evaluatiing circuit resistance tests

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Engineering EncyclopediaSaudi Aramco DeskTop Standards

Evaluating Circuit Resistance Tests

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGES

LOW-RESISTANCE TEST SETS: CONSTRUCTIONS AND OPERATIONALPRINCIPLES.................................................................................................................................... 1

Constructions ...................................................................................................................... 1

Kelvin Bridge ........................................................................................................ 1

Ducter.................................................................................................................... 3

Digital Low-Resistance Ohmmeter (DLRO) ......................................................... 5

Current Injection Test Sets (for Millivolt Drop Tests)........................................... 6

Operational Principles......................................................................................................... 7

Kelvin Bridge ........................................................................................................ 7

Ducter.................................................................................................................... 9

Digital Low-Resistance Ohmmeter (DLRO) ....................................................... 12

Current Injection Test Sets (for Millivolt Drop Tests)......................................... 13

EVALUATING PRIMARY-CIRCUIT RESISTANCE TESTS ..................................................... 14

Primary-Circuit-Resistance Test Methods ........................................................................ 14

Selecting an Effective Ampere Output Option .................................................... 14

Test Hookups ...................................................................................................... 15

Evaluation Factors ............................................................................................................ 16

Loss of Contact Pressure ..................................................................................... 16

Physical Damage ................................................................................................. 16

Chemical Contamination..................................................................................... 16

EVALUATING CIRCUIT CONTINUITY TESTS ........................................................................ 17

Circuit Continuity Tests: Applications and Methods ....................................................... 17

Control Circuits................................................................................................... 17

Heat Tracing Circuits .......................................................................................... 19

Neutral Grounding Resistors ............................................................................... 19

Grounding Systems ............................................................................................. 19




Evaluating Equipment Faults ............................................................................................ 20

Short Circuit ........................................................................................................ 20

Open Circuit ........................................................................................................ 20

Intermittent Circuit .............................................................................................. 20

EVALUATING TERMINAL-TO-TERMINAL RESISTANCE TESTS ........................................ 21

Terminal-to-Terminal Resistance Tests: Applications and Methods................................. 21

Resistance Temperature Detectors....................................................................... 21

Motor and Generator Windings........................................................................... 23

Transformer Windings ........................................................................................ 23


Lead Resistance................................................................................................... 26

Average Conductor Temperature ........................................................................ 27

Time and Temperature ........................................................................................ 27

Trend ................................................................................................................... 28

EVALUATION OF EARTH-RESISTANCE TESTS ..................................................................... 29

Earth-Resistance Test Sets: Constructions and Operational Principles ............................ 29

Constructions ...................................................................................................... 29

Operational Principles ......................................................................................... 32

Earth-Resistance Tests: Theories and Methods................................................................ 35

Theories............................................................................................................... 35

Spheres of Influence............................................................................................ 36

Fall-of-Potential Profiles ..................................................................................... 36

Methods .............................................................................................................. 36

Soil Resistivity (Four-Terminal) Test.................................................................. 36

Fall-of-Potential (Three-Terminal) Test .............................................................. 37

Direct (Two-Terminal) Test................................................................................. 40





Climate and Recent Weather Conditions............................................................. 41

Soil Temperature ................................................................................................. 42

Soil Composition ................................................................................................ 43

Integrity of Grounding System Components....................................................... 43

WORK AID 1: RESOURCES USED TO EVALUATE A PRIMARY-CIRCUITRESISTANCE TEST...................................................................................................................... 44

Work Aid 1A: Non-Mandatory Test Report P-009, Circuit Breakers - MediumVoltage (Handout 8) ......................................................................................................... 44

Work Aid 1B: Manufacturers Literature .......................................................................... 44

Work Aid 1C: Applicable Procedural Steps...................................................................... 46

WORK AID 2: RESOURCES USED TO EVALUATE A CIRCUIT CONTINUITY TEST ......... 47

Work Aid 2A: Non-Mandatory Test Report P-013, Grounding Systems (Handout14)..................................................................................................................................... 47

Work Aid 2B: Table of Circuit Resistance Values for Motor ContactorElectromagnet Coils.......................................................................................................... 47

Work Aid 2C: Applicable Procedural Steps...................................................................... 48

WORK AID 3: PROCEDURES FOR EVALUATING A TERMINAL-TO-TERMINALRESISTANCE TEST...................................................................................................................... 49

WORK AID 4: RESOURCES USED TO EVALUATE AN EARTH-RESISTANCE TEST ......... 52

Work Aid 4A: Non-Mandatory Test Report P-013, Grounding System (Handout15)..................................................................................................................................... 52

Work Aid 4B: Applicable Procedural Steps...................................................................... 52

GLOSSARY ................................................................................................................................... 54



Saudi Aramco DeskTop Standards 1

LOW-RESISTANCE TEST SETS: CONSTRUCTIONS AND OPERATIONALPRINCIPLES

ConstructionsThis section explains the constructions of the following circuit-resistance test sets:

Kelvin Bridge. Ducter. Digital Low-Resistance Ohmmeter (DLRO). Primary-Current Injection Test Set (Millivolt Drop Test).

These are the test sets that are most often used to perform circuit resistance tests.

Kelvin BridgeA Kelvin bridge is a single-unit test set that exists in bench-type or portable versions. Figure 1 is an illustrationof the front panel of a Kelvin bridge. The names of the front panels devices are identified on this illustration.A Kelvin Bridge has three accessory devices:

Test leads that are connected to the terminal posts marked C1, P1, P2, and C2. An optional external battery that can be connected to the terminal posts marked BAT and + (Most

models of Kelvin bridge have a built-in battery). An external detector that can be connected at the external detector receptacle.

A Kelvin bridge is distinguished from other low-resistance test sets by its superior accuracy. The limits of errorare typically 0.03% of the dial indication plus 0.03 micro-ohm. A Kelvin bridge measures resistance in therange of 0.01 micro-ohm to approximately 1000 ohms.




Figure 1. Front Face of a Kelvin Bridge




DucterA Ducter is a two-part portable test set. Figure 2 is an illustration of the metering part of a Ducter. The namesof the top panel devices are identified on this illustration. The second part can be a low-voltage battery or arectifier-type DC power supply. A Ducter has no built-in battery. The only accessory device is a set ofcalibrated test leads.

A Ducter is distinguished from other low-resistance test sets by its ruggedness and simple operation. It canmeasures resistance in the range of 1 micro-ohm to 1 ohm. Ducter is a trade mark that is the property of AVOBiddle Instruments.




