With the promotion of ring network power supply, load switch-fuse combination electrical appliances are widely used. The load switch opens and closes the working current, and the fuse breaks the short-circuit current. However, there is an overcurrent area in between. Overload protection can be successfully completed by properly coordinating the load switch and the fuse. There are two ways to coordinate the load switch and the fuse: impactor operation and trip unit operation. (1) Coordination between the load switch and the current-limiting fuse with impactor. When the fuse blows, the built-in impactor is triggered (usually through mechanical transmission), causing the three phases of the load switch to open simultaneously. This is called impactor operation. Combination electrical appliances with impactor operation can interrupt any current, but the basic characteristic is that the fuse blows to trigger the impactor to operate on the load switch. When the current is greater than the transfer current, the fuse will break it (the load switch will then trip without current). The transfer current area will be broken by the load switch and the fuse together. The current below the minimum breaking current range of the fuse (generally 2.5-3 times the rated current) will be broken by the load switch (at this time, the fuse cannot guarantee breaking, but the fuse blows and the impactor is triggered). The transfer current is the three-phase symmetrical current value when the fuse and the load switch switch the breaking function. There is a time difference between the three-phase fuses. When the fuse first breaks the phase, the impactor is triggered. When the three phases of the load switch are tripped at the same time, there is a phenomenon that the fuses of other phases transfer the breaking function to the load switch. The disadvantage of the impactor operation is that the breaking of both fault and overload currents comes at the cost of damaging three fuses, and the operating current and time cannot be determined manually. (2) In order to overcome the problems of the impactor operation mode and meet the needs of actual operation, the manufacturer can provide a shunt trip device in the load switch for remote operation and overload protection tripping. The combination of a load switch with a shunt trip unit and a current-limiting fuse with an impactor is called the trip unit operation mode. The combination of the load switch and the fuse operated by the trip unit can be regarded as a two-level protection. The first level is the load switch and the second level is the fuse. If the current is less than the current drawdown, the relay protection can start the shunt trip unit to trip the load switch. If the current drawdown is greater than the current drawdown, the fuse will break the circuit. The current drawdown is the current value corresponding to the intersection of the time-current characteristics of the two overcurrent protection devices (load switch and fuse). The trip unit operation mode can better solve the problem of coordination between the upper and lower level protections, and can avoid damaging the fuse in overload protection. (3) Example application of overload protection without damaging the fuse. For example, in a 500kVA distribution transformer project, the user requires the operation to be 0.5s when the current is 6 times the rated current and 0s when the current is 10 times the rated current (i.e., fast operation is required). Obviously, the impactor operation alone cannot meet the user's requirements. The trip unit operation mode must be selected. The configuration uses a vacuum load switch (equipped with a shunt trip unit), with a rated current of 630A, and a 50A current-limiting fuse with an impactor. The primary and secondary circuits are equipped with current transformers with a transformation ratio of 30/5A. The secondary winding of the current transformer is connected to a DL-13/50 type overcurrent relay A (set value 28.9A), which, after a delay by a DS-123 type time relay (set value 0.5s), trips. Another DL-13/50 type overcurrent relay B (set value 48.1A) trips directly. Verification results: ① The transformer's rated current is 28.9A. Considering the actual operating conditions of the ring network and allowing sufficient heat dissipation margin, a 50A fuse is selected. ② Checking the time-current characteristic curve of the 50A current-limiting fuse (Xirong Factory product), under an inrush current of 346.8A, the duration is 0.7s, which is greater than 0. ③ When the current is 6 times the rated current, the transformer circuit current is 28.9 x 6 = 173.4 A, and the secondary current of the current transformer is 173.4 ÷ 30 / 5 = 28.9 A. When this current is reached, relay A operates and, after a 0.5-second delay, the load switch trips. At this time, the fuse's arc-precession time is 90 seconds, while the load switch operates after a 0.5-second delay, which is much shorter than the fuse's arc-precession time, so the fuse will not burn out. ④ When the current is 10 times the rated current, the transformer circuit current is 289 A, and the secondary current of the current transformer is 48.1 A. Relays A and B operate simultaneously, and relay D directly activates the load switch shunt trip unit. The total breaking time of the load switch (relay operating time + inherent tripping time + arcing time) is approximately 0.06 seconds. At this time, the fuse's arc-precession time is 2.1 seconds, which is much longer than the load switch's breaking time, so the fuse will not burn out.