Principles an Practice of Earth Electroe Measurements Document Information: Author: Whitham D. eeve evision: 0.0 (initial raft 06/18/2002) 0.1 (working raft 10/05/2003) 0.2 (aitional work 10/07/2003) 0.3 (eitorial revisions 05/09/2004) 0.4 (ae Slope Fiel Data Form, 10/01/2004) 1.0 (ae appenices an issue, 10/13/2004) 1.1 (eitorial revisions an ae soil resistivity section, 08/01/2008)
1. Introuction This application note escribes principles an methos for measuring the resistance to remote earth of grouning systems (earth electroe systems) use in telecommunications facilities. This note oes not escribe all earth electroe measurement techniques but only those foun through fiel experience to be most useful an consistent. While test equipment is in the fiel for resistance measurements, it is convenient to also measure soil resistivity; therefore, this application note also escribes soil resistivity measurements. This information largely is base on the author s experience an may or may not be correct, safe or useful. None of the information provie in this ocument is professional engineering avice. The most important things to remember are: Use common sense an make your measurements in a safe manner. Do not rely on this ocument for safety information. Always be alert an conscious of your surrounings. You use the information in this ocument at your own risk. Information in this ocument is provie as-is. It is only for the reaer s consieration an stuy an inclues no claim whatsoever as to usefulness, accuracy of content, or accuracy of information. 2. Groun Measurement Principles The object of an earth electroe system is to provie a low resistance to foreign currents that may cause injury or amage or isrupt equipment. The currents will issipate safely when properly conucte to earth via the electroe. There are three components to the resistance (Fig. 1): esistance of the electroe materials an connections to them Contact resistance between the electroe an the soil surrouning it esistance of the surrouning earth. Fig. 1 Equivalent esistance Circuit of an Earth electroe System Electroe Materials an Connections esistance to emote Earth Soil Contact esistance The resistance of the electroe materials is purposely mae small so their contribution to the total resistance is negligible. Generally, copper materials are use throughout. Groun ros usually are copper-coate steel for strength, although galvanize steel groun ros are foun in applications where corrosion is a problem. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 1
The contact resistance between the electroe an soil is negligible if the electroe materials are clean an unpainte when installe an the earth is packe firmly. Even ruste steel groun ros have little contact resistance because the iron oxie reaily soaks up water an has less resistance than most soils (however, ruste groun ros may eventually rust apart in which case their effectiveness is greatly reuce). Generally, the resistance of the surrouning earth will be the largest of the three components. An earth electroe system burie in the earth raiates current in all irections an eventually issipates some istance away epening on the soil s resistance to current flow, as inicate by its resistivity. An earth electroe system consists of all interconnecte burie metallic components incluing groun ros, groun gris, burie metal plates, raial groun systems an burie horizontal wires, water well casings an burie metallic water lines, concrete encase electroes (Ufer grouns), an builing structural steel. The earth electroe can be thought of as being surrouning by shells of earth, each of the same thickness (Fig. 2). The shell closest to the electroe has the smallest surface area an offers the greatest resistance. The next shell has larger area an lower resistance, an so on. A istance eventually will be reache where the aitional earth shells o not a significantly to the resistance. Earth electroe resistance is measure to remote earth, which is the earth outsie the electroe s influence. A larger electroe system requires greater istance before its influence ecreases to a negligible level. Another way of thinking about the earth shells is as parallel resistances. The closest shell has some unit resistance. The next larger shell has more surface area so it is equivalent to several unit resistances in parallel. Each larger shell has smaller equivalent resistance ue to more parallel resistances. The resistance of the surrouning earth epens on the soil resistivity. Soil resistivity is measure in ohm-meters (ohm-m) or ohm-centimeters (ohm-cm) an is the resistance between two opposite faces of a 1 meter or 1 centimeter cube of the soil material. The soil resistivity epens on the type of soil, salt concentration an its moisture content an temperature. Frozen an very ry soils are a goo insulators (have high resistivity) an are ineffective with earth electroes. The resistivity of many soil types is provie in Section 8. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 2
Fig. 2 Concentric Earth Shells Aroun an Earth electroe Groun Electroe Earth Shells Each larger shell contributes a smaller percentage of the total earth resistance Equivalent esistance or each shell 3 5 7 11 9 15 13 17 21 25 29 3. Test Equipment Three typical test sets are shown in Fig. 3. A typical test set has four terminals two current terminals marke C1 an C2 an two potential terminals marke P1 an P2. Other markings are use on test sets originally esigne outsie the Unite States. Cheap test sets may have only three terminals; these shoul be avoie because of their limite usefulness an questionable urability. Also, it has been foun in the fiel that some expensive igital test sets are worthless, particularly some of the DETx/x moels that have been sol by Bile over the years. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 3
