Many high-current power system equipment, preventive tests and trial handover require accurate loop resistance measurement. Circuit breaker is an important equipment in the power system. National standards GB763, GB50150, and power industry DL/T596 specify the following conditions for circuit breaker resistance measurement: the voltage drop should be measured with a DC current not less than 100A. 

      Circuit breaker conductivity depends on the contact resistance between the dynamic and static contacts of the circuit breaker. The contact resistance increases the power loss when the conductor is energized, causing the temperature at the contact to rise. Its value directly affects the normal current-carrying capacity and, to some extent, the ability to interrupt short-circuit currents. Therefore, the resistance value of the conductive circuit in each phase of the circuit breaker is an important data for installation, maintenance, and quality acceptance of the circuit breaker. 

There are various methods to measure contact resistance. Japanese scholars proposed using a superconducting quantum device to measure contact resistance, while others used electrolytic cells. Polish scholar Jerzy Kaczarek proposed measuring contact resistance using the third harmonic. These methods are usually conducted under laboratory conditions. In practice, the four-terminal method is often used to measure the actual contact resistance. 

       Previously, the DC double bridge was commonly used to measure the contact resistance of circuit breakers. However, due to the weak current in the bridge measurement circuit, it is difficult to remove the oxide film resistance, resulting in measured values that are too high. But the oxide film can be easily punctured by large currents, which does not hinder the normal passage of current. Therefore, the test method adopts the DC voltage drop test, and the current should not be too small.

       High-voltage circuit breaker conductive circuit resistance mainly depends on the contact resistance between the moving and static contacts. The presence of contact resistance increases the loss when the conductor is energized, raising the temperature at the contact. Its value directly affects the normal current-carrying capacity and, to some extent, the ability to interrupt short-circuit currents. Therefore, the resistance value of the conductive circuit in each phase of the circuit breaker is an important data for installation, maintenance, and quality acceptance.  

Since there is an oxide film between the switch contacts, if a small current is used for testing, the test result is generally much larger due to the influence of the oxide film. However, the oxide film can be broken down under large current. Theoretically, as long as the test current does not exceed the rated current, the larger the better. But when formulating the regulations, considering the production level of relevant domestic instruments at that time, a provision was made that the current should not be less than 100A.   Power equipment and ground grid continuity resistance tester 

      The reliable and effective connection between the grounding down conductor of power equipment and the ground grid is the fundamental guarantee for the safe operation of power equipment and the personal safety of operators. Although necessary anti-corrosion treatment has been done at the connection of the grounding down conductor during the production of the grounding device, the connection point in the soil is still affected by moisture and other factors for a long time, leading to corrosion, disconnection, loosening, etc., which increases the resistance value at the contact point between the grounding down conductor and the ground grid, failing to meet the requirements of power regulations, causing potential safety hazards in equipment operation, and in severe cases, causing equipment to operate disconnected from the ground grid. It can be used for precise measurement of the continuity resistance between the grounding down conductor of power equipment and the grounding grid or adjacent equipment. 

The ground grid continuity resistance tester is used to measure the continuity resistance value of the grounding down conductor, and to determine the fault point by comparing historical test values and adjacent point test values. The working power of the ground grid continuity resistance tester is provided by an internal lithium battery, no external AC power supply is needed. The test process is controlled by a microcontroller, featuring simple operation, fast test speed, stable test data, and portability.  

    Since the circuit breaker contact resistance is very small, only by using a high current for testing can a certain accuracy be guaranteed, and the anti-interference ability is also stronger.  

    Used for high-precision measurement of circuit breaker contact resistance and current-carrying conductor resistance, it can output large current stably for a long time, complying with the relevant provisions of DLT845.4-2004 "General Technical Conditions for Resistance Measuring Devices Part 4: Loop Resistance Tester".