AEMC 4620 User manual
DIGITAL GROUND
RESISTANCE AND SOIL
RESISTIVITY TESTERS
ENGLISH User Manual
4620
4630
Statement of Compliance
Chauvin Arnoux®, Inc. d.b.a. AEMC®Instruments
certifies that this instrument has been calibrated
using standards and instruments traceable to
international standards.
We guarantee that at the time of shipping your
instrument has met its published specifications.
An NIST traceable certificate may be
requested at the time of purchase, or obtained
by returning the instrument to our repair and
calibration facility, for a nominal charge.
The recommended calibration interval for this
instrument is 12 months and begins on the date of
receipt by the customer. For recalibration, please
use our calibration services. Refer to our repair
and calibration section at www.aemc.com.
Serial #: ________________________________
Catalog #: ______________________________
Model #: 4620 / 4630
Please fill in the appropriate date as indicated:
Date Received: _________________________________
Date Calibration Due: _______________________
Chauvin Arnoux®, Inc.
d.b.a AEMC®Instruments
www.aemc.com
Digital Ground Resistance Tester Model 4620 and 4630
1
Table of Contents
1. INTRODUCTION ................................................................................. 3
1.1 International Electrical Symbols................................................4
1.2 DenitionofMeasurementCategories .....................................4
1.3 ReceivingYourShipment..........................................................4
1.4 OrderingInformation.................................................................5
1.4.1 Kits,AccessoriesandReplacementParts ....................6
2. PRODUCT FEATURES......................................................................... 7
2.1 ControlandConnectorFeatures(Model4620) .......................7
2.2 ControlandConnectorFeatures(Model4630) ........................8
2.3 FaultIndicatorLEDs .................................................................9
2.3.1 X-ZFault .......................................................................9
2.3.2 Xv-YHighResistance ...................................................9
2.3.3 Xv-YHighNoise............................................................9
2.4 Buzzer.....................................................................................10
2.5 Over-rangeIndication .............................................................10
2.6 FaultLEDIndication–TipsandSolutions ..............................10
3. SPECIFICATIONS ............................................................................. 11
3.1 Electrical .................................................................................11
3.2 Mechanical..............................................................................12
3.3 Environmental.........................................................................13
3.4 Safety......................................................................................13
3.5 Auto-ranging ...........................................................................13
4. GROUNDING THEORY....................................................................... 14
4.1 GroundingElectrodeResistance ............................................14
4.1.1 EffectofElectrodeSize/DepthonResistance ............16
4.1.2 EffectsofSoilResistivityonElectrodeResistance.....17
4.1.3 FactorsAffectingSoilResistivity .................................17
4.1.4 EffectofGroundRodDepthonResistance................20
4.2 GroundResistanceValues .....................................................21
4.3 GroundResistanceTestingPrinciple.....................................23
4.3.1 PositionofAuxiliaryElectrodesinMeasurements ......24
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Digital Ground Resistance Tester Model 4620 and 4630
4.4 MeasuringResistanceofGroundElectrodes ........................26
4.4.1 AuxiliaryElectrodeSpacing ........................................28
4.5 MultipleElectrodeSystem ......................................................28
5. OPERATION..................................................................................... 30
5.1 GroundResistanceMeasurementProcedure .......................30
5.2 2-PointMeasurement(SimpliedMeasurement) ...................31
5.3 ContinuityMeasurement.........................................................32
5.4 SoilResistivityMeasurements................................................32
5.4.1 PurposesofSoilResistivity ........................................32
5.4.2 TypesofResistivityMeasurements ............................33
5.5 SoilResistivityMeasurementProcedure(4-Point) .................34
5.6 How to Use 25ΩCalibrationChecker(accessory) ................36
6. MAINTENANCE................................................................................ 37
6.1 Warning..................................................................................37
6.2 Disassembly ...........................................................................37
6.3 PowerSupply..........................................................................38
6.3.1 TestingtheBattery ......................................................38
6.3.2 ReplacingtheBattery(Model4620) ...........................39
6.3.3 RechargingtheBattery(Model4630).........................39
6.3.4 ReplacingtheSafetyFuse..........................................40
6.4 Cleaning..................................................................................40
6.5 Storage ...................................................................................40
RepairandCalibration ....................................................................41
TechnicalandSalesAssistance......................................................41
Limited Warranty .............................................................................42
WarrantyRepairs ............................................................................42
Digital Ground Resistance Tester Model 4620 and 4630
3
CHAPTER 1
INTRODUCTION
WARNING
“It should be impressed on all personnel that a lethal potential can exist
between the station ground and a remote ground if a system fault involving
the station ground occurs while tests are being made. Since one of the
objects of tests on a station ground is the establishment of the location of
an effectively remote point for both current and potential electrodes, the
leads to the electrodes must be treated as though a possible potential
could exist between these test leads and any point on the station ground
grid.”
