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Getting Down
to Earth
Commonwealth Ave - Woburn MA 01801
A practical
guide to
earth resistance
testing
Getting Down to Earth
Introduction
Nothing is quite so common or abundantly available throughout the
world as the earth’s soil. While you likely think of soil as something to
be tilled for planting or to be excavated for a building foundation, it also
has an electrical property—conductivity (or low resistance)—that is put to
practical use every day in industrial plants and utilities.
Broadly speaking, “earth resistance” is the resistance of soil to the
passage of electric current. In reality, the earth is a relatively poor
conductor of electricity compared to normal conductors like copper wire.
However, if the area of a path for current is large enough, resistance can
be quite low and the earth can be a good conductor. It is the earth’s
abundance and availability that make it an indispensible component of a
properly functioning electrical system.
Earth resistance is measured in two ways for two important fields of use:
1. Determining effectiveness of “ground” grids and connections that are
used with electrical systems to protect personnel and equipment.
2. Prospecting for good (low resistance) “ground” locations, or obtaining
measured resistance values that can give specific information about what
lies some distance below the earth’s surface (such as depth to bed rock).
It is not the intent of this manual to go too deeply into the theory and
mathematics of the subject. If you’re interested in learning more, there is
a list of Additional Resources—found at the back of this booklet—which
cover these in extensive detail. Rather,“Getting Down to Earth” is written
in a simple and easy-to-understand format for all industry users.
Testing covered in this manual can be carried out on large, complex earth
systems, including communications earth systems and other difficult
test environments. Testing is carried out in accordance with BS 7430
(Earthing), BS-EN-62305 (Lightning Protection) and IEEE Standard 81.
From years of experience—supplying instruments for the tests involved—
Megger can provide advice to help you perform specific tests. Upon
request, we would also be happy to have a representative call you to
discuss your specific application question(s).
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TABLE OF CONTENTS
Introduction ........................................................................................................ 1
Safety .................................................................................................................. 5
SECTION I
Earth Resistivity .................................................................................................. 6
How Earth Resistivity is Measured............................................................... 6
Practical Example of Test Method ............................................................... 8
Type of Soil Affects Resistivity ..................................................................... 9
Resistivity Decreases with Moisture and Dissolved Salts .......................... 10
Effect of Temperature on Earth Resistivity ............................................... 12
Seasonal Variations in Earth Resistivity..................................................... 12
Determining a Good Electrode Location .................................................. 14
SECTION II
Measuring Earth Resistance for Electrical Grounding Systems ..................... 16
Factors That Can Change Your Minimum Earth Resistance ..................... 17
Some Basic Definitions ............................................................................... 17
Factors Influencing Requirements for a Good Grounding System .......... 19
National Electrical Code Maximum Values ............................................... 21
Nature of an Earth Electrode..................................................................... 21
Principles Involved in Earth Resistance Testing ....................................... 23
Basic Test Methods for Earth Resistance ................................................... 26
Effects of Different Reference Probe Locations ....................................... 30
Bonding and Continuity............................................................................. 35
Test Leads.................................................................................................... 36
Lazy Spikes .................................................................................................. 37
Supplementary Tests .................................................................................. 38
How to Improve Earth Resistance ............................................................. 39
Additional Test Methods:
Clamp-On Method .................................................................................... 44
Advantages of Stakeless Testing ........................................................ 47
Limitations of Stakeless Testing ......................................................... 48
Cell Tower Ground Testing ................................................................. 49
Attached Rod Technique (ART) ................................................................ 50
4
Star Delta Method ..................................................................................... 54
Determining Touch and Step Potential ................................................... 57
SECTION III
Accurately Measuring Earth Resistance for Large Ground Systems .............. 59
Testing Challenges in Large Ground Systems ........................................... 59
Addressing the Testing Challenges in Large Ground Systems ................. 60
Measurement of the Resistance of Large Earth-Electrode Systems:
Intersecting Curves Method ....................................................................... 62
Test at a Large Substation .......................................................................... 63
General Comments ...................................................................................... 64
Slope Method .............................................................................................. 66
Four Potential Method ............................................................................... 69
APPENDIX I
Nomograph Guide to Getting Acceptable Earth Resistance ......................... 72
APPENDIX II
Ground Testing Methods Chart ....................................................................... 74
GROUND TESTERS AVAILABLE FROM MEGGER ............................................. 76
GROUND TESTING ACCESSORIES AVAILABLE FROM MEGGER ..................... 78
REFERENCES ...................................................................................................... 79
ADDITIONAL RESOURCES ................................................................................ 80
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Getting Down to Earth
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Safety
There is an inherent safety problem in earth resistance testing that
requires care and planning by the user of the test set.
