Understanding Earth Ground Resistance by AEMC
Contents
1. Ground shield me oJ e e ere Qu round gt ye Float shield Float shield strip Connect all three Fa z shields together Y electrode Z electrode xD Ground rod Vx Ss ees FIGURE 25 NOTES Understanding Ground Resistance Testin Q m 19 NOTES Excessive Auxiliary Rod Resistance The inherent function of a fall of potential ground tester is to input a constant current into the earth and measure the voltage drop by means of auxiliary electrodes Excessive resistance of one or both auxiliary electrodes can inhibit this function This is caused by high soil resistivity or poor contact between the auxiliary electrode and the surrounding dirt Figure 26 To ensure good contact with the earth stamp down the soil directly around the auxiliary electrode to remove air gaps formed when inserting the rod If soil resistivity is the problem pour water around the auxiliary electrodes This reduces the auxiliary electrode s contact resistance without affecting the measurement A ZZZ 7 7 7777777 7 7 asa o Cree eee eee ft te if Air gaps EARTH FIGURE 26 Tar or Concrete Mat Sometimes a test must be performed on a ground rod that is surrounded by a tar or concrete mat where auxiliary electrodes cannot be driven easily In such cases metal screens and water can be used to replace auxiliary electrodes as shown in Figu2. a Proper operation of electrical equipment b Safety c Meet National Electrical Code requirements If a 5 8 inch ground rod is supposed to measure 25Q and the local soil resistivity measures 20kQ cm approximately how deep must the rod be driven a 10 feet b 25 feet c 40 feet d 50 feet Fall of potential ground resistance measure ments are recommended when a The ground under test can be conveniently disconnected b Ground faults are likely to occur near the round under test c The power system cannot be shut down 32 wr ssCs lt A rn Understanding Ground Resistance Testing 11 12 13 14 15 When performing fall of potential tests the ground electrode should be a Inservice and energized b Disconnected and de energized c It makes no difference What is the minimum number of measure ments needed to accurately perform a fall of potential test 1 e o S b 2 3 5 If when making a fall of potential test each test result is significantly different in value from previous measurements on the same rod what corrective action should be attempted a Position the Z electrode farther from the rod under test b Position the Z electrode closer to the rod under test What is the maximum ground resistance required by the National Electrical Code a 5Q b 159 c 259 d 10 When testing a multiple electrode grid auxiliary electrode spacing is determined by a Depth of the d
3. between the electrodes must then be equivalent to the depth at which average resistivity is to be determined 15 ft or 450 cm Using the more simplified Wenner formula p 2 AR the electrode depth must then be 1 20th of the electrode spacing or 8 7 8 22 5 cm FIGURE 7 Oz Understanding Ground Resistance Testing NOTES Lay out the electrodes in a grid pattern and connect to the Model 4500 as NOTES shown in Figure 8 Proceed as follows e Remove the shoring link between X and Xv C1 P1 e Connect all four auxiliary rods Figure 7 For example if the reading is R 15 p resistivity 27 x Ax R A distance between electrodes 450 cm p 6 28 x 15 x 450 42 390 Q cm GROUND ELECTRODES The term ground is defined as a conducting connection by which a circuit or equipment is connected to the earth The connection is used to establish and maintain as closely as possible the potential of the earth on the circuit or equipment connected to it A ground consists of a grounding conductor a bonding connector its grounding electrode s and the soil in contact with the electrode Grounds have several protection applications For natural phenomena such as lightning grounds are used to discharge the system of current before per sonnel can be injured or system components damaged For foreign potentials due to faults in electric power systems with ground returns grounds help ensure rapid op
4. Observe all safety requirements since dangerously high voltage is present Locate and number all rods usually only a single rod is present If the ground rods are inside the enclosure refer to Figure 34 and if they are out side the enclosure refer to Figure 35 If a single rod is found within the enclosure the measurement should be taken on the conductor just before the bond on the ground rod Often more than one ground conductor is tied to this clamp looping back to the enclosure or neutral In many cases the best reading can be obtained by clamping the 3711 3731 onto the ground rod itself below the point when the ground conductors are attached to the rod so that you are measuring the ground circuit Care must be taken to find a conductor with only one return path to the neutral Enclosure Open door Open door e mae C2 Concentric Underground service neutral Ground rod s FIGURE 34 Enclosure Underground service Ground FIGURE 35 2 Understanding Ground Resistance Testing Transmission Towers NOTES Observe all safety requirements since dangerously high voltage is present Locate the ground conductor at the base of the tower Note Many different configurations exist Care should be taken when searching for the ground conductor Figure 36 shows a single leg mounted on a concrete pad with an external ground conductor The point at which you clamp the ground tester should be above all splices and connection
5. UNDERSTANDING GROUND RESISTANCE TESTING Ammeter 1 Voltimeter E Auxiliary Auxiliary potential current electrode electrode Ground rod and clamp 1 2 58 3 4 1 11 4 11 2 13 4 Rod diameter inches Contact resistance between rod and soil Soil Resistivity Concentric shells of Ground Resistance 3 Point Measurements 4 Point Measurements e Clamp on Measurements TABLE OF CONTENTS Soil RESISEVILY sssandossacdncespiapesscercassntrnintniaanerustiviachoteeloninuisannsinguuslieduntpedtateieiuatss 2 Soil Resistivity Measurements 4 Point Measurement 4 Ground Eleodes umu nan a Sun asawa qatun E S sasi 6 Ground Resistance Values 9 Ground Resistance Testing Principle Fall of Potential 3 Point Measurement 11 Multiple Electrode Syste s isccscincianssenesttnastionenesesbesincatenaninenereiaiana nines 16 Two Point Measurement Simplified Method 17 Continuity M eas reme tu sasana sayan yaaa sus ss assays 17 Tech TIPS ua asua aaa aapasaya Gaqiya swkakiakaqiawshisiashakkhakasaaqdasyaqw iqu ss akikasadsqa 18 Touch Potential Measurement cecscscceeeeceeseeeeeeeseeeeeaeseeeeeaeeeeeeaeenee 21 Clamp on Ground Resistance Measurement Models 3710 and 3731
