Main protective relay shall be a ground directional relay in order to enable selective trip for earth fault. Since operating sensitivity of the ground directionalrelay is maybe insufficient for earth fault with large ground resistance at the fault point, ground over-current relay shall be equipped on the power line as a back-up relay

Article 563. Special Protection

a) Regarding important load which is considered not to be interrupted in case of earth fault, the power line with isolated neutral point connected to an insulation transformer should supply electric power to such load. In such power supply method, the interruption of load in case of earth fault is not necessary in accordance with the item b) of Article564. However, in such power supply method, the alarm signal in case of earth fault shall be transmitted to dispatching control center.

b) In low-voltage power line with directly grounded neutral point, earth faultntcurreshall be interrupted quickly in order to avoid adverse affect on communication lines caused by the earth fault current.

c) If ground resistance of a portable electric tool does not satisfy the required value, such portable electric tool shall be used onan insulated base or double insulation shall be applied to such electric tool.

Article 564. Breaking Time in the event of Earth Faults

In grounding system, step voltage, touch voltage and transition voltage shall be restrained under the permissible values based on duration of interruption of earth faultIn. this case, this duration is summation of the operating time of main protective relay and operating time of circuit breaker.

On the other hand, regarding the voltage applied to electrical equipment in case of earthfault, the duration of interruption of the earth fault is summation of operating time of backupprotective relay and operating time of circuit breaker.

Regarding the earth fault to be considered in the above cases, refer to Article 574.

Chapter 6-6 Earth Resistance Requirements of Earthing System

Article 565. Earth Resistivity Measurement

Soil characteristic shall be investigated through geological survey.

Because measured soil resistivity varies according to time of measurement, measurement of soil resistivity shall be done at several times and at several positions in a substation

Generally speaking, soil resistivity increase when weight of moisture content .in the soil exceeds 22% of weight of the soil. Therefore, if moisture content in the soil varied excessivelya insubstation,

grounding conductor shall be buried more deeply to avoid the affect of variation of the moisture content.

If grounding electrodes need to be buried deeply to obtain required grounding resistance, soil resistivity at several points in vertical direction shall be measured. This measurement of soil resistivity

is carried out in the following way. The distance between measurement points is about from 0.2m to 0.5m.

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As mentioned above, soil resistivity varies according to the moisture content einsoilth and the temperature of the soil. In this regard, soil resistivity can be considered to vary all through the year. Therefore, the soil resistivity for design of grounding system should be the highest among the measured values all through the year.

Article 566. Assurance of Security

Voltage of grounding equipmentshall not be higher than10kV in case that earth-fault current flows through the grounding equipment. If power transmission from grounding equipment to outside of building and fencessurrounding the electrical equipment doesn’t occur, voltage of grounding equipment is allowed to be higher than10kV. If the voltage of grounding equipment is higher than 5kV, measures to protect insulation of communication cable lines and remote controlling system from electrical equipment shall be arranged. Additionally, the measures on prevention of transmitting dangerous voltage to outside of boundary of electrical equipment shall be also available.

When a low-voltage grounding system is adjacent to a high-voltage grounding system, potential of the low-voltage grounding system increases excessively due to potential rise of -thevoltagehigh grounding system in case of earth fault in Init. such case, the voltage applied to the low-voltage electrical equipment shall not be morethan permissible applied voltage of the low-voltage electrical equipment.

(1) In case that low-voltage grounding system is surrounded by high-voltage grounding system

The low-voltage grounding system shall be connected to the voltagehighgrounding system completely.

(2)In case that low-voltage electrical equipment adjacent to high-voltage grounding system supply electricity outside the high-voltage grounding system (for example, low-voltage distribution equipment installed in high-voltage substation)

In order to check whether the low-voltage grounding system and the high-voltage grounding system can be connected to each other, the voltage applied to the low-voltage electrical equipment shall be considered based on the permissible values stipulated in Article552. According to grounding method of the low-voltage electrical equipment, items shown below shall be considered.

- In case that low-voltage grounding system is TT method

In TT method, touch voltage is not necessary considered, because grounding of low-voltage electrical equipment is separated from high-voltage electrical equipment and accordingly potential rise of highvoltage electrical equipment in case of earth fault notis applied. On the other hand, the voltage applied to the low-voltage electrical equipment shall satisfy the permissible value in Table 552.