The possibility exists that a fault in the power system will cause a high
current to flow into the ground system while the test is in progress. This
may cause unexpected high voltages to appear at the current and voltage
probes and also at the terminals of the test set.
This risk must be evaluated by the person responsible for the tests,
taking into account the fault current available and expected step-andtouch potentials. IEEE Standard 80 entitled “IEEE Guide for Safety in AC
Substation Grounding” fully covers this subject. (Other standards may
prevail elsewhere in the world.)
We recommend that the operator wear rubber protective gloves (ANSI/
ASTM D120 or equal) while handling connections and use a rubber safety
mat (ANSI/ASTM D178 or equal) while operating the test set.
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SECTION I
Earth Resistivity
The term “earth resistivity” expressed in ohm-centimeters (abbreviated
ohm-cm) is one basic variable affecting resistance to earth of an electrode
system. Fortunately, the actual value of earth resistivity does not need to
be measured in order to check the electrode earth resistance. Let’s first
consider other fields where the value of resistivity is measured, as well as
some of the factors affecting it that are of interest in earth testing.
Earth resistivity measurements can be used conveniently for geophysical
prospecting — to locate ore bodies, clays, and water-bearing gravel
beneath the earth’s surface. The measurement can also be used to
determine depth to bed rock and thickness of glacial drift.
Measurements of earth resistivity are also useful for finding the best
location and depth for low resistance electrodes, as well. Such studies are
made, for example, when a new electrical unit is being constructed, such
as: a generating station, substation, transmission tower, or telephone
central office.
Finally, earth resistivity may be used to indicate the degree of corrosion
to be expected in underground pipelines for water, oil, gas, gasoline,
etc. In general, spots where the resistivity values are low tend to increase
corrosion. This same kind of information is a good guide for installing
cathodic protection.
How Earth Resistivity is Measured
To measure earth resistivity, a four-terminal instrument is used, along with
four small-sized electrodes - driven down to the same depth and equal
distances apart in a straight line (Fig. 1). Four separate lead wires connect
the electrodes to the four terminals on the instrument, as shown. Hence,
the name of this test: the four-terminal method.
8
Fig. 1: Four-terminal method of measuring earth resistivity
Dr. Frank Wenner of the U.S. Bureau of Standards (now NIST) developed
the theory behind this test in 1915 [1]. He showed that, if the electrode
depth (B) is kept small compared to the distance between the electrodes
(A)1, the following formula applies:
ρ = 2π AR
where ρ is the average soil resistivity to depth A in ohm-cm, π is the
constant 3.1416, A is the distance between the electrodes in cm, and R is
the Megger earth tester reading in ohms.
In other words, if the distance A between the electrodes is 4 ft, you obtain
the average earth resistivity to a depth of 4 ft as follows:
1. Convert the 4 ft to centimeters to obtain A in the formula:
4 x 12 x 2.54 cm = 122 cm
2. Multiply 2 π A to obtain a constant for a given test setup:
2 x 3.14 x 122 = 766
Now, for example, if your instrument reading is 60 Ω, the earth resistivity
would be 60 x 766, or 45,960 ohm-cm.
There are other methods for measuring soil resistivity such as the
Schlumberger method. However, the Wenner method is the most popular
in the electric power industry.
1
B = 1/20A is generally recommended
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Practical Example of Test Method
A petroleum company had a 10-in. pipeline 6300 ft long running through
rugged terrain [2]. After a corrosion leak, they wanted to check out earth
resistivity along the line. Low-resistance spots would most likely require
attention. They used a Megger earth tester to make a survey along the
line.
First, the average depth of the pipeline was found from a profile map.
It was 4 ft, so four electrodes were tied together 4 ft apart with strong
cotton cord. They decided to check soil resistivity every 20 ft along the
line. Fig. 2 shows a portion of the results; pit depth corrosion and Megger
earth tester readings are plotted for points along the pipeline. Note that
for low resistance readings, more corrosion was found.
Fig. 2: Earth resistivity survey of pipeline shows where corrosion is most
likely to occur [2].
10
Type of Soil Affects Resistivity
Whether a soil is largely clay or very sandy, for example, can change the
earth resistivity a great deal. It isn’t always easy to define exactly what’s in
a given soil; “clay” can cover a wide variety of soils. Therefore, we cannot
say that any given soil has a resistivity of so many ohm-cm. Tables I and II
are taken from two different reference books and show the wide range in
values. Note also the spread of values for the same general types of soil.
See Fig. 3 also.
48 OHMS
REDUCTION
Fig. 3: Deeper earth electrodes lower the resistance. These graphs show the relation
between character of soil and resistance of driven electrode at increased depths.
Table I: Resistivities of Different Soils
2
Resistivity (Ohm-cm)
Soil Avg Min Max
Fills: ashes, cinders, brine wastes 2,370 590 7,000
Clay: shale, gumbo, loam 4,060 340 16,300
Same: varying proportions of sand/gravel 15,800 1,020 135,000
Gravel, sand, stones with little clay/loam 94,000 59,000 458,000
2
US Bureau of Standards Report 108
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Resistivity Decreases with Moisture and Dissolved Salts
In soil, conduction of current is largely electrolytic. Therefore, the amount
of moisture and salt content of soil radically affects its resistivity. The
amount of water in the soil varies, of course, with the weather, time of
year, nature of sub-soil, and depth of the permanent water table. Table IV
shows typical effects of water in soil; note that when dry, the two types
of soil are good insulators (resistivities greater than 1000 x 106 ohmcm). With a moisture content of 15 percent, however, note the dramatic
decrease in resistivity (by a factor of 100,000). Actually, pure water has
an infinitely high resistivity. Naturally occurring salts in the earth, dissolved
in water, lower the resistivity. Only a small amount of salt3 can reduce
earth resistivity quite a bit. (See Table IV.) As noted in Section I, this effect
can be useful to provide a good low-resistance electrode, in place of an
expensive, elaborate electrode system.
Table II: Resistivities of Different Soils
Resistivity
Soil Ohm-cm (Range)
Surface soils, loam, etc. 100 - 5,000
Clay 200 - 10,000
Sand and gravel 5,000 - 100,000
Surface limestone 10,000 - 1,000,000
Shales 500 - 10,000
Sandstone 2,000 - 200,000
Granites, basalts, etc. 100,000
Decomposed gneisses 5,000 - 50,000
Slates, etc. 1,000 - 10,000
3
By “salt” we don’t mean the kind used to season food (sodium chloride), though this kind
can occur in soil. Other kinds include copper sulphate, sodium carbonate, and others (see
“Treatment of Soil,” Section II, page 40 ).
4
Evershed & Vignoles Bulletin 245
12
4
Table III: Effect of Moisture Content on Earth Resistivity
5
Moisture Content, Resistivity (Ohm-cm)
Percent by Weight Top Soil Sandy Loam
0.0 1,000 x 10
6
1,000 x 10
2.5 250,000 150,000
5.0 165,000 43,000
10.0 53,000 22,000
15.0 21,000 13,000
20.0 12,000 10,000
30.0 10,000 8,000
6
Table IV: Effects of Salt Content on Earth Resistivity
6
Added Salt
Percent by Weight of Moisture Resistivity (Ohm-cm)
0.0 10,700
0.1 1,800
1.0 460
5.0 190
10.0 130
20.0 100
5
From “An Investigation of Earthing Resistance” by P.J. Higgs, I.E.E. Journal, vol. 68, p.
736, February 1930
6
For sandy loam; moisture content, 15% by weight; temperature 63º F (17º C)
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Effect of Temperature on Earth Resistivity
Not much information has been collected on the effects of temperature.
Two facts lead to the logical conclusion that an increase in temperature
will decrease resistivity: (1) water present in soil mostly determines the
resistivity, and (2) an increase in temperature markedly decreases the
resistivity of water. The results shown in Table V confirm this. Note that
when water in the soil freezes, the resistivity jumps appreciably; ice has
a high resistivity. The resistivity continues to increase as temperatures go
below freezing.
Table V: Effect of Temperature on Earth Resistivity
Temperature
C F Resistivity (Ohm-cm)
20 68 7,200
10 50 9,900
0 32 (water) 13,800
0 32 (ice) 30,000
-5 23 79,000
-15 14 330,000
Seasonal Variations in Earth Resistivity
7
We have seen the effects of temperature, moisture, and salt content
upon earth resistivity. It makes sense, therefore, that the resistivity of soil
will vary considerably at different times of year. This is particularly true
in locations where there are more extremes of temperature, rainfall, dry
spells, and other seasonal variations.
