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如圖3所示，IT系統(tǒng)中的接地故障電流通過線路導體電容流向電源中性點。因此，一次接地故障的特點是電流值極低，以防止過電流保護斷開；導出的接觸電壓非常低。圖3：IT系統(tǒng)中的接地故障根據IEC 60364-4，僅當滿足以下條件時，無需在發(fā)生一次接地故障時自動斷開電路：RA·Id≤ 50 Va.c.RA·Id≤ 120 Vd。c、 式中：RA是裸露導電部件的接地電極和保護導體的電阻之和，單位為歐姆；I d是線路導體和外露導電部分之間阻抗可忽略不計的一個故障2的故障電流，單位為安培；該值考慮了泄漏電流和電氣裝置的總接地阻抗。如果滿足此條件，則故障發(fā)生后，外露導電部件上的接觸電壓將超過50 V（交流），這是人體可以無限期承受的。在IT系統(tǒng)安裝中，應提供絕緣監(jiān)測裝置，以指示故障發(fā)生后是否存在異常情況。符合IEC 61557-8標準的絕緣監(jiān)測裝置是一種持續(xù)監(jiān)測電氣裝置絕緣的裝置。其目的是發(fā)出裝置絕緣水平顯著降低的信號，以便在二次故障發(fā)生之前找到降低的原因，從而防止電源斷開
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
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