Table 572-1 The smallest size of the ground wire and neutral protection wire
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Copper |
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Aluminum |
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Steel |
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Name |
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Indoor |
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Outdoor |
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Indoor |
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Indoor |
Outdoor |
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Under |
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Under |
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Outdoor |
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ground |
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ground |
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Bare conductor: |
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Cross-section-area,mm2 |
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4 |
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6 |
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Diameter,mm |
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5 |
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6 |
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10 |
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Cross-section-area isolated wire,mm2 |
1.5(*) |
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2.5 |
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- |
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- |
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Cross-section--area earthing core and |
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earthed neutral of cable or multi-core |
1 |
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2.5 |
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- |
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wire in common cover with ph |
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core,mm2 |
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Wing thickness of steel corner,mm |
- |
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2 |
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2,5 |
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4 |
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Steel Cross-section-area bar,mm2 |
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24 |
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48 |
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48 |
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3 |
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4 |
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4 |
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Thickness,mm |
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2.5 |
2.5 |
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2.5 |
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3.5 |
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Tube: |
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1.5 |
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2.5 |
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Not |
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allowed |
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Note(*): When placing the wire in the tube, |
the -sectioncross-area of the neutral protecting is |
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permitted by 1mm2 if the phase wire has the same cross-section. |
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Table 572-2 Minimum size of grounding electrode |
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Minimum size |
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Material |
Kind of conductor |
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Diameter |
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Cross- |
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Thickness |
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sectional area |
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(mm) |
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(mm) |
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(mm2) |
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Rectangular plate |
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90 |
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3 |
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Pipe |
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25 |
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2 |
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Zinc galvanized |
Round steel |
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16 |
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Round steel |
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10 |
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Steel |
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(horizontal) |
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With lead sheath |
Round steel |
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8 |
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(horizontal) |
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With copper |
Round steel |
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15 |
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sheath |
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Rectangular plate |
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50 |
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2 |
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Bear |
Round shape |
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25 |
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(horizontal) |
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Copper |
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Pipe |
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20 |
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2 |
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Zinc galvanized |
Rectangular plate |
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50 |
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2 |
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With copper |
Round shape |
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25 |
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sheath |
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Article 573. Temperature Rise Requirements
Grounding conductor shall enable earth fault current to flow throughsafelyit. Especially, at connection point of grounding conductor, its thermalcharacteristic as well as its mechanical strength shall be considered.
Connection point of grounding conductor shall withstand temperature rise due to earth fault current without any problem.
Fastening bolt, pressure bonding and welding can be adopted as connection method of grounding conductor.
However, soldering method cannot be adopted as connection method of grounding conductor because melting point of solder is about 180°C.
Article 574. Cross-section of Earthing Conductors at Isolated Neutral
Refer to Article 576.
In production workshop, steel bar used as main grounding conductor shall have cro-ssectional area not less than 100mm2. As for round steel bar, it shall have the same cross-sectional area.
Article 575. Cross-section of Earthing Conductors |
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Effective |
Earthed |
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Neutral |
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Regarding cross-sectional area of grounding conductor, the requirement in Article576 and the |
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requirements below shall be satisfied. |
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Permissible current of the grounding conductor shall not be lower than |
3 times |
of the |
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nominal current at the nearest fuse |
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- Permissible current of the grounding conductorshall not |
be lower than 3 |
times of |
the |
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maximum breaking current |
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And half of minimum cross-sectional area of the phase conductors |
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In addition to the above,n iorder to protect power gird by fast-operating |
circuit breaker, cross- |
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sectional area of grounding conductor shall be determined so that permissible current of the grounding |
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conductor is not lower than thetransient current multiplied by dispersion coefficient |
and reserve |
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factor. The reserve factor shall be 1.1. As for the dispersion coefficient, unlessotherwise specified, it |
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shall not be lower than 1.4 in case that the rated current of the circuit breaker is up to 100A, or shall not be lower than 1.25 in case that the rated current of the circuit breaker is more than 100A.
