Ground testing

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready 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, EngineeringServices, Saudi Aramco.

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Engineering EncyclopediaSaudi Aramco DeskTop Standards

Ground System Testing

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGE

Basis For Testing Ground Systems............................................................................ 1

Specifying Steps Required To Perform And Evaluate Earth ResistivityMeasurements ............................................................................................................ 3

Specifying Steps Required To Perform And Evaluate Ground GridResistance Measurements For A Ground Grid......................................................... 14

Work Aid 1: Formulas And References For Performing EarthResistivity Measurements..................................................................... 23

Work Aid 2: Formulas And References For Performing GroundResistance Measurements ................................................................... 26

Glossary.................................................................................................................... 28



Saudi Aramco DeskTop Standards 1

BASIS FOR TESTING GROUND SYSTEMS

The sizing of a ground grid involves a number of assumptions. The ground grid is installedfor the safety of both personnel and equipment. Testing of the ground grid resistanceprovides the only concrete proof that the preliminary assumptions were accurate and that thesystem is adequate. Measurements of ground resistance or impedance and potential gradientsthat are on the surface of the earth and that are due to ground currents are necessary for thefollowing reasons:

_ To verify the adequacy of a new grounding system.

_ To detect changes in an existing grounding system.

_ To determine hazardous step and touch voltages.

_ To determine ground potential rise (GPR) in order to design protectionfor power and communication circuits.

Earth resistivity measurements are useful for the following:

_ To estimate the ground resistance of a proposed substation ortransmission tower.

_ To estimate potential gradients, including step and touch voltages.

_ To compute the inductive coupling between neighboring power andcommunication circuits.

_ To design cathodic protection systems.

_ To conduct geological surveys.




BASIS FOR TESTING GROUND SYSTEMS (Cont’d.)

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

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

_ If a direct metallic path exists for distribution systems above 600 V, theground resistance from any point of grounding connection should notexceed two ohms.

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

_ For systems 600 V and below, the NEC Article 250-84 specifies 25ohms 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 (whengrounded supply conductors are disconnected).




SPECIFYING STEPS REQUIRED TO PERFORM AND EVALUATE EARTHRESISTIVITY MEASUREMENTS

In this section, the Engineer will become proficient in measuring earth resistivity by becomingfamiliar with the following topics:

_ Types of testing instruments_ Method of testing_ Measurement reliability_ Analyzing results

Types of Testing Instruments

Commercially-available, portable testing instruments provide the most convenient andsatisfactory means for measurement of the resistance of connections to earth. Instruments thatare used to measure insulation resistance are not suitable because they cannot sufficientlymeasure low resistance values. Ordinary, low-resistance ohmmeters lack sufficient voltagefor this purpose. Measurements are typically taken through use of a Biddle null balance earthtester 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 strayvoltages of more than 10 V.

The double-balance bridge is a cumbersome method of testing, particularly under constructionconditions. The single-balance transformer method is relatively insensitive to stray voltagesand is unwieldy to use.

An induced-polarization receiver and transmitter unit is a highly sensitive apparatus that iswell suited for earth resistivity and resistance measurements. The instrument is a four-terminal type with separate measuring circuitry and power source.

The main advantage of an induced polarization unit is that this unit allows the field engineerto operate the receiver on the survey lines and allows the use of multiple receivers with onetransmitter, thus greatly enhancing the efficiency of the survey. Due to the inherent




Types of Testing Instruments (Cont'd)

capability of this system to suppress noise, surveys can be conducted much closer to sourcesof spurious electrical noise (such as power lines) and deeper, effective penetration can beobtained without increases to power requirements, and the coupling between leads can beeliminated. The light-weight and low-power requirements allow for the maximum fieldmobility 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 theconnections for this testing method.




Method of Testing (Cont'd)

Four small electrodes are inserted into the earth at equal distances apart, in a straight line, andconnected, as shown in Figure 1. The null balance earth megger is connected to each of thefour electrodes from terminals C1, P1, P2, C2. A test current is passed between the two outerelectrodes. The instrument measures the resistance directly by dividing the voltage betweenthe two inner electrodes by the current that is passing between the two outer electrodes.

