DS215UCVEH2A
如图3所示，IT系统中的接地故障电流通过线路导体电容流向电源中性点。因此，一次接地故障的特点是电流值极低，以防止过电流保护断开；导出的接触电压非常低。图3：IT系统中的接地故障根据IEC 60364-4，仅当满足以下条件时，无需在发生一次接地故障时自动断开电路：RA·Id≤ 50 Va.c.RA·Id≤ 120 Vd。c、 式中：RA是裸露导电部件的接地电极和保护导体的电阻之和，单位为欧姆；I d是线路导体和外露导电部分之间阻抗可忽略不计的一个故障2的故障电流，单位为安培；该值考虑了泄漏电流和电气装置的总接地阻抗。如果满足此条件，则故障发生后，外露导电部件上的接触电压将超过50 V（交流），这是人体可以无限期承受的。在IT系统安装中，应提供绝缘监测装置，以指示故障发生后是否存在异常情况。符合IEC 61557-8标准的绝缘监测装置是一种持续监测电气装置绝缘的装置。其目的是发出装置绝缘水平显著降低的信号，以便在二次故障发生之前找到降低的原因，从而防止电源断开

As represented in Figure 3, the earth fault current in an IT system flows through the line conductor capacitance to the power supply neutral point. For this reason, the first earth fault is characterized by such an extremely low current value to prevent the overcurrent protections from disconnecting; the deriving touch voltage is very low. Figure3: Earth fault in IT system According to IEC 60364-4, the automatic disconnection of the circuit in case of the first earth fault is not necessary only if the following condition is fulfilled: RA · Id ≤ 50 Va.c. RA · Id ≤ 120 Vd.c. where: RA is the sum of the resistance, in ohms, of the earth electrode and protective conductor for exposed-conductive- parts; I d is the fault current, in amperes, of the first fault2 of negligible impedance between a line conductor and an exposed-conductive-part; such value takes account of the leakage currents and of the total earthing impedance of the electrical installation. If this condition is fulfilled, after the fault, the touch voltage on the exposedconductive-part will be than 50 V (in alternating current), which is tolerable by the human body for an indefinite time. In IT system installations, an insulation monitoring device shall be provided to indicate the presence of an anomalous condition after the occurrence of a fault. An insulation monitoring device complying with Std. IEC 61557-8 is a device constantly monitoring the insulation of an electrical installation. It is aimed at signaling any remarkable reduction of the insulation level of an installation in order to find the cause of this reduction before a second fault occurs, thus preventing disconnection of the power supply