Ground testing

Eng nginee ineerin ring g Enc Encycl yclop ope edia Saudi Sa udi A ramco DeskTop Standards

Ground System Testing Testing

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : El Electrical File Reference: EEX20503

For additional information on this subject, contact W.A. Roussel on 874-1320

Engineering Encyclopedia

Electrical Ground System Testing

C O N T E NT S

P AG E

Basis For Testing Ground Systems ................................................ .......................................................................... ............................ .. 1 Specifying Steps Required To Perform And Evaluate Earth Resistivity Resistivity Measurements Measure ments ............................................... ......................................................................... ................................................ ................................... ............. 3 Specifying Steps Required To Perform And Evaluate Ground Grid Resistance Resistanc e Measurements For A Ground Grid........................................................ Grid.......................................................... 14 Work Aid 1: Formulas And References For Performing Earth Resistivity Measurements Measur ements ................................................ ..................................................................... ..................... 23 Work Aid 2: Formulas And References For Performing Ground Resistance Resistanc e Measurements Measur ements .............................................. ................................................................... ..................... 26 Glossary Gloss ary ................................................. .......................................................................... ................................................... .......................................... ................ 28

Saudi Aramco DeskTop Standards



C O N T E NT S

P AG E

Basis For Testing Ground Systems ................................................ .......................................................................... ............................ .. 1 Specifying Steps Required To Perform And Evaluate Earth Resistivity Resistivity Measurements Measure ments ............................................... ......................................................................... ................................................ ................................... ............. 3 Specifying Steps Required To Perform And Evaluate Ground Grid Resistance Resistanc e Measurements For A Ground Grid........................................................ Grid.......................................................... 14 Work Aid 1: Formulas And References For Performing Earth Resistivity Measurements Measur ements ................................................ ..................................................................... ..................... 23 Work Aid 2: Formulas And References For Performing Ground Resistance Resistanc e Measurements Measur ements .............................................. ................................................................... ..................... 26 Glossary Gloss ary ................................................. .......................................................................... ................................................... .......................................... ................ 28




BA SIS FOR TESTING GROUND GROUND SYSTEMS The sizing of a ground ground grid involves a number number of assumptions. The ground grid is installed installed for the safety safety of both personnel personnel and equipment. equipment. Testing of the ground grid resistance resistance provides the only concrete proof that the preliminary assumptions were accurate and that the system is adequate. Measurements of ground resistance resistance or impedance and potential potential gradients that are on the surface of the earth and that are due to ground currents are necessary for the following reasons: _

To verif erify y the the adeq dequacy uacy of a new grou roundi nding syste ystem. m.

_

To det detect ect cha chang ngees in in an an ex existi isting ng grou groun nding ding syst ystem. em.

_

To det determi ermin ne haz hazaardou rdouss st step and and tou touch ch vol voltage tages. s.

_

To dete determ rmin inee gro groun und d pot poten enti tial al rise rise (GPR (GPR)) in in ord order er to desi design gn prot protec ecti tion on for power and communication circuits. circuits.

Earth resistivity measurements are useful for the following: _

To esti estima mate te the the gro groun und d res resis ista tanc ncee of of a pro propo pose sed d sub subst stat atio ion n or or transmission tower.

_

To esti estima mate te pote potent ntia iall gra gradi dien ents ts,, inc inclu ludi ding ng step step and and tou touch ch volt voltag ages es..

_

To comp comput utee the the ind induc ucti tive ve cou coupl plin ing g bet betwe ween en nei neigh ghbo bori ring ng powe powerr and and communication circuits.

_

To design ca cathodic pr protection systems.

_

To co conduct ge geological su surveys.


1



BASIS FOR TESTI NG GRO UND SYSTE MS (Cont’d.)

The ohmic values of ground resistance for transmission systems should be as follows: _

The electrode configuration (ground grid plus other electrodes) will be governed by voltage gradient considerations, but the resultant ohmic value usually will be very low, possibly as low as 0.1 ohm.

_

If a direct metallic path exists for distribution systems above 600 V, the ground resistance from any point of grounding connection should not exceed two ohms.

_

If no direct metallic path exists, or if there is a risk that such a path will be interrupted, the voltage gradient consideration will predominate, as for transmission systems.

_

For systems 600 V and below, the NEC Article 250-84 specifies 25 ohms as a maximum value for an electrode at a consumer's premises (when disconnected from the grounded supply conductor).

_

At system grounding points, two ohms should not be exceeded (when grounded supply conductors are disconnected).


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SPECIFYING STEPS REQUIRED TO PERFORM AND EVA LUA TE EARTH RESISTIVITY MEASUREMENTS In this section, the Engineer will become proficient in measuring earth resistivity by becoming familiar with the following topics: _ _ _ _

Types of testing instruments Method of testing Measurement reliability Analyzing results

Types of Testing Instr umen ts

Commercially-available, portable testing instruments provide the most convenient and satisfactory means for measurement of the resistance of connections to earth. Instruments that are used to measure insulation resistance are not suitable because they cannot sufficiently measure low resistance values. Ordinary, low-resistance ohmmeters lack sufficient voltage for this purpose. Measurements are typically taken through use of a Biddle null balance earth tester or similar instrument. The following instruments also can be used to measure ground resistivity: _ _ _ _

Ratio ohmmeter Double-balance bridge Single-balance transformer Induced-polarization receiver and transmitter

The effects of stray voltages can be eliminated through use of the ratio ohmmeter. However, it may be different to obtain a reading when the ground is less than 0.5 ohms with stray voltages of more than 10 V. The double-balance bridge is a cumbersome method of testing, particularly under construction conditions. The single-balance transformer method is relatively insensitive to stray voltages and is unwieldy to use. An induced-polarization receiver and transmitter unit is a highly sensitive apparatus that is well suited for earth resistivity and resistance measurements. The instrument is a fourterminal type with separate measuring circuitry and power source. The main advantage of an induced polarization unit is that this unit allows the field engineer to operate the receiver on the survey lines and allows the use of multiple receivers with one transmitter, thus greatly enhancing the efficiency of the survey. Due to the inherent


3



Types of Testing Instru ments (Cont'd )

capability of this system to suppress noise, surveys can be conducted much closer to sources of spurious electrical noise (such as power lines) and deeper, effective penetration can be obtained without increases to power requirements, and the coupling between leads can be eliminated. The light-weight and low-power requirements allow for the maximum field mobility and versatility of operation. Method of Testing

The three basic methods of measuring earth resistivity are as follows: _ _ _

Four Terminal Method Variation of Depth Method Two-Point Method

Saudi Aramco uses the four-terminal method of measuring soil resistivity. Figure 1 shows the connections for this testing method.


4




5



Method of Testing (Cont' d)

Four small electrodes are inserted into the earth at equal distances apart, in a straight line, and connected, as shown in Figure 1. The null balance earth megger is connected to each of the four electrodes from terminals C1, P1, P2, C2. A test current is passed between the two outer electrodes. The instrument measures the resistance directly by dividing the voltage between the two inner electrodes by the current that is passing between the two outer electrodes. Thus: Where: Where: (rho) R A B

= = = =

the resistivity of the soil in ohm-meters the resistance value measured in ohms the distance between adjacent electrodes (meters) the depth of the electrodes (meters)

When "B" is small compared with "A" (e.g., "A" is greater than 20 times "B"), the equation can be simplified to: A second method that is used by Saudi Aramco to measure ground resistance is known as the unequally-spaced or Schlumberger-Palmer Arrangement. The arrangement that is shown in Figure 2 can be used successfully to measure resistivities over a large area. The potential probes are brought closer to the corresponding current probes to increase the potential value to a level that can be measured. It may be difficult to obtain a reading if the electrodes are spaced too far apart. Sufficient test reliability will be obtained through use of the configuration shown in Figure 2. The probe positions shown in Figure 2 are the keys to sufficient reliability. The following guidelines are used to determine proper probe positioning: _

The potential probe must be positioned so that the distance between the potential probes does not exceed 80% of the distance between the current probes.

