AEMC® – Understanding Ground Resistance Testing

In an electrical process, robust groundingensures a nontoxic working environment as good as appropriate gear performance. This is why electrical utilities strive toprovide sufficient grounding of the electrical distribution procedure, so consumers can enjoythe advantages of electric power in security. Energy utilities attempt to supply and deliverhigh exceptional non-fluctuating electrical vigour. Excellent electrical grounding of transmissionand distribution techniques contributes instantly to the fine of the energy being supplied. Grounding of constructions and other structuresis done for a style of factors. Radio and telecommunication towers are groundedfor premier transmission efficiency. They are also grounded for defense againstlightning strikes. Electrical apparatus mounted on utility polesis additionally susceptible to lightning, which is why poles that elevate electrical equipmentshould have designated attention paid to the efficiency of their grounding conductors andelectrodes. This video is an introduction to groundingsystems. Our goal is to provide you with a just right basicunderstanding of why grounding is predominant, the environmental factors to do not forget whendesigning a grounding method, and the testing required to make sure your grounding is properlyinstalled and performing to standards.On this video, a floor is outlined asa conducting connection during which an electrical circuit or apparatus is attached to earth. This connection is used to establish and maintainas carefully as viable the potential of the earth on the circuit or apparatus connectedto it. This connection to the earth presents a lowimpedance direction for electrical currents to journey underneath fault stipulations. For example, most folks be aware of that the thirdpin on the road twine of an electric equipment is the bottom connection.When a section of electrical apparatus is pluggedinto a wall receptacle, the bottom terminal connects all uncovered metal surfaces of theequipment to a long-established connection, called the apparatus ground, in the buildingselectric carrier panel. From this point the bottom connection typicallyexits the constructing and is hooked up to the approach ground on the constructing service entrance. The system ground is usually a easy metallic roddriven into the ground as shown here, a grid such as more than one electrodes, or anothertype of grounding approach. Many electrical faults that occur are causedby nature, at the same time others are man-made. But most occur because of corrosion and generaldegradation of equipment over time. When these disruptions of traditional electricaloperation do arise, it’s very predominant for the grounding procedure to function effectually.For this to be possible, the grounding circuitmust have a low resistance connection to the surrounding earth. And a grounding method will only participate in properlyif it is designed and installed with nearby soil stipulations taken into consideration. So to evaluate the predicted efficiencyof a grounding system, we must comprehend regional soil resistivity. Soil resistivity measurements are valuable formany causes. For example, information from soil evaluation is usedto make geological surveys as an support in determining ore areas, depth of bedrock, and otherphenomena. Resistivity additionally has a direct have an effect on on thecorrosion in underground pipelines. A minimize in resistivity pertains to an increasein corrosive pastime. And principal for todays video, soilresistivity immediately impacts the design of grounding systems. Soil resistivity is the key factor that determineswhat the resistance of a grounding electrode might be, to what depth it ought to be pushed inorder to receive low earth resistance, and even the style of grounding approach that shouldbe designed for this location. The resistivity of the soil varies widelythroughout the arena and changes seasonally.Soil resistivity is set generally bythe content of electrolytes which includes moisture, minerals, and dissolved salts. A dry soil may have high resistivity if itcontains no soluble salts. Let’s take a appear at a quantity of differentsoils and their relative resistivity. Because the chart on the monitor shows, soilscontaining clay, cinders, or loam have a very low resistivity in comparison with sand, gravel,and stone. Moisture additionally significantly impacts soil resistivity. Bear in mind two samples of soil, a topsoil anda sandy loam. When fully dry, each are excellent insulatorswith totally high resistivity. Nonetheless, when moisture is brought to both samples,resistivity alterations speedily.Resistivity of the soil is also influencedby temperature. For instance, this chart indicates the resistivityof sandy loam containing 15% moisture with temperature changes from 20 degrees to minus15 degrees Celsius. In this temperature range the resistivityvaries appreciably. With these motives in mind, soil resistivitymeasurements are the key to picking out the magnitude of effort which may be required inconstructing an effective grounding approach. The easiest strategy to measure soil resistivityis referred to as the Wenner procedure. This entails inserting four equally spacedand in-line electrodes into the ground. The 2 outer electrodes, called the currentelectrodes, inject current into the soil.The two inside electrodes, referred to as the potentialelectrodes, measure voltage, which is then used to calculate soil resistance. In our example, now we have spaced the electrodes10 ft apart and 6 inches deep. For this test, we’re utilizing the AMEC Model6471, a four-pole ground resistance tester. The 2 outer electrodes are connected tothe instruments outer terminals. The inner