Understanding Ground Resistance Testing & Soil Resistivity

 

U N D E R S TA N D I N G G R O U N D NOTES R E S I S TA N C E T E S T I N G S O I L R E S I S T I V I T Y Why Measure Soil Resistivity? Soil resistivity measurements have a threefold purpose. First, such data ar e used to make sub-surface geophysical surveys as an aid in identifying or e locations, depth to bedr esis- ock and other geological phenomena. Second, r tivity has a direct impact on the degree of corrosion in under ground pipelines.A decrease in resistivity relates to an incr ease in corrosion activity and therefore dictates the protective treatment to be used. Thir d, soil resis- tivity directly affects the design of a gr ounding system, and it is to that task that this discussion is dir When designing an extensive gr ected. ounding system, it is advisable to locate the ar esistivity in or ea of lowest soil r der to achieve the most economical gr ounding installation. Effects of Soil Resistivity on Ground Electrode Resistance Soil resistivity is the key factor that determines what the esistance of a r grounding electrode will be, and to what depth it must be driven to obtain low ground resistance. The r oughout esistivity of the soil varies widely thr the world and changes seasonallySoil resistivity is determined lar . gely by its content of electr which consist of moistur minerals and dissolved olytes, e, salts. A dry soil has high r e esistivity if it contains no soluble salts (Figur1). Resistivity (a pprox), Ω-cm Soil M in. Average Max. Ashes, c inders, brine,waste 590 2,370 7,000 C lay, sha le,gumbo, loam 340 4,060 16,300 Same, with varying proportions of sand and gravel 1,020 15,800 135,000 Grave l,sand, stones with little clay or loam 59,000 94,000 458,000 F IGURE 1 Factors Affecting Soil Resistivity Two samples of soil, when thor oughly dried, may in fact become very good insulators having a resistivity in excess of 1 ohm-centimeters. The r 09 esistiv- ity of the soil sample is seen to change quite rapidly until appr oximately 20% or greater moisture content is reached (Figure 2). Mo isture content Resistivity Ω-cm NOTES % by we ight Top soil Sandy loam 0 >109 >109 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 F IGURE 2 The resistivity of the soil is also infl uenced by temperature. Figure 3 shows the variation of the r e, esistivity of sandy loam, containing 15.2% moistur with temperature changes from 20° to -15°C. In this temperatur e range the resistivity is seen to vary fr om 7200 to 330,000 ohm-centimeters. Temperature Res ist iv ity C F Ohm-cm 20 68 7,200 10 50 9,900 0 32 (water) 13,800 0 32 (ice) 30,000 -5 23 79,000 -15 14 330,000 F IGURE 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. F IGURE 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 In some locations, the resistivity of the earth is so high that low-resistance grounding can be obtained only at considerable expense and with an elaborate grounding system. In such situations, it may be economical to use a ground rod NOTES system of limited size and to reduce the ground resistivity by periodically increasing the soluble chemical content of the THE EFFECT OF SALT* CONTENT ON soil. Figure 5 shows the THE RESISTIVITY OF SOIL substantial re- duction (Sandy loam, Mo isture content, 15% by we ight, Temperature,17°C) in resistivity of sandy Added Salt Res ist iv ity loam brought about by (% by weight of moisture) (Ohm-cent imeters) an increase in chemical 0 10,700 salt content. 0.1 1,800 1.0 460 5 190 Chemically treated 10 130 soil is also subject to 20 100 considerable variation F IGURE 5 of resistivitywith temper- ature changes, as shown in Figure 6. If salt treatment is employed, it is necessary to use gr ound rods which will resist chemical corrosion. *Such as copper sulfate, sodium carbonate, and others. THE EFFECT OF TEMPERATURE ON THE RESISTIVITY OF SOIL CONTAINING SALT* (Sandy loam, 20% moisture. Salt 5% of weight of moisture) Temperature Res ist iv ity (Degrees C) (Ohm-cent imeters) 20 110 10 142 0 190 -5 312 -13 1,440 F IGURE 6 Salts must be EPA or local ordinance approved prior to use. SOIL RESIS TIVI TY ME ASUREMEN TS (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-point 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. 