1997 goals and plans - ise expo
TRANSCRIPT
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Grounding&
Testing
A Communications Perspective
Ed Rousselot
National Telecom Sales Engineer
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Agenda
• What is a “Ground” and why is it important to have a
“good” one
• Soil resistivity and soil resistance
• Considerations with installing a Ground Electrode
System (for our purposes, “rods”)
• Measure the effectiveness of the ground electrode
system by means of ground testing
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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
Low impedance conductor used to provide a safe path
for the dissipation of:
- fault currents
- lightning strikes
- static charges
- EMF/RFI signals
Simply put….
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Simply Put…
The characteristic of a grounded system is that there
is a steady flow of current that is going to the path of
least resistance.
We do not want that path of least resistance to be through
someone or some delicate equipment.
We want it to be through a
“ground electrode system”.
This current can be measured using
an ammeter that measures milliamps.
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Brief Look at Benefits of
Proper Grounding
Safety – Us and our Equipment
Noise on our Circuits
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Ground Resistance
vs.
Soil Resistivity
• Ground Resistance: The resistance (opposition to
current flow) of an installed electrode system
• Measured in Ohms
• Measured using three or four-point stake testers
or a clamp-on tester
• Soil Resistivity: The electrical properties of the soil for
conducting current
• Measured in Ohm-cm (Ohm centimeters)
• Measured using a four-point stake tester
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Resistivity
• Soil Resistivity: The electrical
properties of the earth for conducting
current
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Why Test Resistivity?
•Tells you how “good” (conductive) your soil is
•Good indication on whether or not a generic
ground specification design will work
•Helps reduce “surprises” at the end of the
installation
•An indication of the degree of corrosion to be
expected on components of the ground
system
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Resistivity of Different Soils
Soil Type Resistivity Range Ohm cm
Loam 100 – 5,000
Clay 200 – 10,000
Shales 500 – 10,000
Limestone 500 – 400,000
Surface Limestone 10,000 – 1,000,000
Slates 1,000 – 10,000
Sandstone 2,000 – 200,000
Sand & Gravel 5,000 – 100,000
Granites, Basalts, etc. 100,000
Evershed & Vignoles
Bulletin 245
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Soil Resistivity Ranges
•100 - 15,000 Ohm cm – Standard Design OK
•15,000- 25,000 Ohm cm - Standard Design Maybe
•25,000 - 50,000 Ohm cm - Special Circumstances
•50,000 + Ohm cm – Perhaps not practical
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Measuring Soil Resistivity
• Use a 4-terminal earth resistivity tester
• Space the rods an equal distance apart –
“a” in next slide
• Insert the rods a distance of 1/20th “a”
into the ground
• Measures the average soil resistivity to a
depth equal to the rod separation
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Measuring Earth Resistivity
a a a
C2P2P1C1
a a/20
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Actual Site Testing Procedures
Test at multiple locations across the site
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Four-Point Resistivity TesterAlso Tests Resistance
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• To check that climatic conditions have not
changed the ground such that it no longer
meets the requirements
• To check for seasonal variations
• To check that changes to buildings, the lot, the
streets, etc. have not affected resistivity
Why Measure Soil Resistivity Periodically?
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Ground Resistance
Ground Resistance: The resistance (opposition
to current flow) of an installed electrode
system
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Typical Permissible Ground Resistance
Values
• Telecom - varies depending on the standard used:- Electronic equip mfg. vary from fraction of an ohm upward
-At premise must equalize with power ground which is 25Ω
• Typical values from an insurance company:- Industrial plant: 5
- Chemical plant: 3
- Computer system: 3
• Typical values for a power company:- Generating station: 1 maximum
- Large sub-station: 1 maximum
- Small sub-station: 5 maximum
Why 25 Ohms?
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25 Ohm ground keeps US safe
NEC Section 250
IEEE STD 81
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Ground Electrode System Components
• Ground “Electrodes”
• Ground Conductors
• Ground Bars
• Bonding Connectors
• Mechanical
• Compression
• Welded
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Ground
Electrodes
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Ground ElectrodesDo Not Have To Be Rods
Ground Electrode Types -
Ground Rods:
Copper Clad Steel
Solid Copper
Galvanized
Stainless Steel
Enhanced
Ground Plates
Ground Mesh
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Ground Electrodes
Types of Grounding Systems
Single Rod
ElectrodeMultiple Pole Grounds
Multiple-Rod Electrode
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Ground Electrodes
Ground
Wire
Ground
RodButt
Plate
Utility
Pole
≥ 12” below ground
?
