-k2级
是并联导体的校正系数，可通过
以下公式：
式中，n是每相并联的导体数量；
-1.2是标准允许的磁阈值公差。
TN系统
小短路电流的计算公式为：
因此：
IT系统
当第二个故障将IT系统变成
TN系统。
有必要单独检查中性点未分布的装置
中性分布。
ABB |电气设备461
4保护人类
4.8保护人类的大保护长度
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Dy L1
L2级
L3级
体育课
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体育课
任
我
k L3 L2 L1 Z
体育课
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k L3 L2 L1
中性点未分配
当发生第二个故障时，公式为：
因此：
中性分布
案例A：中性点分布式IT系统中的三相电路
公式为：
因此：
案例B：IT系统中的三相+中性电路，中性点分布
公式为：
因此：
462电气设备| ABB
4保护人类
4.8保护人类的大保护长度
1SDC010045F0001
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L2级
L3级
N
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我
k L3 L2 L1 N Z
体育课
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k L3 L2 L1
A、B
表格使用说明
已定义显示大保护长度（MPL）的表格
考虑以下条件：
-每相一根电缆；
-额定电压等于400 V（三相系统）；
-铜电缆；
-中性非分布式，仅适用于IT系统；
-保护导体横截面符合表1：
表1：保护导体横截面
相导体截面S保护导体截面SPE
[平方毫米
][平方毫米
]
s≤ 16秒
16<秒≤ 35 16
S>35秒/秒
注：相导体和保护导体具有相同的绝缘和导电材料
每当使用电子脱扣器的S功能（延迟短路）时
对于大保护长度的定义，有必要验证
跳闸时间低于第4.5章表1中报告的时间值
对于TN系统和第4.6章表1中的IT系统。
对于与参考条件不同的条件，以下修正系数
应适用。

- k2
 is the correction factor for conductors in parallel, obtainable by the
following formula:
 where n is the number of conductor in parallel per phase;
- 1.2 is the magnetic threshold tolerance allowed by the Standard.
TN system
The formula for the evaluation of the minimum short circuit current is:
and consequently:
IT system
The formulas below are valid when a second fault turns the IT system into a
TN system.
It is necessary to separately examine installations with neutral not distributed
and neutral distributed.
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4 Protection of human beings
4.8 Maximum protected length for the protection of human beings
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PE
PE
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k L3 L2 L1 Z
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k L3 L2 L1
Neutral not distributed
When a second fault occurs, the formula becomes:
and consequently:
Neutral distributed
Case A: three-phase circuits in IT system with neutral distributed
The formula is:
and consequently:
Case B: three-phase + neutral circuits in IT system with neutral distributed
The formula is:
and consequently:
462 Electrical devices | ABB
4 Protection of human beings
4.8 Maximum protected length for the protection of human beings
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PE
PE
PE
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PE
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A B
Note for the use of the tables
The tables showing the maximum protected length (MPL) have been defined
considering the following conditions:
- one cable per phase;
- rated voltage equal to 400 V (three-phase system);
- copper cables;
- neutral not distributed, for IT system only;
- protective conductor cross section according to Table 1:
Table 1: Protective conductor cross section
Phase conductor cross section S Protective conductor cross section SPE
 [mm2
] [mm2
]
S ≤ 16 S
16 < S ≤ 35 16
S > 35 S/2
Note: phase and protective conductors having the same isolation and conductive materials
Whenever the S function (delayed short-circuit) of electronic releases is used
for the definition of the maximum protected length, it is necessary to verify that
the tripping time is lower than the time value reported in Chapter 4.5 Table 1
for TN systems and in Chapter 4.6 Table 1 for IT systems.
For conditions different from the reference ones, the following correction factors
shall be applied.