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Soil Resistivity and Grounding System Testing · PDF fileSoil Resistivity and Grounding System...
Transcript of Soil Resistivity and Grounding System Testing · PDF fileSoil Resistivity and Grounding System...
Soil Resistivity andGrounding System Testing
Roy “Chip” WhittenSenior Applications Engineer/Education SpecialistLyncole Grounding Solutions, LLC
What Is Grounding ?
An 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 earth.
Reasons For Grounding
– Personnel safety and equipment protection by providing a path to safely dissipate any unwanted charges or potentials.
– Ensure equipment performance and protection
– Satisfy manufacturer’s warranty
Electrical Protection Pyramid®
RFAC Surge
Telco / DataLightning
Grounding
Resistance To Earth
20v 12v 10.6v11v16v 10.18v10.5v 10.45v
25 Ohms
0
Grounding System Components
Equipment
SoilResistivity
Basics
Soil Resistivity
Resistance of earth to current flow, measured in Ohms-Meter, between opposite faces of a cube of earth, one cubic meter in volume
Earth
1 Cubic Meter
Signal Generator
~
Soil Resistivity
Key Variable in System Design
– Determines grounding system resistance– Changes from Site to Site– Dependent on:
– Soil type– Moisture– Electrolytes– Temperature
Soil Resistivity Comparison
Surface Soils 100 - 5,000Clay 200 - 10,000Sand and Gravel 5,000 - 100,000Surface Limestone 10,000 - 1,000,000Limestone 500 - 400,000Shale 500 - 10,000Sandstone 2,000 - 200,000Granites, Basalts, etc 100,000Decomposed Gneisses 5,000 - 50,000Slates, etc 1,000 - 10,000
Soil Type Resistivity (ohm-cm)
Effect of Moisture on Resistivity
020406080
100120140160180
0 5 10 15 20 25 30Moisture Content (% by weight)
Res
istiv
ity (o
hm-c
m)
x Th
ousa
nd
Effect of Electrolytes (Salt)on Resistivity
Effect of Temperature on Resistivity
0
50100150200250300350
68 50 40 32 14Temperature F
32
25
Res
istiv
ity (o
hm-c
m)
x Th
ousa
nd
Water Ice23
SoilResistivity
Testing
4-Pt. WENNER METHOD
Soil Resistivity Testing
– Key Variable in System Design– 4 Point Wenner Method
– Most Accurate– Multiple Depth Testing
4-Pt. Wenner Method
– Visually Survey Lease Area to Determine Location and Direction For Test– Not parallel to buried metallic objects– Not parallel overhead power lines– Sufficient straight line distance to allow for test
– Minimal distance 300 feet
4-Pt. Wenner Method
15 ft 10 ft 5 ft 0 ft
C1P1P2C2
A
4-Pt. Wenner Method
Site Area
Direction of Test
Example’s of 4-Point Testing
4-Pt. Wenner Method
Drive test probes into soil0 ft; 5 ft; 10 ft; 15 ft0 ft; 10 ft; 20 ft; 40 ft0 ft; 20 ft; 40 ft; 60 ft
Connect wire leads from meter to probesC1 to 0 ftP1 to 5 ftP2 to 10 ftC2 to 15 ft
C1 to 0 ftP1 to 10 ftP2 to 20 ftC2 to 30 ft
C1 to 0 ftP1 to 20 ftP2 to 40 ftC2 to 60 ft
Soil Resistivity Testing4 Point Data Interpretation
ρ π=
++
−+
41 2
42 2 2 2
ARA
A BA
A B( ) ( )ρ = Resistivity A = Spacing of probesB = Depth of probes R = Resistance
(reading from meter)If A>20B, then ρ = 2π AR (ohm-ft)
= 1.915 AR (ohm-m)
(2π / 3.28 = 1.915 )
4-Pt. Wenner Method
Probe Spacing Meter Reading Calculated Resistivity(Feet) (Ohms) (Ohm-Meter)
5 52.00 497.90
10 19.68 370.87
15 10.16 292.00
20 6.53 250.10
30 4.30 247.04
40 10.80 827.28
60 7.40 850.26
80 5.58 855.60
100 4.44 850.26
ρ
= 1.915 AR
Typical Grounding Electrode System
Resistance Requirements
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 3 OHMS
– Essilor 3 OHMS
– GE Medical Systems 2 OHMS
25Ω Grounding System
Lightning
Strike
18,000APotential Rise will be
~450KV at the site
5 Ohm Grounding System
Lightning
Strike
18,000APotential Rise will be
~90KV at the site
Grounding System Resistance Testing
Grounding System Testing
– Why Test Grounds?
