AEMC 3640 User manual
DIGITAL GROUND
RESISTANCE TESTERS
ENGLISH User Manual
3640
4610
Ω
Press To
Measure
X-Z
Xv-Y
Xv-Y
ZYX
C1 P2 C2
Fault
Hi Resistance
Hi Noise
GROUND RESISTANCE TESTER
MODEL 3640
AUTORANGING
REFER TO USER MANUAL
FOR FAULT WARNING LIGHT
EXPLANATIONS
!
®
INSTRUMENTS
Ω
Press To
Measure
X-Z
Xv-Y
Xv-Y
ZYXXv
C1 P1 P2 C2
Fault
Hi Resistance
Hi Noise
GROUND RESISTANCE TESTER
MODEL 4610
AUTORANGING
REFER TO USER MANUAL
FOR FAULT WARNING LIGHT
EXPLANATIONS
!
®
INSTRUMENTS
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 #: 3640 / 4610
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 3640 and 4610
1
Table of Contents
1. INTRODUCTION............................................................................... 3
1.1 International Electrical Symbols................................................3
1.2 DenitionofMeasurementCategories .....................................4
1.3 ReceivingYourShipment..........................................................4
1.4 OrderingInformation.................................................................4
1.4.1 Kits,AccessoriesandReplacementParts ....................5
2. PRODUCT FEATURES ...................................................................... 6
2.1 3640ControlandConnectorIdentication ...............................6
2.2 4610ControlandConnectorIdentication ...............................7
2.3 FaultIndicatorLEDs .................................................................8
2.3.1 X-ZFault .......................................................................8
2.3.2
X-YHighResistance(3640)
Xv-YHighResistance(4610)............................................... 8
2.3.3 X-YHighNoise(3640)
Xv-YHighNoise(4610) ...................................................8
2.4 Over-rangeIndication ...............................................................9
2.5 FaultLEDIndication–TipsandSolutions ................................9
3. SPECIFICATIONS........................................................................... 10
3.1 Electrical .................................................................................10
3.2 Mechanical.............................................................................. 11
3.3 Environmental......................................................................... 11
3.4 SafetySpecications ..............................................................11
3.5 Auto-ranging ...........................................................................12
4. OPERATION .................................................................................. 13
4.1 GroundingElectrodeResistance ............................................13
4.1.1 EffectofGroundElectrodeSizeandDepth
on Resistance .............................................................15
4.1.2 EffectsofSoilResistivityonGroundElectrode
Resistance ..................................................................16
4.1.3 FactorsAffectingSoilResistivity .................................17
4.1.4 EffectofGroundRodDepthonResistance................19
2
Digital Ground Resistance Tester Model 3640 and 4610
4.2 GroundResistanceValues .....................................................21
4.3 GroundResistanceTestingPrinciple.....................................23
4.3.1 PositionofAuxiliaryElectrodesinMeasurements ......24
4.4 MeasuringResistanceofGroundElectrodes (62%Method)..25
4.4.1 AuxiliaryElectrodeSpacing ........................................27
4.5 GroundResistanceMeasurementProcedure (3-Point)..........28
4.6 MultipleElectrodeSystem ......................................................29
4.7 2-PointMeasurement(SimpliedMeasurement)............. 31
4.8 ContinuityMeasurement.........................................................32
4.9 SoilResistivityMeasurements(Model4610Only) .................33
4.10SoilResistivityMeasurementProcedure(4-Point) .................34
4.11 HowtoUse25ΩCalibrationChecker.....................................36
5. MAINTENANCE ............................................................................. 37
5.1 Warning...................................................................................37
5.2 Cleaning..................................................................................37
5.3 ReplacingtheBattery .............................................................37
5.4 ReplacingtheSafetyFuse......................................................38
RepairandCalibration...........................................................................39
TechnicalandSalesAssistance ............................................................39
Limited Warranty ...................................................................................40
WarrantyRepairs...................................................................................40
Digital Ground Resistance Tester Model 3640 and 4610
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
Thesesafetywarningsareprovidedtoensurethe safetyofpersonnel
andproperoperationoftheinstrument.
• The instrument must not be operated beyond its specied operating
range.
• Safetyistheresponsibilityoftheoperator.
• Allmetalobjectsorwiresconnectedtotheelectricalsystemshouldbe
assumedtobelethaluntiltested.Groundingsystemsarenoexception.
• Use extreme caution when using the instrument around energized
electricalequipment.
• Neverattempttousetheinstrumenttotwistorprythegroundelectrode
orgroundwireawayfromtheequipmentbeinggrounded.
• The use of rubber gloves is an excellent safety practice even if the
equipmentisproperlyoperatedandcorrectlygrounded.
