A Transcat Application Note
The Top 5 Questions About Ground Testing
1. What is meant by the term "ground"?
Exact definitions are available from the National Electric Code and IEEE. It is significant to remember, however, that the term tends to be user-specific; that is to say, it "means" what the speaker intends. When a dialogue is in progress, always be careful that the participants are referring to the same thing.
Ultimately, a "ground" is an intentional electrical connection with the earth, by means of an electrode, which references the connected electrical system to earth (i.e., zero) potential. However, the term is also used to refer to the electrode itself, as well as the conductors that extend throughout the system to tie the equipment to "ground". By extension, "ground" may simply be taken to mean the third wire in a typical "hot-neutral" line cord. It may also be used to indicate an unintentional connection to earth, where a piece of equipment has faulted and "shorted out". So before any meaningful dialogue proceeds, always be sure to determine the manner in which "ground" is being used.
2. How does a ground tester perform its test?
A dedicated ground resistance test instrument has a unique design, specifically fashioned to deal with the features that set ground testing apart from more familiar types of measurement. Unlike familiar test items (motors, transformers, wire and cable), the earth is obviously not a designed circuit, does not confine current flow to a specific path, and has no "beginning" or "end". Furthermore, a ground resistance test is a volumetric measurement, that must be capable of measuring the resistance of a volume of soil that surrounds the ground electrode full-circle, not in a straight-line path, as in a familiar circuit.
To accomplish this, the tester has a current circuit, and a potential circuit. By means of metal probes driven into the soil, a test current is established between the ground electrode and the current probe. Similarly, a voltage probe contacts the soil at a distance commensurate with the volumetric resistance under test, and a measurement made of the associated voltage drop. With these two parameters, voltage and current, the instrument calculates and displays, via Ohm’s Law, the resistance exhibited by the soil to a flow of current from the ground electrode. For purposes of visualization, the "item under test" can be thought of as not just the electrode, but the "ball" or "block" of soil surrounding it, to a distance beyond which the rest of the earth, because of its vastness, contributes no more appreciable resistance.
3. Can I use an insulation tester to do the same test?
No. This is a common error, and the "victims" are often field operators who are issued from stores a "Megger"®, without its being checked to determine whether it's an insulation or ground tester. Insulation testers are designed to measure at the opposite end of the resistance spectrum from a ground tester. No one wants "grounds" that measure in MegOhms! To do this, insulation testers employ high test voltages, in the kV range. Ground testers are limited, for operator safety, to low voltages. Insulation testers do commonly have low-voltage, low-resistance continuity functions, and these are frequently misused to make "jiffy" ground tests. But a continuity test can only make an arbitrary measurement between an installed electrode and a reference ground, which is assumed to have low (i.e., negligible) resistance. This does not, except by luck, afford a reliable measurement of the resistance the earth offers to a ground fault current. Moreover, even this arbitrary measurement may not be reliable, since a DC continuity test can be influenced by soil transients, the electrical "noise" that is generated by utility ground currents trying to get back to the transformer, as well as other sources.
4. The required measurement is of resistance; why can't I use a multimeter?
For the same reasons that a continuity range on an insulation tester should not be used. Measurements made with a DC multimeter are subject to distortion by electrical "noise" in the soil, and a multimeter offers no means of verifying that the resistance displayed represents anything other than an arbitrary measurement between two convenient points. That is to say, with a multimeter, one can measure the resistance of the soil between a ground electrode and some reference point, such as the water pipe system, but a fault current may encounter a higher resistance. Genuine ground testers are amenable to field-developed standard procedures that have built-in cross checks that expose insufficient test conditions and a reference ground, which is assumed to have low (i.e., negligible) resistance.
5. What is the difference between a "2-point", "3-point", and "4-point" test?
Literally, the number of points in contact with the soil. More specifically, these are commonly used terms that correspond to what are also called, respectively, "Dead Earth", "Fall of Potential", and "Wenner Method" tests. In the "Dead Earth" method, contact is made at just two points: the ground electrode under test, and a convenient reference ground, such as the water pipe system or a metal fence post. In the "Fall of Potential" method, a genuine ground tester makes contact via the test electrode, plus the current and potential probes. With the Wenner Method, no ground electrode is involved, but rather the independent electrical properties of the soil itself can be measured, using a four-probe setup and a recognized standard procedure.
Information for this article supplied compliments of Megger