– Confirm Design Spec Satisfied
– Satisfy Warranty Reqs
– Determine Baseline
– Ensure Equip Protection & Performance
– Validate Construction
– Two Test Methods
− Fall of Potential (Three Point) Test
− Clamp-on Test
Testing Methods
Required Equipment3 / 4 Pole Tester
Test KitProbesConductorTape MeasureHammer
Ground System Testing
Fall-of-Potential Method
P2 Auxiliary Voltage ElectrodeGround Rod
Under Test
Earth
C2
RemoteCurrent
ElectrodeC1P1
1 ft 10%20% 30% 40% 50%60%70%80% 90%
10 x depth of electrode
P2 Distance P2 Distance Resistance Amt. of Change( %) ( feet ) ( Ohms ) (Ohms)
1 1 ft 67.0 -10 % 10 ft 114.5 47.520 % 20 ft 119.6 5.130 % 30 ft 121.3 1.740 % 40 ft 122.2 0.950 % 50 ft 122.5 0.360 % 60 ft 123.1 0.6 70 % 70 ft 123.7 0.680 % 80 ft 126.6 2.990 % 90 ft 141.8 15.2
100 % 100 ft C2 -
Fall-of-Potential Test Results
Res
ista
nce
(ohm
s)
5565758595
105115125135145
0 10 20 30 40 50 60 70 80 90 100 P2 Probe Spacing
Fall of Potential
– Why 10+ Samples?– Single Point Could Be Misinterpreted– Data Must Be Plotted
– Visual Plateau– Confirms Test Validity
– #1 Reason– 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
Fall Of Potential Test
Why Invalid?
Ground Resistance Testing Data Plot
Invalid Test Graph
Ohms
Distance (Ground-Probe)
Test With Neutral Connected
Plot For 5 Ohm Ground
Plot For 25 Ohm Ground
Plot For 100 Ohm Ground
Plot For 1000 Ohm Ground
5
15
10
20
Fall Of Potential Test
Why Invalid?
– #2 Reason– Insufficient Probe Spacing
– Req’d to Avoid the Spheres of Influence
Spheres of Influence
Readings Not Impacted by Spheres of Influence
2025
15100500
Plateau System Resistance
Fall of Potential Test
CurrentProbe
50-100ft Spacing
Fall of Potential Test
– Spacing For Current Probe? – Single Electrode
– Minimum 5X Length of Rod– Ideal, 10X Length of Rod
– 10 Foot Rod, 50-100 Feet Away – 200 Foot Well, 1000-2000 Feet Away
Ground Resistance Testing
Clamp-on Resistance Testing Clamp-on Ground Resistance Meter– Convenient, Quick, Easy– Does Not Require Disconnecting
Equipment– Measures Current on the Ground
May Read Ground Loops vs. Ground Resistance
Clamp -on Meter Operation
Current Flow
Current Flow
R = E / I??? ohms
2 Control Xformers
Service Meter
Ground Conductor
Ground Rods
3 Phase Utility Line
Induced Current Flow
Current From Other Grounds Current From Other Grounds
Clamp-On Resistance Testing Example
Clamp-On Resistance Testing Example
Rx R2R1 R3 Rn-1 Rn
VI
Rx R1
Rn-1 Rn
Neutral-GroundBond
Grounding Conductor
Clamp-On Meter Operation
???Current Flow
Utility Neutral Line
Series Loop
Parallel Paths
Current Path
Reading: < 1 OhmInvalid
Invalid Clamp-on Reading
- 48vdcSwitchGear
Use Extreme Caution
Qualified Electricians Only
Clamp-On Use
Service Entrance
THANK YOU FOR YOUR ATTENTION Please enjoy the rest of your time at the
2015 BICSI Fall Conference & ExhibitionRoy “Chip” Whitten
Senior Applications Engineer/Education SpecialistLyncole Grounding Solutions, LLC
800-962-2610