• Always inspect the instrument and leads prior to use. Replace any
defectivepartsimmediately.
1.1 International Electrical Symbols
This symbol 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 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
4
Digital Ground Resistance Tester Model 3640 and 4610
1.2 Definition of Measurement Categories
CAT I: Formeasurementson circuits not directly connected to theAC
supply wall outlet such as protected secondaries, signal level,
andlimitedenergycircuits.
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
with the packing list. Notify your distributor of any missing items. If the
equipmentappearstobedamaged,leaclaimimmediatelywiththecar-
rierandnotifyyourdistributoratonce,givingadetaileddescriptionofany
damage.Savethedamagedpackingcontainertosubstantiateyourclaim.
1.4 Ordering Information
Ground Resistance Tester Model 3640............................ Cat. #2114.92
Includes soft carrying case, batteries and a user manual.
Ground Resistance Tester Model 3640 Kit (150 ft) ......... Cat. #2135.13
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, batteries, carrying bag, user manual and warranty card.
Ground Resistance Tester Model 3640 Kit (300 ft) ......... Cat. #2135.14
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, batteries, carrying bag, user manual and
warranty card.
Ground Resistance Tester Model 3640 Kit (500 ft) ......... Cat. #2135.15
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, batteries, carrying bag,
user manual and warranty card.
Ground Resistance Tester Model 4610............................ Cat. #2114.94
Includes soft carrying case, batteries and a user manual.
Digital Ground Resistance Tester Model 3640 and 4610
5
Ground Resistance Tester Model 4610 Kit (150 ft) ......... Cat. #2135.16
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, batteries, carrying bag, user manual and warranty card.
Ground Resistance Tester Model 4610 Kit (300 ft) ......... Cat. #2135.17
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, batteries, carrying bag, user manual and
warranty card.
Ground Resistance Tester Model 4610 Kit (500 ft) ......... Cat. #2135.18
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, batteries, carrying bag,
user manual and warranty card.
1.4.1 Kits, Accessories and Replacement Parts
Test Kit for 3-Point Testing ............................................... 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,
carrying bag.
Test Kit for 4-Point Testing ............................................... 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, carrying bag.
Test Kit for 4-Point Testing ............................................... 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, 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,
carrying bag.
Set of 2, T-Shaped Auxiliary Ground Electrodes............ Cat. #2135.39
Ground Tester Video/Workbook Set ................................ Cat. #2130.64
25Ω Calibration Checker................................................... Cat. #2130.59
Tape Measure – AEMC 100 ft ............................................ Cat. #2130.60
Fuse – Set of 5, 0.1A, >250V, 0.25 x 1.25"........................ Cat. #2970.12
Order Accessories and Replacement Parts Directly Online
Check our Storefront at www.aemc.com/store for availability
6
Digital Ground Resistance Tester Model 3640 and 4610
CHAPTER 2
PRODUCT FEATURES
2.1 3640 Control and Connector Identification
Ω
Press To
Measure
X-Z
Xv-Y
Xv-Y
ZYX
C1 P2 C2
Fault
Hi Resistance
Hi Noise
GROUND RESISTANCE TESTER
MODEL 3640
AUTORANGING
REFER TO USER MANUAL
FOR FAULT WARNING LIGHT
EXPLANATIONS
!
2 3 4 5
6
78910
1
Figure 1
1. Press-to-Measurebutton
2. InputterminalX(C1)
3. InputterminalY(P2)
4. Groundresistanceshortinglink
5. InputterminalZ(C2)
6. Display
7. Lowbatteryindicator
8. Xv-Yhighnoiseindicator
9. Xv-Yhighresistanceindicator
10. X-Zfaultindicator
Digital Ground Resistance Tester Model 3640 and 4610
7
2.2 4610 Control and Connector Identification
Ω
Press To
Measure
X-Z
Xv-Y
Xv-Y
ZYXXv
C1 P1 P2 C2
Fault
Hi Resistance
Hi Noise
GROUND RESISTANCE TESTER
MODEL 4610
AUTORANGING
REFER TO USER MANUAL
FOR FAULT WARNING LIGHT
EXPLANATIONS
!
2 3 4 5 6
7
891011
1
Figure 2
1. Press-to-Measurebutton
2. InputterminalX(C1)
3. InputterminalXv(P1)
4. InputterminalY(P2)
5. Groundresistanceshortinglink
6. InputterminalZ(C2)
7. Display
8. Lowbatteryindicator
9. Xv-Yhighnoiseindicator
10. Xv-Yhighresistanceindicator
11. X-Zfaultindicator
8
Digital Ground Resistance Tester Model 3640 and 4610
2.3 Fault Indicator LEDs
The three indicators described below conrm the correct measurement
beingtakenifnoneofthemarelit.
