Repeated grounding is in the neutral point directly grounded system, in the zero trunk of one or more places with a metal conductor connected to the grounding device. In the low-voltage three-phase four-wire system neutral point directly grounded line, the construction unit in the installation, the distribution line should be zero trunk and branch line terminal grounding.
Grounding is done every 1 kilometer on the zero trunk line. For the grounding more than 50 meters of distribution lines, access to the user at the zero line should still be repeated grounding, repeated grounding resistance should not be greater than 10 ohms.
Requirements:
1, independent lightning protection grounding resistance should be less than or equal to 10 ohms;
2, independent safety protection grounding resistance should be less than or equal to 4 ohms;
3, independent AC grounding resistance should be less than or equal to 4 ohms;
4, independent DC working grounding resistance should be less than or equal to 4 ohms;
5, anti-static grounding resistance is generally required to be less than or equal to 100 ohms.
Repeated grounding of the zero line can shorten the duration of the fault, reduce the voltage drop loss on the zero line, reduce the danger of phase and zero line reversal. In the protection of the zero line break, when the insulation of electrical equipment is damaged or phase line touch shell, the zero line repeated grounding can also reduce the fault electrical equipment to ground voltage, reduce the risk of electric shock accident.
Therefore, the zero repeated grounding in the power supply network has a very important role, but this role is often overlooked.
Expanded information:
In the TN-S (three-phase, five-wire) system, the zero line (the work) is not allowed to be re-grounded. zero wire) is not allowed to be repeatedly grounded. Do not allow repeated grounding because if the neutral line repeated grounding, three-phase five-wire leakage protection detection is inaccurate, and can not play an accurate role in protection. Therefore, the zero line is not allowed to repeat grounding, in fact, leakage detection point can not be repeated after grounding.
Grounding characteristics:
In the neutral point directly grounded in the low-voltage power distribution system, in order to ensure safe and reliable line operation, to prevent the hazards caused by the zero line breakage, the system in addition to the system (work) grounding, but also in the lead to the zero line in the other locations for the necessary repeat grounding.
Grounding location:
(1) outdoor overhead lines should be centralized repeated grounding; overhead line terminals, branch line length of more than 200 meters at the branch and coastal line every 1 kilometer, the zero line should be repeated grounding.
(2) high-voltage lines and low-voltage lines erected on the same pole, the same pole section of the two ends of the low-voltage zero line should also be repeated grounding.
Safety protection grounding is the non-electrified metal part of the electrical equipment and grounding body to make a good metal connection. That is, the building's electrical equipment and equipment near some of the metal components, and PE wire connection, but it is strictly prohibited to PE wire and N wire connection.
In order to make the accuracy of the various electronic equipment, high stability, in addition to the need for a stable power supply, but also must have a stable reference potential. A larger cross-sectional area of insulated copper core wire can be used as a lead, one end of which is directly connected to the reference potential, and the other end of which is used for DC grounding of the electronic equipment.
Reference: