Earth Ground
Resistance
The Basics
- Diagnose intermittent
electrical problems
- Avoid unnecessary
downtime
- Learn earth ground testing
principles
1
Table of Contents
Earth Ground - Foundation
•
Why Ground? Why Test?
•
Grounding basics
•
Methods of earth ground testing
•
Measuring ground resistance
Earth Ground – Sales Information
•
Who to Target
•
Size of the Opportunity
•
Why we will win
•
Sales Tools
2
Earth Ground –
Foundation
3
What is a ground?
What is ground?
What does it do?
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.
Why test grounding systems?
What is ground?
Catch the problem before it happens!
It’s estimated that at least 15% of power quality problems are
related to grounding, ensuring good grounding by testing can
make certain these problems don’t 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 customer’s basic PdM program.
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)
Components of a ground
electrode
Ground conductor
Connection between the ground conductor and
ground electrode
Ground electrode
Spheres of Influence
There must be proper spacing between ground electrodes and earth
stakes to reduce or eliminate their spheres of influence
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
Types of Ground Systems
Depending on the
soil type and the
level of earth
ground resistance
you are trying to
achieve…..
Single Ground Rod
Multiple Ground Electrodes
Ground Plate
Ground Mesh
You may choose
to install any one
of the following
ground systems.
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.
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.
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 Ohm’s Law
(V=IR), the Fluke tester
automatically calculates the soil
resistance.
Soil Resistivity - Calculation
Resistivity Measurement
From the indicated resistance value RE, the soil resistivity is
calculated according to the equation :
E = 2  . a . R E
E
RE
a
...... mean value of soil resistivity (W.m)
...... measured resistance (W)
...... probe distance (m)
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.
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.
Selective Testing
The selective method is based on
the Fall of Potential test, however
it‘s 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.
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.
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.
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
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.
21
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 aren‘t used at all.
22
Stakeless Testing
The Fluke testers work on the
principle that the parallel/multigrounded 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.
23
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.
24
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.
25
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.
26
Selecting a Test Method
Advantages
Drawbacks
Fall-of-Potential
• Widely accepted
• You have to disconnect ground
• The stakes may not be easy to drive
• There may not be space around the
ground electrode to drive the stakes
Selective
Method
• Don’t have to disconnect electrode
• The stakes may not be easy to drive
• There may not be space around the
ground electrode to drive the stakes
Stakeless
Method
• Don’t have to disconnect electrode
• Convenience
• Assumes a low-impedance parallel path
• Possible to get very low readings by
mistakenly measuring on a hard-wired
loop
Two-pole
Method
• Convenience
• Impossible to judge the integrity of the
“auxiliary electrode.”
• Can’t be sure you are outside the area
of influence
Introducing the new Fluke 1623
& 1625 Earth Ground Testers
Choosing the right instrument
Feature
3-pole earth measurement
1623
1625


4-pole earth measurement


Specific earth resistance (soil-resistivity according to Wenner)


2-pole resistance measurement DC

4-pole resistance measurement DC

2-pole resistance measurement AC


Selective earth measurement (1 clamp)


Stakeless earth measurement (2 clamps)


Earth impedance of high voltage pylons (55 Hz)

Measuring voltage 20/48 V

Measuring voltage <= 48 V

Automatic frequency control (AFC) (94 ... 128 Hz)
Measuring frequency 128 Hz


Programmable limits, settings

One button measurement concept


Protective rubber holster

