Working Principle
In the power system, many high-current electrical equipment need to accurately measure the resistance value of the circuit during preventive tests and handover tests. Circuit breakers are important electrical equipment in the power system. According to the latest standard "JJG1052-2009 Loop Resistance Verification Regulation", national standards GB763, GB50150, and power industry standard DL/T596, the measurement of the conductive circuit resistance of circuit breakers is specified: the DC voltage drop method should be used, with a current not less than 100A.
The resistance of the conductive circuit of a circuit breaker mainly depends on the contact resistance between the moving and static contacts. There are many methods to measure contact resistance. Japanese scholar Isao Minowa proposed using superconducting quantum devices to measure contact resistance, H. Archi proposed using the electrolytic cell method, and Polish scholar Jerzy Kaczarek proposed using the third harmonic method. These methods are generally used in laboratory conditions for electrical contact research. In engineering, the four-terminal method is usually used to measure the contact resistance of actual contacts.
Previously, a DC double bridge was usually used to measure the contact resistance of circuit breakers. However, when using a double bridge to measure the conductive circuit resistance of a circuit breaker, because the measurement circuit of the double bridge passes a weak current, it is difficult to eliminate the oxide film with large resistance, and the measured resistance value is too large. However, the oxide film is easily broken down under a large current and does not hinder the normal current flow. Therefore, when using the DC voltage drop method for testing, the current should not be too small.
The resistance of the conductive circuit mainly depends on the contact resistance between the moving and static contacts of the circuit breaker. The existence of contact resistance increases the loss when the conductor is energized, raises the temperature at the contact point, and its value directly affects the current-carrying capacity during normal operation and, to a certain extent, the breaking capacity. Therefore, the resistance value of each phase of the circuit breaker's conductive circuit is an important data for installation, maintenance, and quality acceptance of circuit breakers.
Due to the oxide film between 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 a large current. Theoretically, as long as the test current does not exceed the rated current, the larger the better. However, when formulating the regulations, considering the production level of relevant domestic instruments at that time, the requirement that the current should not be less than 100A was made. Power equipment
The reliable and effective connection between the grounding down conductor of power equipment and the grounding 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 applied to the connection point of the grounding down conductor during the production of the grounding device, the connection point in the soil will still be affected by moisture and other factors for a long time, resulting in poor connection.