6. In such situations it may THE EFFECT OF SALT CONTENT ON be economical to use a THE RESISTIVITY OF SOIL ground rod system of Sandy loam Moisture content 15 by weight limited size and to Temperature 17 C reduce the ground Added Salt Resistivity resistivity by periodi by gs of moisture ees cally increasing the sol 1 1 800 uble chemical content 1 460 of the soil Figure 5 190 shows the substantial 130 reduction in resistivity 100 of sandy loam brought FIGURE 5 about by an increase in chemical salt content Chemically treated soil is also subject to considerable variation of resistivity with temperature changes as shown in Figure 6 If salt treatment is employed it is necessary to use ground rods which will resist chemical corrosion THE EFFECT OF TEMPERATURE ON THE RESISTIVITY OF SOIL CONTAINING SALT Sandy loam 20 moisture Salt 5 of weight of moisture Temperature Resistivity Degrees C Ohm centimeters 20 110 10 142 0 190 5 312 13 1 440 FIGURE 6 Such as copper sulfate sodium carbonate and others Salts must be EPA or local ordinance approved prior to use SOIL RESISTIVITY MEASUREMENTS 4 Point Measurement Resistivity measurements are of two types the 2 point and the 4 point method The 2 point method is simply the resistance measured between two points For most applications the most accurate method is the 4 point method which is used in the Model 4610 or Model 4500 Ground Tester The 4 p
7. by a level comparator If the clamp is not closed properly an open Jaw annunciator appears on the LCD FIGURE 30 Understanding Ground Resistance Testin Q m 23 24 NOTES Examples Typical In Field Measurements Pole Mounted Transformer Remove any molding covering the ground conductor and provide sufficient room for the Model 3711 3731 jaws which must be able to close easily around the conductor The jaws can be placed around the ground rod itself Note The clamp must be placed so that the jaws are in an electrical path from the system neutral or ground wire to the ground rod or rods as the circuit provides Select the current range A Clamp onto the ground conductor and measure the ground current The maximum current range is 30A If the ground current exceeds 5A ground resistance measurements are not possible Do not proceed further with the measurement Instead remove the clamp on tester from the circuit noting the location for maintenance and continue to the next test location After noting the ground current select the ground resistance range Q and measure the resistance directly The reading you measure with the 3711 3731 indicates the resistance not just of the rod but also of the connection to the system neutral and all bonding connections between the neutral and the rod Note that in Figure 31 there is both a butt plate and a ground rod In this type of circuit the instrument must be pla
8. already available In congested areas where finding room to drive the two auxiliary rods may be a problem the two point measurement method may be applied The reading obtained will be that of the two grounds in series Therefore the water pipe or other ground must be very low in resistance so that it will be negligible in the final measurement The lead resistances will also be measured and should be deducted from the final measurement This method is not as accurate as three point methods 62 method as it is particularly affected by the distance between the tested electrode and the dead ground or water pipe This method should not be used as a standard procedure but rather as a back up in tight areas See Figure 20 Grounding conductor Terminals shorted Auxiliary rod Ground rod with jumper wire Y Z shorted Utility pole Ground level AEG Butt plate SD FIGURE 20 CONTINUITY MEASUREMENT Continuity measurements of a ground conductor are possible by using two terminals Figure 21 FIGURE 21 Understanding Ground Resistance Testing 17 NOTES TECH TIPS Excessive Noise Excessive noise may interfere with testing because of the long leads used to perform a fall of potential test A voltmeter can be utilized to identify this problem Connect the X Y and Z cables to the auxiliary electrodes as for a standard gro
9. pair In order to protect against an overvoltage situation on the telephone wires a protector block is installed inside the NID This protector has TIP RING two internal devices that conduct only when poe iQ unwanted overvoltages K Oo are present In order for 11 Bond protector the protector to function Ry properly it must have a low resistance path for any fault to conduct to hi satel ria earth This bonding and ground resistance poten tial can be verified by using the clamp on ground resistance tester Simply take a short piece of wire and temporarily jumper the tip side CO ground to the ground connector on the protector block By clamping around this jumper wire you will now test the ground resistance potential including all terminations at this location The return signal path required for the clamp on ground tester to make this measurement will be the CO ground FIGURE 41 Overhead Telephone Distribution Telephone systems delivered on overhead points must also be bonded to the MGN This is typically performed by supplying a No 6 copper wire connected to the grounding strand above telephone space If power is not supplied on these points driven ground rods must be installed at required point intervals and subsequently tested Note Coil wire for attachment to power company MGN __ Grounding conductor Ground level Butt
10. Connections to fence NOTES Anticipated fault point Driven ground rods FIGURE 28 Consider the following scenario If the buried cable depicted in Figure 28 experienced an insulation breakdown near the substation shown fault currents would travel through the earth towards the substation ground creating a voltage gradient This voltage gradient may be hazardous or potentially lethal to personnel who come in contact with the affected ground To test for approximate touch potential values in this situation proceed as follows Connect cables between the fence of the substation and C1 and P1 of the four pole earth resistance tester Position an electrode in the earth at the point at which the ground fault is anticipated to occur and connect it to C2 Ina straight line between the substation fence and the anticipated fault point position an auxiliary electrode into the earth one meter or one arm s length away from the substation fence and connect it to P2 Turn the instrument on select the 10 mA current range and observe the measurement Multiply the displayed reading by the maximum fault current of the anticipated fault By positioning the P2 electrode at various positions around the fence adjacent to the anticipated fault line a voltage gradient map may be obtained 22 Understanding Ground Resistance Testing CLAMP ON GROUND NOTES RESISTANCE MEASUREMENT Models 3711 and 3731 This measurement metho
