UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Electrification Administration

**REA BULLETIN 1751F-802**

**SUBJECT: [Electrical Protection Grounding Fundamentals]{.underline}**

**TO:** All Telephone Borrowers

REA Telephone Staff

**EFFECTIVE DATE:** Date of Approval

**EXPIRATION DATE:** Seven years from effective date.

**OFFICE OF PRIMARY INTEREST:** Engineering Standards Branch,
Telecommunications Standards Division

**PREVIOUS INSTRUCTIONS:** This bulletin replaces Telecommunications
Engineering and Construction Manual (TE&CM) Section 802.

**FILING INSTRUCTIONS:** Discard TE&CM Section 802 and replace with this
bulletin. File this bulletin with 7 CFR 1751 and reanet.

**PURPOSE:** To provide technical information for use in the design,
construction and operation of REA borrowers\' telephone systems. The
basic factors affecting earth resistivity and grounding are discussed.
Information is also provided on the selection of an appropriate location
for the installation of electrodes. Further, techniques are presented
for measuring soil resistivity and resistance to ground of an electrode.

[Wally Beyer]{.underline} [4/12/94]{.underline}

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Administrator Date

**TABLE OF CONTENTS**

.Begin Table C.

1\. GENERAL 7

1.1 \"Ground\" 7

1.2 Using Grounds in Protection Applications 7

1.3 Ground Resistance 7

2\. PHENOMENA AFFECTING GROUND RESISTANCE 7

2.1 Introduction 7

3\. CHARACTERISTICS OF VERTICAL ELECTRODES 9

3.1 Single Vertical Electrode Buried in Earth 9

3.2 Variation of Resistance With Depth 9

3.3 Variation of Resistance with Diameter 10

3.4 Electrode Material 10

3.5 Multiple Vertical Electrodes Buried in Earth 11

3.6 Multiple Vertical Electrodes Buried in a Straight Line 11

3.7 Multiple Vertical Electrodes Buried in a Ring 13

3.8 Multiple Vertical Electrodes Buried in a Rod Bed 13

4\. CHARACTERISTICS OF HORIZONTAL ELECTRODES 14

4.1 Horizontal Electrode Buried in Earth 14

4.2 Horizontal Electrode Buried in a Straight Line 15

4.3 Horizontal Electrode Buried in a Ring 15

4.4 Horizontal Electrodes Buried in a Radial Configuration 16

4.5 Horizontal Electrodes Buried in a Grid Configuration 17

5\. MUTUAL RESISTANCE BETWEEN VERTICAL AND HORIZONTAL ELECTRODES 17

5.1 Introduction 17

5.2 Mutual Resistance, Vertical Electrodes in a Straight Line 18

5.3 Mutual Resistance, Vertical Electrodes in a Ring 18

5.4 Mutual Resistance, Rod Bed of Vertical Electrodes 18

6\. COMBINED RESISTANCE OF VERTICAL AND HORIZONTAL ELECTRODES 18

6.1 Introduction 18

6.2 Combined Resistance of Vertical and Horizontal Electrodes in a
Straight Line 18

6.3 Combined Resistance of Vertical and Horizontal Electrodes in a Ring
18

6.4 Combined Resistance of Vertical and Horizontal Electrodes in a Grid
19

7\. DESIGN OF CENTRAL OFFICE GROUNDING SYSTEMS 19

7.1 Introduction 19

7.2 Site Survey 20

7.3 Design Procedure 21

8\. DESIGN OF ISOLATED GROUNDING SYSTEMS 25

8.1 Introduction 25

8.2 Site Survey 25

8.3 Design Procedure 26

9\. INSTALLATION PROCEDURES 29

9.1 Introduction 29

9.2 Wire 29

9.3 Grounding Well 30

9.4 Bonds 30

9.5 Grounding Electrodes 30

9.6 Driving Electrodes 30

9.7 Measurements of the Grounding System Resistance-to-Ground 30

9.8 Remeasuring the Grounding System 31

10\. REFERENCES 31

10.1 Publications 31

APPENDIX A

CONVERSION TABLES 33

APPENDIX B

GROUNDING EQUATIONS 35

APPENDIX C

MEASUREMENT OF SOIL RESISTIVITY 48

APPENDIX D

MEASUREMENT OF RESISTANCE-TO-GROUND 50

APPENDIX E

CONCRETE ENCASED ELECTRODES 55

.End Table C.

TABLE

Table 1: Resistivity of Various Soils 8

APPENDIX FIGURES

Figure B1: Value of Coefficient K1 39

Figure B2: Value of Coefficient K2 42

APPENDIX TABLES

Table A1: Commonly Used Measurements 33

Table A2: English to Metric Conversions 33

Table C1: Test Electrode Depth for Various Distances

between Electrodes 49

Table D1: Reference Electrode Location 53

FIGURES SECTION (NOT AVAILABLE ON REANET)

Figure 1: Components of Resistance in a Ground

Resistance 58

Figure 2: Estimated Average Earth Resistivity in U.S. 59

Figure 3: Typical Variation of Soil Resistivity with

Moisture 60

Figure 4: Typical Effect of Mineral Salt on Earth

Resistivity 61

Figure 5: Typical Variation of Soil Resistivity with

Temperature 62

Figure 6: Resistance-to-Ground Variation with Electrode

Depth 63

Figure 7: Resistance-to-Ground Variation in Multiple Soil

Layers 64

Figure 8: Resistance-to-Ground Variation with Electrode

Diameter 65

Figure 9: Percent Resistance Variation Multiple Electrodes

in a Straight Line Interconnected with Wire 66

Figure 10: Percent Resistance Variation for a Ring of

Multiple Electrodes Interconnected with Wire 67

Figure 11: Resistance-to-Ground for Multiple Electrodes in

a Rod Bed 68

Figure 12: Resistance-to-Ground Variation with Length of

Hortizontal Electrode 69

Figure 13: Resistance-to-Ground Variation with Different

Sized Horizontal Electrodes 70

Figure 14: Resistance-to-Ground Variation with Depth of

Horizontal Electrodes 71

Figure 15: Resistance-to-Ground for Various Horizontal

Rings 72

Figure 16: Resistance-to-Ground Variation between

Configurations 73

Figure 17: Resistance-to-Ground Variation for Various

Radial (Star) Configurations 74

Figure 18: Resistance-to-Ground Variation for Different

Wire Grid Areas 75

Figure 19: Comparsion of Resistance-to-Ground between Wire

Grids and Rodbeds 76

Figure 20: Soil Resistivity Site Survey, Small Site - Less

than 2500 Sq. Ft. (232 Sq. m) 77

Figure 21: Soil Resistivity Site Survey, Large Site - More

than 2500 Sq. Ft. (232 Sq. m) 78

Figure 22: Grounding Systems Design Example 79

Figure 23: Resistance-to-Ground of 10 Foot Sectional

Electrodes 80

Figure 24: Typical Grounding Hand Hole 81

Figure C1: \"Four-Terminal\" Method for Measurement of Soil

Resistivity 82

Figure C2: \"Four-Terminal\" Method for Measuring Soil

Resistivity 83

Figure D1: Triangulation Method for Measuring the

Resistance of a Ground Electrode 84

Figure D2: Direct Method for Measuring the Resistance of a

Ground Electrode 85

Figure D3: Fall of Potential Method 86

Figure D4: Example of Earth Resistance Curve 87

Figure D5: Effect with C2 Located far from Earth

Electrode 88

Figure D6: Effect with C2 Located close to Earth

Electrode 89

Figure D7: Earth Resistance Curve for Large Area Example 90

Figure D8: Earth Resistance Curve for Large Area Example 91

Figure D9: Intersection Curves for Figures D8 92

Figure E1: Concrete Encased Electrodes in a Square Ring

Configuration 93

Figure E2: Concrete Footing 94

Figure E3: Basic Elements for Calculation 95

Figure E4: Variation with Wire Size 96

Figure E5: Variation with Depth for Concrete Encased

Electrodes 97

Figure E6: Comparsion between Encased and Direct Buried

Electrodes with Different Concrete Resistivities 98

Figure E7: Variation with Concrete Diameter for Concrete

Encased Electrodes 99

INDEX:

Grounding

Grounding, Fundamentals

Protection, Electrical

**DEFINITIONS**

Information contained herein in italics is copyright information
extracted from IEEE Std 100-1992, *The New IEEE Standards Dictionary of
Electrical and Electronics Terms,*

copyright-1992 by the Institute of Electrical and Electronics Engineers,
Inc. The IEEE takes no responsibility for and will assume no liability
for damages resulting from the placement and context in this
publication. Information is reproduced with the permission of the IEEE.

***Current*** *The time rate of change of charge.*

***Electrode*** *An electric conductor for the transfer of charge
between the external circuit and the electroactive species in the
electrolyte.*

***Fault Current*** *A current that flows from one conductor to ground
or to another conductor owing to an abnormal connection (including an
arc) between the two conductor.*

*Note: A fault current flowing to ground may be called a ground fault
current.*

***Grounding connector*** *The conductor that is used to establish a
ground and that connects an equipment, device, wiring systems, or
another connector (usually the neutral conductor) with the grounding
electrode of electrodes.*

***Grounding electrode*** *A conductor used to establish a ground.*

***Lightning*** *An electric discharge that occurs in the atmosphere
between clouds or between clouds and ground.*

***Ohm*** *The unit of resistance (and of impedance) in the
International System of Units (SI). The ohm is the resistance of a
conductor such that a constant current of one ampere in it produces a
voltage of one volt between its ends.*

***Resistance*** *A property opposing flow of energy and involving loss
of potential (voltage).*

***Resistivity*** *The ability to resist the flow of current.*

**1. GENERAL**

**1.1** **[Ground]{.underline},** is defined as a conducting connection
by which a circuit or equipment is connected to the earth. The
connection is used for establishing and maintaining the potential of the
earth, or approximately that potential, on the circuit or equipment
connected to it. A \"ground\" consists of a grounding conductor, a
grounding electrode, a grounding connector which attaches the grounding
conductor to the ground electrode, and the soil in contact with the
ground electrode.

**1.2** **[Using Grounds in Protection Applications]{.underline}:**

**1.2.1** For natural phenomena, such as lightning, grounds are used to
discharge the system of current before customer or personnel can be
injured or vulnerable system components can be damaged.

**1.2.2** For potentials due to faults in electric power systems with
ground return, grounds aid in ensuring rapid operation of the power
system protective relays by providing additional low resistance fault
current paths. The low resistance path provides the means for the
removal of the potential as rapidly as possible. The ground should drain
the potential before personnel are injured or the telephone system
damaged.

**1.3** **[Ground Resistance]{.underline}:** Ideally, a ground should be
of zero ohms resistance. In reality, this value cannot be obtained due
to the series resistances shown in Figure 1: Components of Resistance in
a Ground Connection. Grounding theory and methods for obtaining a ground
of the smallest practical resistance will be discussed in subsequent
paragraphs.

**2. PHENOMENA AFFECTING GROUND RESISTANCE**

**2.1** **[Introduction]{.underline}:** A grounding electrode cannot be
driven into the soil with the expectation of obtaining a good, low
resistance, ground. Many factors, both natural and human, may affect
results. Some of the factors include:

**2.1.1 [Earth Resistivity]{.underline}:** The electrical resistivity of
the earth (resistance of the earth to the flow of current) is of major
importance. The unit of earth resistivity, the ohm·meter, is defined as
the resistance, in ohms, between opposite faces of a cube of earth one
cubic meter in volume. An alternative unit of measurement, the
ohm·centimeter, is defined as the resistance in ohms, between opposite
faces of a one centimeter cube of earth. To convert ohm·meters to
ohm·centimeters, multiply by the former by 100.

**2.1.1.1** Earth resistivity varies over a considerable range. Within
the United States it varies from a few ohm·meters along some coasts to
many thousands of ohm·meters in rocky, mountainous country. Figure 2:
Estimated Average Earth Resistivity in U.S. provides very general data
on average surface earth resistivity throughout the United States.

**2.1.1.2** In addition to regional variations, earth resistivity may
vary widely within very small distances due to local soil conditions.
Table I lists typical ranges of earth resistivity for various soil
types. This table should be useful in selecting locations at which a
ground is to be installed.

