1 Grounding and Ground Testing
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Grounding and Ground Testing
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GROUND RESISTANCE TESTING
What is a ground?
“An electrical connection intentionally made
between an electrical body or system and a
metallic body in the earth.”
Source: NBS Technical Paper 108; June, 1918
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Low resistance Connection to Earth to
Drain Away Energy and Engage Protective
Devices
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GROUND RESISTANCE TESTING
What is a ground?
“A conducting connection, between an electrical
circuit or equipment and the earth, or to some
conducting body that serves in place of the
earth.”
Source: NFPA 70-1981; National Electrical Code
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Ability to Drain Away Energy in Sufficient
Manner is also Important. Not Simply
Making a Ground Connection
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GROUND RESISTANCE TESTING
What is a ground?
“A ground is a conducting connection by which
an electrical circuit or equipment is connected
to the earth or some conducting body.”
Source: IEEE Standard 81
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Ground Standards
There is not one standard ground resistance threshold
that is recognized by all agencies.
NFPA and IEEE have recommended a ground resistance
value of 5.0 ohms or less.
The NEC has stated to "Make sure that system impedance
to ground is less than 25 ohms” specified in NEC 250.56.
The Telecommunications industry has often used 5.0 ohms
or less as their value for grounding and bonding.
Communication requires lower signal level
with higher frequency characteristics than
60 Hz Utility requirements
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Loose Neutral Effect on other Phases
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What defines a good ground?
A conductor with resistance low enough to
dissipate fault currents, lightning strikes, etc. into
the earth.
Does a good ground ensure good power
quality??
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Voltage imbalance variation is in
amplitude (peak – rms).
N = 0
Balanced load,
no neutral current
required
3 Phase System, Zero Sequence
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60 Hz, balanced with no zero sequence
current
Ground Plane
Net Current Flow
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5th Harmonic (300 Hz), no neutral current
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Percents of Fundamental
• 3rd = 91%
• 5th = 74%
• 7th = 56%
• 9th = 35%
Fundamental RMS Current
• 0.13A
Fundamental RMS Current
Plus RMS of Harmonics
• 0.23A
Example of 140% TDD
Switch Mode Power Supply – Square Wave Generation
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3rd Harmonic – Neutral Current is 3 Times
Phase Current – When Load is Balanced
Neutral Current
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Neutral Phases have become working
Phases
For example - a
90% electronic load
will require the
neutral to carry 1.6
times the respective
phase current –
even when all three
phases are balanced
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Neutrals have become working phases;
Not simply connections for emergency
situations.
“McMinnville”
Analogy of Water usage. Large pipe for human
service usage. < Quality is Acceptable
Human Consumption Pipe. Quality Necessary
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GROUND RESISTANCE TESTING
Consider Ohm’s Law:
V = R x I
Where:
- V is Volts
- R is the resistance in Ohms
- I is the current in Amperes
Is Determining a Ground
Quality simply sticking a
DMM in the Dirt?
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Components of System Ground How far do we need to run the DMM leads?
1) Electrical properties of Grid up to ground rod
2) Ground Rod Sphere of Electrical Influence
3) Infinite Earth plane
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1) Facility Ground Plane
(copper is assembled)
A facility can spend a large amount of resources to
establish a good “ground” in a substation location -
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Metal to Metal grid is constructed
Low resistance ohm meter used to
quantify grid resistance and
construction quality
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2) MEASURING GROUND to EARTH
RESISTANCE
Bottle Neck of all Grounds
R = r L
a Where:
r is the resistivity of the earth in ohm-cm
L is the Depth of the conducting path
a is the cross-sectional area of the path
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System Resistance
Effective electrical conductivity area is minimal at
connection point where ground rod contacts soil.
Facility Ground Plane Ground Rod
Earth
Resistance
Flow At some point
the area is so
great, R adds
little
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∆
Move from a
very good
conductor, to
a poor
conductor
with a small
conduction
area
R = ρ L
A
∆
∆
Earth Shells
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Earth Resistance Lowers by Increasing
Shell Areas Next layer
of ground
has
increased
surface
area
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Where metal meets the dirt (rod interface)
1‟ Deep; 1”
Diameter
Ground Rod
Surface Area at
Ground Rod
Interface (G) is
0.27 SQ FT
G
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Electrons Move from Metal bus work to
Dirt (Bottle Neck Begins)
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First Layer of Soil, 1” in Thickness
Area at Ground (1”) of
soil shell thickness is
0.90 SQ FT
R = 5.13 Ω
Add 5.13 Ω to all
prior bus work
for 1” of soil
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Ground Resistance at 2” of Soil Thickness
Area at Ground Rod (4) 2” of soil shell is
1.66 SQ FT; Ground Rod was 0.27 SQ FT
6 Fold increase in surface area for 2”
distance.
