Earth resistance measurement - fall of potential method with clamps

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.

Pic. 4 The grounding of a low-voltage pylon

A low-voltage line is shown in pic.4. The groundings of individual pylons are connected to each other with PEN cable. As we look at pic. 3, we may suppose it is enough to put clamps on a test joint and take the measurement. Unfortunately we can’t do this. The bonding conductor of the pylon is connected to its reinforcement. The concrete itself contains moisture. The water content of the concrete combined with the mineral salts present create an electrolyte conducting the electrical current. When we induce the flow of the current, it will appear in all the line. Clamps should measure the value of the current that flows through the grounding of the pylon. But there is also the current that flows through the pylon reinforcement and the current that flows through the concrete, which the pylon is made of. The latter one is minute but we have to remember about it. The meter will measure the value of the voltage drop for the sum of the currents flowing through the reinforcement, the concrete and the measured grounding. But the meter will measure current resistance only for the current measured with the clamps. If this is the case, a higher current than the one, which was measured, induces the voltage drop. As a result the obtained value of the earth resistance will be significantly higher than the standard parameters. It is not a problem when it comes to protection against electrocution. However, it might lead to an unnecessary modernization of the grounding that would incur additional costs. The grounding can be disassembled of course. Unfortunately it is forbidden in the case of live power lines. The safety regulations ban this kind of procedure. Shutting down the line is costly and troublesome. Even bigger problems are related to high-voltage lines. Crate pylons are large conducting elements themselves. It is impossible to use such clamps on them. Additionally air-terminations systems for crate pylons are connected to their surrounding rims. So the current flows in the metal rim. It makes it impossible to analyse the results of the measurement, which were taken only on the bonding conductor. Thus we would like to introduce to you, maybe not revolutionary but a groundbreaking method of measurement with clamps. Sonel Company designed flexible clamps and implemented them to the common use. They are significantly thinner and they are up to 5metres long. Clamps that have been used so far can’t have such a diameter. The new clamps work differently than the old ones. The new working method caused design changes to the input measurement circuits. Sonel doesn’t want to force customers to buy new meters. If the customers want to use this innovatory method, they can buy an intermediate module that works between the meter and the clamps. It is called Sonel ERP-1 adapter.

Pic. 5 Flexible clamps (Rogowski coil) and an adapter Sonel ERP-1

Clients can use for testing, well known for them, flexible clamps type F, dedicated FS clamps (more sensitive) and customized FSX (highly sensitive used in extremely difficult conditions). Sonel ERP-1 is sold with FS clamps 4 meters long as a standard offer. The adapter has the option to adjust the clamps. Pressing the chosen button does it and the choice is signaled by a led diode. Pressing another button, we select the number of coils that the measured grounding consists of.

As an example we can use the situation shown in pic. 6 and pic. 4.).

Pic.6 Correct measurement of low-voltage pylon grounding without disconnecting a test joint and ERP-1.

As it is shown in the picture, surrounding the whole pylon and the grounding with clamps enables us to measure all current flowing in the circuit to the ground. The result of the measurement will be correct then. The difficulty that appeared earlier in such situation, doesn’t matter any longer. It was directly related to the electrical properties of the circuit. Spun concrete pylons are used more often nowadays. Double spun concrete poles are also used, e.g for 20/04 stations. It used to be very difficult to measure the earth resistance for such a construction. Using the flexible clamps and Sonel ERP-1 adapter, it is no longer difficult. 

Pic. 7 Measurement for spun concrete poles. 

Using clamps, we can surround two poles at the same time. We don’t have to worry if the two groundings are connected underground. The clamps measure the total value of the current flowing through the groundings.

Testing on crate pylons is a much more complicated matter. The solution offered by Sonel may become a groundbreaking one. It enables the tester to use the fall of potential method without the need to disconnect the high-voltage line. Using Sonel MRU-200 and Sonel ERP-1 it is possible to diagnose the grounding of crate pylons. It hasn’t previously been possible when the single clamps were used (on live power line).

The rules of the measurement regard the fall of potential method. The procedure slightly differs. 

Pic. 8 Measurement of the crate pylon grounding.

We surround the whole leg of the pylon when testing. It is possible to make more than one coil to achieve more accurate result. We choose the testing procedure by selecting Sonel ERP-1 module on an operating unit. Then we select the number of legs that the pylon has got (1,2,3 or 4). We start the measurement after connecting the meter. We must remember that when we change the position of clamps, we must also change the position of the test joint that induces flow of current E. The meter will show on its display earth resistance for the whole pylon after completing the series of tests. Automatic procedure of calculating earth resistance for a crate pylon is available only on MRU-200. It should be emphasized that the meter checks the direction of the current for an individual test. That’s why the clamps must be joined in the same direction every time a test is taken. The meter can recognize damage such as physical break-down (or total corrosion) of the bonding connector connected to the bonding ring of such a pylon. This function of the meter in unique and no other meter available on the market offers it.

Roman Domański