DS215SLCCG2AZZ01A
如示例所示，由于采用了更灵敏的剩余电流装置，从实际角度来看，将更容易实现与装置自身特性相协调的接地系统。表2显示了剩余电流装置可获得的大接地电阻值，并参考了常见环境（50V）：通过过电流保护装置进行保护。应通过适当协调断开时间来选择自动装置，以防止相间接地故障和间接接触故障回路的阻抗。因此，必须满足以下条件：Zs·Ia≤ U0，其中：ZS是故障回路的阻抗（单位：欧姆），包括源；-直至故障点的线路导体；-外露导电部件的保护导体；-接地导体；-装置的接地电极；-电源的接地电极；I a是表1所示时间内的断开电流，对于电流不超过32A的终电路，或对于配电电路和电流超过32A的终电路，断开电流在1秒内；U0是标称交流均方根对地电压（V）。应通过适当协调断开时间和故障回路阻抗来选择自动装置。Zs·Ia关系≤ U0可表示为：I a≤ U0=IkL对地Zs，其中IkL对地是相对地故障电流。因此，可以说明，当保护装置的跳闸电流Ia（在表1所示的时间内或1s内）低于要保护的外露导电部分的相间接地故障电流IkL时，可验证间接接触保护。需要强调的是，在TT配电系统中，使用剩余电流装置可以使接地装置具有易于获得的接地电阻值，而只有在接地电阻值RA较低的情况下（实践中很难获得）才能使用自动断路器；此外，在这种情况下，可能很难计算故障回路（Zs）的阻抗，因为中性点的接地电阻不能视为可忽略不计（事实上，它可以达到相同数量的接地电阻值）。

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).