[TABLE I: RESISTIVITY OF VARIOUS SOILS]{.underline}

[SOIL RESISTIVITY RANGE (ohm·meters)]{.underline}

Loam 5 - 50

Clay 4 - 100

Sand/Gravel 50 - 1,000

Limestone 5 - 10,000

Sandstone 20 - 2,000

Granite 1,000 - 2,000

Slates 600 - 5,000

**2.1.2 [Soil Moisture]{.underline}:** Nearly any soil, with a zero
moisture content, is an insulator. Fortunately, this condition is rarely
encountered except in desert areas or during periods of extreme drought.
Figure 3: Typical Variation of Soil Resistivity with Moisture
illustrates the typical affect of moisture on soil resistivity. It
should be noted that above 17% moisture by weight additional moisture
has little effect. Below this figure [resistivity]{.underline} rises
rapidly until, at 2% moisture it reaches 100 times its value at 17%
moisture. Thus, a good ground connection should always be in contact
with soil having a ground water content in excess of 17%. Local well
drillers should be able to provide information concerning the depth of
the water table in their areas. Water content alone does
[not]{.underline} provide a good ground in many areas so do not be
misled by moisture depth only. (See Soil Mineral Content section to
follow).

**2.1.3 [Soil Mineral Content]{.underline}:** Water with no mineral salt
content is nearly as good an insulator as soil with no moisture content.
Figure 4: Typical Effect of Mineral Salt on Earth Resistivity
illustrates the effect of mineral salt content on soil resistivity.
Soils which lack adequate soluble mineral salts may be encountered from
time to time.

**2.1.4 [Temperature]{.underline}:** As the temperature of soil
decreases, resistivity increases. When the soil temperature drops below
the freezing point of water, resistivity increases rapidly, as shown in
Figure 5: Typical Variation of Soil Resistivity with Temperature.

**3. CHARACTERISTICS OF VERTICAL ELECTRODES**

**3.1 [Single Vertical Electrode Buried in Earth]{.underline}:** The
majority of ground electrodes installed in telecommunications systems
consists of a single electrode driven vertically into the earth. An
equation for calculating the approximate resistance-to-ground of a
vertically driven electrode is given in Appendix B, Paragraph 2.1. The
resistance-to-ground with this type of electrode is dependent to a large
degree upon the electrode length and to a lesser degree upon the
electrode diameter.

**3.2 [Variation of Resistance With Depth]{.underline}:** The
theoretical resistance-to-ground of various electrodes driven vertically
into homogeneous soil has been calculated and plotted in Figure 6:
Resistance-to-Ground Variation with Electrode Depth. These curves
illustrate the resistance variation with length for electrodes of
different diameters. The curves show that the electrode
resistance-to-ground decreases rapidly during the first few fractions of
a meter the electrode is driven into the earth. Theoretically there is
little gained by driving an electrode more than 3 to 3.7 meters.
However, earth resistivity sometimes decreases with increased depth
since the earth may not always be homogeneous. Therefore, driving a deep
test rod at the site of a proposed grounding system may sometimes
provide valuable resistivity information. Electrodes should be driven
well below the frost line so that the resistance-to-ground will not be
greatly increased by freezing of the surrounding soil.

**3.2.1** Homogeneous soil conditions are rarely encountered. The
conditions illustrated in Figure 7: Resistance-to-Ground Variation in
Multiple Soil Layers are more typical where multiple soil layers
prevail. Ground electrode resistance, under these conditions, will
decrease with depth until the water

table is reached. Resistance decreases rapidly again as increasing
lengths of rod are exposed to the moist soil as the electrode is driven
deeper.

**3.2.2** The desirable electrode length is a balance between that which
can be installed with reasonable effort and that which will produce the
objective resistance value. The objective resistance for outside plant
is typically 25 ohms or less, and for central offices 5 ohms or less.
The possibility of electrode bending increases with electrode length.

**3.2.3** Installation of 5-foot (1.5 meter) or 8-foot (2.4 meter)
electrodes is normally easy. Additionally, it may be more desirable to
install several shorter electrodes, where space is available, than to
attempt installation of one or more long electrodes (rods or pipes).
Installation of multiple electrodes is likely to be the optimum choice
at locations with rocky soil where frost or low water table are not a
problem. Conversely, in areas of sand with a deep water table, a single
long electrode might provide the best solution. The engineer should
examine [all]{.underline} factors and select the option best suited for
a particular situation.

**3.3 [Variation of Resistance with Diameter]{.underline}:** The
calculated, theoretical resistance-to-ground of electrodes having
different diameters and driven vertically into earth of homogeneous soil
is plotted in Figure 8: Resistance-to-Ground Variation with Electrode
Diameter. The curves in Figure 8 show the resistance variation with
diameter for electrodes of different lengths. The curves show that the
resistance decreases only slightly as the diameter of the electrode is
increased. The slight reduction in resistance seems even less productive
when considering the volume of earth displaced as a ground electrode is
driven into the earth increases as the square of the diameter. As a
result, the effort required to install an electrode increases
significantly with increased diameter. Calculations show that two
electrodes of equal diameter will provide a lower resistance-to-ground
than a single electrode of twice that diameter. Based on this, an
electrode of only sufficient diameter to withstand the strain of driving
should be used.

**3.4 [Electrode Material]{.underline}:** The material composition of a
grounding electrode is based primarily on corrosion and strength
(durability) concerns. As far as resistance-to-earth is concerned an
electrode could be made of most any metal because the majority of the
total resistance

occurs in the [earth]{.underline} immediately adjacent to the electrode
surface. Corrosion studies have shown that copper performs well in most
soils. Unfortunately, solid copper electrodes would be too soft for
driving in most soils. As a result, copper is usually used as a cladding
material over steel and mates strength with good corrosion performance.

Steel has also proven to be an effective grounding electrode. However,
since steel corrodes more readily than copper, steel electrodes are
almost always galvanized (coated) with a special formulation of zinc.

Because the exposure of steel to soil through a holiday (bare spot) on a
copper-clad steel rod could result in rapid corrosion deterioration of
the steel, REA insists that clad rods meet strict minimum and average
copper thickness standards (13 mils minimum and 15 mils average). All
REA listed copper-clad steel ground rods have been found to meet these
thickness requirements.

**3.5 [Multiple Vertical Electrodes Buried in Earth]{.underline}:**
There are locations where the objective resistance to earth is less than
the value attainable with a single electrode. Among such locations might
be the sites of carrier system repeaters, central office switching
systems, remote switching terminals, concentrators, etc. [The
installation of two or more electrodes connected in parallel provides a
means of reducing the grounding system resistance]{.underline}. Multiple
vertical electrode systems can be installed in a straight-line,
circular, square, or rectangular configuration. The electrodes may be
interconnected with either insulated or bare buried conductors. The
specific grounding system design is usually based on the space available
for installation and the desired resistance-to-ground objective.

**3.6 [Multiple Vertical Electrodes Buried in a Straight
Line]{.underline}:** Grounding systems of this type are typically used
at installations have to be completed along a road or within the limits
of a narrow right-of-way. Spacing of vertically installed electrodes is
important. Close spacing will result in considerable mutual resistance
which adds to the overall resultant parallel resistance of a multiple
rod installation. Placing vertical ground electrodes with a separation
of twice the electrode length will minimize the unwanted effect from the
mutual resistance between electrodes on the grounding system.

**3.6.1** Equations for calculating the approximate resistance-to-ground
of two or more electrodes installed along a straight line are given in
Paragraphs 2.2.1.1 and 2.2.1.2 of Appendix B. The equation in
Paragraph 2.2.1.1 should be used when the distance between the
electrodes is equal to or greater than the electrode length. When the
distance between electrodes has to be less than the electrode length the
equation in Paragraph 2.2.1.2 should be used.

**3.6.2** The approximate resistance (calculated for various numbers of
multiple vertical ground electrodes in a straight line) for various
electrode spacing is illustrated in Figure 9: Percent Resistance
Variation Multiple Electrodes in a Straight Line Interconnected with
Wire. These curves provide the percentage of the single electrode
resistance that can be expected by installing various numbers of
vertical electrodes interconnected by bare wire and separated by
one-half, one, two and four times the electrode length. For example, a
grounding system with a resistance-to-ground of 25 ohms or less is
required at a carrier repeater location in an area having a mean earth
resistivity of 200 ohm·meters. The grounding system will have to be
placed in a straight line between the side of the road and the edge of
the road right-of-way. The calculated approximate resistance-to-ground
of a single 5/8 inch x 10 foot (1.6 cm x 3.0 m) electrode at the site is
66.2 ohms and of a single 5/8 inch x 20 foot (1.6 cm x 6.1 m) electrode
is 36.7 ohms. The 25 ohm objective is \[(25/66.2)\*100\] 37.8 percent of
the resistance-to-ground with a single 10 foot (3.0 m) electrode and
\[(25/36.7)\*100\] 68.1 percent of a single 20 foot (6.1 m) electrode
resistance-to-ground. The 38 percent horizontal line in Figure 9
intersects the 1·curve between the vertical lines designating three and
four electrodes. Thus, with a distance between the electrodes equal to
the length of a single 10 foot (3.0 m) electrode, the installation of
four electrodes in a straight line should provide a resistance-to-ground
less than the 25 ohm objective. Extending the electrode separation to
four times the single electrode length reduces the requirement to two
since the 4 curve is intersected between the one and two electrode
vertical lines. This design would reduce the number of electrodes by one
but increase the interconnecting wire required from 20 feet (6.1 m) to
40 feet (12.2 m). Following the same procedure for the 68.1 percent of a
20 foot (6.1 m) electrode shows there is some possibility that two
electrodes with a separation equal to the electrode length will provide
25 ohms resistance-to-ground. If the measured resistance-to-ground after
installation was greater than

25 ohms, addition of a third electrode with the same separation should
produce an acceptable resistance-to-ground.

**3.7 [Multiple Vertical Electrodes Buried in a Ring]{.underline}:**
Ring grounding systems may be installed in a square, rectangular, or
circular configuration. Systems of this type are typically installed at
central office and other locations where sufficient area is available
for installation of the grounding system. One form of ring ground is
around the perimeter of a central office building. The perimeter ground
configuration is also used for concentrators and remote switching
terminals housed in buildings.

**3.7.1** An equation for calculating the approximate
resistance-to-ground of four or more electrodes installed in a ring
configuration is given in Paragraph 2.2.2 of Appendix B.

**3.7.2** The approximate resistance variation due to spacing calculated
for various numbers of vertical electrodes in a ring configuration is
shown in Figure 10: Percent Resistance Variation for a Ring of Multiple
Electrodes Interconnected with Wire. These curves provide the percentage
of the single electrode resistance that will be attained by installing
various numbers of vertical electrodes interconnected with bare wire and
separated by various distances, i.e., one-half, or two times the
electrode length. The curves are applied in the same manner as the
curves of Figure 9 which were discussed in Paragraph 3.6.2.

**3.8 [Multiple Vertical Electrodes Buried in a Rod Bed]{.underline}:**
Ground systems of this type are usually installed to complement a
horizontal grid of buried bare wire. The principal application of these
systems is for grounding electrical power stations and substations where
it is essential to provide a low resistance-to-ground and minimize
voltage gradients along the earth\'s surface (step and touch
potentials).

**3.8.1** The equation for calculating the approximate
resistance-to-ground for a multiple buried vertical electrode rod bed is
given in Paragraph 2.2.4 of Appendix B.

**3.8.2** The approximate resistance-to-ground calculated for various
numbers of vertical electrodes in rod beds of selected sizes is shown in
Figure 11: Resistance-to-Ground for Multiple

Electrodes in a Rod Bed. The curves show the resistance-to-ground for
electrodes interconnected with insulated wire. Points are identified on
each curve indicating the number of electrodes that provides a perimeter
ground and a full rod bed where the electrode separation is ten feet
(three meters) or greater.

**3.8.3** Study of the curves in Figure 11 shows that the maximum
reduction in resistance-to-ground is nearly achieved when the electrodes
are placed around the perimeter of an area. Placing additional rods
inside the perimeter to complete the rod bed will reduce the overall
resistance-to-ground four tenths of an ohm, or less. This provides
confirmation that rod beds, while valuable for control of step and touch
potentials along the earth\'s surface, do not provide significant
improvement of the overall resistance-to-ground.

Note: Step potential is defined as the voltage differential between two
points on the ground separated by the distance of one pace, which is
assumed to be 3 feet (0.9 meters). Touch potential is defined as the
voltage differential between both feet on the ground and an object that
can be touched with a hand.

**3.8.4** Increasing the area perimeter does not provide a significant
improvement of the overall resistance-to-ground. For example, changing a
20 x 20 ft. (6.1 x 6.1m) to a 30 x 30 ft. (9.1 x 9.1m) square is a
125 percent increase in area which will reduce the resistance-to-ground
by 26.5 percent. A significant improvement of the overall
resistance-to-ground is defined as a reduction that lowers the
resistance below the objective value. The final 5.2 ohms ground of this
example will not meet the 5.0 ohm central office objective while the
original 7.1 ohms is less than 25 ohms (TE&CM Section 810, proposed
conversion to 1751F-810).