Fig. 3 Typical Test Sets (l-r: Bile Megger 240241, AEMC 4630, Bile DET5/2) One current terminal (C1) is connecte to the earth electroe uner test an the other (C2) to a probe riven in the earth some istance away. The test set injects a current into the earth between the two current terminals. One potential terminal (P1) also is connecte to the earth electroe but the other potential terminal (P2) is connecte to a separate probe riven in the earth between the electroe an the current probe (C2). The potential probes etect the voltage ue to the current injecte in the earth by the current terminals. The test set measures both the current an the voltage an internally calculates an then isplays the resistance V I ohms Some test sets use a resistive brige circuit that requires manual ajustment of the resistance to balance the brige as inicate by a null meter. When the meter has a null reaing, the value is taken from the resistance controls. Most moern test sets require no manual ajustment. The resistance inicate by the test set is the resistance between the earth electroe an the potential probe. If the current an potential probes both are far enough away from the electroe, that is, outsie the influence of the electroe, the reaing correspons to the resistance of the electroe to remote earth. When measuring the resistance of earth electroe systems, C1 an P1 are connecte together either at the test set or at the electroe. If connecte at the test set, say with a jumper, the resistance of the wire from the test set to the earth electroe as to the reaing. The value of this resistance can be measure easily with the test set: (1) Short C1 an P1 together with a short jumper; (2) Short P2 an C2 together with a short jumper; (3) Connect the test lea between P1 an P2 an measure the resistance; (4) Subtract the reaing from all electroe resistance measurements subsequently taken. To avoi having to make these aitional measurements an calculations, always use a short, low-resistance conuctor between the test set an the electroe when C1 an P1 are shorte at the test set (Fig. 4 left), or always connect C1 an P1 to the electroe with separate leas (Fig. 4 right). If the two terminals are connecte together at the electroe, the length an size of the conuctor oes not affect the measurement. The size of the wires connecte to the other two terminals, P2 an C2, are not important from an electrical stanpoint but shoul be large enough eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 4
to provie the neee physical strength. Aitional etails concerning the probes, leas an test equipment are provie in Section 7. Fig. 4 Test Equipment to Earth electroe Connections Short, Heavy Lea Separate Leas Shorting Strap C1P1 P2 C2 C1P1 P2 C2 Groun Electroe Uner Test Groun Electroe Uner Test Test sets with four terminals also may be use to measure soil resistivity (test sets with three terminals cannot be use to measure soil resistivity). Each terminal is connecte to an inepenent probe resulting in two potential an two current probes. Soil resistivity measurements are covere in Sect. 8. 4. Fall-of-Potential Metho The Fall-of-Potential metho (sometimes calle the Three-Terminal metho) is the most common way to measure earth electroe system resistance, but it requires special proceures when use to measure large electroe systems (see Measuring Large Electroes below). For small electroes, such as one or a few groun ros or a small loop, the Fall-of-Potential metho requires only simple proceures (see Measuring Small Electroes below). 4.1 Basic Proceure The basic proceure for the Fall-of-Potential metho is to first connect the test set terminals C1 an P1 to the earth electroe uner test, connect the test set C2 terminal to a current probe locate some istance from the earth electroe an finally connect the test set P2 terminal to a potential probe locate a variable istance between. The two probes normally are locate in a straight line. At each potential probe location, the resistance is recore (a form is provie in eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 5
Appenix I for this purpose). The results of these measurements are then plotte to graphically etermine the electroe resistance. 4.2 Measuring Small Electroes 1. Connect C1 an P1 terminals on the test set to the earth electroe (Fig. 5). 2. Drive a probe into the earth 100 to 200 feet from the center of the electroe an connect to terminal C2. This probe shoul be riven to a epth of 6 inches. 3. Drive another probe into the earth miway between the electroe an probe C2 an connect to terminal P2. This probe shoul be riven to a epth 6 inches. 4. ecor the resistance measurement. 5. Move the potential probe 10 feet farther away from the electroe an make a secon measurement. 6. Move the potential probe 10 feet closer to the electroe an make a thir measurement. 7. If the three measurements agree with each other within a few percent of their average, then the average of the three measurements may be use as the electroe resistance. 8. If the three measurements isagree by more than a few percent from their average, then aitional measurement proceures are require (see Measuring Large Electroes). Fig. 5 Small Electroe Measurements C1 P1 P2 C2 C1 P1 Groun Electroe Uner Test P2 C2 10 ft. 10 ft. 50-100 ft. 50-100 ft. 4.3 Measuring Large Electroes A quick set of measurements can be mae as follows (Fig. 6). This metho eliminates many teious measurements but may not yiel goo accuracy unless the current an potential probes are outsie the electrical influence of the electroe system: eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 6
1. Place the current probe 400 600 feet from the electroe 2. Place the potential probe 61.8% of the istance from the electroe to the current probe 3. Measure the resistance 4. Move the current probe farther away from its present position by, say, 50 100 feet 5. epeat steps 2 an 3 6. Move the current probe closer by, say, 50 100 feet 7. epeat steps 2 an 3 8. Average the three reaings Fig. 6 Quick Measurements of Large Electroe Systems C1 P1 P2 C2 C1 P1 Groun Electroe Uner Test P2 C2 61.8% x D 50-100 ft. 50-100 ft. D = 400-600 ft. As iscusse in Sect. 1, the earth electroe can be thought of being surroune by concentric shells of earth. The current probe also is surroune by earth shells but with a commensurately smaller influence. It is necessary to locate the current probe far enough away so the influential shells o not overlap (Fig. 7). eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 7
Fig. 7 Overlapping an Non-Overlapping Shells of Earth Groun Electroe P2 C2 Plot of P2 esistance versus Distance C1 P1 Groun Electroe P2 C2 Current Probe esistance Groun Electroe esistance eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 8