_

The distance between the probes must be significantly longer than the depth of the probes.

The following formula is used to calculate ground resistance with the test configuration (unequaling-spaced electrodes), shown in Figure 2:


6



Method of Testing (Cont' d)

Where:

(rho) R A B D

= = = = =

the resistivity of the soil in ohm-meters the resistance value measured in ohms the distance between the adjacent electrodes (meters) the depth of the electrodes (meters) the distance between the two center electrodes

Method for Increasing Instrument Reading Figur e 2


7



IEEE Standard 81 lists several other methods of measuring resistivity. One method, called the Variation of Depth Method, is a ground-resistance test that is carried out several times. Each time that the test is conducted, the depth of burial of the tested electrode is increased by a given increment. The purpose of this method is to force more test current through the deep soil. The measured resistance value will then reflect the variation of resistivity at increased depths. The tested electrode is usually a rod. The Variation of Depth Method fails to predict earth resistivity at distances greater than five times the driven rod length from the area in which the test rod is embedded. The two-point method is suited for determination of the resistivity of small volumes of soil. This method employs an apparatus that consists of one small and one smaller iron electrode. Both of these electrodes are attached to an insulating rod. The positive terminal of a battery is connected through a milliammeter to the smaller electrode, and the negative terminal is connected to the other electrode. The instrument can be calibrated to directly read in ohmcentimeters at nominal battery voltage. This type of apparatus is easily portable. Measur ement of Reliability

The measurements of earth resistivities, ground impedances, and potential gradients introduce a number of complexities that were not encountered in other resistance measurements. Because stray currents and other factors usually interfere with the measurements, it may be necessary to make multiple measurements and to plot trends. Selection of a suitable direction or location for a resistance test has become increasingly difficult with industrial growth near power substations. Moreover, the connections of overhead ground wires, buried water pipes, and cable sheaths physically distort and enlarge the ground grid. The reliability of the measurement may be affected by a number of factors, including the following: _ _ _ _

Electrode Spacing Buried Conductors Stray Currents Probe Contact Resistance

Electrode Spacing

Measured resistivity values will vary with different electrode spacings. However, a single resistivity value is required to compute the length of the buried conductor in a ground grid design.


8



Experimental tests have been carried out to determine which electrode spacings give the most reliable measurements of resistivity. Resistivity measurements at electrode spacings of 15m to 45m (50 ft to 150 ft) most closely approximated the actual values.


9



Measur ement of Reliability (Cont' d)

Resistance readings for a given soil will decrease proportionally when electrode spacing is increased. Eventually, a value will be reached that is below the range of the instrument. Unequal electrode spacing may be used if it is difficult to obtain a reading where electrode spacing was equal to the approximate diameter of the proposed grid. Unequal electrode spacing provides an increase in the instrument readings that are six to seven times higher than readings obtained from equally spaced electrodes. Buried Conductors

Buried, bare conductors that are in contact with the soil can invalidate test readings if they are close enough to alter the test current flow pattern. Because buried bare conductors invalidate test results, soil resistivity measurements are of little value in areas where grid conductors have already been installed. Shallow depth measurements in or near the center of a very large mesh rectangle can be validated. In such cases, a few approximate readings might be taken in a short distance outside the grid, through placement of the probes so that the effect of the grid on the current flow pattern is minimized. Though not conclusive as to conditions inside the grid, such readings may be used for an approximation, especially if there is reason to believe that the soil in the entire area is reasonably homogeneous. Buried metallic objects can cause problems that are similar to those caused by buried conductors. Objects or substances that are partially or completely buried (such as rails, water, or industrial metallic pipes) will influence the measurement results. In earth resistivity tests, a sharp drop in the measured value is often caused by the presence of a metallic object that is buried close to the test location. The magnitude and extent of the drop gives an idea of the importance of the depth of the buried material. The measured resistance of a ground electrode that is located close to a buried metallic object can be significantly lower than its value would be if buried metal objects were not present. When the location of a buried metallic structure is known, the influence of these structures on the soil resistivity measurement can be minimized through alignment of the test probes in a direction that is perpendicular to the routing of this structures. Also, the location of the test probes should be as far away as possible from the buried structure.