From the preceding discussion, you can see that earth resistivity is a
very variable quantity. If you want to know what the value is at a given
location, at a given time of year, the only safe way is to measure it. When
you use this value for survey work, the change in the value, caused by
changes in the nature of the sub-soil, is the important thing; from the
variations in resistivity you can obtain useful survey results.
7
For sandy loam, 15.2% moisture
14
As will be covered in Section II, the other main reason for measuring earth
resistivity is to design earth-electrode systems for electrical power systems,
lightning arresters, and so on. The measured resistivity values are used
in standard engineering formulas that calculate factors like number and
depth of rods necessary to achieve a required ground resistance, thus
reducing the amount of trial and error in the installation of an effective
ground. Earth resistance varies directly with earth resistivity and it is
helpful to know what factors affect resistivity.
The curves of Fig. 4 illustrate several worthwhile points. They show the
expected change in earth resistance (due to resistivity changes) over a
1-1/2 year period; they also show that the deeper electrode gives a more
stable and lower value. We conclude that the moisture content and
temperature of the soil become more stable at greater distances below
the earth’s surface. Therefore, the earth electrode should reach a deep
enough level to provide:
n Permanent moisture content (relatively speaking).
n Constant temperature (below frost line; again, relatively speaking).
Fig. 4: Seasonal variation of earth resistance with an electrode of 3/4” pipe in stony
clay soil. Depth of electrode in earth is 3 ft for Curve 1 and 10 ft for Curve 2 [3].
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Determining a Good Electrode Location
A good, low-resistance earth electrode depends upon a low-resistivity
soil in a spot where you can drive in your electrodes. There are two
approaches to picking your location:
1. Drive rods in various locations to such depths as may be required and
test their resistances while they are being driven.
2. Measure the earth resistivity before driving ground rods. Then calculate
the number and length of rods required.
To get a low-resistance electrode in an unfavorable location, lay out
straight lines 10 ft apart, covering the area. Drive four stakes 10 ft apart,
but not more than 6 in. deep, along a line a-b-d-c, as shown in Fig. 5.
Measure the resistance R between stakes b and c, using the method
described for earth resistivity. Then, shift the stakes along the line in
question to points b-c-d-e, c-d-e-f, etc. (see Fig. 5) and test until the
entire line has been covered. Next, move to the next line and repeat
Fig. 5: Method of prospecting for best earth electrode location to a depth a.
Location giving lowest reading on the Megger ground tester is the most desirable.
16
the process until the whole chosen area has been covered. The location
giving the lowest value for R has the lowest specific resistance for the soil
to the chosen depth of 10 ft. The spot is likely to give you the best earth
electrode.
If you want results affected by the average earth resistivity to a depth of
20 ft, repeat the survey on lines 20 ft apart and with stakes spaced 20 ft
apart. Such surveys do not require much time and can pay off in ensuring
a good grounding system.
Alternate Method: Another way is to drive rods or pipes in various
locations to such depths as may prove practicable, testing their resistance
while they are being driven. In this manner, you can usually tell at once
when moisture or other good conducting earth is reached. However, the
work involved is apt to be much more than with the first method.
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SECTION II
Measuring Earth Resistance
for Electrical Grounding Systems
The simplest and somewhat misleading idea of a good ground for an
electrical system is a section of iron pipe driven into the earth with a wire
conductor connected from the pipe to the electrical circuit (Fig. 6). This
may, or may not, be a suitable low resistance path for electric current to
protect personnel and equipment.
Fig. 6: A simplified grounding system in an industrial plant
A practical earth electrode that provides a low ground resistance is not
always easy to obtain. But from experience gained by others you can learn
how to set up a reliable system and how to check the resistance value
with reasonable accuracy. As you will see, earth resistivity (refer to Section
I) has an important bearing on electrode resistance, as does the depth,
size and shape of the electrode.
The principles and methods of earth resistance testing covered in this
section apply to lightning arrester installations as well as to other systems
that require low resistance ground connections. Such tests are made in
power-generating stations, electrical-distribution systems, industrial plants,
and telecommunication systems.