11. ake a reading at each location See Figure 16 If the auxiliary potential rod Y is in an effective resistance area or in both if they overlap as in Figure 14 by displacing it the readings taken will vary noticeably in value Under these conditions no exact value for the resistance to ground may be determined NOTES Understanding Ground Resistance Testin Q m 11 On the other hand if the auxiliary potential rod Y is located outside of the NOTES effective resistance areas Figure 15 as Y is moved back and forth the reading variation is minimal The readings taken should be relatively close to each other and are the best values for the resistance to ground of the ground X The readings should be plotted to ensure that they lie in a plateau region as shown in Figure 15 The region is often referred to as the 62 area See page 13 for explanation x Y Y Y Z S Effective resistance i areas overlapping Resistance 52 62 72 BP _ TA of total distance from X to Z Reading variation y FIGURE 14 Effective resistance areas no overlap of total distance from X to Z Reading variation gt Resistance FIGURE 15 22 EE SE _ E E_ _ _ _ _ _ Understanding Ground Resistance Testing Measuring Resistance of Ground Electrodes 62 Method The 62 method has been adopted after graphical consideration and after actual test It is the most accurate metho
12. ced above the bond so that both grounds are included in the test For future reference note the date ohms reading current reading and point number Replace any molding you may have removed from the conductor Note A high reading indicates one or more of the following A poor ground rod B open ground conductor C high resistance bonds on the rod or splices on the conductor watch for buried split butts clamps and hammer on connections Grounding conductor Ground level FIGURE 31 Understanding Ground Resistance Testing Service Entrance or Meter Follow basically the same procedure as in the first example Notice that Figure 32 shows the possibility of multiple ground rods and in Figure 33 the ground rods have been replaced with a water pipe ground You may also have both types acting as a ground In these cases it is necessary to make the measurements between the service neutral and both grounded points Building wall gt Pole mounted transformer 4 Service box Ground level FIGURE 32 Ground rods Building wall gt lt 4 Pole mounted transformer Service meter Understanding Ground Resistance Testing lt Water pipe FIGURE 33 NOTES 25 Pad Mounted Transformer Note Never open transformer enclosures They are the property of the electrical utility This test is for high voltage experts only NOTES
13. ction Association ee Understanding Ground Resistance Testing GROUND RESISTANCE TESTING PRINCIPLE Fall of Potential 3 Point Measurement The potential difference between rods X and Y is measured by a voltmeter and the current flow between rods X and Z is measured by an ammeter Note X Y and Z may be referred to as X P and C in a 3 point tester or C1 P2 and C2 in a 4 point tester See Figure 13 By Ohm s Law E RI or R E I we may obtain the ground electrode resistance R If E 20 V and I 1 A then R E 20 20 It is not necessary to carry out all the measurements when using a ground tester The ground tester will measure directly by generating its own current and displaying the resistance of the ground electrode Current Ammeter I Voltimeter E Ground Auxiliary Auxiliary electrode X potential Y current zZ under test N electrode electrode 777 ZZ 7 Z ZZ sZ ZZ v EARTH FIGURE 13 Position of the Auxiliary Electrodes on Measurements 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 ground electrode and the auxiliary current electrode The best way to find out if the auxiliary potential rod Y is outside the effective resistance areas is to move it between X and Z and to t
14. ctrodes will be driven and connected in parallel by a cable Very often when two three or four ground electrodes are being used they are driven in a straight line when four or more are being used a hollow square configuration is used and the ground electrodes are still connected in parallel and are equally spaced Figure 19 In multiple electrode systems the 62 method electrode spacing may no longer be applied directly The distance of the auxiliary electrodes is now based on the maximum grid distance i e in a square the diagonal in a line the total length For example a square having a side of 20 ft will have a diagonal of approximately 28 ft Multiple Electrode System Max Grid Distance Distance to Y Distance to Z 6 ft 78 ft 125 ft 8 ft 87 ft 140 ft 10 ft 100 ft 160 ft 12 ft 105 ft 170 ft 14 ft 118 ft 190 ft 16 ft 124 ft 200 ft 18 ft 130 ft 210 ft 20 ft 136 ft 220 ft 30 ft 161 ft 260 ft 40 ft 186 ft 300 ft 50 ft 211 ft 340 ft 60 ft 230 ft 370 ft 80 ft 273 ft 440 ft 100 ft 310 ft 500 ft 120 ft 341 ft 550 ft 140 ft 372 ft 600 ft 160 ft 390 ft 630 ft 180 ft 434 ft 700 ft 200 ft 453 ft 730 ft 16 s mW Understanding Ground Resistance Testing TWO POINT MEASUREMENT NOTES SIMPLIFIED METHOD This is an alternative method when an excellent ground is
15. d Resistance Testing GROUNDING NOMOGRAPH Ground rod Soil resistivity Rod depth Rod diameter resistance ohms ohm centimeters feet inches D 100 K R 90 100 80 DIA 90 70 8 80 P 60 7 70 100000 50 6 60 50000 40 5 50 40000 30 4 40 30000 3 30 20000 20 2 15000 ia 20 10000 5 15 15 5000 10 1 10 4000 9 3000 3 4 8 2000 7 5 5 8 6 1000 1 2 4 5 500 3 4 2 1 4 3 2 1 Represents example of a 209 20 foot ground rod Select required resistance on R scale Select apparent resistivity on P scale Lay straightedge on R and P scale and allow to intersect with K scale Mark K scale point Lay straightedge on K scale point and DIA scale and allow to intersect with D scale D Q e Ww NY P gt Point on D scale will be the rod depth required for resistance on R scale Understanding Ground Resistance Testing m 35 FALL OF POTENTIAL PLOT Instrument Mfg Name of Operator Model Location Date Ground System Type Serial I I Single Rod Multiple Rods Longest dimension ft Voltage Electrode distance Resistance from Gnd Test Conditions under Test ad Temp Soil Moist Dry FEET OHMS Soil Type Loam Sand amp Gravel Shale Clay Limestone Sandstone A Granite S
