这些配电系统用于特定电厂，其中供电的连续性是一项基本要求，没有供电可能会对人员造成危害或相当大的经济损失，或者需要一次接地故障的低值。在TT装置中，中性导线连接到电源星形中心，它通常是分布式的，具有使相电压（例如230 V）可用的功能，可用于单相负载供电。相反，裸露的导电部分单独或整体地局部连接到地面。TT系统通常用于民用设施。TN系统通常用于将电源分配给具有自己变电站的负载。中性导线在变电站中直接接地；外露导电部分连接到中性导线的同一接地点，并可局部接地。根据中性导线和保护导线的布置，考虑三种类型的TN系统：1。TN-C中性点和保护功能组合在一根导线（PEN导线）中；2.TN-S中性导线和保护导线始终分开；3.TN-C-S中性和保护功能组合在系统（PEN）一部分的单个导体中，并在另一部分（PE+N）中分离。在TT系统中，在TN系统内，故障电流通过固态金属连接流向电源中性点，几乎不涉及接地电极（图2）。在这些情况下，应提供绝缘监测装置，用于发出可能的接地故障或所供设备故障的光或声信号。图2:TN系统中的接地故障图3:IT系统中的接地故障IT系统没有直接接地的带电部件，但它们可以通过足够高的阻抗接地。外露导电部件应单独、成组或集体接地至独立接地电极。接地故障电流通过接地电极和线路导体电容流向电源中性点（图3）。

In TT installations, the neutral conductor is connected to the supply star center, it is usually distributed and has the function of making the phase voltage (e.g. 230 V) available, useful for single-phase load supply. The exposed conductive parts, on the contrary, singularly or collectively, are locally connected to earth. TT systems are generally used for civil installations. TN systems are typically used when the power supply is distributed to loads having their own electrical substation. The neutral conductor is directly earthed in the substation; the exposed conductive parts are connected to the same earthing point of the neutral conductor, and can be locally earthed. Three types of TN system are considered according to the arrangement of neutral and protective conductors: 1. TN-C neutral and protective functions are combined in a single conductor (PEN conductor); 2. TN-S neutral and protective conductors are always separated; 3. TN-C-S neutral and protective functions are combined in a single conductor in a part of the system (PEN) and are separated in another part (PE + N). In TT systems, theIn TN systems, the fault current flows towards the power supply neutral point through a solid metallic connection, practically without involving the earth electrode (Figure 2). These distribution systems are used for particular plants, where the continuity of supply is a fundamental requirement, where the absence of the supply can cause hazards to people or considerable economical losses, or where a low value of a first earth fault is required. In these cases, an insulation monitoring device shall be provided for optical or acoustic signalling of possible earth faults, or failure of the supplied equipment. Figure 2: Earth fault in TN systems Figure 3: Earth fault in IT systems IT systems have no live parts directly connected to earth, but they can be earthed through a sufficiently high impedance. Exposed conductive parts shall be earthed individually, in groups or collectively to an independent earthing electrode. The earth fault current flows towards the power supply neutral point through the earthing electrode and the line conductor capacitance (Figure 3).