Fall-of-Potential measurements are base on the istance of the current an potential probes from the center of the electroe uner test. However, the location of the center of the electroe selom is known, especially if it consists of numerous types of bone electroes such as groun ros, gris, concrete-encase electroes (Ufer grouns), water well casings an burie raial wires. In these cases, special proceures, such as the Intersecting Curves or Slope methos, are require. These are escribe in later sections. The farther a probe is place from the electroe, the smaller the influence. The best istance for the current probe is at least 10 to 20 times the largest imension of the electroe (Table 1). However, terrain or obstructions, such as builings an pave roas, can greatly limit the space available for measuring larger systems. Nevertheless, acceptable accuracy can be obtaine by using smaller istances in most situations. Table 1 Current Probe Distance from Electroe Maximum Electroe Dimension (Feet) Distance to Current Probe from Electroe Center (Feet) 15 300 30 450 60 600 It can be shown (Appenix II) that the actual electroe resistance is measure when the potential probe is locate 61.8% of the istance between the center of the electroe an the current probe (Fig 8). For example, if the current probe is locate 400 feet from the electroe center, then the resistance can be measure with the potential probe locate 61.8% x 400 = 247 feet from the electroe center. The 61.8% measurement point assumes the current an potential probes are locate in a straight line an the soil is homogeneous (same type of soil surrouning the electroe area an to a epth equal to 10 times the largest electroe imension). While the latter conition is almost never known with certainty, the 61.8% measurement point still provies suitable accuracy for most measurements if other cautionary measures are taken as escribe below. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 9
Fig. 8 61.8% Distance for esistance Measurement C1 P1 P2 C2 C1 P1 Groun Electroe Uner Test P2 C2 Distance = 61.8% x D Distance = D Center of Groun Electroe As mentione above, the electroe center location selom is known. In this case, at least three sets of measurements are mae, each with the current probe a ifferent istance from the electroe, preferably in ifferent irections. However, when space is not available or obstructions prevent measurements in ifferent irections, suitable measurements usually can be mae by moving the current probe in a line away from or closer to the electroe. For example, the measurement may be mae with the current probe locate 200, 300 an 400 feet along a line from the electroe. Each set of measurements involves placing the current probe an then moving the potential probe in 10 feet increments towar or away from the electroe, epening on the starting point. The starting point is not critical but shoul be 20 to 30 feet from the electroe connection point, in which case the potential probe is move in 10 feet increments towar the current probe, or 20 to 30 feet from the current probe, in which case the potential probe is move in 10 feet increments back towar the electroe. The spacing between successive potential probe locations is not particularly critical, an oes not have to be 10 feet, as long as the measurements are taken at equal intervals along a line between the electroe connection an the current probe. Larger spacing means quicker measurements at the expense of fewer ata points; smaller spacing means more ata points at the expense of slower measurements. Base on the above, the basic steps for accurately measuring earth electroes of the type foun at telecommunications facilities are as follows. If obstructions o not permit using the probe eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 10
spacings specifie, make as many measurements as possible with the current probe as far an in ifferent irections from the electroe uner test as possible: 1. Place the current probe (C2) 200 feet from the electroe uner test 2. Place the potential probe (P2) 20 feet from the point at which the test set is connecte to the earth electroe 3. Measure an recor the resistance 4. Move the potential probe (P2) farther away by 10 feet 5. epeat steps 3 an 4 until the potential probe (P2) is within 20 or 30 feet of the current probe (C2) 6. Move the current probe (C2) 300 feet from the electroe uner test 7. epeat steps 2 through 5 8 Move the current probe (C2) 400 feet from the electroe uner test 9. epeat steps 2 through 5 Once all measurements have been mae, the ata is plotte with the istance from the electroe on the horizontal scale an the measure resistance on the vertical scale. The curves for each ata set shoul be smooth with no significant peaks or valleys. If there are epartures from a smooth curve, these ata points can be re-measure, ignore or replace artificially by interpolating between two goo ata points. If the current probe is outsie the electroe s influence, the curves will rise as the potential probe is move away from the electroe, increase with a slight positive (increasing) slope an then level off just beyon the mi-point between the electroe an current probe. As the potential probe approaches the current probe, the slope will increase sharply. If the curves o not level off in the mile but have a small slope, the probes were only partially influence by the electroe, an the resistance can be rea from the curve at a point that is 61.8% of the istance to the current probe. If the curves have a steep slope, the electroe influence the measurements an reaing the resistance at the 61.8% point usually gives a resistance that is higher than actual. In this case, the Intersecting Curves an Slope proceures may be use to manipulate the ata to yiel a more accurate resistance value. Example: The resistance ata in Table 2 were measure at a site near Sewar, AK. The earth electroe consists of #2 AWG bare copper gri with overall imensions of 40 feet by 60 feet. Gri spacing is 10 feet, an total conuctor length is 580 feet. Burial epth is approximately 4 feet below grae (unerneath the builing founation). Measurements in Columns (2), (3) an (4) were taken with the C2 probe at 200, 300 an 400 feet from the connection to the earth electroe. Columns (5) through (11) are for separate intersecting curve calculations that will be use in the next section. The Fall-of-Potential curve corresponing to the measurements with C2 at 200 feet is fairly steep, which inicates influence by the earth electroe (Fig. 9). The influence with C2 at 300 eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 11