**4. CHARACTERISTICS OF HORIZONTAL ELECTRODES**

**4.1 [Horizontal Electrode Buried in Earth]{.underline}:** Installation
of vertical ground electrodes is not always practical at locations where
grounding systems are required. A horizontal ground electrode provides
an effective alternative. The horizontal electrode might be a ground rod
or, more typically, a length of bare copper wire buried in the earth.
Where a horizontal electrode is used in lieu of a vertical electrode,
burial depth should be below the deepest frost penetration. (Reference
Paragraph 2.5). Horizontal electrodes are often used to interconnect a
system of

multiple vertical electrodes for further reduction of the overall system
resistance-to-ground. A horizontal electrode configuration can be either
a straight line, a ring, or perimeter (square, circular, or
rectangular), a grid (square or rectangular), or a star radiating from a
single point.

**4.2 [Horizontal Electrode Buried in a Straight Line]{.underline}:**
Grounding systems of this type are utilized at locations where vertical
electrodes cannot be installed along a narrow path, such as a road
right-of-way, due to a shallow rock substructure. (Another common
configuration is the interconnection of a series of vertical electrodes
installed in a straight line.) The equation for calculating the
approximate resistance-to-ground of a horizontal electrode buried in a
straight line is given in Paragraph 3.1 of Appendix B. The approximate
resistance-to-ground calculated for various lengths of #2 solid copper
wire buried in a straight line is shown in Figure 12:
Resistance-to-Ground Variation with Length of Horizontal Electrode.

**4.2.1** The electrode diameter has minimal effect on the
resistance-to-ground of a buried horizontal electrode but is of major
importance to the physical strength of the configuration. The calculated
approximate resistance of straight horizontal electrodes of different
lengths for different wire sizes is illustrated in Figure 13:
Resistance-to-Ground Variation with Different Sized Hortizontal
Electrodes.

**4.2.2** There is no significant reduction of the horizontal electrode
resistance-to-ground from increased burial depth at the commonly used
depths for buried wire grounding systems in the telecommunications
industry. Greater reduction can be attained with a deep buried
horizontal electrode but this is neither practical nor economical. The
calculated resistance of horizontal electrodes buried at various depths
is shown in Figure 14: Resistance-to-Ground Variation with Depth of
Horizontal Electrodes.

**4.3 [Horizontal Electrode Buried in a Ring]{.underline}:** A common
application for a buried horizontal electrode in a ring configuration is
the interconnection of a ring of buried vertical ground electrodes
installed around the perimeter of a central office building. Two
equations for calculating the approximate resistance-to-ground of a
buried bare wire ring are given in Paragraphs 3.2.1 and 3.2.2 of
Appendix B. The equation from Paragraph 3.2.1 is convenient for
calculating the resistance-to-ground of a square

or rectangular ring configuration. Calculation of the
resistance-to-ground with a circular ring configuration is more
convenient by the equation given in Paragraph 3.2.2 of the Appendix B.

**4.3.1** The approximate resistance-to-ground calculated for buried
wire rings with various perimeter lengths is shown in Figure 15:
Resistance-to-Ground for Various Horizontal Rings. Curves for three wire
sizes are provided to illustrate the small resistance variation relative
to conductor size. Resistance-to-ground variations with burial depth for
ring configurations are similar to those for straight horizontal
electrodes discussed in Paragraph 4.2.2 and illustrated in Figure 14.

**4.3.2** The resistance-to-ground of a buried wire ring is greater than
that of a straight buried wire of the same length. This difference is
illustrated in Figure 16: Resistance-to-Ground Variation between
Configuration. The resistance-to-ground of a wire ring with an 80 foot
(24.4 meter) perimeter is about 9 percent greater than that of a
straight wire of the same length.

**4.4 [Horizontal Electrodes Buried in a Radial
Configuration]{.underline}:** Buried radially extending bare wire
grounding systems are employed typically for protecting radio antenna
tower installations. These systems may also be used for protecting
similar installations where it is convenient to extend the grounding
conductors radially from a common point. Radial grounding systems have
the advantage of a lower initial surge impedance to lightning surge
currents (relative to the direct current resistance) than a single or
group of parallel horizontal wires.

**4.4.1** Equations for calculating the approximate resistance-to-ground
for bare wires buried in a radial configuration are given in
Paragraphs 3.4.1 thru 3.4.4, of Appendix B. These equations provide a
calculation means for the calculations for the following types of
installations:

Three-branched Star - Paragraph 3.4.1

Four-branched Star - Paragraph 3.4.2

Six-branched Star - Paragraph 3.4.3

Eight-or more branched Star - Paragraph 3.4.4

**4.4.2** The approximate resistance-to-ground calculated for buried
bare wire radial ground systems with various numbers and lengths of
conductors is shown in Figure 17: Resistance-to-Ground Variation for
Various Radial (Star) Configurations.

These curves show there is little advantage in designing a radial
grounding system with more than six branches.

**4.5 [Horizontal Electrodes Buried in a Grid
Configuration]{.underline}:** Grounding systems of this design are
commonly used at locations where it is essential to minimize voltage
gradients along the earth\'s surface. The grid configuration is most
frequently used for grounding systems associated with electric power
stations and substations. A typical design consists of a grid of
horizontal electrodes solidly connected at each crossing installed at or
close to the earth\'s surface. The resistance-to-ground of a grid
depends mainly on the area covered by the grid and to a lesser extent on
the total length of wire used in constructing the grid.

**4.5.1** The equation for calculating the approximate
resistance-to-ground for a buried bare wire grid is given in
Paragraph 3.3 of Appendix B.

**4.5.2** The approximate resistance-to-ground calculated for bare wire
grid grounding systems for different grid sizes with various total wire
lengths is illustrated in Figure 18: Resistance-to-Ground Variation for
Different Wire Grid Areas. These curves show there is no significant
reduction of the resistance-to-ground when a buried wire perimeter
grounding system is converted to a grid system of the same area. For
example, Curve C of Figure 18 is for a 900 square foot (83.6 square
meter) buried wire grounding system. The perimeter ground conductor is
120 feet (36.6 meters) long and as shown by the X at the left end of the
curve has a resistance-to-ground of about 6 ohms. Adding 120 feet
(36.6 meters) of buried conductor will convert the perimeter ground to a
grid of 10 foot (3.0 meter) squares. The resistance-to-ground will be
about 5.8 ohms. The buried conductor length has been increased by
100 percent to obtain an 6 percent reduction in resistance-to-ground.
The 0.3 ohm reduction will not significantly improve the performance of
the grounding system so there is no justification for the additional
wire, except when needed for safety purpose to minimize step and touch
potentials

**5. MUTUAL RESISTANCE BETWEEN VERTICAL AND HORIZONTAL ELECTRODES**

**5.1** **[Introduction]{.underline}:** Vertical electrodes of a
grounding system are usually interconnected with buried bare horizontal
wire. The net combined resistance-to-ground of

the interconnected vertical and horizontal electrodes will be greater
than the calculated parallel equivalent of each type\'s separate
resistance but less than the resistance of either electrode system
alone. This is due to the interaction between the electrodes called
mutual resistance. The contribution to total resistance from this mutual
resistance is addressed in Paragraph 6.

**5.2 [Mutual Resistance, Vertical Electrodes in a Straight
Line]{.underline}:** The approximate mutual resistance between vertical
ground electrodes placed in a straight line and an interconnecting
buried bare wire may be calculated using the equation in Paragraph 4.2
of Appendix B. The wire length for this equation is the total conductor
distance between the first and last vertical electrode.

**5.3 [Mutual Resistance, Vertical Electrodes in a Ring]{.underline}:**
The equation for calculating the approximate mutual resistance between a
ring of vertical ground electrodes and interconnecting buried bare wire
is given in Paragraph 4.3 of Appendix B. Wire length for this equation
is the total length of the ring.

**5.4 [Mutual Resistance, Rod Bed of Vertical Electrodes]{.underline}:**
The equation for calculating the approximate mutual resistance between a
rod bed of vertical electrodes and a grid of interconnecting buried bare
wire is given in Paragraph 4.4 of Appendix B. The wire length is the
total length required to form the grid.

**6. COMBINED RESISTANCE OF VERTICAL AND HORIZONTAL ELECTRODES**

**6.1** **[Introduction]{.underline}:** The combined parallel
resistance-to-ground for vertical electrodes interconnected with bare
wire may be calculated by the equation in Paragraph 5.1 of Appendix B. A
guide is provided in Paragraph 5.2 of the Appendix showing the
appropriate equations for calculating the resistance-to-ground with
various grounding system configurations.

**6.2 [Combined Resistance of Vertical and Horizontal Electrodes in a
Straight Line]{.underline}:** The approximate resistance-to-ground
variation calculated for multiple vertical ground electrodes placed in a
straight line and interconnected with buried bare horizontal conductors
is shown in Figure 9.

The combined resistance is given as a percentage of the
resistance-to-ground for a single electrode.

**6.3 [Combined Resistance of Vertical and Horizontal Electrodes in a
Ring]{.underline}:** The calculated approximate resistance-to-ground
variation for multiple 5/8 inch x 10 foot (1.6 cm x 3.0 m) vertical
ground electrodes placed in a square, rectangular, or circular ring and
interconnected with buried bare horizontal conductors is illustrated in
Figure 10. The combined resistance is given as a percentage of the
resistance-to-ground for a single electrode. A set of curves that would
apply to all electrode dimensions could not be produced due to the
effects of mutual resistance with ring configurations.

**6.4 [Combined Resistance of Vertical and Horizontal Electrodes in a
Grid]{.underline}:** The approximate resistance-to-ground calculated for
rod beds of various area and interconnected with a grid of buried bare
horizontal conductors is shown in Figure 19: Comparsion of
Resistance-to-Ground between Wire Grids and Rodbeds. The curves show
that due to the mutual resistance effects adding vertical electrodes to
a grounding grid does not produce a significant reduction of the overall
resistance-to-ground. The horizontal wire of grounding grids are usually
installed near the earth\'s surface since the principal purpose is to
control step and touch potentials. This location is well above the frost
line in colder parts of the country. During the winter months the grid
resistance-to-ground will increase significantly. The vertical
electrodes are installed to a depth well below the frost line in these
areas and help to ensure maintaining a low resistance during the periods
when the ground is frozen.

**7. DESIGN OF CENTRAL OFFICE GROUNDING SYSTEMS**

**7.1** **[Introduction]{.underline}:** The ground electrode system
establishes the electrical connection between the central office
facility and the earth. This connection is essential for lightning
protection, power fault protection, and to a lesser degree, the
minimization of noise. The system should be tailored for the physical
characteristics of the site and the objective resistance-to-ground. A
grounding system has to be properly installed; follow-up action is
essential to assure the system continues to provide a low resistance
connection to ground throughout the office\'s life. The procedure for
achieving these objectives is as follows: first, determine the physical
and electrical properties of the site; second, design an electrode
system appropriate for the site; third, install the

system in accordance with recommended procedures, and; finally, measure
the resistance-to-ground of the completed system to verify that the
objectives have been met. Then periodically measure the
resistance-to-ground of the completed system to ensure the objectives
continue to be met.

**7.2 [Site Survey]{.underline}:** A thorough survey of the proposed
grounding electrode system site should be conducted before starting the
design. The survey should determine the soil resistivity and any
significant geological features that might influence the design. Local
climate effects should also be reviewed. If a building is already in
place, review architectural and landscape features that might influence
the system design. Ideally, this survey should be conducted before the
final site selection so that troublesome locations can be avoided.

**7.2.1 [Soil Resistivity:]{.underline}** The initial step of a site
survey is the measurement of soil resistivity at [several]{.underline}
points in the area occupied by and surrounding the central office
building. For small sites up to 2500 square feet (232.3 square meters),
complete one measurement at the center of the site and at each of the
four corners as shown in Figure 20: Soil Resistivity Site Survey, Small
Site Less than 2500 Sq. Ft. (232 Sq m). At each of the locations a
measurement should be made with 12 foot (3.7 meter) and 22 foot
(6.7 meter) test probe spacing. The recorded results of the
[five]{.underline} readings with each probe spacing are averaged to
obtain the soil resistivity for the site. The method for measurement of
earth resistivity is discussed in Appendix C. For larger sites, divide
the area into two or more smaller areas with sides of 50 feet
(15.2 meters) or less. Complete earth resistivity measurements at each
corner and at the center of each smaller area as described above. Two
examples of larger sites are illustrated in Figure 21: Soil Resistivity
Site Survey, Large Site - More than 2500 Sq. Ft.(232 Sq. m). Average the
recorded results of all measurements for each probe spacing to obtain
the soil resistivity for the site.