- excerpted from IEEE Std. 81-1962
Thesesafetywarningsareprovidedtoensurethesafetyofper-
sonnelandproperoperationoftheinstrument.
• The instrument must not be operated beyond its specied
operatingrange.
• Safetyistheresponsibilityoftheoperator.
• Allmetalobjectsorwiresconnectedtotheelectricalsystem
shouldbeassumedtobelethaluntiltested.Groundingsys-
temsarenoexception.
• Useextremecautionwhenusingtheinstrumentaroundener-
gizedelectricalequipment.
• Neverattempttousetheinstrumenttotwistorprytheground
electrode or ground wire away from the equipment being
grounded.
• AEMC®Instrumentsconsiderstheuseofrubberglovestobe
anexcellentsafetypracticeeveniftheequipmentisproperly
operatedandcorrectlygrounded.
• Alwaysinspecttheinstrumentandleadspriortouse.Replace
anydefectivepartsimmediately.
4
Digital Ground Resistance Tester Model 4620 and 4630
1.1 International Electrical Symbols
Signifies that the instrument is protected by double or reinforced insulation.
This symbol on the instrument indicates a WARNING and that the operator must refer
to the user manual for instructions before operating the instrument. In this manual,
the symbol preceding instructions indicates that if the instructions are not followed,
bodily injury, installation/sample and/or product damage may result.
Risk of electric shock. The voltage at the parts marked with this symbol may be
dangerous.
In conformity with WEEE 2002/96/EC
1.2 Definition of Measurement Categories
CAT II: For measurements performed on circuits directly connected to
the electrical distribution system. Examples are measurements
onhouseholdappliancesorportabletools.
CAT III: For measurements performed in the building installation at
the distribution level such as on hardwired equipment in xed
installationandcircuitbreakers.
CAT IV: For measurements performed at the primary electrical supply
(<1000V) such as on primary overcurrent protection devices,
ripplecontrolunits,ormeters.
1.3 Receiving Your Shipment
Uponreceivingyourshipment,makesurethatthecontentsareconsistent
withtheorderinginformation.Notifyyourdistributorofanymissingitems.
Iftheequipmentappearstobedamaged,leaclaimimmediatelywiththe
carrierandnotifyyourdistributoratonce,givingadetaileddescriptionof
anydamage.Savethedamagedpackingcontainertosubstantiateyour
claim.
Donotuseequipmentwhichisdamagedorappearstobedamaged.
Digital Ground Resistance Tester Model 4620 and 4630
5
1.4 Ordering Information
Ground Resistance Tester Model 4620............................ Cat. #2130.43
Includes ground tester workbook on USB stick, 8 C cell batteries and user manual.
Ground Resistance Tester Model 4620 Kit (150 ft) ......... Cat. #2135.19
Includes ground tester, two 150 ft color-coded leads on spools (red/blue), one 30 ft lead
(green), two T-shaped auxiliary ground electrodes, set of 5 spaded lugs, one 100 ft AEMC®
tape measure, ground tester workbook on USB stick, batteries, carrying bag and user manual.