4πAR ρ = NOTES 1 + 2A - 2A √(A2 + 4B2) √(4A2 + 4B2) Where: A = distance between the electr odes in centimeters B = electrode depth in centimeters If A > 20 B, the formula becomes: ρ = 2π AR (with A in cm) ρ = 191.5 AR (with A in feet) ρ = Soil resistivity (ohm-cm) This value is the average r ound at a depth equivalent to the esistivity of the gr distance “A” between two electr odes. Soil Resistivity Measur ements with the Model 4500 Given a sizable tract of land in which to determine the optimum soil resistiv- ity some intuition is in order. Assuming that the objective is low resistivity , preference should be given to an ar ea containing moist loam as opposed to a dry sandy area. Consideration must also be given to the depth at whichesis- r tivity is required. Example After inspection, the ar owed down to a plot of ea investigated has been narr ground approximately 75 square feet (7 m2). Assume that you need to deter- mine the resistivity at a depth of 15 feet (450 cm). The distance “A” between the electrodes must then be equivalent to the depth at which averageesis- r tivity is to be determined (15 ft, or 450 cm). Using the mor simplified e Wenner formula (ρ = 2π AR), the electrode depth must then be 1/20th of the electrode spacing or 8-7/8” (22.5 cm). F IGURE 7 F IGURE 8 Lay out the electrodes in a grid pattern and connect to the Model 4500 as NOTES shown in Figure 8. Proceed as follows: •Remove the shoring link between X and Xv (C1, P1) •Connect all four auxiliary r ods (Figure 7) For example, if the r eading is R = 15 ρ (resistivity) = 2π x A x R A (distance between electr odes) = 450 cm ρ = 6.28 x 15 x 450 = 42,390 Ω-cm G R O U N D E L E C T R O D E S The term “ground” is defined as a conducting connection by which a cir cuit 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 cuit or ir 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, gr ent before per- ounds are used to discharge the system of curr sonnel can be injur eign potentials ed or system components damaged. For for due to faults in electric power systems with gr ound returns, grounds help ensure rapid operation of the pr otection relays by providing low resistance fault current paths. This pr ovides for the removal of the foreign potential as quickly as possible. The gr should drain the for potential befor ound eign e personnel are injured and the power or communications system is damaged. Ideally to maintain a reference potential for instr safety protect , ument , against static electricityand limit the system to frame voltage for operator , safety, a ground resistance should be zer o ohms. In reality, as we describe further in the text, this value cannot be obtained. ® Last but not least, low gr ound resistance is essential to meet NEC, OSHA and other electrical safety standar ds. Figure 9 illustrates a grounding rod. The resistance of the electr ode has the following components: (A) the resistance of the metal and that of the connection to it. (B) the contact resistance of the surr ode. ounding earth to the electr (C) the resistance in the surrounding earth to current flow or earth resistivity which is often the most signifi cant factor. More specifically: (A) Grounding electrodes are usually made of a very conductive metal (copper or copper clad) with adequate cr esis- oss sections so that the overall r tance is negligible. (B) The National Institute of Standar ds and Technology has demonstrated NOTES that the resistance between the electr - ode and the surrounding earth is negli gible if the electr ode is free of paint, grease, or other coating, and if the earth is firmly packed. (C) The only component remaining is the r esistance of the surrounding earth. The electrode can be thought of as being surrounded by concentric shells of earth or soil, all of the same thick - ness. The closer the shell to the electrode, the smaller its surface; hence, the gr eater its resistance. The farther away the shells are from the elec- trode, the greater the surface of the shell; hence, the lower F IGURE 9 the resistance. Eventually, adding shells at a distance from the grounding electrode will no longer noticeably af the overall fect earth resistance surrounding the electr fect ode. The distance at which this ef occurs is referred to as the effective resistance area and is directly dependent on the depth of the gr ounding electrode. In theory, the ground resistance may be derived fr om the general formula: ρ L Length R = ––– Resistance = Resistivity x ––––––– A Area This formula illustrates why the shells of concentric earth decr ease in resis- tance the farther they ar e from the