Thus:Where:Where:

(rho) = the resistivity of the soil in ohm-metersR = the resistance value measured in ohmsA = the distance between adjacent electrodes (meters)B = the depth of the electrodes (meters)

When "B" is small compared with "A" (e.g., "A" is greater than 20 times "B"), the equationcan be simplified to:

A second method that is used by Saudi Aramco to measure ground resistance is known as theunequally-spaced or Schlumberger-Palmer Arrangement. The arrangement that is shown inFigure 2 can be used successfully to measure resistivities over a large area. The potentialprobes are brought closer to the corresponding current probes to increase the potential valueto a level that can be measured. It may be difficult to obtain a reading if the electrodes arespaced 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 followingguidelines are used to determine proper probe positioning:

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

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

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




Method of Testing (Cont'd)

Where: (rho) = the resistivity of the soil in ohm-metersR = the resistance value measured in ohmsA = the distance between the adjacent electrodes (meters)B = the depth of the electrodes (meters)D = the distance between the two center electrodes

Method for Increasing Instrument ReadingFigure 2




IEEE Standard 81 lists several other methods of measuring resistivity. One method, called theVariation of Depth Method, is a ground-resistance test that is carried out several times. Eachtime that the test is conducted, the depth of burial of the tested electrode is increased by agiven increment. The purpose of this method is to force more test current through the deepsoil. The measured resistance value will then reflect the variation of resistivity at increaseddepths. The tested electrode is usually a rod. The Variation of Depth Method fails to predictearth resistivity at distances greater than five times the driven rod length from the area inwhich 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 isconnected through a milliammeter to the smaller electrode, and the negative terminal isconnected to the other electrode. The instrument can be calibrated to directly read in ohm-centimeters at nominal battery voltage. This type of apparatus is easily portable.

Measurement of Reliability

The measurements of earth resistivities, ground impedances, and potential gradients introducea number of complexities that were not encountered in other resistance measurements.Because stray currents and other factors usually interfere with the measurements, it may benecessary to make multiple measurements and to plot trends.

Selection of a suitable direction or location for a resistance test has become increasinglydifficult with industrial growth near power substations. Moreover, the connections ofoverhead ground wires, buried water pipes, and cable sheaths physically distort and enlargethe ground grid.

The reliability of the measurement may be affected by a number of factors, including thefollowing:

_ Electrode Spacing_ Buried Conductors_ Stray Currents_ Probe Contact Resistance

Electrode Spacing

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




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




Measurement of Reliability (Cont'd)

Resistance readings for a given soil will decrease proportionally when electrode spacing isincreased. 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 electrodespacing was equal to the approximate diameter of the proposed grid. Unequal electrodespacing provides an increase in the instrument readings that are six to seven times higher thanreadings obtained from equally spaced electrodes.

Buried Conductors

Buried, bare conductors that are in contact with the soil can invalidate test readings if they areclose enough to alter the test current flow pattern. Because buried bare conductors invalidatetest results, soil resistivity measurements are of little value in areas where grid conductorshave already been installed. Shallow depth measurements in or near the center of a very largemesh rectangle can be validated. In such cases, a few approximate readings might be taken ina short distance outside the grid, through placement of the probes so that the effect of the gridon the current flow pattern is minimized. Though not conclusive as to conditions inside thegrid, such readings may be used for an approximation, especially if there is reason to believethat the soil in the entire area is reasonably homogeneous.

Buried metallic objects can cause problems that are similar to those caused by buriedconductors. 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 ofa metallic object that is buried close to the test location. The magnitude and extent of the dropgives an idea of the importance of the depth of the buried material. The measured resistanceof a ground electrode that is located close to a buried metallic object can be significantlylower 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 onthe soil resistivity measurement can be minimized through alignment of the test probes in adirection that is perpendicular to the routing of this structures. Also, the location of the testprobes should be as far away as possible from the buried structure.