an 400 feet is consierably less as inicate by the comparatively flat slope an overlap of the two curves. The earth electroe resistance can be estimate by examining the curves or fiel ata at the 61.8% point, or a P2 istance of 4, 185 an 247 feet on the three curves, respectively. For P2 = 4 feet an C2 = 200 feet, the estimate resistance is approximately 16 ohms. The other two curves yiel approximately 15.4 an 15.6 ohms, respectively. An average of the three is 15.7 ohms. Since the secon an thir ata sets (C2 = 300 an 400 feet) yiel resistances that are closer together than the first an secon ata sets, they probably are more accurate. The average of these two is 15.5 ohms. Fig. 9 Example Fall-of-Potential Curves 40 35 Measure esistance (Ohms) 30 25 20 15 10 5 0 0 50 100 150 200 250 300 350 400 Potential Probe P2 Distance, D (Feet) eeve Engineers 2008, File: Groun Testing 1-1.oc, Page
Table 2 Example Data Sets (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) C2 = 200 ft. C2 = 300 ft. C2 = 400 ft. P2 (ft) C2=200 ft C2=300 ft C2=400 ft X (ft) P' (ft) ' (ohms) P' (ft) ' (ohms) P' (ft) ' (ohms) (ohms) (ohms) (ohms) 0 4 15.96 185 15.55 247 15.61 10 10 0 15.63 182 15.40 243 15.49 20 5.00 5.00 5.00 20 116 15.34 178 15.26 240 15.40 30 8.30 8.90 8.20 30 1 15.05 174 15.18 6 15.32 40 9.90 9.80 9.40 40 108 14.74 170 15.10 2 15.24 50 10.70 10.30 10.30 50 105 14.50 166 14.98 228 15.16 60 11.70 11.10 11.20 60 101 14.18 162 14.86 224 15.08 70.20.00 11.80 70 97 13.89 159 14.78 220 15.00 80.90 13.00.30 80 93 13.61 155 14.70 217 14.94 90 13.40 13.50.50 90 89 13.35 151 14.62 213 14.80 100 14.10 13.40 13.00 100 85 13.15 147 14.54 209 14.79 110 14.90 13.90 13.30 110 82 13.00 143 14.26 205 14.75 0 15.63 14.00 13.40 0 78.28 140 14.40 201 14.71 130 16.47 14.20 13.60 130 74.24 136 14.32 198 14.66 140 17.35 14.40 13.70 140 70.20 132 14.24 194 14.58 150 18.28 14.60 13.80 150 66.00 8 14.16 190 14.50 160 20.80 14.80 14.00 160 62 11.80 4 14.08 186 14.42 170 27.20 15.10 14.20 170 59 11.60 0 14.00 182 14.34 180 41.80 15.30 14.30 180 55 11.20 117 13.97 178 14.28 190 76.90 15.80 14.50 190 51 10.80 113 13.93 175 14.25 200 16.20 14.70 200 47 10.46 109 13.85 171 14.21 210 16.30 14.80 210 43 10.14 105 13.65 167 14.14 220 17.10 15.00 220 40 9.90 101 13.45 163 14.06 0 17.80 15.20 0 36 9.26 98 13.42 159 13.98 240 19.50 15.40 240 32 8.62 94 13.46 156 13.92 250 21.20 15.70 250 28 7.64 90 13.50 152 13.84 260.90 15.80 260 24 6.32 86 13.30 148 13.78 270 29.10 16.10 270 20 5.00 82 13.10 144 13.74 280 39.00 16.30 290 16.30 300 17.00 310 17.30 320 17.60 330 18.40 340 19.30 350 21.30 360 25.00 370 33.70 380 52.10 eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 13
5. Intersecting Curves Proceures The intersecting curves calculations assume the center of the grouning system is at some istance X from the actual grouning system connection point. When using the Fall-of-Potential metho, the actual grouning system resistance is theoretically measure at a point that is 61.8% of the istance to the C2 probe from the center of the grouning system. If the C2 probe is place at istance D from the connection point, then the actual resistance ' is measure at a point P' = 61.8%(X + D) X from the connection point (Fig. 10). Fig. 10 Dimensions for Intersecting Curves Metho Groun Electroe Uner Test Connection Point (C1, P1) P2 C2 X Distance = 61.8% x (X + D) - X Distance = 61.8% x (X + D) Distance = D Distance = X + D Center of Groun Electroe The value of X is varie over a positive (or negative) range. To fin the value of X that gives the actual resistance, it is necessary to calculate P' for various values of X an then interpolate the value of ' corresponing to each value of P' from the ata set. This process is repeate for each ata set. The results are plotte in the Intersecting Curves graph. The point at which the curves intersect is the actual earth electroe system resistance. For example, assume the C2 current probe is locate D = 300 feet from the connection point, then the probe is X + 300 feet from the electroe center. The resistance woul be measure at 61.8% of this istance, that is, 61.8% (X + 300 feet). Next, assume that X is varie from 0 to 100 feet in 10 feet increments. When X is 0, the 61.8% point is 61.8% x (0 + 300) = 185 feet from the electroe center. The measure resistance corresponing to (185 0) = 185 feet istance from the connection point to the current probe is the first ata point for the Intersecting Curves plot. The next point woul be 61.8% x (10 + 300) = 192 feet from the electroe center. The measure resistance corresponing to (192 10) = 182 feet istance from the connection eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 14
point to the current probe is the next ata point for the plot. This sequence is repeate until the last point where the istance is 61.8% x (100 + 300) = 247 feet from the electroe center to the current probe, an the measure resistance corresponing to (247 100) = 147 feet istance from the connection point to the current probe is use as the last ata point to be plotte. The above proceures are repeate for each measurement set. At least three measurement sets are require for a reliable intersection. When the curves are plotte, they will intersect at the actual electroe resistance. In many cases, the curves o not intersect at exactly the same point but form a small irregular box. The size of the box inicates the uncertainty, an the smaller the box, the smaller the uncertainty. It is possible the curves o not all intersect or only two intersect. If so, the measurements probably are unreliable because of a faulty test set, reaing errors, poor connections or inavertent shorts to groun or because of transcription errors in the ata or in ata reuction. Sometimes, burie conuctive objects, such as burie power or telecommunications cables, engine blocks, or other large pieces of metallic junk can corrupt the measurements. If one or more curves are horizontal or almost horizontal, the current probe probably was locate outsie the earth electroe influence. Example: The ata use in this example are the same as in Section 4. The calculations of P for various values of X [Column (5)] an C2 =200 feet are in Columns (6), an the resistances corresponing to the P istances, as taken from the original ata for C2 = 200 feet, are in Column (7). Similar calculations for C2 = 300 feet are in Columns (8) an (9) an for C2 = 400 feet are in Columns (10) an (11). The calculate ata in Columns (7), (9) an (11). A plot of the resistance as a function of X is shown in Fig. 11. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 15