electrodes are related the innerterminals. Lets pause a second to talk about how theseterminals are labeled. According to worldwide necessities, thecurrent terminals are called H and E. Some producers label these terminals Z andX, at the same time others use C1 and C2. Similarly, global requisites definethe knowledge terminals as S and Es. On some instruments these are labeled Y andXv, and on others P1 and P2. In this video, we can refer to the currentterminals as Z and X, and the abilities terminals Xv and Y. The mannequin 6471 has the potential to automaticallycalculate and show soil resistivity, a value represented by using the emblem rho.To measure this value, turn the dial to therho surroundings, and enter the distance between electrodes, as prompt by the instrumentsuser manual. Then push the button to take a measurement. After a few moments, the resistivity readingappears. Should you commonly have got to participate in soil resistivitytesting, don’t forget purchasing an instrument such because the mannequin 6471 that automaticallycalculates this value. This may shop time and eliminate potentialmath error if your instrument doesnt offer automaticsoil resistivity calculation, with ease take a resistance reading. For example, in case you dont enter the electrodedistance, the rho environment on the model 6471 displays soil resistance, which as you cansee is 15.1 ohms.We will now use this price to manually calculatethe resistivity of the soil. The entire formula for soil resistivityis shown on the monitor. Nevertheless, a extra simplified components can beused when the auxiliary electrodes are driven to a depth of approximately 1/twentieth of theelectrode spacing distance, as weve carried out in our demonstration. In this formula, soil resistivity equals 2multiplied with the aid of A, the gap between the electrodes in meters, improved with the aid of R, thereading got for the period of measurement. So to calculate soil resistivity we multiply2, or 6.28, via 3.05 meters, the metric equivalent of 10 ft. We then multiply this with the aid of our studying of 15.1ohms to acquire a soil resistivity price of 289 ohm-meters. A grounding process more often than not includes agrounding conductor, a bonding connector, its grounding electrode (quite often a rod orgrid approach), and the soil in touch with the electrode. An electrode can be proposal of as being surroundedby concentric shells of earth or soil, all the equal thickness, with every successive shellhaving a better pass-sectional price over which the resistance is dispensed. Be aware that growing the diameter of the groundingelectrode does not drastically curb its resistance.For example, doubling the diameter reducesthe resistance by way of less than 10%. However, driving the ground rod deeper intothe earth does greatly minimize resistance. As a normal rule, doubling the depth to whichthe rod is pushed into the ground reduces its resistance via up to 40%. Nonetheless, this has its realistic limits. When a ground electrode is set up, itsessential to experiment its resistance to make sure it is functioning readily. The 3-point fall-of-advantage experiment is a commonlyaccepted procedure for measuring the resistance of single grounding rod or grid systems. The diagram proven on the reveal illustratesthe inner circuitry of a general fall-of-skills tester. Packaged inside a average instrument are twoindependent circuits. One circuit is a regular present generator,the other a high impedance voltmeter that measures the voltage drop and then calculatesand shows resistance. By means of inserting auxiliary electrodes into theearth at exact distances from the electrode below scan, and then connecting them to theinstrument, the operator units up a bridge circuit with the earth. Current flows into the earth via auxiliaryelectrode Z, and returns to its source via the rod underneath scan.A voltage drop is developed between thesepoints and is measured with the aid of the Y auxiliary electrode. With the aid of positioning the Y electrode at a numberof exclusive elements between the rod below experiment and the Z electrode, measurements canbe plotted and a characteristic curve turns into evident. In a full fall-of-competencies test, this involvestaking a dimension at every 10% increment of the space between the rod underneath testand Z. To be certain an correct dimension, be sureto position the present electrode a long way adequate from the ground rod under scan so that thecentral measurements of the expertise electrode might be external the sphere of influence ofboth the bottom rod and the present injecting electrode.The nice strategy to investigate whether or not or not thecurrent injecting electrode is placed effectively is to participate in a simplified type of fall-of-potentialtest known as a sixty two% experiment. This involves taking measurements at 52%,62%, and 72% of the gap between the grounding rod and current injecting electrode. If the spheres of influence of the groundrod and current injecting electrode are in collision, readings taken at these pointswill vary significantly.When the ground rod and current injectingelectrode are placed effectively, readings will differ simplest slightly as measurements aremade. To demonstrate, we will use the AEMC Model3640 floor tester to participate in a 62% scan. Our experiment discipline is an eight-foot ground rod. Earlier than connecting the ground rod under testto the instrument, the rod ought to be disconnected and remoted from carrier. To measure our floor rod resistance, we connectthe rod to the devices X terminal. We then situation the present injecting electrodea minimum of 10 occasions the rod depth, or 80 feet, from the rod and attach it to the instrumentsZ terminal. Finally, we