit which usually supplies equipment (for example an aspirator, a bridge crane, etc.) • distribution circuit: it is a circuit which supplies a distribution board to which other final circuits are connected. In a TT system, to achieve a correct protection against indirect contact through the automatic disconnection of the circuit, it is necessary to respect one of the following conditions (in compliance with IEC 60364-4): Protection by means of residual current devices By assuming 50V as limit voltage (standard environments), to achieve protection against indirect contact by means of residual current devices it is necessary to satisfy the following condition: RA · I?n ≤ 50V then: RA ≤ 50V I ?n 1 The resistance of the earth electrode is in series with that of the protective conductor, which is negligible if compared with the resistance RA; as a consequence, in the formula it is possible to take into consideration only the resistance of the earth electrode of the user’s plant. where: RA is the total resistance (in ohm) of the earth electrode and of the protective conductor of the exposed-conductive-parts1 ; I ?n is the rated residual operating current of the residual current circuitbreaker. ABB | Electrical devices 449 4 Protection of human beings 4.4 TT System 50V400V s System a.c. d.c. a.c. d.c. a.c. d.c. a.c. d.c. TT 0.3 Note 1 0.2 0.4 0.07 0.2 0.04 0.1 Uo is the nominal a.c. or d.c. line to earth voltage. From the above, it is evident that the value of the resistance RA of the earthing arrangement results to be different by using residual current circuit-breakers with different sensitivity, since the current quantity at the denominator in the above mentioned relationship is different. In fact, by using a residual current device with 30mA sensitivity, an earthing resistance value lower than Table 1: Maximum disconnection times for final circuits not exceeding 32A As regards the disconnection times, the Standard distinguishes two possibilities; • final circuits with rated currents not exceeding 32A: in this case it is necessary that the above mentioned condition with the times shown in Table 1 (values referred to fault currents significantly higher than the rated residual current of the residual current circuit-breakers typically 5·I?n ) is fulfilled; • distribution circuit or final circuit with rated currents exceeding 32A: in this case it is necessary that the above mentioned condition is fulfilled with a time not exceeding 1 s (conventional time).