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如示例所示，由于采用了更靈敏的剩余電流裝置，從實際角度來看，將更容易實現(xiàn)與裝置自身特性相協(xié)調(diào)的接地系統(tǒng)。表2顯示了剩余電流裝置可獲得的大接地電阻值，并參考了常見環(huán)境（50V）：通過過電流保護裝置進行保護。應通過適當協(xié)調(diào)斷開時間來選擇自動裝置，以防止相間接地故障和間接接觸故障回路的阻抗。因此，必須滿足以下條件：Zs·Ia≤ U0，其中：ZS是故障回路的阻抗（單位：歐姆），包括源；-直至故障點的線路導體；-外露導電部件的保護導體；-接地導體；-裝置的接地電極；-電源的接地電極；I a是表1所示時間內(nèi)的斷開電流，對于電流不超過32A的終電路，或?qū)τ谂潆婋娐泛碗娏鞒^32A的終電路，斷開電流在1秒內(nèi)；U0是標稱交流均方根對地電壓（V）。應通過適當協(xié)調(diào)斷開時間和故障回路阻抗來選擇自動裝置。Zs·Ia關(guān)系≤ U0可表示為：I a≤ U0=IkL對地Zs，其中IkL對地是相對地故障電流。因此，可以說明，當保護裝置的跳閘電流Ia（在表1所示的時間內(nèi)或1s內(nèi)）低于要保護的外露導電部分的相間接地故障電流IkL時，可驗證間接接觸保護。需要強調(diào)的是，在TT配電系統(tǒng)中，使用剩余電流裝置可以使接地裝置具有易于獲得的接地電阻值，而只有在接地電阻值RA較低的情況下（實踐中很難獲得）才能使用自動斷路器；此外，在這種情況下，可能很難計算故障回路（Zs）的阻抗，因為中性點的接地電阻不能視為可忽略不計（事實上，它可以達到相同數(shù)量的接地電阻值）。

As shown in the example, thanks to a more sensitive residual current device, from a practical point of view it will be easier to realize an earthing system coordinated with the characteristics of the device itself. The Table 2 shows the maximum values of earth resistance which can be obtained with residual current devices and making reference to a common environment (50V):Protection by means of overcurrent protective devices The choice of the automatic device for the protection against phase-to-earth faults and indirect contact shall be carried out by coordinating properly the disconnection times with the impedance of the fault loop. As a consequence, it is necessary to fulfill the following condition: Zs · Ia ≤ U0 where: ZS is the impedance (in ohms) of the fault loop comprising - the source; - the line conductor up to the point of the fault; - the protective conductor of the exposed-conductive-parts; - the earthing conductor; - the earth electrode of the installation; - the earth electrode of the source; I a is the disconnection current in the times shown in Table 1 for final circuits with currents not exceeding 32A or within 1 second for distribution circuits and for final circuits with currents exceeding 32A; U0 is the nominal a.c. r.m.s. voltage to earth (V). The choice of the automatic device shall be made by coordinating properly the disconnection times with the impedance of the fault loop. The relationship Zs · Ia ≤ U0 may be expressed as : I a ≤ U0 = IkL-to earth Zs where IkL-to earth is the phase-to-earth fault current. Therefore, it is possible to state that the protection against indirect contact is verified when the tripping current Ia of the protective device (within the times shown in Table 1 or within 1s) is lower than the phase-to-earth fault current IkL-to earth at the exposed-conductive-part to be protected. It is to underline that in TT distribution systems the use of a residual current device allows to have an earthing arrangement with an earth resistance value which can be easily obtained, whereas the use of automatic circuit-breakers is possible only in case of low earth resistance values RA (very difficult to be obtained in practice); besides, in such circumstances, it could be very difficult to calculate the impedance of the fault loop (Zs), because the earthing resistance of the neutral cannot be considered negligible (in fact it could reach values of the same quantity of the earth resistance).