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 DenitionofMeasurementCategories .....................................4
1.3 ReceivingYourShipment..........................................................4
1.4 OrderingInformation.................................................................5
1.4.1 Kits,AccessoriesandReplacementParts ....................6
2. PRODUCT FEATURES......................................................................... 7
2.1 ControlandConnectorFeatures(Model4620) .......................7
2.2 ControlandConnectorFeatures(Model4630) ........................8
2.3 FaultIndicatorLEDs .................................................................9
2.3.1 X-ZFault .......................................................................9
2.3.2 Xv-YHighResistance ...................................................9
2.3.3 Xv-YHighNoise............................................................9
2.4 Buzzer.....................................................................................10
2.5 Over-rangeIndication .............................................................10
2.6 FaultLEDIndication–TipsandSolutions ..............................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 GroundingElectrodeResistance ............................................14
4.1.1 EffectofElectrodeSize/DepthonResistance ............16
4.1.2 EffectsofSoilResistivityonElectrodeResistance.....17
4.1.3 FactorsAffectingSoilResistivity .................................17
4.1.4 EffectofGroundRodDepthonResistance................20
4.2 GroundResistanceValues .....................................................21
4.3 GroundResistanceTestingPrinciple.....................................23
4.3.1 PositionofAuxiliaryElectrodesinMeasurements ......24

2 Digital Ground Resistance Tester Model 4620 and 4630
4.4 MeasuringResistanceofGroundElectrodes ........................26
4.4.1 AuxiliaryElectrodeSpacing ........................................28
4.5 MultipleElectrodeSystem ......................................................28
5. OPERATION..................................................................................... 30
5.1 GroundResistanceMeasurementProcedure .......................30
5.2 2-PointMeasurement(SimpliedMeasurement) ...................31
5.3 ContinuityMeasurement.........................................................32
5.4 SoilResistivityMeasurements................................................32
5.4.1 PurposesofSoilResistivity ........................................32
5.4.2 TypesofResistivityMeasurements ............................33
5.5 SoilResistivityMeasurementProcedure(4-Point) .................34
5.6 How to Use 25ΩCalibrationChecker(accessory) ................36
6. MAINTENANCE................................................................................ 37
6.1 Warning..................................................................................37
6.2 Disassembly ...........................................................................37
6.3 PowerSupply..........................................................................38
6.3.1 TestingtheBattery ......................................................38
6.3.2 ReplacingtheBattery(Model4620) ...........................39
6.3.3 RechargingtheBattery(Model4630).........................39
6.3.4 ReplacingtheSafetyFuse..........................................40
6.4 Cleaning..................................................................................40
6.5 Storage ...................................................................................40
RepairandCalibration ....................................................................41
TechnicalandSalesAssistance......................................................41
Limited Warranty .............................................................................42
WarrantyRepairs ............................................................................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
Thesesafetywarningsareprovidedtoensurethesafetyofper-
sonnelandproperoperationoftheinstrument.
• The instrument must not be operated beyond its specied
operatingrange.
• Safetyistheresponsibilityoftheoperator.
• Allmetalobjectsorwiresconnectedtotheelectricalsystem
shouldbeassumedtobelethaluntiltested.Groundingsys-
temsarenoexception.
• Useextremecautionwhenusingtheinstrumentaroundener-
gizedelectricalequipment.
• Neverattempttousetheinstrumenttotwistorprytheground
electrode or ground wire away from the equipment being
grounded.
• AEMC®Instrumentsconsiderstheuseofrubberglovestobe
anexcellentsafetypracticeeveniftheequipmentisproperly
operatedandcorrectlygrounded.
• Alwaysinspecttheinstrumentandleadspriortouse.Replace
anydefectivepartsimmediately.

