Choosing the Right Ground Resistance Tester Ram Meter, Inc.

Choosing the Right Ground Resistance Tester

May 14th, :
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 article, 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.

For more information, please visit Hengfeng.

Do you need to regularly test sites for soil resistivity?

The first, and probably most important, question is whether your current or future needs require soil resistivity testing, because it will determine the type of ground resistance tester you will need. 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 4-pole testing (also referred to as 4-point testing) is required for this application. A basic low-cost 4-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 built-in formulas for calculating soil resistivity using the Wenner or the Schlumberger method. If you regularly need to perform 4-pole testing, consider purchasing an instrument that automatically calculates soil resistivity. This will save time and eliminate potential math errors.

What type(s) of ground systems will you test?

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 or two, driven into the ground and connected to the service entrance.

In the above illustration, 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 electrode rod driven 8 feet deep requires at least 80 to 100 foot test leads. If more ground rods are used, the distance requirement increases. Ground resistance test kits are available that include the measurement instrument, the auxiliary electrodes, and leads. Typical lead lengths provided in these kits are 150, 300, 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.

Does the test site have high soil resistivity and/or requires long test leads?

Another 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 and/or soil resistivity tests, you must consider the instrument’s injection current and test voltage. Typical injection currents range from a few milliamps up to several hundred mA. 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 10mA test current; so in this circumstance we recommend an 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.

The company is the world’s best Resistance Tester Electric supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

Is electromagnetic interference (EMI) present?

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 128Hz. 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 AEMC clamp-on models also offer test frequency selection.

Note that in some high-inductive environments lower test frequencies can produce more reliable results.

How will you use the measurement data?

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 3- and 4-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 or text message.

AEMC DataView® Ground Tester Resistance vs Frequency report (above, left) and AEMC Model Android™ app (above, right)

Do you need to test the bonding of grounding system components?

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 of 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 few 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 high test currents up to 200A. This can expose problem areas not always revealed when testing with milliamp-range currents.

In Conclusion

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.
• Bear in mind how you plan to use the measurement data.
• If you need to measure complex grounding systems, consider an instrument that can perform continuity checking on the bonding between components.

I'm trying to understand the complete reasoning behind performing an earth resistance test.

I very limited knowledge is that it is necessary to perform this test to ensure that any source has a low resistance path to ground requred by the NEC. I understand that this is necessary in order for the appropriate amount of ground fault current to flow to the source during a ground fault.

But what if there is a high resistance to ground at the location where the ground fault actually takes place. How come earth resistance testing is not required at equipment location, and is only required at the source such as a ground grid around a transformer?

Am I missing anything in regards to earth resistance testing?

I understand that this is necessary in order for the appropriate amount of ground fault current to flow to the source during a ground fault.


Am I missing anything in regards to earth resistance testing?

NO! This is not the purpose of GROUNDING of an electrical service. See Section 250.4(A)(1) and (2) for understanding of WHY we ground electrical systems.

Earth resistance testing is purely a design consideration that really does not indicate anything. The NEC never requires earth resistance testing, and for the most part doesn't even care what the earth resistance is. If earth resistance testing is required as apart of a contract on specified on plans, it means probably one of two things.

1. The engineer really doesn't know what grounding does and what grounding's limitations are.

2. A higher AHJ (insurance, underwriter, ect) is requesting this information because they don't know what grounding does and what grounding's limitations are.

Oh no, here we go again.

What is the application? Are you talking about a ground rod or 2 for a resi service or are we talking about a ground grid for a large industrial of generation station?

The application is various substations in a cement plant. Each one of the substations has a ground grid around it which requires an earth resistance test per our job specifications. We are governed by MSHA who requires this test and I've known this test to be used for several other reasons.

The type of test that I am familiar with for this is the "Fall of potential Test" or 3-point test. From information I have found this test seems to be a very popular test, so I find it hard to believe that earth resistance testing is not necessary if so much information is published about this test.

The application is various substations in a cement plant. Each one of the substations has a ground grid around it which requires an earth resistance test per our job specifications. We are governed by MSHA who requires this test and I've known this test to be used for several other reasons.

The type of test that I am familiar with for this is the "Fall of potential Test" or 3-point test. From information I have found this test seems to be a very popular test, so I find it hard to believe that earth resistance testing is not necessary if so much information is published about this test.

That wil be debated here as it has many times over and over, is it necessary for a house to be <25Ohms, not really, but your system has other reasons. If you dont mind do a search for the old threads where this has been discussed and I will be happy to answer any questions you have. You can also download "Getting down to earth" from the megger website for more information. Basically in your situation the low resistance is necessary for lightning protection systems and personel safety, not for relay operation. Mull what you are missing is high voltage vs low voltage systems. NEC applications are low voltage installations and earth resistance doesn't mean a lot, you could never get it low enough to operate most OCPD's. The NEC forbids earth to be used as a fault or load current path. For NEC applications the earth reference is mainly to provide a path for lightning, and accidental contact with high voltages like a transformer from primary to secondary fault, or primary distribution lines falling on secondary distribution lines.

High voltage is a different animal al together. Utilities use earth as both a fault clearing path and load current path.

All you have to do is examine Ohm's Law and run a few equations to figure out what is happening.

For example lets say your home GES measure 10 ohms. What will be the fault current of say 120 volts on a 20 -amp breaker? Would it be 12 amps. What would 12 amps do on a 20 amp breaker? Fall of Potential tests of grid resistance-to-earth for small sub ground grids in an industrial facility probably give erroneous results due to interference from underground piping, structural steel and other metallic objects that are interconnected with the grounding system either by design or accident.

To get good results with a FOP test, the current probe should be away from the grid a distance about 10 x the grid?s largest dimension. In a cement plant, you will probably have a lot of interconnected grids and structures easily resulting in an effective grid dimension of 500 - feet or more. You will need a two mile test lead. It is best if the ground between the remote current probe and the grid under test is homogenous - all the same type of soil, no streams or rivers, no underground metallic pipes, no overhead transmission lines with grounded static lines or grounded neutrals, etc, etc. (You also have to keep passer-bys from tripping over your miles of test leads or stealing it for the copper.)

Bottom line, it is difficult to get accurate FOP tests on a large grid or on a small grid that is inside a large industrial facility. There are test methods and analysis that can give a good indication of the grid resistance but it takes training and experience to perform the tests and interpret the results.

In my experience, an engineer or a government document specifies some minimum resistance to earth and a testing company is hired to do a FOP test on the completed grid. The test is done with short leads (>200 feet) to probes driven in whatever available soil can be found around the facility. The errors caused by the items I mentioned above tend to give a lower grid resistance reading than actual. So the test technician writes down the reading that is less than the required ohms and everyone is happy.

Is that a problem? Probably not. Usually the amount of copper in the ground is sufficient to provide a safe installation for most applications. If it is a high voltage substation (>35 kV) with potential for serious step and touch voltage issues, the engineers and owners require other tests to verify a safe installation.

In my experience, an engineer or a government document specifies some minimum resistance to earth and a testing company is hired to do a FOP test on the completed grid. The test is done with short leads (>200 feet) to probes driven in whatever available soil can be found around the facility. The errors caused by the items I mentioned above tend to give a lower grid resistance reading than actual. So the test technician writes down the reading that is less than the required ohms and everyone is happy.

Then they dont know what they are doing, an accurate FOP reading is proven with multiple readings graphed and showing a plautau region. OR if an accurate reading cannot be acheived using the 3 point FOP method, other proven methods can be employed. Either way, if you get a single reading and that is it, you just wasted your time and money.