Ground testing | Secrets of the soil - Part I

8 July 2019


Rocky terrain? Urban surroundings? Huge ground system? We’ve got a test for that.

When it comes to protecting the electrical system, grounding electrodes are essential. These buried metal conductors (rods, plates, grids, etc..) divert fault currents out of the electrical system and lock the voltage rating at specific values, keeping the electric grid safe and secure, every day.

Whether you are installing a ground electrode for the first time or performing routine maintenance, choosing the proper ground test is the first step. When performing a test, ideally, you would like to see a resistance as close to zero as possible, since the efficiency of a grounding electrode is inversely proportional to the resistivity of the earth.

The secret is in the soil (not the sauce, sorry). The soil is what makes or breaks a ground connection. When fault current travels through an electrode, it is dispersed in every direction through the surrounding soil. If the soil can accommodate this pattern of dispersal, then you’ve got a great connection. If not, your electrode is going to need some work.  


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Unlike tests performed in a lab, factory, or another closed environment, ground testing can be a beast. The test subject is planet earth, and I think we can all agree that earth is not predictable. In a ground test, the soil and geological conditions are running the show. We, as the test operators, must adapt, which can be tricky.

Keep in mind, you are not on your own here. Ground Testing Equipment measures current, performs calculations automatically, and throws resistance values back at you. Although you are not personally doing the math, understanding what’s going on behind the scenes will give you a clearer insight, as you comprehend your results and assess the efficiency of your ground system.   

When it comes to ground testing, there are quite a few methods available. While some are more popular than others, each method (typically) has a specific application that goes along with it. Indecisive? Don’t worry. We are going to break down all of the methods for you. Let’s dig in.

Fall of Potential

The Fall of Potential Method - it’s a classic and the only ground testing method that conforms to the IEEE 81. A reliable, highly accurate test for any size ground system – what more could you ask for? Plus, the operator is in complete control of the test set-up and can easily check his/her results by changing the spacing of the probes. On the other hand, it can be very time consuming and labor intensive. Particularly for large systems, which require long distances and test probes. 

Let’s break down the actual test. You have three points of contact – one with the electrode under test and two placed in the soil. In the soil, one probe will act as a current source, establishing a circuit through the soil, while the other probe will provide potential, measuring a voltage gradient established between the test current and surrounding soil resistance. Now, to get those test results. Imagine if it was as simple as just stretching out the test leads as far as they go, placing the probes in the ground, and boom – you have your results! If you do happen to get an accurate test this way, you’re very lucky. Perhaps, you may want to even consider buying a lottery ticket on the way home. For the rest of us, a reliable and accurate test comes from walking the potential probe at regular intervals, while recording a series of readings.

Wondering what those results would look like? We graphed them for you below. Your ground electrode’s resistance is the value at the flat, level point on the curve (B). When the potential probe is within the influence of the test electrode or the current probe, you will see the rises on the graph at points A and C, respectively. If there is not proper spacing between the current probe and the ground electrode, the potential probe will never fall outside the influence of the other probes and the graph will never become horizontal. If the shape of the graph doesn’t look like the one below, the current probe must be moved further out and the test needs to be repeated. Sorry, everyone.


Simplified Fall of Potential

Now that you (hopefully) understand the Fall of Potential Method, let’s talk about the simplified version. Please understand, the simplified method should only be used under circumstances where gathering enough data to plot a full curve of resistance vs. distance is impossible, since this method may compromise the accuracy of your results.

So here are the steps:

1. Take a reading (R1) with the potential probe (P) halfway (50%) between the earth electrode and the current probe (C).

2. Move reference potential probe (P) to a location that is 40% of the distance to C and take a reading (R2).

3. Repeat at 60% for reading R3.

4. Average these results (R1, R2, and R3). 

Now, time for some real math. You may be thinking, this doesn’t seem very “simplified” to me. Don’t worry, we are thinking this too! To make things easier, we suggest following along with the example below.  

5. Find the reading that is furthest away from the average of all the readings. In our case, it’s 55 Ω.

6. Determine the maximum deviation from the average. 

If 1.2 times this percentage (circled in red) is LESS than your desired test accuracy (which is 5% in our example), the average of the results can be used as your test result. Does this make sense? If your result is NOT within the desired accuracy, you must move the current probe further away and repeat the test.  


The 61.8% Rule

Moving right along – the 61.8% Rule. It’s simple. All you need to do is make one measurement with the potential probe at a distance of 61.8% of the length between the ground electrode under test and the current probe. Since this requires the least amount of exercise (to move the probes around), very little math, and the most basic procedure on the planet – you’re probably thinking, why doesn’t everyone do this? Well, it has quite a few limitations. To begin, it assumes that you are working under perfect conditions, with perfect, homogenous soil. It is also less accurate than both the Fall of Potential Methods that we previously discussed.

So, who is using this? Not everyone. Basically, if your testing site is very well known and highly proofed, the 61.8% is a great backup method of testing. Everywhere else, probably not your best bet. Since this is based on an ideal model, its actual application to real-world testing may fall short. You never know what may be lurking beneath the soil either; pipes, power cables, and irregularities in the soil composition will impact the accuracy of your test.

Okay, that’s all for today. Stay tuned for more secrets of the soil. Next time, we will be looking at the slope, intersecting curves, and dead earth methods of ground testing.