10



Measur ement of Reliability (Cont' d) Stray Current

Stray currents in the soil can be DC or AC. The conduction of electricity in the soil is electrolytic, and direct current results in chemical action and in a polarization potential difference. Direct potentials are produced by galvanic action between various types of soil and between soil and metal. Galvanic potentials, polarization and, if present, stray direct currents may seriously interfere with earth resistance measurements. When DC tests are being conducted, the test current should be increased or the test current should be periodically reversed to overcome the interfering effects of stray DC earth currents. A regularly pulsed current is occasionally used to make measurements. However, when periodically reversed direct current is used for resistance measurements, the resulting values will be fairly close. These values, however, may not be accurate for alternating current applications. Caution must be exercised in areas that are subject to solar-induced currents (quasi-DC). Stray alternating currents in the earth, in the grounding system under test, and in the test electrodes can present an additional complication. The effects of stray alternating current may be mitigated in ground resistance measurements through use of a frequency that is not present in the stray current. Most measuring devices use frequencies that are within a range of 50 Hz to 1200 Hz. The use of filters or narrow band measuring instruments is often required to overcome the effects of stray alternating currents. Probe Contact Resistance

Theoretically, the resistances of the test electrodes do not influence measurements because these resistances are taken into consideration by the method of measurement. In practice, however, the resistance values of the electrodes should not exceed a maximum value that would cause insufficient test current in the measuring instrument. Insufficient test current is defined as follows: _ _

Current that is lower than the instrument sensitivity. Current that is in the order of magnitude of the stray current in the earth.

The only corrective action that is available at the site of the measurement to overcome a current that is lower than the instrument sensitivity is to increase the test current. This increase in test current can be done by either through an increase to the voltage of the power supply or through a decrease to the test electrode resistances. It is not always possible to increase the power supply voltage, especially with hand-driven generators incorporated in the measuring instrument. Care must be taken to avoid dangerous potentials if the power supply voltage is increased. A maximum of 100V is considered safe if special precautions (such as the use of insulating gloves or shoes) are taken.


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Measur ement of Reliability (Cont' d)

The most effective way to increase the test current is to decrease the current electrode resistance. To accomplish this decrease, the rod can be driven deeper into the soil; water can be poured around the rod; or additional rods can be driven into the soil, and the rods can be interconnected in parallel. As a rule, the resistance values of the current and potential electrodes should meet the requirements of the instruments used. A potential electrode resistance of 1000 _ may be used with commercial instruments. Some manufacturers claim that their instruments will permit 10,000 _ potential electrodes. The current electrode resistance should be less than 500 _. This resistance value is a function of the voltage that is generated by the power supply and the desired test current. The ratio of the generated voltage to the current electrode resistance determines the test current that is flowing in the current-indicating element of the instrument being used. A guideline that can be used is that the ratio between the current electrode resistance and the ground resistance being tested should never exceed 1000 to 1, preferably 100 to 1 or less. Analyzing Results

Sample resistance readings from a hypothetical earth resistivity measurement are shown below:

Resistance (Ohms) R 2 1.9 2.2 1.23 0.35 0.21 0.14

Assume that:

Electr ode Spacing (Meters) A 15 25 30 40 55 75 100

"A" is greater than 20 x "B"

where:

B

=

depth of the electrodes (meters)

then:

rho

=

6.3 AR Ohm-Meters

A plot of the calculated values of resistivity versus electrode spacing is shown in Figure 3.


