soil resistivity and grounding system … resistivity... · key variable in system design ... –...
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Soil Resistivityandand
Grounding System TestingGrounding System Testing
Roy “Chip” WhittenSenior Applications Engineer/Education SpecialistLyncole Grounding Solutions, LLC
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What Is Grounding ?What Is Grounding ?
A l t i l ti h th i t ti l id t l b tAn electrical connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earthbody that serves in place of the earth.
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Reasons For GroundingReasons For Grounding
–Personnel safety and equipment protection by providing aPersonnel safety and equipment protection by providing apath to safely dissipate any unwanted charges or potentials.
Ensure equipment performance and protection– Ensure equipment performance and protection
– Satisfy manufacturer’s warrantySatisfy manufacturer s warranty
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Electrical Protection PyramidElectrical Protection Pyramid®®
RFRFAC SurgeAC Surge
Telco / DataTelco / DataLightningLightning
GroundingGroundingGroundingGrounding
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Resistance To EarthResistance To Earth
25 Ohms
20v 12v 10.6v11v16v 10.18v10.5v 10.45v 0
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SoilR i ti itResistivity
BasicsBasics
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Soil Resistivity
Key Variable in System Design
– Determines grounding system resistance– Changes from Site to Site– Dependent on:
– Soil type– Moisture– Electrolytes– Temperature
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Soil Resistivity Comparison
SurfaceSurface SoilsSoils 100100 -- 5,0005,000Soil TypeSoil Type Resistivity (ohmResistivity (ohm--cm)cm)
SurfaceSurface SoilsSoils 100 100 5,0005,000ClayClay 200 200 -- 10,00010,000Sand and GravelSand and Gravel 5,000 5,000 -- 100,000100,000Surface LimestoneSurface Limestone 10,00010,000 -- 1,000,0001,000,000Surface LimestoneSurface Limestone 10,000 10,000 1,000,0001,000,000LimestoneLimestone 500 500 -- 400,000400,000ShaleShale 500 500 -- 10,00010,000SandstoneSandstone 2,000 2,000 -- 200,000200,000,, ,,Granites,Granites, Basalts, etcBasalts, etc 100,000100,000Decomposed Decomposed GneissesGneisses 5,000 5,000 -- 50,00050,000Slates, etcSlates, etc 1,000 1,000 -- 10,00010,000,, ,, ,,
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SoilR i ti itResistivity
TestingTesting
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Soil Resistivity Testing
– Key Variable in System Design– 4 Point Wenner Method
– Most Accurate– Multiple Depth Testing
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4-Pt. Wenner Method
Visually Survey Lease Area to Determine Location– Visually Survey Lease Area to Determine Location and Direction For Test
Not parallel to buried metallic objects– Not parallel to buried metallic objects– Not parallel overhead power lines
Sufficient straight line distance to allow for test– Sufficient straight line distance to allow for test
– Minimal distance 300 feet
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4-Pt. Wenner Method
C1P1P1P2P2C2C2
15 ft15 ft 10 ft10 ft 5 ft5 ft 0 ft0 ft15 ft15 ft 10 ft10 ft 5 ft5 ft 0 ft0 ft
AA
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4-Pt. Wenner Method
Site AreaSite Area
Direction of TestDirection of Test
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44--Pt. Wenner MethodPt. Wenner Method
Probe SpacingProbe Spacing Meter ReadingMeter Reading Calculated ResistivityCalculated Resistivity(Feet)(Feet) (Ohms)(Ohms) (Ohm(Ohm Meter)Meter)
ρρ
= = 1.915 1.915 ARAR
(Feet)(Feet) (Ohms)(Ohms) (Ohm(Ohm--Meter)Meter)5 52.00 497.90
10 19.68 370.87
15 10.16 292.00
20 6.53 250.10
30 4 30 247 0430 4.30 247.04
40 10.80 827.28
60 7.40 850.26
80 5.58 855.60
100 4.44 850.26
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T i lTypical Grounding Electrode SystemGrounding Electrode System
Resistance Requirementsq
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Typical Resistance Requirements