16. d but is limited by the fact that the ground tested is a single unit NOTES This method applies only when all three electrodes are in a straight line and the ground is a single electrode pipe or plate etc as in Figure 16 DIGITAL GROUND RESISTANCE TESTER MODEL 4 lt Y Electrode Z Electrode gt I I 10 3rd 10 2nd ey Measurement _ Measurement Serres v S aia 5h i Ground rod Y Electrode Z Electrode x I I z 0 52 62 72 100 of distance of total distance from X to Z between X and Z FIGURE 16 Consider Figure 17 which shows the effective resistance areas concentric shells of the ground electrode X and of the auxiliary current electrode Z The resistance areas overlap If readings were taken by moving the auxiliary potential electrode Y towards either X or Z the reading differentials would be great and one could Ground Auxiliary Auxiliary not obtain a reading electrode potential current ne under test electrode electrode within a reasonable gor band of tolerance The x Y z sensitive areas overlap J and act constantly to increase resistance as Y is moved away from X Overlapping effective resistance areas Resistance gt Distance from Y to ground electrode FIGURE 17 Understanding Ground Resistance Testin Q m B Now consider Figure 18 where the X and Z electrodes are sufficiently spaced so that th
17. d equipment is comparable to the size of the ground to be tested The footprint is the outline of the part of equipment in contact with the earth Neither fall of potential resistance measurements nor touch potential measurements tests the ability of grounding conductors to carry high phase to ground fault currents Additional high current tests should be performed to verify that the grounding system can carry these currents When performing touch potential measurements a four pole ground resistance tester is used During the test the instrument induces a low level fault into the earth at some proximity to the subject ground The instrument displays touch potential in volts per ampere of fault current The displayed value is then multiplied by the largest anticipated ground fault current to obtain the worst case touch potential for a given installation For example if the instrument displayed a value of 100 when connected to a system where the maximum fault current was expected to be 5000A the maximum touch potential would be 5000 x 1 500 volts Touch potential measurements are similar to fall of potential measurements in that both measurements require placement of auxiliary electrodes into or on top of the earth Spacing the auxiliary electrodes during touch potential measurements differs from fall of potential electrode spacing as shown in Figure 28 on the following page Understanding Ground Resistance Testin Q m 21
18. d is innovative and quite unique It offers the ability to measure the resistance without disconnecting the ground This type of measurement also offers the advantage of including the bonding to ground and the overall grounding connection resistances Principle of Operation Usually a common distribution line grounded system can be simulated as a simple basic circuit as shown in Figure 29 or an equivalent circuit shown in Figure 30 If voltage E is applied to any measured grounding point Rx through a special transformer current I flows through the circuit thereby establishing the following equation 1 E I Rx where usually 1 Rx gt gt x A Rk z 1 k 1 y Rk k 1 Therefore E I Rx is established If I is detected with E kept constant measured grounding point resistance can be obtained Refer again to Figures 29 and 30 Current is fed to a special transformer via a power k amplifier from a 2 4 kHz constant voltage oscillator This current is detected by a detection CT Only the 2 4 kHz signal frequency is amplified by a filter amplifier This occurs before the A D conversion and after synchronous eS rectification It is then displayed Rx R1 R2 Rn t Rn on the LCD FIGURE 29 k The filter amplifier is used to cut off both earth current at commercial frequency and high frequency noise Voltage is detected by coils wound around the injection CT which is then amplified rectified and compared
19. d tester can be utilized to ensure that this connection has been successfully terminated The low resistance return path for the Understanding Ground Resistance Testin Q m 29 instrument to make this measurement will be from this bond wire under test NOTES to the MGN back through all other bonds up and or down stream theory of parallel resistance The clamp on ground tester also is a True RMS ammeter Power transformer Bond cable shield to multi grounded pad or pedestal neutral system at A All above ground closures B All pedestal and or transformer locations C At least every 1 000 feet NOTE A bond MUST be made at any above ground closure within 10 feet of any above ground Tel power apparatus enclosure No 6 YV FAS w Grd wire P AS Tel cable and wire ait ower cables Electric company shall make bond connection to power cable and or power apparatus Buried tel Grd wire enclosure PL NC Bee Y Susana 7 we Ba irae u Bond cable shield _ 7 to multiground SL Z 7 d neutral 7 z 7 Telephone cable and wire Buried telephone bre enclosure SS top view _ G sa 565 JOINT BURIED CONSTRUCTION RANDOM SEPARATION FIGURE 40 30 ws rmwnrr Understanding Ground Resistance Testing Network Interface Device NID with a Protector Block NOTES The typical customer connection is achieved with the tip and ring drop cable
20. e areas of effective resistance do not overlap If we plot the resis tance measured we find that the measurements level off when Y is placed at 62 of the distance from X to Z and that the readings on either side of the initial Y setting are most likely to be within the established tolerance band This tolerance band is defined by the user and expressed as a percent of the initial reading 2 5 10 ete NOTES Ground Auxiliary Auxiliary electrode potential current under test electrode electrode eT N AN x yoy x Y z J 62 of D gt 38 of D gt Effective resistance areas do 2 Resistance of not overlap auxiliary current a electrode e eee Resistance of earth electrode i Distance from Y to ground electrode FIGURE 18 Auxiliary Electrode Spacing No definite distance between X and Z can be given since this distance is relative to the diameter of the electrode tested its length the homogeneity of the soil tested and particularly the effective resistance areas However an approximate distance may be determined from the following chart which is given for a homogeneous soil and an electrode of 1 in diameter For a diameter of 1 2 reduce the distance by 10 for a diameter of 2 increase the distance by 10 Approximate distance to auxiliary electrodes using the 62 method Depth Driven Distance to Y Distance to Z 6 ft 45 ft 72 ft 8 ft 50 ft 80 ft 10 ft 55 f