**7.2.2 [Geological Features:]{.underline}** Attempt to identify the
presence of geological features at the site that might influence the
grounding system design. Such features include the distribution of major
soil types, major rock formations, and depth of water table. Information
relating to the geological structure of an area can be obtained from
local construction companies, well drillers, utilities (gas, water, and
power), and local maps and site inspections. Test borings can be
utilized when adequate information is not available from other sources.
Review this information to determine the presence of factors that

may influence the design or installation of the grounding system.

**7.2.3 [Physical Features:]{.underline}** Identify other physical
features that might influence the location, shape or type of grounding
system. Study the planned or existing location of the building(s) or
structure(s) together with the location of existing and proposed parking
lots, paved roadways, and sidewalks. Buried structures, such as pipes or
tanks, should also be located.

**7.2.4 [Climate Conditions:]{.underline}** Determine the annual amount
and seasonal distribution of rainfall, the relative incidence of
lightning, and the typical soil depth of freezing (frost line) for the
area. Rainfall and frost line information is available from the local
weather service. The relative lightning incidence can be obtained from
the isokeraunic map in (TE&CM Section 801, proposed conversion to
Bulletin 1751F-801).

**7.3 [Design Procedure]{.underline}**

**7.3.1 [Grounding System Configuration]{.underline}**: Determine the
type of grounding system appropriate for the office building and
property on which it is located. A perimeter grounding system, installed
around the outside building walls, is recommended. Electrodes should be
placed at a 3 feet (0.9 meters) or greater distance from the outside
wall of the building foundation and outside the roof drip line to ensure
wetting of the earth surrounding the electrodes by precipitation. The
conductor between electrodes should be the same size as the conductor
that is to be used to connect the grounding system to the central office
master ground bar (See TE&CM Section 810, proposed conversion to
Bulletin 1751F-810). Major portions of the grounding system should not
be located under paved areas such as roads, parking lots, or sidewalks.
The location of the building relative to property lines may make
installation of a perimeter ground impractical. The property should be
studied further to determine the area available for installation of the
grounding system and the configuration best suited to the area. As
previously mentioned, perimeter configuration is recommended even if the
proposed grounding system will not be installed around a building.
Installation of additional ground rods to convert a perimeter to a rod
bed grounding system will not produce a significant reduction of the
overall resistance-to-ground. This is due to the increasing mutual
effects between the electrodes as they are added inside the perimeter.
For example, consider a grounding system of 16, 5/8 inch by 10 foot
(1.6 centimeter by 3.0 meter) electrodes, interconnected with insulated
wire, installed around the perimeter of a 1600 square foot (148.6
square meter) area.

Assuming a 200 ohm·meter earth resistivity, the calculated
resistance-to-ground is 7.7 ohms. Addition of 9  electrodes will convert
the perimeter system to a square rod bed containing 25 electrodes with
10 foot (3.0 meter) separation between electrodes. The calculated
resistance-to-ground for the rod bed is 7.2 ohms. This one-half ohm
reduction of the resistance-to-ground generally would not justify the
added cost of installing nine additional electrodes and the wire
required to interconnect them. Further, if bare #2 conductors are
utilized for interconnecting the electrodes, both configurations have a
calculated resistance-to-ground of 7.2 ohms. Installation of a rod bed
array should be limited to locations where control of step and touch
potentials is required.

**7.3.2 [Calculation of Earth Resistance:]{.underline}** Once the
appropriate configuration is chosen, the number and size of electrodes
should be determined for the initial calculation of the anticipated
system resistance-to-ground. The minimum electrode diameter is 1/2 inch
(1.3 centimeter). This is practical for most installations. Electrodes
5/8 inch (1.6 centimeters) in diameter are recommended, where the soil
is extremely hard, to resist bending of the electrode during
installation. The electrode length selected is based on
[three]{.underline} factors:

**7.3.2.1** The depth of rock formations in the area determines the
maximum depth that an electrode can be driven into the earth. The
electrode length selected for the initial resistance to earth
calculation should typically be shorter than this maximum depth. When an
acceptable resistance-to-ground cannot be attained in the available area
with electrodes driven to the rock depth, a well drilled through the
rock formation may prove to be an acceptable alternative.

**7.3.2.2** Where the depth of the water table is near the surface with
only small variation from year to year, selection of an electrode length
that will penetrate to a depth 5 feet (1.5 meters) below the lowest
water table level is appropriate. This ensures that the electrodes will
be in contact with moist soil most of the time.

**7.3.2.3** The frost line depth is an important factor for
determination of electrode length. Earth resistivity will increase three
to four hundred percent as the earth freezes (Reference Figure 5). For
example, an electrode measuring

35 ohms to ground in the summer is located in an area where the frost
line penetrates to a depth of one-half the electrode length. During the
winter when the earth is frozen the resistance-to-ground will increase
to about 87 ohms. The portion of total electrode length that extends
below the frost line should have a calculated resistance-to-ground
meeting the desired objective (when the portion of electrode length
above the frost line is disregarded). Further, buried bare conductors
interconnecting the vertical electrodes should not be included in
calculations of resistance-to-ground in areas where the frost line
exceeds a depth of 12 inches (30 centimeters). The goal is to provide a
grounding system that will meet the objective resistance-to-ground
during the entire year.

**7.3.2.4** The initial calculation of the grounding system
resistance-to-ground should, where possible, be based on a spacing
between electrodes of twice the electrode length. If the calculated
resistance-to-ground meets the design requirement, the design may be
implemented. When the objective resistance-to-ground is not met,
alternative configurations should be considered. There are two
alternatives available when a spacing of twice the electrode length is
used initially. One is to double the length of the electrode and the
second is to add electrodes of the initial length to reduce the spacing
to the electrode length. The average earth resistivity at depths equal
to the proposed electrode lengths should indicate the best alternative.

**7.3.2.5** To illustrate this design procedure, assume that a 40 foot x
60 foot (12.2 meter x 18.3 meter) rectangular configuration, as shown in
Figure 22: Grounding System Design Example, will accommodate a perimeter
grounding system around a proposed central office building. Also, assume
that the soil resistivity measurements made during the site survey show
an average resistivity of 900 ohm·meters to a depth of 12 feet
(3.7 meters) and 200 ohm·meters to a depth of 22 feet (6.7 meters). In
addition, the site survey indicated that all rock formations are at
depths greater than 25 feet (7.6 meters); the water table never drops
below 6 feet (1.8 meters); and the frost line extends only 1 foot
(0.3 meters) below the surface. Because of the high soil resistivity,
10 foot (3 meter) electrodes are selected for the initial evaluation.
The design steps follow:

**7.3.2.5.1** Determine the resistance-to-ground of a single ground
electrode from Figure 23: Resistance-to-Ground of 10 Foot

Sectional Electrodes. The resistance-to-ground for a 5/8 inch x 10 foot
(1.6 centimeter x 3.0 meter) electrode in 900 ohm·meter soil is obtained
from Figure 23. Since the curve for 10 foot (3.0 meter) length does not
intersect the 900 ohm·meter line, divide 900 by 10 and determine the
resistance-to-ground for the electrode in 90 meter· ohm soil. This value
is about 29.8 ohms, which, multiplied by 10, equals 298 ohms, the
desired 900 ohm·meter value.

**7.3.2.5.2** Assume an initial spacing of 20 feet (6.1 meter) or twice
the electrode length between electrodes. Figure 22 shows that
10 electrodes are required to provide the perimeter ground. Use
Figure 10 to determine the equivalent value of the resistance of one
electrode that is produced by 10 electrodes in parallel. The answer is
about 9.3 percent. Thus the expected resistance of 10 electrodes in
900 ohm·meter soil is:

R = 298 x 0.093 = 27.7 ohms

While the objective resistance-to-ground for a central office grounding
system is 5 ohms or lower, a system with a resistance-to-ground of
25 ohms or lower is acceptable where the resistance cannot
[economically]{.underline} be reduced to meet the objective. This
configuration exceeds 25 ohms so an alternate configuration should be
considered.

**7.3.2.5.3** There are [two]{.underline} alternatives readily available
based on the same physical dimensions. One is to place 10 additional
electrodes, reducing the spacing to the electrode length. Figure 10
shows that 20 electrodes in 900 ohm·meter soil will have an expected
resistance-to-ground of about 8.2 percent of that of a single electrode.

R = 298 x 0.082 = 24.4 ohms

The 24.4 ohms resistance-to-ground is less than 25 ohms and is
acceptable [unless]{.underline} there is a means to provide a lower
resistance with essentially the [same]{.underline} expenditures. The
[second]{.underline} alternative should [also]{.underline} be studied
before making a final decision.

**7.3.2.5.4** The second alternative is to place electrodes twice the
initial length providing a system of 10 electrodes with the spacing
equal to the length of the electrode. The earth resistivity at the
greater depth is 200 ohm·meters and from Figure 23 the
resistance-to-ground of a 5/8 inch x 20 feet (1.6 centimeter x
6.1 meter) electrode in 200 ohm·meter soil is 36.7 ohms. From Figure 10,
the resistance-to-ground of 10 electrodes with spacing equal to the
electrode length is

about 14 percent that of a single electrode.

R = 36.7 x 0.14 = 5.1 ohms

The second alternative design will provide a grounding system having an
expected resistance-to-ground near the [5 ohm]{.underline} objective.
Since there should be little difference in the cost of installing either
of the two alternative designs the second is recommended for
installation.

**7.3.3** Ideal sites will not always be encountered. For example, had
the water table been deeper in the area of the illustration discussed in
Paragraph 7.3.2.2 (causing higher earth resistivity) it would not have
been possible to provide a grounding resistance of 25 ohms. Where there
is not sufficient property available for enlarging the grounding system,
it may be necessary to drill a [well]{.underline} that extends below the
lowest yearly water table level.

**8. DESIGN OF ISOLATED GROUNDING SYSTEMS**

**8.1** **[Introduction]{.underline}:** An isolated ground electrode
system establishes the electrical connection between electronic
equipment and the earth. These systems are typically installed along
cable routes at the location of span line and voice frequency repeaters,
carrier terminal equipment, and small-enclosure mounted concentrator
equipment. This grounding connection is vital for lightning and power
fault protection. The system should be tailored for the physical
characteristics of the site and the objective resistance-to-ground. A
grounding system has to be properly installed and periodic measurement
is essential to assure the system continues to provide an acceptable
connection to ground. The procedure for achieving the objectives are as
follows: first, determine the physical and electrical properties of the
site; second, design an electrode system appropriate for the site;
third, install the system in accordance with recommended procedures,
and; finally, measure the resistance-to-ground of the completed system
to verify that the objectives have been met.

**8.2 [Site Survey]{.underline}:** Before starting the design, a
thorough survey should be conducted at the site where the ground
electrode system is to be installed. This survey should determine the
soil resistivity and any significant geological features that might
influence the design. The majority of isolated grounding systems will be
located along roadways or private right-of-ways. This limits the design
options available.

**8.2.1 [Soil Resistivity:]{.underline}** The initial step of the site
survey is the measurement of soil resistivity at several points over the
area available for the grounding system. Where the corridor along which
the telecommunications cable is placed has a width of 10 feet
(3.0 meters) or greater, two designs can be considered. One design is a
four-branch star configuration of buried bare wire with a vertical
electrode installed at the end of each branch. The second design is a
straight line configuration with the vertical electrodes installed
parallel to the right-of-way boundary. Earth resistivity measurements
should be completed so that either design can be considered for
implementation. At each point, measurements should be made with 12 foot
(3.7 meter) and 22 foot (6.7 meter) probe spacing. The method for
measuring earth resistivity is discussed in Appendix C.

**8.2.2 [Geological Features:]{.underline}** Attempt to identify the
presence of geological features at the site that might influence the
grounding system design. Such features include the distribution of major
soil types, major rock formations, and depth of water table. Information
relating to the geological structure of an area can be obtained from
local construction companies, well drillers, utilities (gas, water, and
power), local maps, and site inspections. Review this information to
determine the presence of factors that may influence the design or
installation of the grounding system.

**8.2.3 [Physical Features:]{.underline}** Identify other physical
features that might influence the location, shape or type of grounding
system. Study the location of existing or proposed roadways and drainage
systems. Buried structures in the area, such as pipes, power conductors,
and communication cables should be precisely located.

