Ground testing | Secrets of the soil - Part III
All good things must come to an end. It’s time for our last installment of Secrets of the Soil. From the Fall of Potential Methods to Large Grounding Systems, we’ve seen it all. This week, we are looking into the final methods of ground testing – Dead Earth, Star Delta, and Clamp-On.
The Dead Earth Method is the quickest and easiest way to make an earth resistance measurement. Before you get too excited though, let’s chat. There are many uncertainties that come along with this method, and we do NOT recommend using it. Unless, of course, it is the last resort. Regardless, we will break it down for you. You can follow along with Figure 1, below. If you are using a four-terminal testing instrument, you will connect the P1 and C1 terminals to the earth electrode under test, and then connect the P2 and C2 terminals to an all-metallic water pipe system. If you are using a three-terminal instrument instead, just connect the X terminal to the earth electrode and P and C to the pipe system.
There are a few limitations to be aware of when you are using the Dead Earth Method. You need to be working with a large waterpipe system with negligible resistance. As long as you are working with an extensive and wide-spread water system, the reading on the instrument should be the resistance of the electrode under test, since the resistance of the water system should only be a fraction of an ohm. The pipe system must also be metallic throughout, with no insulating couplings or flanges. Finally, the earth electrode under test must be far enough away, so it doesn’t fall within the water pipe system’s sphere of influence.
What happens if your earth electrode is too close to the water pipe system? Don’t panic. You can actually still run the test, as long as you have an extensive waterpipe system that is metallic throughout – no exceptions. If those criteria are met, you can connect to the waterpipe system and obtain a suitable earth electrode that is the right distance away. In this case, you should also install an earth electrode, as a precaution against potential changes in the resistance of the water pipe system.
Figure 1. Diagram of the Dead Earth Method from Getting Down to Earth.
If you are working with a small amount of space and limited room to move probes (like a city, perhaps), then we may have the answer. Better yet, if you’ve already tried the Slope Method or Intersecting Curves and you are getting unintelligible results – listen up. Named for the configuration of its test probes and lines of measurement, the Star-Delta Method is a space-saver, implementing a tight configuration of three probes around the test ground, as shown in Figure 2 below.
Figure 2. Star Delta Method Testing Configuration
If you are following along with Figure 2, the ground electrode under test is E and the three current probes are P2, P3 and P4. Each of the current probes are placed equidistant from E, with a 120° angle between them. Now, we will completely abandon everything we knew about potential and current probes and proceed with a series of two-point measurements. You will be making measurements between all pairs of probes and between all probes to the electrode under test.
Is it just us or does this remind you of elementary school math? If Alex has 3 shirts (red, blue, and yellow), 2 pairs of pants (black and tan), and 2 pairs of shoes (blue and white), how many different outfit combinations can he make? (If we did the math right, it’s 12 outfits.) For the Star Delta method, it’s basically the same thing, but you will be making 6 measurements (shown in orange and purple).
In isolation, the electrode under test (E) and each of the probes have their own resistance – R1, R2, R3, and R4, respectively. The resistance between them is the sum of their individual resistances. After making the six measurements, it’s time for a bit of a mathematical crunch time.
If the results from equation 1 matches the results from the other three equations, then you are good to go. Unfortunately, if one of the probes has been placed improperly and its resistance area overlaps with that of E or another probe, then you most likely obtained a false reading. If this happens, you are going to have to redo the test – sorry folks.
Lucky for you, with just a “little” bit of math, you can determine which probe is at fault. Just use the equations below to figure out who to blame…
You made it. It’s time for the final method of ground testing – the Clamp-On Method. Let’s start out with the obvious. You absolutely must have a clamp-on ground tester to do this one. Additionally, you’ll need to have a complete circuit already in place that includes the earth in the return loop. This method doesn’t utilize probes, so you will not be setting up your own test circuit, as we have done in the previous methods.
The clamp-on method is based on Ohm’s Law (R = V/I). A known voltage is applied to a circuit, the resulting current flow is measured, and the resistance of the circuit is calculated. Easy as pie! Best of all, the clamp-on tester does it all, thanks to a transmit coil that applies the voltage and a receive coil that measures the current.
By the way, this method assumes that only the resistance of the ground electrode under test contributes significantly to the resistance.
There are quite a few advantages that come along with this method. First, it is quick and easy, requires no probes, and the ground rod can stay connected to the system. The clamp-on method can also give you information about the bonding and overall connection resistance of the system, as well as leakage current, which is not available among the other tests.
So, why aren’t we all just using clamp-on testers all the time? Unfortunately, this method is only effective if multiple grounds are in parallel, and it cannot be used on isolated grounds. Thus, it is not a suitable test for installation checks or commissioning new grounding sites, sadly. Additionally, you cannot use the clamp-on method if there is an alternate lower resistance return not involving the soil (think: cellular towers or substations). Bummer. Noise can also be a problem with clamp-on testers, as they are very susceptible to noise from nearby electrical devices. The operator needs to be on his/her A-game, as well, since a thorough understanding of the electrode system at large is vital for performing this method.
With the fall of potential methods, you can check your results by changing the spacing of the probes. With the clamp-on method, there is no way to ensure that you have correctly identified the resistance of your ground electrode. You just need to have faith that you got the right result. Terrifying, right?
But, when used in combination with other methods of ground testing, the clamp-on method is a great tool to have in your box. It can quickly identify problems, confirm results, and allow the operator to save precious time.