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Analyzing R esults (Cont' d)

An average value of soil resistivity that is measured at probe spacings that are equal to the approximate diameter of the proposed grid, should be used for Saudi Aramco systems. If it is assumed from Figure 3, that the grid diameter equals 100 meters, resistivity equals 90.

Example of Resistivity Versus Electrode Spacing Figure 3 The curve in Figure 3 gives an indication of the soil structure. For example, another layer is reached at a depth that is equal to any electrode separation at which a break or change in curvature occurs. As an approximation, the depth to the lower layer is taken at 2/3 the electrode separation at which the point of inflexion occurs. Reference to Figure 3 shows that the curve changes at two points; at electrode spacing of 30 meters and 55 meters.


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SPECIFYING STEPS REQUIRED TO PERFORM AND EVA LUA TE GROUND GRID RESISTANCE MEASUREMENTS FOR A GROUND GRID The Engineer should understand that only approximate results can usually be expected from a precalculation of station ground impedance. A careful measurement of the impedance of the installation, as constructed, is desirable. Extreme precision is not always possible in measurement, but the results should be more dependable than values that have been calculated. Types of Testing Instr umen ts

The instruments that are used for ground resistance measurements are identical to those that are used for earth resistivity measurements. Method s of Testing

The four basic methods of measuring ground resistance are listed below: _ _ _ _

Fall of Potential Method Two-Point Method Three-Point Method Ratio Method

The method that is used by Saudi Aramco to measure ground grid resistance is the Fall of Potential Method. Figure 4 shows the circuit connections for this method of testing.


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Methods of Testing (Cont'd )

Measurement Arrangement Figur e 4 The potential probe (PP) and the current probe (CP) are inserted into the earth in a straight line at distances P and C from the ground grid (E) to be measured. A test current is passed through the ground grid via the current probe CP. The voltage that is produced between the ground grid and the surface of the ground is measured by the potential probe (PP). The instrument directly measures the resistance of the ground grid by dividing the measured voltage by the test current. For small, low voltage systems, the Two-Point or Ammeter-Voltmeter Method could be used. In this method, the total resistance of the unknown ground and of an auxiliary ground are measured. The resistance of the auxiliary ground is presumed to be negligible in comparison with the resistance of the unknown ground, and the measured value (in ohms) is called the resistance of the unknown ground. The usual application of this method is to determine the resistance of a single rod-driven ground that is near a residence and that also has a common municipal water supply system that uses a metal pipe without insulating joints. The water pipe is the auxiliary ground. The ground resistance of the water pipe is assumed to be in the order of 1 _ and must be low in relation to the permissible driven ground maximum Saudi Aramco DeskTop Standards

15



resistance, which is usually in the order of 25 _. This method is subject to large errors for low-valued driven grounds but is very useful and adequate where a "go/no-go" type of test is all that is required.


16



Methods of Testing (Cont'd )