– NFPA 70 NEC 25 OHMS or Two Rods
– IEEE Standard 142 & 1100 Equipment Dependent
– Motorola Standard R-56 10 OHMS (Design Goal)
– Telecommunications 5 to 10 OHMS
– Emerson DeltaV 3 OHMS
– Essilor 3 OHMS
– GE Medical Systems 2 OHMS
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25 Ohm Grounding System
Lightning
StrikeStrike
18,000APotential Rise will be
~450KV at the site
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5 Ohm Grounding System
Lightning
StrikeStrike
18,000APotential Rise will be
~90KV at the site
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Grounding System Resistance Testing
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Grounding System Testing
– Why Test Grounds?y
– Determine Baseline Validate Construction
– Confirm Design Spec Satisfied
– Validate Construction
– Satisfy Warranty Reqs– Ensure Equip Protection & Performance
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Testing Methods
– Two Test Methods
− Fall of Potential (Three Point) TestFall of Potential (Three Point) Test− Clamp-on Test
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Fall-of-Potential Method
RemoteRemoteCurrentCurrent
ElectrodeElectrodeC1C1P1P1
P2P2 Auxiliary Voltage ElectrodeAuxiliary Voltage ElectrodeGround Ground RodRod
EarthEarth
C2C2P1P1
11 ftft 110%0%220%0% 330%0% 440%0% 550%0%660%0%770%0%880%0% 990%0%RodRodUnder Under
TestTest
11 ftft 110%0%220%0% 330%0% 440%0% 550%0%660%0%770%0%880%0% 990%0%
1010 x depth of electrodex depth of electrode
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Fall of Potential
– Why 10+ Samples?– Single Point Could Be Misinterpreted– Data Must Be Plotted
– Visual Plateau– Confirms Test Validity
ohm
s)
125135145
y
ista
nce
(o
758595
105115125
Res
556575
0 10 20 30 40 50 60 70 80 90 100 P2 Probe Spacing
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Fall Of Potential Test
Why Invalid?– #1 Reason
l d
Why Invalid?
– Not Isolating System Under Test– Meter is a constant amperage meter– Part of the current travels through the connection – The ground system is part of a parallel network
– Test Is Invalid Unless Disconnected
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Fall Of Potential Test
Why Invalid?Why Invalid?
– #2 Reason– Insufficient Probe Spacing
– Req’d to Avoid the Spheres of Influenceq p
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Fall of Potential Test
S i C b ?– Spacing For Current Probe? – Single ElectrodeS g e ect ode
– Minimum 5X Length of Rod– Ideal, 10X Length of Rod
– 10 Foot Rod, 50-100 Feet Away , y– 200 Foot Well, 1000-2000 Feet Away
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Ground Resistance Testing
Clamp-on Resistance Testing Clamp on Ground Resistance MeterClamp-on Ground Resistance Meter– Convenient, Quick, Easy
i i i– Does Not Require Disconnecting Equipment
– Measures Current on the Ground
May Read Ground Loops vs. Ground ResistanceMay Read Ground Loops vs. Ground Resistancey py p
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Clamp -on Meter Operation
2 Control Xformers
Current Flow
R = E / I
Current Flow??? ohms
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ClampClamp--On Resistance Testing ExampleOn Resistance Testing Example3 Phase Utility 3 Phase Utility LineLine
Service MeterService Meter
Induced Current FlowInduced Current FlowInduced Current FlowInduced Current Flow
Ground ConductorGround Conductor
Ground RodsGround RodsCurrent From Other GroundsCurrent From Other Grounds C t F Oth G dC t F Oth G dGround RodsGround RodsCurrent From Other GroundsCurrent From Other Grounds Current From Other GroundsCurrent From Other Grounds
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Invalid Clamp-on Reading
Reading: < 1 Ohm
Current Path
gInvalid
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THANK YOU FOR YOUR ATTENTION Please enjoy the rest of your time at thePlease enjoy the rest of your time at the
2013 BICSI Fall Conference & ExhibitionRoy “Chip” Whitten
Senior Applications Engineer/Education Specialistpp g / pLyncole Grounding Solutions, LLC
800-962-2610