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For the TN-S system, repeated grounding refers to the repeated grounding of the PE wire, and its functions are as follows: (1) If repeated grounding is not carried out, when the PE wire breaks, the system is in an unprotected state with no connection to either the neutral or the ground. After performing repeated grounding on it, when the PE is in normal condition, the system is in a zero-grounding protection state ； When the PE wire breaks, if the break occurs before the repeated grounding point, the system is in a grounded protection state. The TN—S system with repeated grounding possesses a very interesting dual-protection feature: in the event of a PE wire break, it switches from the TN—S configuration to a TT system protection mode (with the PE wire break occurring on the side before the repeated grounding). (2) When the phase wire breaks and shorts to ground, the presence of fault current causes the potential of the PE wire to rise; when the resistance between the break point and ground is low, the potential of the PE wire can very well exceed the safe voltage level. This hazardous voltage is transmitted along the PE wire to the enclosures of various electrical devices, thereby posing a risk to human safety. After implementing repeated grounding, since the equivalent resistance resulting from the parallel connection of the repeated grounding resistance and the power supply’s working grounding resistance is smaller than the latter, the voltage shared by the grounding resistance at the point where the phase wire is broken increases. This effectively reduces the voltage of the PE wire with respect to ground, thereby minimizing the risk of electric shock. (3) The repeated grounding of the PE wire can reduce the voltage of the equipment’s enclosure relative to ground in the event of a short circuit between a phase wire and the enclosure. In such a situation, the voltage of the enclosure relative to ground is equal to the voltage between the fault point P and the transformer’s neutral point. Assuming that the specifications of the phase wire and the PE wire are the same, the voltage of the equipment enclosure with respect to ground is 110V. After the PE wire is grounded repeatedly, starting from the fault point P, the impedance of the PE wire is in parallel with the resistance resulting from the series connection of the repeated grounding resistance RE and the working grounding resistance RA. Under normal circumstances, since the resistance resulting from the series connection of the repeated grounding resistance RE and the working grounding resistance RA is much larger than the impedance of the PE wire itself, the equivalent impedance from P to the transformer’s neutral point remains close to the impedance of the PE wire itself from P to the transformer’s neutral point. If the specifications of the phase wire and the PE wire are the same, the voltage UPO between P and the transformer’s neutral point remains approximately 110V. In this case, the voltage UP across the equipment’s enclosure with respect to ground is only a portion of the voltage UPO between the faulty point P and the transformer’s neutral point; it can be expressed as: UP = UPO × RERA + RE. Assuming that the resistance for repeated grounding is RE = 10Ω and the resistance for normal grounding is RA = 4Ω, then UP = 78.6V. If grounding is done only on the N wire, it does not have the effects described in points (1) and (3) above, but only the effect described in point (2). In a TN–S system, the equipment’s enclosures are connected to the PE wire, not to the N wire. Therefore, what is more important to us is the potential of the PE wire, rather than that of the N wire; the repeated grounding in the TN—S system is not a repeated grounding of the N wire. If the PE wire and the N wire are grounded together, since the PE wire and the N wire meet at the repeated grounding point, the PE wire and the N wire on the side before the repeated grounding (the side close to the transformer’s neutral point) become indistinguishable from each other. All of the neutral current that originally flowed through the N wire is now shared by both the N wire and the PE wire (with a small portion being diverted through the repeated grounding). It can be considered that at this point, there is no longer a PE wire on the front side of the repeated grounding; only a PEN wire composed of the original PE wire and the N wire in parallel exists. The original TN–S system has effectively become a TN–C–S system, and the advantages associated with the original TN–S system are lost. Therefore, it is not possible to ground the PE wire and the N wire together. In engineering practice, for TN—S systems, it is rare to ground the N wire and the PE wire separately. The main reasons are as follows: 1) Grounding the N wire and the PE wire separately only provides an additional benefit compared to grounding only the PE wire; it helps to reduce the shift in the neutral point potential that occurs when the N wire is broken, thereby enhancing the safety of electrical equipment. However, this effect is not necessarily very significant. Moreover, once the working neutral wire is grounded separately, leakage protection cannot be used on the circuit connected to it. 2) If it is necessary to ground the N wire and the PE wire separately, in order to maintain a stable potential for the PE wire and prevent it from being affected by the potential of the N wire, the grounding point for the N wire must be kept at a sufficient distance from the grounding point for the PE wire, as well as from all other grounding elements, metal components, and the underground parts of metal pipes that are connected together to achieve equal potential – ideally at a distance of more than 20 meters. However, this is difficult to achieve in actual construction. This post was last edited by wopale3 on 2009-4-7 16:53]