Ground Resistance Tester Model 4620 Kit (300 ft) ......... Cat. #2135.20
Includes ground tester, two 300 ft color-coded leads on spools (red/blue), two 100 ft color-
coded leads (hand-tied, green/black), four T-shaped auxiliary ground electrodes, set of
5 spaded lugs, one 100 ft AEMC®tape measure, ground tester workbook on USB stick,
batteries, carrying bag and user manual.
Ground Resistance Tester Model 4620 Kit (500 ft) ......... Cat. #2135.21
Includes ground tester, two 500 ft color-coded leads on spools (red/blue), two 100 ft color-
coded leads (hand-tied, green/black), one 30 ft lead (green), four T-shaped auxiliary ground
electrodes, set of 5 spaded lugs, one 100 ft AEMC®tape measure, ground tester workbook
on USB stick, batteries, carrying bag and user manual.
Ground Resistance Tester Model 4630............................ Cat. #2130.44
Includes ground tester workbook on USB stick, rechargeable battery, AC power cord and
user manual.
Ground Resistance Tester Model 4630 Kit (150 ft) ......... Cat. #2135.22
Includes ground tester, two 150 ft color-coded leads on spools (red/blue), one 30 ft lead
(green), two T-shaped auxiliary ground electrodes, set of 5 spaded lugs, one 100 ft AEMC®
tape measure, ground tester workbook on USB stick, batteries, carrying bag and user manual.
Ground Resistance Tester Model 4630 Kit (300 ft) ......... Cat. #2135.23
Includes ground tester, two 300 ft color-coded leads on spools (red/blue), two 100 ft color-
coded leads (hand-tied, green/black), four T-shaped auxiliary ground electrodes, set of
5 spaded lugs, one 100 ft AEMC®tape measure, ground tester workbook on USB stick,
batteries, carrying bag and user manual.
Ground Resistance Tester Model 4630 Kit (500 ft) ......... Cat. #2135.24
Includes ground tester, two 500 ft color-coded leads on spools (red/blue), two 100 ft color-
coded leads (hand-tied, green/black), one 30 ft lead (green), four T-shaped auxiliary ground
electrodes, set of 5 spaded lugs, one 100 ft AEMC®tape measure, ground tester workbook
on USB stick, batteries, carrying bag and user manual.
6
Digital Ground Resistance Tester Model 4620 and 4630
1.4.1 Kits, Accessories and Replacement Parts
Test Kit for 3-Point Testing (150 ft) .................................. Cat. #2135.35
Includes two 150 ft color-coded leads on spools (red/blue), one 30 ft lead (green), two T-
shaped auxiliary ground electrodes, set of 5 spaded lugs, one 100 ft AEMC®tape measure
and carrying bag.
Test Kit for 4-Point Testing (300 ft) .................................. Cat. #2135.36
Includes two 300 ft color-coded leads on spools (red/blue), two 100 ft color-coded leads
(hand-tied, green/black), four T-shaped auxiliary ground electrodes, set of 5 spaded lugs,
one 100 ft AEMC®tape measure and carrying bag.
Test Kit for 4-Point Testing (500 ft) .................................. Cat. #2135.37
Includes two 500 ft color-coded leads on spools (red/blue), two 100 ft color-coded leads
(hand-tied, green/black), one 30 ft lead (green), four T-shaped auxiliary ground electrodes,
set of 5 spaded lugs, one 100 ft AEMC®tape measure and carrying bag.
Test Kit for 3-Point Testing (Supplementalfor4-PointTesting)... Cat. #2135.38
Includes two 100 ft color-coded leads (hand-tied, green/black), one 30 ft lead (green), two
T-shaped auxiliary ground electrodes, set of 5 spaded lugs, one 100 ft AEMC®tape measure
and carrying bag.