18
Factors That Can Change Your Minimum Earth Resistance
We will discuss later what value of earth resistance is considered low
enough. You’ll see that there is no general rule usable for all cases. First,
however, consider three factors that can change the earth electrode
requirements from year to year:
n A plant or other electrical facility can expand in size. Also, new plants
continue to be built larger and larger. Such changes create different
needs in the earth electrode. What was formerly a suitably low earth
resistance can become an obsolete “standard.”
n As facilities add more modern sensitive computer-controlled equipment,
the problem of electrical noise is magnified. Noise that would not affect
cruder, older equipment can cause daily problems with new equipment.
n As more nonmetallic pipes and conduits are installed underground,
such installations become less and less dependable as effective, lowresistance ground connections.
n In many locations, the water table is gradually falling. In a year or so,
earth electrode systems that formerly were effective may end up in dry
earth of high resistance.
These factors emphasize the importance of a continuous, periodic
program of earth-resistance testing. It is not enough to check the earth
resistance only at the time of installation.
Some Basic Definitions
First, let’s define our terms. As early as 1918, the terms ground,
permanent ground, and ground connections were defined to mean
“electrical connections intentionally made between electrical bodies (or
conducting bodies in close proximity to electrical circuits) and metallic
bodies in the earth — such as rods, water pipes, plates, or driven pipes
[4].”
The metallic body in the earth is often referred to as an electrode
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even though it may be a water-pipe system, buried strips or plates, or
wires. Such combinations of metallic bodies are called a grid. The earth
resistance we’re concerned with is the resistance to current from the
electrode into the surrounding earth.
To appreciate why earth resistance must be low, you need only use
Ohm’s Law: E = R x I where E is volts; R, the resistance in ohms; and I,
the current in amperes. Assume that you have a 4000-V supply (2300 V
to ground) with a resistance of 13 Ω (see Fig. 7). Now, assume that an
exposed wire in this system touches a motor frame that is connected to a
grounding system which has a 10-ohm resistance to earth.
By Ohm’s Law, there will be a current of 100 A8 through the fault (from
the motor frame to the earth). If you happen to touch the motor frame
and are grounded solidly to earth, (by standing in a puddle) you
could be subjected to 1000 V (10 Ω x 100 A).
As you’ll note from point 2 in the following, this may be more than
enough to kill you instantly. If, however, the earth resistance is less than
1 Ω, the shock you’d get would be under 100 V (1 x 100) and you’d
probably live to correct the fault.
Equipment can also be damaged similarly by overvoltages caused by highresistance ground systems.
Fig. 7: Example of an electrical circuit with too high an earth resistance
8
I = E/R = 2,300/ (10 + 13) = 100 amperes
20
Factors Influencing Requirements for a Good Grounding
System
In an industrial plant or other facility that requires a grounding system,
one or more of the following must be carefully considered (see Fig. 8):
1. Limiting to definite values the voltage to earth of the entire electrical
system. Use of a suitable grounding system can do this by maintaining
some point in the circuit at earth potential. Such a grounding system
provides these advantages:
n Limits voltage to which the system-to-ground insulation is subjected,
thereby more definitely fixing the insulation rating.
n Limits the system-to-ground or system-to-frame voltage to values
safe for personnel.
n Provides a relatively stable system with a minimum of transient
overvoltages.
n Permits any system fault to ground to be quickly isolated.
2. Proper grounding of metallic enclosures and support structures that
are part of the electrical system and may be contacted by personnel.
Fig. 8: Typical conditions to be considered in a plant ground system
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Also to be included are portable electrically operated devices. Consider
that only a small amount of electric current — as little as 0.1 A for one
second — can be fatal! An even smaller amount can cause you to lose
muscular control. These low currents can occur in your body at voltages
as low as 100 V, if your skin is moist.
3. Protection against static electricity from friction. Along with this are
the attendant hazards of shock, fire and explosion. Moving objects
that may be inherent insulators - such as paper, textiles, conveyor belts
or power belts and rubberized fabrics - can develop surprisingly high
charges unless properly grounded.
4. Protection against direct lightning strokes. Elevated structures, such as
stacks, the building proper, and water tanks may require lightning rods
connected into the grounding system.
5. Protection against induced lightning voltages. This is particularly a factor
if aerial power distribution and communications circuits are involved.
Lightning arresters may be required in strategic locations throughout
the plant.
6. Providing good grounds for electric process control and communication
circuits. With the increased use of industrial control instruments,
computers, and communications equipment, accessibility of lowresistance ground connections in many plant locations — in office and
production areas — must be considered.