ground rod: R = Resistivity of Soil x Thickness of Shell –––––––––––––––– Area In the case of gr esistivity through- ound resistance, uniform earth (or soil) r out the volume is assumed, although this is seldom the case in natur e. The equations for systems of electr essed odes are very complex and often expr only as approximations. The most commonly used formula for single gr ound electrode systems, developed by Professor H. R. Dwight of the Massachusetts Institute of T echnology, is the following: ρ {(ln 4L) -1} R = x 2 π L r R = resistance in ohms of the gr ound rod to the earth (or soil) L = grounding electrode length r = grounding electrode radius ρ = average resistivity in ohms-cm. Effect of Ground Electrode Size NOTES and Depth on Resistance Size: Increasing the diameter of the r educe its resis- od does not materially r tance. Doubling the diameter r e 10). educes resistance by less than 10% (Figur F IGURE 10 Depth: As a ground rod is driven deeper into the earth, its r - esistance is sub stantially reduced. In general, doubling the r od length reduces the resistance by an additional 40% (Figur e 11). The NEC (1987, 250-83-3) requires a mini- mum of 8 ft (2.4 m) to be in contact with the soil. The most common is a 10 ft (3 m) cylindrical r od 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 cop- per or copper clad steel r - ods (NEC 1987, 250-83-2). Minimum practical diam eters for driving limitations for 10 ft (3 m) r ods are: • 1/2 inch (1.27 cm) in average soil • 5/8 inch (1.59 cm) in moist soil • 3/4 inch (1.91 cm) in hard soil or more than 10 ft driving depths F IGURE 11 NOTES F IGURE 12 Grounding Nomograph 1. Select requ ired 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 & DIA scale, and allow to intersect with D scale. 6. Point on D scale will be rod depth requ ired for resistance on R scale. G R O U N D R E S I S TA N C E V A L U E S NEC® 250-84 (1987): Resistance of man-made electr odes: “A single electrode consisting of a r od, pipe, or plate which does not have a resistance to ground of 25 ohms or less shall be augmented by one addition - al rod of any of the types specifi e multi- ed in section 250-81 or 250-83. Wher ple rod, pipe or plate electr equirements of this odes are installed to meet the r 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 ohms. This is an upper limit and guideline, since much lower resistance is required in many instances. “How low in resistance should a gr ound be?” An arbitrary answer to this in ohms is difficult. The lower the gr ound resistance, the safer; and for positive protection of personnel and equipment, it is worth the ef fort to aim for less than one ohm. It is generally impractical toeach such a low resistance along r a distribution system or a transmission line or in small substations. In some regions, resistances of 5 ohms or less may be obtained without much tr ouble. NOTES In other regions, it may be dificult to bring r ounds esistance of driven gr below 100 ohms. Accepted industry standards stipulate that transmission substations should be designed not to exceed 1Ω. In distribution substations, the maximum rec- ommended resistance is for 5 ohms or even 1 ohm. In most cases, the buried grid system of any substation will pr ovide the desired resistance. In light industrial or in telecommunication central of 5Ω is often the fices, accepted value. For lightning pr otection, the arrestors should be coupled with a maximum ground resistance of 1Ω. These parameters can usually be met with the pr application of basic oper grounding theory There will always exist cir . cumstances which will make it difficult to obtain the gr ound 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 ods, and chemical r treatment of the soil. Additional methods discussed in other published data are buried plates, buried conductors (counterpoise), electrically connected building steel, and electrically connected concr ete reinforced steel. Electrically connecting to existing water and gas distribution systems was often consider to yield low ground resistance; however recent design ed , changes utilizing non-metallic pipes and insulating joints have made this method of obtaining a low esistance ground questionable and in many r 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 elec- trode 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 Protection Association.

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