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When the voltage applied to the low-voltage electrical equipment during interrupting time of earth fault satisfy the requirements for this low-voltage electrical equipment regarding the applied voltage, the neutral conductor of low-voltage electrical equipment can be connected to the -highvoltage grounding system. (See Figure 570-1)

Figure 566-1 Grounding method of neutral conductor of low-voltage electrical equipment

However, when the above-mentioned condition is not satisfied, the neural wire shall be grounded through grounding electrode isolated from high-voltage grounding system. (See Figure 570-2)

Figure 566-2 Grounding method of neutral conductor of low-voltage electrical equipment

- In case that low-voltage grounding system is TN method

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Touch voltage shall satisfy permissible value stipulated in Table552. Usually, the value, X, is selected as 2. When the neutral conductor of the low-voltage electrical equipment is grounded at only high-voltage grounding system, X is selected as 1. When the neutral conductor of the -low voltage electrical equipment is grounded at several points in addition to the high-voltage grounding system, X can be selected as from 2 to 5, which is dependent on the soil condition in the substation. UT shall be decided based on Figure. The figure below indicates the case that the neutral conductor

is grounded at only high-voltage grounding system. In this case, X is selected as 1.

In addition to the above requirement, when the transition voltage from the high-voltage electrical equipment, which is applied to the enclosure of low-voltage electrical equipment, is interrupted within the time shown in Figure572-3, the neutral conductor can be connected to the highvoltagegrounding system.

Time

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AC voltage (root-mean-

Figure 566-3 Grounding method of neutral conductor of low-voltage electrical equipment

However, when the above-mentioned condition is not satisfied, the neural wire shall be grounded through grounding electrode isolated from high-voltage grounding system. (See Figure 566-4)

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Figure 566-4 Grounding method of neutral conductor of low-voltage electrical equipment

- In case that low-voltage grounding system is IT method

Regarding touch voltage, when the protective conductor is installed, the same requirements as TN method shall be satisfied. Otherwise, touch voltage is not necessarily considered.

In addition to the above, the voltage applied to the -lowvoltage electrical equipment in case of short-circuit between the low-voltage electrical equipment and the -voltagehigh electrical equipment shall be considered and restrained under the permissible voltage of the-voltagelow electrical equipment, because, in such case, high voltage is applied to the -lowvoltage electrical equipment directly.

- Protection of communication cable

Insulation protection measures for communication cable lines and remote controlling system from electrical equipment must be arranged. Additionally, the measures on prevention of transmitting dangerous voltage to outside of boundary of electrical equipment must be available too.

Article 567. Touch and Step Voltages

Permissible values of touch voltage and step voltage can be calculated by the following equationsIn. these equations, a body weight is estimated at 50kg.

Step voltage

Estep = (RB + 2Rf ) IB

For body weight of 50kg

Estep50 = (1000 + 6Cs ρs )0.116

ts

For body weight of 70kg

Estep70 = (1000 + 6Cs ρs )0.157

ts

Where

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RB: Resistance of human body in Ω

Rf: Ground resistance of one foot (with presence of the substation grounding system ignored) in

Ω

IB: Rms magnitude of the current through body in A Cs: surface layer derating factor

Ts: duration of shock current in seconds

Cs is calculated by the following equation.

Cs =1− 0.09 1− ρρs

2hs + 0.09

Where

ρ: surface material resistivity in Ωm

ρs : resistivity of earth beneath surface material in Ωm

hs: thickness of surface material in m

For body weight of 50kg

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Figure 567 Touch voltage and step voltage

Article 568. Resistance of Grounding System

Grounding system of substation and electrical equipment with voltage of more than 1kV or equal shall be designed in accordance with the following requirements

Touch voltage and step voltage shall be calculated based on the specifications of actual grounding system. These calculation results shall not be exceed the permissible touch voltage and step voltage.

The common procedure is as follows:

(1) Determine the soil model and the grounding grid area

Soil model at construction sector is determined based on the results of the geological survey in the area. Then the grounding grid areais determined taking into account the layout of electrical equipment in substations and other conditions

(2) Calculate the resistivity of the soil

Since the resistivity of the soil is essential for the calculation of the touch voltage and step voltage as well as the resistance of the soil by the following equations in the model of land has been defined in clause (1).

Soil resistivity model and the two-layer soil model is the most common use. Two-layer soil model is often a good approximate estimate of soil structure is the multi-layer soil model can be used for more complex soil conditions.

