Measuring the earth resistance is a time-consuming process. Taking measurements requires a lot of commitment from the staff responsible. It depends on the grounding structure, terrain and many other objective factors. Both physical and mental commitment is needed. It is necessary to measure the grounding accurately without omitting any element of the measuring procedure. Taking shortcuts may result in such big errors that the whole measurement will have no metrological importance.
It is obvious that any device that simplifies or makes testing easier and can be used in this case is particularly desirable. Groundings, regardless of their properties, must be disconnected if they are a unit consisting of many air-termination systems in order to measure the resistance selectively. This is not necessary if we use clamps for fall of potential method. The time needed for the measurement is reduced significantly. We need to determine two factors if it is possible to take the measurement using the clamps. The factors are the electrical circuit of the grounding and its design. Let’s remind ourselves of the rules of earth resistance measurement with the fall of potential method in order to expand on the subject.
Pic. 1. Principles of the fall of potential method
If we want to measure the earth resistance “E”, we must force the current to flow through it. To do this, an auxiliary test probe H has to be put into the ground at a certain distance from the grounding to be tested. This way the electrical circuit of our unit is created. Induced by the current supply in the meter, alternating current flows and electrical potentials are created around the grounding and the auxiliary test probe H. It happens when the alternating current flows in H circuit through the ground and the tested E grounding. A fall of voltage will occur because of a certain resistance of the tested grounding. It is enough to build a voltage circuit and measure the value of the voltage fall to determine the earth resistance. We use the second auxiliary test probe S. We put it into the ground between the tested grounding and the auxiliary current probe. The method of testing is illustrated in Pic. 1. It looks simple. We have to remember about a few necessary rules though. The test probe has to be far away enough so that the potential surrounding the measured grounding doesn’t overlap the potential of the auxiliary probe H. Voltage auxiliary probe S has to be placed in the area of zero potential. At this stage this is the first element related to the accuracy and time-consumption of measurements. One test doesn’t guarantee that the measurement is correct. At least two more tests are required in order to verify the accuracy of the measurement. It should be done by placing the voltage test probe a few meters closer to the tested grounding and then moving it closer to the auxiliary current probe. We can consider the measurement as the correct one only when three results of earth testing are the same or very close.
This method is commonly used but the principles of how to use it are very often forgotten. In case of single groundings there are no practical problems when this method is applied. A pylon of medium-voltage line can be an example of a single earthing electrode.
Pic. 2. Medium-voltage line pylon
It is a typical single grounding because groundings of line pylons aren’t connected to each other. The use of a method other than the one described above can cause nothing but measurement errors. Using clamps in such cases is forbidden. Let’s explain when the fall of potential method can be applied when clamps are used.
If we take the measurement of the resistant unit determining individual resistance values for each air-termination system, we have to disconnect the measured part of the grounding from the whole system. It is possible to determine what current flows through the grounding element and how big the voltage fall is without disconnecting test-joints. It can be done using clamps. So it is exactly an earth resistance measurement with fall of potential method shown in pic. 1. The only difference is that we measure the current flowing through a single air-termination with clamps.
Pic. 3 Principles of ground resistance testing using clamps
The lightning conductor of the building is shown in picture 3. There are four air-termination systems connected to each other on the roof. The current, that we induce, flows through the whole circuit but clamps can measure the value in an individual element of the grounding system. Undoubtedly, it is a very convenient method. Unfortunately it can’t be used everywhere. The reason for this is the design of the clamps themselves. The clamps have specific dimensions. We must take into account the thickness and the angle of the clamps arms. We are not able to place them anywhere. It is difficult in newly built or modernized (insulated) houses. Construction companies cover the grounding installation with Styrofoam and install small inspection windows on test joints. It is difficult to put a hand inside or reasonably big clamps. This is the first limitation, the size of the inspection window. The second limitation is the electrical circuit of the grounding.