Stray Current

Stray currents in the soil can be DC or AC. The conduction of electricity in the soil iselectrolytic, and direct current results in chemical action and in a polarization potentialdifference. Direct potentials are produced by galvanic action between various types of soiland between soil and metal. Galvanic potentials, polarization and, if present, stray directcurrents may seriously interfere with earth resistance measurements.

When DC tests are being conducted, the test current should be increased or the test currentshould be periodically reversed to overcome the interfering effects of stray DC earth currents.A regularly pulsed current is occasionally used to make measurements. However, whenperiodically reversed direct current is used for resistance measurements, the resulting valueswill be fairly close. These values, however, may not be accurate for alternating currentapplications. 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 testelectrodes can present an additional complication. The effects of stray alternating currentmay be mitigated in ground resistance measurements through use of a frequency that is notpresent in the stray current. Most measuring devices use frequencies that are within a rangeof 50 Hz to 1200 Hz. The use of filters or narrow band measuring instruments is oftenrequired to overcome the effects of stray alternating currents.

Probe Contact Resistance

Theoretically, the resistances of the test electrodes do not influence measurements becausethese 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 thatwould cause insufficient test current in the measuring instrument. Insufficient test current isdefined 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 acurrent that is lower than the instrument sensitivity is to increase the test current. Thisincrease in test current can be done by either through an increase to the voltage of the powersupply or through a decrease to the test electrode resistances. It is not always possible toincrease the power supply voltage, especially with hand-driven generators incorporated in themeasuring instrument. Care must be taken to avoid dangerous potentials if the power supplyvoltage is increased. A maximum of 100V is considered safe if special precautions (such asthe use of insulating gloves or shoes) are taken.





The most effective way to increase the test current is to decrease the current electroderesistance. To accomplish this decrease, the rod can be driven deeper into the soil; water canbe poured around the rod; or additional rods can be driven into the soil, and the rods can beinterconnected in parallel.

As a rule, the resistance values of the current and potential electrodes should meet therequirements of the instruments used. A potential electrode resistance of 1000 _ may be usedwith commercial instruments. Some manufacturers claim that their instruments will permit10,000 _ potential electrodes.

The current electrode resistance should be less than 500 _. This resistance value is a functionof the voltage that is generated by the power supply and the desired test current. The ratio ofthe generated voltage to the current electrode resistance determines the test current that isflowing in the current-indicating element of the instrument being used. A guideline that canbe used is that the ratio between the current electrode resistance and the ground resistancebeing 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 shownbelow:

Resistance(Ohms)

R

Electrode Spacing(Meters)

A2

1.92.2

1.230.350.210.14

152530405575

100

Assume that: "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.




Analyzing Results (Cont'd)

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

Example of Resistivity Versus Electrode SpacingFigure 3

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




SPECIFYING STEPS REQUIRED TO PERFORM AND EVALUATE GROUNDGRID RESISTANCE MEASUREMENTS FOR A GROUND GRID

The Engineer should understand that only approximate results can usually be expected from aprecalculation of station ground impedance. A careful measurement of the impedance of theinstallation, as constructed, is desirable. Extreme precision is not always possible inmeasurement, but the results should be more dependable than values that have beencalculated.

Types of Testing Instruments

The instruments that are used for ground resistance measurements are identical to those thatare used for earth resistivity measurements.

Methods 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 ofPotential Method. Figure 4 shows the circuit connections for this method of testing.




Methods of Testing (Cont'd)

Measurement ArrangementFigure 4

The potential probe (PP) and the current probe (CP) are inserted into the earth in a straightline at distances P and C from the ground grid (E) to be measured. A test current is passedthrough the ground grid via the current probe CP. The voltage that is produced between theground grid and the surface of the ground is measured by the potential probe (PP). Theinstrument directly measures the resistance of the ground grid by dividing the measuredvoltage 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 aremeasured. The resistance of the auxiliary ground is presumed to be negligible in comparisonwith the resistance of the unknown ground, and the measured value (in ohms) is called theresistance of the unknown ground. The usual application of this method is to determine theresistance of a single rod-driven ground that is near a residence and that also has a commonmunicipal water supply system that uses a metal pipe without insulating joints. The waterpipe is the auxiliary ground. The ground resistance of the water pipe is assumed to be in theorder of 1 _ and must be low in relation to the permissible driven ground maximum