Figure 2. Top View of a Ducter




Digital Low-Resistance Ohmmeter (DLRO)A digital low-resistance ohmmeter (DLRO) is a portable test set that measures resistance in the range of 1micro-ohm to 60 ohms. A DLRO is the modern test set that performs the same functions as a Ducter. DLRO isa trade mark that is the property of AVO Biddle Instruments.

Figure 3 is a top-view illustration of a DLRO. The names of the DLROs metering and control devices areidentified on this illustration.

Figure 3. Top View of a Digital Low-Resistance Ohmmeter




Current Injection Test Sets (for Millivolt Drop Tests)The construction of a current injection test set is described in Module EEX 107.05. A brief description ofconstruction as it relates to a millivolt drop test is given here.

A current injection test set is a high-current low-voltage source of AC current and associated control equipmentthat are enclosed within a metallic structure, and mounted on wheels. Current injection test sets are as large as0.90 meters in height, 1.35 meters wide, and 0.7 meters deep and they are as much as 550 kilograms in weight.Figure 4 is an outline diagram of a current injection test set, a circuit breaker under test, and a millivolt meterconnected together for performing a millivolt drop test.

Figure 4. Circuit Diagram for a Millivolt Drop Test




Operational PrinciplesThis section explains the operational principles of the test sets whose constructions were described in theprevious section.

Kelvin BridgeA Kelvin bridge operates on the principle of a double bridge. Referring to Figure 5, the resistance X is theunknown value of resistance. The bridge is in a balanced condition whenever the multiplier dial and themechanically-linked measuring dials are adjusted to achieve a zero voltage differential between nodes m and n,as indicated by the detector-circuit meter. When the bridge is balanced, the ratio of resistance values is:

XS

AB

=

Where X is the unknown resistance, S is the resistance of the standard resistor, A is the resistance of theadjustable arm A, and B is the resistance of the fixed arm B.

Each operating knob of each measuring dial is marked with numbers. When the bridge is balanced, the value ofthe unknown resistance X is indicated by the series of numbers that appear in the windows of the measuringdials multiplied by 10-5 and multiplied by the number indicated on the multiplier dial.

The resistance values R1 and R2 shown in Figure 5 are parts of the measured circuit but do not add to theresistance indicated on the bridge. The leads L1, L2, L3 and L4 have resistance, but these do not add to theresistance indicated. As in the case with other four-terminal resistance measurement instruments, the resistancemeasured is only that resistance between the points of the probes P1 and P2.

When a Kelvin bridge is making a measurement, a significant amount of current flows in the measured circuit.The amount of current can be as large as 5 amperes but depends on the specific model of bridge being used andthe ohmic value of the measured circuit . This current will, in some cases, heat the circuit components of themeasured circuit. Consequently, this warming will change the resistance of the measured circuit.




Figure 5. Schematic Circuit Diagram of a Kelvin Bridge




DucterA Ducter contains a two-coil moving coil meter whose deflection is proportional to the magnitude of voltageacross its deflection coil divided by the magnitude of current in its control coil. Figure 6 is a representation ofthe operational elements of a Ducters meter. These elements are:

The deflecting coil, A. This coil is also called the voltage coil. The reference coil, B. This coil is also called the current coil. The north pole N and south pole S of the permanent magnet. The fixed coaxial magnet core C. The ligaments L. The ligaments conduct current to the deflecting coil and reference coil while not

producing a significant torque on the coil assembly. The bearings J support the coil assembly.

Figure 6. Meter Elements of a Ducter




Figure 7 is a schematic diagram of the circuit of a Ducter. The range selector connects built-in resistors whoseohmic values are appropriate for different ranges. The model of Ducter most often found in industrial plantshas ranges of 100, 1000, 10,000, 100,000, and 1,000,000 micro-ohms. 1,000,000 micro-ohms is equal to oneohm. As in the case with other four-terminal resistance measurement instruments, the resistance measured isonly that resistance between the points of the probes P1 and P2. A Ducter differs from a Kelvin bridge becausethe resistance of the test leads adds to the value of resistance that is displayed. All Ducters are specificallycalibrated using test leads that each have 0.04 ohm resistance. If leads of different resistance values are used, acorrection factor must be applied to the scale reading.




Figure 7. Schematic Circuit Diagram of a Ducter




Digital Low-Resistance Ohmmeter (DLRO)Figure 8 is a simplified schematic diagram of the circuit of a digital low-resistance ohmmeter (DLRO).Batteries B1 and B2 drive current through the resistance under test RX. The set of resistance elements selectedby the S1 range selector produce a voltage EREF which is proportional to the amount of current flowing in theresistance under test RX. The voltage EREF is input to the digital meter. The voltage drop EIN across RX is asecond input to the digital meter. The meter displays a number that is proportional to E IN divided by EREF.The calibrating resistor RC is calibrated at the factory so that the digital meter displays the value of resistanceRX in milliohms.Because the values of resistors RA and RA are sufficiently large, test leads of different values of resistance donot change the value of RX displayed on the digital meter.

Figure 8. Schematic Circuit Diagram of a Digital Low-Resistance Ohmmeter




Current Injection Test Sets (for Millivolt Drop Tests)Current injection test sets are used primarily to test the correct functioning of the automatic trip devices of low-voltage circuit breakers. A current injection test set can also be used to supply current to one interrupter of acircuit breaker (Figure 4). Current from the test sets output transformer causes a voltage drop that is typicallymillivolts in magnitude. The millivolt meter is used to measure this voltage drop. The resistance of theinterrupter can be calculated using the following mathematical formula:

RV

Imicro ohmsmillivolts

amperes( )

( )

( )- =

1000

Where R is the resistance of the interrupter circuit in micro-ohms, V is the measured drop at the terminalsof the interrupter in millivolts, and I is the interrupter current in amperes.




EVALUATING PRIMARY-CIRCUIT RESISTANCE TESTSThe primary circuit of a circuit breaker includes its interrupters and their main current-carrying contacts.Primary-circuit resistance tests are measurements of the values of resistance between each pair of terminals ofeach individual interrupter.

The general purpose of a primary-circuit resistance test is to detect a loss of contact pressure, physical damageto the surfactes of the contacts, or chemical contamination on the surfaces of the contacts.

Primary-Circuit-Resistance Test MethodsThis section explains the following methods for conducting primary-circuit resistance tests on power circuitbreakers:

Selecting an effective ampere output option. Test hookups.

Selecting an Effective Ampere Output OptionA precise measurement of the resistance of an electrical contact requires a minimum magnitude of current to beflowing in the contact during the measurement. The amount of current needed to maintain precision is differentaccording to design variables. The minimum amount of current required for a particular model of circuitbreaker is usually specified in the manufacturers instructional literature. Because almost all primary-circuitresistance tests require 10 amperes or more, they are performed using a Ducter or a DLRO.