Apparent resistivity of a homogeneous soil can be calculated from the following equation

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In that:

ρa(1) + ρa(2) + ρa( ) + + ρa(n): Apparent resistivity measured at different intervals in

the four-pin plug or method at different depths in place spindles on the ground method, in Ωm

n: total number of measurements

Apparent resistivity of the two soil layers is calculated from the following equation::

In that:

h: The first layer height

ρ1 : The first layer resistivity ρ2 : Deep layer resistivity

K= ((ρ2 − ρ1 )): Reflection coefficient

ρ2 + ρ1

Figure 568-1 Two-layer land

(3) The calculation of the ground wire

The temperature increase briefly in the grounding wire or the wire size required is a function of the current in the wire, can be calculated from the following equation:

In that:

I: Effective 1-phase short-circuit current (rms), kA

Amm2: wire cross section,mm2

Tm: temperature allows the largest,

Ta: ambient temperature,

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Tr: the reference temperature for the material constants,

α0 : Temperature coefficient of resistivity at 0°C, 1 /

αr : Temperature coefficient of resistivity at reference temperature Tr, 1 /

rr : The resistivity of the ground wire at the reference temperature Tr, K0: 1/α0 or (1/α0 )- Tr , o C

tc: the current time, s

TCAP: heat capacity / unit volume selected from Table 572, J / (cm3)

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The above equations reflect two basic assumptions:

a)Complete heat is retained in the wire (adiabatic process)

b)The results of the specific heat (SH) and specific gravity (SW), TCAP, is approximately constant for SH increase and SW decreased at speed nearly equal. For most materials, these assumptions can be applied on a wide range of reasonable time of the incident in a few of seconds

TCAP can be calculated for the whole materials not listed in Table572 from the specific heat and specific gravity. Specific heat, SH, cal /grams( ×o C ) and SW, gram/cm3 related to the heat

capacity per unit volume, J / ( cm3 ×o C ) as follows.

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na = 2LLc

p

nb = 1 for the square grid

nc = 1 for square, rectangles mesh

nd = 1 for squares, rectangles mesh and L-shaped

mesh with other pictures

0.7 A

nc = Lx ALy Lx Ly

Lc: the total length of wire in the horizontal grid, m Lp: length of the perimeter of the grid, m

A: area of the grid, m2

Lx: the greatest length of the grid in the X direction, m Ly: the greatest length of the grid in the Y direction, m

Dm: the greatest distance between any two points on the grid, m D: distance between two parallel conductors, m

h: depth of the wires of the grid, m d: diameter of the wires of the grid, m

Not rule coefficients, Ki, is used along with the number n above is:

Ki = 0.644 + 0.148 n

For grid no ground piles ornetwork only a little ground piles scattered in the grid, but not in the corners or along the perimeter of the grid, the effective buried length, LM, is

LM=LC+LR in that

LR: total length of all ground piles, m

For ground grid with piles arranged at the corners, as well as along the perimeter and throughout the grid, length buried utility, LM, is

in that

Lr: length of each piles, m

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Calculated Voltage step is the product of the geometric coefficient, Ks, the adjustment coefficient, Ki, ground resistivity, and the average current per unit length of wire buried grounding system (IG / LS).

IG = SfDfCpInm

In which:

For grids with or without ground piles <useful length of the wire, LS, is

LS = 0.75 LC + 0.85 LR

Largest voltage step is assumed to occur over a distance of 1 m, starting at and stretching out wire perimeter at the corner divided by 2 of the most far corner of the net. For the buried length is 0.25m <h <2.5 m, Ks, is

(6) Evaluation of the touch voltage and step voltage was calculated

Results calculated touch voltage and step voltage are compared with their permitted values.

Whether the principle of the above, the resistance of the earthing device must not be higher than 0.5 in the area of ground resistivity of not more than 500 throughout the year.This requirement does not apply to equipment grounding pole of the OPL and substations with voltage of 35kV or less.

For substations with voltage of 35kV or less, in measured ground resistance throughout the year must be calculated by the following formula, but not higher than 10:

a) If the ground is used for both high-voltage electrical equipment with more or less than 1kV

Rnd = 125

Icd

In this case, to satisfy the requirements for grounding electrical equipment with voltage up to 1kV. b) If the device is only used for ground electrical equipment with voltages from 1kV or more

Rnd = 250

Icd

In it,

Rnd: The largest ground Resistor when taking into account the change in resistivity of the soil throughout the year, Ω

ICD: Current earth fault calculations,A.

Grounding resistance of electric equipment with voltage less than or equal to1kV must comply with the following requirements.

Neutral ground resistance of the generator or transformer or power output of one phase in the whole year, not more than 2Ω or 4Ω, corresponding to voltage 660V or 380V (phase voltage is 380V or 220Vin the case of single-phase power supply).Resistor value also applies to many points for ground the neutral wire of OPL.

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