The Three-Point Method is sometimes used for ground resistance measurements. This method involves the use of two test electrodes, with the resistances of the test electrodes designated r 2 and r3, and with the electrode to be measured designated r 1. The resistance between each pair of electrodes is measured and designated r 12 , r 13 , and r23 : where r12 = r 1 + r2, etc. From the solutions the simultaneous equations, it follows that: The value of r 1 may be established through measurement of the series resistance of each pair of ground electrodes and through substitution of the resistance values in the equation. If the two test electrodes are of materially higher resistance than the electrode that is under test, the errors in the individual measurements will be greatly magnified in the result. For the measurement, the electrodes must be at some distance from each other; otherwise, absurdities may arise in the calculations (such as zero or even negative resistance.) In measurements of the resistance of a singledriven electrode, the distance between the three separate ground electrodes should be at least 5m, with a preferable spacing of 10m or more. For larger area grounding systems, which are presumably of lower resistances, spacings in the order of the dimensions of the grounding systems are required as a minimum. The Three-Point Method becomes awkward for large substations, and some form of the Fallof-Potential Method is preferred if more accuracy is required. Another form of ground resistance measurement is known as the Ratio Method. In this method, the resistance of the electrode that is under test is compared with a known resistance, usually through use of the same electrode configuration as in the Fall-of-Potential Method. Because this is a comparison method, the ohm readings are independent of the test current magnitude if the test current is high enough to give adequate sensitivity. Staged, high-current tests may be required for cases in which specific information is desired on a particular grounding installation. Also, a ground impedance determination can be obtained at the time of actual ground faults through use of an oscillograph. This determination can be used as auxiliary information Measur ement Reliability

High-quality measuring instruments should be selected in order to obtain reliable data. Accuracy also can be related to difficulties that are encountered on the site. The most frequent problem during testing is caused by stray current flowing in the earth and by mutual coupling between leads. The more common problems and their possible solutions are as follows: _ _ _ _

Probe Spacing Stray Current Self and Mutual Impedance Probe Resistance


17



Measurement Reliability (Cont'd) Pr obe Spacing

Probe spacing is critical when the Fall-of-Potential Method is used. description will show why spacing is critical.

The following

If the current probe is placed at a fixed distance from the ground grid, the resistance value that is measured will vary with the variation of the potential probe spacing between the ground grid and the current probe. Figure 5 shows typical curves of the resistance values which are plotted against the variations of the potential probe spacing. The "true" resistance is the value of the resistance at the center position of the curve, in which the curve tends to the horizontal (between P1 and P2 of curve "a"). In this horizontal area, the influence of the potential gradient of the ground grid and the current probe is minimum. The current probe must be placed as far away as is practicable from the ground grid for an accurate measurement.


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Measurement Reliability (Cont'd)

If the current probe (CP) from the ground grid is insufficiently spaced, a resistance curve will be forwarded without the flat zone (curve "b"). This forwarding of a resistance curve occurs because the current flow does not diverge sufficiently to allow the current density to become zero before the current flow begins to converge toward the current probe. The probes must be placed outside of the potential gradient area of the system in order to minimize error in the measurement. This error can be avoided through placement of the probes at a distance as large as is practicable from the ground grid. Stray Current

The conduction of current through the soil is electrolytic in nature, and back voltages develop at the auxiliary electrodes. An easy way to eliminate electrolytic effects is to alternating test currents. If the test current is of power frequency, electrolysis is eliminated, and stray alternating current at power frequencies may influence the results. higher frequencies, electrolysis is negligible.

can use not At

If direct current is used, the problem can be solved through periodic reversal of the direct current. Periodically reversed direct current, with a complete break in the circuit between reversals, is the best power source for resistance or resistivity measurements. Self and Mutu al Impedance

At higher frequencies, the self and mutual impedance of the leads are increased, and errors may be introduced. Also, if an impedance test is performed, the reactance component will be different from the 60 Hz value. Usually, a compromise that uses frequencies in the order of 80 Hz is considered adequate. The error that is introduced due to the self and mutual impedances can be eliminated through use of direct current. Pr obe Resistance

The current electrode resistance is in series with the power source and is one of the factors governing the testing current. If this current is low, it may be necessary to drive additional ground rods in order to obtain a lower current electrode resistance. It is a good practice to drive rods at an angle with respect to the vertical in rocky soil. Inclined rods will slide over the top of a rock. The device that is used to measure the potential difference should have an internal resistance that is large compared with the potential electrode resistance. If the internal resistance is not relatively large, additional ground rods may be required to lower the potential-electrode resistance.