Figure 9. Connection Diagram for a Primary-Circuit Resistance Test

Test HookupsFigure 9 is a connection diagram for performing a primary-circuit resistance test. This example showns aDucter. The hookup is similar for tests that are conducted using a DLRO. Note that the voltage probes areplaced inside of the current connections. This arrangement is important for making an accurate measurement ofresistance. Additionally, the voltage probes are placed so that all of the conducting components of theinterrupter are between the voltage probes. Any components that are not secured with their normal pressure ortorque are not included between the voltage probes. For example, the voltage probes are placed on the bushingcaps of the circuit breaker shown in Figure 9 rather than being placed on the loosened screw-eyes of thebushings.




Evaluation FactorsThis section explains the factors that influence the evaluation of primary-circuit resistance tests and the readingsobtained from these tests. These Evaluation factors are:

Loss of contact pressure. Physical damage to contacts. Chemical contamination of contact surfaces.

Loss of Contact PressurePressure is maintained on the mating surfaces of circuit breaker contacts by springs. If these springs becomeoverheated for any reason, they will loose their ability to maintain pressure. This loss of pressure reduces thecurrent-carrying capacity of the interrupter. Because this loss of pressure causes an increase in the resistance ofthe contact mating surfaces, it is detectable as an increased primary-circuit resistance.

Physical DamagePhysical damage to interrupter components often causes an increase in primary-circuit resistance. This increasein resistance is related to incorrect alignment of moving contacts with respect to stationary contacts.

Chemical ContaminationChemical contaminants on contact surfaces cause an increase in primary-circuit resistance. Typicalcontaminants are rust, copper sulfate, silver oxide, burned oil, sand particles, and improperly applied lubricationmaterials.




EVALUATING CIRCUIT CONTINUITY TESTS

Circuit Continuity Tests: Applications and MethodsThe purpose of a circuit continuity test is to verify that a complete conductive path exists between two terminalsof an electrical circuit. A second purpose of a circuit continuity test is to verify the isolation of two electricalcircuits. Circuit continuity tests are almost always performed when circuits are disconnected from their normalsources of electrical power.

The applications and methods of the following circuit-continuity tests are briefly explained below:

Control-circuit tests. Heat-tracing circuit tests. Neutral-grounding circuit tests.

Control CircuitsIndividual circuits of an electrical control system are tested for continuity after control components are installedin their permanent locations and before the initial energization of the control system. The testing of theseindividual circuits is also called point-to-point wiring checks. If the functional operation of the control systemcomponents was verified at the manufacturers plant, only that wiring which was installed at the equipmentusers site needs to be tested for continuity.

In order to keep an account of which point-to-point circuits have already been tested for continuity, a copy of aconstruction document (called an interwiring diagram) is marked with a red pencil. The interwiring diagramalso identifies the correct terminals between which each conductive path is supposed to exist.

The test equipment that can be is used to perform continuity tests includes test lamps, buzzers, and multimeters.Figure 10 shows the circuit diagrams for tests that use these three types of continuity testers. A test lamp is adevice that has two probes, a low-voltage isolation transformer, and a lamp connected in a series circuit. Whenthe probes are touched to the two terminals of a continuous circuit, the lamp becomes illuminated. A buzzer issimilar to a lamp tester except that it has an audio signaling device rather than a lamp. Many models of digitalmultimeters have a built-in audio signaling device that sounds when the two probes of the multimeter aretouched to the two terminals of a continuous circuit.




Figure 10. Circuit Diagrams for Control Circuit Continuity Tests




A test lamp, buzzer, or digital multimeter will signal that a circuit is continuous if its resistance is less thanapproximately 150 ohms.

Heat Tracing CircuitsAn open circuit is the most likely mode of failure for heat tracing. Heat tracing circuits are periodically testedfor continuity. Because heat tracing circuits sometimes have a resistance greater than 150 ohms, theircontinuity cannot be tested with a test lamp, buzzer, or the audio signal of a digital multimeter. A heat tracingcircuit is tested by measuring its resistance with an ohmmeter.

Neutral Grounding ResistorsAn open circuit is the most likely mode of failure for a neutral grounding resistor. Other possible failure modesare the development of a high-resistance connection or a short circuit between resistor elements. Neutralgrounding resistors are periodically tested for continuity. The continuity of a neutral grounding resistor istested by measuring its resistance with an ohmmeter, a Kelvin Bridge, or a digital low-resistance ohmmeter.

Grounding SystemsIn addition to establishing plant electrical apparatus at earth potential, the conductors of a grounding systemalso connect together the frames of individual apparatus. Continuity tests are periodically conducted to detectopen-circuit faults and high-resistance connections in these grounding system conductors. Grounding systemconductors are tested using a special test set called a safety-ground test set. In a safety-ground test, 5 to 300amperes of direct current are made to flow in the grounding system conductors between the frames of twopieces of electrical apparatus or between a frame and a derived neutral grounding point. The ability ofgrounding conductors to carry 300 amperes for a short period of time proves their integrity.

Grounding system conductors are sometimes tested while substation apparatus or plant apparatus is energized.To avoid electrical hazards, take special care not to disconnect any of the grounding system circuits. Althoughvoltage might not can be detected, handle test leads as if they are energized with several hundred volts.




Evaluating Equipment FaultsThe characteristic indications of the following types of circuit faults are explained below:

Short-circuit faults. Open-circuit faults. Intermittant-circuit faults.

Short CircuitShort-circuit faults between control circuits can be discovered using a test lamp, buzzer, or a digital multimeter.

Short-circuit faults between series-connected components of a control circuit, heat tracing circuit, neutralgrounding resistor, or grounding system cause a decrease in circuit resistance. Circuit resistance measurementsmade using an ohmmeter, Kelvin bridge, or digital low-resistance ohmmeter are compared with previousresistance measurements to discover this decrease in resistance.

Open CircuitBecause control circuits, heat tracing circuits, neutral grounding resistors, and grounding system circuits usuallyhave parallel paths, an open circuit in a single path does not cause an open circuit resistance indication in acircuit-continuity test. To detect open circuits in parallel paths, the circuit resistance is compared with previousresistance measurements to detect an increase in resistance.

An increase in resistance compared to previous resistance measurements might also indicate the existence of ahigh-resistance connection.