R = 3.37 Ω additional (2” – 1” shell)
8.50 Ω Total resistance (5.13 Ω + 3.37 Ω )
3
4
2”
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Resistance at 5” of Ground Soil Thickness
Area at Ground Rod (G5) 5”
of soil shell is 4.74 SQ FT;
R = 1.53 Ω additional (5” @
4” shell);
G5 5” from ground rod edge
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Ground resistance at 5” of Soil
13.28 Ω Total resistance
through first 5” of soil
2" 5.13
3" 3.37
4" 1.85
5" 1.54
6" 1.40
13.28 Ω
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Shell Resistance per 1” Increments
Area at Ground Rod (7) 7” of
soil shell is 10.3 SQ FT;
R = 1.17 Ω additional (7” – 6”
shell); 3.75 Ω
7” of Thickness
SQ FT
1" 5.13 0.90
2" 3.37 1.66
3" 1.85 2.56
4" 1.54 3.58
5" 1.40 4.74
6" 1.28 6.03
7" 1.18 7.44
R Ω
15.75 Ω
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Total Resistance through first foot of soil
1" 5.13
2" 3.37
3" 1.85
4" 1.54
5" 1.28
6" 1.18
7" 1.09
8" 1.01
9" 0.94
10" 0.87
11" 0.81
Total 19.07 Ω
0.90
1.66
2.56
3.58
4.74
6.03
7.44
9.00
10.70
12.50
14.42
SQ FT
Ω
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Measured Ground Value – 33
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11
Resistance Verses Distance
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Combination of all three Ground components
Example: 300 feet from ground,
shell area is 128,800 SQ FT or 9
football fields of area
R= 26 µΩ; Resistance contributes
to overall ground resistance (1”
shell)
R = 33.07 Ω + 0.0000266 Ω
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30 35 40 45 50
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3) Infinite Earth Ground Potential
Once the electron fights its way through the Ground
Rod Sphere of Influence. Infinite Earth potential
exists. Once an electron leaves the zone of
influence of a ground, there is no effective
resistance of that electron to move across the
Earth.
For testing purposes, need to measure ground rod
up to the infinite Earth barrier. The distance after
that does not matter, why??.
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Infinite Earth Potential
How can an electron moving 50„ - have a
net resistance of 50 Ohm, yet have zero
resistance from one side of the planet to
the other, once in the Earth Potential
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Deep Space Asteroid
Gravitational force is equal to mass, Inverse
Distance Squared
An object never truly leaves the sun‟s field of
influence, but its effects are dampened by distance
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Diminished Field or Free Space
An electron will behave like the
asteroid. Once the distance is so
far from the sun, other bodies have
an equivalent gravitational effect –
close neighbor asteroids, near
planets such as Pluto, even other
stars and galaxies.
Our electron never leaves the ground sphere of influence,
it is just the influence is dampened to the point where
other influences are just as prominent (the ground
becomes noise).
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TESTING METHODS
Most Popular Testing
Methods:
Fall of Potential Method
(Wenner)
- Full
- Simplified
Slope Method
And others
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4-Terminal Earth Test
(2 Terminals of 4 – Current)
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4-Terminal Earth Test
Circuit is complete, Current
Flows, Voltage is known –
therefore can read a resistance
value – RIGHT?
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4-Terminal Earth Test
Using Ohms Law; Δ V and I are known.
Variables –ground sphere of influence
(Ground of interest) & Test rod C2
Two variables, one equation
C2 Needs to be eliminated somehow
Δ V= I * (GEuT + C2)
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Additional 2 wires (Potential Wires)
4 WIRE TEST
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Physical Outline of Test Procedure
Equipotential
Circles
GEuT
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Should You Accept This Result
A location has been known to have an infinite Earth
distance of about 60 Feet. A crew comes back
after testing with the following set up & results;
Ground value 72 Ohms, Measurement distance of
65 Feet. Do you accept the result?
Ground
Under Test
Temporary Test
Rod
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The Problem of Limited Distance/Space
Distance of Potential Probe from X (dp)
Res
ista
nce
in
Oh
ms
Current Probe (C)
Potential Probe (P)
Ground Electrode
Under Test (X)
Do not want
to be in
Current
Probes Zone
of Influence
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Theoretical Background - Fall of Potential
Current Probe
Position
Distance of Potential Probe from X (dp) Ground
Electrode Position
X C
Res
ista
nce
in
Oh
ms
Current Probe (C)
Potential Probe (P) Positions
Ground Electrode
Under Test (X)
Want to
determine this
point
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Fall of Potential Method - Advantages
Extremely reliable: - Results can be checked by testing at different probe spacings.
