Research on the Principle, Method and Application of Contact Loop Resistance Test for High-Voltage Switches

Abstract: As a critical control and protection device in power systems, the contact loop resistance of high-voltage switches directly determines the equipment’s conductive performance, heating characteristics, and operational reliability. Excessively high contact loop resistance is prone to causing local overheating, contact burnout, and even equipment failure, which seriously endangers the safe and stable operation of power grids. Based on the demand for condition-based maintenance of power systems, this paper systematically elaborates on the core principles, standard test procedures, and key technical points of contact loop resistance testing for high-voltage switches. It also analyzes the error sources and corresponding prevention measures during the testing process, and discusses the innovative applications and development trends of testing technology in conjunction with the industry’s technological frontier in 2026. This study provides a scientific basis and practical guidance for the condition assessment, fault diagnosis, and operation and maintenance optimization of high-voltage switches.
Keywords: High-voltage switch; Contact loop resistance; DC voltage drop method; Four-wire measurement; Condition-based maintenance; DRM technology

1. Introduction

Against the backdrop of the accelerated construction of new power systems, power grids have imposed higher requirements on the reliability and safety of electrical equipment. The transformation from traditional "post-fault maintenance" to "predictive maintenance" and "condition-based maintenance" has become an industry consensus. High-voltage switches are widely applied in the power generation, transmission, and distribution links of power grids at all levels, undertaking the important functions of circuit switching and fault clearance. The conductive performance of their contact loops (including contacts, busbar connections, wiring terminals, and other components) is a core indicator to ensure the normal operation of the equipment. Contact loop resistance refers to the resistance generated when current passes through contact parts, which is mainly composed of contact resistance and conductor resistance. Among them, contact resistance is affected by factors such as contact material, contact pressure, and surface condition, and it is the primary cause of abnormal loop resistance.
When the contact loop resistance exceeds the specified limit, significant Joule heat will be generated under rated current, leading to an increase in contact temperature, accelerating contact oxidation and aging, and even triggering serious faults such as contact welding and insulation damage, which may result in power grid blackouts. Therefore, regularly conducting contact loop resistance testing of high-voltage switches, accurately grasping the value and variation trend of loop resistance, and timely identifying potential hazards are important contents of high-voltage switch operation and maintenance work. This practice is of great engineering value and scientific significance for ensuring the safe and stable operation of power systems.

2. Core Principles of Contact Loop Resistance Testing for High-Voltage Switches

The testing of contact loop resistance for high-voltage switches is essentially a low-resistance measurement. Since the contact loop resistance is usually in the range of microohms (μΩ) to milliohms (mΩ), conventional measurement methods are easily affected by lead resistance, contact resistance, and electromagnetic interference, making it difficult to ensure measurement accuracy. Currently, the "DC voltage drop method" is widely adopted as the standard testing method in the industry, combined with four-wire measurement technology to effectively avoid interference and ensure the accuracy of measurement results. Its core principles and key technical points are as follows.

2.1 Principle of the DC Voltage Drop Method

Based on Ohm’s Law (R=U/I), the DC voltage drop method calculates the loop resistance by applying a constant high DC current to the measured contact loop and measuring the voltage drop generated by the current across the loop. The core advantage of this method is that the use of high test current can effectively break down the oxide film and contaminants on the contact surface, avoiding the impact of false contact resistance on measurement results—traditional small-current (mA-level) measurement is difficult to penetrate the oxide film, which tends to lead to significantly larger measurement values that fail to reflect the actual contact state.
According to industry standards, the test current is usually not less than 100A. On the premise of not exceeding the rated value of the equipment, the larger the current, the better the breakdown effect and the higher the measurement accuracy. During the test, it is necessary to ensure the stability of the DC current, and the calculation should be performed after the current and voltage drop values stabilize to avoid measurement errors caused by transient current.