25Ω CalibrationChecker .....................................................Cat. #2130.59
TapeMeasure–AEMC100ft..............................................Cat. #2130.60
GroundTesterVideo/WorkbookSet .................................... Cat. #2130.64
Setof2,T-ShapedAuxiliaryGroundElectrodes ................. Cat. #2135.39
Inverter–12VDCto120VAC200Wattforvehicleuse........ Cat. #2135.43
Fuse–Setof5,0.1A,>250V,0.25x1.25" ......................... Cat. #2970.12
Replacement9.6VRechargeableBatteryPack(4630)....... Cat. #2960.21
115VPowerCord ................................................................Cat. #5000.14
Order Accessories and Replacement Parts Directly Online
Check our Storefront at www.aemc.com/store for availability
Digital Ground Resistance Tester Model 4620 and 4630
7
CHAPTER 2
PRODUCT FEATURES
2.1 Control and Connector Features
(Model 4620)
Ω
1. Low battery indicator
2. Input terminal Z (H)
3. Input terminal Y (S)
4. Input terminal Xv (ES)
5. Input terminal X (E)
6. X-Z Fault indicator
Figure 1
7. Xv-Y High Resistance indicator
8. Xv-Y High Noise indicator
9.
Display (with backlight - lights up when
the TEST button is pressed)
10. Fuse holder
11. TEST button
(when released, turns the unit OFF)
2
1
3
4
5
11
10
9
867
8
Digital Ground Resistance Tester Model 4620 and 4630
2.2 Control and Connector Features
(Model 4630)
Ω
1. Low battery indicator
2. AC power input
3. Input terminal Z (H)
4. Input terminal Y (S)
5. Input terminal Xv (ES)
6. Input terminal X (E)
Figure 2
7. X-Z Fault indicator
8. Xv-Y High Resistance indicator
9. Xv-Y High Noise indicator
10. Display (with backlight)
11. Fuse holder
12. Test button
3
2
1
4
5
6
12
11
10
978
Digital Ground Resistance Tester Model 4620 and 4630
9
2.3 Fault Indicator LEDs
Thethreeindicatorsconrmthatthecorrectmeasurementisbeingtaken,
ifnoneofthemarelit.
2.3.1 X-Z Fault
ThisLEDsignalsthatthevoltagebetweenterminalsXandZexceeds30V
peak.
Therearefourpossiblecauses:
• theresistanceofthecurrentcircuitbetweenXandZistoohigh
• interferencevoltageinthecurrentcircuitistoohigh
• thefuseisblown
• thecircuitisopen(leadnotconnected)
2.3.2 Xv-Y High Resistance
ThisLEDsignalsthattheresistanceinthevoltagecircuit(betweenXvand
YorXandY)istoohigh(approx50kΩ)orthatthecircuitmaybeopen.
• Flashing will continue throughout the measurement, even if the
resistance drops below the threshold (e.g. after reconnecting
or lowering auxiliary rod resistance). In this case, you must
releasethepush-buttonandpressagainafterthefaulthasbeen
corrected.
• Occasionally, a stray voltage above 4.5V may also set off this
light.
• Checktheleadsforapossiblesolution.
2.3.3 Xv-Y High Noise
This LED signals the presence of excessive electrical noise (13V peak
approx)inthevoltagecircuit(betweenXvandYorXandY).
• Oneremedyistouseshieldedleadsfromtheinstrumenttothe
auxiliaryelectrodes.
• Connectalltheshieldstotherodundertest.
10
Digital Ground Resistance Tester Model 4620 and 4630
2.4 Buzzer
TheModels4620and4630featureabuzzer,whichwillsoundwhenthe
terminals of the tester are connected to a voltage source. The sound
volume is proportional to the voltage up to 30V approximately, then
becomes stable.
2.5 Over-range Indication
Over-rangeisindicatedwhenthedisplayreads1,orwhenthedisplayis
blinkingandtheindicatorislit.
2.6 Fault LED Indication – Tips and Solutions
TheLEDindicatorsshowexcessiveelectroderesistanceandexcessive
transientnoiseand/orstraycurrent.
Intheeventofanincorrectmeasurementindication:
• Improvethequalityoftheconnectiontoearthofauxiliaryground
electrodesYandZ.Zisthemostlikelysourceofproblemscaused
byexcessiveelectroderesistance.
