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Sep 2 2019

The Analysis of Function and Measurement Method of Ground Resistance


This video will introduce the function of ground resistance and explain the importance of grounding, environment factors and testing

Introduction

Over time, corrosive soils with high humidity and salt content, as well as high temperatures, all of which can reduce grounding rods and their connections. Therefore, although the ground resistance of the grounding system is low at the initial installation, the resistance of the grounding system will increase if the grounding rod is corroded.

Ground tester is an indispensable troubleshooting tool to help you maintain uptime. It is recommended that all grounding and grounding connections be checked at least once a year as part of the normal predictive maintenance plan. If the resistance measured during these periodic inspections is increased by more than 20%, the technician should investigate the root causes of the problem and correct by replacing or adding the grounding system of the earth bar to reduce the resistance.

Article Core

Ground Resistance

Purpose

Introduce function and measurement method of Ground Resistance

Application

Semiconductor industry

Keywords

Resistor


Catalog

Introduction

I What is Ground Resistance

1.1 Definition

1.2 What is a Safe Ground Resistance Value

1.3 Specification requirements for standard grounding resistance

1.4 Measurement method of Ground Resistance

II Method for using ground resistance tester

III About the ground resistance value

3.1 Hazards of excessive resistance of ground resistance

3.2 Reasons for excessive resistance of transformer grounding resistor

3.3 Precautions to prevent excessive ground resistance


What is Ground Resistance

1.1 Definition

Ground resistance is the resistance encountered when the current flows from the grounding device to the earth and then flows through the earth to another grounding body or diffuses to a distance. The ground resistance value reflects the good degree of contact between the electrical device and the``ground'' and the scale of the grounding grid.

The ground resistance is an important parameter used to measure whether the ground state is good or not. It is the resistance that the current flows from the grounding device to the ground and then flows to the other earth body or to the far end. And it includes the resistance of the ground wire and the ground body itself, the contact resistance between the ground body and the earth's resistance, and the resistance of the earth between the two grounding bodies or the earth resistance of the earth body to the infinite distance. The size of the ground resistance directly reflects the good degree of contact between the electrical device and the "ground", and also reflects the scale of the grounding grid. The concept of ground resistance is only suitable for small grounding grid; However, with the increase of the land area of the grounding network and the decrease of the resistivity of the soil, the effect of the inductive component in the ground impedance is becoming larger and larger, and the large-scale ground network should be designed with the grounding impedance.

The effect of the grounding resistance is shown in: In the cable-powered system, the ground capacitance current is large. Arc grounding overvoltage will occur when the current is greater than the specified value. The purpose of using the neutral point resistance grounding method is to inject a resistive current into the fault point, so that the ground fault current has a resistance-capacitance property, reduce the phase difference with the voltage, and reduce the re-ignition rate after the zero-crossing of the fault point current. The overvoltage is limited to 2.6 times of the phase voltage, and the sensitivity of the relay protection is increased to trip, thereby effectively protecting the normal operation of the system.

Ground Resistance Testing 

1.2 What is a Safe Ground Resistance Value

The grounding resistance must be as small as possible, and the equipment requirements are different.

1) In a direct grounding system with a neutral point below 1000v, the grounding resistance is less than or equal to 4 ohms, and the repeated grounding resistance is less than or equal to 10 ohms.

2) In a neutral ungrounding system with a voltage below 1000V, the grounding resistance is generally 4 EU.

Therefore, according to the actual installation experience, the grounding resistance in street lamp lighting system should be less than or equal to 4 euros. Single point grounding usually uses the following style of instrument to test the grounding resistance:

3) Working grounding (zero protection), protecting grounding:

In 380 V 220 V low voltage system, the grounding current is very small and generally does not exceed a few amperes, so the specified connection resistance is not more than 4 ohms. When the capacity is below 100 KVA, the grounding resistance can also be relaxed to no more than 10 ohms.

4) Iterative earthing:

In the low-voltage power grid with direct grounding of the neutral point according to the relevant regulations, the zero line at the terminal of the main line and the branch line of the overhead line and the zero line at each kilometer along the line shall be grounded repeatedly, and each repeated grounding resistance shall not be more than 10 ohms; Where the working ground resistance is allowed to be 10 ohms, each repeated ground resistance should not be greater than 30 ohms, but the repeat ground should not be less than 3 ohms.

1.3 Specification requirements for standard grounding resistance

1) The independent lightning protection grounding resistance shall be less than or equal to 10 ohms;

2) The grounding resistance of independent safety protection should be less than or equal to 4 EU;

3) The grounding resistance of independent safety protection should be less than or equal to 4 EU;

4) The independent DC working grounding resistance should be less than or equal to 4 EU;

5) The anti-static grounding resistance is generally required to be less than or equal to 100 ohms.

