There are a wide variety of ground resistance testers available on the market today.
These vary in design, features, and complexity, and include small handheld models as well
as larger field instruments that are often packaged as part of a complete kit.
These products also range in price, from a few hundred up to several thousand dollars.
In this video, we discuss several critical questions to consider when selecting a ground
resistance tester. Our goal is to help guide you in choosing the instrument best suited
to your specific application and requirements.
The first – and probably most important – question is whether your current or future
needs require soil resistivity testing. For example, if your work involves the design
and/or installation of new grounding systems, soil resistivity testing is a necessity.
An instrument designed for four-pole testing (also referred to as four-point testing) is
required for this application. A basic low-cost four-pole tester provides measurement results
in ohms. You can then use this reading to manually calculate soil resistivity, which
is usually expressed in ohm-centimeters or ohm-meters.
More sophisticated instruments include a built-in formula for calculating soil resistivity using
the Wenner or the Schlumberger method.
If you regularly need to perform four-pole testing, consider purchasing an instrument
that automatically calculates soil resistivity. This will save time and eliminate potential
math errors.
Assuming you do need to perform soil resistivity testing, the obvious follow-up question involves
the types of grounding systems you will test. Will this include small systems such as residential,
or larger and more complex systems such as commercial, industrial, telecommunication,
or electric utilities?
To illustrate the importance of this question, let’s consider a typical small site with
a grounding system consisting of a copper rod, driven into the ground and connected
to the service entrance. In this illustration shown on the screen, if the house has not
yet been connected to the power line, a basic 3-pole ground resistance tester (or a 4-pole
instrument configured for 3-pole testing) will suffice for measuring the resistance
of the house ground rod. If the house has been connected to the power line, a clamp-on
ground resistance tester can measure the house ground rod resistance.
If you choose a 3- or 4-pole instrument for this, one point to bear in mind is the distances
required for auxiliary rod placement. For example, performing a Fall-of-Potential test
on a single ground rod driven 8 feet deep requires at least 80 to 100 foot test leads.
If more ground rods are present, the distance requirement increases. Ground resistance test
kits are available from AEMC that include the measurement instrument, the auxiliary
electrodes, and leads. Lead lengths provided in these kits are 150 feet, 300 feet, and
500 feet. We suggest selecting a ground resistance test kit with leads at least one size longer
than your immediate need. So if 150 feet is required, a kit that includes 300 foot leads
will provide a good margin of error. For larger sites with multiple rods or ground grids,
consider kits that provide 500 foot leads.
A related question is whether the soil resistivity is high in the area that you will be testing,
or whether the distance required for the auxiliary rods to perform Fall-of-Potential testing
is unusually long. If the answer to either or both of these questions is yes, and you
intend to perform Fall-of-Potential testing, you must consider the instrument’s injection
current and test voltage. Typical injection currents range from a few milliamps up to
a couple hundred milliamps. High soil resistivity usually produces high contact resistance for
the auxiliary electrodes. This can be of concern when using lower-cost instruments that typically
provide 10 milliamp test current; so in these circumstance we recommend a four pole instrument
capable of delivering higher test current.
Before we leave the topic of auxiliary electrodes, note that clamp-on instruments do not require
any auxiliary rods or leads. Another advantage is that you do not need to take the grounding
system out of service to perform the test.
Another subject to consider is whether electromagnetic interference, or EMI, is present at the test
site. EMI can result in unstable or inaccurate readings, particularly at lower test frequencies.
The most common test frequency is 128 Hz. Instruments that feature automatic test frequency
selection can find the “cleanest” available frequency, which provides an advantage in
high-EMI environments. Clamp-on instruments can also be effective in such locations, since
they typically test at higher frequencies. Newer clamp-on models available from AEMC
also offer test frequency selection.
Note that in some high-inductive environments lower test frequencies can produce more reliable
results.
The choice of instrument can also depend on how you intend to use the data you obtain.
For example, if you plan to save, analyze, and distribute your test results, data storage
and report generation become important considerations. Newer and more advanced instruments, both
three and four pole testers and clamp-on models, can store test results in internal memory.