2.2 Four-Wire Measurement Technology

To eliminate the interference of the test lead resistance itself and the contact resistance between the leads and the test points on the measurement results, the four-wire measurement method must be adopted for the contact loop resistance test of high-voltage switches. This method divides the test circuit into two independent lines, which are used for current application and voltage measurement respectively. The specific wiring method is as follows:
1. Current circuit: Connect the high-current output terminals (I+, I-) of the tester to both ends of the measured loop through special test wires with sufficient cross-sectional area (usually ≥16mm²), which is responsible for applying a constant high DC current to the loop;
2. Voltage circuit: Connect the high-sensitivity voltage measurement terminals (V+, V-) of the tester to the inner side of the current wire clamps, as close as possible to the measured contact (usually with a distance ≤50mm). This terminal is responsible for measuring the voltage drop across the measured contact, avoiding the inclusion of lead resistance in the measurement results.
By separating the current circuit from the voltage circuit, the four-wire measurement method enables the voltage measurement terminal to only collect the voltage drop at the measured contact part, effectively eliminating the interference of lead resistance and contact resistance. It is a key technology to ensure microohm-level measurement accuracy.

2.3 Principles of Auxiliary Technologies

With the development of testing technology, auxiliary technologies such as Dynamic Resistance Measurement (DRM) and temperature rise compensation algorithms have been gradually applied in testing practice, further improving the depth and accuracy of testing. Among them, the DRM technology collects resistance curves in real time during the opening and closing process of circuit breakers, and analyzes the continuity, fluctuation, and transition time of the curves. This technology can effectively diagnose potential problems such as the degree of contact burnout, the synchronization of mechanical transmission mechanisms, and the spring energy storage state. It makes up for the deficiency that traditional static resistance measurement can only reflect the stable state and has become a key indicator for the condition-based maintenance of high-voltage circuit breakers.
The temperature rise compensation algorithm automatically corrects the resistance deviation caused by changes in ambient temperature, ensuring the consistency and comparability of test data in different seasons and regions. It is particularly suitable for outdoor operation scenarios. The law of metal resistance changing with temperature can be corrected by the formula: R₂₀=Rt×(1+α(20−t)), where α is the temperature coefficient of the conductor material (e.g., α of copper is approximately 0.00393/℃), Rt is the measured resistance value, t is the test ambient temperature, and R₂₀ is the resistance value at the standard temperature (20℃).

3. Standard Test Procedures for Contact Loop Resistance of High-Voltage Switches

The contact loop resistance test of high-voltage switches must strictly comply with national and industry standards, such as DL/T 845.4-2019 General Technical Specifications for Resistance Measuring Devices, GB/T 11022-2020 Common Technical Requirements for High-Voltage AC Switchgear and Control Equipment, and DL/T 596-2021 Code for Preventive Test of Power Equipment. Combined with on-site actual conditions, the test procedures should be standardized to ensure test safety and data accuracy. The complete test process is divided into five stages: pre-test preparation, on-site wiring, parameter setting, formal testing, and post-test processing.

3.1 Pre-Test Preparation

Pre-test preparation is the basis for ensuring test safety and accuracy, mainly including three aspects: safety preparation, equipment preparation, and on-site preparation:
1. Safety preparation: Strictly implement the safety procedures of "power cut, power verification, discharge, and grounding wire installation". Confirm that the measured high-voltage switch has been completely powered off, and install grounding wires (with a cross-sectional area not less than 25mm²) on both sides of the equipment. Demarcate a safety warning area, hang warning signs such as "High Voltage Danger" and "No Switching On, Testing in Progress", and arrange special personnel for monitoring. Single-person operation is strictly prohibited. Test personnel shall wear full sets of insulating protective equipment (insulating gloves, insulating shoes, safety helmets, etc.).
2. Equipment preparation: Select a loop resistance tester that meets standard requirements, ensuring its measurement accuracy ≤±1%, test current ≥100A, and measurement range covering 0-2000μΩ. Conduct a power-on self-test in advance to confirm that the equipment is free of faults and the data display is accurate. Check that the test wires are free of damage or broken strands, and the wiring terminals are free of oxidation or looseness. Prepare auxiliary tools (sandpaper, alcohol, cotton cloth, torque wrench, etc.) for cleaning the test contact surface and tightening the connecting bolts.
3. On-site preparation: Clean the test site and remove surrounding debris to avoid the impact of strong electromagnetic interference sources (such as large transformers and frequency converters) on the test results. Check the state of the measured switch, open the cabinet door, clean the dust and oil stains on the contacts and busbar connection parts, polish the contact surface with sandpaper to remove the oxide layer, and tighten the connecting bolts according to the manufacturer’s specified torque if they are loose. Confirm that the test environment meets the requirements: temperature -10℃～40℃ (ideal temperature 20±5℃), relative humidity ≤80%, and no severe weather such as thunderstorms or strong winds.

