July 2019
Ground Testing on Large Systems | Secrets of the Soil - Part II

Ground Testing on Large Systems | Secrets of the Soil - Part II

18 July 2019


We are back. If you missed last week’s blog or you need a refresher, you can find it here. This week, we are looking at large electrode systems – the giants of ground testing. For the big guys, we can either use really, really long leads or find a new way to approach the situation. Que, the slope, intersecting curves, and four-potential methods of ground testing.

Let’s kick things off with the slope method.

Slope Method

If you are working with a large grounding system, we are talking thousands of feet, using the fall of potential method is often unfeasible or nearly impossible. With the slope method, you do not need to find the “flat” part of the curve, synonymous with the fall of potential method, illustrated below.

Figure 1. Graph of the Fall of Potential Method of Ground Testing.

To implement the slope method, readings are measured at 20%, 40% and 60% of the current probe distance. Even if you are working in non-homogenous soil, the slope method provides accurate and satisfactory results with a relatively easy technique.

Let’s go through the steps. First, you need to, obviously, connect the Earth Tester you are using to the grounding electrode rod under test (E). In a large earth electrode system, there may be many parallel rods forming the complex, so chose the one that is most convenient – maybe on the edge or the corner of the grounding system. The current probe should then be inserted at a distance of Dc away from E. Check out the visual below (Figure 2) . Usually, this distance is around 2 to 3 times the maximum dimension of the grounding system.


Figure 2. Diagram of the Slope Method. 

After that is set up, potential probes are inserted into the ground at distances of 20%, 40%, and 60% of Dc. Then, you simply measure the earth resistance at each of these potential probes (step 1 below) and record the resistance (R1, R2, and R3). After you have these measurements, you are ready to do some math. In step 2, you will see the formula for calculating µ. This value represents the change of slope of the resistance/distance curve.


When you have this value, head to Table 1, shown below. This table (and complete set of instructions) can also be found in our handy Earth Testing Guide (Getting Down to Earth). Find the value of DP/DC that corresponds to the µ that you calculated in step 2. By the way, DP stands for distance of the potential probe. Since we already know the DC (distance to the current probe), we can easily calculate a new DP (distance of the potential probe), using the ratio we found from the table in step 2. See step 3 for this formula.

Table 1. Values of DP/Dfor various values of µ. 


Now, measure the earth resistance again at this new distance of the potential probe. That measurement will be referred to as the “true” resistance. Still following along? We are almost finished. At this point, you must do everything all over again, we are sorry. But it’s easy, just move the current probe further away, so DC is at a larger value. After recording the earth resistance, it’s time to make some comparisons.

If the “true” resistance decreases appreciably as DC is increased, the current probe distance needs to be increased even further. If you keep repeating this process and plotting the “true” resistance for each test, your curve will begin to decrease less – indicating a more stabilized reading. Once the curve is stable, you’ve found the resistance of your grounding system.

There are also some disclaimers that come along with this method. First, and this is more of a general note, if the calculation of µ is greater than the values listed on the table, you need to move your current probe further away. Next, it is recommended that you repeat this test in multiple directions with various spacing arrangements. Ideally, the results should match up with a reasonable amount of agreement, so you can have some confidence in the accuracy of your results. Finally, noise can quickly become a problem in large testing systems. You should be using an instrument with advanced technical capabilities that can easily overcome the effects of significant noise.

Intersecting Curves

When you are working with an earth-electrode system that consists of many rods connected in a parallel grid, spread out over a large area, you will start to run into problems when it comes to earth testing. Some methods may ask you to run a lead as far as 3000 feet away from the electrode under test. How convenient is that? Intersecting curves method to the rescue!

To help you along in this journey, please refer to the figure below (Figure 3). Things are about to get a little more complicated. First, let’s get to know the players. You have an arbitrary starting point (O), which all measurements are made from. The distance from O to the potential electrode or current electrode will be referred to as P and C, respectively. A curve (like abc below) of resistance against the potential can then be plotted. Then, just for kicks, let’s say that the true electrical center of the earth-electrode system is actually at point D, which is a distance X from O. Are you lost yet?

Now that we have all of the variables defined, we know that the distance from the true center to the current probe is C + X, right? Correct. And, the true resistance occurs when the potential probe is at a distance of 0.618 (C + X) from D. So, the value of P, must be equal to 0.618 (C + X) – X. If you know the values of X, you can calculate the values of P and then read the resistance, right off of the curve.

Almost there. Time to plot a new curve. This time, these resistances (from above) can be plotted against the values of X. Once you have your new curve, it’s time to do it all over again at a new value of C. Ouch.

At this point, you will have two curves of resistance against X. When you overlay these curves, the intersection point is the resistance of your ground electrode system. If you are uncertain about the validity of your results, you can repeat the process a third time. Easy as pie!


Figure 3. Diagram of the Intersecting Curves Method

Four Potential Method

If you are still with us, it’s time to talk about our last method – the Four Potential Method. It is based on the Fall of Potential Method, so if you read our last blog, then you should have a head start.

To start, you’ll need to set up the test probes, as they are shown in the figure below (Figure 4). You will be making measurements from the edge of the electrical system and setting the current probe at a suitable distance away from the grounding system. This distance can range up to 2,000 feet for large grounding systems or areas of very low resistance, which is a major drawback to this method. But, if you can get past that small detail, you are golden!

The potential probe is then placed at a distance equal to 20% of C (the current probe) and the resistance is measured. This test is repeated for distances equal to 40%, 50%, 60%, 70%, and 80% of the distance to the current probe. No need to go to the gym tonight, you will get plenty of exercise moving the potential probe back and forth! The resistance values obtained are then plugged into the following four formulae… (Note: R1 = 20%, R2 = 40%, R3 = 50%, R4 = 60%, R5 = 70%, R6 = 80%).


Once you’ve finished the math, each of the values for R should substantially agree, and an average for the four results can be calculated. Because of the assumptions made for this theory, it is possible that the results from equation (1) will not be as accurate as the others. If this is the case and (1) looks like an outlier comparatively, it can be ignored and the average can be calculated from the other 3 results.


Figure 4. Diagram of the Four Potential Method


You made it. We've covered the basics, but the choice is ultimately yours. When you are working with a large system of grounding electrodes, you can always rely on the Four Potential, Intersecting Curves, or Slope Methods of ground testing. It may require a calculator, or perhaps an energy drink, but you can do it. 

Next time, we will be looking at the final three methods of ground testing: Dead Earth (Two Point), Star Delta, and Clamp-On.