Ground Resistance & Measurement

Ground Resistance

and Measurement

Terry Klimchak

Revised 11/19/13

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Presentation Outline

• Applicable IEEE Standards• Soil Resistivity• Measuring Soil Resistivity• Measuring Ground Resistance• Lowering Ground Resistance

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IEEE Standard

• ANSI/IEEE Std 81-2012“IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System”

5IEEE is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.

IEEE Std 81-2012

• Scope– Establishing safe testing conditions– Measuring earth resistivity– Measuring the power system frequency resistance or impedance of

the ground system to remote earth– Measuring the transient (surge) impedance of the ground system

to remote earth– Measuring step and touch voltages– Verifying the integrity of the grounding system– Reviewing common methods and procedures for performing

ground testing– Reviewing instrumentation characteristics and limitations– Reviewing various factors that can distort test measurements


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IEEE Std 81-2012

• Purpose– The purpose of this guide is to present practical instrumentation

methods that may be used for measuring soil resistivity, the impedance to remote earth, step and touch voltages, and current distributions in ground grids associated with electric utility facilities.

– These grids typically consist of interconnected grounding systems ranging in complexity from a few ground rods to large grids with many ground rods or wells, buried conductors, and external ground connections. External ground connections may include overhead shield/ground/neutral wires, underground cable sheaths/neutrals, counterpoises, grid tie conductors, metallic pipes, and other connections that provide additional paths to remote earth.


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IEEE Std 81-2012 (Cont.)

• Purpose– This guide is intended to assist the engineer or technician in

obtaining and interpreting accurate, reliable data. The factors that influence the choice of instruments are discussed along with a presentation of field techniques for various types of measurements. These factors include the purpose of the measurement, the accuracy required, the types of instruments available, the possible sources of error, and the nature of the ground or grounding system under test. It also describes test procedures that promote the safety of personnel and property, and it seeks to minimize operating interferences with neighboring facilities."


IEEE Std 81- 2012

• Testing Methods Covered Include:– Measurement of Earth Resistivity

– Measurement of the Resistance and Impedance to Earth (From Small Rods and Plates to Large Substation Grounding Systems Stations)

– Measurement of Step and Touch Voltages and Potential Contour Surveys

– Integrity of Grounding Systems


Earth Resistivity

Soil Resistivity - Effect of Moisture

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Soil Resistivity - Effect of Temperature

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Resistance Variation With Seasons

Seasonal Variation of Earth Resistance With an Electrode of 3/4" Pipe in Stony Clay Soil

0

20

40

60

80

100

JAN

MAR

MAY

JUL

SEPNO

VJA

NM

ARM

AY

Re

sis

tam

ce

(O

hm

s)

Buried 3'

Buried 10'

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Measuring Soil Resistivity

Wenner’s or Four Pin Method

• Also Known as Equally Spaced Method• Most Widely Used Method • Small Test Probes Are Installed in Depth “L” and

Spaced in Intervals “A”• Test Current is Passed Between the Outer

Electrodes. The Voltage Drop Is Measured With a High Impedance Volt Meter Connected to the Inner Electrodes

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20I

VR

aR

la

a

la

aaR 2

4

21

4

2222


• Test Probes (Pins) Placed in a Straight Line at Intervals “a” Driven to a Depth “l” (“l” < 0.1”a”)

• Assume “l” = 0 and Formula is Simplified as Follows:

• The Formula Provides the Average Soil Resistivity (Apparent Resistivity) of the Soil to Depth “a”

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aR 2


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Measuring Ground

Resistance

Measuring Ground Resistance

• Purpose: – To Determine Actual Resistance Ground Connections– To Check Calculations– To Determine GPR That Results From Ground a Fault

Current in a Power System– To Determine the Suitability of a Grounding

Connection for Lightning Protection, – To Obtain Data Required for the Design of Protection

for Buildings, Equipment and Personnel

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Fall-of-Potential or 3-Point Method

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FALL OF POTENTIAL TEST

• Must have complete isolation from utility ground system

• Measure DIAGONAL length of ground system (10m x 10m grid would be 14m diagonal)

• Probe Z is driven at a distance of 10X diagonal

• Probe Y is driven at 1X Diagonal, then 2X,3X etc. up to 9x

• Results are plotted, FALL of resistance should be 62%

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Fall-of-Potential or 3-Point Method

Plot shows 62% of homogenous soil 32

Stakeless Measurements

• Simple and fast• No auxiliary rods or

spacing requirements• Direct reading of

continuity and ground loop resistance.

