Abstract: Currently, in low-voltage power distribution systems such as rural power grids, ordinary current-type electronic residual current protection devices, including residual current relays and residual current circuit breakers, are widely used. Besides meeting relevant national product standards and obtaining qualified 3C certification, the operating characteristics of these products during online operation after installation, such as operating current and operating time, should also meet the requirements of relevant national installation and operation management standards. Keywords: Residual current protection device, Online operation discussion. Currently, in low-voltage power distribution systems such as rural power grids, ordinary current-type electronic residual current protection devices, including residual current relays and residual current circuit breakers, are widely used. Besides meeting relevant national product standards and obtaining qualified 3C certification, the operating characteristics of these products during online operation after installation, such as operating current and operating time, should also meet the requirements of relevant national installation and operation management standards. 1. Impact of Three-Phase Leakage Current Imbalance Factors During normal operation of low-voltage power distribution systems such as rural power grids, differences in three-phase loads and insulation levels objectively lead to an imbalance in three-phase leakage current. The leakage current of power supply lines and electrical equipment is related to factors such as climate, temperature, load characteristics, and load size, resulting in a wide range of variations and making it difficult to control. From the perspective of residual current protection technology, when the leakage current of the power supply line is large, sudden application of the same instantaneous fault current on different phases will result in different residual current values. For ordinary current-type products, although the instantaneous ground fault current on a certain phase may be equal to or greater than the rated operating value, the resulting residual current may be less than the rated operating value of the product. In this case, the residual current protection device may not operate, which is a common phenomenon in three-phase power supply systems where there is a phase with insensitive residual current operation. Testing the residual current protection device under normal operating conditions involves checking its operating characteristics during online operation using a dedicated residual current protection device testing instrument, in addition to using the test button on the protection device itself. For example, when the rated operating current I△n is 300 mA during online operation, leakage current exists in the line at a certain moment, as shown in Figure 1. If 300 mA is used as the test fault current, and tests are conducted on phases A, B, and C respectively, different residual currents will be synthesized. In Figure 1, I△ is synthesized from the leakage current of the phase line and the neutral line passing through the residual current transformer. Its amplitude and phase angle are random variables affected by it. Generally speaking, the special test instrument for residual current protection devices has purely resistive load characteristics. Therefore, when using it to test the trip on phases A, B, and C respectively, I△n can be considered to be in phase with UA, UB, and UC. From Figure 1, the residual currents I△(A), I△(B), and I△(C) synthesized on the three phases with different amplitudes and phase angles can be calculated. Taking the amplitude as an example, we can obtain I△(A)≈400 mA, I△(B)≈240 mA, and I△(C)≈280 mA. In other words, the test using IΔn yielded three different results: Phase A IΔ(A) > UΔn, indicating correct operation; Phase B IΔ(B) < IΔn, indicating failure to operate, which can be considered an operational failure; Phase C IΔ(C) < IΔn, but may be close to the product's set operating value, placing it in a critical state between operation and non-operation. While these are specific examples, careful analysis reveals that many areas in the vector diagram are insensitive due to actual leakage current. From the perspective of safe and reliable operation of low-voltage power distribution systems, transient faults such as single-phase grounding (tree branches touching the wire, broken wires falling to the ground, grounding short-circuit arcs, etc.) account for a certain proportion. Residual current protection devices selected for low-voltage power distribution systems such as rural power grids, used for indirect contact electric shock protection and prevention of electrical fires and equipment damage, are required by relevant national product standards to detect suddenly applied or slowly rising residual currents, and to meet the breaking time requirements at the rated operating value. Therefore, considering the current status of China's low-voltage power distribution system, when selecting residual current protection devices, it is crucial to ensure that their operating characteristic parameters, such as operating current, maximum breaking time, and ultimate non-driving time, still meet the standards during online operation. From a technical perspective, this is of greater practical significance for the safety and reliability of residual current protection in low-voltage power distribution systems. 2. The Influence of Operating Time Factors According to relevant national product standards for residual current protection devices, they are classified into two main categories based on breaking time: general type and time-delay type. The S (selective) type, as a type of time-delay type, can sometimes be listed as a separate category. For example, in the JB/T 8756-1998 standard "Residual Current Operated Protection Relays," the breaking time is divided into three categories: general type, time-delay type, and S type. Residual current relays combined with AC contactors or various low-voltage circuit breakers with electric opening and closing functions form combined residual current protection devices. These devices have advantages such as high technical content and good maintainability, and are widely used in low-voltage power distribution systems such as rural power grids. The S-type residual current relay, a type of time-delay relay, can be used in conjunction with general-type products for selective graded protection and can be configured with an automatic reclosing function. Because its breaking time at rated operating current is much longer than that of the general-type, it has strong anti-interference capabilities against spike interference caused by lightning strikes or system overvoltages, significantly reducing the chance of maloperation. Furthermore, since most residential or single-unit equipment uses miniature residual current circuit