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
onhouseholdappliancesorportabletools.
CAT III: For measurements performed in the building installation at
the distribution level such as on hardwired equipment in xed
installationandcircuitbreakers.
CAT IV: For measurements performed at the primary electrical supply
(<1000V) such as on primary overcurrent protection devices,
ripplecontrolunits,ormeters.
1.3 Receiving Your Shipment
Uponreceivingyourshipment,makesurethatthecontentsareconsistent
withtheorderinginformation.Notifyyourdistributorofanymissingitems.
Iftheequipmentappearstobedamaged,leaclaimimmediatelywiththe
carrierandnotifyyourdistributoratonce,givingadetaileddescriptionof
anydamage.Savethedamagedpackingcontainertosubstantiateyour
claim.
Donotuseequipmentwhichisdamagedorappearstobedamaged.

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 (Supplementalfor4-PointTesting)... 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 CalibrationChecker .....................................................Cat. #2130.59
TapeMeasure–AEMC100ft..............................................Cat. #2130.60
GroundTesterVideo/WorkbookSet .................................... Cat. #2130.64
Setof2,T-ShapedAuxiliaryGroundElectrodes ................. Cat. #2135.39
Inverter–12VDCto120VAC200Wattforvehicleuse........ Cat. #2135.43
Fuse–Setof5,0.1A,>250V,0.25x1.25" ......................... Cat. #2970.12
Replacement9.6VRechargeableBatteryPack(4630)....... Cat. #2960.21
115VPowerCord ................................................................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
Thethreeindicatorsconrmthatthecorrectmeasurementisbeingtaken,
ifnoneofthemarelit.
2.3.1 X-Z Fault
ThisLEDsignalsthatthevoltagebetweenterminalsXandZexceeds30V
peak.
Therearefourpossiblecauses:
• theresistanceofthecurrentcircuitbetweenXandZistoohigh
• interferencevoltageinthecurrentcircuitistoohigh
• thefuseisblown
• thecircuitisopen(leadnotconnected)
2.3.2 Xv-Y High Resistance
ThisLEDsignalsthattheresistanceinthevoltagecircuit(betweenXvand
YorXandY)istoohigh(approx50kΩ)orthatthecircuitmaybeopen.
• 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
releasethepush-buttonandpressagainafterthefaulthasbeen
corrected.
• Occasionally, a stray voltage above 4.5V may also set off this
light.
• Checktheleadsforapossiblesolution.
2.3.3 Xv-Y High Noise
This LED signals the presence of excessive electrical noise (13V peak
approx)inthevoltagecircuit(betweenXvandYorXandY).
• Oneremedyistouseshieldedleadsfromtheinstrumenttothe
auxiliaryelectrodes.
• Connectalltheshieldstotherodundertest.

10 Digital Ground Resistance Tester Model 4620 and 4630
2.4 Buzzer
TheModels4620and4630featureabuzzer,whichwillsoundwhenthe
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-rangeisindicatedwhenthedisplayreads1,orwhenthedisplayis
blinkingandtheindicatorislit.
2.6 Fault LED Indication – Tips and Solutions
TheLEDindicatorsshowexcessiveelectroderesistanceandexcessive
transientnoiseand/orstraycurrent.
Intheeventofanincorrectmeasurementindication:
• Improvethequalityoftheconnectiontoearthofauxiliaryground
electrodesYandZ.Zisthemostlikelysourceofproblemscaused
byexcessiveelectroderesistance.
• Checkconnectionsforcontinuitybetweenleadsandelectrodes.
• Be sure that electrodes are properly inserted; they should be
buriedasmuchaspossible.
• Ifhighelectroderesistancestillexistsafterproperlyinsertingaux-
iliaryelectrodesintotheearth,trypouringwateronandaround
theauxiliaryelectrodes.Thiswillimprovetheirelectricalconnec-
tiontoearth.
• Ifstraycurrentsaresuspected,onesolutiontoreducetheirinu-
enceistomovebothYandZelectrodesinanarcrelativetotheX
electrode(try,e.g.a90°shift),andtestagain.
• Displayof0.00:XvandYareshort-circuited.
• Displayof<0:XandZorXvandYrodsarereversed.
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-ranging0to2000Ω
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:128Hzsquarewave
Max. Auxiliary Rod Resistance:
Range 20Ω200Ω2000Ω
Current Circuit 3kΩ30kΩ50kΩ
Voltage Circuit 50kΩ
Response Time: 4to8secondsapproxforastabilizedmeasurement
Interference:
TheModels4620&4630aredesignedtorejecthighlevelsofinterference
voltage(DC,50/60Hz,harmonics)
• DCvoltageinserieswithX:20V
• ACvoltageinserieswithY:13Vpeak(@16.67,50,60,400Hz)
• ACvoltageinserieswithZ:32Vpeak(@16.67,50,60,400Hz)
Accuracies and specications 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:20to250VACbetweentheXandZterminals
orbetweentheXvandZterminals.
Frequency: DCat450Hz
Voltage Withstanding:Bothmodelsarefuseprotected.Intheeventofa
systemfault,theunitscanwithstand250VACor100VDC.
Fuse Protection: Highbreakingcapacity-0.1A,>250V,0.25x1.25"

