Fall-of-potential testing is extremely reliable, highly accurate, conforms to IEEE 81 and gives the operator complete control over the set-up. Unfortunately, it is exceedingly time consuming and labor intensive, and requires that the individual ground electrodes be disconnected from the system.
The clamp-on testing is quick and easy, but has many limitations. It requires a good return path, is susceptible to noise, has reduced accuracies and cannot be used on isolated grounds. It is not applicable for installation checks or commissioning new sites and has no built in proof.
Attached Rod Technique (ART)
Mix of clamp-on testing and fall-of-potential testing
The Attached Rod Technique (ART) method of testing provides some of the advantages of clamp-on testing (not having to disconnect the ground electrode) while remaining true to the theory and methodology of fall-of-potential testing.
Figures 1 and 2 show the three measurements that would be made.
#1 – The first step is to measure the resistance (RT) of the entire system using a typical fall-of-potential configuration. In this example, the reading for RT is 1.9 Ω.
#2 – Step two involves measuring the total current (IT) being injected into the system from C1. For this example, IT is 9.00 mA. The next step is to measure the amount of current (IU) flowing to the service. In this case, IU is 5.00 mA.
With these measurements, the voltage drop from the selected volume of soil to the point of the P2 can be determined as follows:
- V = IT × RT
- V = 0.009 A × 1.9 Ω
- V = 0.017 V
The current through the ground electrode (IG) can also be determined as follows:
- IG = IT – IU
- IG = 9.00 mA – 5.00 mA
- IG = 4.00 mA
Using the voltage drop and the current through the ground electrode, the resistance of the ground electrode (RG) can be determined.
- RG = V / IG
- RG = 0.017 V / 0.004 A
- RG = 4.25 Ω
As noted, this is a theoretical approach that requires perfect conditions. Any additional current flowing from the service through the ground electrode would reduce the accuracy of the measurement.
The earth leakage clamp meter would have to filter out all but the current generated by the instrument through C1 to ensure accuracy. Additionally, this approach requires that a number of mathematical calculations be made.
The test is a fall-of-potential test, meaning that all the “rules” still apply. Ideally, the operator would take ten measurements and plot the results to determine true resistance. Proper probe spacing remains critical, and fall-of-potential procedure and methodology must be followed. As with a traditional fall-of-potential test, the results can be proofed by increasing the probe spacings.
The Attached Rod Technique is based on the theory outlined above. Figure 3 shows an ART test being made.
Ground testers that are designed to make ART measurements include a special built-in current clamp that is placed between the C1 connection and the earth. This type of instrument includes noise protection and digitally filters out all currents other than that generated by the instrument.
The instrument’s microprocessor automatically performs all the calculations necessary to generate a resistance measurement for the ground electrode.
The advantage of the ART method over traditional fall-of-potential testing is that the ground electrode under test does not have to be disconnected from the system.
Using ART method with Megger DET3TC to test commercial ground without disconnecting the system
Reminder! How Earth Resistivity is Measured?
A four terminal instrument is used to measure earth resistivity. Now, however, you use four small-sized electrodes driven down to the same depth and equal distances apart in a straight line. Four separate lead wires connect the electrodes to the four terminals on the instrument, as shown.
Hence, the name of this test: the four terminal method. Keep reading:
Electrode test (VIDEO)
Reference // Getting down to earth by MEGGER