Intermittent CircuitAn intermittent circuit is a circuit that becomes continuous and discontinuous at irregular intervals. There areseveral methods for evaluating an intermittent circuit: elevating or lowering the temperature of circuitcomponents to cause the intermittent circuit to become continually continuous or discontinuous, applyingimpact force to components, applying mechanical stress to components, and replacing suspect components.




EVALUATING TERMINAL-TO-TERMINAL RESISTANCE TESTS

Terminal-to-Terminal Resistance Tests: Applications and MethodsThe purposes of terminal-to-terminal tests for new equipment are to detect shipping damage, verify factory testdata, and verify specified performance. The purpose of terminal-to-terminal tests for in-service equipment is todetect open circuits and high-resistance connections in windings.

Terminal-to-terminal resistance tests are applied to resistance temperature detectors, motors, generators, andtransformers. The methods of performing terminal-to-terminal resistance tests on the following equipment areexplained briefly below:

Resistance temperature detector. Motor and generator windings. Transformer windings.

Resistance Temperature DetectorsTerminal-to-terminal resistance tests are performed on resistance temperature detectors (RTDs) using a Kelvinbridge. The purpose of these tests is to verify that an RTD has the correct ohmic value of resistance for thetemperature it is sensing.

The resistance measurements for two, three, and four-terminal RTDs are illustrated in Figure 11.

Two-Terminal - A single measurement of resistance, M12, is made for a two-terminal RTD. Terminals C1 andP1 of the Kelvin bridge are connected to RTD terminal 1. Terminals C2 and P2 of the Kelvin bridge areconnected to RTD terminal 2. M12 is the measured resistance between terminal 1 and terminal 2.

Three-Terminal - Three measurements of resistance, M12, M23, and M31, are made for a three-terminal RTD.For each of these three measurements, the Kelvin bridge is connected to two RTD terminals.

Four-Terminal - One direct measurement of resistance is made for a four-terminal RTD. Terminals C1, P1,P2, and C2 are connected to RTD terminals 4, 2, 1, and 3 respectively. The value of resistance measured usingthis method does not include the resistance of the RTD leads.




Figure 11. Terminal-to-Terminal Resistance Measurements for RTDs




Motor and Generator WindingsA measurement of terminal-to-terminal resistance for a motor or generator is made using a digital low-resistance ohmmeter, a Ducter, or a Kelvin bridge. Before making these measurements, winding circuits areelectrically isolated if provisions to isolate them are available in the machines connection compartment. Ifwinding circuits cannot be isolated, one resistance measurement is made between each pair of windingterminals. The average winding temperature is measured using an imbedded winding temperature detector or aglass thermometer placed into a ventilation duct. This temperature should be recorded with the terminal-to-terminal resistance measurements. Work Aid 2 contains a procedure for calculating temperature-correctedvalues of winding resistance.

Transformer WindingsA measurement of terminal-to-terminal resistance for a transformer is made using a digital low-resistanceohmmeter, Ducter, or Kelvin bridge. Individual winding circuits of power transformers typically cannot beisolated by changing external connections. The resistance of the individual circuits of a delta or a wye-connected winding circuit can be resolved mathematically. Work Aid 2 contains a procedure for calculating theresistance of individual winding circuits. The average winding temperature, as indicated by the windingsembedded detector or the transformers liquid temperature gauge, should be recorded with the terminal-to-terminal resistance measurements.Measuring Wye-Connected Winding Circuits - Refer to Figure 12a. For wye-connected windings that havea neutral terminal, the following values of terminal-to-terminal resistance are measured:

MAN, the measured resistance from terminal A to terminal N. MBN, the measured resistance from terminal B to terminal N. MCN, the measured resistance from terminal C to terminal N.

Refer to Figure 12b. For wye-connected windings that do not have a neutral terminal, the following values ofterminal-to-terminal resistance are measured:

MAB, the measured resistance from terminal A to terminal B. MBC, the measured resistance from terminal B to terminal C. MCA, the measured resistance from terminal C to terminal A.




Measuring Delta-Connected Winding Circuits - Refer to Figure 12c. For delta-connected windings, thefollowing values of terminal-to-terminal resistance are measured:

MAB, the measured resistance from terminal A to terminal B. MBC, the measured resistance from terminal B to terminal C. MCA, the measured resistance from terminal C to terminal A.




Figure 12. Terminal-to-Terminal Resistance Measurements for Power Transformer WindingCircuits




Evaluation FactorsThe sections below explain the factors that affect the evaluation of terminal-to-terminal resistance tests:

Lead resistance. Average conductor temperature. Time and temperature. Trends.

Lead ResistanceThe resistance of the leads of RTDs, motors, generators, and transformers influence the results of terminal-to-terminal resistance measurements in different ways.

The resistance of the leads of a two-terminal RTD (see Figure 11a) will add ohmic value to the measurement ofthe RTDs resistance. For example, a two-terminal platinum RTD has approximately 112 ohms of resistance at30 C temperature. If the RTDs lead cables each have 0.1 ohm resistance, the measured terminal-to-terminalresistance would be 112.2 ohms. This additional ohmic value represents a 0.18% addition. This extraresistance corresponds to an extra 0.5 degree of temperature registration for this example RTD.

The exact resistance of a three-terminal RTD (see Figure 11b) is determined mathematically using the dataresulting from terminal-to-terminal resistance tests. The RTD resistance value is calculated using the followingformula, which eliminates the resistance of the leads:

RM M

M=+

-12 13 232 Where R is the resistance of the RTD, M12 is the resistance measured between terminal 1 and terminal 2,

M13 is the resistance measured between terminal 1 and terminal 3, and M23 is the resistance measuredbetween terminal 2 and terminal 3.

The exact resistance of a four-terminal RTD is measured directly using a Kelvin bridge. The resistance of theRTD leads does not affect the result of this terminal-to-terminal resistance measurement.

The resistance of motor leads, generator leads, or transformer leads is a significant portion of the measuredvalue of terminal-to-terminal resistance. This error does not, however, prevent accurate evaluation ofcommissioning or maintenance tests.




Average Conductor TemperatureIn order to accurately evaluate a terminal-to-terminal resistance measurement, the measurement must becorrected to a standard temperature. The methods for temperature correction and the values of standardtemperature are different for resistance measurements made on copper RTDs, nickel RTDs, platinum RTDs,motors, generators, and transformers. A direct measurement or a reasonable estimate of the averagetemperature of the circuit conductors is needed for the calculation of its temperature-corrected resistance.Terminal-to-terminal resistance changes in direct proportion to the average temperature of the conductors in thecircuit.