3.2 On-Site Wiring

Wiring must strictly follow the requirements of the four-wire measurement method to avoid wrong or missing wiring. The specific steps are as follows:
1. Connect the current wires: Connect the I+ and I- terminals of the tester to both ends of the contact loop of the measured high-voltage switch (e.g., the incoming and outgoing terminals of the circuit breaker) through special high-current test wires. Ensure firm and good contact, and the current wires should be as short as possible to avoid winding;
2. Connect the voltage wires: Connect the V+ and V- terminals of the tester to the inner side of the current wire clamps, close to the measured contact (distance ≤50mm), through shielded voltage wires. Arrange the voltage wires and current wires separately without winding to avoid electromagnetic interference;
3. Ground connection: Reliably ground the grounding terminal of the tester through a special grounding wire. The grounding wire should be as short and straight as possible to ensure reliable grounding and reduce the impact of electromagnetic interference.
After the wiring is completed, the supervisor shall review the correctness of the wiring. Only after confirming that there is no wrong or missing wiring can the next operation be carried out.

3.3 Parameter Setting

Set the test parameters according to the model, rated parameters, and test standards of the measured high-voltage switch:
1. Test current: Generally set to 100A, and can be adjusted to 200A according to the manufacturer’s requirements for special equipment to ensure that the current can effectively break down the contact oxide film;
2. Test time: Set to 30-60 seconds to ensure the stability of the current, fully break down the oxide film, and make the measurement data tend to be stable;
3. Other parameters: Enable the automatic data recording function and set overcurrent and overvoltage protection parameters. Some high-precision instruments can enable temperature rise compensation and anti-interference filtering functions to improve measurement accuracy.

3.4 Formal Testing

1. Turn on the power of the tester and preheat it for 1-2 minutes to ensure that the instrument is in a stable working state;
2. After the test personnel confirm the on-site safety and the presence of the supervisor again, start the test. The instrument automatically outputs the set high DC current, collects the voltage drop data in real time, and calculates the loop resistance value;
3. During the test, closely observe the current and resistance values displayed on the tester, and pay attention to whether the measured switch has abnormal sounds or heating. If any abnormality occurs, immediately press the emergency stop button to cut off the test power supply, troubleshoot the hidden dangers, and then conduct the test again;
4. Repeat the test 2-3 times at the same measured point, and take the average value as the final test result to reduce measurement errors and ensure the authenticity and reliability of the data;
5. Record the test data in detail, including the loop resistance value, test current, ambient temperature and humidity, test date, test personnel, equipment number, and other information. Falsification or tampering of test data is strictly prohibited.

3.5 Post-Test Processing

1. After the test is completed, first reduce the test current to the minimum, turn off the power of the tester, wait for the instrument to fully discharge (about 3-5 minutes), and then remove the test wires in the reverse order (first remove the voltage wires, then the current wires);
2. Clean the test points of the measured high-voltage switch, wipe the wiring parts with cotton cloth, check that there are no remaining tools or wires inside the switch, confirm that the contacts and connection parts are intact, and close the cabinet door of the switchgear;
3. Remove the grounding wire (removal order: first remove the conductor end, then the grounding end), clean the test site, remove the warning signs, barriers, and other safety equipment, and sort out the test equipment and tools;
4. Hand over with the on-duty operation personnel, submit the test records, complete the work ticket termination procedures, and finish the test work.

4. Test Error Sources and Prevention Measures

The accuracy of the contact loop resistance test of high-voltage switches directly affects the accuracy of equipment condition assessment. Due to the influence of various factors during the test, measurement errors are likely to occur. Combined with on-site test practice, common error sources are mainly divided into four categories. Taking targeted prevention measures can effectively improve measurement accuracy.

4.1 Contact-Related Errors

Such errors are mainly caused by poor contact between the test wires and the measured contacts, as well as oxidation or contamination of the contact surface, which leads to an increase in contact resistance and affects the measurement results. Prevention measures: Thoroughly clean the measured contacts and test wire terminals with sandpaper and alcohol before the test to remove oxide layers, oil stains, and dust; ensure that the test wires are firmly connected to the contacts to avoid virtual contact; select test wires with sufficient cross-sectional area to reduce contact resistance; observe whether the current and voltage values are stable during the test. If fluctuations occur, promptly check the contact state of the wiring.