21. eepest rod b Maximum internal grid dimension c The VA rating of equipment being grounded 16 17 18 19 Touch potential measurements are recom mended when a Itis physically impossible to disconnectthe subject ground from service b Determining the degree of electrical safety under fault conditions is considered to be more important than measuring actual ground resistance c The grounding system is extensive and undocumented d All of the above The clamp on test method cannot be used on high tension towers due to their spacing a True b False The clamp on tester must be clamped around the ground rod only a True b False The clamp on tester can be used only if the system under test is energized a True b False 20 The clamp on method of testing should not be performed a When testing large substation grounds b On ground electrodes disconnected from service c On single point lightning protection grounds d All of the above Understanding Ground Resistance Testing m 33 REFERENCES IEEE Std 81 1983 IEEE Guide for Measuring Earth Resistivity Ground Impedance and Earth Surface Potentials of Ground Systems IEEE Std 142 1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems Blackburn American Electric Co Memphis TN 38119 A Modern Approach to Grounding Systems 34 w auum sn rrrsKs rs v nm Understanding Groun
22. eration of the protection relays by providing low resistance fault current paths This provides for the removal of the foreign potential as quickly as possible The ground should drain the foreign potential before personnel are injured and the power or communications system is damaged Ideally to maintain a reference potential for instrument safety protect against static electricity and limit the system to frame voltage for operator safety a ground resistance should be zero ohms In reality as we describe further in the text this value cannot be obtained Last but not least low ground resistance is essential to meet NEC OSHA and other electrical safety standards Figure 9 illustrates a grounding rod The resistance of the electrode has the following components A the resistance of the metal and that of the connection to it B the contact resistance of the surrounding earth to the electrode C the resistance in the surrounding earth to current flow or earth resistivity which is often the most significant factor More specifically A Grounding electrodes are usually made of a very conductive metal copper or copper clad with adequate cross sections so that the overall resistance is negligible ee Understanding Ground Resistance Testing B The National Institute of Standards and Technology has demonstrated that the resistance between the electrode and the surrounding earth is NOTES negligible if the electrode is f
23. ivity through out the volume is assumed although this is seldom the case in nature The equations for systems of electrodes are very complex and often expressed only as approximations The most commonly used formula for single ground electrode systems developed by Professor H R Dwight of the Massachusetts Institute of Technology is the following p 4L R 1 27L a S 1 R resistance in ohms of the ground rod to the earth or soil L grounding electrode length r grounding electrode radius p average resistivity in ohms cm Understanding Ground Resistance Testin Q m 7 Effect of Ground Electrode Size and Depth on Resistance NOTES Size Increasing the diameter of the rod does not materially reduce its resis tance Doubling the diameter reduces resistance by less than 10 Figure 10 Resistance in a ofS 1 2 5 8 3 4 1 11 4 11 2 13 4 Rod diameter inches FIGURE 10 Depth As a ground rod is driven deeper into the earth its resistance is substantially reduced In general doubling the rod length reduces the resis tance by an additional 40 Figure 11 The NEC 1987 250 83 3 requires a minimum of 8 ft 2 4 m to be in contact with the soil The most common is a 10 ft 3 m cylindrical rod which meets the NEC code A minimum diameter of 5 8 inch 1 59 cm is required for steel rods and 1 2 inch 1 27 cm for copper or copper clad steel rods NEC 1987 250 83 2 Minimum practical diameters for driving limi
24. late Other 50 100 45 90 40 80 35 70 I 3 60 25 50 20 40 15 30 10 20 5 10 a as x1 x10 Circles Scales x1 10 20 30 40 50 60 70 80 90 100 amp Multipliers Used x10 5 10 15 20 25 30 35 40 45 50 Distance in Feet from Found under Test to Voltage Electrode 36 m Understanding Ground Resistance Testing P OZ q 61 q 81 q ZT q 91 q ST 9 PT e ET ZT q TL e 01 6 q g q Z p 9 q Pp t q 7 q 1 st wsuv Understanding Ground Resistance Te Sti a ms a TH amaaz 37 Chauvin Arnoux Inc d b a AEMC Instruments 200 Foxborough Blvd Foxborough MA 02035 e USA 508 698 2115 e 800 343 1391 e Fax 508 698 2118 sales aemc com www aemc com
25. mperature changes from 20 to 15 C In this temperature range the resistivity is seen to vary from 7 200 to 330 000 ohm centimeters Temperature Resistivity C F Ohm cm 68 7 200 50 9 900 32 water 13 800 32 ice 30 000 23 79 000 14 330 000 FIGURE 3 Because soil resistivity directly relates to moisture content and temperature it is reasonable to assume that the resistance of any grounding system will vary throughout the different seasons of the year Such variations are shown in Figure 4 Since both temperature and moisture content become more stable at greater distances below the surface of the earth it follows that a grounding system to be most effective at all times should be constructed with the ground rod driven down a considerable distance below the surface of the earth Best results are obtained if the ground rod reaches the water table 80 60 40 Curve 1 i Curve 2 re z gt gt ZZ z x Z gt s 232 3 e 2 s 2 3 FIGURE 4 Seasonal variation of earth resistance with an electrode of 3 4 inch pipe in rather stony clay soil Depth of electrode in earth is 3 ft for Curve 1 and 10 ft for Curve 2 Understanding Ground Resistance Testin Q m 3 In some locations the resistivity of the earth is so high that low resistance NOTES grounding can be obtained only at considerable expense and with an elaborate grounding system