Fig. 11 Example Intersecting Curves 18 16 14 esistance (Ohms) 10 8 6 4 2 0 0 50 100 150 200 250 300 350 X Distance (Feet) Examination of the Intersecting Curves shows that the three curves cross at a resistance of aroun 15.5 ohms, which very closely agrees with the estimation mae in Section 4. The istance X at the curve crossing is about 20 feet, inicating that the center of the earth electroe was about 20 feet from the connection point as was actually the case. 6. Slope Proceures If the time available to make resistance measurements is limite, the Slope metho is much faster than the Fall of Potential metho because fewer measurements are neee. The Slope metho may use the same ata sets as iscusse above, which provies a crosscheck of the two methos. With the Slope metho the ata is manipulate in a ifferent way. If the istance from the earth electroe to the current probe is D, the resistance is rea from the ata sets at potential probe locations corresponing to 0.2 x D, 0.4 x D an 0.6 x D. Call these resistance reaings 1, 2 an 3, respectively (Fig. ). The change in slope of the resistance curve,, is calculate from 3 2 2 1 eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 16
Fig. Potential Probe P2 Locations for Slope Metho Groun Electroe Uner Test Connection Point (C1, P1) P2 (1) P2 (2) P2 (3) C2 Distance = 0.2 x D Distance = 0.4 x D Distance = 0.6 x D Distance = D Center of Groun Electroe P The table of values in Appenix III is then use to fin the value of D corresponing to the D calculate value of. If the calculate value of is not covere in the table, then the current probe is too close to the electroe system an aitional measurements are require with a greater istance. P D is the istance from the connection point to the potential probe where the true resistance woul be measure. P D is calculate by multiplying the quantity D P D (taken from the Slope table) by the value D actually use. The resistance may then be foun from the ata set at the calculate istance PD. If the ata set oes not inclue a measurement at the exact value of PD, the ata may be interpolate, or a separate measurement can be mae at the calculate value of P D. A fiel ata form is provie in Appenix IV for the Slope Metho. For example, assume that D = 300 feet an the calculate value of = 0.635. From the table in P D Appenix II, = 0.6079. Multiplying by D gives 0.6079 x 300 feet = 182 feet. This value D woul be use to fin the measure resistance from the original ata with interpolation if necessary, or a resistance measurement woul be mae with the P 2 probe at 182 feet. Similar calculations shoul be mae for each ata set an the resulting resistances shoul agree within a few percent. The resistances so calculate can be plotte on a new graph with the istance D on the horizontal scale (Fig. 11). Where the resistance significantly ecreases as the istance D increases (Data Set 1 an Data Set 2 in the example), the resistance values are higher eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 17
than actual. Where the resistances are reasonably close (Data Set 2 an Data Set 3 in the example), the values can be use with confience. Example: The ata for this example is the same ata that was use in Section 4. For C2 = 200 feet, the 0.2 xd = 40 feet, 0.4 x D = 80 feet an 0.6 x D = 0 feet. The corresponing resistances from the original ata at these istances are 1 = 9.90 ohms, 2 =.30 ohms, an 3 = 15.63 ohms. Calculating for this ata set gives 3 2 2 1 = 1.388. From Appenix II, PD/D = 0.4350. Since D = 200 feet, PD = 0.4350 x 200 feet = 87 feet. The measure resistance corresponing to P2 istance of 87 feet, as interpolate from the original measurements for C2 = 200 feet, is approximately 15.1 ohms. A similar set of calculations is mae for C2 = 300 feet an C2 = 400 feet. These are summarize along with C2 = 200 feet in Table 3. The average of the three is 15.4 ohms, which closely agrees with the resistance estimate in the previous example. Table 3 Example Slope Calculations 0.2 x D 1 0.4 x D 2 0.6 x D 3 P D/D P D C2 = 200 feet 40 feet 9.90 80 feet.90 0 feet 15.63 0.910 0.5598 1 feet 15.1 C2 = 300 feet 60 feet 11.10 0 feet 14.00 180 feet 15.30 0.448 0.6363 191 feet 15.8 C2 = 400 feet 80 feet.30 160 feet 14.00 240 feet 15.40 0.824 0.5759 0 feet 15.2 7. Fiel Measurements an Data euction 7.1 Cautions: The electric utility multi-groune neutral usually (but not always, epening on how well the electric utility grouns their system) contributes the most to a low system resistance. The resistances of parallel electroes, such as several interconnecte groun ros, o not combine like parallel resistors because of the mutual resistance between them. The resistance of interconnecte parallel electroes always will be higher than the parallel combination of the equivalent iniviual resistances. In permafrost areas, the central office grouning system usually is better (lower resistance) than other service grouning systems. Currents from electric utility faults in the neighborhoo will flow through the central office groun. Never isconnect operating telecommunications equipment from the earth electroe system. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 18