If above-mentioned requirements are not satisfied, additional measures shall be takenShortening. breaking time of earth fault by quick operating protective relay can be considered.
Chapter 6-9 Installation Method of Earthing Systems
Article 576. Earthing Electrodes and Earthing Rings
When earth fault current flows into grounding electrode, electric potential is distributed on the ground surface around the grounding electrode, and accordingly, the electric potential gradient is formed on the ground surface. Such electric potential gradient shall be reduced from the viewpoint of security for people and electrical equipment because excessive electric potential cause shock hazard.
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Since electric potential distribution and electric potential gradient vary depending on shape and arrangement of grounding electrode, these factors shall be consideredThe. following shapes of grounding electrode can be selected.
-Half-sphere
-Plate
-(Vertical) Rod
-(Horizontal) Bar
Article 577. Material of Electrodes
Grounding electrode shall satisfy the conditions below.
(1)Grounding electrode has sufficient conductivity not to cause potential difference in the grounding electrode.
(2)Grounding electrode does not melt down by earth fault current.
(3)Grounding electrode withstands erosion and corrosion.
Article 578. Voltage Leveling
Electric potential gradient in case of earth fault becomes bigger as earth fault current which flows into
the grounding electrode becomes biggerIn. order to reduce the electric potential gradient, the
following items are considered.
(1) around electrical equipment
Auxiliary grounding mesh is installed. The distance between conductors of the auxiliary grounding
mesh is smaller than that of main grounding mesh.
(2) around surrounding fence of substation
-the surrounding fence is installed at the location distant from the outer edge of main grounding mesh
-the grounding electrodes are buried deeply around the outer edge of main grounding mesh
Grounding equipment shall be installed according to the following requirements.
-Grounding conductors shall be installed at the depth of not less than 0.8m.
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One grounding ring shall be arranged horizontally,surrounding the |
grounded neutral of |
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transformer. |
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When grounding equipment is installed out of protective fence |
for electrical equipment, |
horizontal grounding electrodes shall be arranged around the grounding equipment at the depth of not less than 1m. In this case, grounding ring shall be in polygonal shape with obtuse angle.
When grounding equipment of industrial electrical equipment is connected to highvoltagegrounding system with effective grounded neutral, measures for voltage leveling around the industrial building or the area where industrial electrical equipment is installed shall be taken according to one of the following requirements.
-One grounding ring surrounding theindustrial building or the are where industrial electrical
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equipment is installed shall be arranged and this grounding ring shall be connected |
to the |
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grounding equipment of industrial electrical equipment. This grounding ring shall be buried at the |
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depth of 1m and 1m distant from the foundation of the building or perimeter of the area where the |
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industrial electrical equipment is installed. |
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Reinforcing bar in concrete foundation can be used as grounding conductor if its |
ground |
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resistance satisfies the required ground resistance. |
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Article 579. Arrangement of Earthing Electrodes
Grounding mesh is appropriate way to reduce ground resistance of substationGrounding. mesh is usually formed by connecting grounding electrodes and buried grounding conductors to each other. The reason is that ground resistance of grounding mesh is lower than that in case that grounding electrodes are not connected to each other.
On connecting grounding electrodes and grounding conductors to each other, the conditions below shall be considered.
(1)Reinforcing bars in buildings andfoundation block, water pipes, etc.with sufficient conductivity are connected to grounding mesh. However, in this case, measures to protect them from electrical corrosion shall be taken.
(2)Auxiliary grounding mesh is installed at the part with large electric potential gradient.
(3)Material with high resistance such as asphalt or gravel is laid on the ground surface to increase touch resistance between human foot and ground.
(4)Material to reduce ground resistance is put into the ground.
(5)If measures to reduce ground resistance cannot be taken, measures to prohibit access to such part shall be taken.
To form grounding mesh, the following manner is applied.
