Earth resistance testing must not be carried out on a live system. Ra is the resistance of the earth rod and the earthing conductor and can only be measured using earth electrode testing equipment with separate test electrodes. Getting down to earth 5 4 getting down to earth safety there is an inherent safety problem in earth resistance testing that requires care and planning by the user of the test set. Earth electrodes earth rods earth rod seal material. While test equipment is in the field for resistance measurements, it is convenient to also measure soil resistivity. The resistance measurement is made between the earth electrode and test spikes inserted into the ground. B the contact resistance of the surrounding earth to the electrode. Timeresistance tests on large rotating electrical machinery especially with high operating voltage require high insulation resistance ranges and a very constant test voltage. Det4 series 4terminal earthground resistance and soil. What is earth electrode resistance test the earth images.
Bureau of standards now nist developed the theory behind this test in 1915 see reference pg. Ground resistance tester ip67 protected ft603103 hioki. If this ground electrode is in parallel or series with other ground rods, the re value is the total value of all resistances. During a periodic inspection of an existing earth electrode, the situation is less certain as there are unlikely to be details of its construction or its buried location. This is the actual value of the ground electrode under test. Another way of thinking about the earth shells is as parallel resistances. This is known as the fall of potential method and it can be seen in figure 3 below that most of the voltage is dropped around the electrodes where, as outlined above, the soil layers are.
The test instrument is connected to the earth grid under test, and to a remote temporary electrode, and also to a second, intermediate temporary electrode. If the current and potential probes both are far enough away from the electrode, that is, outside the influence of the electrode, the reading corresponds to the resistance of the electrode to remote earth. It was approved on behalf of the council of standards australia on 26 october 2007 and on behalf of the council of standards new zealand on 23 october 2007. Normally, the outer test electrode, or current test stake, is driven into the ground 30 to 50 metres away from the earth system, although this distance will depend on the size of the system being tested refer to table 1 and the inner electrode, or voltage test stake, is then driven into the ground midway between the earth electrode and the current test stake, and in a direct line between them.
This method is referenced in all the electrical installation testing standards and can be used to measure the earth resistance both accurately and safely. Earth resistance measurements with 4, 3 and 2 wires earth. The ft603103 is a ip67 dustproof and waterproof two or threepole ground resistance tester with large, easytoread display, highly sensitive earthing rods, and innovative cable winder that significantly reduces the amount of time needed to complete. Earth electrode conductive part, which may be embedded in the soil or in a specific conductive medium, e. For land electrodes, the electrode site occupies a large enough, flat land where underground water is abundant and the resistivity of surface soil is preferably less than 100.
Numerically, it is equal to the ratio of the potential of the earth electrode to the current dissipated by it. How to determine the resistance of earth electrode. 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. Designing for a low resistance earth interface grounding roy b. It is the purpose of this guide to describe and discuss the present state of the technique of measuring ground resistance and impedance, earth resistivity, and potential gradients resulting from currents in the earth and the prediction of the magnitudes of ground resistance and potential gradients from scale model tests. Earthing or grounding of electrical equipment is essential to maintain safety, protecting lives, as well as prevent damage to equipment. Ieee recommended guide for measuring ground resistance and. Second, the tester is connected to the earth electrode.
R is the earth electrode resistance reading in ohms. Using a proprietary earth electrode test instrument the earth electrode resistance test is conducted as follows. Earth electrode and earth loop impedance testing theory and applications as two of the most common testing subjects debated amongst electricians as well as generating the highest percentage of calls to the megger technical support desk this series of presentations and supporting booklet hope to clarify some of the obstacles faced by an. The goal in precisely measuring the resistance to ground is to place the auxiliary current electrode z far enough from the ground electrode under test so that the auxiliary potential electrode y will be outside of the effective resistance areas of both. The earth electrode resistance test is used to confirm that the earth electrode provides a satisfactory path to earth. The most commonly used method of measuring the earth resistance of an earth electrode is the 3point measuring technique shown in figure 1. Checking ground electrode impedance for commercial. The earth electrode under test should be isolated from the circuit it is protecting before testing commences. This joint australiannew zealand standard was prepared by joint technical committee el001, wiring rules. A practical guide to earth resistance testing weschler instruments. The resistance of earth electrode depen ds on the earth surrounding i t.
Multiple, user selectable test frequencies resistance measurement range to. Using ohms law v ir, the tester automatically calculates the resistance of the earth electrode. This instrument makes earth resistance measurements with fallofpotential method, which is a method to obtain earth resistance value rx by applying ac constant current i between the measurement object e earth electrode and hc current electrode, and finding out the potential difference v between e earth electrode. Ground resistance depends on grounding electrode selection, soil resistivity, soil contact, and other factors. Seasonal variation of earth resistance with an electrode of 34inch pipe in rather stony clay soil. The earth loop impedance test includes the suppliers earth electrode which has a maximum impedance of 21 ohms see first page of onsite guide eric. Introduction a good grounding system also known as an earth electrode system is important for the protection of an overall system facility. Lanzoni october 2007 introduction grounding or earthing is the art of making an electrical connection to the earth.