12 Digital Ground Resistance Tester Model 4620 and 4630
Model 4620
Power Source: Eight1.5VCcellbatteries
Battery Life: 4500measurementsof15secondseach
Low Battery Indicator: Ifthe indicatorlightsup,thebatteriesare
losingpower.
Model 4630
Power Source:NiMHrechargeablecell(4630)
Battery Life: 2000measurementsof15secondseach
Battery Charging:
Externalrecharge-120-230V/50-60Hz,20VA
Chargetime-6hrstoattain80%ofthebatterycapacity
Low Battery Indicator: Ifthe“CHARGE”indicatorturnsonsolidRED,
thebatteryneedstobecharged.
3.2 Mechanical
Connection:
Color-codedterminalsacceptspadelugswithminimumgapof6mmor
standard4mmbananajacks.
Display:
2000-count7segmentLCD,approx1"high(3-1/2digit)
LCDalsoindicatesoverrange,testleadshortsandleadreversals.
Blueelectroluminescentbacklight.
Dimensions:
10.8x9.7x5.0"(273x247x127mm)
Weight:
Model4620:6.28lbs(2.85kg)
Model4630:7.38lbs(3.35kg)
Colors:
Case-safetyyellow;Frontpanel-gray
Mechanical Protection:
TheModels4620and4630havesuccessfullyundergonealltherequired
mechanicaltestsandthereforemeetalltherequirementsofthestandards
EN61557andEN61010-1.

Digital Ground Resistance Tester Model 4620 and 4630 13
3.3 Environmental
Operating Temperature:
14°to131°F(-10°to55°C),0to90%RH
Storage Temperature:
-40°to158°F(-40°to70°C),0to90%RHwithbatteriesremoved
3.4 Safety
Electrical:
EN61010-1+A2(ed.95)
EN61557(ed.97)
30Vrms,CATIII,PollutionDegree2
Electromagnetic Compatibility:
EN61326-1(ed.98)
*Specications are subject to change without notice
3.5 Auto-ranging
Theselectionofthemeasurementcurrentisdependingontheresistance
tomeasure.
WhentheinstrumentisturnedON,themeasurementstartsonthesmallest
currentrange(100µA).Ifthemeasurementisbetween185and1950cts,
therangestaysthesame(100µA).Ifthemeasurementisunder185cts,
thecurrentismultipliedby10(within10mAmax).Ifitisabove1950cts,the
currentisdividedby10(withoutgoingunder100µA).
Thisisdonetoavoidswitchingbackandforthbetweenrangeswhenyou
aremeasuring190Ω.Itispossibletodisplay190.0or190Ωdependingon
theautomaticrangeselection.

