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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
應按照以下方式斷開電源：
a） 如果外露導電部件成組或單獨接地，則
保護條件與TT系統相同：
如果外露導電部件成組或單獨接地，
應滿足以下條件：
RA·Ia≤ 50V電壓
其中：
RA是接地電極和保護電極的電阻之和
外露導電部分的導體；
我
a是導致保護裝置自動斷開的電流
與TT系統一致的時間；
b） 如果外露導電部件通過集體接地的保護導體互連，則TN系統的條件適用；特別是，
應滿足以下條件：
如果中性點未分配：
Zs公司≤ U
2·Ia
如果分配了中性點：
Z的≤ U0級
2·Ia
該標準建議不要在IT系統中分配中性導體。
其中一個原因是在滿足
雙故障回路Z的阻抗。事實上，在存在
對于分布的中性導體，阻抗必須小于
阻抗Zs，在兩個
階段；這樣，很明顯，在
與自動斷開裝置協調，該裝置必須跳閘，以提供
防止間接接觸。
此外，重要的是，對于相當復雜的工業(yè)裝置
中性點分布可能會在任何時候發(fā)生意外連接的風險
指向地面，從而消除IT系統提供的優(yōu)勢。
應仔細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
通過線路導體電容到電源中性點的電流
點（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
456電氣設備| ABB
4保護人類
1SDC010041F0001
R
T
L編號
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
接地故障電流，通過包含所有
帶電導體，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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