22
National Electrical Code Maximum Values
The National Electrical Code, Section 250-56 states that a single electrode
with a resistance to ground greater than 25 Ω shall be augmented by one
additional electrode. (Other standards may prevail elsewhere in the world.)
We recommend that single-electrode grounds be tested when
installed and periodically afterward.
Resistance to earth can vary with changes in climate and temperature.
Such changes can be considerable. An earth electrode that was good
(low-resistance) when installed may not stay that way; to be sure, you
must check it periodically.
We cannot tell you what your maximum earth resistance should be. For
specific systems in definite locations, specifications are often set. Some
call for 5 Ω maximum; others accept no more than 3 Ω. In certain cases,
resistances as low as a small fraction of an ohm are required.
Nature of an Earth Electrode
Resistance to current through an earth electrode actually has three
components (Fig. 9):
1. Resistance of the electrode itself and connections to it.
2. Contact resistance between the electrode and the soil adjacent to it.
3. Resistance of the surrounding earth.
Electrode Resistance: Rods, pipes, masses of metal, structures, and other
devices are commonly used for earth connections. These are usually of
sufficient size or cross-section that their resistance is a negligible part of
the total resistance.
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Electrode-Earth Contact Resistance: This is much less than you might
think. If the electrode is free from paint or grease, and the earth is packed
firmly, contact resistance is negligible. Rust on an iron electrode has little
or no effect; the iron oxide is readily soaked with water and has less
resistance than most soils. But if an iron pipe has rusted through, the part
below the break is not effective as a part of the earth electrode.
Fig. 9: Components of earth resistances in an earth electrode
Resistance of Surrounding Earth: An electrode driven into earth of
uniform resistivity radiates current in all directions. Think of the electrode
as being surrounded by shells of earth, all of equal thickness (see Fig. 9).
The earth shell nearest the electrode naturally has the smallest surface
area and so offers the greatest resistance. The next earth shell is
somewhat larger in area and offers less resistance. Finally, a distance from
the electrode will be reached where inclusion of additional earth shells
does not add significantly to the resistance of the earth surrounding the
electrode. It is this critical volume of soil that determines the effectiveness
of the ground electrode and which therefore must be effectively
24
measured in order to make this determination. Ground testing is distinct
when compared to more familiar forms of electrical measurement, in
that it is a volumetric measurement and cannot be treated as a “point”
property.
Generally, the resistance of the surrounding earth will be the largest of
the three components making up the resistance of a ground connection.
The several factors that can affect this value are discussed in Section II on
Earth Resistivity. From Section II, you’ll see that earth resistivity depends
on the soil material, the moisture content, and the temperature. It is far
from a constant, predictable value ranging generally from 500 to 50,000
ohm-cm9.
Principles Involved in Earth Resistance Testing
The resistance to earth of any system of electrodes theoretically can be
calculated from formulas based upon the general resistance formula:
L
R = ρ
A
where ρ is the resistivity of the earth in ohm-cm, L is the length of the
conducting path, and A is the cross-sectional area of the path. Prof. H.
B. Dwight of Massachusetts Institute of Technology developed rather
complex formulas for the calculation of the resistance to earth for
any distance from various systems of electrodes [4]. All such formulas
can be simplified a little by basing them on the assumption that the
earth’s resistivity is uniform throughout the entire soil volume under
consideration.
Because the formulas are complicated, and earth resistivity is neither
uniform or constant, a simple and direct method of measuring earth
resistance is needed. This is where we come in with our Megger Ground
Resistance Tester, a self-contained portable instrument that is reliable and
easy to use. With it, you can check the resistance of your earth electrode
while it is being installed; and, by periodic tests, observe any changes with
time.
9
An ohm-centimeter (abbreviated ohm-cm) is defined as the resistance of a cube of material
(in this case, earth) with the cube sides being measured in centimeters.
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To understand the principle of earth testing, consider the schematic
diagram in Fig. 10a. Bear in mind our previous observation with reference
to the earth shell diagram in Fig. 9: with increased distance from an
electrode, the earth shells are of greater surface area and therefore of
lower resistance. Now, assume that you have three rods driven into the
earth some distance apart and a voltage applied, as shown in Fig. 10a.
The current between rods 1 and 2 is measured by an ammeter; the
potential difference (voltage) between rods 1 and 3 is measured by a
voltmeter.
(a)
ROD 3
(P)
ROD 2
(C)
Fig. 10: Principle of an earth resistance test
26
(b)
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