PART 6 EARTHING

Chapter 6-1 Purpose of earthing

Article 546. Applicable Scope

The regulation in this part stipulates the requirements on grounding system of AC power grid and electrical equipment. Grounding system of DC power grid and electrical equipment shall be designed on a case-by-case basis.

Article 547. Need for Earthing Systems

The requirements on performance of grounding system are as follows.

1.Agrounding conductor enables earth fault current to pass through it without any trouble about its limit on temperature rise and mechanical strength.

2.Agrounding conductor can bear mechanical force and possess a resistance to corrosion.

3.Agrounding system has to restrain temperature rise, voltage potential difference and fault current passing through the grounding system to protect electrical equipment from damage.

4.Agrounding system has to restrain step voltage, touch voltage and transition voltage potential within their permissible values.

The performance of a grounding system about the above-mentioned issues is dependent on the grounding method of the neutral point and amplitude of earth fault current and its duration. And a grounding system shall ensure safety for persons including the public.

The specification of a grounding system such as size of grounding conductor, arrangement of grounding electrodes and etc. is decided based on the conditions below.

1.Amplitude of earth fault current

2.Duration of earth fault

3.Soil characteristic

Article 548. Common Earthing Systems

If high-voltage grounding system is located adjacent to low-voltage grounding system, potential of the low-voltage grounding system increase according to the potential rise of the high-voltage grounding system in case of earth fault in the high-voltage grounding system. The measures to avoid such potential rise of the low-voltage grounding system are as follows.

- All high-voltage grounding systems and all low-voltage grounding system are connected mutually.

electrical equipment in low-voltage grounding system shall be restrained within permissible values respectively.

[Low-voltage electrical equipment located in a high-voltage substation]

If low-voltage grounding system is surrounded by only high-voltage grounding system, both of the grounding systems shall be connected electrically.

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[Low-voltage electrical equipment connected to a high-voltage substation]

If low-voltage electrical equipment installed in the high-voltage substation supplies electricity to outside the high-voltage substation, the connection of both of the grounding systems shall obey the requirements shown in the following table.

Table 548 Minimum requirements for interconnection of low-voltage and high-voltage grounding systems

tF: duration from outbreak of earth fault to its interruption UT: permissible touch voltage shown in Figure 548

EPR: Electrical Potential Rise

Figure 548 Permissible touch voltage UT

X is usually selected as 2. If neutral conductor of low-voltage grounding system is grounded at only high-voltage grounding system, X is selected as 1. If neutral conductor of-vloltagew grounding system is grounded at several points in addition to high-voltage grounding system, X is selected as from 2 to 5 according to soil characteristic such as soil resistivity.

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If low-voltage grounding system is TT method, voltage applied to low-voltage electrical equipment shall be considered mainly. If low-voltage grounding system is TN method, touch voltage and transition voltage shall be considered mainly. If low-voltage grounding system is IT method, touch voltage and voltage applied to low-voltage electrical equipment in case of short-circuit between highvoltage electrical equipment and low-voltage electrical equipment.

Chapter 6-2 Components to be Earthed in Power Networks

Article 549. Earthing Method of Neutral of Overhead Lines with voltage up to 1kV

In low-voltage power system (up to 1kV) connected to high-voltage power system, touch voltage shall be restrained within the permissible valuewhich is stipulated at Article 567. In addition to this, earth

The above-mentioned permissible earth potential rise on low-voltage grounding system is as follows.

-Impressed voltage: In case that faulttime is not more than 5 seconds, EPR=<1,200V. In case that fault time is more than 5 seconds, EPR=<250V.

-Touch voltage: EPR=< X ×UT

X can be selected from 1 to 5 according to neutral grounding method and soil characteristic such as soil structure as follows.

- Neutral conductor or protective neutral conductor is grounded at only-voltagehigh grounding system : X=1

-Other than the above : X=2

-Neutral conductor or protective neutral conductor is grounded additionally : X is from 2 to 5. UT can be obtained from Figure 548.

Figure 549 shows the case that neutral conductor is grounded additionallyIn. such case, X can be selected as larger value.

In this regard, if grounding resistance of low-voltage grounding system is difficult to obey the abovementioned grounding resistance by grounding the neural conductor independently due to soil characteristic, the neutral conductor shall be grounded repeatedlyThe. grounding conductor shall be

put into an electric conduitwhich has weather resistance and self extinguishment performance from 75cm underground to 2m above ground from the viewpoint of avoidance of damage and public safety.

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Fault location

Difference of grounding potential is low: X ≤ 5

Difference of grounding potential is high: X < 5

Grounding resistance: High

Grounding resistance: Low

Figure 549 Grounding method of neutral conductor

Article 550. Earthing of Power Cable Lines

shall not be less than 6mm2.

Metal parts and steel bars of power cableinesl and other cable structures shall be connected each other and connected to the covers of their connection boxes by flexible copper conductors.

