Repeated grounding refers to reconnecting one or more of the zero wires to the ground through a grounding device. It is a technical measure to prevent electric shock that is often used in the protection zero system. Its security role generally has 5 points:
1) Reduce the risk of electric shock on the zero-connected equipment when the PE wire or PEN wire is accidentally broken or the contact is poor.
2) Reduce the load neutral drift when the PEN line is disconnected
3) Further reduce the ground voltage of accidentally charged equipment during the duration of the fault
4) Shorten the duration of leakage faults
5) Improved lightning protection performance of overhead lines
However, if we further analyze the protection of the repeated grounding system, we will find that it has the following three potential risks in the TN-C system (the system in which the neutral line and the protection line are commonly used today).
1. The existence of repeated grounding may hide the occurrence of zero-mainline disconnection faults. Single-phase electrical equipment continues to operate by repeating grounding to form a new electrical circuit, extending the duration of the fault.
2. After the zero-mainline disconnection fault occurs, if the electrical equipment continues to be used, the zero line zero drift will occur due to the existence of the working grounding resistance and the repeated grounding resistance. Two zero-to-ground voltages appear before and after the fault point, resulting in the PEN line. It is live and has long existed with the work of electrical equipment.
3. Under normal circumstances, the exposed conductive part of the device is connected with the PEN line to form a zero-connection protection. However, after the zero-main line breakpoint, the connected electrical equipment actually turns into a grounding protection mode, forming a zero-protection and grounding protection hybrid system. There is a risk of electric shock.
Of course, repeated grounding is still a good system protection method in the three-phase four-wire system. Its protection effect on the system is significant, and its advantages outweigh the disadvantages (three-phase five-wire system should be adopted in important occasions). However, the above analysis also shows that repeated grounding is not perfect, the system still has the risk of electric shock, should always check whether the system has abnormal phenomena (such as zero line electrification, electrical equipment can not reach the rated power, etc.), early detection and elimination of zero trunk line break Line failure.
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