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Several methods of grounding resistance test

time:2020/8/12   source:华天电力  reading:254 time

Several methods of grounding resistance test


The ability to correctly measure ground resistance is essential to prevent costly downtime due to service interruption caused by poor grounding. Here are the four most common methods used by test technicians:


Earth Resistance Tester.png


Two-point method


In areas where it may be impractical to drive ground rods, the two-point method can be used. With this method, the resistance of a series of two electrodes is measured by connecting the P1 and C1 terminals to the ground electrode under test, and P2 and C2 are connected to Separate all-metal grounding points (such as water pipes or construction steel).


Dead place


The simplest way to obtain ground resistance readings, but not as accurate as the three-point method, and can only be used as a last resort. It is the most effective for quickly testing the connections and conductors between the connection points.


Note: The ground electrode under test must be far enough away from the auxiliary ground point to exceed its influence range.


Three-point method


The three-point method is the most thorough and reliable test method; it is used to measure the ground resistance of the installed ground electrode.


Four-point method


This method is the most commonly used method to measure soil resistivity, which is important for designing electrical grounding systems. In this method, four small-sized electrodes are driven into the earth at the same depth and equal distance (straight line). The soil moisture and salt content fundamentally affect its resistivity, and the soil resistivity measurement will also be affected by the existing nearby ground electrode. If the buried conductive object in contact with the soil is close enough to change the test current mode, It will invalidate the reading, especially for large or long objects.


The four-point method is the most commonly used technique in soil resistivity measurement


Clamping method


The uniqueness of the clamping method is that it can measure resistance without disconnecting the ground system. It is fast and simple, and also includes the grounding and overall ground connection resistance in the measurement. The uniqueness of the clamping method is that it can be used in The resistance is measured without disconnecting the ground system, and the tester is "clamped" around the ground electrode to be measured. Similar to the method of measuring current using a multimeter current clamp, the tester applies a known voltage through the transmitter coil without directly It is electrically connected, and the current is measured through the receiving coil. The test is performed at a high frequency to make the transformer as small and practical as possible.


In order for the clamping method to be effective, there must be a complete grounding circuit, the complete resistance path (loop) used by the tester to measure the signal, all elements of the loop are measured in series, and the operator must understand the limitations of the test method. So that he/she will not abuse the instrument and obtain false or misleading readings.


Some limitations of the clamp method include:


• Only valid when there are multiple parallel connections.


• Cannot be used on isolated ground, not suitable for installation inspection or commissioning of new sites.


• If there are alternative low resistance circuits that do not involve soil, such as cellular towers or substations, they cannot be used.


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