Never isconnect live ac service equipment from the earth electroe system this is for your safety. In a new central office, with no live equipment an no live ac service, it will be possible to test iniviual grouning system components (well casing, gri, groun ros, builing structural steel, etc.) if they can be isolate from each other. Otherwise, leave everything interconnecte an measure the overall system. Usually, telecommunications facility earth electroe systems consist of several components an the facility operates normally with all the components interconnecte. Therefore, if your goal is to measure only the telecommunications facility earth electroe system, you woul not measure them iniviually. Instea, you woul make sure all components, except the electric utility MGN, are bone as they woul be in the final operating configuration an measure them as a complete system. If you leave all earth electroe components interconnecte for safety, you are measuring the composite resistance of all earth electroe components together, incluing the electric utility multi-groune neutral (MGN). Usually, the electric utility MGN will contribute the most to a low resistance reaing. emember: This kin of test (with everything interconnecte, incluing the electric utility MGN) will not yiel vali results if your goal is to measure only the builing or enclosure earth electroe system. If you can safely isconnect an iniviual electroe component from the grouning electroe system, then you can correctly measure that component. Frozen groun is an insulator. eaings taken uring the summer when the active soil layer is thawe are meaningless for winter. Where the winter frost epth excees the epth of earth electroes, the electroes are insulate from remote earth an the resistance of the electroe system will be very high. To reuce potential ifferences for safety an operation, all metallic components of the telecommunications facility must be bone together. 7.2 Test Instruments: Battery operate test sets with an analog null meter are the best, provie the most consistent measurements an are the most reliable. The han-crank test set by Bile an other manufacturers works fine but are relatively ifficult an awkwar to use. Some igital test sets most often o not yiel usable ata. This is particularly the case if the earth electroe system being measure is near a power plant or substation. Always test your test set before taking it into the fiel. The manual tells you how to o this. Take an extra set of batteries. If the test set uses rechargeable batteries, make sure they are not worn out an have enough charge for the measurements to be mae. Occasionally check calibration especially if the test set has not been taken care of in the fiel or in storage or if it has been in storage for a long time. The manual tells you how to o this (you will nee some resistors). For earth electroe measurements, always short the P1 an C1 terminals together or use separate test leas, one from P1 to the earth electroe an one from C1 to the earth electroe. The latter provies the best accuracy. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 19
7.3 Test Leas General Information: Use a heavy-gauge (14-16 AWG) test lea wire an lots of it. The lea connecte to the current probe (C2) shoul have a total overall length of 300-400 ft or more The lea connecte to the potential probe (P2) shoul have a total overall length of 300-400 ft or more (it shoul have the same overall length as the C2 lea) The lea connecte from C1/P1 to the grouning electroe system shoul be as short as possible, preferably less than 10 ft long. If this lea is too long, the reaings will not be reliable. The resistance of this lea irectly affects the measure resistance. One way to eliminate this measurement error is to use a separate lea from the C1 terminal to the grouning electroe system an a separate lea from the P1 terminal to the grouning electroe. 7.4 Test Lea Kit: You will nee approximately 600 ft 800 ft of test lea wire (300 ft 400 ft for P2 an 300 ft 400 ft for C2). Make up two test leas, each 100 ft long, with a large (2 in. or 3 in.) alligator clip at one en an 1/4 in. fork terminal lug at the other. One will connect to the C2 terminal an the other to the P2 terminal on the test set. Use rubber insulating boots over the alligator clips Use the remaining test lea wire to make 100 ft sections with large insulate alligator clips at each en. Make up one test lea, 10 ft long, with large alligator clip at one en an 1/4 in. fork terminal lug at the other. This will be use to connect P1/C1 terminal on the test set to the grouning electroe system. Also make up extra test leas if it is necessary to locate the test set some istance from the connection point at the earth electroe (one test lea for C1 an one for P1). Put test leas on in. or 14 in. Corwheels. These are available from harware stores (Home Depot, Lowes) an are esigne for coiling extension cors but work well with test leas. Put spare terminal lugs an alligator clips in the kit along with a crimping tool, screwriver an any other tools you may nee to repair the test leas in the fiel. Be sure to inclue a set of goo gloves. The test lea kit also shoul inclue safety glasses, gloves, an a short-hanle, 4 lb., ouble-hea hammer, an test probes (see below). 7.5 Test Leas Connections: When stringing out the test leas, make sure the insulation is not amage. Use gloves to hanle the wire, especially if the leas are strung below or above power lines. Where test lea sections are connecte together to make a longer lea, be sure to insulate the alligator clips from the earth an foliage. Use 8 in. lengths of 1-1/2 in. or 2 in. flexible non-metallic conuit or other insulating material as insulator sleeves (these are slippe over the alligator clips to keep them from touching the groun). Be sure to clean the point of connection to the grouning electroe system. Use ScotchBrite or wire brush. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 20