• Checkconnectionsforcontinuitybetweenleadsandelectrodes.
• Be sure that electrodes are properly inserted; they should be
buriedasmuchaspossible.
• Ifhighelectroderesistancestillexistsafterproperlyinsertingaux-
iliaryelectrodesintotheearth,trypouringwateronandaround
theauxiliaryelectrodes.Thiswillimprovetheirelectricalconnec-
tiontoearth.
• Ifstraycurrentsaresuspected,onesolutiontoreducetheirinu-
enceistomovebothYandZelectrodesinanarcrelativetotheX
electrode(try,e.g.a90°shift),andtestagain.
• Displayof0.00:XvandYareshort-circuited.
• Displayof<0:XandZorXvandYrodsarereversed.
NOTE: Accuracy may be affected by auxiliary ground rod (Ry, Rz) resistance
levels and by stray signal levels (earth currents).
Digital Ground Resistance Tester Model 4620 and 4630
11
CHAPTER 3
SPECIFICATIONS
3.1 Electrical
Measurement Range:Auto-ranging0to2000Ω
Range 20Ω200Ω2000Ω
Measurement 0 to 19.99Ω20 to 199.9Ω200 to 1999Ω
Resolution 10mΩ100mΩ1Ω
Test Current 10mA 1mA 0.1mA
Accuracy 2% of Reading ± 1ct ± 5% of Reading ± 3cts
Open Voltage <42V peak
Operating Frequency:128Hzsquarewave
Max. Auxiliary Rod Resistance:
Range 20Ω200Ω2000Ω
Current Circuit 3kΩ30kΩ50kΩ
Voltage Circuit 50kΩ
Response Time: 4to8secondsapproxforastabilizedmeasurement
Interference:
TheModels4620&4630aredesignedtorejecthighlevelsofinterference
voltage(DC,50/60Hz,harmonics)
• DCvoltageinserieswithX:20V
• ACvoltageinserieswithY:13Vpeak(@16.67,50,60,400Hz)
• ACvoltageinserieswithZ:32Vpeak(@16.67,50,60,400Hz)
Accuracies and specications are given for an ambient temperature of 23°C ±3°K, RH of
45 to 55%, battery power at 9.5V, auxiliary resistance at the measurement terminals = 0, no
stray voltage, an electrical eld < 1V/m, and a magnetic eld from 0 to 40A/m.
Voltage Detection Range:20to250VACbetweentheXandZterminals
orbetweentheXvandZterminals.
Frequency: DCat450Hz
Voltage Withstanding:Bothmodelsarefuseprotected.Intheeventofa
systemfault,theunitscanwithstand250VACor100VDC.
Fuse Protection: Highbreakingcapacity-0.1A,>250V,0.25x1.25"
12
Digital Ground Resistance Tester Model 4620 and 4630
Model 4620
Power Source: Eight1.5VCcellbatteries
Battery Life: 4500measurementsof15secondseach
Low Battery Indicator: Ifthe indicatorlightsup,thebatteriesare
losingpower.
Model 4630
Power Source:NiMHrechargeablecell(4630)
Battery Life: 2000measurementsof15secondseach
Battery Charging:
Externalrecharge-120-230V/50-60Hz,20VA
Chargetime-6hrstoattain80%ofthebatterycapacity
Low Battery Indicator: Ifthe“CHARGE”indicatorturnsonsolidRED,
thebatteryneedstobecharged.
3.2 Mechanical
Connection:
Color-codedterminalsacceptspadelugswithminimumgapof6mmor
standard4mmbananajacks.
Display:
2000-count7segmentLCD,approx1"high(3-1/2digit)
LCDalsoindicatesoverrange,testleadshortsandleadreversals.
Blueelectroluminescentbacklight.
Dimensions:
10.8x9.7x5.0"(273x247x127mm)
Weight:
Model4620:6.28lbs(2.85kg)
Model4630:7.38lbs(3.35kg)
Colors:
Case-safetyyellow;Frontpanel-gray
Mechanical Protection:
TheModels4620and4630havesuccessfullyundergonealltherequired
mechanicaltestsandthereforemeetalltherequirementsofthestandards
EN61557andEN61010-1.