Conforms to IEEE 81; only approved method.
Operator has complete control of the test set-up.
Can be used to test any size system.
Highly accurate: - 4-wire configuration/no additional loop resistances included.
- Significant for low resistance (1-2) grounds.
Tester uses a unique source frequency, non 60 Hz, so active
power fields will not interfere with testing.
.
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Fall of Potential Method – Disadvantages
Extremely time consuming and labor intensive. - Temporary probes must be placed.
- Cables must be run to make connections.
Space constraints can make it hard to place
remote probes.
Must disconnect individual ground electrodes to
measure them (only return must be ground)
Must “know” of other Grounds in system.
Substation Testing requiring low resistance values
can run 4000‟ or greater test lengths.
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Field Tricks
A customer complaint about grounding quality
occurs. In order to test the ground, must throw
service cutout, terminate power, disconnect utility
feed ground from service ground.
Alternate – Place a new ground rod 6‟ from service
rod. Test the rod only to determine 3 or 4 wire
grounding value. When measured, attach new
ground to old in service ground. Ground is at least
that value (or better). No need to interrupt service.
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Abbreviated 4 Wire – 3 Wire Test
V Potential
Probe
carries
current, so
it will have
its own
voltage
drop
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MEASURING GROUND RESISTANCE
(testing methods)
61.8% Rule/Method:
Based on the theory behind the full Fall of
Potential method.
Take measurement at only one point.
Quality check – “First Down Rule”
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MEASURING GROUND RESISTANCE
(testing methods)
61.8% Rule/Method:
Advantage: Extremely quick and easy.
Disadvantage: Assumes that conditions are
perfect (adequate probe spacing and soil
homogeneity). The ground behavior needs to
be known before testing begins.
“Kansas Test”
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MEASURING GROUND RESISTANCE
(testing methods)
Slope Method:
Based on the theory behind the Fall of
Potential method;
- for complex grounding systems and/or
- situations where lead lengths prohibitive
Use three measurements in calculation; can
take more; 40%, 50%, 60%
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MEASURING GROUND RESISTANCE
(testing methods)
Slope Method:
Advantage: Provides an approach for dealing
with complex systems.
Disadvantage: Makes assumptions about soil
resistance in region not tested
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Ground Test Clamp on Method
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Clamp-On/Stakeless Methodology
Based on Ohm‟s Law (R=V/I): - Apply known voltage to a complete circuit. Measure resulting current
- Calculate resistance of the entire circuit.
Apply signal and measure current without direct electrical
connection: - Grounds do not need to be lifted for testing
- Power does not need to be disconnected for testing.
Clamp includes transmit coil (applies voltage) and receive
coil (measures the current).
Measurement Loop is Directional.
It will first give continuity, to determine if a ground
exits to being with
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Clamp-On/Stakeless Methodology
For accuracy, more return paths, the better results
For 1 return electrode = average of the two.
For 6 similar electrodes with a resistance of 10: - Rloop = 10 + 2 = 12
For 60 similar electrodes with a resistance of 10: - Rloop = 10 + 0.17 = 10.17
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Governing Equation – Parallel Circuits
Rloop = RTest + (1/(1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5))
Note: The resistance of the ground under test will
always be higher than the actual ground resistance
value. Worst case approach leaves a safety margin
of error (if infinite Earth potential is reached)*
If Rtest is greater than acceptable, prove value with
4 wire test unit.
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10 Transmission Tower Under Test
0
2
4
6
8
10
12
14
16
18
20
0 20 40 60 80 100
20 Tower Value 10.5
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No basis for the test in standards – no objective
reference for the test results
Less effective for very “low” grounds: - Extraneous elements in reading become comparatively large.
There is no built-in proof for the method - results must
be accepted on “faith”.
The returns must be well clear of the Infinite Earth
Potential zone. This is the greatest cause of
testing result failure.
Must be aware of other grounds tied to the system,
that are in close proximity.
Clamp-On Method - Disadvantages
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Recommendation. On a tower ground, 3 or 4 wire test
to determine distance to Infinite Earth Distance (note
the resistance verses distance measured value)
Assure that tower distance is at least 2X this distance.
Note that measured tower ground point values will
always be higher than actual.
If a tower is out of spec, 3 or 4 wire test to confirm if
rework is required.
Clamp-On Method - Strategy
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Analogy – The length of a wire spool is to
be tested to determine length
A four wire test is analogous
to stretching the wire out, and
measuring. Know resistance
per foot, measure the
resistance.
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The short cut measurement
method
Know – 1 per Foot.
Measurement is 40 ; therefore 40‟ correct?
Only item knows from test results is there is at least
40‟ of wire, that is my minimum spool wire length.