**8.2.4 [Climatic Conditions:]{.underline}** Determine the annual amount
and seasonal distribution of rainfall, the relative incidence of
lightning, and the typical depth of freezing (frost line) for the area.
Rainfall and frost line information is available from the local weather
service. The relative incidence of lightning can be obtained from the
isokeraunic map in TE&CM Section 801 (proposed conversion to Bulletin
1751F-801).

**8.3 [Design Procedure]{.underline}**

**8.3.1 [Grounding System Configuration:]{.underline}** Determine the
type of grounding system appropriate for the area available for the
installation.

The area in which isolated grounding systems are typically installed
limits design flexibility. Isolated grounding systems are usually
required at points along a telecommunications cable route. These routes
may be located along a roadway in either public or private right-of-way
or along private right-of-way not adjacent to a traveled roadway. Thus,
the use of some form of elongated system is indicated. A series of two
or more vertical electrodes installed parallel to the public or private
right-of-way may provide an acceptable grounding electrode. Although
there exists no theoretical limit to the number of vertical electrodes
that may be placed in a straight line, there may be some practical
limits. Eight to ten electrodes is a reasonable maximum number of
electrodes in a straight line. Where the width is 10 feet (3 meters) or
greater, a rectangular configuration might be used. If the installation
is adjacent to a roadway, the rectangular configuration should not be
considered where one side would be placed at the edge of a drainage
ditch (where it might be damaged during road maintenance). The specific
grounding system will be determined during calculations of
resistance-to-ground.

**8.3.2 [Calculation of Resistance-to-Ground:]{.underline}** The first
step in calculating the resistance-to-ground is to determine the number
and size of electrodes that will provide the objective system
resistance-to-ground. The minimum electrode diameter is 1/2 inch
(1.3 centimeter) (this is practical for most installations). When the
soil is extremely hard, electrodes 5/8 inch (1.6 centimeter) in diameter
are recommended to resist bending of the electrode during installation.
The electrode length selected is based on [three]{.underline} factors:

**8.3.2.1** The depth of rock formations in the area determine the
maximum depth that an electrode can be driven into the earth. The
initial electrode length selected will typically be less than the
maximum available depth.

**8.3.2.2** Where the water table is near the surface with only small
variation from year to year, selection of an electrode length that will
penetrate to a depth 5 feet (1.5 meters) below the lowest water table
level is appropriate. This ensures that the electrodes will be in
contact with moist soil.

**8.3.2.3** The frost line depth is an important factor for
determination of electrode length. Earth resistivity will increase three
to four hundred percent as the earth freezes (Reference Figure 5).
Buried bare conductors interconnecting the vertical electrodes should
not be included in calculations of resistance-to-ground in areas where
the frost line exceeds a depth of 12 inches (30 centimeters).

The goal is to provide a grounding system that will meet the objective
resistance-to-ground during the entire year.

**8.3.2.4** The objective resistance-to-ground for a grounding system
protecting electronic equipment installed at a location remote from the
central office is 25 ohms. The majority of isolated ground electrode
systems will be located along road or private right-of-ways. This limits
the design to a series of electrodes in a straight line along the right
of way. The use of spacing between the electrodes equal to the electrode
length is recommended.

**8.3.2.5** To illustrate this design procedure, assume a grounding
system for the protection of a span line repeater along a road right of
way. Further, assume that soil resistivity measurements made during the
site survey show an average resistivity of 400 ohm·meters to a depth of
12 feet (3.7 meters) and 250 ohm·meters to a depth of 22 feet
(6.7 meters). In addition, the site survey indicated that no rock
formation in the area will interfere with the installation of electrodes
to either depth; the water table is about 30 feet (9.1 meters); and the
frost line extends 1.5 feet (0.5 meter) below the surface. A 10 foot
(3 meter) sectional electrode is selected for the initial calculation,
to have the option of subsequently driving a 20 foot rod to take
advantage of the lower resistivity at 12 feet (3.7 meters) to 22 feet
(6.7 meters) below the surface. The design steps follow:

**8.3.2.5.1** Determine the resistance-to-ground of a single 5/8 inch x
10 foot (1.6 centimeter x 3.0 meter) electrode in 400 ohm·meter soil
from Figure 23. Since the curve for a 10 foot (3.0 meter) length does
not intersect the 400 ohm·meter line, divide 400 by 10 and determine the
resistance-to-ground for the electrode in 40 ohm·meter soil. This value
is about 13.2 ohms, which, multiplied by 10, equals 132 ohms, the
desired 400 ohm·meter value.

**8.3.2.5.2** Determine what percentage of the resistance-to-ground of a
single electrode equals the objective resistance-to-ground (25/132 x 100
= 18.9%). Use Figure 9 to determine the number of 10 foot (3.0 meter)
electrodes that will provide a resistance-to-ground of 25 ohms or less.
The horizontal line representing 18.9 percent intersects the curve (s =
1) just before the vertical line indicating 7 electrodes. This shows
that a grounding system with 7 electrodes extended in a straight line is
expected to provide a resistance-to-ground less than 25 ohms.

The system would extend 60 feet (18.3 meters) and require at least that
length of wire for interconnection. An alternate plan should be studied
to determine if the system could be installed more economically.

**8.3.2.5.3** Determine the resistance-to-ground of a single 5/8 inch x
[20 foot]{.underline} (1.6 centimeter x 6 meter) electrode in
250 ohm·meter soil. From [Figure 23]{.underline}, this is 45.9 ohms. The
objective ground resistance is 100 times 25 divided by 45.9 equals
54.5 percent of the single electrode resistance-to-ground. Use Figure 9
to determine the number of 20 foot (6 meter) electrodes that will
provide a resistance-to-ground of 25 ohms, or less. The horizontal line
representing 54.3 percent intersects the curve between the vertical
lines indicating one and two electrodes, respectively. Thus, a grounding
system with two 20 foot (6 meter) electrodes extended in a straight line
should provide a resistance-to-ground of less than 25 ohms. This system
would extend 20 feet (6.1 meters) and require at least that length of
wire for interconnection. This plan is more economical and should be
recommended for installation.

**8.3.3** Ideal sites for isolated grounding systems will not always be
available. Since the location of the electronic equipment to be
protected is determined by the transmission facility to which it is
connected only minor relocation is possible. Thus, relocation to a more
desirable site may be impossible. Where the expense of drilling a well
cannot be justified, in extremely rocky terrain the burial of horizontal
electrodes at a depth below the frost line may be the only means of
providing adequate protection.

**9. INSTALLATION PROCEDURES**

**9.1** **[Introduction]{.underline}:** The installation of the
electrode system should be scheduled so that needed excavation, such as
hole and trench digging, can be performed while other excavating
associated with building construction is in progress. If the system is
installed prior to other earth moving operations, necessary precautions
should be taken to assure the components are not damaged.

**9.2** **[Wire]{.underline}:** Wire provided for interconnection of
vertical electrodes should be buried at least 2.5 feet (0.76 meter)
below grade level. The tops of the vertical electrodes should be a
minimum of one foot (0.3 meter) below grade level. This will minimize
resistance variations caused by surface drying of the soil.

The possibility of mechanical damage will also be reduced. Resistance
variations caused by freezing of the soil will be minimized in those
areas where the frost line is one foot (0.3 meter) or less below grade
level.

**9.3** **[Grounding Well]{.underline}:** Access to the grounding system
at a building site should be provided through the installation of a hand
hole or [grounding well]{.underline}. This provides access for periodic
resistance checks of the ground electrode system. Either clay pipe or
poured concrete may be used, as illustrated in Figure 24: Typical
Grounding Hand Hole, with a removable access cover.

**9.4** **[Bonds]{.underline}:** All Bonds in concealed locations have
to be brazed or welded. While the bonding of dissimilar metals should be
avoided, there will be occasions where it becomes necessary. Any bonds
between dissimilar metals, such as between a copper wire and cast iron
on steel pipe, have to be thoroughly sealed against moisture intrusion
to minimize corrosion. Bolted clamp connections should be made only in
manholes, handholes or grounding wells where they are readily available
for the verification of integrity.

**9.5** **[Grounding Electrodes]{.underline}:** Grounding electrodes
should only be driven into undisturbed earth or thoroughly compacted
filled areas. Electrodes and interconnecting conductors should be placed
in the backfill around new building foundations only after the soil has
been compacted or has had adequate time to settle. Electrodes should not
be driven or laid in gravel beds which have been installed for drainage
purposes unless the electrodes extend through such beds far enough to
provide a minimum of 8 feet (2.4 meters) of contact with the undisturbed
earth underneath. Horizontal bare interconnecting conductors should not
be placed in such beds under any circumstances.

**9.6** **[Driving Electrodes]{.underline}:** Electrodes may be driven
either by hand sledging or with power drivers. Hand sledging may be
preferable where only a limited number of electrodes are installed in
earth of moderate compactness. Driving nuts should be used to prevent
damage to the driven end, particularly, if two or more sections are to
be joined. Deep driven electrodes or those driven into hard or rocky
soil generally require the use of power drivers with special driving
collars to prevent damage to the electrode.

**9.7** **[Measurements of the Grounding System
Resistance-to-Ground]{.underline}:** Should be done as soon as
installation is completed by using the procedure outlined in
[Appendix D]{.underline}.

This initial resistance-to-ground value will probably exceed the
calculated values by as much as twenty percent since the disturbed soil
has not had time to settle and provide good contact with electrode
surfaces. Remeasuring on a monthly basis is recommended until a steady
value is established.

**9.8** **[Remeasuring the Grounding System]{.underline}:** After the
resistance of the grounding system has stabilized, the
resistance-to-ground should be remeasured annually to determine that the
system is still adequate for protection purposes. Although resistance
fluctuations will occur, any increase in the resistance-to-ground of
20 percent or more should be investigated and the necessary work
completed to reduce it to the objective value.

**10. REFERENCES**

**10.1** **[Publications]{.underline}** listed below were utilized
during the preparation of this practice. They are recommended for study
to those individuals desiring further knowledge of grounding theory.

1\. F. Wenner, \"A Method of Measuring Earth Resistivity,\" Report No.
258, Bulletin of the Bureau of Standards, Volume 12, No. 3, October 11,
1915.

2\. O. S. Peters, \"Ground Connections for Electrical Systems,\"
Technological Paper No. 108, Bureau of Standards, June 20, 1918.

3\. H. B. Dwight, \"Calculation of Resistance-to-ground,\" Electrical
Engineering, Volume 55, December 1936, pp. 1319-1328.

4\. S. J. Schwarz, \"Analytical Expressions for the Resistance of
Grounding Systems,\" AIEE Transactions, Volume 73, Part III-B, 1954, pp.
1011-1016.

5\. \"Guide for Safety in Alternating-Current Substation Grounding,\"
IEEE Standard 80-1986, IEEE, New York.

6\. E. D. Sunde, \"Earth Conduction Effects in Transmission Systems,\"
Dover Publications, Inc., New York, 1968.

7\. G. F. Tagg, \"Measurement of the Resistance of an Earth-Electrode
System Covering a Large Area,\" IEEE Proceedings, Volume 116, March
1969, pp. 475-479.

8\. \"Getting Down to Earth . . .,\" Manual 25T, James G. Biddle, Co.,
Plymouth Meeting, Pennsylvania, October 1970.

9\. \"Guide for Measuring Earth Resistivity, Ground Impedance, and Earth
Surface Potentials of a Ground System,\" IEEE Standard 81-1983, IEEE,
New York.

10\. \"Standard Dictionary of Electrical and Electronics Terms\"
ANSI/IEEE Standard 100-1988, IEEE, New York.

[APPENDIX A CONVERSION TABLES]{.underline}

(English to Metric)

**1. GENERAL**

**1.1** **[Introduction]{.underline}:** The tables provided in this
Appendix may be used for converting measurements from English units to
metric so converted values may be used in the equations found in
Appendix B.

[TABLE A1: COMMONLY USED MEASUREMENTS]{.underline}

[English]{.underline} [Metric]{.underline}

1/2 inch → 1.3 centimeters

5/8 inch → 1.6 centimeters

5 feet → 1.5 meters

8 feet → 2.4 meters

10 feet → 3.0 meters

20 feet → 6.1 meters

30 feet → 9.1 meters

40 feet → 12.2 meters

[TABLE A2: ENGLISH TO METRIC CONVERSIONS]{.underline}

**Multiply**

**[To convert from by To get]{.underline}**

Feet 0.3048 Meters

Inches 2.54 Centimeters

Square Feet 0.09290 Square Meters

**1.2** **[Example:]{.underline}** The following example illustrates how
one can convert from English to Metric units and use the equations in
Appendix B. A 30 x 40 foot rectangular ring configuration is installed.
A site survey has been preformed and the soil resistivity has been
measured at 100 ohm·meters. The ground rods used in this system are
5/8 inch x 10 feet and are spaced 10 feet apart. The rods are connected
by #2 g auge wire, which has a diameter of 0.2576 inches. The grounding
system is buried 2 feet deep.

