By hvhipot 
Analysis of inaccurate grounding resistance readings
1. Reasons for inaccurate detection of ground resistance or unstable value or even negative value
Because the ground resistance measuring instrument is composed of many precision electronic components and has a relatively long detection line, under the influence of bad environment and operation, measurement errors are often caused, and it is difficult to confirm the accurate value of the measured ground resistance. The main ones are as follows factor:
(1) There are large potentiometers on the surface, and there are independent grounding in many places, such as the grounding of transformers in factories and complex buildings. Due to various reasons, the grounding resistance becomes larger, the insulation of the transformer itself deteriorates, and leakage occurs. There is a potential difference around the ground electrode. If the detection rod is placed around it, it will affect the measurement accuracy.
(2) The tested ground electrode itself has alternating current (the electrical equipment is not well insulated, the leakage phenomenon caused by partial short circuit, and there is interference from the parallel high-voltage power supply near the down conductor); the early structure of the building was rather chaotic. The wiring is disorderly, and sometimes even the potential difference of the ground neutral line is above 100V, which directly affects the measurement error of the ground resistance.
(3) Poor contact (including the instrument itself): The wiring connection of the ground resistance tester is easy to break due to frequent bending and use, and it is difficult to find due to the existence of the protective cover, which causes the phenomenon of disconnection and connection; in addition, due to The detection rod and alligator have been used for a long time, and there are oxidation and corrosion phenomena, which can also cause poor contact; if the measured ground electrode is seriously oxidized and the embroidery is not good, it will also affect the measurement reading.
(4) There are strong electromagnetic fields emitted by transmitters, antennas, etc. nearby: Introduce high-power equipment near high-power transmitting bases, such as mobile, microwave, and BP machines, high-voltage substations and high-voltage lines. Frequent starting locations.
(5) When the grounding device and the metal pipeline are buried in a complex place, it may also cause poor or unstable grounding resistance measurement, such as gas stations, chemical plants, etc., due to the complicated layout of the underground metal pipelines, the underground metal appearance is unsteady when connected according to the normal detection. The existence of, actually changes the direction of current at each end of the measuring instrument, and often causes the phenomenon that the measured value is zero or negative. If there are different soil resistivities in the same site, this phenomenon can also be caused.
(6) When detecting high-rise buildings, a too long detection line induces a voltage and causes a detection error. At the same time, the long line itself also has a cable resistance.
(7) When using sandy soil with high soil resistivity and poor water absorption as the foundation cushion of a whole-story building, the ground resistance measured is often too large.
(8) The operation is not carried out in accordance with the method specified in the instruction manual, the instrument itself is improperly maintained, and the sick or over-checked instrument is used.
Two, avoid method
(1) When testing the grounding resistance and electrostatic grounding resistance of gas stations, liquefied gas stations and high-rise buildings, the layout of buried metal (oil, gas) pipes, grounding devices and metal devices is not very correct on the map Marked out, so the placement direction and distance of the test meter rod when testing grounding resistance have a great impact on the measured value. Usually, the value is different with the direction and distance. Sometimes the measured value may even appear negative. Especially the detection of buried metal pipelines in gas stations and other places often occurs. The solution is to understand the layout of underground metal pipes before testing, not only to check the grounding device diagram, but also to check the layout diagrams of other underground metal pipes, and choose the place where the impact is as small as possible to place the P and C grounding electrodes.
(2) Where there is a disconnect card in the grounding down conductor, disconnect it as much as possible for testing to avoid the influence of other equipment on the testing.
(3) If there is an abnormality during the test, the cause should be found out, or the test should be compared at different times, in different directions and locations, and the correct test value should be obtained.
(4) In order to avoid electromagnetic interference of the lead under high electromagnetic field, the detection lead should be shortened relatively, and the inner diameter of the lead should use qualified multi-stranded metal wire.
(5) When testing the grounding resistance in the place of the sand-gravel cushion with high resistivity, the P and C grounding electrodes should be placed in a moist and well-conducted place with the earth, so that the measured grounding resistance is relatively correct.
(6) Testing should be carried out in accordance with operating procedures, testing instruments should be maintained regularly, and over-tested instruments should not be used.