This data can then be downloaded and analyzed using software running on a computer, or via
mobile apps for smartphones and tablets. This can be a very powerful tool for contractors
conducting tests for clients. An added advantage for a mobile app is the ability to immediately
send test results as an e-mail or text message.
Finally, if you’re planning to test complex grounding system consisting of many components
including a ground mat or grid, you will need to test the continuity across the bonding
between the various elements. This test is most often conducted using DC voltage and
current. Several ground resistance testers provide this capability, with test currents
up to a couple hundred milliamps. In addition, a more complete test can be performed with
a micro-ohmmeter. The advantage in using this instrument is its ability to test at higher
test currents, up to 200 amps. This can expose problem areas not always revealed when testing
with milliamp-range currents.
Whatever your ground resistance testing needs, AEMC has an instrument that meets your requirements.
If you need to perform basic ground resistance testing with no soil resistivity testing required,
consider the AEMC Ground Resistance Tester Models 3620 and 3640. Both instruments measure
the resistance of grounding systems via the 2- and 3-point methods. And both feature simple
“one-button” operation. The Model 3640 also provides auto-ranging. Each instrument
can be purchased standalone or as part of a kit that includes leads, auxiliary electrodes,
and other accessories.
In addition, the hand-held Clamp-On Ground Resistance Tester Models 6416 and 6417 measure
ground rod and grid resistance without requiring auxiliary rods.
To perform basic ground resistance testing with 4-point soil resistivity measurement,
the AEMC Models 4620 and 4630 are good choices. Each instrument is rugged, easy to use, and
can be purchased standalone or as part of a complete ground testing kit.
For ground resistance testing with high soil resistivity present, you need an instrument
that can inject test currents greater than the 10 milliamp current typically provided
by lower-cost instruments. The Models 6470-B, 6471, and 6472 provide test currents up to
250 milliamps. These instruments also offer a number of other advanced ground resistance
capabilities.
If you have a need for testing continuity across the bonding between components in complex
grounding systems, you’ll also require an instrument that can inject higher test currents.
In addition to the Ground Resistance Testers shown on the screen, AEMC offers micro-ohmmeters
that can be used in this application, for example the Model 6292, with test currents
up to 200 amps.
For ground resistance testing in high EMI environments, the best way to minimize these
effects is to use instruments that automatically select the “cleanest” test frequency for
the environment. The Models 6470-B, 6471, and 6472 provide this capability, as do the
Models 6416 and 6417 Clamp-On Ground Testers. Note that the Models 6416 and 6417 are the
only clamp on ground resistance testers on the market that offer test frequency selection.
If you plan to perform high-end ground resistance testing, including pylon testing, AEMC’s
GroundFlex Field Kit is your best option. Pylon or tower testing usually requires disconnecting
the overhead ground conductor – a time consuming, expensive, and potentially hazardous operation.
The GroundFlex Field Kit offers the unique capability of testing pylon resistance without
disconnecting the overhead ground conductor. The kit includes the Model 6472 as well as
the GroundFlex Adapter Model 6474, and comes with all the sensors, leads, and other accessories
needed to perform pylon testing and other ground resistance testing applications.
And if you want report generation capability, AEMC’s free DataView software enables you
to download test results from the instrument to a computer. You can then generate reports
from this data, using templates provided with the software or custom templates you create
to suit your specific requirements. These reports can be analyzed, shared, and stored
for future review. DataView supports the Models 6470-B, 6471, and 6472. It also supports the
Model 6417 Clamp-On instrument. A free Android app is also available for interacting with
the Model 6417.
Let’s take a moment to review. When deciding which ground resistance tester is right for
you, consider whether or not you need an instrument that can measure soil resistivity. If so,
think about what types of ground systems you are likely to test. Take into consideration
the environmental factors at your potential test sites, such as high soil resistivity
or EMI. Also bear in mind how you plan to use the measurement data. And if you need
to test bonding in complex grounding systems, consider an instrument that can perform continuity
checking with high test currents.
These are just a few important points to consider when choosing a ground resistance tester.
For more information on ground resistance testers provided by AEMC, see our web site.
And be sure to check out our YouTube channel for instructional videos on other topics in
electronics, including the many products offered by AEMC