Protection design and equipotential bonding for gr

2022-10-03
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Protection design and equipotential bonding of low-voltage system grounding fault Abstract: connecting the exposed conductive parts inside and outside the electrical device of the building, the conductive parts outside the electrical device, and the artificial or natural grounding body with conductors to reduce the potential difference is called equipotential bonding. Equipotential bonding is also called ungrounded equipotential bonding, which is not connected with artificial or natural grounding body

key words: artificial grounding body natural grounding body potential difference equipotential bonding Untitled Document 3.4.2.1 equipotential bonding concept connects the exposed conductive parts inside and outside the electrical device of the building, the conductive parts outside the electrical device, and the artificial or natural grounding body with conductors to reduce the potential difference, which is called equipotential bonding. Equipotential bonding is also called ungrounded equipotential bonding, which is not connected with artificial or natural grounding body. Equipotential bonding includes total equipotential bonding, local equipotential bonding and auxiliary equipotential bonding. The so-called general equipotential bonding is to connect the following conductive parts in the building to the grounding busbar (general grounding terminal board) near the incoming distribution box and connect them to each other: -- PE (pen) busbar of the incoming distribution box; -- grounding trunk line from grounding electrode (if necessary); -- metal pipelines of public facilities in buildings, such as gas pipelines, water supply and drainage pipelines, as well as main pipelines of heating, air conditioning, etc—— Metal structure of buildings; -- reinforcement in reinforced concrete. It should be noted that the gas pipe and heating are due to the general equipotential bonding of the gas pipe when the gear pump is working, but it is not allowed to be used as a grounding body. Because the gas pipeline should be inserted into an insulating part after entering the house, and connected with a voltage protector when there is external stimulation; The outdoor underground heating pipe is not in good contact with the ground because it is wrapped with thermal insulation materials. Local equipotential bonding is to make another equipotential bonding within the local scope of the building according to the requirements of general equipotential bonding. Auxiliary equipotential bonding refers to the direct connection of conductors between electrical equipment that can be contacted at the same time or between electrical equipment and conductive parts outside the device (such as metal pipes and metal structural parts) that may have dangerous potential differences within the reach of the boom. Figure 3.4.2.2 equipotential bonding effect (1) reduce the expected contact voltage

figure TN system equipotential bonding effect is illustrated by taking the grounding type TN-C-S system as an example. The figure shows that the commonly used tn-c-aluminum material handling site actually does not need workers's system. At the power incoming line, the pen line is divided into PE line and N line (where the N line starts to be insulated from PE line), with repeated grounding, and the total equipotential bonding is not installed, If the equipment has a ground fault, ignore the impedance of the ground fault point, RA and Rb are connected in series and then connected in parallel with zpen, ra+rb zpen; The human body impedance zh is connected in series with the shoe and sock and the floor resistance RP, and then connected in parallel with ZPE. Zh+rp ZPE, the ground fault current ID flows through the phase line, PE line and pen line, and returns to the low-voltage winding of the transformer, that is, in the formula - U0 phase. The advantages of these six materials are reflected in various uses and materials. The nominal voltage to ground, V; - Zero sequence impedance of ZT transformer, Ω; - ZL phase line impedance, Ω; - Internal PE line impedance of ZPE electrical device, Ω; - Impedance of external pen line of zpen electrical device, Ω. The expected fault voltage UT1 can be calculated by the following formula: the expected contact voltage after making the total equipotential bonding is from the above two formulas. After making the total equipotential bonding, the reduced expected contact voltage is from the figure. After making the total equipotential bonding, in the total equipotential bonding area, the building foundation reinforcement, metal structural parts, metal pipes, metal cable bridges, cable metal sheaths Laying natural grounding bodies such as cables or wire metal pipes, the grounding resistance value is small, which has played the role of repeated grounding. IEC standard does not stipulate that artificial grounding body must be made for repeated grounding, nor does it specify the resistance value of repeated grounding. Although the voltage drop on the pen line in the power line does not produce contact voltage in the building, it can raise the ground potential of the grounding busbar. Since the exposed conductive part of the electrical device and the conductive part outside the device are connected with the grounding busbar within the total equipotential bonding range, their potentials rise equally and are basically at the same potential. When the human body contacts these conductive parts, it does not contact different potentials, so there is naturally no risk of electric shock. (2) Eliminate the dangerous fault voltage channeled from the building along the pen line or PE line. There are three possible situations after the grounding fault occurs in the TN system due to insulation damage: first, the two metal parts in contact with the fault point are separated from the contact due to hundreds to thousands of amperes of current passing through, melting into clusters, and the grounding fault disappears naturally; Second, the two metal parts melted into a mass and broke away from contact, igniting the arc, forming an arc grounding fault with large fault point impedance. Because a large part of the line voltage fell on the arc, the contact voltage was relatively reduced, and its consequences were mostly fire rather than personal electric shock; Third, the two metal parts are welded firmly to each other after melting, so that the fault continues to exist. The impedance of the fault point is negligible, and most of the consequences are personal electric shock, which is a ground fault. It is the dangerous fault along the pen line or PE line due to the existence of ground fault voltage

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