6) The ground wire of the lightning rod belongs to the lightning protection grounding. If the grounding resistance of the lightning rod and the anti-static grounding resistance are set as required, the ground wire of the anti-static device can be connected with the ground wire of the lightning rod. Because the grounding resistance of the lightning rod is 10 times smaller than the static grounding resistance, most of the lightning will be released from the lightning rod when a lightning accident occurs, and the current passing through the anti-static ground can be ignored.

Grounding can be divided into three kinds of protection grounding: metal shell, concrete, pole, etc., of electrical equipment. Due to insulation damage may be charged, in order to prevent this situation from endangering personal safety, the grounding resistance is less than 1 Ω. The protective earth resistance of low-voltage electrical equipment is not more than 4 Ω. The ground resistance of high voltage protection with small grounding short circuit current (less than 500A) is not more than 10 Ω. High - voltage protective ground resistance of large earthing short - circuit current (above 500 A) is not greater than 0.5 Ω. The neutral ground resistance of transformer is not more than 4 Ω, and the repeated ground resistance is not more than 10 Ω. If the soil resistance is too high, the methods of external grounding, soil chemical treatment, changing soil quality, deep burying, prolonging the grounding body, and network ground device can be used to reduce the grounding resistance.

Anti-static grounding: to prevent the influence of static hazards, the grounding of fuel, natural gas storage tanks and pipelines, electronic equipment and the like can be prevented.

Lightning protection grounding: In order to introduce the lightning into the ground, the grounding end of the lightning protection equipment (lightning rod, etc.) shall be connected with the earth, so as to eliminate the grounding of the hazard of the lightning overvoltage to the electrical equipment and personal property, and also refer to the overvoltage protection grounding.

1.4 Measurement method of Ground Resistance

There are many factors affecting ground resistance: the size of grounding pole (length, thickness), shape, quantity, buried depth, surrounding geographical environment (such as flat ground, ditch, sloping land is different), soil moisture, texture and so on. In order to ensure the good grounding of the equipment, it is necessary to use the instrument to measure the ground resistance.

The measurement methods of grounding resistance can be divided into voltage and current meter method, ratio meter method and bridge method. According to the specific measuring instrument and the number of the pole, it can be divided into one hand-shaking table method, a clamp-shaped ground-resistance method, a voltage-current meter method, a three-pole method and a four-pole method.

When measuring the grounding resistance, some factors cause the grounding resistance to be inaccurate:

1) The soil composition around the ground network is inconsistent, the geology is different, the degree of dryness and humidity is different. And the stray current on the ground surface, especially the overhead ground wire, underground water pipe, cable skin and so on, has a great influence on the test. The solution is to take different points for measurement and finally take the average

2) The test line is not in the right direction and the distance is not long enough. The solution is to find the test direction and distance.

3) The auxiliary grounding pole resistance is too large. The solution is to find the direction and distance of the quasi-test.

4) The contact resistance of the test clip and the ground measurement point is too large. The solution is to polish the contact points with a trowel or sandpaper and fully clamp the polished contacts with the test leads.

5) Interference effect. The solution is to adjust the direction of the line and try to avoid the direction of the interference, so that the meter reading is reduced.

6) Meter usage issuesbattery power shortage. The solution is to replace the battery

7) Meter accuracy drops. The solution is to recalibrate to zero.

The accuracy of the test value of the ground resistance is one of the important factors to judge whether the ground is good. Once the test value is not accurate, it will not waste manpower and material resources (measured value is too large), or it will bring safety hazards to the grounded equipment (measured value is too small).

Ground Resistance Tester 


Method for using ground resistance tester

(1) Preparation of work using a ground resistance tester

1) To read the instructions of ground resistance tester and understand the structure, performance and application method of the instrument.

2) The tool and all instrument accessories necessary for the preparation and measurement shall be cleaned, and the instrument and ground probe shall be wiped clean, especially the grounding probe, and the dirt and rust stain on the surface of the instrument must be cleaned.

3) To disconnect the grounding trunk from the grounding body connection point or the grounding main line connection point, so that the grounding body is separated from any connection and becomes an independent body.

(2) Measurement steps for using grounding resistance tester

1) Two grounding probes are inserted into the ground at a distance of 20 m and 40 m respectively along the radiation direction of the grounding body, and the insertion depth is 400mm, as shown in the following figure.

2) The ground resistance tester is placed in the vicinity of the grounding body and the wiring is carried out. The wiring method is as follows:

  Actual operation of grounding resistance test

a) Actual Operation of Grounding Resistance Test

b) Equivalent Principle of Grounding Resistance Test

① The shortest special wire is used to connect the grounding body to the terminal of the grounding measuring instrument "E1" (the measuring instrument of the three-terminal button) or to the "C2" short-circuited common terminal (four-terminal knob meter).

② To connect the measuring probe (current probe) from the grounding body 40m to the measuring knob "C1" of the measuring instrument with the longest dedicated wire.

③ To Connect the measuring probe (potential probe) from the grounding body 20.m to the terminal "P1" of the measuring instrument with a dedicated wire centered on the remaining length

b)Equivalent principle of grounding resistance test

3) After the measuring instrument is placed horizontally, check that the pointer of the galvanometer points to the center line, otherwise, adjust the "zero-position regulator" to point the meter pointer to the center line.