**1.2.1** Determining the resistance of the ground rods: Before one can
use Equation B5 of Appendix B to find the resistance of the rods, one
has to convert the electrode length, diameter, and

spacing to Metric units. Using Table A1 one finds that the rods are
3.0 meters in length, have a diameter of 1.6 centimeters, and are spaced
3.0 meters apart. Now Equation 5 can be used to find that the resistance
of the ground rods is 4.00 ohms.

**1.2.2** Determining the buried wire resistance: Table A2 is used to
convert the, English units wire length, diameter, and depth to metric
units. To convert the wire length from English to Metric, multiplying
the wire length, 140 feet, by 0.3048 to get 43 meters. Next, the wire
diameter, 0.2576 inches, is multiplied by 2.54 to obtain
0.6543 centimeters. The wire depth, 2 feet, is converted in the same
manner as wire length and found to be 0.6 meters. Next, these metric
values are substituted into Equation B8 of Appendix B and one finds that
the buried wire has a resistance of 2.30 ohms.

**1.2.3** Determining the mutual and total resistance: Since the rod
length and wire length have already been converted, one can go straight
to Equation B16 of Appendix B to calculate the mutual resistance of the
grounding system, 2.89 ohms. Using Equation B18 one can calculate the
total resistance of the system, 1.59 ohms.

[APPENDIX B GROUNDING EQUATIONS]{.underline}

**1. GENERAL**

**1.1** **[Introduction]{.underline}:** The equations provided in this
Appendix may be used for calculating the approximate anticipated
resistance-to-ground of various grounding configurations. These
calculations allow an engineer to determine the probability for
obtaining the desired objective resistance-to-ground at a location prior
to electrode installation. The equations are based on the assumption
that the soil is homogeneous. Even though soil is seldom of uniform
resistivity, the calculated results are close enough to determine if
additional electrodes should be driven or another design studied.

**1.2** **[A Site Survey]{.underline}** should first be completed to
determine the area available for installation of the grounding system.
(Refer to Paragraph 7.2 of Bulletin text). Measurement of soil
resistivity throughout the available area provides the average
resistivity of the soil for use in the calculations. (Refer to
Appendix C).

**1.3** **[The Equations]{.underline}** are derived from those published
by Erling D. Sunde, \"Earth Conduction Effects on Transmission
Systems,\" Dover Publications, Inc., New York, 1968 and ANSI/IEEE Std.
80-1986. Equations have been simplified to permit use of electrode sizes
in units commonly used for identification, i.e., 2.5 cm x 3 m. ground
rod.

**2. VERTICAL GROUND ELECTRODES (ROD OR PIPE)**

**2.1 [Single Vertical Ground Electrode]{.underline}:** The equation for
calculating the approximate resistance-to-ground of a single vertical
ground electrode driven into the earth is:

$R_{r} = \frac{\rho}{2\pi\ell r}\text{ln}\frac{\text{294}\text{.}\text{30}\ell_{r}}{d_{r}}$
(B1)

Where: Rr = Electrode resistance-to-ground, ohms

ρ = Earth resistivity, ohmmeters

r = Electrode length, meters

dr = Electrode diameter, centimeters

π = 3.1416

**2.2 [Multiple Vertical Ground Electrodes]{.underline}:** Multiple
ground electrode systems can be placed in several configurations, such
as straight line, circular or rectangular ring, or rectangular bed. The
individual electrodes contained in a system may be interconnected with
either insulated or bare conductors. Equations (B2), (B3), (B4) and (B5)
apply to multiple electrode systems interconnected with insulated
conductors. The equations are based on the assumption that the
electrodes are uniformly spaced.

**2.2.1 [Multiple Vertical Ground Electrodes in a Straight
Line]{.underline}:** There are two possible conditions relative to
parallel grounding electrodes in a straight line:

1\. The distance between the electrodes is equal to or

greater than the length of the electrodes.

2\. The distance between the electrodes is less than the

length of the electrodes.

Because of these conditions there are two equations for calculating the
resistance-to-ground for multiple electrodes in a straight line.

**2.2.1.1** When the distance between the electrode (S) is equal to or
greater than () the length of the electrode, the approximate
resistance-to-ground may be calculated by the following equation:

S = r

$R_{R}\  = \ \frac{1}{n}\left( R_{r\ } + \ \frac{\rho}{\pi S}\ (\frac{1}{2}\  + \ \frac{1}{3}\  + \ \text{.}\text{.}\text{.}\  + \ \frac{1}{n}) \right)$
(B2)

Where: RR = Parallel electrodes resistance-to-ground, ohms

n = Number of electrodes

Rr = Single electrode resistance-to-ground, ohms

(From Equation (B1), Paragraph 2.1)

ρ = Earth resistivity, ohm·meters

r = Electrode Length, meters

S = Spacing between electrodes, meters

π = 3.1416

**2.2.1.2** The approximate parallel resistance-to-ground, where the
distance (S) between the electrodes is less than the length () of the
electrode may be calculated by the following equation:

S \< r

$R_{R}\  = \ \frac{1}{n}\ (R_{r}\  + (n - 1)\ R_{m})\ \text{ohms}$ (B3)

Where: RR = Parallel electrodes resistance-to-ground, ohms

n = Number of electrodes

Rr = Single electrode resistance-to-ground, ohms

(From Equation (B1), Paragraph 2.1)

Rm = Mutual resistance between electrodes, ohms

(From Equation (B4) below)

r = Electrode length, meters

The mutual resistance between two ground electrodes at a designated
spacing for completing equation (B3) may be calculated by the following
equation:

$R_{m}\  = \ \frac{\rho}{2\pi\ell_{r}}\text{ln}\frac{1\text{.}\text{475}\ell_{r}}{S}$
(B4)

Where: Rm = Mutual resistance between electrodes, ohms

ρ = Earth resistivity, ohm·meters

r = Electrode length, meters

S = Electrode spacing, meters

π = 3.1416

**2.2.2 [Multiple Vertical Ground Electrodes in a Circular
Ring]{.underline}:** The equation for calculating the approximate
resistance-to-ground of a grounding system with multiple vertical ground
electrodes placed in a circle is:

$R_{R}\  = \ \frac{\rho}{2\pi n\ell_{r}}\ \left( \text{ln}\ \frac{\text{294}\text{.}3\ell_{r}}{d_{r}} + \frac{2\ell_{r}}{S} + \ \text{ln}\ \frac{2n}{\pi} \right)$
(B5)

Where: RR = Parallel electrode resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

n = Number of electrodes

r = Electrode length, meters

dr = Electrode diameter, centimeters

S = Electrode spacing, meters

π = 3.1416

**2.2.3 [Multiple Vertical Ground Electrodes in a Square or Rectangular
Ring]{.underline}:** Calculation of resistance-to-ground for these
configurations is the same as for a circular ring in Paragraph 2.2.2.

**2.2.4 [Multiple Vertical Ground Electrodes in a Rod
Bed]{.underline}:** The approximate resistance-to-ground of a grounding
system with multiple vertical ground electrodes arranged in a rod bed
may be calculated by:

$R_{\text{GR}}\  = \ \frac{\rho}{2\pi n\ell_{r}}\left( \text{ln}\frac{\text{294}\text{.}\text{30}\ell_{r}}{d_{r}} + \ \frac{2K_{1}\ell_{r}}{\sqrt{A}}\ {(\sqrt{n}\  - 1)}^{2} \right)$
(B6)

Where: RGR = Parallel electrodes resistance-to-ground, ohms

ρ = Earth resistivity, ohmÊmeters

n = Number of electrodes

r = Electrode length, meters

dr = Electrode diameter, centimeters

A = Area covered by electrodes, square meters

K1 = Coefficient from Figure 1, below

π = 3.1416

t

X - For Depth h ˜ 0

Y - For Depth h ? 0.1$\sqrt{A}$

Z - For Depth h ? 0.17$\sqrt{A}$

**3. BURIED BARE WIRE**

**3.1 [Bare Wire Buried in a Straight Line]{.underline}:** The equation
for calculating the approximate resistance-to-ground of a single bare
wire buried in a straight line is:

$R_{w}\  = \ \frac{\rho}{\pi\ell_{w}}\text{ln}\ \frac{7\text{.}\text{3576}\ell_{w}}{\sqrt{d_{w}h}}$
(B7)

Where: Rw = Bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

w = Wire length, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

π = 3.1416

**3.2 [Bare Wire Buried in a Ring Configuration]{.underline}:** There
are two equations available for calculation of the approximate
resistance-to-ground for a bare wire buried in a ring configuration. The
equations produce identical results when the same parameter values are
used. Both equations are included in this practice. One, Equation (B8),
pertains to a square or rectangular ring since it is based on the wire
length (perimeter of the ring). The second, Equation (B9), pertains to a
circular configuration since it is based on the ring diameter.

**3.2.1** Bare wire buried in a square or rectangular ring: The
approximate resistance-to-ground of a single bare wire buried in a ring
may be calculated by the following equation:

$R_{w}\  = \ \frac{\rho}{\pi\ell_{w}}\text{ln}\ \frac{\text{12}\text{.}\text{732}\ell_{w}}{\sqrt{d_{w}h}}$
(B8)

Where: Rw = Bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

w = Wire length, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

**3.2.2 [Bare wire in a circular ring]{.underline}:** Another equation
for calculating the approximate resistance-to-ground of a single bare
wire buried in a ring is:

$R_{w\ }\  = \ \frac{\rho}{2\pi^{2}\ D}\ \left( \text{ln}\ \frac{\text{800}D}{d_{w}}\  + \ \text{ln}\ \frac{2D}{h} \right)$
(B9)

Where: Rw = Bare wire resistance-to-ground, ohms

D = Diameter of circular ring, meters

ρ = Earth resistivity, ohm·meters

dw = Wire diameter, centimeters

h = Wire depth, meters

π = 3.1416

**3.3 [Bare Wire Buried in a Square or Rectangular Grid]{.underline}:**
The approximate resistance-to-ground of a grounding system with bare
wire buried in a square or rectangular grid can be calculated by the
following equation:

$R_{w}\  = \ \frac{\rho}{\pi\ell_{w}}\ \left( \text{ln}\ \frac{\text{20}\ell_{w}}{\sqrt{d_{w}h}}\  + \ \frac{K_{1}\ell_{w}}{\sqrt{K}}\  - \ K_{2} \right)$
(B10)

Where: RGW = Bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

w = Total wire length in grid, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

A = Area covered by wire, square meters

K1 = Coefficient from Figure B1

K2 = Coefficient from Figure B2, below

π = 3.1416

t

X - For Depth h ˜ 0

Y - For Depth h ? 0.1$\sqrt{A}$

Z - For Depth h ? 0.17$\sqrt{A}$

**3.4 [Radial Bare Buried Wires]{.underline}:** There are a series of
equations for calculating the resistance-to-ground of radial wire

grounding systems. The equations are for three-branched, four-branched,
six-branched, and six- or more branched stars.

**3.4.1 [Radial Bare Buried Wires, Three-Branched Star]{.underline}:**
The equation for calculating the approximate resistance-to-ground of a
grounding system with three bare buried wires extending radially from a
point is:

$R_{\text{WN}}\  = \ \frac{\rho}{\text{18}\text{.}\text{85}\ell_{w}}\ \left( \ \text{ln}\ \frac{\text{1154}\text{.}5\ell_{w}^{2}}{d_{w}h} \right)$
(B11)

Where: RWN = Radial bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

w = Wire length of single radial branch, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

**3.4.2 [Radial Bare Buried Wires, Four-Branched Star]{.underline}:**
The approximate resistance-to-ground of a grounding system with four
bare buried wires extending radially from a point may be calculated by
the following equation:

$R_{\text{WN}}\  = \ \frac{\rho}{\text{25}\text{.}\text{133}\ell_{w}}\ \left( \ \text{ln}\ \frac{\text{7416}\text{.}5\ell_{w}^{2}}{d_{w}h} \right)$
(B12)

Where: RWN = Radial bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

w = Wire length, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

**3.4.3 [Radial Bare Buried Wires, Six-Branched Star]{.underline}:** The
equation for calculating the approximate resistance-to-ground of a
grounding system with six bare buried wires extending radially from a
point is:

$$R_{\text{WN}}\  = \ \frac{\rho}{\text{37}\text{.}\text{70}\ell_{w}}\ \left( \ \text{ln}\ \frac{\text{381356}\ell_{w}^{2}}{d_{w}h} \right)$$

Where: RWN = Radial bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

w = Wire length, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

**3.4.4 [Radial Bare Buried Wires, Six- and Greater Branched
Stars]{.underline}:** The equation for calculating the approximate
resistance-to-ground of a grounding system with six or more bare buried
wires extending radially from a point is:

$$R_{\text{WN}}\  = \ \frac{\rho}{n\pi\ell_{w}}\ \left( \ \text{ln}\ \frac{7\text{.}\text{3576}\ell_{w}}{\sqrt{d_{w}h}}\  + \ 1\text{.}\text{228}(n - 1) - \text{ln}\ n\  \right)$$

Where: RWN = Radial bare wire resistance-to-ground, ohms

ρ = Earth resistivity, ohm·meters

n = Number of radial wires

w = Wire length of single radial branch, meters

dw = Wire diameter, centimeters

h = Wire depth, meters

π = 3.1416

**4. MUTUAL RESISTANCE**

**4.1** **[Introduction]{.underline}:** When a multiple vertical ground
electrode system is interconnected with bare buried conductors, the
total resistance-to-ground (RT) is influenced by the mutual resistance
(RWR) between the vertical electrodes and the bare wire.