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




Methods of Testing (Cont'd)

The Three-Point Method is sometimes used for ground resistance measurements. Thismethod involves the use of two test electrodes, with the resistances of the test electrodesdesignated r2 and r3, and with the electrode to be measured designated r1. The resistancebetween each pair of electrodes is measured and designated r12, r13, and r23: where r12 = r1+ r2, etc. From the solutions the simultaneous equations, it follows that: The value of r1 maybe established through measurement of the series resistance of each pair of ground electrodesand through substitution of the resistance values in the equation. If the two test electrodes areof materially higher resistance than the electrode that is under test, the errors in the individualmeasurements will be greatly magnified in the result. For the measurement, the electrodesmust 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 single-driven electrode, the distance between the three separate ground electrodes should be at least5m, with a preferable spacing of 10m or more. For larger area grounding systems, which arepresumably of lower resistances, spacings in the order of the dimensions of the groundingsystems are required as a minimum.

The Three-Point Method becomes awkward for large substations, and some form of the Fall-of-Potential Method is preferred if more accuracy is required.

Another form of ground resistance measurement is known as the Ratio Method. In thismethod, 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 currentmagnitude 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 desiredon a particular grounding installation. Also, a ground impedance determination can beobtained at the time of actual ground faults through use of an oscillograph. Thisdetermination can be used as auxiliary information

Measurement 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 mostfrequent problem during testing is caused by stray current flowing in the earth and by mutualcoupling between leads. The more common problems and their possible solutions are asfollows:

_ Probe Spacing_ Stray Current_ Self and Mutual Impedance_ Probe Resistance




Measurement Reliability (Cont'd)

Probe Spacing

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

If the current probe is placed at a fixed distance from the ground grid, the resistance value thatis measured will vary with the variation of the potential probe spacing between the groundgrid and the current probe.

Figure 5 shows typical curves of the resistance values which are plotted against the variationsof the potential probe spacing. The "true" resistance is the value of the resistance at the centerposition 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 thecurrent probe is minimum. The current probe must be placed as far away as is practicablefrom the ground grid for an accurate measurement.




Measurement Reliability (Cont'd)

If the current probe (CP) from the ground grid is insufficiently spaced, a resistance curve willbe forwarded without the flat zone (curve "b"). This forwarding of a resistance curve occursbecause the current flow does not diverge sufficiently to allow the current density to becomezero before the current flow begins to converge toward the current probe. The probes must beplaced outside of the potential gradient area of the system in order to minimize error in themeasurement. This error can be avoided through placement of the probes at a distance aslarge as is practicable from the ground grid.

Stray Current

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

If direct current is used, the problem can be solved through periodic reversal of the directcurrent. Periodically reversed direct current, with a complete break in the circuit betweenreversals, is the best power source for resistance or resistivity measurements.

Self and Mutual Impedance

At higher frequencies, the self and mutual impedance of the leads are increased, and errorsmay be introduced. Also, if an impedance test is performed, the reactance component will bedifferent from the 60 Hz value. Usually, a compromise that uses frequencies in the order of80 Hz is considered adequate. The error that is introduced due to the self and mutualimpedances can be eliminated through use of direct current.

Probe Resistance

The current electrode resistance is in series with the power source and is one of the factorsgoverning the testing current. If this current is low, it may be necessary to drive additionalground rods in order to obtain a lower current electrode resistance. It is a good practice todrive rods at an angle with respect to the vertical in rocky soil. Inclined rods will slide overthe top of a rock.

The device that is used to measure the potential difference should have an internal resistancethat is large compared with the potential electrode resistance. If the internal resistance is notrelatively large, additional ground rods may be required to lower the potential-electroderesistance.