Time and TemperatureThe terminal-to-terminal resistance measurement of a motor, generator, or transformer winding is sometimesmade soon after the apparatus is shut down. The purpose of this type of resistance measurement is to determinethe average temperature that the winding conductors realized while the apparatus was running. There is aninaccuracy in this method of temperature determination due to the fact that the temperature of the windingchanges as soon as the apparatus is stopped. The accuracy of the temperature determination is improved bymaking several measurements of terminal-to-terminal resistance after the apparatus is stopped and graphicallyextrapolating a value of resistance for the point time that the apparatus was running. Figure 13 is an example ofan extrapolated plot produced by this time and temperature method.




Figure 13. Extrapolated Time Plot of Resistance and Temperature Values

TrendTrend analysis is the method used to evaluate terminal-to-terminal resistance tests that are made for purposes ofmaintenance. The most recent resistance measurement is compared with past measurements to discover anydeteriorating trends. Rising values of resistance generally indicate the development of high-resistance joints oropen-circuit faults. Declining values of resistance generally indicate the development of short-circuit faults.




EVALUATION OF EARTH-RESISTANCE TESTS

Earth-Resistance Test Sets: Constructions and Operational Principles

ConstructionsThe construction types of earth-resistance test sets are:

The voltmeter-ammeter type. The single-balance transformer type. The direct-reading Megger type. The induced-polarization receiver and transmitter type.

Most of the Saudi Aramco earth-resistance test sets are either the single-balance transformer type, having thetrade names Megger, Vibrometer, or Ground Ohmer, or the direct-reading Megger types which are used for lesscritical tests.

Figure 14 is a line drawing of a single-balance transformer type of earth-resistance test set. The single-balancetransformer test set has four terminals marked C1, P1, P2, and C2 similar to a Ducter or a Kelvin Bridge. Therange switch and the measurement dials are changed until a null-balance indication shows on the detector. Theohmic value of earth resistance is read from the numbers of the dials. The model shown in Figure 14 has ahand-crank generator. Other models have a battery source of current rather than a generator.

Figure 15 is a line drawing of a direct reading Megger test set. The direct-reading test set has three terminals,X, P, and C. The ohmic value of earth resistance is read directly from the scale of the test set.

Induced-polarization receiver and transmitter test sets are specialized electronic types which are used to measurethe earth resistance of the grounding components of high-voltage transmission towers.




Figure 14. Single-Balance Transformer Earth-Resistance Test Set




Figure 15. Direct Reading Megger Earth-Resistance Test Set




Operational PrinciplesWith the availability of single-balance transformer and direct-reading test sets, voltmeters and ammeters areseldom used in industry for earth-resistance testing. A circuit diagram, such as the diagram in Figure 16, of avoltmeter-ammeter test circuit is, however, useful for explaining the operational principles of all earth-resistance test sets.

The electrode X is the permanently installed grounding electrode, sometimes called the made electrode, whoseresistance RX with respect to remote earth is to be measured. A low-voltage (6 to 50 volts AC) transformerdrives current into electrode X. This current, whose magnitude is indicated by ammeter A1, flows through theearth to the test probe Z. A voltmeter indicates voltage V1 between electrode X and probe Y. The test probes Zand Y also have resistance values RZ and RY with respect to remote earth. Because RY is significantly smallerthan the impedance of the voltmeter, no appreciable error is introduced in the measurement due to the smallvoltage drop across RY. The voltage drop across Rz is not in the loop of the voltmeter circuit. The resistance ofthe grounding electrode RX is, therefore, equal to voltage V1 divided by current A1.

Figure 16. Circuit Diagram of a Voltmeter andAmmeter Earth-Resistance Test




The internal and the external circuit of a single-balance transformer test set is shown in Figure 17. Thegenerator drives an AC current into the primary winding of the test sets transformer and into electrode X. Thiscurrent returns to the generator through earth probe Z. The earth voltage between probe Y and electrode X isbalanced in the resistor network that consists of RA, RB, and the decade resistors against the voltage induced inthe secondary winding of the test sets transformer. When the detector indicates a null balance, the ohmic valueof RX can be read from the measurement dials. The mechanical rectifier is mechanically linked to the generatorand makes the detector sensitive only to the output frequency of the generator. This synchronizing of thedetector with the generator makes a single-balance transformer test set insensitive to power-frequency earthcurrents. Because this type of test set is a null-balance type, the accuracy of the earth-resistance measurement isnot sensitive to the impedance of the test leads or the impedance of the probes.

Figure 17. Circuit Diagram of a Single-Balance Transformer Earth-Resistance Test Set




The operational principle of a direct-reading Megger test set similar to that of a Ducter or a megohmmeter. Thetest sets circuit diagram (Figure 18) shows its control coil and its deflecting coil. Earth resistance is readdirectly from the scale of a direct-reading Megger test set.

Figure 18. Circuit Diagram of a Direct-Reading Megger TypeEarth-Resistance Test Set




Earth-Resistance Tests: Theories and Methods

TheoriesAn electrode buried in earth that has uniform resistivity radiates current in all directions. Shells of earth can beimagined as surrounding the electrode. All the shells are of equal thickness (see Figure 19). The earth shellnearest an electrode has the least surface area and, therefore, offers the greatest resistance to current flow. Thenext earth shell has a larger area and offers less resistance. At some distance away from the electrode,additional shells of earth do not add significantly to the resistance of the earth surrounding the electrode.

Figure 19. Fall-of Potential Profiles




Spheres of InfluenceFigure 19a shows a diagram of three rods driven into the ground. Earth shells are drawn around the electrodeX, which is the electrode that is permanently installed for grounding purposes. The outermost earth shellsdefine spheres of influence for electrode X and probe Z. If probe Y is moved in a straight line to various pointsbetween electrode X and probe Z, a set of earth-resistance measurements would reveal a directly proportionalrelationship between the distance of Y from X to the ohmic value of earth resistance. This relationship is adirect proportion because probe Y is always within the spheres of influence of electrode X and probe Z.

Fall-of-Potential ProfilesFigure 19b shows a diagram of three rods driven into the ground but spaced at greater distances. If probe Y ismoved in a straight line to various points, a set of earth-resistance measurements would reveal ohmic valuesincreasing with distance but with a discernible flattening of the of the curvilinear relationship for positions ofprobe Y that are not within the spheres of influence of either electrode X or probe Z. An earth-resistance-versus-distance plot of this kind is called a fall-of-potential profile. The flat portion of the profile is usuallyabout 62% of the distance between electrode X and probe Z.

MethodsThe three tests most often performed with an earth-resistance test set are the soil-resistivity test, fall-of-potentialearth-resistance test, and direct earth-resistance test.

