Overview Of Earth Resistance Measurement Methods

Earthing is used in various electrical system projects. The very concept of \"earthing\" is schematically considered by connecting a section of electrical circuit to the earth potential.

The earthing circuit contains a conductor and an electrode embedded deep in the ground. A traditional action in electrical practice is to measure the earthing resistance of networks that are just starting up and those that are already in operation. We describe how and in what way this important activity is carried out.

What is the article about:

  • What are the measurements for?
  • Overview of measuring methods
    • 3-point determination system
    • Measuring with \"62%\"
    • A simplified 2-point method
    • Accurate 4-point measurements
    • Measuring with a C-measure. A6415 (6410, 6412, 6415)
    • Instructions for measurement with the C.A6415
  • General recommendations for USG measurements
    • Option #1: single layer soil
    • Option #2: multi layer soil
  • Conclusions and useful video on the subject

What are the measurements for?

The ideal zero resistance in an earthing circuit achieves the following objectives:

  1. To avoid the occurrence of voltages on the case of processing machines.
  2. To obtain an effective reference potential of the electrical apparatus.
  3. To eliminate static currents completely.

Electrical experience shows that it is not possible to obtain a result under perfect zero.

Overview of earth resistance measurement methods

Perform the necessary measurements with an earthing bus resistance measuring device. These procedures are carried out according to a schedule approved by the management of the earth electrode

In any case the earth electrode gives a resistance of some kind.

  • the resistance of the electrode at the point of contact with the conductor bar;
  • the contact area between the earth electrode and the ground;
  • the ground structure giving a different resistance.

Practice in measuring the resistance of an earthing circuit indicates that the first two factors may well be neglected, but under logical conditions:

  1. The earth electrode is made of metal with high electrical conductivity.
  2. The body of the electrode prong is carefully cleaned and tightly planted in the ground.

There remains the third factor, the resistive surface of the ground. It is seen as the main calculation detail for measuring the resistance of the grounding circuit.

Calculated by the formula:

R = pL / A,

where: p is the resistivity of the ground, L is the conventional burial, A is the working area.

To keep home/apartment owners safe, all types of powerful home electrical equipment must be grounded:

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All types of household power-dependent equipment operated in apartments and homes must be connected to self-contained or public grounding systems

In order to connect electrical appliances to a grounding system, receptacles with grounding contacts equipped with either copper brackets extending outside the housing or a third hole

All kinds of refrigerating equipment (refrigerators, freezers, MBPs, electric stoves, washing machines

Connection to the earthing circuit must be made according to the scheme enclosed by the technical products manufacturer, using the means recommended by him

It is obligatory to make earthing of the whirlpool bath, as it is not necessary to use a grounding system. к. it uses electrical appliances

All types of network machines, from the home desktop computer to server cabinets, including electrical cabinets for fuse boxes and RCDs

All models of energy-dependent gas boilers should be grounded without fail: both floor-standing and wall-mounted

All grounding lines are laid in a parallel circuit,

Patterns for earthing contact

Socket outlet with earthing contact

Earthing of kitchen appliances

Power washing machine connected to earth

Earthing connection the grounding of the hot tub

Method for grounding the mains equipment

grounding the floor standing gas boiler

Wiring the grounding lines to the bus

Test the resistance for each of the grounding lines separately. The resistance between the grounding element and each non-conductive part of the electrical equipment that may be energized should be less than 0.1 ohm.

Overview of measuring methods

There are several ways of measuring the resistance of an earthing circuit and each of these allows a fairly accurate determination of the quantity in question.

3-point determination system

Such as the 3-point technique, based on the potential drop effect, is often used.

Overview of earth resistance measurement methods

The graphical scheme of the so-called 3-point system, which is quite often used when one wishes to measure the resistance value of an earthing circuit

Measurements are made in three basic steps:

  1. Measure the voltage at electrode E1 and probe E2.
  2. Measurement of the current at electrode E1 and probe E3.
  3. Calculation (formula R = E / I) of the resistance of the earth electrode.

