By Jeff Jowett

Fall-of-potential testing is the only ground testing method that conforms to IEEE 81. It is extremely reliable, highly accurate and can be used to test any size ground system. Additionally, the operator has complete control of the test set-up and can check or proof his/her results by testing at different probe spacings. Unfortunately, the Fall of Potential method also comes with several significant disadvantages:

• It is extremely time consuming and labor intensive; and
• Individual ground electrodes must be disconnected from the system to be measured.

The clamp-on ground testing method, although it does not conform to IEEE 81, does provide the operator with the ability to make effective measurements under the right conditions. The clamp-on methodology is based on Ohm’s Law (R=V/I). A known voltage is applied to a complete circuit and the resulting current flow is measured. The resistance of the circuit can then by calculated. The clamp-on ground tester applies the signal and measures the current without a direct electrical connection. The clamp includes a transmit coil that applies the voltage and a receive coil that measures the current.

For the clamp-on method to work, there must be a complete circuit already in place, as the operator has no probes and, therefore, cannot set up the desired test circuit. The operator must be certain that earth is included in the return loop. The tester measures the complete resistance of the path (loop) the signal is taking. All elements of the loop are measured in series. The method assumes that only the resistance of the test ground contributes significantly.

Figure #1 shows the basic methodology. The tester is clamped over R X. All test current travels through R X, but divides between the remaining resistances to return. Note that R X is assumed to be much, much greater than the remaining resistance. In a multiple ground system, the circuit can be considered a loop consisting of the individual ground electrode, a return path via all other electrodes and the mass of earth. The single electrode will have a higher resistance than the remainder of grounds connected in parallel.

Figure # 2 shows a practical example of where the clamp-on method is highly effective. The application is an interconnected parallel ground, like a lighting string. The system neutral completes the return. The resistance of the loop could be calculated by:

R loop = R 6 + (1/(1/R 1 + 1/R 2 + 1/R 3 + 1/R 4 + 1/R 5 ))

For six similar electrodes with a resistance of 10 W, the loop resistance measured when testing each electrode would be:

R loop = 10 W + 2 W = 12 W

 

For 60 similar electrodes with a resistance of 10 W, the loop resistance measured when testing each electrode would be:

R loop = 10 W + 0.17 W = 10.17 W

 

If one of six electrodes has a resistance of 100 W, and the rest have a resistance of 10 W, the loop resistance measured when testing the high resistance electrode would be:

R loop = 100 W + 2 W = 102 W

The loop resistance measured when testing each of the five other electrodes would be:

R loop = 10 W + 2.4 W = 12.4 W

The more returns, the smaller the contribution of extraneous elements to the reading and, therefore, the greater the accuracy. Even a “bad” (high resistance) element among many low resistance returns is not enough to defeat the measurement. But, if the returns are few, or all the elements are “high”, the error is large.

There are several major advantages to the clamp-on method, but also a number of disadvantages. It is important for the operator to understand the limitations of the test method so that he/she does not misuse the instrument and get erroneous or misleading readings. A clamp-on tester is an important tool in the bag of a test technician, but cannot be the only instrument used.

The primary advantage of the clamp-on method is that it is quick and easy, as no probes have to be driven and the ground rod does not have to be disconnected from the system. The clamp-on method also includes the bonding and overall connection resistance of the system, and can measure the leakage current flowing through the system, information that is not available from a fall-of-potential test. Fall-of-potential measures only the ground electrode, not the bonding of the system (leads must be shifted to make a bonding test). With the clamp-on method, an “open” or high resistance bond will show up in the reading because the clamp-on tester uses the grounding conductor as part of the return.

The clamp-on method is only effective in situations with multiple grounds in parallel. It cannot be used on isolated grounds, as there is no return path, making it not applicable for installation checks or commissioning new sites. In addition, it cannot be used if an alternate lower resistance return exists not involving the soil, such as with cellular towers or substations.

The operator must also be aware of the subtleties of the test method to ensure accurate results and analysis. If another part of the ground system is in the “resistance area” of the electrode under test, the result will be lower than the true resistance of the electrode, which could lead to a false sense of security. The test is carried out at a high frequency to enable the transformers to be as small and practical as possible. The downside is that this approach is less representative of a fault at power frequency than the traditional ground testing frequency of 128 Hz.

A good return path is required for more accurate readings. A poor return path may give high readings. The connection must be on the correct part of the loop for the electrode under test, as a wrong connection can give a faulty reading. The operator must have a thorough understanding of the system to know exactly what is being measured. The method is susceptible to noise from nearby electrical apparatus and is less effective for very “low” grounds (extraneous elements in the reading become comparatively large).

A final disadvantage of the clamp-on ground tester is that there is no built-in proof for the method. With fall-of-potential testing, the operator can check the results by increasing the probe spacings. With the clamp-on method, the results must be accepted on “faith”.

As noted previously, a clamp-on ground tester should not be the only test instrument used. It is, however, an important part of the ground testing tool kit, along with a fall-of-potential tester. The clamp-on tester can be used to identify problems quickly. A fall-of-potential tester can then be used confirm those problem results. This approach allows the operator to save time but improves accuracy.

Following is an application where the clamp-on method is often misused. This example will help show why knowledge of the system is critical to making the correct test.