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指示一次接地故障的發(fā)生；如果發(fā)生二次故障，則
應(yīng)按照以下方式斷開(kāi)電源：
a） 如果外露導(dǎo)電部件成組或單獨(dú)接地，則
保護(hù)條件與TT系統(tǒng)相同：
如果外露導(dǎo)電部件成組或單獨(dú)接地，
應(yīng)滿(mǎn)足以下條件：
RA·Ia≤ 50V電壓
其中：
RA是接地電極和保護(hù)電極的電阻之和
外露導(dǎo)電部分的導(dǎo)體；
我
a是導(dǎo)致保護(hù)裝置自動(dòng)斷開(kāi)的電流
與TT系統(tǒng)一致的時(shí)間；
b） 如果外露導(dǎo)電部件通過(guò)集體接地的保護(hù)導(dǎo)體互連，則TN系統(tǒng)的條件適用；特別是，
應(yīng)滿(mǎn)足以下條件：
如果中性點(diǎn)未分配：
Zs公司≤ U
2·Ia
如果分配了中性點(diǎn)：
Z的≤ U0級(jí)
2·Ia
該標(biāo)準(zhǔn)建議不要在IT系統(tǒng)中分配中性導(dǎo)體。
其中一個(gè)原因是在滿(mǎn)足
雙故障回路Z的阻抗。事實(shí)上，在存在
對(duì)于分布的中性導(dǎo)體，阻抗必須小于
阻抗Zs，在兩個(gè)
階段；這樣，很明顯，在
與自動(dòng)斷開(kāi)裝置協(xié)調(diào)，該裝置必須跳閘，以提供
防止間接接觸。
此外，重要的是，對(duì)于相當(dāng)復(fù)雜的工業(yè)裝置
中性點(diǎn)分布可能會(huì)在任何時(shí)候發(fā)生意外連接的風(fēng)險(xiǎn)
指向地面，從而消除IT系統(tǒng)提供的優(yōu)勢(shì)。
應(yīng)仔細(xì)選擇剩余電流裝置閾值，以避免
意外跳閘，也是由于一次故障后的特定路徑
通過(guò)線路導(dǎo)體電容到電源中性點(diǎn)的電流
點(diǎn)（而不是故障線路，另一條具有更高電容的聲音線路
可能受到更高故障電流值的影響）。
4.6 IT系統(tǒng)
456電氣設(shè)備| ABB
4保護(hù)人類(lèi)
1SDC010041F0001
R
T
L編號(hào)
剩余電流斷路器概述
剩余電流釋放的工作原理基本上是檢測(cè)
接地故障電流，通過(guò)包含所有
帶電導(dǎo)體，包括分布的中性線。
indicate the occurrence of a first earth fault; in the event of a second fault, the
supply shall be disconnected according to the following modalities:
a) where exposed conductive parts are earthed in groups or individually, the
conditions for protection are the same as for TT systems:
where the exposed-conductive-parts are earthed in groups or individually,
the following condition shall be fulfilled:
RA · Ia ≤ 50V
where:
RA is the sum of the resistance of the earth electrode and of the protective
conductor for the exposed-conductive-parts;
I
a is the current causing automatic disconnection of the protective device in
a time complying with that for TT systems;
b) where exposed conductive parts are interconnected by a protective conductor collectively earthed, the conditions of a TN system apply; in particular,
the following conditions shall be fulfilled:
if the neutral is not distributed:
Zs ≤ U
2·Ia
if the neutral is distributed:
Z’s ≤ U0
2·Ia
The Standard suggests not to distribute the neutral conductor in IT systems.
One of the reasons is the real difficulty in fulfilling the condition prescribed for
the impedance of the double fault loop Z’s. As a matter of fact, in the presence
of a neutral conductor distributed, the impedance must be 58% smaller than
the impedance Zs, which is verified in the event of a double fault between the
phases; in this way, it becomes evident that there is a greater difficulty in the
co-ordination with the automatic disconnection device which must trip to provide
protection against indirect contact.
Moreover, above all for quite complex industrial installations, the presence of the
neutral distributed may involve the risk of an accidental connection of it at any
point to earth, thus eliminating the advantages offered by IT systems.
The residual current device threshold shall be carefully chosen in order to avoid
unwanted tripping, due also to the particular path followed by the first fault
current through the line conductor capacitance to the power supply neutral
point (instead of the faulted line, another sound line with higher capacitance
could be affected by a higher fault current value).
4.6 IT System
456 Electrical devices | ABB
4 Protection of human beings
1SDC010041F0001
R
T
L N
Generalities on residual current circuit-breakers
The operating principle of the residual current release is basically the detection
of an earth fault current, by means of a toroid transformer which embraces all
the live conductors, included the neutral if distributed.
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