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Another type of butt ground
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Soil Resistivity - Some soils, (such as sandy soils), have
such high resistivity that conventional ground rods or
ground electrode systems may be unable to attain the
desired ground resistance.
•Enhanced ground electrodes or ground enhancement
materials may be required to meet the grounding
specification.
Some of these are…
Ground Electrodes… Considerations
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Ufer Grounds - Concrete
encased electrode tying
into tower footing or
building-pad rebar
Enhanced Ground Electrodes
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Enhanced Grounding Material
Low Resistance
Reduces seasonal variation
Carbon-based
Not permanent
Washes away over time
Only consider when deep or
multiple rods are not practical
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Enhanced Grounding Material
Soil treating material
placed in circular trench
and covered with earth
Ground rod
Appx 1 ft.
Appx 18 in.
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Enhanced Grounding Material
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Soil Ph - affects the rate of corrosion of metal ground
components that are in contact with the soilcont.
Ground Electrodes… Considerations
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Soil Characteristics - Some sites may have only a few
inches of soil (or none) sitting on top of bedrock.
Consider-
Ground mesh & plates
Horizontal rods
(Don’t drill into bedrock such as granite. Such rock does
not make a good ground.)
cont.
Ground Electrodes… Considerations
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Ground Rods… Considerations
Ground Rod Diameter –
•Doubling diameter of ground rod reduces
resistance only 10%.
•Using larger diameter ground rods is mainly a
strength issue. In rocky conditions, a larger
diameter ground rod might be advantageous.
Cont.
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Ground Rods… Considerations
Ground Rod Length - Doubling length theoretically reduces
resistance 40%.
Actual reduction depends on soil resistivities of multi-layered
soils.
Cont.
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Ground Rods…Considerations
• Multiple Ground Rods – Two well-spaced rods driven
into the ground provide parallel paths. They are, in
effect, two resistances in parallel.
• The rule for two resistances in parallel does not apply
exactly. (The resultant resistance is not one-half of one
of the rod’s resistance.)
• The reduction for equal resistance rods is about:
• 40 percent for 2 rods
• 60 percent for 3 rods
• 66 percent for 4 rods
–Spaced apart greater than their length
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What Else Can YOU
Control?
Torque
Bullet Bonds
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Which kind does your
locate contractor use?
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Causes of Ground System Deterioration
• Weather influences exert mechanical strain on
ground rods
• Cable locates
• Corrosion over time
• Catastrophic events like lightning strikes or large
fault currents can cause instant degradation that
may not be visible
• Soil resistivity can change over time due to
environmental conditions
• Facility expansion can create different needs in
the ground system
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Risks from Ground System Deterioration
• Potentially deadly electrical shock situations
• Plant-wide equipment damage
• Disruption in the performance of sensitive
equipment with tight voltage parameters
• Heat build-up on a single piece of electrical
equipment and, eventually, fire
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Ground Electrode System Testing
• Ok, the system is designed and
installed.
• Let’s Test!
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Choose the Proper Instruments
Use a dedicated ground tester
- designed to measure grounds
• Don’t use a generalized ohmmeter or multimeter
• Don’t use an insulation resistance tester
• Don’t use the ground test of a telecom multi-
function test set
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Safety First!
This is not a
safety course
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Three-Terminal Ground-Resistance Testing
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Review – Ohm’s Law
• Resistance = Voltage ÷ Current
• Ohms = Volts ÷ Amps
• If we know Voltage and Current, we can calculate
Resistance
• So, If we have 50 Volts and 2 Amps, we have 25
Ohms (25 = 50 ÷ 2)
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Three-Terminal Ground Resistance Tester
Current Supply
Ammeter (I)
Voltmeter (E)
P C
X
EarthEarth
CurrentProbe
PotentialProbe
GroundRod
Under Test(Isolated)
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Fall of Potential
(Isolated)
Ground Rod
Under Test (X)
Ground Rod
Position
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Resistance Curve
CurrentProbe
Position
Distance of Potential Probe from X Ground
RodPosition
X C
Resis
tan
ce i
n O
hm
s
True Resistance
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“Rules of Thumb” on “C” Probe Spacing
From Ground Rod “X”
Single ground rod - 50 feet
Small grid of 2 ground rods - 100-125 feet
Large system (several rods or plates in parallel)
>200 feet
Complex systems (large number of rods or other
electrodes and other metallic structures bonded
together) - far greater distances are required
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Insufficient Probe Spacing
Distance of Potential Probe from X
Resis
tan
ce i
n O
hm
s
CurrentProbe (C)
PotentialProbe (P)
GroundRod
Under Test (X)
(Isolated)
No Flattening
No True Resistance
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Advantages/Disadvantages of
Fall of Potential Testing
Advantage:
Conforms to IEEE 81 - only approved method.