4.2 Electromagnetic Interference Errors

Strong electromagnetic interference (such as power frequency interference and harmonic interference in substations) usually exists at the high-voltage switch test site, which will affect the collection of voltage and current signals and lead to deviations in measurement values. Prevention measures: Select a loop resistance tester with anti-interference functions, and adopt high-frequency sampling (e.g., 20kHz), digital filtering, and multiple shielding technologies to suppress electromagnetic interference; keep the test wires at a safe distance from high-voltage lines and busbars, and use shielded wires for voltage wires; ensure reliable grounding of the tester to reduce the impact of common-mode interference; avoid testing near strong electromagnetic interference sources, and adopt the double-grounding state test method if necessary to eliminate induced electric interference.

4.3 Environmental Factor Errors

Changes in ambient temperature and humidity will affect the values of conductor resistance and contact resistance, leading to measurement errors. Prevention measures: Conduct the test under standard environmental conditions. If the ambient temperature deviates significantly from 20℃, use the temperature rise compensation algorithm to correct the test data; avoid testing in high-humidity or condensation environments to prevent an increase in resistance caused by moisture on the contact surface; conduct outdoor tests on days without rain, snow, or strong winds to ensure that the test equipment and test wires are not affected by the environment.

4.4 Instrument and Operation Errors

Insufficient instrument accuracy, improper parameter setting, or non-standard operation will lead to measurement errors. Prevention measures: Select a high-precision loop resistance tester that meets industry standards, and conduct regular calibration to ensure qualified instrument performance; strictly set test parameters in accordance with test procedures to avoid errors caused by insufficient test current or too short test time; standardize the operation process, repeat the test and take the average value to reduce human operation errors; preheat and self-test the instrument before the test to promptly detect instrument faults.

5. Analysis of Test Results and Engineering Applications

The core purpose of the contact loop resistance test of high-voltage switches is to evaluate the contact state of the equipment, identify potential hazards, and provide a basis for equipment operation and maintenance by analyzing the test data. The analysis of test results needs to conduct a comprehensive judgment combined with standard requirements, historical data, and equipment operation status. Its engineering applications are mainly reflected in three aspects: condition assessment, fault diagnosis, and operation and maintenance optimization.

5.1 Judgment Standards for Test Results

The judgment of test results shall follow the following principles to ensure the scientificity and accuracy of the assessment:
1. Comparison with factory values: The current measured value should generally not exceed 120% of the factory value. If it exceeds this limit, it indicates that there is an abnormality in the contact loop, and further investigation of hidden dangers is required;
2. Comparison with historical data: Pay attention to the variation trend of the resistance value. If the resistance value increases significantly in a short period of time (e.g., the increase exceeds 20%), it indicates the deterioration of the contact point, which may be caused by contact wear, looseness, oxidation, and other problems;
3. Comparison with equipment of the same type: High-voltage switches of the same model and similar working conditions should not have significant differences in loop resistance values (the difference should not exceed 20%). If the difference is too large, it is necessary to investigate the contact state of the abnormal equipment;
4. Reference to standard limits: Combine the equipment technical documents and industry standards to judge whether the measured value is within the specified limit range. For example, the contact loop resistance of 10kV high-voltage circuit breakers is usually required to be no more than 100μΩ, and that of equipment above 35kV is required to be no more than 50μΩ (the specific limit shall be adjusted according to the manufacturer’s requirements).

5.2 Engineering Application Scenarios

1. Condition assessment: Regularly conduct contact loop resistance testing, establish equipment test files, track the variation trend of resistance values, evaluate the health status of high-voltage switches, provide data support for condition-based maintenance, realize "maintenance only when necessary and maintenance well when performed", and avoid excessive or insufficient maintenance;
2. Fault diagnosis: If the test result exceeds the standard or the resistance value fluctuates greatly, the fault cause can be diagnosed by combining the DRM test curve. For example, an excessively high resistance value may be caused by contact oxidation, bolt loosening, contact wear, etc., and an unstable resistance value may be caused by poor contact of test wires, high ambient humidity, or contamination of the contact surface, providing a precise direction for fault handling;
3. Operation and maintenance optimization: For the problems found in the test, take targeted operation and maintenance measures, such as polishing contacts, tightening bolts, and replacing damaged components, to reduce the contact loop resistance and improve the conductive performance of the equipment; combine the test data to optimize the operation and maintenance cycle, and shorten the test cycle and strengthen monitoring for equipment with a rapid increase in resistance value;
4. Adaptation to new scenarios: For equipment with complex structures such as GIS, adopt new and efficient test devices to realize direct measurement without disassembling the grounding connection piece, shorten the test time, and reduce the risk of busbar shutdown; in some power-off maintenance sites, adopt the double-grounding state test method to improve test safety and accuracy.