There maybe more.
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Outcomes from the Clamp on
Tester
Measure 15 at a meter entrance. Spec calls for
25 . The service is 15 or greater – known.
Measure 45 at a service. We know the ground
is 45 or greater. Remedial action required
Measure 250 at a pole mount. Know there is
continuity, along with potential rework.
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Applications & Limitations – Service
Entrance/Meter
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Incorrect Reading
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Grounding
Conductor
Ground
Rod Butt
Plate
Utility
Pole
Applications – Pole Grounds
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Applications & Limitations – Pad Mount
Transformer Facility with Multiple Ground Points
Underground buried concentric
Neutral
Ground
Rods
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MEASURING SOIL RESISTIVITY
* 3 Meter rod, 33 Ω; 3‟ foot rod ~ 109 Ω
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Super Ground PISA style helix
with extensions
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Variables in Grounding Quality
Temperature
Moisture
Ionification (Salt Adders)
Ground Rod Diameter
Ground Rod Depth
Number of Ground Rods
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Ground Resistance of Farming Clay Loom
ρ = 100 Ω – M
3’ Ground Rod
106 Ohms
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0 10 20 30 50 60 40
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
Variation of Soil Resistivity
with Temperature
Soil Contained 18.6% Moisture
Re
sis
tivit
y o
f S
oil
Temperature - Degrees F
Variation of Soil
Resistivity with
Temperature
Liquid Water
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SOIL MOISTURE VERSES CONDUCTIVITY
0 10 20 30 35 50 55 60 40 45
400,000
300,000
200,000
100,000
80,000
Variation of Soil Resistivity
with Moisture Content
Red Clay Soil
Re
sis
tivit
y o
f S
oil
Per cent Moisture in Soil
60,000
40,000
20,000
5 15 25 65
Variation of Soil
Resistivity with
Moisture Content
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MEASURING SOIL RESISTIVITY
Effect of salt content on soil resistivity:
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MONTHS OF YEAR
130
120
110
100
90
80
70
60
50
40
30
0
20
10
5 6 7 8 9 10 11 12 2 1 3 4 5 6 7 8 9
Chemical treatment reduces seasonal variations
5/8 x 8’ ROD
(SOIL UNTREATED)
5/8 x 8’ ROD
(SOIL TREATED)
RE
SIS
TA
NC
E, O
HM
S
IMPROVING YOUR GROUNDING SYSTEM
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Solution of Proposed Items to this Point
Warm Soil
Moist Soil
Salted Soil
Great for ground electron conduction, not so
great for metallic existence
Ground Rod Diameter Effects
Ground Rod Length Effects
Number of Ground Rods Effects
84
100
90
80
0.50 0.75 1.00 1.25 1.5 1.75 2.00
IMPROVING YOUR GROUNDING
SYSTEM
Doubling Rod diameter, decreases
resistance by only 10%
10% increase for four times the
material usage. Increase diameter
used for mechanical strength & rod
survivability (time)
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DEPTH OF ROD, FEET
160
140
120
100
80
60
40
20
0 1 2 3 4 5 6 7 8 9 10 11
RE
SIS
TA
NC
E,
oh
ms
IMPROVING YOUR GROUNDING SYSTEM
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What steps can be taken if there is a problem
in the grounding system
Use longer ground rods.
Chemically treat the soil
Use multiple ground rods.
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Effects of Multiple Ground Rods
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Effects of Multiple Ground Rods
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What causes a ground system to deteriorate
(and become ineffective)?
Corrosion and weather influences exert mechanical
strain on ground rods and cause metallic corrosion
over time (as a ground rod corrodes, its resistance
rises and it loses its effectiveness) - TILLAMOOK
soil resistivity can vary considerably with changes
in climate and temperature
Water Tables
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Last Variable – The Macro Environment
Results depend on time of year of test; Worst
Case or not
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MEASURING GROUND RESISTANCE
(testing methods) 61.8% Rule Method Probe Placement: •Determine depth of ground electrode to be tested
•Distance of C > 4 x Depth of electrode to be tested
•Place P probe at 61.8% of the distance of C
•Take the measurement
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Maximum Ground Resistance Targets
Typical values for a power company: - Generating station: 1 maximum
- Large sub-station: 1 maximum
- Small sub-station: 5 maximum
Water pipe ground should be less than 3 and
frequently less than 1 .
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An electron really doesn‟t move to China from Ohio,
with zero resistance. It is deposited in a infinite pool
of available electrons, while an electron is picked
from the pool for service in China. There is a net
movement of free space electrons but it is <<
negligble.
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Anchoring
Lazy Spikes
Unique Test Frequency (105 to 160 Hz)