Soil Resistivity (Four-Terminal) TestSoil-resistivity measurements are made to provide information that is needed to design a grounding system.The circuit of a soil-resistivity test is shown in Figure 20. To measure the average resistivity of the soil for adepth L, four probes are driven into the ground at equal distances L. The electrode depth D is made smallcompared to the distance L. A measurement of resistance R is made with a single-balance transformer test set.The resistivity of the soil is determined as follows:

r = 2 p L R Where R is the measured resistance, L is the distance between probes in centimeters, and r is resistivity

in ohm-centimeters.




Figure 20. Soil Resistivity Test Circuit

Fall-of-Potential (Three-Terminal) TestEarth-resistance tests of substation grounding systems are usually made using the fall-of-potential method.Figure 21 shows the circuit of an earth-resistance test using this method. The test can be performed using asingle-balance transformer test set or a direct reading Megger test set. Probe Z is placed at a distance C that isestimated to be well away from the sphere of influence of electrode X. Saudi Aramco Design Practice SADP-P-111 suggests that this distance should be 100 meters. Probe Y is driven into the ground at several locations in astraight line between electrode X and probe Z. SADP-P-111 suggests that each probe should be driven at least0.6 meters into the soil and at locations 0.2, 0.4, and 0.6 of distance C. At each location the earth-resistance isread from the test set and plotted on a resistance-versus-distance graph (see Figure 21).




Figure 21. Fall-of-Potential Earth-Resistance Test Circuit




The plotted curve should have a relatively flat horizontal portion. The flatness of this horizontal portion isevaluated by calculating its slope variation coefficient:

m =--

R RR R

0 6 0 4

0 4 0 2

. .

. .

Where m is the slope variation coefficient, R0.6 is the earth-resistance at distance 0.6C, R0.4 is the earth-resistance at distance 0.4C, and R0.2 is the earth-resistance at distance 0.2C.

If m is greater than 1.59, the earth-resistance values must be measured using a greater distance C.

If m is within the range of 0.4 to 1.59, a distance PT is calculated. Distance PT is the distance that probe Yshould be placed from electrode X to measure the true earth-resistance value of electrode X.

PT PT C C= ( / )

Where PT is the distance to place probe Y for the final measurement, PT/C is a ratio that is read from atable of values that are a function of the value of u, and C is the distance of probe Z from electrode X.

Figure 22 is an abbreviated table of values for the ratio PT/C. The complete table of values for PT/C can befound in SADP-P-111, chapter 13.

m PT/C m PT/C m PT/C0.40 0.643 0.80 0.580 1.20 0.4940.50 0.629 0.90 0.562 1.30 0.4650.60 0.613 1.00 0.542 1.40 0.4300.70 0.597 1.10 0.519 1.50 0.389

1.59 0.341

Figure 22. Table of Values for PT/C




Direct (Two-Terminal) TestMeasurements of earth resistance that are taken for the purpose of proving that a grounding electrode meets aminimum required ohmic value can be performed using the direct test method. Figure 23 shows the circuit ofan earth-resistance test using this method. A single-balance transformer or a direct-reading test set can be usedto make this measurement. One set of P and C terminals is connected to a metallic structure that hascomponents buried in earth and, additionally, is not connected to the electrode under test. As an example, aconnection to a metallic water system is shown in Figure 23. The other terminal or terminals of the test set areconnected to the electrode under test. The resistance measured is the sum of the buried structures resistanceand the electrodes resistance. If the measured sum is less than the minimum required ohmic value, it can beassumed that the earth-resistance of the grounding electrode is less than the minimum required.

Figure 23. Direct Earth-Resistance Test Circuit




Evaluation FactorsFactors that affect the evaluation of earth-resistance tests include climate condition, recent weather conditions,soil temperature, soil composition, and the integrity of grounding system components.

Climate and Recent Weather ConditionsSoil resistivity is generally greater in a dry climate. In order to meet minimum design criteria for earth-resistance, grounding electrodes used in dry climate locations are driven more deeply and have more elementsthan those used in wet climate locations.

Recent rain causes earth resistance to decrease. When comparing a recent measurement of earth-resistance withpast measurements, an approximate correction must be estimated to account for recent weather that might havebeen unusually dry or unusually wet. Several measurements taken during a time interval of one year can beanalyzed to account for seasonal variations. Figure 24 shows the seasonal variation of earth resistance for twodifferent electrodes buried within the same substation. If the minimum required resistance criteria for each ofthe electrodes is 50 ohms, only the electrode corresponding to Curve 2 meets the minimum criteria. This is trueeven though the most recent data point for Curve 1 is less than 50 ohms.

Figure 24. Plot of Seasonal Variation in Earth-Resistance




Soil TemperatureSoil resistivity becomes greater when soil becomes cooler. Figure 25 is a table relating soil temperature to soilresistivity for a sandy loam soil having 15% moisture content. The earth resistance of a grounding electrodewill change with temperature but is expected to meet minimum resistance requirements under any condition ofsoil temperature. Whenever the water in soil freezes, soil resistivity becomes significantly greater.

Soil Temperature Typical Soil Resistivity, ohm-cm

20 720010 99000 (water) 13,8000 (ice) 30,000-5 79,000-15 330,000

Figure 25. Table of Soil Resistivity-versus-Temperature




Soil CompositionSoils of various compositions will have different values of resistivity. However, the earth resistance of agrounding system and its individual components should meet their respective minimum requirements of ohmicvalue without regard to variability in soil composition.

Sandy soil has a greater resistivity.

Moist soil has a reduced resistivity.

Electrolytes that are found naturally in soil or that are added to soil decrease resistivity. Typical electrolytesadded to soil to intentionally decrease its resistivity are magnesium sulfate, calcium chloride, copper sulfate,and sodium chloride.

Integrity of Grounding System ComponentsGrounding system components dissolve into the soil with time. A grounding system might become ineffectivein as little as three years in an extreme case. Most systems endure long past the useful lifetime of the industrialfacility in which they are installed. The most frequent faults in grounding systems are:

Open circuits caused by conductors dissolving in the soil. Open circuits unintentionally caused by excavations. Loss of intimate contact with soil due to soil shrinkage. Open circuits caused by fault currents.

Earth-resistance tests are conducted for the purpose of discovering these faults.




WORK AID 1: RESOURCES USED TO EVALUATE A PRIMARY-CIRCUITRESISTANCE TEST

The procedure for Evaluating a primary-circuit resistance test is simple and direct. Use the Work Aidsand the procedure described below to evaluate a primary-resistance test for any type of power circuitbreaker.