For this technique the accuracy of the measurements is logically related to the installation site of probe E3. It is recommended to insert it into the ground at a distance, optimally beyond the so-called ESE (effective electrode resistance) area of E1 and E2.

Measurements according to \"62%\"

If the soil structure under the placing of the earth electrode is of homogeneous contents, the \"62%\" technique for determination of the earth loop resistances promises a good result.

The scheme under the interesting name \"62%\". However, the name is taken from the optimum value of the indentation between the electrodes at which an acceptable result is obtained

The method is applicable under schemes with a single grounding electrode. The accuracy of the readings here is due to the possibility to position the working probes  in a rectilinear area, with respect to the earthing electrode.

Positioning points of the work probes

Probe depth, mProbe distance E1, mProbe distance E2, м
2,415,2524, 4
5, 521,635,0
9,026,242, 65

Simplified two-point method

The application of this method of measurement requires another quality ground besides the one to be investigated. The technique is relevant in densely populated areas where it is often not possible to operate auxiliary working electrodes extensively.

Overview of earth resistance measurement methods

The simplified method of measurement is made by a two-point technique. This technique requires less manipulation of the equipment and calculations, but the accuracy of the calculations is low

The two-point measurement method is different in that it gives results simultaneously for two earthing devices connected in series. This explains the requirement for a high quality performance of the second earth electrode, so that its resistance is not taken into account.

The resistance of the earth rail is also measured to make the calculations. The result is subtracted from the total measurement results.

The accuracy of this method leaves much to be desired compared to the two above. The distance between the earth electrode whose resistance is measured and the second earth electrode plays a significant role here. This technique is not used as standard. It is a kind of alternative when other measuring methods cannot be used.

Four-point measurements

For most resistance measurements, the 4-point technique is considered the best method, in addition to 2 and 3-point measurements. Devices like the 4500 series tester have this measurement technology. Judging from the name of the method, four working electrodes are placed on the working platform in one line and at equal distances.

Overview of earth resistance measurement methods

The most accurate measurements are made under this scheme, the four-point. Modern apparatus is used and it is possible to work without disconnecting the earth circuit

The current generator of the instrument is connected to the outermost electrodes, as a result of which a current of known value flows between them. On the other terminals of the device the two internal working electrodes are connected.

The voltage drop value is present on these terminals. The final result of the measurement is the earth resistance (in ohms), the value of which is shown on the display.

The 4500 series instruments are often used to measure the touch voltage. A small voltage is generated in the ground by a special module, simulating a cable fault.

The current flowing in the ground circuit is indicated at the same time on the scale of the device. The readings on the screen are taken as a basis and multiplied by the assumed value of the current in the ground. In this way the touching voltage is calculated.

Overview of earth resistance measurement methods

Performing measures to monitor the condition of electrical equipment and earthing lines. A Type 4500

For example, the maximum value of expected current in the fault area is 4000A. A value of 0,100 is marked on the screen of the meter. Then the magnitude of the touch voltages is 400V (4000*0.100).

Measurements with the C.A6415 (6410, 6412, 6415)

What is unique about this method is that measurements can be made without disconnecting the earth electrode. It is also to be pointed out here that it is preferable to measure the total resistance of an earthing device by including in the earthing circuit the resistive component of all connections.

The working principle is approximately as follows:

  1. A special transformer creates a current in the circuit.
  2. The current flows in the formed circuit.
  3. The measured signal is recorded with a synchronous detector.
  4. The received signal is converted by a digital converter.
  5. The result is displayed on the LCD.

The device is equipped with a module (selective amplifier) thanks to which the useful signal is efficiently cleared from all kinds of disturbances - both h and noise. In the articulated state the clamp paws form an excited circuit which covers the ground conductor.

Measuring instructions for the C.A6415

The operating sequence of the C.A6415 series is clearly described in the operating instructions supplied with this unique device.

