The earthing resistance measurement is a crucial aspect of ensuring electrical systems’ safety and efficiency. Among the various methods to measure the earthing resistance, the Fall of Potential (FOP) method stands out for its precision and reliability. However, several misconceptions surround this method, particularly regarding the measurement point and the position of the voltage electrode. Let’s dive into the Fall of Potential method, debunk common myths, and provide a comprehensive understanding of its correct application.
What is the Fall of Potential Method?
The Fall of Potential method is a technique used to measure the resistance of an earthing system. It involves driving a current through the earth between two electrodes (a current electrode and a earthing electrode which is the object of interest) and measuring the resulting voltage drop (potential), preferably at various points, to determine the resistance. This method is widely recognized for its accuracy and is commonly used in electrical engineering and safety inspections.
Debunking the 61.8% Myth
A prevalent misconception about the Fall of Potential method is that the voltage electrode must always be placed at 61.8% of the distance between the current and earthing electrodes. Yes, in theory the placement at 61.8% of the distance would be correct, but there are two important aspects that should be taken into account.
1. The influence of soil layers
The 61.8% rule is derived from the concept of the potential gradient, where the potential drop is assumed to be linear between the electrodes. This approximation works well in uniform soil conditions, but it is not universally applicable. The actual potential gradient can vary significantly depending on soil resistivity, moisture content, and other environmental factors. In practice this means that if a layered soil is present at the location, and the layers have (very) different resistivity values, the ideal point may be varying between 50% and 70%.
2. Mathematics
Fortunately, in the past there were some clever people that captured the electric behaviour of soil and objects into formula’s. So there happens to be a formula that allows for converting measurements taken at any position of the voltage electrode to the equivalent measurement at the 61.8% distance. This formula ensures that the results are consistent and accurate, regardless of the actual position of the voltage electrode.
The Conversion Formula
The conversion formula is based on the principle of proportionality and takes into account the actual distances involved. Let:
- Rmeasured be the measured resistance
- ρ be the distance from the current electrode to the voltage l electrode
- D be the distance between the electrodes (with D12 the total distance, D1 and D2 the distance of the voltage electrode to the subject of measurement and the current return electrode.
The resistance at the 61.8% distance, denoted as R61.8%, can be calculated using the formula:
This formula adjusts the measured resistance to what it would be if the voltage electrode was placed at 61.8% of the total distance. By applying this conversion, engineers can achieve accurate and consistent results without being constrained by the 61.8% rule. The user shall estimate the value of the local (uniform) soil resistivity.
Practical Considerations
While the conversion formula provides a robust solution, there are practical considerations to keep in mind:
- Soil Conditions: there it is again. The formula takes a soil resistivity value for a uniform soil configuration, but often a multi layer soil is present. And non-uniform soil conditions can still affect the accuracy of measurements.
- Electrode Placement: Ensure that the current and earthing electrodes are placed at sufficient distances to minimize interference and achieve reliable readings. And also very important: placing the voltage electrode close to either the measured object or the current return electrode may lead to inaccurate measurements. The reason for that is that the formula is based on hemispherical shapes of the soil potential around the objects, but at very close distances this is not accurate (also see the figure below).
- Environmental Factors: Be aware of environmental factors such as moisture content, temperature, and soil composition, as they can influence earthing resistance measurements.
It is essential to take multiple readings at different positions and use averaging techniques to mitigate these effects.
Conclusion
The Fall of Potential method remains a highly reliable technique for measuring earthing resistance, provided it is applied correctly. The 61.8% rule, while useful in theory, should not be rigidly followed without considering soil conditions and other factors. By using the conversion formula, engineers can ensure accurate measurements regardless of the voltage electrode’s position. In addition, this enables the user to take multiple readings and look for an average value. This will provide additional insight and a higher trust in the result.
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