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The 50522 isolating switch model of Wenzhou Electric Power Bureau's 500kV Ouhai substation is SSBⅢ-AM-550/3150. It is manufactured by MG Company of the Netherlands. It was shipped from the factory in 2000 and put into operation in June 2001. This is a Group 1 isolating switch manufactured by Siemens imported from China. Since it was put into operation, there have been many failures in opening and closing, causing the No. 2 main transformer of Ouhai Substation to be shut down for maintenance many times, but the existing problems have not been completely solved. On January 2, 2004, the contacts of the isolating switch burned out during operation, seriously affecting the safe operation of the power grid. 1 The fault occurred on 2004-0l-02. Due to the expansion work of Ouhai Substation, the operating 500kV I bus and II bus were required to be changed to busbars for maintenance. At that time, the operation mode of the 500kV system in Ouhai Substation was one line and one change. The No. 2 main transformer, Shuangou 5463 line, and switch intervals of 5051, 5052, and 5053 were all in operation. The main wiring of the 500kV system was shown in Figure 1. East China dispatch required that switches 5051 and 5053 be changed from operation to cold standby. After opening the 5051 switch in Ouhai substation, it was found that the contact of the 50522 isolating switch A on the side of the switch was arcing, and the heating temperature reached 400°C (infrared temperature measurement). The load at that time was 400,000 kW. Ouhai Substation reported to East China Dispatch and re-closed the 5051 switch, opened the 5052 switch, and changed the 5052 switch interval to cold standby at 07:22. 2 Fault Analysis After the power outage, it was found that the B and C phases of the 50522 isolating switch were intact, the contacts of phase A on the side of the No. 2 main transformer were intact, and the dynamic and static contacts of phase A on the side of the 5052 switch were severely burned, as shown in Figure 2. From the burnt contact part, it can be found that the lower side of the movable contact guide disc and the lower contact finger of the static contact near the guide disc are particularly severely burned. This proves that arcing begins to occur at the contact point between the lower side of the guide disc and the lower contact finger of the static contact, and then the arc gradually spreads to the surroundings. From this analysis, the contact between the dynamic and static contacts here is unreliable, the contact surface is too small, and the contact resistance is very high. When operating normally, the load of Shuangou 5463 line can be sent to the No. 2 main transformer through two ways. The first way is, Shuangou 5463 line - 5051 switch - 500kV I bus - I, II bus short connection - 500kV II bus - 5053 switch - No. 2 main transformer ； The second way is, Shuangou 5463 line-50521 isolating switch-5052 switch-50522 isolating switch-No. 2 main transformer. After opening the 505l switch, it is equivalent to disconnecting the first path. The load of 400,000 kW of the Shuangou 5463 line is all sent to the No. 2 main transformer through the second path. Because the contact resistance of the 50522 switch A-phase 5052 switch side contact is very large, it causes overheating and arcing. 3 Cause Analysis and Treatment 3.1 Cause Analysis By analyzing the structural characteristics and action principle of the 50522 isolating switch, the isolating switch has a horizontally opening double-break structure. During the closing process, the relative position requirements of the moving and static contacts are very strict. Only when the guide disk of the moving contact and the groove of the nylon guide block of the static contact are exactly aligned, and a certain gap is ensured, the isolating switch can be closed normally, as shown in Figure 3. If the gap here is not adjusted well, or the isolating switch is affected by factors such as lead tension and foundation subsidence, the isolating switch porcelain bottle tilts slightly, causing the movable contact guide disc to not enter the groove of the static contact nylon guide block normally when the isolating switch is closed. Instead, it runs to the convex edge of the static contact nylon guide block. On the one hand, the closing resistance increases, causing the isolating switch to fail to close properly. ； On the other hand, if the movable contact is tilted downward, the upper contact surface has almost no contact. Although the disc end of the lower contact surface is in close contact, the contact surface gradually decreases inward. The burning situation shown in Figure 2 is caused by a sudden increase in current under this condition, which first generates heat at the close contact, generates arcing, and then spreads to the surroundings. 3.2 Treatment plan (1) Notify the manufacturer to analyze the cause of the failure, formulate a maintenance plan, and prepare spare parts for the isolating switch. (2) Since this isolating switch is not available in China, phase A of the isolating switch is temporarily short-circuited and then put into operation. (3) After the isolating switch spare parts arrive, arrange for a power outage and replace the static contact bases, moving contact blades on both sides of phase A of the isolating switch, and the upper porcelain bottle on the burnt static contact side of phase A. 4 Conclusion After the damaged parts were replaced and re-adjusted, everything was normal after the isolating switch was put into on-site operation. However, the problems exposed by this accident deserve serious consideration. (1) When selecting equipment, try to choose an isolating switch with good operating performance and reliable quality, and consider appropriate spare parts. (2) During the equipment installation process, problems must be discovered and solved promptly, and defects must not be left until after the equipment is put into operation. (3) Regularly maintain the isolating switch to improve the quality of maintenance and ensure reliable operation of the knife switch. (4) When operating, the operator must carefully check whether the closing process of the isolating switch is normal on site and confirm that the contacts are in reliable contact. If conditions permit, you can judge whether the isolation switch contact is reliable by observing the three-phase current readings on the secondary side. Only by doing all the work thoroughly and meticulously can the reliability of the isolation switch and the safe operation of the power grid be ensured.