Soil Resistivity and Grounding System Testing Resistivity and Grounding System Testing ... – Visually Survey Lease Area to Determine Location ... Drive test probes into soil 0 ft; 5 ft; ...

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  • 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 manufacturers 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

  • Examples 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

    =+

    +

    +

    41 2

    42 2 2 2

    ARA

    A BA

    A B( ) ( ) = Resistivity A = Spacing of probesB = Depth of probes R = Resistance

    (reading from meter)If A>20B, then = 2 AR (ohm-ft)

    = 1.915 AR (ohm-m)(2 / 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

    = 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

  • 25 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

    Reqd 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

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