Earth Ground Resistance The Basics

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Earth Ground Resistance The Basics
- Diagnose intermittent electrical problems -
Avoid unnecessary downtime - Learn earth ground
testing principles
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Table of Contents
Earth Ground - Foundation

• Why Ground? Why Test?
• Grounding basics
• Methods of earth ground testing
• Measuring ground resistance
• Who to Target
• Size of the Opportunity
• Why we will win
• Sales Tools

Earth Ground Sales Information
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Earth Ground Foundation
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What is a ground? What does it do?
What is ground?
NEC, Article 100, A conducting connection,
whether intentional or accidental, between an
electrical circuit or equipment and the earth, or
to some conducting body that serves in place of
earth Beyond the actual definition of a ground,
the important point is - a connection to earth
can be intentional or accidental. Ground
testing ensures that the intentional grounds are
functioning If the intentional path to ground
fails and accidental path to ground is disturbed,
systems can fail intermittently.
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Why test grounding systems?
What is ground?
Catch the problem before it happens! Its
estimated that at least 15 of power quality
problems are related to grounding, ensuring good
grounding by testing can make certain these
problems dont escalate (source -
EPRI) Lightning strikes on equipment with poorly
maintained protection systems destroy millions of
dollars of equipment and lost production every
year (source www.copper.org) Earth Ground
testing should be a part of a customers basic
PdM program.
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What is a good ground value?
What is ground?

• Ideally the ground resistance of a system is zero
  ohms.
• But in reality, the goal is to achieve the lowest
  ground resistance possible that makes sense
  economically and physically.
• NFPA IEEE Recommends a ground resistance
  value of 5.0 ohms or less.
• Telecommunications Industry Often uses 5.0
  ohms or less as their value for grounding or
  bonding
• NEC Make sure the system to ground is 25.0
  ohms or less. In facilities with sensitive
  equipment, it should be 5.0 ohms or less. (source
  NEC 250.56)

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Components of a ground electrode
Ground conductor Connection between the ground
conductor and ground electrode Ground electrode
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Spheres of Influence
There must be proper spacing between ground
electrodes and earth stakes to reduce or
eliminate their spheres of influence
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What affects ground resistance?

• The NEC code requires a minimum ground electrode
  length of 2.5 meters (8.0 feet) to be in contact
  with the soil. But, there are four variables
  that affect the ground resistance of a ground
  system
• Length / Depth of the ground electrode double
  the length, reduce ground resistance by up to 40
• Diameter of the ground electrode double the
  diameter, lower ground resistance by only 10
• Number of ground electrodes for increased
  effectiveness, space additional electrodes at
  least equal to the depth of the ground electrodes
• Ground system design single ground rod to
  ground plate

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Types of Ground Systems
Depending on the soil type and the level of earth
ground resistance you are trying to
achieve.. You may choose to install any one of
the following ground systems.
Single Ground Rod
Multiple Ground Electrodes
Ground Mesh
Ground Plate
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Methods of Testing

• Soil Resistivity uses four stakes
• Fall of Potential uses two stakes
• Selective Testing uses one clamp and two
  stakes
• Stakeless Testing uses only two clamps
• These are the only types of earth ground test
  methods available today.