26. oint method Figures 7 and 8 as the name implies requires the insertion of four equally spaced and in line electrodes into the test area A known current from a constant current generator is passed between the outer electrodes The potential drop a function of the resistance is then measured across the two inner electrodes The Model 4610 and Model 4500 are calibrated to read directly in ohms 1 su Fv ssrn m Understanding Ground Resistance Testing 4nAR 1 2A 2A A 4B 4A 4B Where A distance between the electrodes in centimeters B electrode depth in centimeters If A gt 20 B the formula becomes p 22 AR with A in cm p 191 5 AR with A in feet p Soil resistivity ohm cm This value is the average resistivity of the ground at a depth equivalent to the distance A between two electrodes Soil Resistivity Measurements with the Model 4500 Given a sizable tract of land in which to determine the optimum soil resistivity some intuition is in order Assuming that the objective is low resistivity preference should be given to an area containing moist loam as opposed to a dry sandy area Consideration must also be given to the depth at which resistivity is required Example After inspection the area investigated has been narrowed down to a plot of ground approximately 75 square feet 7 m Assume that you need to determine the resistivity at a depth of 15 feet 450 cm The distance A
27. on to the person making the test Even more important the clamp on ground test method allows the user to make this necessary reading without the risky business of removing the ground under test from service In many applications the ground consists of bonding the two Utilities together to avoid any difference of potentials that could be dangerous to equipment and personnel alike The clamp on Ohm meter can be used to test these important bonds Here are some of the solutions and clamp on procedures that have applications to the telephone industry Telephone Cabinets and Enclosures Grounding plays a very important role in the maintainance of sensitive equipment in telephone cabinets and enclosures In order to protect this equipment a low resistance path must be maintained in order for any over voltage potentials to conduct safely to earth This resistance test is performed by clamping a ground tester Model 3711 3731 around the driven ground rod below any common telephone and power company bond connections AC panel board Power meter 120 240V power service Telephone cable Bonding integrity or conduit Ground resistance FIGURE 37 Understanding Ground Resistance Testing To avoid any high voltage potentials between the telephone and power companies a low resistance bond is established Bonding integrity is NOTES performed by clamping around the No 6 copper wire between WATT hour the ma
28. plate Ground wire doubled under ground clamp GROUNDING STRAND WITH NO OVERHEAD POWER FIGURE 42 Understanding Ground Resistance Testin Q m 31 SUMMARY QUIZ 1 When using the simplified Wenner formula 6 p 2nAR for determining soil resistivity four pole electrode depth should be 1 2 of the electrode spacing 1 20 of the electrode spacing 2 times the electrode spacing Equal to the electrode spacing aoe 2 What factors determine soil resistivity 7 Soil type Amount of moisture in soil Amount of electrolytes in soil Temperature All of the above choos 3 When doing a soil resistivity test and placing auxiliary rods at a spacing of 15 feet what depth of earth is being measured 7 5 feet 15 feet 30 feet 60 feet aoe 4 What results can be obtained by doing a soil resistivity measurement Geophysical surveys Corrosion analysis Electrical grounding design All of the above Roo 5 As the temperature of the soil decreases what 10 happens to the soil resistance a Decreases b Increases c No change Doubling the diameter of the rod has what effect on the potential resistance of a ground rod to be installed 100 reduction 50 reduction 25 reduction Less than a 10 reduction e o S b As a general rule doubling the depth of the rod length reduces the resistance by a 100 b 40 c Less than 10 What is the most important reason for good grounding practices
29. re 27 Place the screens on the floor the same distance from the ground rod under test as you would auxiliary electrodes in a standard fall of potential test Pour water on the screens and allow it to soak in These screens will now perform the same function as would driven auxiliary electrodes Ae ad Ground rod ARNG FIGURE 27 Screens 20 m Understanding Ground Resistance Testing TOUCH POTENTIAL NOTES MEASUREMENTS The primary reason for performing fall of potential measurements is to observe electrical safety of personnel and equipment However in certain circumstances the degree of electrical safety can be evaluated from a different perspective Periodic ground electrode or grid resistance measurements are recommended when 1 The electrode grid is relatively small and is able to be conveniently disconnected 2 Corrosion induced by low soil resistivity or galvanic action is suspected 3 Ground faults are very unlikely to occur near the ground under test Touch potential measurements are an alternative method for determining electrical safety Touch potential measurements are recommended when 1 Itis physically or economically impossible to disconnect the ground to be tested 2 Ground faults could reasonably be expected to occur near the ground to be tested or near equipment grounded by the ground to be tested 3 The footprint of grounde
30. ree of paint grease or other coating and if the earth is firmly packed C The only component e D remaining is the resistance of Ground red the surrounding earth The and clamp electrode can be thought of as being surrounded by i Contact concentric shells of earth or Perens soil all of the same thick between rod ness The closer the shell to the electrode the smaller its shells of I l l l 44 p11 Concentric I l l I I surface hence the greater its ean earth Sue resistance The farther away Scie P q the shells are from the elec Aa R PN Se 2 ZC 32 trode the greater the surface T ec eee EEA YYA cal nee eee Pid of the shell hence the lower SUS ecco FIGURE 9 the resistance Eventually adding shells at a distance from the grounding electrode will no longer noticeably affect the overall earth resistance surrounding the electrode The distance at which this effect occurs is referred to as the effective resistance area and is directly dependent on the depth of the grounding electrode In theory the ground resistance may be derived from the general formula pL Length R a Resistance Resistivity x Sen Area This formula illustrates why the shells of concentric earth decrease in resistance the farther they are from the ground rod R Resistivity of Soilx Thickness of Shell Area In the case of ground resistance uniform earth or soil resist
