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故障回路不影響接地系統(tǒng)，基本上由相導(dǎo)體和保護(hù)導(dǎo)體串聯(lián)而成。根據(jù)IEC 60364-4規(guī)定，為提供自動(dòng)斷開(kāi)電路的保護(hù)，應(yīng)滿(mǎn)足以下條件：Zs·Ia≤ U0，其中：Zs是故障回路的阻抗，包括電源、故障點(diǎn)之前的帶電導(dǎo)體以及故障點(diǎn)和電源之間的保護(hù)導(dǎo)體[?]; U0是標(biāo)稱(chēng)交流r.m.s.接地電壓【V】；I a是保護(hù)裝置在表3中規(guī)定的時(shí)間內(nèi)的斷開(kāi)電流（安培），作為電流不超過(guò)32A的終電路的額定電壓U0的函數(shù)，或配電電路和電流超過(guò)32A的終電路在5秒內(nèi)的函數(shù)（對(duì)于電路類(lèi)型的描述，應(yīng)參考TT系統(tǒng)給出的指示）.應(yīng)通過(guò)適當(dāng)協(xié)調(diào)斷開(kāi)時(shí)間和故障回路阻抗，選擇自動(dòng)裝置，以防止相位PE故障和間接接觸。在TN系統(tǒng)中，低壓側(cè)的螺栓故障相PE通常會(huì)產(chǎn)生類(lèi)似于短路的電流，接地故障電流流經(jīng)線路導(dǎo)體（或多個(gè)導(dǎo)體），而保護(hù)導(dǎo)體不會(huì)影響接地布置。Zs·Ia關(guān)系≤ U0可表示為：I a≤ U0=IkLPE Zs，其中IkLPE是相位PE故障電流。因此，可以說(shuō)明，當(dāng)保護(hù)裝置的跳閘電流I a（在表3所示時(shí)間內(nèi)或5s內(nèi)）低于待保護(hù)外露導(dǎo)電部分的相PE故障電流IkLPE時(shí)，可驗(yàn)證間接接觸保護(hù)。在TN系統(tǒng)中，以下裝置用于防止間接接觸：•帶熱磁脫扣器的斷路器；•帶電子脫扣器的斷路器剩余電流裝置（僅TN-S）。
The fault loop does not affect the earthing system and is basically formed by the connection in series of the phase conductor and of the protective conductor. To provide a protection with automatic disconnection of the circuit, according to IEC 60364-4 prescriptions, the following condition shall be fulfilled: Zs · Ia ≤ U0 where: Zs is the impedance of the fault loop comprising the source, the live conductor up to the point of the fault and the protective conductor between the point of the fault and the source [?]; U0 is the nominal ac r.m.s. voltage to earth [V]; I a is the disconnection current in amperes of the protective device within the times defined in Table 3 as a function of the rated voltage U0 for final circuits with currents not exceeding 32A or within 5 seconds for distribution circuits and for final circuits with current exceeding 32A (for a description of the circuit typologies reference shall be made to the indications given for TT systems).The choice of the automatic device for the protection against phase-PE faults and against indirect contact shall be carried out by coordinating properly the disconnection times with the impedance of the fault loop. In TN systems a bolted fault phase-PE on the LV side usually generates a current similar to that of a short-circuit and the earth fault current flowing through the line conductor (or conductors) and the protective conductor does not absolutely affect the earthing arrangement. The relationship Zs · Ia ≤ U0 may be expressed as: I a ≤ U0 = IkLPE Zs where IkLPE is the phase-PE fault current. Therefore, it is possible to state that the protection against indirect contact is verified when the tripping current I a of the protective device (within the times shown in Table 3 or within 5s) is lower than the phase-PE fault current IkLPE at the exposed-conductive-part to be protected. In TN systems the following devices are used for protection against indirect contact: • circuit-breakers with thermomagnetic releases; • circuit-breakers with electronic releases; • residual current devices (TN-S only).
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