Work Aid 1A: Non-Mandatory Test Report P-009, Circuit Breakers - Medium Voltage(Handout 8)

For the contents of Test Report Form P-009, refer to Handout 8. Note: Handout 8 was used in Work Aid 1 ofthe preceding Module.

Work Aid 1B: Manufacturers LiteratureA representative excerpts from manufacturers literature are reproduced on the following page:




6.6 PRIMARY CIRCUIT RESISTANCE CHECK If desired, the d-c- resistance of the primary circuitmay be measured as follows: close the breaker, pass atleast 100 amps d-c current through the breaker. With alow resistance instrument, measure resistance across thestuds on the breaker side of the disconnects for eachpole. The resistance should not exceed 60 mW, 40 mWand 20 mW for 1200 amp, 2000 amp and 3000 ampbreakers respectively.

Westinghouse Type VCP-W Vacuum-Interrupter Circuit Breaker

Contact resistance terminal to terminal measured with[Ducter] or equivalent 100 amp d-c source. 120 micro-ohms maximum for used contacts (2000A) or 135 micro-ohms maximum for used contacts for the 1200 Amperebreaker.

Westinghouse Type SP Gas Circuit Breaker (Periodic Inspection)

Contact Resistance Terminal to Terminal Measured with[Ducter] or Equivalent 100 amp d-c Source (80 micro-ohms max. (2000A) or 90 micro-ohms max. (1200A)) fornew contacts.

Westinghouse Type SP Gas Circuit Breaker (Commissioning Inspection)

Figure 29. Excerpts from Manufacturers Instruction Books




Work Aid 1C:Applicable Procedural Steps1. Determine from the manufacturers instructional literature the maximum primary-circuit resistance

allowed for the particular model of circuit breaker that was tested. Figure 29 is an excerpt from amanufacturers instruction book that contains evaluation criteria for a primary circuit resistance test. Ifno recommendation is given in the manufacturers instructional literature, go to step 3.

2. Determine from the manufacturers instructional literature the minimum magnitude of current requiredfor performing a primary-circuit resistance test on the particular model of power circuit breaker thatwas tested. Figure 29 is an excerpt from a manufacturers instruction book that contains a specificationfor a magnitude of test current. If the manufacturers instructional literature does not contain aspecification for a minimum magnitude of test current, use 10 amperes as a minimum recommendedvalue.

3. If the manufacturers literature has no recommended value for maximum primary circuit resistance, usethe evaluation criteria given in Project Acceptance Committee Form P-000. Form P-000 states that anyvalue greater than 200 micro-ohms must be investigated.

4. Compare the maximum recommended value of resistance determined from step 1 or step 3 to eachvalue of primary circuit resistance given on the test record. If all test-record values are less than therecommended value, the evaluation ends at this step.

5. Compare the minimum recommended value of test current determined in step 2 to the value of testcurrent given on the test record. The use of less than recommended current is a possible reason forfailing to meet the maximum criteria for a primary-circuit resistance test. If less than the recommendedcurrent was used, the circuit breaker should be re-tested using the recommended value.

6. If the primary circuit resistance of any pole of the circuit breaker is greater than the maximumrecommended value, a physical inspection of main contacts, main contact springs, flexible shunts, andcurrent-carrying joints should be recommended.




WORK AID 2: RESOURCES USED TO EVALUATE A CIRCUIT CONTINUITYTEST

Work Aid 2A: Non-Mandatory Test Report P-013, Grounding Systems (Handout 14)For the contents of Test Report Form P-013, refer to Handout 14.

Work Aid 2B: Table of Circuit Resistance Values for Motor Contactor ElectromagnetCoils

A representative example of a manufacturers table of resistance values for electromagnet coils of motorcontactors is reporduced below.

CoilVoltage

Resistance Value in Ohms

Sizes 00, 0, 1,and 2

Size 3 Size 4 Size 5 Size 6 Sizes 7and 8

Size 9

120 AC 1846 800 686 450 343240 AC 7385 3200 2743 1800 1371480 AC 29538 12800 10971 7200 5486

48 DC 128 66 66 72 115125 DC 868 446 446 488 781 39 45250 DC 3472 1786 1786 1953 3125 156 179

Figure 30. Resistance Values of Electromagnet Coils for Westinghouse A200 MotorContactors




Work Aid 2C: Applicable Procedural Steps1. If the circuit continuity test was conducted on a heat tracing circuit, compare the measured value of

resistance with the values recorded during commissioning tests. An increase of more than 20% of theoriginal value indicates a high-resistance or open-circuit fault in the heat tracing circuit.

2. If the circuit continuity test was conducted on a neutral grounding resistor, compare the measuredvalue of resistance to the value stamped on the nameplate of the resistor. The measured value shouldbe the same as the nameplate value within plus or minus 10%. A greater resistance value indicates ahigh-resistance or open-circuit fault. A lesser resistance indicates a short-circuit fault.

3. If the circuit continuity test was conducted on the conductors of a grounding system, the continuity ofthe circuit is evaluated by its ability to conduct 300 amperes of alternating current for a short period oftime. An inability to carry 300 amperes indicates a high-resistance or open-circuit fault in a grounding-system conductor, a fused connection, or a clamped connection.

4. If the circuit continuity test was conducted on an electromagnet coil of a motor contactor, compare themeasured resistance value with the value read from the coil manufacturers table of values. Themeasured value should be equal to the table value within 10. A representative example of amanufacturers table is given in Work Aid 2B.




WORK AID 3: PROCEDURES FOR EVALUATING A TERMINAL-TO-TERMINAL RESISTANCE TEST

1. All terminal-to-terminal resistance test measurements must be corrected to a standard temperature beforebeing evaluated. Use the following formula to calculate the terminal-to-terminal resistance valuescorrected to a standard reference temperature:

( )( )

( )RT K R

T KSS M

M

=+

+

RS is the resistance corrected to the standard reference temperature.

RM is the measured terminal-to-terminal resistance.

TS is the standard reference temperature. TS is equal to 25C for motor windings and generatorwindings. TS is equal to the rated average temperature rise plus 20C for power transformers.

TM is the average temperature of the circuit conductors at the time the terminal-to-terminal resistance wasmeasured.

K is 234.5 for a copper conductor. K is 225.0 for an aluminum conductor.

2. If the circuit tested has only two terminals, compare the single value of temperature-corrected terminal-to-terminal resistance to the temperature-corrected value of resistance that was recorded duringcommissioning tests or to the value that was recorded at the manufacturers factory.