2.3.1 X-Z Fault
ThisLEDsignalsthatvoltagebetweenterminalsXandZexceeds30Vpeak.
Therearefourpossiblecauses:
• theresistanceofthecurrentcircuitbetweenXandZistoohigh
• interferencevoltageinthecurrentcircuitistoohigh
• thefuseisblown
• thecircuitisopen(leadnotconnected)
2.3.2
X-Y High Resistance (3640) - Xv-Y High Resistance (4610)
ThisLEDsignalsthattheresistanceinthevoltagecircuit(betweenXvand
YorXandY)istoohigh(approx50kΩ)orthatthecircuitmaybeopen.
• Flashing will continue throughout the measurement, even if the
resistancedrops belowthethreshold(e.g. afterreconnectingor
loweringauxiliaryrodresistance).
• Inthiscase,youmustreleasethepush-buttonandpressagain
afterthefaulthasbeencorrected.
• Occasionally, a stray voltage above 6VDC may also set off this
light.
• Checktheleadsforapossiblesolution.
2.3.3 X-Y High Noise (3640) - Xv-Y High Noise (4610)
ThisLEDsignalsthepresenceofexcessivenoise(approx13Vpeak)in
thevoltagecircuit(betweenXvandYorXandY).
• Oneremedyistouseshieldedleadsfromtheinstrumenttothe
auxiliaryelectrodes.
• Connectalltheshieldstotherodundertest.
Digital Ground Resistance Tester Model 3640 and 4610
9
2.4 Over-range Indication
Over-rangeisindicatedwhenthedisplayreads1,orwhenthedisplayis
blinkingandtheindicatorislit.
2.5 Fault LED Indication – Tips and Solutions
TheLEDindicatorsshowexcessiveelectroderesistanceand excessive
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).
10
Digital Ground Resistance Tester Model 3640 and 4610
CHAPTER 3
SPECIFICATIONS
3.1 Electrical
Measurement Ranges: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 3% of Reading ± 3cts
Open Voltage <42V peak
Operating Frequency:128Hzsquarewave
Max. Auxiliary Rod Resistance:
Range 20Ω200Ω2000Ω
Current Circuit 3kΩ30kΩ50kΩ
Voltage Circuit 50kΩ
Response Time: Approximately6secondsforastabilizedmeasurement
Interference: Models3640&4610aredesignedtorejecthighlevelsof
interferencevoltage(DC,50/60Hz,harmonics)
• DCvoltageinserieswithX:20V
• ACvoltageinserieswithY:13Vpeak
• ACvoltageinserieswithZ:32Vpeak
Accuracies and specications are given for an ambient temperature of 23°C ± 3°, RH of 45
to 55%, battery power at 8V, auxiliary resistance at the measurement terminals <200Ω, no
stray voltage and a magnetic eld from 0 to 40A/m.
Power Source: Eight1.5V“AA”batteries;Alkalinerecommended.
Battery Life: 180015-secondmeasurements
Low Battery Indicator: Ifthe“LoBat”indicatorlightsup,thebatteries
arelosingpower.T
heavailableoperatingtimeremainingis10015-second
measurements(approx).
Fuse Protection: Highbreakingcapacity-0.1A,>250V,0.25x1.25"
Digital Ground Resistance Tester Model 3640 and 4610
11
3.2 Mechanical
Connection:Color-codedterminalsacceptspadelugswithminimumgap
of6mmorstandard4mmbananajacks
Display:7segmentLCD,.71"(18mm)high(3-1/2digit);2000cts
LCDalsoindicatesoverrange,testleadshortsandleadreversals.
Dimensions:8.7x5.4x5.9"(220x136x150mm)
Weight: 2.9lbs(1.3kg)
Case: Heavy-duty,ABS
Colors: Case-safetyyellow;Frontpanel-gray
Mechanical Shock: IEC 68-2-27
Vibration: IEC 68-2-6
Drop Test: IEC 68-2-32
Case Material: UL94
Environmental: O-ringsealedagainstdustandwatertoIP50
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 Specifications
Electrical: EN61010-1,CATlII,PollutionDegree2,42V
Electromagnetic Compatibility: Emission:EN50081-1
Immunity:EN50082-1
*Specications are subject to change without notice.
12
Digital Ground Resistance Tester Model 3640 and 4610
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.
Digital Ground Resistance Tester Model 3640 and 4610
13
CHAPTER 4
OPERATION
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
14
Digital Ground Resistance Tester Model 3640 and 4610
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.