**4.2 [Multiple Vertical Ground Electrodes in a Straight
Line]{.underline}:** The mutual resistance between multiple vertical
ground electrodes in a straight line and bare buried interconnecting
conductors may be calculated by the equation:

$R_{\text{WR}}\  = \ \frac{\rho}{\pi\ell_{w}}\ \text{ln}\ \frac{2\ell_{w}}{\ell_{r}}$
(B15)

Where: RWR = Mutual resistance between electrodes and wire,

ohms

ρ = Earth resistivity, ohm·meters

w = Interconnecting wire length, meters

r = Electrode length, meters

π = 3.1416

**4.3 [Multiple Vertical Ground Electrodes in a Ring]{.underline}:** The
equation for calculating the mutual resistance between multiple vertical
ground electrodes in a circular, square or rectangular ring and a bare
buried interconnecting conductor is:

$R_{\text{WR}}\  = \ \frac{\rho}{\pi\ell_{w}}\ \text{ln}\ \frac{3\text{.}\text{461}\ell_{w}}{\ell_{r}}$
(B16)

Where: RWR = Mutual resistance between electrodes and wire,

ohms

π = Earth resistivity, ohm·meters

w = Interconnecting wire length, meters

r = Electrode length, meters

π = 3.1416

**4.4 [Multiple Vertical Ground Electrodes in a Rod Bed]{.underline}:**
The mutual resistance between a rod bed of multiple vertical ground
electrodes and an interconnecting grid of bare buried conductors may be
calculated by the following equation:

$R_{\text{GWR}}\  = \ \frac{\rho}{\pi\ell_{w}}\ \left( \text{ln}\ \frac{5\text{.}\text{4366}\ell_{w}}{\ell_{r}}\  + \ \frac{K_{1}\ell_{w}}{\sqrt{A}}\  - K_{2} \right)$
(B17)

Where: RGWR = Mutual resistance between electrodes and wire,

ohms

ρ = Earth resistivity, ohm·meters

w = Total wire length in grid, meters

r = Electrode length, meters

K1 = Coefficient from Figure B1

K2 = Coefficient from Figure B2

A = Area covered by wire, square meters

π = 3.1416

**5. COMBINED RESISTANCE, VERTICAL AND HORIZONTAL ELECTRODES**

**5.1** **[The Equation]{.underline}** for calculating the approximate
combined resistance-to-ground of a grounding system with multiple
vertical electrodes interconnected by bare buried wire is:

$R_{T}\  = \ \frac{R_{W}R_{R}\  - \ R_{\text{WR}}^{2}}{R_{W}\  + \ R_{R}}$
(B18)

Where: RT = Aggregate resistance of wire and electrodes,

ohms

RW = Bare wire resistance-to-ground, ohms

(From Equations (B7), (B8), (B9), or (B10), as

appropriate)

RR = Parallel vertical electrodes resistance to

ground, ohms

(From Equations (B2), (B3), (B5), or (B6), as

appropriate)

RWR = Mutual resistance between wire and electrodes,

ohms

(From Equations (B15), (B16), or (B17), as

appropriate)

**5.2** **[Guide for Calculating the Approximate Resistance-to-ground of
Various Grounding System Configurations]{.underline}:**

Electrode Straight Square or Rod bed

[Configuration]{.underline} [Line]{.underline} [Circular]{.underline}
[Rectangle]{.underline} [and Grid]{.underline}

Find RW from Equation (B7) (B8), (B9) (B8) (B10)

Find RR from Equation (B2), (B3) (B5) (B5) (B6)

Find RWR from Equation (B15) (B16) (B16) (B17)

Find RT from Equation (B18) (B18) (B18) (B18)

[APPENDIX C MEASUREMENT OF SOIL RESISTIVITY]{.underline}

**1. GENERAL**

**1.1** **[Soil Resistivity Measurements]{.underline}** are commonly
made with a test instrument that uses the four-terminal fall of
potential method. The test instrument has four terminals that are
connected to four electrodes arranged at equal distances along a
straight line (shown in [Figure C1]{.underline}: \"Four-Terminal\"
Method for Measurement of Soil Resistivity). Internally the instrument
contains a current circuit and a voltage circuit. The current source can
be a hand-driven a.c. generator or a voltage reversing vibrator that
causes a current to flow between the two outer electrodes (Terminals C1
and C2). A potential is measured between the inner electrodes
(Terminals P1 and P2). The voltage and current circuits are coupled
within the test set to provide a reading in ohms.

**1.2** **[The Theory for This Measurement]{.underline}** was developed
by Dr. Frank Wenner of the U.S. Bureau of Standards in 1915 and
published in Report No. 258, Bulletin of Bureau of Standards, Vol. 12,
No. 3, October 11, 1915, \"A Method of Measuring Earth Resistivity.\"
Dr. Wenner established that, if the test electrode depth is small
compared to the distance between the electrodes, the following equation
applies to determine the average soil resistivity to a depth equal to
the distance between the electrodes:

ρ = 2πAR = 6.28AR

Where: q = Average soil resistivity to depth equal to A, in

ohm·centimeters

π = 3.1416

A = Distance between electrodes, in centimeters

R = Test instrument resistance reading, in ohms

Note: Divide ohm·centimeters by 100 to convert to ohm·meters.

**1.3** **[Ground Test Instruments]{.underline}** generally use an
alternating voltage source with a frequency not related to power system
fundamental frequencies or their harmonics. This avoids the effects of
polarization and foreign earth currents which could produce erroneous
results.

**1.4** **[Objectives of Soil Resistivity Measurements]{.underline}:**
The first is to determine the type of earth connection required to
provide the objective resistance to earth. The second is to define any
geological limitations that might be present, such as a rock layer, that
would restrict installation of the grounding system.

**2. BASIC SOIL RESISTIVITY MEASUREMENT**

**2.1** **[Introduction]{.underline}:** The depth to which the average
soil resistivity is desired determines the distance (A) between the test
electrodes. This distance will typically be the length of the ground
electrode to be installed plus the depth below the earth\'s surface to
which it will be driven. A measurement should be taken with test
electrode spacings of one-half, one, two and four times the length of
the proposed ground electrode. This will identify the presence of large
deviations in the soil resistivity. Place four test electrodes along a
base line in relation to the proposed vertical ground electrode location
as shown in Figure C2: \"Four-Terminal\" Method for Measuring soil
Resistivity. The test electrodes should be driven into the soil to a
depth equal to A/20. Depths for test electrodes for various distances
(A) are shown in Table C-I.

[TABLE C-I]{.underline}

[Test Electrode Depths for Various Distances Between
Electrodes]{.underline}

Distance Between Electrodes (A) Test Electrode Depth (B)

[Feet]{.underline} [Meters]{.underline} [Inches]{.underline}
[Centimeters]{.underline}

5 1.52 3.0 8

8 2.44 5.0 13

10 3.05 6.0 15

16 4.88 10.0 25

20 6.10 12.0 30

30 9.14 18.0 46

40 12.19 24.0 61

50 15.4 30.0 76

**2.2** **[Performing the Measurement]{.underline}:** Connect the leads
from the four test electrodes to the proper terminals on the test set,
C1, P1, P2 and C2. Complete the measurement as described by the
manufacturer of the test equipment. Calculate the soil resistivity by
the equation in Paragraph 1.2 and record results.

[APPENDIX D MEASUREMENT OF RESISTANCE-TO-GROUND]{.underline}

**1. GENERAL**

**1.1** **[Introduction]{.underline}:** Measurement of a new grounding
system\'s resistance-to-ground is needed to determine if the design
criteria have been met. Measurement is essential when the grounding
system has been installed for the protection of sensitive electronic
equipment, such as, digital central offices, line concentrators, carrier
terminals and carrier repeaters.

**1.2** **[Remeasuring Existing Grounding Systems]{.underline}:** The
resistance-to-ground of existing grounding systems should be
periodically measured to determine if the system continues to meet the
original design limits. Where the resistance-to-ground is found to have
increased significantly, expansion of the system may be desirable to
restore the original effectiveness.

**2. METHODS**

**2.1** **[Introduction]{.underline}:** There are three methods for
measuring grounding system resistance-to-ground. They are the
triangulation, direct (two terminal), and fall-of-potential (three
terminal) methods.

**2.2 [Triangulation method]{.underline}:** The series resistance of the
electrode under test (RX) and the auxiliary electrodes (Ra, Rb) are
measured two at a time as illustrated in Figure D1: Triangulation Method
for Measuring the Resistance of a Ground Electrode. The unknown
resistance can then be calculated by the formula:

$R_{x}\  = \ \frac{\left( R_{x}\  + \ R_{a} \right)\  + \ \left( R_{x}\  + \ R_{b} \right)\  - \ \left( R_{a}\  + \ R_{b} \right)}{2}$

The series resistances may be measured with a bridge, ohmmeter, or a
voltmeter. The current source may be either alternating or direct
current. The auxiliary electrodes should have a resistance-to-ground on
the same order of magnitude as the unknown for accuracy. The electrodes
have to be some distance apart to avoid errors, such as zero or negative
resistances, in the calculations. The recommended distance between each
pair of the three separate ground electrodes, when measuring a single
driven electrode, should be 10m or more. For larger areas the minimum
spacing should be on the order of the dimensions of the grounds.

**2.3 [Direct method (two terminal)]{.underline}:** This technique
utilizes an instrument with four terminals (P1, P2, C1, C2) for ground
resistance tests. The instrument includes: (1) a voltage source, (2) an
ohmmeter to measure resistance directly, and (3) a switch or switches to
change the resistance range. The voltage source can be either a
hand-driven a.c. generator or a voltage reversing vibrator. Connect
terminals P1 and C1 to the ground electrode to be measured and connect
terminals P2 and C2 to an all-metallic water-pipe system as illustrated
in Figure D2: Direct Method for Measuting the Resistance of a Ground
Electrode. (The resistance-to-ground of a metallic water system covering
a large area should be less than one ohm.) The instrument reading can be
accepted as being the resistance-to-ground of the electrode under test.

Although the direct method of measuring two electrodes in series is the
simplest way to make a ground resistance test, it has important
limitations. The water-pipe system has to be extensive enough to have
only negligible resistance-to-ground. The water-pipe system has to be
entirely metallic without insulating couplings or flanges. The electrode
being tested has to be located far enough from the water-pipe system to
be outside its sphere of influence.

NOTE: \"Getting Down-to-Earth\", Manual on Earth-Resistance Testing for
the Practical Man, published by the James G. Biddle Company indicates
the distance from the electrode to the water-pipe system should be about
ten times the radius of the electrode or grid to provide a measurement
accuracy of ± 10 percent.

**2.4 [Fall-of-Potential Method (Three terminal)]{.underline}:** The
three-terminal test, illustrated in Figure D3: Fall of Potential Method,
is the method most commonly used to measure the electrode
resistance-to-ground. The P1 and C1 terminals of the test instrument are
connected together and to the electrode being tested. A reference
electrode C2 should be driven into earth as far as practical from the
electrode being tested. The distance between the electrode under test
and the reference electrode may be limited by the length of wire
available or the physical characteristics of the surrounding area.
Reference electrode P2 is then driven at a number of points along a
straight line between the ground electrode and C2. Resistance readings
are taken and recorded for each point. A curve of resistance-to-ground
versus distance can then be plotted similar to Figure D4: Example of
Earth Resistance Curve. The correct resistance-to-ground is shown on the
curve at a distance about 62 percent of

the total distance from the ground electrode to C2.