breakers with breaking times less than 0.1 s and rated operating currents generally around 50 mA, combining it with an S-type residual current protection device (minimum non-driving time of 0.13 s) to form graded protection can meet the requirement that the lower-level limit non-driving time is greater than the upper-level operating time. Practical application has proven that the S-type residual current relay is more suitable for widespread use in rural power grids in my country. A comparison of the operating times of the S-type residual current relay with general-type and time-delay relays is shown in Table 1. Currently, many widely used current-type electronic products use analog circuit technology composed of resistive and capacitive components to set the operating time. Under normal temperature conditions, when conducting type tests or other tests, they can meet the relevant product standard requirements. However, after a prolonged period of installation and operation, it should be considered whether the breaking time and ultimate non-driving time requirements at I△n, 2I△n, and 5I△n can still be met. GB 13955-2005, "Installation and Operation of Residual Current Operated Protective Devices," specifically states regarding product operation management: to verify the operating characteristics and changes of residual current protection devices during operation, the operation management unit should configure dedicated testing instruments and conduct regular operating characteristic tests, including testing the operating current value, breaking time, and ultimate non-driving time. This has strong pertinence and practical significance, and plays a good supervisory role in assessing the quality of residual current protection devices and guiding correct selection and operation management. It should be given high priority by power supply management departments and manufacturers. 4. Technical Improvement Measures for Current-Type Residual Current Relay Relays From the above analysis, it can be seen that existing ordinary electronic current-type residual current protection devices exhibit a phenomenon of insensitive residual current operation in three-phase power supply systems. Furthermore, using analog circuits to set the operating current value, breaking time, and ultimate non-driving time settings results in large dispersion, making it difficult to fully meet the technical requirements for safe and reliable operation of low-voltage power distribution systems such as rural power grids. With the rapid development of microcontroller technology, its information processing capabilities have been greatly enhanced. Introducing microcontroller technology to upgrade existing current-type residual current protection devices can overcome current product defects and significantly improve the accuracy of action time and current setting, thus meeting the higher requirements for the safe and reliable operation of low-voltage power distribution systems. The affiliated factory of Nanjing Institute of Technology has done some work in this area, developing products such as the LJM microcomputer-controlled S-type residual current relay. Based on the ordinary S-type product, it adds microcontroller technology, employing appropriate mathematical processing methods to perform dynamic phase detection, digital filtering, and real-time calculation on the suddenly applied or slowly rising residual current signal detected by the residual current transformer. Furthermore, it does not require directional characteristics of the transformer terminals. This ensures that the residual current protection value is unaffected by leakage current within a 360° phase angle range, solving the problem of insensitive phases in ordinary current-type products when there is a large three-phase unbalanced leakage current in low-voltage power distribution systems. Furthermore, the adoption of digital and software setting technology significantly improves the accuracy of the rated setting values ​​for operating current and operating time, ensuring the reliability and stability of the low-voltage power distribution system and overcoming the performance drift problem inherent in analog electronic circuits. When the leakage current of the protected low-voltage power distribution system is less than the rated residual operating current, the same grounding test current (the rated residual operating current indicated on the product nameplate) is used for each phase to test the operating characteristics of the residual current protection device, ensuring accurate operation. When a momentary ground fault occurs in any phase of the line, resulting in a sudden applied leakage current, the product can accurately detect it and reliably operate when the rated operating value is reached, unaffected by the imbalance of three-phase leakage current. This ensures consistent residual current operating values ​​for single-phase ground faults and provides a single reclosing function, guaranteeing safe and reliable operation. When the gradually changing residual current of the protected low-voltage power distribution system reaches the rated operating value, it trips immediately and also provides a single reclosing function. If the fault persists, the trip is locked, requiring the residual current relay power supply to be turned off and the switch to be closed again for operation to resume. • Residual current transformers have no directional installation requirements, and there is no one-to-one correspondence between the transformer and the device, resulting in good interchangeability and facilitating mass production, installation, and maintenance. • Real-time digital display of residual current with optional alarm function. • Recording and storage of residual current action values ​​during ground faults aids in fault cause analysis. • Optional communication interface functionality. GB 13955-2005, "Installation and Operation of Residual Current Operated Protective Devices," focuses on installation and use. After its publication and implementation, it will form a complete standard chain with existing national product standards, perfecting the standard system for the entire process of residual current protection devices from manufacturing to installation and operation management. In summary, my country's existing low-voltage power distribution systems suffer from three-phase unbalanced leakage current with relatively large values, and the operating characteristic parameters of many currently used products are still insufficient. Therefore, it is essential to emphasize the technological upgrading and improvement of residual current protection devices, eliminating defects from a technical perspective, improving the accuracy and stability of operating characteristic parameter settings, and perfecting product application functions to meet the higher requirements of national standards and the current state of low-voltage power distribution systems.