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How To Measure Ground Resistance?

Author: Apogeeweb Date: 2 Sep 2019  5386

Ground Resistance Testing

I Introduction

Ground resistance testing methods are usually the following: two-line method, three-line method, four-line method, single-clamp method and double-clamp method. Each has its own characteristics. In an actual test, we have to choose the correct testing method to make the test results accurate.

 

This article will mainly introduce several methods of ground resistance testing, including the principle of testing, how to use ground resistance tester, and so on.

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

Catalog

I Introduction

II What is Ground Resistance

III Voltmeter-Ammeter Method

IV Using a Ground Resistance Tester

 4.1 Introduction of Ground Resistance Tester

 4.2 How To Use The Ground Resistance Tester

 4.3 Precautions For Using The Ground Resistance Tester

V Two-line Method

VI Three-line Method

VII Four-line Method

VIII Single Clamp Measurement

IX Double Clamp Method

X One Earthing / Grounding Quiz Question

 10.1 Question

 10.2 Answer

XI FAQ

II What is Ground Resistance

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 a 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.

Ground Resistance Testing 

Figure1. Ground Resistance Testing

III Voltmeter-Ammeter Method

(1) Scope of application: suitable for measuring grounding devices less than 0.5 ohms.

 

(2) When a single ground electrode is used, the measured single ground electrode, current ground electrode, and voltage ground electrode should be arranged in a straight line of 20M-40M.

 

(3) When the grounding device is a grounding network, the measured grounding network G, current grounding electrode C, and voltage grounding electrode P should also be arranged in a straight line. The distance between the current ground electrode C and the edge of the measured ground grid G should be DGC = (4-5) D, and the distance between the measured ground grid G​and the voltage ground electrode P should be DGP = 90.5-0.618)

 

(4) D is the maximum diagonal length of the ground grid G to be measured. The voltage ground electrode P is placed in the actual zero potential areas of the current field in the ground. To find the actual zero potential areas of the current field in the ground, the voltage ground electrode P can be moved three times in the direction of the GC connection. The distance of each move is about 5% of the DGC. Measure the voltage between PG.

 

(5) If the error between the three times indicated by the voltmeter does not exceed 5%. The middle position can be used as the position of the voltage electrode for measurement.

 

(6) The quotient of the indicated value of the voltmeter and the indicated value of the ammeter is the ground resistance of the ground network G to be measured.

Measurement of Low Resistance 

Figure2. Measurement of Low Resistance

IV Using a Ground Resistance Tester

4.1 Introduction of Ground Resistance Tester

Ground Resistance Tester, also generally outputs an AC power supply with a no-load voltage of 6V, and an AC source with a constant current of 10A or 25A is added between the two points to be measured. The tester can test the voltage drop between the two points, and according to Ohm's law directly shows the resistance between the two points being measured.

 

4.2 How To Use The Ground Resistance Tester

(1) Preparation of using a ground resistance tester

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

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

3) To disconnect the grounding trunk from the grounding body connection point or the grounding mainline 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.

 

  • 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

Figure3. 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

The Wiring Method

Figure4. The Wiring Method

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

 

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 centerline), the cranks are swung to make the speed reach 120r/min or more,  and the "measuring scale" is adjusted to point the pointer to the centerline.

 

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.

 

4.3 Precautions For Using The Ground Resistance Tester

(1) When measuring the ground resistance with a ground resistance tester, the product manual requires the use of a 20-40 meter pole method. The ground resistance testers are equipped with 20M and 40M dedicated wires.

 

(2) In order to eliminate the influence of mutual resistance, the distance between the voltage ground electrode P and the current ground electrode C is not less than 20M. If the current ground electrode C is located away from the voltage ground electrode P, the current ground electrode C cannot be arranged.

 

(3) The current grounding electrode C and the voltage grounding electrode P can be arranged perpendicular to the ground grid G to be tested, or the current grounding electrode C and the voltage grounding electrode P, and the ground grid G to be tested is formed into a triangle, and each side is 20 meters in length.

 

(4) When the surrounding ground grid G is covered with asphalt or concrete pavement, two flat steel plates (250MM × 250MM) can be placed on the pavement and watered between them. The test clip is clamped on the steel plate. A cloth material that can hold water can also be placed on the road surface, and the cloth material with water surrounds the auxiliary ground electrode.

 

(5) It is also possible to pile sand and release water on the road surface, and the auxiliary ground electrode is placed in the sand puddle.

Ground Resistance Tester 

Figure5. Ground Resistance Tester

V Two-line Method

(1) Conditions

There must be a well-grounded ground, such as PEN. The measured result is the sum of the resistances of the measured and known grounds. If the ground is known to be much smaller than the resistance of the measured ground, the measurement result can be used as the result of the measured ground.

