In recent years, load switch-fuse combination appliances have been widely used in the selection of protection and control switches for 10kV distribution transformers due to their advantages over circuit breakers, such as simple structure, convenient operation and maintenance, low cost, and reliable operation. In practical applications, the correct selection of combination appliances and the reasonable matching of parameters between load switches, fuses, and transformers are crucial issues related to whether the combination appliances can function effectively and ensure the safe operation of the system. 1. Verification of Transfer Current Because the melting of the three-phase fuses in the combination appliance has a time difference, after one phase of the three-phase fuse breaks first, the striker operates. At this time, the other two phase fuses may not have extinguished their arcs and broken, while the striker strikes, causing the load switch to interrupt the fault current. That is, the breaking task originally undertaken by the fuse is transferred to the load switch. Therefore, the transfer current refers to the three-phase symmetrical current when the fuse and load switch switch switch functions. When it is lower than this value, the current of the first phase is interrupted by the fuse, and the current of the other two phases is interrupted by the load switch. When the current exceeds this value, the three-phase current is interrupted only by the fuse. Transfer current is an important indicator to consider when selecting combined electrical appliances. If an inappropriate selection is made, the load switch's transfer current capacity will be insufficient, making it unable to interrupt two-phase short-circuit currents and potentially causing the switch to explode. Load switches are generally divided into two types: general-purpose and frequent-use. Gas-generating and compressed air-insulating load switches are general-purpose, while vacuum and SF6 load switches are frequent-use. Different load switches have different transfer current specifications. General-purpose load switches have a transfer current of around 800–1000A, while frequent-use load switches can reach 1500–3150A. The transfer current also varies depending on the capacity of the distribution transformer; the actual transfer current can be estimated from the transformer capacity. Generally, the transfer current of an S9-800×10 type distribution transformer is 978A. According to the definition of transfer current and considering the breaking time and characteristics of the load switch, the transfer current of the load switch should avoid the maximum short-circuit current and be controlled within 70% of the maximum short-circuit current. That is, the actual transfer current is approximately 978 × 70% = 685A. Based on the analysis of the technical coefficients of domestically produced load switches and fuses, and considering the product variability, and according to the calculation results of the transfer current, based on our city's experience, for transformers with a capacity of 800kVA or less, air-insulated general-type load switches can be selected. For transformers with a capacity in the range of 800–1250kVA, vacuum or SF6 insulated frequent-current load switches are generally selected. Transformers with a capacity greater than 1250kVA require circuit breakers for protection and control. From the years of operation of the combined electrical equipment in our city, it has been safe, reliable, and in good condition, with no accidents caused by improper selection. 2. Selection of Cross-Circuit Current Parameters Some load switches are equipped with shunt trip units for overload and other protection tripping. This means that in the event of an overload, the load switch trips via relay protection without burning out the fuse; the fuse only provides short-circuit protection. The intersection of the relay protection's operating characteristics (operated by the shunt trip unit) and the fuse's time-current characteristics is called the "cross-circuit current." Cross-circuit current is an overcurrent value. Overcurrents below the cross-circuit current cause the shunt trip unit to trip the load switch; overcurrents above the cross-circuit current cause the fuse to trip. Therefore, selecting a load switch with a higher cross-circuit current parameter can effectively reduce the number of fuse trips, thus significantly reducing the number of fuse replacements, which has certain technical and economic significance. For vacuum and SF6 load switches, a relatively higher cross-circuit current value can increase the cross-circuit current to approach the transfer current, fully utilizing the strong breaking capacity advantage of these frequently tripping load switches. 3. Selection of Current-Limiting Fuses In load switch-fuse combination electrical appliances, the load switch is responsible for interrupting normal or transferred current, while the fuse is responsible for interrupting overload and short-circuit currents. The breaking capacities of these two appliances must work together to ensure a smooth and complete interruption. Therefore, the selection of current-limiting fuses is crucial. The selected current-limiting fuse should possess characteristics such as high breaking capacity, low minimum breaking current, low operating temperature, steep time-current characteristic curve, and small characteristic curve error. It should also meet requirements such as aging resistance, versatility in installation methods, and suitable dimensions. Furthermore, it should be noted that the power loss of the fuse should not exceed 75W at an ambient temperature of 40℃. When selecting a fuse, the rated current of the fuse must match the capacity of the transformer. Some people believe that selecting a fuse with a larger rated current is safer, which is incorrect. This not only results in economic waste but also deteriorates the fuse's time-current characteristic, reduces protection speed, and affects the fuse's correct interruption protection. According to IEC standards, in a 10kV system, the rated current of the fuse can generally be selected according to Table 1 for transformers of different capacities: 4 Other issues to be noted (1) For a system with multiple transformers operating in parallel, special attention should be paid to the verification of transfer current when selecting combined electrical appliances. As mentioned in the verification calculation above, if two transformers of the same model and capacity are operating in parallel, and if the secondary terminals of the transformer are short-circuited, the transformer impedance will be only half that of a single transformer system, thereby doubling the maximum three-phase short-circuit current on the high-voltage side, and the possible transfer current will also double. Therefore, for a system with multiple transformers operating in parallel, the transfer current should be verified when selecting combined electrical appliances, and combined electrical appliances with transfer current indicators that meet the requirements should be selected. (2) If any of the following requirements exist, the combined electrical appliances should be equipped with a shunt trip device to realize the fast electric tripping of the load switch: ① Oil-immersed transformers that require heavy gas protection. Generally, oil-immersed transformers with a capacity of 800kVA and above must be equipped with heavy gas trip protection. ② Over-temperature trip protection for dry-type transformers. ③ Tripping protection for accidental energization and opening of the valve on a dry-type transformer with a casing. ④ For applications requiring remote operation and control. 5. Conclusion In summary, the selection of load switch-fuse combination electrical appliances should be based on the actual application, the transformer capacity and operating mode, and the various technical parameters and breaking capacities of different types of load switches. The transfer current and transfer current should be calculated, and the parameters of the load switch, fuse, and transformer should be rationally selected to ensure the correct choice of the combination electrical appliance and guarantee its safe and reliable operation.