The megger det family of test instruments offers a unique solution to the measurement of earth or ground electrode rod resistance and soil resistivity. These test electrodes can be of lesser quality higher earth resistance but must be electrically independent of the electrode to be measured. Earth electrodes earth rods earth rod seal material. 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. Earthground measurement guide chauvin arnoux metrix. First, the earth electrode of interest must be disconnected from its connection to the site. Checking ground electrode impedance for commercial. 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. 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. Earth electrode conductive part, which may be embedded in the soil or in a specific conductive medium, e. The instrument terminals should be connected to the system under test through isolation switches.
Earth electrode resistance is measured to remote earth, which is the earth outside the electrode s influence. The measurement of earth resistance is made by the potential fall method. 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. 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. The resistance measurement is made between the earth electrode and test spikes inserted into the ground. The resistance of earth electrode depen ds on the earth surrounding i t. This joint australiannew zealand standard was prepared by joint technical committee el001, wiring rules. 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. 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. A the resistance of the metal and that of the connection to it. Earth electrode and looping, theory and applications.
The resistance area of the earth electrode is the area of the soil around which a voltage gradient is measured with a commercial instrument. Lanzoni october 2007 introduction grounding or earthing is the art of making an electrical connection to the earth. Designing for a low resistance earth interface grounding roy b. The earth electrode under test should be isolated from the circuit it is protecting before testing commences. Using a proprietary earth electrode test instrument the earth electrode resistance test is conducted as follows. The most commonly used method of measuring the earth resistance of an earth electrode is the 3point measuring technique shown in figure 1. The process is a combination of science and art as opposed to pure science, because it is necessary to test the. C the resistance in the surrounding earth to current flow or earth. This test provides a measurement of the resistance of a simple earth grid. 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. This test requires a 3point ground resistance tester such as aemcs 4620 or 4630 ground resistance tester. 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.
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 earth resistivity test ert of soil was carried out at site for studies work for andhrapradesh medtech zone amtz, vishakhapatnam, andhrapradesh. 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. Timeresistance tests on large rotating electrical machinery especially with high operating voltage require high insulation resistance ranges and a very constant test voltage. For the love of physics walter lewin may 16, 2011 duration. The ground tester will measure directly by generating its own current and displaying the resistance of the ground 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. The test measures the earth resistance and also detects the voltage between the auxiliary. In the figure shown below e is the earth electrode under rest, and a is an auxiliary earth electrode positioned so that two. In addition to performing standard 3 and 4pole ground resistance measure ments, an innovative process accurately measures individual earth electrode. Using ohms law v ir, the tester automatically calculates the resistance of the earth electrode.
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. Second, the tester is connected to the earth electrode. But the actual value of earth resistivity need not be measured to check the electrode earth resistance. The earth resistance mainly implies the resistance between the electrode and the point of zero potential. Designing for a low resistance earth interface grounding. Det4 series 4terminal earthground resistance and soil. 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. What is earth electrode resistance test the earth images.
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. The most common methods of measuring the resistance of an. Pdf guideline for earthing of buildings and industrial. Leon markwell, senior engineer at the iet, discusses the inspection and testing of earth electrodes. 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. Description each kg earth rod seal assembly es300 earth rod seal and membrane seal 0. Ground resistance depends on grounding electrode selection, soil resistivity, soil contact, and other factors. B the contact resistance of the surrounding earth to the electrode. This ensuresan accurate earth resistance measurement, otherwise the measurement will resultin higher resistance readings dueto summationof the zones of influence of the earth. 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. 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. Consider other fields where the value of resistivity is measured. This method is referenced in all the electrical installation testing standards and can be used to measure the earth resistance both accurately and safely.
If this ground electrode is in parallel or series with other ground rods, the re value is the total value of all resistances. The earth electrode resistance test is used to confirm that the earth electrode provides a satisfactory path to earth. If the deviation exceeds 5%, further tests must be carried out with a larger separation between the earth electrode under test and spike t1. Numerically, it is equal to the ratio of the potential of the earth electrode to the current dissipated by it. 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. A larger electrode system requires greater distance before its influence decreases to a negligible level. 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. The resistance indicated by the test set is the resistance between the earth electrode and the potential probe. If this is not possible, art may be used to measure electrode resistance. These graphs show the relation between character of soil and resistance of driven electrode at increased depths. 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.
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 ra 72 ohms. How to determine the resistance of earth electrode. Practically only few meters of ea rth around the electrode influences the resistance. 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.
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. While test equipment is in the field for resistance measurements, it is convenient to also measure soil resistivity. Measurement of earth resistance by potential fall method. Seasonal variation of earth resistance with an electrode of 34inch pipe in rather stony clay soil. Fluke 1625 geo earth ground tester manual pdf instrumart. Principles and practice of earth electrode measurements reeve. Another way of thinking about the earth shells is as parallel resistances.
Earth resistance measurements on installations with a single earth electrode it is important to point out that the earth resistance measurement of reference is the 2stake method. Earth resistance measurements with 4, 3 and 2 wires earth. A practical guide to earth resistance testing weschler instruments. This is the actual value of the ground electrode under test.
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. A is the spacing in metres between the test electrodes. Multiple, user selectable test frequencies resistance measurement range to. An earth electrode and earth electrode resistance are defined in bs 7671 as. The family has seven variants supporting 2, 3 and 4wire measurements. Introduction a good grounding system also known as an earth electrode system is important for the protection of an overall system facility. Earthing or grounding of electrical equipment is essential to maintain safety, protecting lives, as well as prevent damage to equipment. The resistance of the electrode has the following components. A ground electrode provides the contact between the electrical system and the earth. R is the earth electrode resistance reading in ohms. 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. The use of a four terminal test instrument such as the seaward ert 1557 eliminates the resistance of the test leads.
402 563 1087 831 632 117 495 1228 614 1002 377 1418 1419 1382 447 1223 965 1127 196 1325 835 816 1352 281 269 863 1374 356 327 301 1071 1376 480 402 741