31. roper application of basic grounding theory There will always exist circumstances which will make it difficult to obtain the ground resistance required by the NEC or other safety standards When these situations develop several methods of lowering the ground resistance can be employed These include parallel rod systems deep driven rod systems utilizing sectional rods and chemical treatment of the soil Additional methods discussed in other published data are buried plates buried conductors counterpoise electrically connected building steel and electrically connected concrete reinforced steel Electrically connecting to existing water and gas distribution systems was often considered to yield low ground resistance however recent design changes utilizing non metallic pipes and insulating joints have made this method of obtaining a low resistance ground questionable and in many instances unreliable The measurement of ground resistances may only be accomplished with specially designed test equipment Most instruments use the fall of potential principle of alternating current AC circulating between an auxiliary electrode and the ground electrode under test The reading will be given in ohms and represents the resistance of the ground electrode to the surrounding earth AEMC has also recently introduced clamp on ground resistance testers Note The National Electrical Code and NEC are registered trademarks of the National Fire Prote
32. s which allow for multiple rods butt wraps or butt plates Central Office Locations The main ground conductor from ground window or ground plane is often too large to clamp around Due to the wiring practices within the central office there are many locations at which you can look at the water pipe or counterpoise from within the building An effective location is usually at the ground buss in the power room or near the backup generator By measuring at several points and comparing the readings both of current flow and resistance you will be able to identify neutral loops utility grounds and central office grounds The test is effective and accurate because the ground window is connected to the utility ground at only one point according to standard practices Concrete we pad Ground rod FIGURE 36 Understanding Ground Resistance Testin Q m 27 28 NOTES TELECOMMUNICATIONS The clamp on ground tester developed by AEMC and discussed in the previous chapter has revolutionized the ability of power companies to measure their ground resistance values This same proven instrument and technology can be applied to telephone industries to aid in detecting grounding and bonding problems As equipment operates at lower voltages the system s ability to remove any manmade or natural overpotentials becomes even more critical The traditional fall of potential tester proved to be labor intensive and left a lot of interpretati
33. ssssciscsscsecctesectesedscceaseeshisk Santi caeaccs eaaa siaaa 23 Telecomm Call Ons eiyan ua ete See Aiea asap ashi 28 Summary DITA aan incnawnronitansaunestaanvananuieebawninaadaeenitanatesininesaantsadiaeisbnnkanarisne 32 References a una ua aaah adware dae N Ka Hd 34 Grounding Nomograph jcpasansssncascersanintinensininnssrssseivesondmamnonninsasaeneet 35 Fall of Potential Plot ua assasi cvevd seniteieneuavinednsteeateasient veenesetinee steeds 36 Models 3711 3731 have replaced Models 3710 3730 Workbook Edition 7 0 950 WKBK GROUND 08 00 NOTES SOIL RESISTIVITY Why Measure Soil Resistivity Soil resistivity measurements have a threefold purpose First such data are used to make sub surface geophysical surveys as an aid in identifying ore locations depth to bedrock and other geological phenomena Second resis tivity has a direct impact on the degree of corrosion in underground pipelines A decrease in resistivity relates to an increase in corrosion activity and therefore dictates the protective treatment to be used Third soil resis tivity directly affects the design of a grounding system and it is to that task that this discussion is directed When designing an extensive grounding system it is advisable to locate the area of lowest soil resistivity in order to achieve the most economical grounding installation Effects of Soil Resistivity on Ground Electrode Resistance Soil resistivity is the key factor that determines
34. ster ground bar MGB Hi and the power company s multi Transe 16 Z mai grounded neutral MGN The switch cabinet resistance value displayed on the tester will also include loose or poorly landed terminations that Lightning may have degraded over time arrester Power co ground Additionally the clamp on Tetee ground tester can be used as a ooma True RMS ammeter Ground rod 8 ft long NOTE If seperate ground rods are used for telephone and power grounds the ground rods must be bonded together using no 6 ground wire FIGURE 38 Pedestal grounds All cable sheaths are bonded to a ground bar inside each pedestal This ground bar is connected to earth by means of a driven ground rod The ground rod resistance can be found by using the instrument clamped around the ground rod or the No 6 cable connecting these two points See Figure 39 1 Remove ground bar 2 Extend pedestal rod lead to the connection to pedestal rod ground bar using a temporary jumper Phone Phone Pedestal pedestal gt Ground Bar Ground bar Sheath connection Sheath Connection Ground level Ground level Grong Note temporary jumper required only if pedestal does not allow tester to fit FIGURE 39 Cable shield bonds to MGN The cable shields in a buried or above ground telephone enclosure may be grounded by means of the power company s multigrounded neutral The clamp on groun
