Abstract: This paper briefly introduces the importance of measuring the grounding resistance of power transmission towers, the concepts of grounding systems and contact resistance, traditional measurement methods, their simple principles, and their respective advantages and disadvantages. It analyzes a new grounding resistance measurement method improved by utilizing the advantages of each method and proposes several methods for handling grounding resistance exceeding the standard value. Keywords: Grounding resistance measuring instrument, power transmission tower CLC Number: TM934.15 Document Code: A Article Number: 1003-0867(2005)07-0046-02 Power transmission line towers must be reliably grounded to ensure that lightning current is discharged into the earth and protect the line insulation. To improve lightning withstand level, protect equipment insulation, and avoid personal injury caused by step voltage, it is necessary to reduce the grounding resistance of the power transmission towers. 1 Introduction to Grounding Devices Grounding devices. This refers to the general term for grounding electrodes and grounding down conductors. A grounding electrode refers to a metal conductor buried in the ground and in direct contact with the earth; for power transmission towers, it refers to metal components such as round steel and angle steel buried underground. The grounding down conductor is the part that connects lightning-attracting equipment (lightning conductor, lightning rod, etc.) to the grounding electrode. For towers, this mainly includes independent grounding down conductors, the reinforcing bars of reinforced concrete poles (non-prestressed), and the steel of the tower. Grounding resistance. Traditional grounding resistance measurements (using a ZC-8 type resistance meter) only measure the grounding resistance of the grounding electrode. However, analysis shows that lightning current flows from the top of the tower through the grounding down conductor into the earth. From the perspective of conducting lightning current, the resistance of the entire discharge channel should be considered, not just the grounding resistance of the grounding electrode. Furthermore, the grounding electrode, grounding down conductor, and lightning conductor are connected by bolts, connecting plates, and welding, and there is contact resistance between them. Therefore, the grounding resistance should be the sum of the resistance of the grounding electrode, the resistance of the grounding down conductor, and the contact resistance. 2. Measurement Method Before Improvement The measurement method using the ZC-8 type grounding resistance meter is simple, and its advantages are accurate measurement of the grounding resistance of the grounding electrode and stable performance. However, this method has a fatal weakness: it can only measure the grounding resistance of the grounding electrode, and all grounding leads must be disconnected before measurement, requiring the laying of tens of meters of wire, resulting in a large workload and low efficiency, with each person only able to measure about 5-6 electrodes per day on average. The advantage of using the CA6411 resistance meter is that, under conditions of good grounding system contact, it can accurately measure the grounding resistance of the entire leakage channel. It is simple to use, saves time and effort, and is highly efficient, with each person able to measure about 15 electrodes per day on average. The disadvantages are that when the grounding system is rusty or has poor contact, the measurement results have a larger error; and because it measures the grounding resistance of the entire leakage channel, it cannot determine the location of the excessive resistance value. 3. Improved Measurement Method Using the ZC-8 resistance meter and the CA6411 resistance meter alone has many advantages and disadvantages. Combining the two can leverage their respective advantages; therefore, an "elimination method" measurement method is proposed. First, use the CA6411 meter to measure; if the grounding resistance is qualified, proceed to the next electrode. If the measurement is unqualified, then use the ZC-8 meter to measure the grounding resistance of the grounding electrode. If the resistance is found to be substandard, it indicates that the problem lies in the grounding down conductor and its connection points. If the ZC-8 measuring instrument also fails the test, the grounding conductor should be treated first until it meets the requirements. Then, a CA6411 measuring instrument is used. If the test is successful, the next tower is tested. If the test is still unsuccessful, it indicates that there is still a cause for the high resistance value in the grounding down conductor. The "elimination method" was used to test the tower grounding resistance. This method improved work efficiency compared to simply using the ZC-8 resistance measuring instrument, allowing each person to test approximately 10 towers per day. Because this method can pinpoint the location of excessive resistance values, it lays the foundation for targeted treatment. 4. Measurement Result Analysis and Processing Methods When using the "elimination method" to test the tower grounding resistance, the following common problems were found in the tower grounding system: In the tower grounding system, there is a large contact resistance at the grounding connection plate. Table 1 shows several towers with large contact resistance values. In the table, for tower No. 137 of the factory-junction line, when measured with a CA6411 grounding resistance meter, the grounding resistance of one leg was normal at 7.2 W; the grounding resistance of the other leg was 1120 W. After disassembling the grounding connecting plate, the grounding electrode resistance of both legs was measured with a ZC-8 measuring instrument, and both legs showed 8.4 W. On-site personnel removed mud and rust from the connecting plate of the leg exceeding the standard, applied conductive paste, and then tightened it with bolts. The CA6411 grounding resistance meter was used for a retest, and the result was 7.5 W. This fully demonstrates the existence of contact resistance and the reliability of the improved measurement method. In the table, towers No. 58 of the Nanji line and No. 133 of the factory-junction line are non-prestressed reinforced concrete poles using steel bars as grounding down conductors. After measuring with the new method, it was found that the grounding electrode connecting plate and bolt connection of No. 58 of the Nanji line were severely corroded. The treatment method adopted was to weld additional grounding bolts to the steel hoop at the pole section connection and extend the grounding electrode to the steel hoop using steel strand or 10 mm diameter round steel. It was discovered that grounding electrode No. 133 on the factory grounding line was severely corroded. The original 10 mm diameter round steel grounding electrode had corroded to only 6-8 mm in diameter. On-site, a 2.5 m long, 45×5 angle steel was vertically driven into the ground. 5. Conclusion Regarding measurement methods, the approach has evolved from being unable to locate locations of excessive resistance using a single measuring tool to being able to pinpoint the exact location using multiple instruments. In tower grounding systems, the concept of contact resistance was introduced. Practice has proven that contact resistance is a weak link in tower grounding systems. After treatment, the tower grounding system can improve its lightning resistance level with only a small investment, which is of great significance for line lightning protection.