DS3800HPTK
指示一次接地故障的发生；如果发生二次故障，则
应按照以下方式断开电源：
a） 如果外露导电部件成组或单独接地，则
保护条件与TT系统相同：
如果外露导电部件成组或单独接地，
应满足以下条件：
RA·Ia≤ 50V电压
其中：
RA是接地电极和保护电极的电阻之和
外露导电部分的导体；
我
a是导致保护装置自动断开的电流
与TT系统一致的时间；
b） 如果外露导电部件通过集体接地的保护导体互连，则TN系统的条件适用；特别是，
应满足以下条件：
如果中性点未分配：
Zs公司≤ U
2·Ia
如果分配了中性点：
Z的≤ U0级
2·Ia
该标准建议不要在IT系统中分配中性导体。
其中一个原因是在满足
双故障回路Z的阻抗。事实上，在存在
对于分布的中性导体，阻抗必须小于
阻抗Zs，在两个
阶段；这样，很明显，在
与自动断开装置协调，该装置必须跳闸，以提供
防止间接接触。
此外，重要的是，对于相当复杂的工业装置
中性点分布可能会在任何时候发生意外连接的风险
指向地面，从而消除IT系统提供的优势。
应仔细选择剩余电流装置阈值，以避免
意外跳闸，也是由于一次故障后的特定路径
通过线路导体电容到电源中性点的电流
点（而不是故障线路，另一条具有更高电容的声音线路
可能受到更高故障电流值的影响）。
4.6 IT系统
456电气设备| ABB
4保护人类
1SDC010041F0001
R
T
L编号
剩余电流断路器概述
剩余电流释放的工作原理基本上是检测
接地故障电流，通过包含所有
带电导体，包括分布的中性线。

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.