The farther the current probe (C2) is from the grouning electroe system, the better. The object is to get the P2 probe out of the influence of the grouning electroe system when P2 is 61.8% of the istance to C2. If the largest imension of the electroe system is x ft, the C2 probe shoul be 10x ft to 20x ft from the electroe system. For example, if the grouning electroe is 20 ft x 20 ft, C2 shoul be at least 200 ft to 400 ft away. Unfortunately, the extent of the grouning system usually is unknown, so you will try to place the C2 probe as far away as possible. To calculate resistance, you almost always will have to use the Intersecting Curves or the Slope metho.. Never isconnect the ac service equipment from the earth electroe system. Never isconnect working telecommunications equipment from groun. In a new central office, with no live equipment an no live ac service, it will be possible to test iniviual grouning system components (well casing, gri, groun ros, builing structural steel, etc.) if they can be isolate from each other. Otherwise, leave everything interconnecte an measure the overall system this is for your safety. 7.6 Test Probes: Test probes shoul be 18 20 in. long an mae from strong galvanize steel. They shoul have a loop at one en an sharpene to a point at the other. Purchase reay-mae probes from Bile (Fig. 15) or make your own but carry extras. It is a goo iea to carry a couple longer probes mae from a cut groun ro, 3 ft 4 ft long, for use where the shorter probes o not make goo enough contact with the earth. Wel a T-bar near the top of these longer ros so you can twist an pull them out of the groun. The probes normally only have to be riven to a epth of 6 in.. in. The eeper the probe the harer it is to pull out. Wear safety glasses when riving the probes. emove probes by twisting an pulling on the loop at the top of the probe; o not hit the sie of the probe with a hammer or kick it to loosen it because all you will o is ben the probe an ruin it. If you have to place the probes in a parking area or roaway with very har groun, concrete or pavement, use metal plates in place of the pointe probes. Lay the plates on the groun an wet them an the area aroun them thoroughly. An alternative is to rill through the pavement or concrete with a hammer rill. Be sure the alligator clip makes a goo connection where it is connecte to the probe. Use 3M ScotchBrite or wire brush to clean the probes where you connect the alligator clips. P2 shoul be place in a straight line between the connection point an C2. Sometimes it will be necessary to place C2 in one irection but move in another irection with P2, although some test set manufacturers recommen against this proceure. If you o this, move P2 in a straight line away from the connection point an away from the electroe system. Avoi placing the probes in line with unergroun facilities (electric, telephone, metallic water an sewer pipes, etc.) but sometimes you have no choice. Be aware of shallow unergroun facilities, such as telephone cables, when you rive the probes. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 21
7.7 eaings: Start your reaings with the P2 probe 10 ft 20 ft from the point of connection. Take a reaing every 10 ft to within 20 ft 30 ft of C2. You will notice the reaings are low when you are close to the point of connection an slowly climb. If C2 is far enough away, the reaings will level off in the mile. As you approach C2, the reaings climb rapily. If C2 is far from the influence of the electroe system, the reaings will level off near the 61.8% point. When P2 is closer to the connection point, the reaings will be lower an when closer to C2, the reaings will be higher. eaings that ecrease as you move away from the connection point inicate unergroun metallic facilities that are interfering with the measurements or that the P2 probe is over the grouning electroe. An occasional ip in the reaings is not unusual, but if several reaings follow this pattern, you have to move C2 an start over. Always take at least three separate sets of reaings. Usually, you can take two sets of reaings with the C2 probe at two ifferent istances along the same irection, say 300 ft an 250 ft, although 400 ft an 300 ft are preferable. Take the thir set of reaings in another irection if possible. Depening on the topography an obstructions, you may have to take all three reaings in the same irection (say, 400 ft, 300 ft an 200 ft or 300 ft, 250 ft an 200 ft). If you are able to move the test set analog meter neele to either sie of null (with the resistance knobs), the probes are eep enough. If the neele moves only to one sie (usually the sie), one or both probes are not eep enough, you have a ba connection, or the alligator clips use to splice lea sections are in contact with the earth. Check all connections an try riving the probes a little eeper or use the longer probes. You also can try pouring water aroun the probe. In any case, you must be able to move the neele to both sies of null with the resistance knobs; otherwise, the reaing is not reliable. With analog instruments, the null sometimes will be lazy (move slowly as you ajust the resistance knobs), which inicates unergroun metallic facilities or inaequate probe contact. For the latter situation, rive the probes eeper or use longer probes. With analog instruments, the meter sometimes will oscillate aroun null. You will have to ajust the resistance knobs so the meter moves an equal istance left an right of null (center) an then take a reaing; this takes practice an a little practice. Digital instruments inclue fault inicators that inicate excessively high probe resistance or ba connections. If you see this inication, check all connections an, if necessary, rive the probes eeper. 7.8 Data euction: The actual electroe system resistance is measure when P2 is 61.8% of the istance C2 is from the center of the electroe system. However, the actual center of the electroe system selom is known except in small systems, so the location of this 61.8% point is never known accurately. A small system woul be a few groun ros that are not connecte to anything else other than each other. If measuring a small system, you can put C2 outsie the influence (10X 20X as mentione above) an then put P2 at a point 61.8% of this istance. The reaing you take at this point will be close to the actual electroe system resistance. Aitional eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 22