Metallic enclosures and covers of the connection boxes or lightning arresters on poles shall be connected to grounding system of the lightning arresters. Metallic covers of cables are prohibited to

be used as grounding conductors.

Article 551. Earthing for Oil-Filled Cable Lines

As stipulated in Technical Regulation.

Article 552. Earthing Neutral Conductors and Poles

Neutral conductors of overhead power lines with voltage up to1kV shall be grounded at intervals from 200m to 250m.

Cross-sectional area of grounding conductors shall comply with the requirements in Chapter86inTechnical Regulation and Guideline, moreover, diameter of the grounding conductor shall not be less than 6mm.

Ground resistance of ground conductors shall comply with the requirements in Technical Regulation and Guideline, furthermore, the ground resistance of the grounding conductor shall not be more than 50 ohm.

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Article 553. Additional Earthing Requirements

Ground resistance of grounding conductor of overhead lines with voltage up to 1kV shall comply with the requirements in Technical RegulationVol.1 and its Guideline, moreover, the ground resistance shall not be more than 30 ohm.

The distance between each grounded point shall not be more than the following values.

-200m: for the area where thunderstorm occurs not more than 40 hours in a year.

-100m: for the area where thunderstorm occurs more than 40 hours in a year.

The distance from grounded points of the end poles to grounded points of overhead power lines shall not be more than the following values.

-100m: for the area where thunderstorm occurs from 10 to 40 hours in a year

-50m: for the area where thunderstorm occurs more than 40 hours in a year The place to be grounded

-The building where many people may gather such as schools, theaters, hospitals, etc.

- The places with high economic values such as animal breeding facilities, warehouses, workshop, etc.

Article 554. Earthing of Overhead Power Lines with Voltage Exceeding 1kV

1. Ground resistance of pole

(1)The ground resistance of polewith lightning conductors or lightning arresters, etc. shall not be more than the value shown in the following table.

(2)The ground resistance of pole of overhead power line with voltage from 6kV to22kV in populated area and overhead power line with35kV shall not be more than the values shown inthe following table.

Table 554-1 Maximum ground resistance of pole

(3)The ground resistance of pole of overhead power line with voltage from 6kV22kVto in nonpopulated area shall not be more than the values shown in the following table.

Table 554-2 Maximum ground resistance of pole

(4)The ground resistance of pole with main transformers, potential transformers,disconnectors, fuses or other devices shall comply with the following requirements.

-The ground resistance of pole of overhead power line with voltage from 6kV35kVto

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accompanied with large earth fault current and overhead power line with voltage from 110kV to 500kV shall not be more than the values shown in Table 554-1.

-The ground resistance of pole of overhead power line with voltage from 6kV35kVto accompanied with low earth fault current shall complywith the requirement in article552 and 568 in Technical RegulationVol.1 and its Guideline.

2.Utilization of reinforced concrete foundation

For overhead power line across the area where resistivity of soil is not more thanΩm500,the reinforcing steel buried in reinforced concretefoundation are permitted to be utilized as neutral ground or to be combined with artificial grounding equipment. However, in case of the area where the resistivity of soil exceeds 500Ωm , the reinforcing steels in the reinforced concrete foundation are prohibited to be utilized as natural grounding conductor, and the ground resistance shall not be more than the value shown in Table 554-1 by installing artificial grounding equipment.

If reinforced concrete foundation is utilized as natural grounding conductor (except the requirement in article 290), it shall comply with the following requirements.

-The foundation shall not be covered with non-conductive paint, etc.

-Metallic connectors shall be applied to connect anchor bolts to foundation framesThe. electrical conductivity of reinforced concrete foundations shall be measured in two months after the foundations are constructed.

3.Utilization of reinforced pole

The reinforcing steels are permitted to be utilized as grounding conductors, which are connected to each other or grounding equipment physically and electrically.

4. Grounding for pedestal and lightning conductor

The pedestal for fixing insulators to arm bars of pole and lightning conductor shall be connected to grounding conductors or reinforced concrete foundations.

5. Grounding conductor for overhead power line with voltage exceeding 1kV

Cross-sectional area of grounding conductors on pole of overhead power line with voltage exceeding 1kV shall not be less than 35mm2. Single-core conductors with diameter not less than 10mm and galvanized single-core steel conductor with diameter not less than 6mm are also permitted to be used. Grounding conductor on reinforced concrete and metal pole shall be connected by welding, bolting or pressure bonding.

Article 555. Earthing in Crossing Spans

The ground resistances of metallic pipelines, bridges, nets, fences, aerial transport cable lines and wires hanging traffic signals in the span where overhead power lines cross shall not be more than 10 ohm.

Article 556. Earthing of Metal Objects near Overhead Power Lines

1.Scope of ground

The scope of ground for metal objects below overhead power line shall complywith the following requirements.

-220kV: The scope is the width space within 25m along the overhead power line measured horizontally from the outer edge of conductor.

-500kV: The scope is the widthspace within 60m along the overhead power line measured horizontally from the outer edge of conductor.

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