14 Digital Ground Resistance Tester Model 4620 and 4630
CHAPTER 4
GROUNDING THEORY
4.1 Grounding Electrode Resistance
Figure3illustratesagroundingrod.Theresistanceoftheelectrodehas
thefollowingcomponents:
• theresistanceofthemetalandthatoftheconnectiontoit
• thecontactresistanceofthesurroundingearthtotheelectrode
• theresistanceinthesurroundingearth
Morespecically:
A) Groundingelectrodesareusuallymadeofaveryconductivemetal
(copper)withadequatecrosssectionssothatoverallresistanceis
negligible.
B) TheNationalInstituteofStandardandTechnology(N.I.S.T.)has
demonstratedthattheresistancebetweentheelectrodeandthe
surrounding earth is negligible if the electrode is free of paint,
greaseorothercoating,andiftheearthisrmlypacked.
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) Theonlycomponentremainingistheresistanceofthesurround-
ingearth.Theelectrodecanbethoughtofasbeingsurrounded
by concentric shells of earth or soil, all of the same thickness.
The closer the shell to the electrode, the smaller its surface;
hence,thegreateritsresistance.Thefartherawaytheshellsare
fromtheelectrode,thegreaterthesurfaceoftheshell;hence,the
lowertheresistance.Eventually,addingshellsatadistancefrom
thegroundingelectrodewillnolongernoticeablyaffecttheoverall
earthresistancesurroundingtheelectrode.Thedistanceatwhich
thiseffectoccursisreferredtoastheeffectiveresistanceareaand
isdirectlydependentonthedepthofthegroundingelectrode.
In theory, the ground resistance may be derived from the general for-
mula:
R = ρ Resistance=Resistivityx
L
A
Length
Area
Thisformulaclearlyillustrateswhytheshellsofconcentricearthdecrease
inresistancethefarthertheyarefromthegroundrod:
R=ResistivityofSoilx ThicknessofShell
Area
Inthecaseofgroundresistance,uniformearth(orsoil)resistivitythrough-
outthevolume isassumed,although thisisseldom the casein nature.
The equations for systems of electrodes are very complex and often
expressedonlyasapproximations.Themostcommonlyusedformulafor
singlegroundelectrodesystems,developedbyProfessorH.R.Dwightof
theMassachusettsInstituteofTechnology,follows:
4L
r
R = ρ
2πLIn -1
R=resistanceinohmsofthegroundrodtotheearth(orsoil)
L=groundingelectrodelength
r=groundingelectroderadius
ρ=averageresistivityinohms-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.Doublingthediameterreducesresistancebylessthan10%.
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
issubstantiallyreduced.Ingeneral,doublingtherodlengthreducesthe
resistancebyanadditional40%.
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®2014250.52(A)(5)requiresaminimumof8ft(2.4m)oftheelec-
trodetobeincontactwiththesoil.Themostcommonofelectrodeisa
10ft(3m)cylindricalrodwhichmeetstheNEC®code,whichrequiresa
minimumdiameterof5/8”(1.59cm).
4.1.2 Effects of Soil Resistivity on Ground Electrode Resistance
Dwight’sformula, citedpreviously,showsthat the resistancetoearthof
groundingelectrodesdependsnotonlyonthedepthandsurfaceareaof
groundingelectrodesbutonsoilresistivityaswell.
Soilresistivityisthekeyfactorthatdetermineswhattheresistanceofa
groundingelectrodewillbe,andtowhatdepthitmustbedriventoobtain
lowgroundresistance.
Theresistivityofthesoilvarieswidelythroughouttheworldandchanges
seasonally.Soilresistivityisdeterminedlargelybyitscontentofelectro-
lytes,consistingofmoisture,mineralsanddissolvedsalts.Adrysoilhas
highresistivityifitcontainsnosolublesalts.
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
Twosamplesofsoil,whenthoroughlydried,maybecomeinfactverygood
insulators,havingaresistivityinexcessof109ohm-centimeters.Theresis-
tivityofthesoilsampleisseentochangequiterapidlyuntilapproximately
twentypercentorgreatermoisturecontentisreached.

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
Theresistivityofthesoilisalsoinuencedbytemperature.Table3shows
thevariationoftheresistivityofsandyloam,containing15.2%moisture,
withtemperaturechangesfrom20°to-15°C.Inthistemperaturerangethe
resistivityisseentovaryfrom7200to330,000ohm-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
systemwillvarythroughoutthedifferentseasonsoftheyear.Suchvaria-
tionsareshowninFigure6below.