Lightning detection in the importance of grounding resistance and its influencing factors
Author: Song Wei Wang Youli Zhang Yanlong Qin Dongxu Liu Jianguo
Keywords Lightning detection; grounding resistance; meteorology; equipment
I. Definition of Grounding Resistance
Grounding resistance actually refers to the current from the grounding device flows to the earth and then to another grounding or to the distance Spread the resistance encountered. Grounding resistance is divided into frequency grounding resistance and impact grounding resistance. Frequency grounding resistance is the grounding body of the flow of current as the frequency current and thus get the grounding resistance; and impact grounding resistance is the grounding body of the flow of current as the impact current and thus get the grounding resistance value, which in the case of lightning current flow through the case is very valuable for research. We usually work in the measured grounding resistance value value for the frequency grounding resistance value, so usually if there is no indication of which type of grounding resistance, are referred to as the frequency grounding resistance. I can calculate the formula to convert the grounding resistance to measure whether it is in line with the requirements of the regulations. Conversion formula for: R = ARi.
Two, lightning detection of the importance of grounding resistance analysis
Testing the merits of the grounding device is an important indicator of the size of the grounding resistance, in general, the smaller the grounding resistance, lightning occurs, the faster the speed of its dispersion, and once the object is struck by lightning, the duration of its resulting high potential time will be the shorter the lightning protection device on the high potential will correspondingly be the same as that of a lightning strike. Lightning high potential is correspondingly lower, reducing the threat to people and various equipment.
According to the relevant electrical principles, when a lightning strike occurs, the lightning current generated through the lightning protection device, grounding resistance on the high voltage and grounding resistance is proportional to the relationship, that is, the impact of the grounding resistance of the value of the smaller, the voltage (voltage backlash across the step voltage and contact voltage) on the threat of the human or object is smaller, it can be seen that the grounding resistance can be used as an important indicator of the merits of the grounding device for Measurement. In the various types of lightning protection code, in different uses at the same time on the grounding resistance requirements are clearer. For example, in the "lightning protection technology standard specification compilation" (hereinafter referred to as "specification compilation"), respectively, the lightning protection type for one, two, three types of lightning protection building grounding resistance for specific provisions, one, two types of resistance should be less than 10 Ω, three types of resistance should not be less than 30 Ω, and power transformers or generators should not be greater than the work of grounding resistance of 4 Ω. Therefore, great importance should be attached to the grounding resistance of the relevant testing work.
At present, with the gradual development of lightning protection and grounding technology, in the process of detecting the grounding resistance, other factors should be considered comprehensively, such as also need to equipotential connection measures and structural attributes of the grounding device and so on whether to comply with the specification requirements of the detailed test. According to the relevant provisions of the Code Compendium, in areas with high soil resistivity, the local economic conditions and the difficulty of the construction of the region for comprehensive consideration, should focus on the iron frame and the lightning rod and public **** grounding system connection between the status of the inspection, and for medical equipment, computer systems should be focused on considering the status of the equipotential connection.
Three, lightning detection in the grounding resistance factors and their countermeasures
(a) factors
1. meteorological conditions. Because there is no specific provision in the code compilation in the grounding resistance test should have meteorological conditions, so when the actual resistance test, the local meteorological conditions (such as humidity, temperature, etc.) to have an understanding of the local meteorological conditions, and then according to these to clarify the correlation between grounding resistance and meteorological conditions. The relationship between grounding resistance and soil resistivity is positively proportional, in other words, the higher the resistivity of the soil, the higher the resistance value of grounding resistance. The chemical composition of the soil, the relative humidity and temperature, and the compactness of the soil will have an effect on the resistivity of the soil, and among these factors, the relative humidity and temperature of the soil are the ones that will have the most serious effect on the resistivity.
