Principles of Grounding; Ground Resistance Testing Principles

 

G R O U N D R E S I S TA N C E NOTES T E S T I N G P R I N C I P L E (Fall of Potential — 3-Point Measurement) The potential diference 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, Yand 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 gr ound electrode resis- tance R. If E = 20 V and I = 1A, then R = E = 20 = 20 ––– ––– I 1 It is not necessary to carry out all the measur ound ements when using a gr tester. The ground tester will measur ent e directly by generating its own curr and displaying the resistance of the ground electrode. F IGURE 13 Position of theAuxiliary Electrodes on Measurements The goal in pr measuring the r ecisely esistance to ground is to place the auxiliary current electrode Z far enough fr om the ground electrode under test so that the auxiliary potential electr will be outside of the ef ode Y fective resistance areas of both the gr electrode and the auxiliary curr ound ent electrode. The best way to fi nd out if the auxiliary potential rod Y is outside the effective resistance areas is to move it between X and Z and to take a reading at each location. If the auxiliary potential od Y is in an ef r fective resistance area (or in both if they overlap, as in Figur e 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. 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). F IGURE 14 F IGURE 15 Measuring Resistance of NOTES Ground Electrodes (62% Method) The 62% method has been adopted after graphical consideration and after actual test. It is the most accurate method but is limited by the fact that the ground tested is a single unit. This method applies only when all thr ee electrodes are in a straight line and the ground is a single electrode, pipe, or plate, etc., as in Figur e 16. F IGURE 16 Consider Figure 17, which shows the efective resistance areas (concentric shells) of the gr ent electrode Z. ound electrode X and of the auxiliary curr 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 not obtain a reading within a reasonable band of tolerance. The sensitive areas overlap and act constantly to increase resistance asY is moved away from X. F IGURE 17 Now consider Figure 18, where the X and Z electrodes are sufficiently spaced NOTES so that the areas of effective resistance do not overlap. If we plot the r esis- tance measured we find that the measurements level of f when Y is placed at 62% of the distance fr om 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 defi ned by the user and expressed as a percent of the initial reading: ± 2%, ± 5%, ± 10%, etc. F IGURE 18 Auxiliary Electrode Spacing No definite distance between X and Z can be given, since this distance is r- el ative to the diameter of the electr ode tested, its length, the homogeneity of the soil tested, and particularlythe effective resistance areas. However, an , approximate distance may be determined fr the following chart which om is given for a homogeneous soil and an electr . (For a ode of 1” in diameter diameter of 1/2”, reduce the distance by 10%; for a diameter of 2” incr ease the distance by 10%.) Approx imate 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 ft 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 M U LT I P L E E L E C T R O D E NOTES S Y S T E M A single driven ground electrode is an economical and simple means of making a good ground system. But sometimes a single r od will not provide sufficient low r esistance, and several ground electrodes will be driven and connected in parallel by a cable. V ery 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 ar e equally spaced (Figure 19). In multiple electr systems, the ode 62% method electrode spacing may no longer be applied dir ectly. The F IGURE 19 distance of the auxiliary electr odes is now based on the maximum griddistance (i.e. in a square, the diagonal; in a line, the total length. For example, a squar e having a side of 20 ft willhave 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 T W O - P O I N T M E A S U R E M E N T NOTES (SIMPLIFIED METHOD) This is an alternative methodwhen an excellent ground is already available. In congested areas where finding room to drive the two auxiliary r ods may be a problem, the two-point measur ead- ement method may be applied. The r ing obtained will be that of the two gr efore, the water ounds in series. Ther pipe or other ground must be very low in r - esistance so that it will be negli gible in the fi esistances will also be measur nal measurement. The lead r ed and should be deducted fr om the final measurement. This method is not as accurate as thr ee-point methods (62% method), as it is particularly affected by the distance between the tested electr and the ode dead ground or water pipe. This method should not be used as a standar d procedure, but rather as a back-up in tight ar eas. See Figure 20. F IGURE 20 C O N T I N U I T Y M E A S U R E M E N T Continuity measurements of a ground conductor are possible by using two terminals (Figure 21). F IGURE 21 T E C H T I P S NOTES Excessive Noise Excessive noise may interfer e with testing because of the long leads used to perform a fall-of-potential test. voltmeter can be utilized to identify this A problem. Connect the “X”, “Y” and “Z” cables to the auxiliary electr odes as for a standard ground resistance test. Use the voltmeter to test the voltage across terminals “X” and “Z” (Figur e 22). F IGURE 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 togetherThis often has the ef . fect of canceling out the common mode voltages between these two conductors (Figur e 23). F IGURE 23 B) If the previous method fails, try changing the alignment of the auxiliary NOTES cables so that they ar ound e not parallel to power lines above or below the gr (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 pr - otect the inner conductor by cap turing the voltage and draining it to gr ound (Figure 25). 