How to Use and Select Clamp-Type Ground Resistance Tester Correctly

    Reducing the grounding resistance of power line towers can improve the lightning resistance of the line, reduce lightning accidents, reduce contact voltage and step voltage near the towers, and prevent electric shock accidents for people and animals. Therefore, the grounding resistance of the tower is an important data. All aspects of design, construction, and operation must be taken very seriously. Its true value must be accurately measured and made lower than the specified value. In the past, grounding megohmmeters were used to measure grounding resistance, but since it was necessary to lead a measuring line of more than 100 meters from the grounding grid to two auxiliary ground electrodes, the workload was large, and it was often restricted by the terrain and environment. The position of the auxiliary ground electrodes could not meet the requirements, so it was difficult to obtain correct measurement results. In recent years, the clamp-type grounding resistance tester has been introduced. Due to its simple measurement method, it is welcomed by the majority of line workers.

    In order to correctly use the clamp-type ground resistance tester to measure ground resistance, first of all, you must understand its measurement principle. The clamp-type ground resistance tester is used to measure the ground resistance of any loop system. The instrument itself can generate a power supply potential, which can generate current in any loop system. Therefore, its measurement principle is simply the Ohm's law of the entire circuit. It measures the loop resistance value of this loop system.

    The method of measuring the grounding resistance of power line towers with a clamp-type grounding resistance tester is simple and the measurement results are highly reliable, but it can only be used on high-voltage lines with overhead ground wires. During measurement, only one grounding down conductor is allowed to exist on the tower to be measured. If the ground grids of each tower foot are not connected, the grounding down conductors of the remaining feet should be disassembled and connected to the down conductor of the measuring foot with temporary wires (the connection point is below the clamp meter). By analyzing the measurement results, it can be determined whether the ground grids of each tower foot are connected and whether there is a hidden danger of poor contact of the grounding down conductor.

1. Development and selection of ground resistance tester

    The ground resistance tester is a commonly used instrument for testing and measuring ground resistance. It is also an indispensable tool for electrical safety inspection and grounding project completion acceptance. In recent years, due to the rapid development of computer technology, the ground resistance tester has also infiltrated a large amount of microprocessor technology. Its measurement function, content and accuracy are incomparable to general instruments. At present, advanced ground resistance testers can meet all grounding measurement requirements. Using the new clamp method, there is no need to pile and lay out lines for direct online measurement. A powerful ground resistance tester is controlled by a microprocessor, which can automatically detect the connection status of each interface and the interference voltage and interference frequency of the ground network, and has unique functions such as value retention and intelligent prompts.

 2. Ground resistance test and development

    Initially, people used the volt-ampere method to measure grounding resistance, which was a very primitive test. Figure 1 shows the measurement method using an ammeter and a voltmeter. When measuring resistance, you must first estimate the current and select an insulated wire with an appropriate cross-section. In the preliminary test, you can use a variable resistor R to adjust the current. When the formal measurement is performed, the variable resistor is short-circuited, and the grounding resistance can be calculated from the values ​​obtained by the ammeter and voltmeter.

     The volt-ampere method for measuring ground resistance has obvious disadvantages. First, it is troublesome, cumbersome, and labor-intensive. During the test, the grounding rod is 20 to 50 meters away from the ground pole, and the auxiliary grounding is at least 40 to 100 meters away from the ground. In addition, it is greatly affected by external interference, and sometimes it is simply impossible to measure in a strong voltage area.

    In the 1950s and 1960s, the Soviet Union's E-type megohmmeter replaced the volt-ampere method. It was easy to carry and a hand-cranked generator, so the workload was simpler than the volt-ampere method. In the 1970s, domestic ground resistance meters came out, such as: ZC-28, ZC-29, no matter in structure, volume, weight, measurement range, graduation value, accuracy, they were better than the "E" type megohmmeter. Therefore, for quite a long time, the ground resistance meter was represented by the ZC series produced by Shanghai Liubiao Factory. Due to the hand-cranked generator, the accuracy of the above instruments is not high.

    The use of digital ground resistance meters in the 1980s brought new life to ground resistance testing. Although the wiring method of the test is no different from that of the ZC series, its stability is much higher than that of the megohmmeter pointer type. A real innovation of ground resistance meters was the birth of clamp-type ground resistance meters in the 1990s, which broke the traditional testing method. For example, the 6411 clamp-type ground resistance tester produced by the French CA company can be regarded as a major revolution in ground resistance testing. The biggest feature of the clamp-type ground resistance test is that there is no need to hit the auxiliary ground rod. As long as the ground wire or ground rod is clamped, the ground resistance can be measured. The above-mentioned ground resistance tester is a single-jaw type, which has its advantages of fast testing and simple operation, but it also has the problem of low accuracy, especially when the ground resistance is less than 0.7Ω, it cannot be distinguished. In addition, the single-jaw ground resistance meter is mainly used to check the multi-electrode grounding network connected above the ground, and query the grounding status of each grounding electrode through the loop ground resistance, but it cannot replace the power frequency ground resistance measurement of the entire network. The measurement range and accuracy of the GEOX double-clamp ground resistance meter have been improved, but because the clamp method uses the principle of electromagnetic induction, it is susceptible to interference and has a large measurement error, which cannot meet the requirements of high-precision measurement. The double-clamp multifunctional ground resistance tester developed and produced by Wuhan Xinchunda Electric Power Equipment Co., Ltd. combines the characteristics of traditional volt-ampere method measurement with the new technology principle of the clamp method, and uses advanced computer control technology to become the leading intelligent ground resistance measuring instrument in the contemporary era. It has the characteristics of high accuracy, complete functions and easy operation. It can be widely used in ground resistance measurement of power telecommunications systems, buildings, airports, railways, oil tanks, lightning arresters, high-voltage iron towers, etc. At present, it has been configured in domestic post and telecommunications, power, aviation and other industries.

 3. Selection of ground resistance tester

    Why is ground resistance still a problem that is ignored by everyone? The main reason is that there is no ideal measuring instrument. The grounding shaker has poor test value accuracy due to well-known reasons. Sometimes the same ground resistance becomes an abstract physical quantity, which is difficult to grasp. With the development of scientific instruments, advanced ground resistance testers have completely mastered the essentials of ground resistance testing, and can make the test value correct. At present, intelligent ground resistance meters are not only powerful, but also can cope with various complex situations on site, such as effectively eliminating interference, automatically tracking the most suitable test conditions, and intelligent prompts when various problems occur. In addition to the above-mentioned ground resistance test instruments, the French CA company 6412, 6415 single-clamp ground resistance meter is also a popular ground resistance tester. Domestic production of similar products includes ET2000, and the basic functions are similar to those of CA. The XJD series ground resistance tester produced by Wuhan Xinchunda Electric Power Equipment Co., Ltd. is the most ideal ground resistance meter for grounding device inspection during the rainy season, or in the middle of the year or at the end of the year. If you need a higher test accuracy and want to be convenient and easy, you can use the XJD series double-jaw ground resistance tester. For example, the ground resistance measurement of grounding devices such as output power poles, microwave towers, lightning rods, and auxiliary devices with good grounding conditions (faucets, water pipes), this double-jaw ground resistance meter can be used to measure the ground resistance. For large-scale system grounding, network grounding, and soil resistivity testing, you should choose XDW-3 and other ground resistance meters. Using three-wire and four-wire measurement methods, due to the unique functions of the instrument, the repeatability and stability of the ground resistance measurement value are guaranteed, and the measurement accuracy of the XDW-3 ground resistance meter is as high as 3%, and its test current is less than 3A.