2. Testing equipment. In the compilation of specifications required to detect the resistance is impact grounding resistance, and in most of the meteorological stations in the use of the Japanese production of the shaking table type ground resistivity meter, through this ground resistivity meter is called the detection of frequency grounding resistance, and the compilation of specifications in the requirements do not meet. Therefore, in the resistance meter test, focus on testing the potential gradient in the soil is approximate to 0, that is, the resistance meter is placed in the region of the zero point, in order to avoid errors, so as to make the tested grounding resistance value is more accurate and effective, but it is difficult to do in the actual test. Most of China's lightning protection testing organizations in the detection of grounding resistance, the more common use of clamp-on grounding resistance meter to test, this resistance meter detection speed is relatively faster and do not have to use the auxiliary grounding rod, more easy to use. In the reality of the grounding body resistance test, can not measure the grounding body as a test pole and to test the distance between the grounding body, in some special circumstances, the distance between the two grounding body is very short, can not meet the measurement standards, and in the case of not yet mastered the internal structure of the grounding device, the two grounding body and the underground electrical communication, at this time to test the resistance value does not have the Reliable risk, the error generated is also very large.
3. Random factors. In the actual detection of grounding body resistance value process, must ensure that there is no interference with unfavorable factors, so that the measured data more accurate and effective. In the grounding resistance test, there will be some random unfavorable factors to the detection process, such as the detection of the ground resistance meter used in the measurement process to produce a small amount of current, will make the measured data is not accurate. In addition to these interfering factors, there will be some human factors on the detection process, for these factors must have enough attention, must maximize the protection of the measurement process is not affected by factors interfere.
In addition, in the detection of grounding resistance, there will be a high-rise building lightning protection equipment grounding resistance detection of the situation, the detection will use a long test line, which will also make the detection error is large, for example, some high-rise building lightning protection project is doing very well, but in the detection of the grounding resistance of a high error. So in order to avoid this situation, the staff should take into account the resistance and inductive impedance generated by the test line that exceeds the standard length and the interference electromotive force brought about by the electric current and other factors.
(2) countermeasures
1. grounding resistance value in a large extent by the test personnel's manipulation of the impact of the test should pay attention to: the tester's three poles should be in a straight line and perpendicular to the ground network; ground network test point and the tester's connecting line length is preferably less than 5 m. If you need to lengthen the line, you should be the measured value of the ground resistance value of the line with the extension of the value of the resistance subtracted, and then fill out the form and so on.
2. Ground resistance is affected by the testing environment, testing, grounding resistance tester grounding leads and other wires should be high and low voltage power supply lines to avoid, to prevent danger and interference; if the ground network charged on the detection of the impact should be the cause of its identification, to solve the problem of charged and then measured, or change the location of the test measurements; if the measurement of high-frequency interference, industrial leakage, stray current and other factors, so that the grounding resistance value of the actual value of the extended line resistance value and then fill in the form. If the measurement because of high-frequency interference, industrial frequency leakage current, stray current and other factors, so that the grounding resistance meter readings are not stable, you can change the ground network test point and the tester's line to a shielded line, or choose to change the test frequency, with a narrow-band filter or frequency amplifier of the grounding resistance meter test, so that its anti-interference capability can be improved; according to the DL475-92 "grounding device measurement of the physical parameters of the measurement of the industrial frequency guidelines," the provisions of the ground when the large-scale grounding device or ground network diagonal D ≥ 60m need to use high-current measurement, applied to the current pole on the frequency current should be ≥ 30A, in order to exclude interference so that the error is reduced.
3. According to the actual test object on the grounding resistance requirements of the accuracy of the selected detection method. Usually can be used three-pole method, but if there is a higher grounding resistance accuracy requirements, it must be used four-pole method, and orientation, multi-point test.
4. In the calibration of the effective period of use of the testing equipment can be used, measuring instruments and testing equipment to comply with national regulations on measurement, testing equipment, see "Building Lightning Protection Device Detection Technical Specification" GB/T21431-2008 Appendix E. At the same time the selection of testing equipment should be based on the actual detection of the object of the grounding method, in the test Note that the ground network to be measured is not a single point of grounding, the measured ground and equipment is not connected, there is no reliable grounding circuit, so as to choose the appropriate measuring instruments.
5. Ground resistance value of the test should be in the soil is not frozen and non-rainy days, the weather and climate conditions should be able to make the normal test can be carried out.
Four, conclusion
In summary, grounding resistance is a measure of lightning protection detection in the performance of grounding devices and the quality of the main indicators of the lightning protection project, in the actual detection process, there are a variety of factors on the detection of the data caused by the interference, which makes the detection of the grounding resistance is not accurate enough, true. The grounding resistance can meet the requirements, is to ensure that the reliability of the lightning protection device is the key, so it can be seen from this, lightning protection detection in the grounding resistance plays a very important role.
References
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