4) To place the "magnification scale" (or the coarse adjustment knob) at the maximum, and slowly turn the generator stem (the pointer starts to shift) while rotating the "measuring scale" (or the fine adjustment knob) to point the galvanometer pointer to the centerline.

5) When the pointer of the galvanometer is close to the balance (the pointer is close to the center line), he crank is swung to make the speed reach 120r/min or more,  and the "measuring scale" is adjusted to point the pointer to the center line.

6) If the reading of the measuring dial is too small (less than 1), it is difficult to read accurately, which indicates that the multiplier scale multiple is too large. At this time, the "magnification scale" should be placed at a small multiple, and the "measuring scale" should be re-adjusted so that the pointer points to the center line and reads the exact reading.

7) The measurement results are calculated, i.e., R = magnification scale x the number of dial readings.


About the ground resistance value

3.1 Hazards of excessive resistance of ground resistance

(1) The grounding resistance value of transformer grounding wire is too large, if accompanied by low voltage phase line insulation damage and grounding (for example, L1 phase line grounding), then there will be current flow in transformer grounding line, and L1 phase voltage will be applied to the earth and ground resistance. The larger the grounding resistance value is, the greater the partial voltage on the ground resistance will be. At this time, if someone mistouches the transformer grounding wire or neutral wire and transformer shell, the human body will form parallel connection with the grounding resistance, then the voltage added to the human body will cause the person to be electrocuted.

(2) When the neutral line grounding resistance of the transformer three-phase four-wire is too large or the neutral line is broken, the neutral point of the transformer will be offset due to the imbalance of the three-phase load, and the grounding point potential is not zero. So that some of the phase voltage rises and burns the electrical equipment.

(3)  When the resistance of the grounding resistance is too large, the transformer batch arrester cannot discharge to the ground normally, causing the arrester or transformer to burn out.

Ground Resistance Tester 

3.2 Reasons for excessive resistance of transformer grounding resistor

 (1) The material of the grounding device is unqualified. Due to the non-standard embedding of the grounding body, the unqualified installation technology, the loose connection between the grounding body and the grounding wire joint, and the excessive dryness of the earth, it is possible to cause the grounding resistance value to be too large.

(2) Due to the insufficient recognition of the importance of the transformer grounding wire, the selection of the neutral wire cross-sectional area is too small, or due to the damage of the external force, the ground wire is stolen, etc., the grounding wire may be broken or the resistance of the grounding resistance is too large.

3.3 Precautions to prevent excessive ground resistance

 (1) Strict construction technology and standardize the embedding of grounding body. The ground device is generally made of steel pipe, angle steel, flat steel and steel strand. The buried depth should be not less than 0.5 ≤ 0.8 m. The construction of the ground device shall be carried out at the same time as the foundation construction, and the specific requirements are as follows.

① The depth of the grounding trough should meet the design requirements, generally 0.5 ≤ 0.8 m, and the arable land should be laid below the depth of cultivation. The width of the grounding slot is generally 0.3 m and 0.4 m, and all the sundries in the slot that affect the contact between the grounding body and the soil should be removed.

② The specification of the steel pipe and the depth of the pipe into the soil should meet the design requirements, and the grounding body should enter the ground vertically and fix it so as not to increase the street resistance value. In mountainous areas and areas with high soil resistivity, pipe-type earthing devices are used as little as possible, and surface-embedded earthing devices are used.

③ The grounding lead line should be laid along the pole and as short and straight as possible to reduce its impact reactance. The straight line distance between the supporting parts is usually 1.0 m 1.5 m, and the turning part is 0.1 m.

④ There shall be no other joint in the grounding lead line other than the break reserved for measuring the grounding resistance value. The connection of the ground device shall ensure the connection between the contacts, and all of which shall be welded. The connection between the grounding lead line and the broken place reserved for measuring the grounding resistance should be bolted, and the connecting bolt should be galvanized and rust-proof.

⑤ After the grounding body is laid, the backfill shall be filled, and the stone and the like shall not be embedded in the sundries in contact with the soil.

 (2) To select the appropriate position on the neutral line of the transformer, and repeat the grounding of the neutral line of the transformer at multiple points. When the neutral line of the transformer breaks at a certain point, because of the multi-point grounding, the neutral line current can still return to the neutral point of the transformer through the earth, the potential of the neutral line is always zero, and the voltage of each phase load is always the normal phase voltage.

 (3) To install a residual current action protector after the user's energy meter. When the user installs the protector, if the ground resistance value of the transformer junction is too large, the geodetic potential will no longer be zero, and then a current will pass through the protector and the earth will flow into the transformer connection site. This current will cause the protector to act and remove the contact point, thus preventing the increase of the geodetic potential. In addition, when the protector is installed, when the person touches the phase line, the protector will also act, so as to ensure the safety of the person.



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