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Soil Resistivity
The purpose of soil resistivity measurements is
to quantify the effectiveness of the earth where
a grounding system will be installed. So, soil
resistivity testing is most necessary when
determining the design of the grounding system
for new installations. Ideally, you would find a
location with the lowest possible
resistance. The soil composition, moisture
content and temperature of the soil all impact
the soil resistivity.
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Soil Resistivity - Testing
To test soil resistivity, connect the ground
tester as shown. The Fluke-1625 generates a
known current through the two outer stakes and
the drop in voltage potential is measured between
the two inner ground stakes. Using Ohms Law
(VIR), the Fluke tester automatically calculates
the soil resistance.
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Soil Resistivity - Calculation
Resistivity Measurement
From the indicated resistance value RE, the soil
resistivity is calculated according to the
equation ?E 2 ? . a . RE   ?E ......
mean value of soil resistivity (W.m) RE ......
measured resistance (W) a ...... probe distance
(m)
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Fall of Potential Testing
First, the ground electrode of interest must be
disconnected from its connection to the
site. Two earth stakes are placed in the soil in
a direct line away from the earth electrode.
Normally, a spacing of 20 meters is
sufficient. A known current is generated by the
Fluke 1625 between the outer stake and the ground
electrode, while the voltage potential is
measured between the inner earth stake and the
earth electrode.
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Fall of Potential Testing (II)
To test the accuracy of the results and to ensure
that the ground stakes are outside the sphere of
influence, reposition the inner stake 1 meter and
take a fresh measurement. If there is a
significant change in the reading (30) you need
to increase the distance between the ground
electrode under test and the earth stakes.
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Selective Testing
The selective method is based on the Fall of
Potential test, however its not necessary to
disconnect the ground electrode under test ! A
current clamp is used to isolate the test current
injected into the electrodes under test, the
current will flow to earth by any path. By
isolating the current, with use of the current
clamp, the ground resistance of individual
elements can be measured without disconnecting.
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Selective Testing (II)
If the total resistance of the ground system
should be measured, then each earth electrode
resistance must be measured by placing the clamp
around each individual earth electrode. Then the
total resistance of the ground system can be
determined by calculation.
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Selective Testing - Application
This application example, at a central office,
shows the benefit of the selective test in a
typical installation. First, position the ground
spikes according to the requirements of the
system under test.
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Selective Testing - Application

• Once the spikes are placed, individual elements
  of the system can be measured. No need to
  disconnect the ground system !
• In this example, measurements are taken at the
• MGN (multi grounded neutral)
• ground field,
• water pipe,
• structural or building steel

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Stakeless Testing

• The stakeless method eliminates the need for
  temporary ground stakes. This is useful in a
  wide range of situations. Examples include
• Inside buildings
• Airports
• Urban locations
• Chemical and industrial plants
• The stakeless method is not available on all
  ground testers. However, it comes standard on
  the Fluke 1623 and 1625 earth ground testers.
• The temporary ground stakes are replaced by two
  current clamps. The first clamp generates a
  voltage on the ground condutor, the second clamp
  measures the current flowing due to the generated
  voltage.

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Stakeless Testing
The Fluke 1623 and 1625 testers are able to
measure earth ground loop resistances for multi
grounded systems using only current clamps. With
this test method, two clamps are placed around
the earth ground rod or connecting cable and each
connected to the tester. Earth ground stakes
arent used at all.
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Stakeless Testing
The Fluke testers work on the principle that the
parallel/multi-grounded sysems, the net
resistance of all ground paths will be extremely
low as compared to any single path (the one under
test). So, the net resistance of all the
parallel return path resistances is effectively
zero. If the ground system is not parallel to
earth then you will either have an open circuit,
or be measuring ground loop resistance.
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Stakeless Testing - Application
The clamps are placed around the ground conductor
The stakeless method does not need a lot of
parallel paths to be present to ensure good
results.
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Stakeless Testing - Details

• If there is only one path to ground, like at many
  residential applications, the stakeless method
  will not provide an acceptable value and the Fall
  of Potential test method must be used.
• An abnormally high reading or an open circuit
  indication on the instrument points to a poor
  connection between two or more of the
  aforementioned critical components.
• An abnormally low reading could indicate the
  instrument is measuring a loop of bonding
  conductors.

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Two Pole ground resistance

• Used where other methods are not available. Uses
  nearby metal structures as a temporary spike.
  Metal water pipes are typically used.
• Drawbacks
• The resistance of the metal pipe should be
  significantly less than the electrode under test.
• Metal pipes are being replaced with plastic.
• Some metal pipes use plastic couplings.

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Selecting a Test Method
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Introducing the new Fluke 1623 1625 Earth
Ground Testers
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Choosing the right instrument