Intheory,thegroundresistancemaybederivedfromthegeneral
formula:
R = ρ Resistance=Resistivityx
L
A
Length
Area
Thisformulaclearlyillustrateswhytheshellsofconcentricearth
decreaseinresistancethefarthertheyarefromthegroundrod:
R=ResistivityofSoilx ThicknessofShell
Area
Inthecaseofgroundresistance,uniformearth(orsoil)resistivitythrough-
outthevolumeisassumed,althoughthisisseldomthe case in nature.
The equations for systems of electrodes are very complex and often
expressedonlyasapproximations.Themostcommonlyusedformulafor
singlegroundelectrodesystems,developedbyProfessorH.R.Dwightof
theMassachusettsInstituteofTechnology,follows:
R = ρ
2πL
{(In 4L) -1}
r
R=resistanceinohmsofthegroundrodtotheearth(orsoil)
L=groundingelectrodelength
r=groundingelectroderadius
ρ=averageresistivityinohms-cm
Digital Ground Resistance Tester Model 3640 and 4610
15
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 1/411/213/4
Rod Diameter (inches)
Resistance in %
Figure 4
Depth: As a ground rod is driven deeper into the earth, its resistance
issubstantiallyreduced.Ingeneral,doublingtherodlengthreducesthe
resistance by an additional 40%.
5 15 25 3540 50 60 70
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
16
Digital Ground Resistance Tester Model 3640 and 4610
TheNEC®2008250.52(A)(5)requiresaminimumof8ft(2.4m)tobein
contactwiththesoil.Themostcommonisa10ft(3m)cylindricalrodwhich
meetstheNEC®code.Aminimumdiameterof5/8"(1.59cm)isrequired
forsteelrodsand1/2"(1.27cm)forcopperorcoppercladsteelrodsNEC®
2008250.52(A)(5)(a)(b).Minimumpracticaldiameterfordrivinglimitations
for10ft(3m)rodsare:
• 1/2"(1.27cm)inaveragesoil
• 5/8"(1.59cminmoistsoil
• 3/4"(1.91cm)inhardsoilormorethan10ftdrivingdepths
4.1.2 Effects of Soil Resistivity on Ground Electrode Resistance
Dwight’sformula,citedpreviously,showsthattheresistance to earth of
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
1,020
59,000
2,370
4,060
15,800
94,000
7,000
16,300
135,000
458,000
Resistivity, Ω-cm
Minimum Average Maximum
Table 1
Digital Ground Resistance Tester Model 3640 and 4610
17
4.1.3 Factors Affecting Soil Resistivity
Twosamplesofsoil,whenthoroughlydried,maybecomeinfactverygood
insulators, having a resistivity in excess of 109 ohm-centimeters. The
resistivityofthesoilsampleisseentochangequiterapidlyuntilapproxi-
matelytwentypercentorgreatermoisturecontentisreached.
0
2.5
5
10
15
20
30
> 109
250,000
165,000
53,000
19,000
12,000
6,400
> 109
150,000
43,000
18,500
10,500
6,300
4,200
Resistivity, Ω-cm
Moisture content,
% by weight Top Soil Sandy Loam
Table 2
Theresistivityofthesoilisalsoinuencedbytemperature.Table3shows
thevariationoftheresistivityofsandyloam,containing15.2%moisture,
withtemperaturechangesfrom20°to-15°C.Inthistemperaturerangethe
resistivityisseentovaryfrom7,200to330,000ohm-centimeters.
18
Digital Ground Resistance Tester Model 3640 and 4610
20
10
0
0
-5
-15
68
50
32 (water)
32 (ice)
23
14
7,200
9,900
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.
Since both temperature and moisture content become more stable at
greaterdistancesbelowthesurfaceoftheearth,itfollowsthatagrounding
system(tobemosteffectiveatalltimes)shouldbeconstructedwiththe
groundroddrivendownaconsiderabledistancebelowthesurfaceofthe
earth.Bestresultsareobtainedifthegroundrodreachesthewatertable.
0
20
40
60
80
Jan.
Mar.
May
July
Sept.
Nov.
Jan.
Mar.
May
July
Curve 1
Curve 2
Figure 6
Seasonal variation of earth resistance with an electrode of 3/4" pipe in
rather stony clay soil. Depth of electrode in earth is 3 ft for Curve 1, and
10 ft for Curve 2.
Insomelocations,theresistivityoftheearthissohighthatlow-resistance
groundingcanbeobtainedonlyatconsiderableexpenseandwithanelab-
orategroundingsystem.
Insuchsituations,itmaybeeconomicaltouseagroundrodsystemof
This manual suits for next models
1
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