Overview of earth resistance measurement methods

A unique clamp meter which makes it relatively simple and easy to measure the earth loop resistance under various conditions

It is necessary to measure the earth resistance of an electric module (transformer, electricity meter, etc.).

Following steps:

  1. Prevent access to the Earthing bar by removing the protective cover.
  2. Grab the earth electrode (bar or electrode directly) with the pliers.
  3. Select measurement mode \"A\" (current measurement).

The maximum current value of the unit is 30A, therefore if this number is exceeded the measurement cannot be carried out. Remove the device and try to measure again at a different point.

Overview of earth resistance measurement methods

The process of measurement with the measurement devices type C.A6415 and 3770. The measurement results are recorded in the table and compared at the next TOU

When the current value obtained on the scale is within the permissible range, the operation can be continued by switching the instrument to the resistance measurement \"?\".

The result displayed will show the total resistance value, including:

  • electrode and grounding bar;
  • neutral to grounding electrode contact;
  • connection contact on line between the neutral and the grounding electrode.

When working with the pliers, keep in mind: overstated ground resistance readings are usually due to poor contact of the ground electrode with the ground.

A high resistance can also be caused by a broken conductor bus. High resistance figures at the connection points (splices) of the conductors can also affect the readings of the device.

General recommendations for measuring the USG

Before constructing a grounding circuit, for example for a gas boiler, it is necessary to get precise information about the area in which the ground electrode will be placed. It is often suggested to refer to existing tables to determine the \"p\" values of the ground.

But this version with tables gives purely indicative data. Therefore, it is not worth relying on them. The true resistivity values of the ground can vary several times.

Option #1: homogeneous soil

If the ground has a homogeneous component, its resistivity is measured by the \"test electrode\" technique.

Overview of earth resistance measurement methods

Structure of homogeneous soil. Under such conditions it is much easier to measure and calculate resistance than to do the same work on layered soils

The method involves a certain procedure in two steps:

  1. Take a rod test probe slightly longer than the depth of the design deposit.
  2. Submerses the probe vertically in the ground strictly to the depth of the design embedment.
  3. The end remaining above the ground is used to measure the Rr resistance.
  4. Determines the USG according to the formula p = Rr * Ψ.

It is advisable to perform the procedure several times at different points in the working area. Alternative measurements help to achieve accurate ground resistivity measurements.

Option #2: multi-layer ground

In this situation the USG measurement is carried out by a step sensing method. That is, the test probe is immersed to the working depth in steps and resistivity measurements are taken at the position of each step. The calculation of the average resistivity is done by means of formulas for each individual measurement.

Overview of earth resistance measurement methods

Multilayer soil. Under such conditions it is necessary to calculate the resistance of each individual layer. Calculations on multilayered soils require more work

Then, based on the climatic conditions of the area, find values for seasonal changes. In this way (rather complicated) the calculated USG values of the upper layers are obtained. The underlying layers are considered as not subject to seasonal variations and, therefore, the calculation for them is limited to a somewhat simplified measurement and calculation.

Work execution requirements

Works of this kind are of course carried out by qualified personnel representing specialized organizations. For example, for the operation of power panels in residential buildings, as a rule, are responsible utilities. It is only permitted to make any measurements at these points by contacting these services.

Electrical circuits are hazardous systems. Although domestic circuits are rated for voltages less than 1000 volts, these voltages are lethal to humans. All necessary safety precautions must be observed when handling electrical equipment. The average person is often unaware of these measures.

In the following article, which contains rules and guidelines for the work, you will find out how to install a grounding system for a bathtub in a city apartment.

Conclusions and useful video on the subject

Practical measurements with the instrument:

The performance of work related to the earth resistance test is required, regardless of the complexity of the electrical circuit and the category of the facility where electrical equipment is installed or installed and operated. Many specialized organizations are willing to provide such services.

Please leave comments in the box below. It is possible that you know a simple and effective way to measure the resistance of ground loops that is not listed in the article. Ask questions, share useful information and pictures on the subject.

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