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Lowering Ground Resistance


• What Affects the Grounding Resistance?– Diameter of the Ground Electrode– Length / Depth of the Ground Electrode– Number of Ground Electrodes– Ground System Design / Electrode

Placement

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Resistance vs. Rod Depth

0%

20%

40%

60%

80%

100%

120%

0.50 0.63 0.75 0.88 1.00 1.13 1.25 1.38 1.50

Resistance (%)

Rod Diameter Has Minor Effect On Resistance

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Resistance vs. Rod Depth

0

100

200

300

400

500

600

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Rod Depth (ft)

Res

ista

nce

(o

hm

s)

1/2" Rod (ohms)

1" Rod (ohms)

Doubling Rod Depth Reduces Resistance 40% In Uniform Soil37


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l

rRadius (r ) in

MetersPercent of Resistance

0.03 25%

0.06 38%

0.09 46%

0.15 52%

0.3 68%

1.5 86%

3 94%

Effective Resistance Area


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Concentric Shell Overlap Decreases

Efficiency of Ground Rod Resistance


• Add More Rods• Deep Driven Rods

– Threaded Couplings– Compression Couplings– Exothermic Ground Rod

Splice

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• Ground Enhancement – Chemical Ground Rods– Bentonite Clay– Ground Enhancement Material

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Ground Enhancement

• Chemicals (Salts) and Chemical Ground Rods– Expensive– May Need to Be Periodically Recharged– Typically Need Bentonite– May Have Environmental Concerns Depending

on Materials Used

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Chemical Ground Rods

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Bentonite Clay

• Advantages– Effective When Wet– Low Initial Cost

• Disadvantage– May Shrink and Pull Away From Rod or Soil

When It Dries

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Ground Enhancement Materials

• Permanent, Stable, Low Resistivity Backfill Material Used to Improve Grounding Effectiveness Regardless of Soil Conditions

• Composed of Chemically Stable Materials

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New IEC 62561-7 Standard for Ground Enhancement Materials

• Leaching test requirements• Sulfur determination requirements• Resistivity determination results• Additional corrosion testing

46IEC is a registered trademark of The International Electrotechnical Commission

IEC 62561-7

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5.2 Leaching Test5.2.3 Passing Criteria:The criteria are given by national or international regulations.

(GEM25A was tested to Environmental Protection Agency)

5.3 Sulfur Determination5.3.2 Passing Criteria:The material is deemed to have passed the test if all measured

values are less than 2%. The recorded value resulting from this test shall be indicated within the product documentation.

IEC is a registered trademark of The International Electrotechnical Commission

IEC 62561-7

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5.4 Determination of Resistivity5.4.4 Passing Criteria:Specimens are deemed to have passed the test if the obtained resistivity value from the three samples are equal or less than the resistivity value claimed by the manufacturer


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IEC 62561-7

5.5.5 Passing Criteria:• For copper-plated earth electrodes, the polarization resistance shall be: > 4 x m2 for non-aggressive environments and > 8 x m2 for aggressive environments

• For galvanized earth electrodes, the polarization resistance shall be: > 3 x m2 for non-aggressive environments and > 7.6 x m2 for aggressive environments


IEC 62561-7

5.6 Marking and IndicationEach package unit shall be marked in an indelible way1.The name of the manufacturer or his trademark2.The type or serial number of the batch of ground enhancing compound3.The installation instructions4.The resistivity value and test apparatus used5.The conformity statement to the present standard

50IEC is a registered trademark of The International Electrotechnical Commission

Ground Enhancement Material Specification

Ground enhancement material must be compliant to IEC 62561-7, must be permanent and maintenance-free (no recharging with salts or chemicals, which may be corrosive) and maintain its earth resistance with time. It must set up firmly and not dissolve or decompose, or otherwise pollute the soil or the local water table. It should not depend on the continuous presence of water to maintain its conductivity, and in its set form will have a resistivity of not more than 2 ohm-cm.

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Ground Resistance & Measurement

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