Disadvantage:
Time consuming
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61.8% Rule
(Isolated)
61.8%
Ground Rod
Under Test (X)
Ground Rod
Position
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61.8% Rule/Method
• Based on the full Fall of Potential method
• Take measurement at only one point
• Advantage – Quick and easy
• Disadvantage – Assumes that conditions
are perfect with respect to:
• Adequate spacing of C and P probes
• Resistivity of the soil being the same
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Ground Testing on Asphalt“Lazy Spike”
MEASURE
3 POLE 4 POLE
MEGGER® DET5/4R
(Isolated)
Does not have to be wet
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Ground Testing Methods
• Fall of Potential Method*
• 61.8% Rule Method*
• Simplified Fall of Potential
• Four Potential Method
• Intersecting Curves Method
• Slope Method
• Dead Earth Method
• Star-Delta Method*Covered here
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Stake-less or Clamp
Ground Testing
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Review – Ohm’s Law
• Resistance = Voltage ÷ Current
• Ohms = Volts ÷ Amps
• If we know Voltage and Current, we can calculate
Resistance
• So, If we have 50 Volts and 2 Amps, we have 25
Ohms (25 = 50 ÷ 2)
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CAUTION
Ammeter does not measure the value of the ground
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Considerations When Using
Stake-less/Clamp-On Method
Effective only in situations with multiple
grounds in parallel such as pole grounds
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Multiple Grounds in Parallel
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Considerations When Using
Stake-less/Clamp-On Method
Effective only in situations with multiple grounds in
parallel such as pole grounds
Requires a good return path so it cannot be
used on isolated grounds
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CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
No Return Path
Clamp Won’t Work
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Considerations When Using
Stake-less/Clamp-On Method
Effective only in situations with multiple grounds in
parallel such as pole grounds
Requires a good return path so it cannot be used on
isolated grounds
Cannot be used if an alternate, lower resistance,
return path exists not involving the soil
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CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
Must Measure at the Correct Part of the Loop
Lightning RodsConnecting Wire
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Multiple ground rods
bonded underground
To power company
ground (Has many
parallel connections to
power company feed.)
Power Company Feed
What is being measured?
NID
6’ Rods are 6’ to 8’ apart
Clamp-on set must be
BELOW this point.
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Reality
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What is Being Measured?
Grounding
Conductor –
Bonded to MGN
Ground
Rod
Butt
Plate
Utility
Pole
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CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
What is Being Measured?Only Connectivity – Not Ground Resistance
Ground Rods
At Each Leg
Buried Wire
Connecting the Rods
Wire Connecting
Each Leg to Rod
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SHIELD CONNECTION
PedestalGround
Bus
Ground
Bar
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Ground
bar
CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
CA
TIV
n600V
Meg
ger®
Measuring
continuity
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This is what the
ground wires look like
that we have to
clamp around to test.
Right?
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Sometimes It’s Not Easy
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What Now?
Could be worse…
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Quick Review
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Addressing Ground System Problems
• Use longer ground rods
• Use multiple ground rods
• Chemically treat the soil
• Place the system in lower resistivity soil if
possible
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• So, we tested our ground system and we
passed!
• Do we ever need to test it again?
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• (You have seen this before but…)
• To determine the effectiveness of ground rods and
connections
• Seasonal changes
• Water table changes
• Changes in the site and/or building
• To check that standards set by codes are still being met
• To check that specific design parameters have been met
• To check that the ground rods and bonds are still present
Why Measure Ground Resistance Periodically?
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Questions