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
6IEEE is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.
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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.
7IEEE is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.
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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."
8IEEE is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.
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
9IEEE is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.
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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Wenner’s or Four Pin Method
20I
VR
aR
la
a
la
aaR 2
4
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4
2222
Wenner’s or Four Pin Method
• 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
Wenner’s or Four Pin Method
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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
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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Lowering Ground Resistance
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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Lowering Ground Resistance
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
Lowering Ground Resistance
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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
Lowering Ground Resistance
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Concentric Shell Overlap Decreases
Efficiency of Ground Rod Resistance
Lowering Ground Resistance
• Add More Rods• Deep Driven Rods
– Threaded Couplings– Compression Couplings– Exothermic Ground Rod
Splice
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Lowering Ground Resistance
• 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
IEC is a registered trademark of The International Electrotechnical Commission
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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 is a registered trademark of The International Electrotechnical Commission
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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