1. Float the shields at the auxiliary electr odes. 2. Connect all three shields together at (but not to) the instr ument. 3. Solidly ground the remaining shield to the gr ound under test. F IGURE 24 F IGURE 25 Excessive Auxiliary Rod Resistance NOTES The inherent function of a fall-of-potential gr tester is to input a ound constant current into the earth and measur e the voltage drop by means of auxiliary electrodes. Excessive resistance of one or both auxiliary electr odes can inhibit this function. This is caused by high soilesistivity or poor r contact between the auxiliary electr ode and the surrounding dirt (Figure 26). To ensure good contact with the earth, stamp down the soil dir ectly around the auxiliary electrode to remove air gaps formed when inserting the r od. If soil resistivity is the pr pour water around the auxiliary electr oblem, odes. This reduces the auxiliary electr ode’s contact resistance without afecting the measurement. F IGURE 26 Tar or Concrete Mat Sometimes a test must be performed on a gr ound rod that is surrounded by a tar or concrete mat, where auxiliary electrodes cannot be driven easilyIn . such cases, metal screens and water can be used to r auxiliary elec eplace - trodes, as shown in Figure 27. Place the screens on the floor the same distance from the ground rod under test as you would auxiliary electr in a standard fall-of-potential test. odes Pour water on the screens and allow it to soak in. These scr eens will now perform the same function as would driven auxiliary electr odes. F IGURE 27 T O U C H P O T E N T I A L NOTES M E A S U R E M E N T S The primary reason for performing fall-of-potential measur is to ements observe electrical safety of personnel and equipment. However certain , in circumstances the degree of electrical safety can be evaluated fr om 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 r esistivity or galvanic action is suspected. 3) Ground faults are very unlikely to occur near the gr ound under test. Touch potential measurements are an alternative method for determining elec - trical safety ements are recommended when: . Touch potential measur 1) It is physically or economically impossible to disconnect the gr ound to be tested. 2) Ground faults could r ound to easonably be expected to occur near the gr be tested, or near equipment gr ounded by the ground to be tested. 3) The “footprint” of gr ounded equipment is comparable to the size of the ground to be tested. (The “footprint” is the outline of the part of equip - ment in contact with the earth.) Neither fall-of-potential resistance measurements nor touch potential measurements tests the ability of gr ounding conductors to carry high phase- to-ground fault currents. Additional high curr ent tests should be performed to verify that the grounding system can carry these curr ents. When performing touch potential measur ements, a four-pole ground resis- tance tester is used. During the test, the instr ument induces a low level fault into the earth at some pr ound. ument oximity to the subject gr The instr displays touch-potential in volts per amper e of fault current. The displayed value is then multiplied by the lar anticipated ground fault current to gest obtain the worst case touch potential for a given installation. For example, if the instr ument displayed a value of .100Ω when connected to a system where the maximum fault current was expected to be 5000 A, the maximum touch potential would be: 5000 x .1 = 500 volts. Touch potential measurements are similar to fall-of-potential measur ements in that both measur odes into or ements require placement of auxiliary electr on top of the earth. Spacing the auxiliary electr odes during touch potential measurements differs from fall-of-potential electr ode spacing, as shown in Figure 28 on the following page. NOTES F IGURE 28 Consider the following scenario: If the buried cable depicted in Figur e 28 experienced an insulation br eakdown near the substation shown, fault currents would travel through the earth towards the substation gr ound, creating a voltage gradient. This voltage gradient may be hazar or dous potentially lethal to personnel who come in contact with the af fected ground. To test for approximate touch potential values in this situation, pr oceed as follows: Connect cables between the fence of the substation and C1 and P1 of the four-pole earth resistance tester ode in the earth at the . Position an electr point at which the gr , ound fault is anticipated to occurand connect it to C2. In a straight line between the substation fence and the anticipated fault point, position an auxiliary electr ode into the earth one meter (or one arm’s length) away from the substation fence, and connect it to P2. T urn the instrument on, select the 10 mAcurrent range, and observe the measur ement. Multiply the displayed reading by the maximum fault curr ent 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.

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