reaings are not necessary, but they shoul be taken anyway to ensure that the ata is vali. If measuring larger systems, you shoul use the Intersecting Curves or Slope metho. Fig. 15 - T-Hanle, Factory-Mae Test Probes in a kit of four 8. Soil esistivity Measurements Soil resistivity is a measure of how well the soil passes electrical current. Soil with high resistivity oes not pass current well. esistivity is efine in terms of the electrical resistance of a cube of soil. The resistance of this cube, as measure across its faces, is proportional to the resistivity an inversely proportional to the length of one sie of the cube. The resistivity of the soil in which the earth electroe is burie is the limiting factor in attaining low electroe resistance. Soil with low resistivity is better for builing low resistance earth electroes than soil with high resistivity. Soil resistivity epens on soil type, salt concentration, moisture content an temperature. Formulas for specific electroe configurations are available in reference hanbooks. Therefore, if the soil resistivity is known, the resistance of many common earth electroe configurations can be reaily calculate. These formulas are beyon the scope of the present article. 1 From an operational stanpoint, soil resistivity measurements allow for initial earth electroe esign base on these formulas an in preiction of the effects of changes to an existing electroe. The resistivities of various soil types are given in Table 4. A wie range of soil types may be foun in any given area or the soil types may vary wiely with epth, or both. The season an weather conitions will greatly affect the soil resistivity. For example, high resistivity rock can be overlain by clays that have low resistivity in the summer but have high resistivities in the winter when frozen. 1 See, for example, MIL-HDBK-419a, Grouning, Boning, an Shieling for Electronic Equipments an Facilities an ural Utilities Service Bulletin 1751F-802, Electrical Protection Grouning Funamentals. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page
esistivity measurements provie an average reaing. The metho almost always use an escribe here assumes the soil is homogeneous (all the same or similar kin). Soil resistivity is measure by injecting a current into the earth, measuring the voltage rop, an then calculating the resistance. Four-terminal test sets o the resistance calculations automatically. If the four probes are place equiistance from each other an in a straight line as shown in Fig. 13, the resistivity can be calculate from (for separation measure in ft) 1. 915a ft ohm-m where a = Soil resistivity, ohm-m = Distance between the four equally space probes, ft = Measure resistance, ohms or (for separation measure in m) 6. 283a m ohm-m where a = Soil resistivity, ohm-m = Distance between each of the four equally space probes, m = Measure resistance, ohms Be careful with units of measurement an be sure to use the correct formula. Most soil resistivity tables are in units of ohm-m or ohm-cm but most istance or length measurements in the Unite States are (unfortunately) in ft. The above formulas are vali only if the probes are burie no more than 1/20 of the separation istance, a. For example, if the probes are space 5 ft (1.5 m), they cannot be riven more than about 3 in. (0.075 m). Every effort shoul be mae to rive all four probes to the same epth. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 24
Fig. 13 - Soil esistivity Measurement Setup This type of measurement provies the average resistivity to a epth equal to the probe separation istance. For example, if the probes are space 10 ft (3.05 m), the measurement provies the average resistivity to a epth of about 10 ft. Generally, soil resistivity measurements are taken at several probe spacings starting at 5 or 10 ft spacing an then increasing in 5 or 10 ft increments. The measurements may inicate that soil resistivity ecreases ramatically as the epth increases. If so, the earth electroes shoul be burie or riven eep to take avantage of the low resistivity. Fig. 14 shows the basic setup with 30 ft probe spacing. Fig. 15 shows measurements taken at a site in Anchorage, AK near Elmenorf Air Force Base. A fiel ata form for recoring soil resistivity measurements is provie in Appenix V. eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 25
Fig. 14 - Basic Soil esistivity Measurement Setup with 30 ft probe spacing eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 26
Fig. 15 - Soil esistivity Measurements at a site near Elmenorf AFB (curve has been smoothe by Excel charting function) 1,200 Average Soil esistivity vs Depth 1,000 Average Soil esistivity (ohm -m) 800 600 400 200 0 0 10 20 30 40 50 60 Depth (ft) eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 27
Table 4 - Soil Characteristics Approximate Soil esistivity Source: Table 2-2, MIL-HDBK-419A Soil Type esistivity (ohm-m) Wet organic soil 10 Moist soil 100 Dry soil 1,000 Be rock 10,000 Detaile Soil esistivity Source: Table 2-3, MIL-HDBK-419A esistivity (ohm-m) Minimum Average Maximum Soil Type Surface soils, loam 1 500 Clay 2 100 San an gravel 50 1,000 Surface limestone 100 10,000 Limestones 5 400 Shales 5 100 Sanstone 20 2,000 Granites, basalts 10,000 Decompose gneiss 50 500 Slates 10 100 Fresh water lakes 200 Tap water 10 50 Sea water 0.2 1 2 Pastoral, low hills, rich soil (typical of Dallas, TX an Lincoln, NE areas) Flat country, marshy, ensely wooe (typical of LA near Mississippi iver) Pastoral, meium hills an forestation (typical of MD, PA, NY, exclusive of mountainous territory an seacoasts) 30 2 100 ocky soil, steep hills (typical of New Englan) 10 500 1,000 Sany, ry, flat (typical of coastal country) 300 500 5,000 City, inustrial areas 1,000 10,000 Fills, ashes, ciners, brine, waste 6 25 70 Clay, shale, gumbo, loam 3 40 200 Same with varying proportion of san an gravel 10 150 1,000 Gravel, sanstones with little clay or loam, granite 500 1,000 10,000 Note: To convert to ohm-cm, multiply by 100 (for example, 100 ohm-m x 100 = 10,000 ohm-cm) 200 eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 28
Appenix I Fall-of-Potential Metho Fiel Data Form Place the C2 probe as far as possible from the groun uner test (typically 200, 300 an 400 ft.). Place the P2 probe at 10 or 20 ft. intervals in a straight line with C2. Measure an recor the resistance reaing on the test set. P2 (ft) (ohms) (ohms) (ohms) Site Info 0 C2 = C2 = C2 = Test Set Moel: 10 Test Set S/N: 20 Groun System Type: 30 Temperature: 40 Weather: 50 Soil Moist or Dry: 60 Soil Type (Circle): 70 Loam 80 San & gravel 90 Shale 100 Clay 110 Limestone 0 Sanstone 130 Granite 140 Slate 150 Other 160 170 180 190 Notes: 200 210 220 0 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 Location: Conitions: Date: By: eeve Engineers 2008, File: Groun Testing 1-1.oc, Page 29