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Analyzing Results

The following is a list of sample resistance readings from a hypothetical earth resistivity measurement:

Apparent Ground Impedance (Ohms) 1.2 1.4 1.5 1.55 1.55 1.60 1.60 1.60 1.65 1.70 1.75 2.0 2.5

Distance Fr om Potential Electr ode To The Station G rid (M) 25 50 75 100 125 150 175 200 225 250 275 300 325

Figure 6 shows a plot of these values. The curve tends to level out between 80 and 240 meters and relates to an ohmic reading of 1.6. This reading is the impedance value of the ground under test.


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Analyzing R esults (Cont' d)


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WORK AID 1: FORMULAS AND REFERENCES FOR PERFORMING EARTH RESISTIVITY MEASUREMENTS This Work Aid is designed to help the Participants in performing Exercise 1.

Saudi Aramco Engineering Standard _

SADP-P-111 : CH. 10

IEEE Standards _ W or k Aid 1A:

IEEE STD 81 CH. 7 E q uip m en t /M a ter ia l Used Measurements

for

p er for m in g G r ou n d R esist an ce

The following equipment will be available for the ground resistance test in Exercise 1A:


_

4 small size electrodes

_

1 null balance earth megger

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WOR K AID 1 (Cont' d) Work Aid 1B:

Formulas for Calculating Earth Resistivity and Curves Showing Acceptable Values

The following formulas and typical resistivity curves may be applied to this exercise: Thus: Where: Where: (rho) R A B

= = = =

the resistivity of the soil in ohm-meters the resistance value measured in ohms the distance between adjacent electrodes (meters) the depth of the electrodes (meters)

When "B" is small compared with "A" (e.g., "A" greater than 20 x "B"), the equation simplifies to:


24



WOR K AID 1 (Cont' d)

Figure 8 shows the effects of electrode spacing on resistivity.

Examples of Resistivity Versus Electrode Spacing Figur e 8


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WORK AID 2: FORMULAS AND REFERENCES FOR PERFORMING GROUND RESISTANCE MEASUREMENTS This Work Aid is designed to help the Participants in performing Exercise 2.

W or k Aid 2A:

E q uip m en t/M a ter ia l Used Measurements

for

P er for m in g G r ou n d R esist an ce

One instrument which comprises of: _ A DC power source _ Voltmeter _ Ammeter _ Capability of dividing measured voltage by the test current and of indicating same as an ohmic reading Two ground probes Flexible test leads, sufficiently long to accomplish the task. W or k Aid 2B:

F or m u la s for C a lcu la tin g Acceptable Values

G r ou n d

R esist an ce

and

L ist

of

Once the slope variation coefficient "u" has been calculated and found to be within the range of 0.4 <= u <= 1.59, obtain value of PT from the table in Figure 9. C PT will be the distance of the potential probe position to the ground grid where the "true" resistance value should measured. The "true" resistance should be measured by inserting the potential probe at the corresponding distance PT.


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WOR K AID 2 (Cont' d)

Values of PT/C for Values of u Figur e 9


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GLOSSARY coupling

The association of two or more circuits or systems in a way that power or signal information can be transferred from one to another.

coupling capacitance

The association of two or more circuits with one another by means of capacitance mutual to the circuits.

dir ect coup ling

The association of two or more circuits by means of self-inductance, capacitance, resistance, or a combination of these characteristics that is common to the circuits.

effective resistivity

A factor in which the conduction current density is equal to the electric field in the material divided by the resistivity.

electric p otential

The potential difference between the point and some equipotential surface, usually the surface of the earth, which is arbitrarily chosen as having zero potential (remote earth).

ground

A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth.

grounded

Provision of a system, circuit, or apparatus with a ground.

ground-return circuit

A circuit in which the earth is utilized to complete the circuit.

grounding conductor

The conductor that is used to establish a ground and that connects an equipment, device, wiring system, or another conductor (usually the neutral conductor) with the grounding electrode or relectrodes.

grounding electrode

A conductor used to establish a ground.


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Ground testing

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