Key Questions to Ask When Ordering ground resistance tester

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.

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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.

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.

Ground Resistance Test Set with leads and accessories. Photo: AEMC

In electrical power distribution systems, a protective ground conductor is an essential part of the safety earthing system. For measurement purposes, the Earth serves as a somewhat constant potential reference against which other potentials can be measured.

Knowing how to properly test an electrical ground system is crucial to ensure that it has an appropriate current-carrying capability to serve as an adequate zero-voltage reference level.

In this article, we take a look at frequently asked questions by test technicians and trainees related to ground-resistance test methods.

Contents

• What is the difference between a two-point, three-point, and four-point ground resistance test?
• How often should ground systems be tested?
• What is considered to be an acceptable ground resistance reading?
• What affect does rain have on a ground resistance test?
• How deep should I drive my test probes?
• Does watering down a ground test probe to improve contact influence my test result?
• Is it possible to perform ground resistance test on concrete or macadam?
• What can I do if there isn’t enough room to run out my test leads?
• Can I test ground rods in sandy or rocky soil?
• Can an insulation tester (Megger) or multimeter be used to perform ground resistance tests?

1. What is the difference between a two-point, three-point, and four-point ground resistance test?

Ground tests are named after the number of points that come in contact with the soil. Commonly used terms refer to dead earth, fall of potential, and Wenner method tests.

• Dead Earth (Two-Point): In the dead earth method, contact is made at just two points: the ground electrode under test and a convenient reference ground, such as a water pipe system or metal fence post.

• Fall of Potential (Three-Point): In the fall-of-potential method, contact is made at the ground electrode under test while the current and potential probes contact the soil at predetermined distances in the test procedure.

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• Wenner Method (Four-Point): With the Wenner Method, no ground electrode is involved, but rather the independent electrical properties of the soil can be measured using a four-probe setup and a recognized standard procedure. This test is also known as soil resistivity.

Related: 4 Important Methods of Ground Resistance Testing

2. How often should ground systems be tested?

Weather conditions and seasons have the biggest effect on ground systems. Most standards recommend testing in odd intervals of 5, 7, or 9 months. Using odd intervals ensures the worst case seasons will be revealed.

3. What is considered to be an acceptable ground resistance reading?

The goal in ground resistance testing is to achieve the lowest ground resistance value possible. The most widely used specification for grounding is found in the National Electric Code, which specifies residential grounds have a resistance of 25 ohms or less.

Some specifications may demand a lower resistance, such as one specified by an engineer, client or equipment manufacturer. The NFPA and IEEE recommend a ground resistance value of 5 ohms or less. Computers, generating stations, and process control equipment may require as little as 1 or 2 ohms.

4. What affect does rain have on a ground resistance test?

Increased moisture from rainfall dissolves salts in the soil and promotes added conductivity – resulting in a lower resistance. If it has rained heavily prior to your test and the electrode barely meets specifications, odds are that it won’t pass when the soil is dry.

5. How deep should I drive my test probes?

It is a common misconception that driving test probes deeper will improve ground-resistance readings. Test probes need to only make a minimum amount of contact with the soil, which can be obtained by observing the test set display.

When using Ground-resistance sets with high resistance tolerance, it may not even be necessary to penetrate the surface in order to meet the threshold tolerance. Simply laying the probes flat and watering down the area will often be sufficient.

6. Does watering down a ground test probe to improve contact influence my test result?

Watering a ground-resistance test probe is a specialized means of improving contact, similar to sanding an electrode before connecting it to a circuit. This method should have no influence on your final reading as long as the electrodes have enough spacing when watering.

7. Is it possible to perform ground resistance test on concrete or macadam?

Since concrete conducts current fairly well, chances are you only need to lay your probes flat on the surface and wet the area to establish contact. Macadam on the other hand does not conduct as well as concrete because of the tar content, but it may be possible to achieve enough contact.

If you are having problems obtaining ground resistance readings with the probes provided with your test set, try using a ground contact mat made of a flexible metallized conductive pad, such as a piece of sheet metal.

8. What can I do if there isn’t enough room to run out my test leads?

If there is not enough room to stretch out your leads for fall of potential testing you will have to try another method, reference the test procedures described in IEEE Standard No. 81. The most-used procedure used in this situation would be the Star-Delta method.

The Star-Delta method is an adaptation of the two-point method. Test probes are arranged in a fairly close triangle around the ground under test and a series of measurements are made between the various two points (probe to ground and probe to probe, for example). Values are then run through a series of specially designed equations in order to obtain a ground-resistance reading.

9. Can I test ground rods in sandy or rocky soil?

It is possible to test ground rods driven in sandy or rocky soil, although its more difficult to test because the moisture that promotes electrical conductivity quickly drains away. Rocky soils especially have poor overall consistency and reduced surface contact electrodes due to the large spaces between each element. In many cases longer and stronger probes may be required to make good contact with the soil.

Related: What is Soil Resistivity and Why Does it Matter?

10. Can an insulation tester (Megger) or multimeter be used to perform ground resistance tests?

No. Insulation testers are designed to measure high levels of resistance and are capable of producing high voltages. Ground testers are designed to measure low resistance and are limited to low voltages for operator safety.

Related: Test Equipment 101: Electrical Testing Fundamentals

With a multimeter, it is possible to measure the resistance of the soil between a ground electrode and an arbitrary reference point (ex. water pipe system), but in a real world situation, ground fault currents may encounter a higher resistance.

Measurements made with a DC multimeter or insulation tester are subject to distortion by electrical noise in the soil. Ground resistance test sets are specifically designed to expose insufficient test conditions.

References