A ground electrode provides the contact between the electrical system and the earth. Principles and practice of earth electrode measurements reeve. The resistance r vi is measured for each electrode separation and apparent resistivity is calculated by multiplying value of r with wenner configuration factor 2 a, where a is the uniform distance between the elctrodes. The resistance indicated by the test set is the resistance between the earth electrode and the potential probe. August 7, 2019 hilman rojak leave a comment on what is earth electrode resistance test 3 ways to test a ground rod wikihow earth pit resistance testing services prediction of earthing resistance the most mon methods of measuring resistance an earth. Earthground measurement guide chauvin arnoux metrix. First, the earth electrode of interest must be disconnected from its connection to the site. The test measures the earth resistance and also detects the voltage between the auxiliary. Consider other fields where the value of resistivity is measured. Measurement of earth resistance by potential fall method. The ground tester will measure directly by generating its own current and displaying the resistance of the ground electrode. The earth resistivity test ert of soil was carried out at site for studies work for andhrapradesh medtech zone amtz, vishakhapatnam, andhrapradesh. This ensuresan accurate earth resistance measurement, otherwise the measurement will resultin higher resistance readings dueto summationof the zones of influence of the earth. The use of a four terminal test instrument such as the seaward ert 1557 eliminates the resistance of the test leads.
If this is not possible, art may be used to measure electrode resistance. The family has seven variants supporting 2, 3 and 4wire measurements. Pdf guideline for earthing of buildings and industrial. C the resistance in the surrounding earth to current flow or earth. The resistance area of the earth electrode is the area of the soil around which a voltage gradient is measured with a commercial instrument. Designing for a low resistance earth interface grounding.
Principles and testing methods of earth ground resistance. An earth electrode and earth electrode resistance are defined in bs 7671 as. If the deviation exceeds 5%, further tests must be carried out with a larger separation between the earth electrode under test and spike t1. Description each kg earth rod seal assembly es300 earth rod seal and membrane seal 0. Earth electrode resistance is measured to remote earth, which is the earth outside the electrode s influence. The simplest and somewhat misleading idea of a good ground for an electrical system is a section of iron pipe driven into the earth with a wire conductor connected from the pipe to the electrical circuit figure 1 a practical guide to earth resistance testing megger on photo. As the three readings all fall within this 5% tolerance, they are acceptable and the average value 72 ohms would be recorded as the earth electrode resistance r a 72 ohms. Then, for the 3pole fallofpotential test, two earth stakes are placed in the soil in a direct line away from the earth electrode. Checking ground electrode impedance for commercial, industrial and residential buildings most facilities have grounded electrical systems, so that in the event of a lightning strike or utility overvoltage, current will find a safe path to earth. Practically only few meters of ea rth around the electrode influences the resistance. For the love of physics walter lewin may 16, 2011 duration. The most common methods of measuring the resistance of an. This test provides a measurement of the resistance of a simple earth grid.
The resistance of the electrode has the following components. The earth resistance mainly implies the resistance between the electrode and the point of zero potential. In the figure shown below e is the earth electrode under rest, and a is an auxiliary earth electrode positioned so that two. The process is a combination of science and art as opposed to pure science, because it is necessary to test the. The megger det family of test instruments offers a unique solution to the measurement of earth or ground electrode rod resistance and soil resistivity. Fluke 1625 geo earth ground tester manual pdf instrumart. One auxiliary electrode is placed at a set distance from the earth electrode, and the other at 62% of the distance between the two in a straight line.
In preparation for the test the installation should be isolated, the earth electrode should preferably be disconnected from the earthing conductor which connects it to the main earthing terminal. These test electrodes can be of lesser quality higher earth resistance but must be electrically independent of the electrode to be measured. This method is derived from the 4point method which is used for soil resistivity measurements the 3point method, called the fall of potential method, comprises the earth electrode to be measured and two other electrically independent. A larger electrode system requires greater distance before its influence decreases to a negligible level. It is essential that the probe be placed outside the sphere of influence of the ground electrode under test and the auxiliary earth to achieve the highest degree of accuracy when performing a 3pole ground resistance test or the effective areas of.
The instrument terminals should be connected to the system under test through isolation switches. A further advantage of this two reading test is that it gives you a clearer picture, even when a spot reading says the insulation looks ok. But the actual value of earth resistivity need not be measured to check the electrode earth resistance. A is the spacing in metres between the test electrodes.
Earth electrode and looping, theory and applications. In addition to performing standard 3 and 4pole ground resistance measure ments, an innovative process accurately measures individual earth electrode. A the resistance of the metal and that of the connection to it. When a new earth electrode is installed the installer will know its construction and location and some details of the surrounding soil condition, but its earth resistance can only be determined by a test measurement.
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