Analyzing Results

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

Apparent Ground Impedance(Ohms)

Distance From Potential ElectrodeTo The Station Grid (M)

1.21.41.5

1.551.551.601.601.601.651.701.752.02.5

255075

100125150175200225250275300325

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




Analyzing Results (Cont'd)




WORK AID 1: FORMULAS AND REFERENCES FOR PERFORMING EARTHRESISTIVITY 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

_ IEEE STD 81 CH. 7

Work Aid 1A: Equipment/Material Used for performing Ground ResistanceMeasurements

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

_ 4 small size electrodes

_ 1 null balance earth megger




WORK AID 1 (Cont'd)

Work Aid 1B: Formulas for Calculating Earth Resistivity and Curves ShowingAcceptable Values

The following formulas and typical resistivity curves may be applied tothis exercise:

Thus:Where:

Where:

(rho) = the resistivity of the soil in ohm-metersR = the resistance value measured in ohmsA = the distance between adjacent electrodes (meters)B = the depth of the electrodes (meters)

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




WORK AID 1 (Cont'd)

Figure 8 shows the effects of electrode spacing on resistivity.

Examples of Resistivity Versus Electrode SpacingFigure 8




WORK AID 2: FORMULAS AND REFERENCES FOR PERFORMING GROUNDRESISTANCE MEASUREMENTS

This Work Aid is designed to help the Participants in performing Exercise 2.

Work Aid 2A: Equipment/Material Used for Performing Ground ResistanceMeasurements

One instrument which comprises of:

_ A DC power source_ Voltmeter_ Ammeter_ Capability of dividing measured voltage by the testcurrent and of indicating same as an ohmic reading

Two ground probes

Flexible test leads, sufficiently long to accomplish the task.

Work Aid 2B: Formulas for Calculating Ground Resistance and List ofAcceptable Values

Once the slope variation coefficient "u" has been calculated and foundto be within the range of 0.4 <= u <= 1.59, obtain value of PT from thetable in Figure 9. C

PT will be the distance of the potential probe position to the ground gridwhere the "true" resistance value should measured.

The "true" resistance should be measured by inserting the potentialprobe at the corresponding distance PT.




WORK AID 2 (Cont'd)

Values of PT/C for Values of uFigure 9




GLOSSARY

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

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

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

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

electric potential The potential difference between the point and some equipotentialsurface, usually the surface of the earth, which is arbitrarily chosenas having zero potential (remote earth).

ground A conducting connection, whether intentional or accidental, bywhich an electric circuit or equipment is connected to the earth or tosome conducting body of relatively large extent that serves in placeof the earth.

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

ground-return A circuit in which the earth is utilized to complete the circuit.circuit

grounding The conductor that is used to establish a ground and that connects anconductor equipment, device, wiring system, or another conductor (usually the

neutral conductor) with the grounding electrode or relectrodes.

grounding A conductor used to establish a ground.electrode




grounding A connection used in establishing a ground. This connectionconsistsconnection of a grounding conductor, a grounding electrode, and either the earth

(soil) that surrounds the electrode or some conductive body thatserves instead of the earth.

ground grid A system of grounding electrodes consisting of interconnected barecables buried in the earth to provide a common ground for electricaldevices and metallic structures.

grounding Consists of all interconnected grounding connections in a specificarea.

system

ground resistance The ohmic resistance between the grounding electrode and a remotegrounding electrode of zero resistance.

inductive coupling The association of two or more circuits with one another by meansof inductance mutual to the circuits or the mutual inductance thatassociates the circuits.

mutual resistance Equal to the voltage change in one of the electrodes produced by aof grounding change of one ampere of direct current in the other. Expressed inelectrodes ohms.

potential profile A plot of potential as a function of distance along a specified path.

resistive coupling The association of two or more circuits by means of resistancemutual to the circuits.

resistivity (material) A factor in which the conduction-current density is equal to theelectric field in the material divided by the resistivity.

step voltage The potential difference between two points on the earth's surfaceseparated by a distance of one pace (assumed to be one meter) in thedirection of maximum potential gradient.

surface-potential The slope of a potential profile, the path of which intersectsgradient equipotential lines at right angles.

Ground testing

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