6. Development Trends of Testing Technology

With the improvement of the intelligence and digitalization level of power systems, the contact loop resistance testing technology for high-voltage switches is developing towards high precision, intelligence, digitalization, and integration. Combined with the industry’s technological frontier in 2026, the main trends are as follows:
1. Improved intelligence: Modern loop resistance testers generally support multiple data transmission methods such as Bluetooth, Wi-Fi, or USB, which can seamlessly connect with smartphone APPs, PC software, or cloud platforms to realize automatic storage, analysis, report generation, and remote management of test data. Some high-end equipment is equipped with a built-in expert diagnosis system, which can automatically evaluate the equipment health status and provide maintenance suggestions based on the test data, greatly improving the operation and maintenance efficiency.
2. Continuous optimization of test accuracy: By adopting more advanced signal processing algorithms, high-stability current sources, and precision voltage acquisition circuits, the measurement accuracy of loop resistance testers has reached 0.1% or higher, and the measurement range has been expanded to 0.1μΩ to several thousand mΩ, covering the test needs of different voltage levels and application scenarios. The in-depth application of DRM technology has realized the transformation from static measurement to dynamic diagnosis, which can capture the instantaneous resistance changes during the opening and closing of contacts and evaluate the equipment state more comprehensively.
3. Enhanced anti-interference capability: Aiming at the strong electromagnetic interference environment such as substations, new testers adopt high-frequency sampling, multiple shielding, common-mode interference suppression, and other technologies, combined with the improved four-wire measurement method, effectively solving the problem of false data caused by open voltage circuits, poor contact, and strong electromagnetic interference. It ensures that stable and accurate measurement data can be obtained even in the strong electric field of 500kV or higher voltage levels. Portable testers generally have IP65 protection level, which can adapt to the operation needs of outdoor and harsh industrial environments.
4. Integration of operation and maintenance: Loop resistance test equipment is gradually integrated with high-voltage switch online monitoring systems and equipment health management platforms, realizing the joint analysis of test data, equipment operation data, and historical maintenance data, and building a full-life-cycle equipment operation and maintenance system. The application of the Internet of Things (IoT) technology makes the test equipment an important part of the equipment health management system, realizing real-time data upload, remote monitoring, and big data analysis, and providing support for predictive maintenance.

7. Conclusions

The contact loop resistance test of high-voltage switches is a key test to ensure the safe and stable operation of high-voltage switches, and its test accuracy directly affects the accuracy of equipment condition assessment and fault diagnosis. Through the systematic research on test principles, standard procedures, error prevention and control, and engineering applications, this paper draws the following conclusions:
1. The DC voltage drop method combined with four-wire measurement technology is the standard method for contact loop resistance testing of high-voltage switches. The application of auxiliary technologies such as DRM technology and temperature rise compensation algorithms further improves the depth and accuracy of the test, which can effectively avoid interference and reflect the actual contact state of the equipment;
2. Strictly following the standard test procedures, doing a good job in pre-test preparation, standardized wiring, reasonable parameter setting, and scientific data analysis are the keys to ensuring test safety and accuracy. Taking targeted prevention measures for contact-related, electromagnetic interference-related, environmental, and instrument operation-related errors can significantly improve measurement reliability;
3. The analysis of test results needs to comprehensively evaluate the equipment contact state by combining factory values, historical data, data of equipment of the same type, and standard limits, providing a scientific basis for fault diagnosis and operation and maintenance optimization. The application of new test technologies can further improve operation and maintenance efficiency and equipment reliability;
4. In the future, with the development of intelligent and digital technologies, the contact loop resistance testing of high-voltage switches will move towards high precision, intelligence, and integration, and gradually realize in-depth integration with equipment online monitoring and big data analysis, providing more powerful support for the safe and stable operation of new power systems.
Future research can further focus on the optimization of test accuracy in complex electromagnetic environments, the engineering application of DRM technology, and the correlation analysis between test data and equipment life prediction, so as to promote the continuous upgrading of high-voltage switch operation and maintenance technology.