 

(2) Application

Areas with dense buildings or concrete floors cannot be used for ground piles.

 

(3) Wiring

E + ES is connected to the measured ground, H + S is connected to the known ground.

 

VI Three-line Method

(1) Conditions

There must be two ground rods: an auxiliary ground and a detection electrode. The distance between each ground electrode is not less than 20 meters.

(2) Principle

The current is added between the auxiliary ground and the measured ground to measure the voltage drop between the measured ground and the detection electrode. The measurement results include the resistance of the cable itself.

(3) Application

Grounding, grounding at construction sites and grounding lightning rod QPZ.

(4) Wiring

S is connected to the detection electrode, H is connected to the auxiliary ground, and E and ES are connected to the measured ground.

 

VII Four-line Method

The four-wire method is basically the same as the three-wire method. It replaces the three-line method when measuring the low ground resistance and eliminating the effect of the measurement cable resistance on the measurement result. E and ES must be connected directly to the ground to be measured separately. This method is the most accurate of all ground resistance measurement methods.

Ground Resistance Tester

Figure6. Ground Resistance Testing

VIII Single Clamp Measurement

(1) Conditions

Measure the ground resistance of each ground point in a multi-point ground. Do not disconnect the ground connection to prevent danger.

(2) Application

Multi-point grounding. Do not disconnect. Measure the resistance of each ground point.

(3) Wiring

Use a current clamp to monitor the current at the measured ground point.

Ground Resistance Clamp Testing on Tower Legs

Figure7. Ground Resistance Clamp Testing on Tower Legs

IX Double Clamp Method

(1) Conditions

Multi-point grounding without measuring auxiliary ground stakes, measuring single grounding.

(2) Wiring

Use the current clamp specified by the manufacturer to connect to the corresponding socket, and clamp the two clamps on the ground conductor. The distance between the two clamps should be greater than 0.25 meters.

 

X One Earthing / Grounding Quiz Question

10.1 Question

In which of the following system, identification of fault is tedious:

  1. Resistance grounding
  2. Solid grounding
  3. Reactance grounding
  4. Ungrounding

10.2 Answer

D

 

 

XI FAQ

1. What is ground resistance?

The resistance offered by the earth electrode to the flow of current into the ground is known as the earth resistance or resistance to earth. ... The resistance between the earthing plate and the ground is measured by the potential fall method.

 

2. What is the resistance of Earth?

The Earth Resistance profile varies between 10 Ohms and 20 Ohms. Soil identifications, as well as programmed intensive field measurements of soil resistivity and earthing system at selected sites, prove that soil resistivity values depend on the type of soil.

 

3. What is the resistance of a ground rod?

25 ohms

Nearly all electricians and electrical inspectors are familiar with the National Electrical Code requirement in Sec. 250-54, which requires the resistance to the ground of a single-made electrode (e.g., ground rod) to be 25 ohms or less.

 

4. How do you use a ground resistance test clamp?

Clamp the Ground Tester around the wire going to the grounding electrode connected to the GREEN wire. Record the reading. Remove the Ground Tester from the wire and re- clamp on the calibration loop. Re-verify the 5.0 reading to validate the ground rod reading.

 

5. What is a ground impedance test?

In order to simulate a single-phase fault, a test current is injected into one of your power lines by using a frequency variable current source and grounding the remote end of the line.

 

6. How do you check ground resistance with a multimeter?

The simple, but somewhat unreliable method uses a long wire and a DMM. Connect one end of the wire to a known, good earth contact (maybe next to the location where your fuse box is installed). Measure the resistance from the other end of the wire to the ground connector of the outlet/device to be tested.

 

7. How many ohms is the good ground?

5.0 ohms

Ideally, the ground should be of zero ohms resistance. There is not one standard ground resistance threshold that is recognized by all agencies. However, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less.

 

8. Can you test a ground rod with a multimeter?

With a multimeter, one can measure the resistance of the soil between a ground electrode and some reference point, such as the water pipe system, but a fault current may encounter a higher resistance.

 

9. Which is the easiest method for measurement of earthing resistance?

The dead earth method is the simplest way to obtain a ground resistance reading but is not as accurate as the three-point method and should only be used as a last resort, it is most effective for quickly testing the connections and conductors between connections points.

 

10. What is the difference between bonding and grounding?

By bonding normally non-current-carrying objects that are part of the electrical installation (such as metal conduit and enclosures) to the grounding system, it ensures that they cannot become energized.

 


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