If the temperature-corrected terminal-to-terminal resistance of a winding circuit has changed by morethan 1%, a set of additional electrical tests should be recommended to determine the nature of thewinding fault.

Note: There are no universally accepted criteria for evaluating a change in the terminal-to-terminalresistance of a motor, generator, or transformer winding. Any evaluation is based on the subjectivejudgment of the evaluating engineer. The International Electrical Testing Associations publicationentitled Maintenance Testing Specifications states that a change in winding resistance should be nogreater than 1%. A 1% or greater change almost certainly indicates the existence of a winding faultbut does not suggest the nature of the fault.




3. If the circuit tested is a winding circuit that has a wye connection with no access to the common(neutral) winding connection, the three, temperature-corrected, terminal-to-terminal resistance valuesand the following formulae should be used to determine the resistance values of the individual windingcircuits:

RM M M

ANAB BC CA=

- +2

RM M M

BNBC CA AB=

- +2

RM M M

CNCA AB BC=

- +2

Where MAB is the measured resistance from terminal A to terminal B, MBC is the measured resistancefrom terminal B to terminal C, and MCA is the measured resistance from terminal C to terminal A.

4.f the circuit tested is a winding circuit that has a closed-delta connection, the three, temperature-corrected,

terminal-to-terminal resistance values and the following formulae should be used to determine theresistance values of the individual winding circuits:

( ) ( ) ( )[ ]

RM M M M M M M M M

M M MABAB BC BC CA CA AB AB BC CA

AB BC CA

=+ + - + +

- + +

2 2 2

2 2 2

2 2 2(

( ) ( ) ( )[ ]

RM M M M M M M M M

M M MBCAB BC BC CA CA AB AB BC CA

AB BC CA

=+ + - + +

- +

2 2 2

2 2 2

2 2 2(

( ) ( ) ( )[ ]

RM M M M M M M M M

M M MCAAB BC BC CA CA AB AB BC CA

AB BC CA

=+ + - + +

+ -

2 2 2

2 2 2

2 2 2(

Where MAB is the measured resistance from terminal A to terminal B, MBC is the measured resistancefrom terminal B to terminal C, and MCA is the measured resistance from terminal C to terminal A.




5. If the values of resistance measured at the factory or measured at the time of commissioning areterminal-to-terminal resistance values, determine the individual winding resistance values using theformulae of step 3 for a wye connection or using step 4 for a delta connection.

6. Compare the three individual winding resistance values calculated in step 3 or step 4 to the valuesmeasured at the factory or at the time of commissioning. If the resistance of any winding has changedby more than 1%, a set of additional electrical tests should be recommended to determine the nature ofthe winding fault.

Note: The purpose of calculating individual winding resistance values is to identify which windingcontains a fault. The faulted winding cannot be identified using the terminal-to-terminal resistancevalues alone because a single fault causes all of the terminal-to-terminal resistance values to change.




WORK AID 4: RESOURCES USED TO EVALUATE AN EARTH-RESISTANCE TEST

Work Aid 4A: Non-Mandatory Test Report P-013, Grounding System (Handout 15)For the contents of Test Report Form P-013, refer to Handout 14.

Work Aid 4B: Applicable Procedural StepsThe Evaluation criteria used in this procedure are based on SADP-P-111, ANSI/IEEE Standards 80 and 81, andthe National Electrical Code (NEC).

1. If the measured earth resistance of a single electrode of a grounding system is greater than 25 ohms,then the installation of an extra ground electrode should be recommended. The National ElectricalCode (NEC) states in its article 250-84, A single electrode consisting of a rod, plate, or pipe that doesnot have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode ofany of the types specified in section 250-81 or 250-83.

2. If the earth-resistance measurement was made on a grounding system after its installation or aftermodification, and the grounding system is associated with a power circuit greater than 600 volts, theearth-resistance should not be greater than 2 ohms. An earth resistance greater than 2 ohms representsa conflict with Saudi Aramco Engineering Standard SAES-P-111 and, therefore, should be resolved bythe Saudi Aramco Chief Engineer.

3. If the earth resistance of a grounding system of a communication facility central office is more than 5ohms, a deviation exists with SADP-P-111. A report of this deviation should be made to the managerof the Consulting Services Department.

4. If the earth resistance of a grounding system of a communication facility remote repeater is more than25 ohms, a deviation exists with SADP-P-111. A report of this deviation should be made to themanager of the Consulting Services Department.




5. If the measured earth resistance of a grounding system is greater than a site-specific criteria, a report ofthis deviation should be made to the manager of the Consulting Services Department.

Note: The development of site-specific earth-resistance criteria is beyond the scope of this training.Site-specific criteria, when they exist, are based on SAES-P-111 paragraph 4.1.4 which states thatground resistance shall not exceed 2 ohms and shall be determined in accordance with touch and steppotentials as defined in ANSI/IEEE standard 80.




GLOSSARY

derived neutral The point of connection of a power distributioncircuits neutral conductor to the winding circuitof a source-of-power transformer or generator.

DLRO Digital low-resistance ohmmeter.

Ducter A trade name for a four-terminal moving-coil typelow-resistance test set made by AVO InternationalCompany.

earth resistance The ohmic value of resistance between agrounding electrode or a grounding system withrespect to a hypothetical remote groundingelectrode of zero resistance.

grounding conductor A conductor used to connect a piece ofequipment, device, wiring system, or anotherconductor to a grounding system.

grounding electrode Any conductor used to establish a connection toground.

grounding system All interconnected components used in anindustrial plant, commercial building, or electricalsubstation to establish a connection to ground.

heat tracing A ribbon made of extrinsic semiconductormaterial or plastic-covered metallic conductor thatis wrapped around pipe to heat the pipe by meansof electric current.

interwiring diagram A construction document that illustrates theconnection of wiring that is installed betweenseparate pieces of control equipment.




Kelvin bridge A four-terminal low-resistance ohmmeter thatoperates on the principle of a double bridgeinvented by the scientist Lord William ThomasKelvin.

made electrode A grounding electrode made of metallic structuresburied in earth that has a specific purpose ofestablishing an earth-potential reference for anelectrical power distribution system.

micro-ohm A value of resistance that is equal to one millionthof an ohm. Micro-ohm is pronounced microohm but is sometimes written as microhm.

primary circuit (circuit breaker) The circuit of a circuit breaker that comprises allof the conductive components that carry loadcurrent.

remote earth Earth at a distance such that the spheres ofinfluence of two grounding electrodes separatedby this distance do not intersect. Earth at adistance such that the mutual resistance of twogrounding electrodes separated by this distance iszero.

evaluatiing circuit resistance tests

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