Digital Ground Resistance Tester Model 4620 and 4630
13
3.3 Environmental
Operating Temperature:
14°to131°F(-10°to55°C),0to90%RH
Storage Temperature:
-40°to158°F(-40°to70°C),0to90%RHwithbatteriesremoved
3.4 Safety
Electrical:
EN61010-1+A2(ed.95)
EN61557(ed.97)
30Vrms,CATIII,PollutionDegree2
Electromagnetic Compatibility:
EN61326-1(ed.98)
*Specications are subject to change without notice
3.5 Auto-ranging
Theselectionofthemeasurementcurrentisdependingontheresistance
tomeasure.
WhentheinstrumentisturnedON,themeasurementstartsonthesmallest
currentrange(100µA).Ifthemeasurementisbetween185and1950cts,
therangestaysthesame(100µA).Ifthemeasurementisunder185cts,
thecurrentismultipliedby10(within10mAmax).Ifitisabove1950cts,the
currentisdividedby10(withoutgoingunder100µA).
Thisisdonetoavoidswitchingbackandforthbetweenrangeswhenyou
aremeasuring190Ω.Itispossibletodisplay190.0or190Ωdependingon
theautomaticrangeselection.
14
Digital Ground Resistance Tester Model 4620 and 4630
CHAPTER 4
GROUNDING THEORY
4.1 Grounding Electrode Resistance
Figure3illustratesagroundingrod.Theresistanceoftheelectrodehas
thefollowingcomponents:
• theresistanceofthemetalandthatoftheconnectiontoit
• thecontactresistanceofthesurroundingearthtotheelectrode
• theresistanceinthesurroundingearth
Morespecically:
A) Groundingelectrodesareusuallymadeofaveryconductivemetal
(copper)withadequatecrosssectionssothatoverallresistanceis
negligible.
B) TheNationalInstituteofStandardandTechnology(N.I.S.T.)has
demonstratedthattheresistancebetweentheelectrodeandthe
surrounding earth is negligible if the electrode is free of paint,
greaseorothercoating,andiftheearthisrmlypacked.
Ground Rod
and Clamp
Contact
Resistance
Between Rod
and Soil
Concentric
Shells of
Earth
Figure 3
Digital Ground Resistance Tester Model 4620 and 4630
15
C) Theonlycomponentremainingistheresistanceofthesurround-
ingearth.Theelectrodecanbethoughtofasbeingsurrounded
by concentric shells of earth or soil, all of the same thickness.
The closer the shell to the electrode, the smaller its surface;
hence,thegreateritsresistance.Thefartherawaytheshellsare
fromtheelectrode,thegreaterthesurfaceoftheshell;hence,the
lowertheresistance.Eventually,addingshellsatadistancefrom
thegroundingelectrodewillnolongernoticeablyaffecttheoverall
earthresistancesurroundingtheelectrode.Thedistanceatwhich
thiseffectoccursisreferredtoastheeffectiveresistanceareaand
isdirectlydependentonthedepthofthegroundingelectrode.
In theory, the ground resistance may be derived from the general for-
mula:
R = ρ Resistance=Resistivityx
L
A
Length
Area
Thisformulaclearlyillustrateswhytheshellsofconcentricearthdecrease
inresistancethefarthertheyarefromthegroundrod:
R=ResistivityofSoilx ThicknessofShell
Area
Inthecaseofgroundresistance,uniformearth(orsoil)resistivitythrough-
outthevolume isassumed,although thisisseldom the casein nature.