**2.4.1** Reference electrode location: The distance between the
reference electrode C2 and the electrode being tested determines the
accuracy of the test results. When the electrodes are located far enough
apart so that the earth shells surrounding them do not overlap the
resistance versus distance curve will flatten as shown in Figure D5:
Effect with C2 Located far from Earth Electrode. A value very close to
the actual resistance-to-ground can be found at the point within this
plateau along the curve representing 62 percent of the total distance.
The value will be erroneous only where soil conditions at the 62 percent
point vary significantly from those at other points. The objective is to
get a degree of flatness along the curve that will provide easy
identification of such variations.

When the electrodes are located so close that the earth shells overlap,
as illustrated in Figure D6: Effect with C2 Located Close to Earth
Electrode, the leveling of the curve does not occur. The shells
surrounding reference electrode (C2) add to the shells around the ground
electrode and the resistance increases linearly.

**2.4.2** Table D1 (see next page) provides the minimum distance between
the reference electrode (C2) and the ground electrode system. The
distance from the ground electrode to reference electrode (P2) that is
about 62 percent of the distance to C2 is also shown. The maximum
dimension is determined as follows: When the grounding system contains
two or more electrodes along a straight line, the maximum dimension is
the distance between the first and last electrode. The maximum dimension
for a grounding system containing electrodes in a circular ring
configuration is the diameter of the circle. The diagonal distance
across a grounding system having a square or rectangular form is the
maximum dimension.

**3. GROUNDING SYSTEMS OF LARGE AREA**

**3.1** **[Problems with Measuring Grounding System]{.underline}:** The
primary problem is the need for placing the reference electrode (C2) at
a considerable distance from the ground electrode system. In addition to
the necessity of transporting sufficient wire to reach this electrode
and the corresponding length for reference electrode (P2) there is often
difficulty in findings clear path of the required length from the
grounding system. G. F. Tagg developed a technique for these
measurements in which such long leads are not necessary.

[TABLE D-I]{.underline}

REFERENCE ELECTRODE LOCATION

Maximum Distance to Distance to

Dimension, Ft. (Meters) P2 Ft. (Meters) C2, Ft. (Meters)

6 (1.8) 80 (24.4) 125 (38.1)

8 (2.4) 90 (27.4) 140 (42.7)

10 (3.0) 100 (30.5) 160 (48.8)

12 (3.7) 105 (32.0) 170 (51.8)

14 (4.3) 120 (36.6) 190 (57.9)

16 (4.9) 125 (38.1) 200 (61.0)

18 (5.5) 130 (39.6) 210 (64.0)

20 (6.1) 140 (42.7) 220 (67.1)

40 (12.2) 200 (61.0) 320 (97.5)

60 (18.3) 240 (73.2) 390 (118.9)

80 (24.4) 280 (85.3) 450 (137.2)

100 (30.5) 310 (94.5) 500 (152.4)

120 (36.6) 340 (103.6) 550 (167.6)

140 (42.7) 365 (111.3) 590 (179.8)

160 (48.8) 400 (121.9) 640 (195.1)

180 (54.9) 420 (128.0) 680 (207.3)

200 (61.0) 440 (134.1) 710 (216.3)

100 (30.5) 310 (94.5) 500 (152.4)

120 (36.6) 340 (103.6) 550 (167.6)

140 (42.7) 365 (111.3) 590 (179.8)

160 (48.8) 400 (121.9) 640 (195.1)

180 (54.9) 420 (128.0) 680 (207.3)

200 (61.0) 440 (134.1) 710 (216.3)

**3.2** **[The Measurement Method]{.underline}** is presented in this
practice since there may be locations where it will be desirable to use
it when measuring systems of moderate area. Depending on the location of
the grounding system, a clear path may not be available for the
reference electrodes and associated wire. The technique described below
may be utilized to complete the desired measurements.

**3.3** **[The Basis of the Technique]{.underline}** is to obtain ground
resistance curves for several reference electrode spacings. Then,
assuming a number of successive positions for the electrical center of
the system, intersection curves are constructed which will give the
earth resistance and the position of the electrical center.

**3.3.1** Assume that all measurements are made from an arbitrary
starting point 0 along the perimeter of the grounding system. The
distance C to the reference electrode (C2) and the variable

distance P to the reference electrode (P2) are measured from point 0. A
curve such as abc in [Figure D7]{.underline}: Earth Resistance Curve for
Large Area Example can be constructed giving the measured resistance
against the value of P. Further, assume the electrical center of the
ground electrode system is point D, at distance X from point 0. The true
distance from the center to C2 is then C + X and the true resistance is
obtained when P2 is at a distance 0.618(C+X) from D. Thus, the value of
P, measured from 0, is 0.618(C+X)-X. If X is given a number of values
the corresponding values of P can be calculated and the resistance read
off the curve. These resistances can be plotted against the values of X
in another curve. When this procedure is repeated for a different value
of C, and another curve of resistance against X plotted, the two curves
should intersect at the required resistance. The procedure can be
repeated for a third value of C as a check. These curves are called
intersection curves. The electrical center (D) is assumed to be an
extension of the testing line (0, C2). Even where this is not the case
only a small error results which is not important.

**3.3.2** Large Area Example: The ground system covers an area 300 ft. x
250 ft. (91 m x 76 m) and consists of a number of ground electrodes
bonded together by copper cables. The testing line extends from a point
approximately halfway along one side with reference electrode placed at
a distance of 400 (122), 600 (183), 800 (244), and 1000 (305) feet
(meters). The resulting curves are shown in Figure D8: Earth Resistance
Curve for Large Area Example. Applying the described method produced the
intersection curves given in Figure D9: Intersection Curves for Figure
D8. The center of the triangle formed by these curves gives the ground
resistance as about 0.146 ohms.

**3.4** **[The Purpose of this Method]{.underline}** is to reduce the
distance from C1 to reference electrode C2 which appears to have been
achieved. There are some additional points to be noted. The distance to
electrode C2 has certain limits. If the grounding system is in the form
of a square, the minimum distance to C2 should not be less than the side
of the square. On the other hand, the maximum distance should not be too
great or the resulting curve is very flat and the intersection point
will be rather indefinite. Again, for a square system, the maximum
distance should not exceed twice the side of the square. For other
shapes of ground electrode systems, suitable minimum and maximum values
for the distance to C2 are based on judgement.

[APPENDIX E CONCRETE ENCASED ELECTRODES]{.underline}

**1. GENERAL**

**1.1** **[Introduction]{.underline}:** A grounding conductor encased in
the concrete footing of a small central office building may provide an
effective grounding system. This type grounding system is sometimes
called a \"Ufer\" ground. Encased conductors should be considered at
locations where there is not sufficient space available for installation
of a ring ground outside the building. They also provide an alternative
where a rock substructure will not permit installation of vertical
electrodes.

**1.2** **[Preventing Structural Damage]{.underline}:** The grounding
conductor may be a copper wire, which is placed in the concrete footing
specifically for grounding, or the reinforcing steel bars provided in
the footing for strength. Reinforcing bars should be used only where the
electrical continuity of the entire reinforcing steel network has been
assured by metal fusing techniques at all contact points. Failure to
provide these solid bonds can result in structural damage during a
lightning stroke due to side flash between the bars.

**1.3** **[Avoid Moisture Barriers]{.underline}:** There should be no
moisture barrier between the concrete footing and the surrounding earth.
Such a barrier will interrupt the path to remote earth and severely
degrade the grounding system.

**1.4** **[Another path to ground]{.underline}** is provided through the
steel reinforcing bars in the concrete floor of a building. Since these
bars are usually near the earth\'s surface in a single story building,
they will not be as effective as those in the building footings, which
are deeper. Further, the mutual effects between the bars in the floor
and in the footings will almost offset the benefits gained in
resistance-to-ground from the floor bars.

**2. CALCULATION OF RESISTANCE-TO-GROUND**

**2.1** **[Equation]{.underline}:** The approximate resistance-to-ground
for a grounding electrode encased in the concrete footing of a building
may be calculated in Metric units by the equation:

$$R_{c}\  = \ \frac{1}{\pi\ell_{w}}\left( \rho_{0}\text{ln}\ \frac{\text{12}\text{.}\text{732}\ell_{w}}{\sqrt{d_{w}h}}\  - \ \rho_{0}\text{ln}\ \frac{\text{12}\text{.}\text{732}\ell_{w}}{\sqrt{d_{c}h}}\  + \ \rho_{1}\text{ln}\frac{\text{12}\text{.}\text{732}\ell_{w}}{\sqrt{d_{c}}h} \right)$$

Where: Rc = Resistance of Ufer ground, ohms.

ρ0 = Resistivity of concrete, ohm·meters.

ρ1 = Resistivity of soil, ohm·meters.

w = Length of grounding conductor, meters.

dw = Diameter of grounding conductor, centimeters.

dc = Diameter of concrete cylinder, centimeters.

h = Depth of grounding conductor, meters.

π = 3.1416

**2.3** **[Resistance of Various Sized Areas]{.underline}:** The
calculated approximate resistance-to-ground of concrete encased ring
systems surrounding differently sized areas with various soil
resistivities is shown in Figure E1: Concrete Encased Electrodes in a
Square Ring Configuration. These curves may be used to determine if a
Ufer ground in the concrete footing of a new building will meet the
resistance objectives when the building area and average soil
resistivity are known.

**3. CHARACTERISTICS**

**3.1 [Resistivity of Concrete]{.underline}:** The resistivity of
concrete has been estimated to be in the range of 30 to 300 ohm·meters,
according to values published by several investigators. The majority of
them favor a range from 30 to 100 ohm·meters. A value of 50 ohm·meters
is recommended for calculations where the actual concrete resistivity is
unknown.

**3.2 [Diameter of Concrete]{.underline}:** A typical building footing
is illustrated in Figure E2: Concrete Footing. For clarity only a single
horizontal wire is shown in the footing. For purposes of approximation,
the effective surface of the concrete is assumed to be equivalent to a
cylinder having a diameter equal to the shortest measurement, height or
width, of the footing. The basic elements for calculating the
resistance-to-ground of a concrete encased ring electrode are shown in
Figure E3: Basic Elements for Calculation.

**3.3** **[The Size of the Encased Conductor]{.underline}** is not a
major factor in the resulting resistance-to-ground with an encased
electrode. Results of calculations, using #2 and 2/0 conductors in
encased grounding systems, have been plotted in Figure E4: Variation
with Wire Size. The curves show the ground resistance with a 2/0

conductor is only slightly lower than that of a #2 conductor for all
values of soil resistivity. The installation of a 2/0 conductor thus
cannot be justified for reduction of the resistance-to-ground.

**3.4** **[Depth of Burial]{.underline}** is also not a significant
factor in the value of resistance-to-ground with encased electrodes.
Figure E5: Variation with Depth for Concrete Encased Electrodes shows
plotted curves of calculated resistance-to-ground for a #2 encased
grounding conductor buried at depths of 2, 4, and 10 feet (0.6, 1.2 and
3.0 meters). The lower resistance-to-ground obtainable with deeper
burial is considered significant only where the reduction will provide a
grounding system meeting the desired objective. The curves show that
this is not the case for most values of soil resistivity.

**3.5** **[Comparison Between Concrete Encased Electrode and
Direct-Buried Electrode]{.underline}:** The resistance-to-ground of the
direct-buried electrode is lower than that of the encased electrode when
the soil resistivity is lower than the concrete\'s resistivity and vice
versa. When the resistivity of both soil and concrete are the same, the
resistance-to-ground of both electrodes will be the same. The concrete
encased electrode is the preferred design in areas of higher soil
resistivity. The curves in Figure E6: Comparsion between Encased and
Direct Buried Electrodes with Different Concrete Resistivities
illustrate the comparison of resistances to ground between an electrode
buried directly in the earth and concrete encased electrodes with
different concrete resistivities. These curves show that at the higher
concrete resistivity values the difference between the
resistance-to-ground of a direct buried and concrete encased electrode
is significant.

**3.6** **[The Effect of Concrete Footing Size]{.underline}:** The
curves in Figure E7: Variation with concrete Diameter for Concrete
Encased Electrodes provide a comparison of the calculated
resistance-to-ground between electrodes encased in differently sized
concrete footings. The curves show that when the concrete\'s resistivity
and that of the surrounding earth are identical, the
resistance-to-ground of all four configurations is the same. When the
resistivity of the surrounding soil is less than that of the concrete,
the conductor encased in concrete of the least volume will have the
lowest resistance-to-ground. Conversely the conductor encased in
concrete having the greatest volume will have the lowest
resistance-to-ground when the resistivity of the surrounding soil is
higher than that of the concrete footing.