- Regarding arrangement of grounding electrodes lengthwise, the grounding electrodes are arranged between electrical equipment at the depth of not less than 0.8m. In this case, the grounding electrodes shall be more than 1.0m distant fromfoundation or base of the electrical equipment.
-Regarding arrangement of grounding electrodes crosswise, the grounding electrodesare arranged at convenient location for installation work between electrical equipment at the depth of not less than 0.8m.
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Grounding electrodes are arranged along the perimeter of the grounding mesh so that the |
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grounding electrodes form a closed ring. |
When grounding equipment of electrical equipmentwith voltage of more than1kV and effective grounded is installed at the areawith its resistivity of higher than 500Ωm all through the year, the measures below shall be considered.
-Grounding electrode is buried deeper if the soil resistivity decreases as the depth is deeper.
-The area where grounding electrodes is extended if the soil with low resistivity exists nearby.
-Material to reduce soil resistivity is used.
Article 580. Prohibited Usage of Soil as an Alternative to Neutral
The ground is not allowed to be used as alternative of phase conductor or neutral conductor because the current which flows through the ground cause electric corrosion to near metal structures in this case.
Article 581. Installation of Earthing Conductors
Grounding conductor and neutral conductor shall have sufficient cross-sectional area so that their mechanical strength is ensured and their temperature rise is restrained within permissible value. Therefore, cable sheath shall not be used foralternative of grounding conductor or neutral conductor. (Refer to Article 576)
All parts of metal structure inside a building shall be grounded in order to avoid potential difference inside the building.
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Article 582. Earthing Conductors on Effective Earthed Neutral
Generally, an insulated wire or a cable is used for neutral conductor of-voltagelow power line. However, if case or support structure of busbar is used as working conductor and/or protective neutral conductor, an insulated wire or a cable is not required for such conductor.
In production hall, normal environment, metal structure, pipe, enclosure and support structure of busbar are allowed to be used as working neutral conductor in the following cases.
-supply of single phase electricity to singe load with voltage up to 42V
-supply of single phase electricity to breaking coil of magnetic contactor or starter
-supply of single phase electricity to lighting equipment or single control circuit
Article 583. Mechanical and Chemical Protection of Earthing Conductors
Chemical erosion effect depends on soil structure and moisture content, acidity andconductivity of soil. Therefore, these factors shall bemeasured before selection of material of grounding electrode. And the cross-sectional area of grounding electrode shall be selected in consideration of adverse affect of chemical erosion.
Article 584. Signage and Paint for Earthing Conductors
Bare grounding conductors and structures of aboveground grounding devices shall be painted in violet
or black. The joint or branch points of grounding conductors shall be painted with two color lines, violet or black, which are 150mm far from each other.
Article 585. Connection of Earthing Conductors
The overlapped length of welded part of the grounding conductor at the connection point shall be more than or equal to2 times of width of connecting rectangularshaped conductor, or 6times of diameter of round shaped conductor.
On overhead power line, the connection method of neutral conductor shall be thesame as the abovementioned method. However, instead of the above-mentioned stipulation, the connection method of phase conductor can be applied to the connection of the neutral conductor.
In humid rooms or roomwith corrosive vapor or gas, grounding conductor shall be connected each other by welding. If connection by welding is impossible, grounding conductor can be connected with bolt. In this case, the connection part shall be painted with protectiveMoreover,paint. such connection part shall be located to make visual check done easily
When grounding conductor is connected to pipes, it should be connected at the terminattachedl to the pipes by welding. If welding cannot be applied, connection by band or bolt can be applied.Before connection, the contact surface of the pipe shall be cleaned.
When grounding conductor is connected to metal structures, it should be connected by welding.When grounding conductor is connected to electrical equipment or machines, it should be connected to them by bolt. If there is much vibration at the connection part, the measures to prevent the bolt from coming loose shall be taken.
If grounding conductor or protective neutral conductor is connected to movable equipment or needs to be removed sometimes, flexible conductor shall be applied.
Neutral point of transformer and generator, which is directly grounded through capacitive current compensator, shall be connected to main grounding system.
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