The equations for systems of electrodes are very complex and often
expressedonlyasapproximations.Themostcommonlyusedformulafor
singlegroundelectrodesystems,developedbyProfessorH.R.Dwightof
theMassachusettsInstituteofTechnology,follows:
4L
r
R = ρ
2πLIn -1
R=resistanceinohmsofthegroundrodtotheearth(orsoil)
L=groundingelectrodelength
r=groundingelectroderadius
ρ=averageresistivityinohms-cm
16
Digital Ground Resistance Tester Model 4620 and 4630
4.1.1 Effect of Ground Electrode Size and Depth on Resistance
Size: Increasing the diameter of the rod does not materially reduce its
resistance.Doublingthediameterreducesresistancebylessthan10%.
0
25
50
75
100
1/25/8 3/411 1/411/
21
3/
4
Rod Diameter (inches)
Resistance in
%
Figure 4
Depth: As a ground rod is driven deeper into the earth, its resistance
issubstantiallyreduced.Ingeneral,doublingtherodlengthreducesthe
resistancebyanadditional40%.
515253540506
07
0
1
2
3
4
5
6
8
10
20
30
40
60
80
100
200
Driven Depth in Feet
Ground Resistance Versus Ground Rod Depth
Resistance in Ohms
1/2" dia.1" dia.
Figure 5
Digital Ground Resistance Tester Model 4620 and 4630
17
NEC®2014250.52(A)(5)requiresaminimumof8ft(2.4m)oftheelec-
trodetobeincontactwiththesoil.Themostcommonofelectrodeisa
10ft(3m)cylindricalrodwhichmeetstheNEC®code,whichrequiresa
minimumdiameterof5/8”(1.59cm).
4.1.2 Effects of Soil Resistivity on Ground Electrode Resistance
Dwight’sformula, citedpreviously,showsthat the resistancetoearthof
groundingelectrodesdependsnotonlyonthedepthandsurfaceareaof
groundingelectrodesbutonsoilresistivityaswell.
Soilresistivityisthekeyfactorthatdetermineswhattheresistanceofa
groundingelectrodewillbe,andtowhatdepthitmustbedriventoobtain
lowgroundresistance.
Theresistivityofthesoilvarieswidelythroughouttheworldandchanges
seasonally.Soilresistivityisdeterminedlargelybyitscontentofelectro-
lytes,consistingofmoisture,mineralsanddissolvedsalts.Adrysoilhas
highresistivityifitcontainsnosolublesalts.
Soil
Ashes, cinders, brine, waste
Clay, shale, gumbo, loam
Same, with varying proportions
of sand and gravel
Gravel, sand, stones with
little clay or loam
590
340
1020
59,000
2370
4060
15,800
94,000
7000
16,300
135,000
458,000
Resistivity, Ω-cm
Minimum Average Maximum
Table 1
4.1.3 Factors Affecting Soil Resistivity
Twosamplesofsoil,whenthoroughlydried,maybecomeinfactverygood
insulators,havingaresistivityinexcessof109ohm-centimeters.Theresis-
tivityofthesoilsampleisseentochangequiterapidlyuntilapproximately
twentypercentorgreatermoisturecontentisreached.
18
Digital Ground Resistance Tester Model 4620 and 4630
0
2.5
5
10
15
20
30
> 109
250,000
165,000
53,000
19,000
12,000
6400
> 109
150,000
43,000
18,500
10,500
6300
4200
Resistivity, Ω-cm
Moisture content,
% by weight Top Soil Sandy Loam
Table 2
Theresistivityofthesoilisalsoinuencedbytemperature.Table3shows
thevariationoftheresistivityofsandyloam,containing15.2%moisture,
withtemperaturechangesfrom20°to-15°C.Inthistemperaturerangethe
resistivityisseentovaryfrom7200to330,000ohm-centimeters.
20
10
0
0
-5
-15
68
50
32 (water)
32 (ice)
23
14
7200
9900
13,800
30,000
79,000
330,000
Resistivity
Ω-cm
Temperature
°C °F
Table 3
Because soil resistivity directly relates to moisture content and temper-
ature, it is reasonable to assume that the resistance of any grounding
systemwillvarythroughoutthedifferentseasonsoftheyear.Suchvaria-
tionsareshowninFigure6below.
Other manuals for 4620
1
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1
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