35. t 88 ft 12 ft 60 ft 96 ft 18 ft 71 ft 115 ft 20 ft 74 ft 120 ft 30 ft 86 ft 140 ft l4 ee Understanding Ground Resistance Testing Multiple Rod Spacing NOTES Parallel multiple electrodes yield lower resistance to ground than a single electrode High capacity installations require low grounding resistance Multiple rods are used to provide this resistance A second rod does not provide a total resistance of half that of a single rod unless the two are several rod lengths apart To achieve the grounding resistance place multiple rods one rod length apart in a line circle hollow triangle or square The equivelent resistance can be calculated by dividing by the number of rods and mutlipling by the factor X shown below Additional considerations regarding step and touch potentials should be addressed by the geometry Multiplying Factors for Multiple Rods Number of Rods X 2 1 16 3 1 29 4 1 36 8 1 68 12 1 80 16 1 92 20 2 00 24 2 16 Placing additional rods within the periphery of a shape will not reduce the grounding resistance below that of the peripheral rods alone Understanding Ground Resistance Testing ee O NOTES MULTIPLE ELECTRODE SYSTEM A single driven ground electrode is an economical and simple means of making a good ground system But sometimes a single rod will not provide V FIGURE 19 sufficient low resistance and several ground ele
36. tations for 10 ft 3 m rods are e 1 2 inch 1 27 cm in average soil e 5 8 inch 1 59 cm in moist soil e 3 4 inch 1 91 cm in hard soil or more than 10 ft driving depths 1 2 dia Resistance in ohms 5 15 25 35 40 50 60 70 Driven depth in feet Ground resistance versus ground rod depth FIGURE 11 G mC PsFsasn Understanding Ground Resistance Testing Ground rod Soil resistivity Rod depth Rod diameter resistance ohms ohm centimeters feet inches NOTES D 100 K R 90 100 80 7 DIA 90 80 P 70 100000 _ 60 F50000 50 40000 40 30000 30 20000 15000 20 10000 15 7 5000 10 4000 9 3000 8 2000 f 6 1000 5 500 1 4 3 2 FIGURE 12 Grounding Nomograph 1 Select required resistance on R scale 2 Select apparent resistivity on P scale 3 Lay straightedge on R and P scale and allow to intersect with K scale 4 Mark K scale point 5 Lay straightedge on K scale point amp DIA scale and allow to intersect with D scale 6 Point on D scale will be rod depth required for resistance on R scale GROUND RESISTANCE VALUES NEC 250 84 1987 Resistance of man made electrodes A single electrode consisting of a rod pipe or plate which does not have a resistance to ground of 25Q or less shall be augmented by one additional rod of any of the types specified in section 250 81 or 250 83 Where m
37. ultiple rod pipe or plate electrodes are installed to meet the requirements of this section they shall be not less than 6 ft 1 83 m apart The National Electrical Code NEC states that the resistance to ground shall not exceed 25 This is an upper limit and guideline since much lower resistance is required in many instances How low in resistance should a ground be An arbitrary answer to this in ohms is difficult The lower the ground resistance the safer and for positive protection of personnel and equipment it is worth the effort to aim for less than one ohm It is generally impractical to reach such a low resistance along a distribution system or a transmission line or in small substations In some Understanding Ground Resistance Testin Q m 9 regions resistances of 5Q or less may be obtained without much trouble NOTES In other regions it may be difficult to bring resistance of driven grounds below 1002 Accepted industry standards stipulate that transmission substations should be designed not to exceed 1 In distribution substations the maximum recommended resistance is for 5Q or even 1Q In most cases the buried grid system of any substation will provide the desired resistance In light industrial or in telecommunication central offices 5Q is often the accepted value For lightning protection the arrestors should be coupled with a maximum ground resistance of 12 These parameters can usually be met with the p
38. und resistance test Use the voltmeter to test the voltage across terminals X and Z Figure 22 Usu VOLTMETER 0 gt ee Y Y Electrode Z Electrode Ground rod FIGURE 22 The voltage reading should be within stray voltage tolerances acceptable to your ground tester If the voltage exceeds this value try the following techniques A Braid the auxiliary cables together This often has the effect of canceling out the common mode voltages between these two conductors Figure 23 gt Ground strip Electrode Z Ground rod FIGURE 23 1O ars Understanding Ground Resistance Testing B If the previous method fails try changing the alignment of the auxiliary cables so that they are not parallel to power lines above or below the ground Figure 24 C If a satisfactory low voltage value is still not obtained the use of shielded cables may be required The shield acts to protect the inner conductor by capturing the voltage and draining it to ground Figure 25 1 Float the shields at the auxiliary electrodes 2 Connect all three shields together at but not to the instrument 3 Solidly ground the remaining shield to the ground under test Disconnect ground under test FIGURE 24
39. what the resistance of a grounding electrode will be and to what depth it must be driven to obtain low ground resistance The resistivity of the soil varies widely throughout the world and changes seasonally Soil resistivity is determined largely by its content of electrolytes which consist of moisture minerals and dissolved salts A dry soil has high resistivity if it contains no soluble salts Figure 1 Resistivity approx Q cm Soil Min Average Max Ashes cinders brine waste 590 2 370 7 000 Clay shale gumbo loam 340 4 060 16 300 Same with varying proportions of sand and gravel 1 020 15 800 135 000 Gravel sand stones with little clay or loam 59 000 94 000 458 000 FIGURE 1 Factors Affecting Soil Resistivity Two samples of soil when thoroughly dried may in fact become very good insulators having a resistivity in excess of 10 ohm centimeters The resistivity of the soil sample is seen to change quite rapidly until approximately 20 or greater moisture content is reached Figure 2 NN r rs rrurrrr Understanding Ground Resistance Testing Moisture content Resistivity Q cm NOTES by weight Top soil Sandy loam 0 gt 10 gt 10 2 5 250 000 150 000 5 165 000 43 000 10 53 000 18 500 15 19 000 10 500 20 12 000 6 300 30 6 400 4 200 FIGURE 2 The resistivity of the soil is also influenced by temperature Figure 3 shows the variation of the resistivity of sandy loam containing 15 2 moisture with te
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