How do you perform the measurement of the earthing resistance of a tower in an overhead line, the so called tower footing resistance? This topic is addressed in different types of literature, is advised by consultancy companies and mentioned by the manufacturers of measurement equipment. In most cases reference is made to the fall of potential method, focusing on a measurement at 61.8% (or 62%) of the current lead length. Although this is not our preferred method (as you can read here) it can definitely provide you with good results. At least, if you know what you are doing. We have found many sources that try to explain how this measurement works, but not all are 100% complete, and some even make incorrect statements.
What is the Fall of potential method?
The fall of potential (FOP) method is the most classic approach and is used worldwide. Typically, the setup makes use of a current loop, for which an electrode is placed at a distance D from the tower and a lead is connected. This way, the current can be injected in the tower and finds its return via this electrode and lead. Then, the potential between the tower and soil is measured by placing a voltage probe (electrode) at 62% of distance D. Then, the resulting earthing resistance is determined with Ohm’s law, dividing the voltage by the current, resulting in a resistance value.
Why do you measure at 62% if you use the fall of potential method? This has to do with the golden ratio.
In this case the current injection and voltage measurement are handled by one (combined) or two (separate) devices. However, this setup only leads to meaningful results if the shielding wires are disconnected. Variations to the setup are possible, for example by measuring the actual current that flows into the earth, or the currents that flows away via any connected shielding wires (or underbuilt wires). In that case, the curve (as shown below) might have a bit of a different shape, since injected current and return current are not completely equal. What is also a common approach, is to use stray currents in the system instead of an injected current. What the best approach is depends on the (safety) policy from the asset owner, and the local situation and conditions, such as the soil type and available space to perform the measurement.
Some advises that you may find in literature or on the internet are described next:
At what distance D shall the current electrode be placed?
– Advice A: Use 10x the depth of the grounding point.
– Advice B: The distance shall be so large that the middle of the curve is flat
– Advice C: At least 8 to 10 times the depth of the grounding system
– Advice D: The variation at 52%, 62% and 72% shall not vary too much
All these points have a common background, but it is (almost) never explained why.
The distance shall be large enough, and the main reason is that for the application of Ohms law, you will have to assume that the current is injected into a very small point, one on each side of the measurement setup. But in fact, both the tower and the electrode are not point injections, they distribute the current throughout the complete earthing arrangement. It takes some distance before you can refer to the equipotential lines near the tower foot as a half-hemisphere. This is illustrated in the figure below.
In other words, because the earthing structure covers a larger area in the earth, following a different shape than a half-hemisphere, the potential distribution close to the tower foot will be greatly influenced by the shape of the structure. Only at a greater distance the tower foot can be considered to behave like a half-hemisphere. Therefore, the required distance depends on the shape of the earthed structure and the local soil characteristics.
So, what to do?
Often it is sufficient to use a distance D equal to or greater than 5 times the maximum diagonal dimension of the earth electrode (tower diameter), if the soil has a uniform soil resistivity, or the earth electrode is completely in the top layer. If no soil information is available, a distance D of 10 times the diagonal dimension is advised. If you are not sure, it is always advisable to take some extra distance.
How many measurements shall be performed?
– Advice A: One measurement at 62% is sufficient. If the current electrode is placed at sufficient distance you will have the answer that you are looking for.
– Advice B: Three measurements shall be performed, at 20%, 40% and 60%, then the slope method shall be used to determine where the actual measurement shall be taken.
– Advice C: One measurement at 50% and two or three additional measurements in both directions.
In theory, advice A would be great. However, this is based on several assumptions. For example, the 62% measurement point is only correct if the soil resistivity is not varying to much (uniform) in the measurement location area. This is seldom the case. In that case the ideal measurement point is not at 62%, but may be anywhere between 50% and 70%. Next to that, you will not be able to see any (single point) measurement inaccuracies. Blindly following the slope method (advice B) can also result in errors. What if one of the three points is disturbed by local deviations in the soil or nearby conductive elements?
How to solve this?
We always recommend to take multiple measurements, which is in line with the listed advices, but adds a bit more to it. This way you are able to correct for any error that may be applicable to a single measurement point. For example, we always measure at multiple distances between 20% and 85% of distance D. More points are better! Then, plotting the results will give you a basic overview of the applicable values. At a later point you can analyse which values may be interesting to actually take into account to determine the earthing resistance value for example with this tool we made.
Which reading(s) shall I use to determine the earthing resistance value?
– Advice A: One measurement at 62% is sufficient. If the current electrode is placed at sufficient distance you will have the answer that you are looking for.
– Advice B: Perform three measurements (at 52%, 62% and 72%) and take the average.
– Advice C: Perform three measurements (at 20%, 40% and 60%) and use the slope method to determine the final measurement point.
These are all different approaches, leading to a variety of results. It may be that the differences are small, but what is the best theoretical approach?
The theory allows you to convert any value (measured at any distance) to the applicable earthing resistance value. For example, if you have a measurement performed at 50%, you can convert it to the value that you would measure at 62%. The only thing you need for this, is the local soil resistance value (as seen by the tower). This means that you can perform multiple measurements and convert all these values to the requested earthing resistance value by changing the soil resistivity parameter. This way you can average multiple measurements, which corrects for any inaccuracies of a single measurement point. As a bonus you will have an indication of the local soil resistivity! Try is with this tool.
If you want to know more about how to do this, please check this course on measurements of tower footing impedances.