place the competencies measurementelectrode at sixty two% of the space from the rod to the current injecting electrode, connectit to the Y terminal, and take a size. We entire the test through taking measurementsat fifty two% and seventy two% of the space between the ground rod and current injecting electrode. As we famous previous on this video, groundingsystems can encompass grids which include more than one rods connected together. Grids are most commonly designed for substationsand equivalent facilities to furnish the lowest feasible earth resistance values, as wellas to create an equipotential zone throughout the entire station.The fence surrounding the substation is usuallyincluded on this zone for security explanations. Multiple rod grids are regularly constructedwith ground rods exothermically welded to copper mesh, making a giant field of zeropotential earth when mounted safely. When performing a fall-of-abilities test ona grid process, we can not use the depth of a single rod to calculate the placement ofthe auxiliary electrodes. As a substitute, the distance must be headquartered on themaximum inside diagonal dimension of the grid. Furthermore to single rods and grid systems,other grounding design choices are to be had. These comprise: Grounding plates. These are frequently thin copper plates buriedin direct contact with the earth. Grounding plates are frequently positioned below polesor similar buildings. There are also concrete-encased programs, oftencalled ufers inside the enterprise. These may also be a number of copper rods, rebar,wire, or mesh encased in concrete, mainly incorporated as a part of the constructions foundation. Also used are chemical rods consisting ofa hole electrode stuffed with electrolytic salts. This alternative can furnish an effective groundsystem in locations the place terrible soil stipulations are present and spacing for electrodes islimited.Chemical rods are generally used in conjunctionwith soil enhancement materials that improve grounding effectiveness. These substances will also be used in othergrounding systems located in soils with negative conductivity. Fall-of-capabilities testers are riskless andcan be used in a number of exceptional functions. However, there are some places, such aswhere house is restrained, entry to the earth is unavailable, or the grounding system cannotbe disconnected, the place fall-of-knowledge checking out is tricky or unattainable. In these occasions, clamp-on floor resistancetesting is fast becoming the substitute. Clamp-on testers offer the expertise of measuringground resistance without disconnecting or de-energizing the procedure. These instruments also allow you to checkboth the resistance of a ground rod, and its integrity to the leisure of the procedure. To take a size, simply open the jaws,clamp onto the rod or the wire leading to the rod, and take a studying.The jaws of clamp-on instruments are designedwith two unbiased shielded magnetic assemblies. One aspect is a transmitter that injects a testsignal into the approach at the ground rod beneath test. The opposite aspect acts as a detector that measuresthe resultant present that flows via the ground rod being inspected. This method presents correct results similarto values received using the autumn-of-expertise procedure. Clamp-on devices can be used to measurethe electrode resistance of pole grounds, pad-established transformer grounds, transmissiontower grounds, and repair entrance grounds, with out the necessity for auxiliary electrodes. They may be able to additionally measure authentic RMS neutral-to-earthleakage currents. Advanced clamp-on instruments, reminiscent of theAEMC items 6416 and 6417, also provide variable test frequency, loop resistance indication,floor voltage detection, and storage of measurements. The mannequin 6417 additionally features Bluetooth wirelesscommunication for connecting to a computer or cell app. Notice that AEMC devices manufactures avariety of products for measuring soil resistivity and grounding system resistance.Anything your floor resistance trying out wants,AEMC has an instrument that meets your necessities. If you would like extra know-how about groundtesters or different AEMC merchandise, please seek advice from our website or call our technical assistancehotline. Lets take a second to check a couple of of themajor points weve protected: Grounding is a valuable element of electricalsystems on account that it helps shield apparatus and customers from electrical faults, lightningstrikes, and other harmful electrical anomalies. Grounding methods can range in complexityfrom a single rod driven into the ground, to tricky grids which includes more than one rodsconnected with wire mesh, to different types of grounding systems incorporating plates, concrete,chemical substances, and soil conductivity enhancers. Abilities of neighborhood soil resistivity is essentialfor designing a grounding system appropriate for the web site. The Wenner procedure is a straightforward and effectiveway for measuring soil resistivity. A fall-of-talents scan, for example the62% experiment, is used for measuring the efficacy of a grounding system. In occasions the place performing a fall-of-potentialtest isn’t possible, clamp-on testers furnish a excellent alternative.As we’ve got visible, there are a quantity of differentmethods on hand for testing the resistance of grounding techniques. Armed with right figuring out of the techniquesand test results, floor resistance trying out may also be performed without difficulty. And be sure to determine our YouTube channel forinstructional videos on different topics in electronics, including the many products furnished through AEMC..

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