This paper enumerates the application of mechanical key lock in the electric switching operation, and avoids the accident of misoperation from the technical measures.The mechanical plug-in key interlock used in electrical equipment is a safe operation system which must be operated in the specified sequence to prevent human operation errors, otherwise, it is impossible to carry out subsequent operations, so as to ensure the safety of personal equipment. The interlocking method is simple, reliable, direct and effective, especially suitable for the interlocking between multiple cabinets or doors. It is easy for operators to identify and eliminate misoperation. It is widely used in power plant, metallurgy and other fields.In this paper, the examples of power supply and shutdown during the maintenance of dry-type transformer, frequency conversion cabinet and electrostatic precipitator in power plant are cited to illustrate how the mechanical key can realize the interlock operation in terms of technical measures.According to the requirements of the electric power operation specification, the dry-type transformer maintenance should first disconnect the low-voltage 380V switch, then disconnect the high-voltage 6kV switch, then ground the 6kV switch, and finally open the transformer door to repair the transformer. According to this operation sequence, mechanical locks are installed on 380V switch cabinet, 6kV switch cabinet grounding knife and transformer door respectively.During operation, one key can be pulled out only after the 380V circuit breaker is disconnected. At this time, the 380V circuit breaker is locked in the off state. Take this key to unlock the grounding knife in the 6kV switch cabinet (Note: here, the self interlock between the 6kV circuit breaker and the grounding knife can be used). The grounding knife is grounded, and another key is pulled out. The grounding knife is locked in the ground state.At this time, it is ensured that the low-voltage and high-voltage switches are de energized and grounded. Finally, take out the key to open the transformer door to repair the transformer. The high and low voltage switch can be powered by reverse operation (as shown in Figure 1).Figure 1Interlocking operation sequence: turn off 380V switch, take out key 2a, and the cabinet is locked in the power-off state → turn off 6kV switch, unlock 6kV side grounding knife with key 2a, and the grounding knife is grounded, pull out key 2B, and the grounding knife is locked in the grounding state → open high-voltage side cabinet door of dry-type transformer with key 2B, and pull out a key 2C → open low-voltage side cabinet door of dry-type transformer with key 2C. After the experience of electricity, the personnel can carry out the maintenance of the transformer.According to the requirements of the electrical switching operation specification, the power on operation of the frequency conversion cabinet (as shown in Figure 2) is to confirm that the bypass switch K3 is disconnected before closing the outgoing switch K2, then closing the incoming switch K1, and finally sending power to the higher-level high-voltage switch. The power-off operation of the inverter cabinet is to disconnect the upper high-voltage switch, the incoming cabinet knife switch, and then the outgoing knife switch.The power on operation of frequency conversion cabinet is to confirm that the incoming line switch and outgoing line switch are disconnected, and the bypass switch is closed to close the upper high-voltage switch. The power frequency power-off operation of frequency conversion cabinet is to disconnect the upper high-voltage switch and then the bypass switch.According to the above operation requirements, a mechanical lock shall be installed on the high-voltage switch cabinet, and then two mechanical locks shall be installed on the incoming line switch, outgoing line switch and bypass switch respectively. One lock realizes the switch opening and the other lock realizes the switch closing. When the inverter cabinet is powered off, first disconnect the high-voltage switch, pull out the lock key of the high-voltage switch, and the high-voltage switch is locked in the power-off state.Figure 2Use this key to unlock the incoming line switch, disconnect the incoming line switch, lock the switch, pull out a key, and the incoming line switch is locked in the off state. Use the extracted key to unlock the outlet switch, disconnect the outlet switch, pull out a key, and the outlet switch is locked in the off state.This key can be used to unlock and power the bypass cabinet knife gate, and then another key on the bypass knife gate can be pulled out to power the high-voltage switch, so as to realize the conversion of the fan from the frequency conversion mode to the power frequency mode.The primary fan B1 is normally in frequency conversion working state: the K1 and K2 knife switches are closed, and the K3 knife switches are open. Frequency power on interlocking operation sequence of inverter bypass:First disconnect the high-voltage side switch of the primary fan, pull out the key A1, lock the primary fan switch in the cold standby state → insert the key A1 pulled out in the previous step into the mechanical lock of K1 knife gate A1, unlock K1 knife gate, separate K1 knife gate and exchange the key A2, K1 knife gate is locked in the off position, insert the key A2 into the mechanical lock of K2 knife gate, separate K2 knife gate and lock K2 knife gate in the off position, pull out the key a 3 → insert the key pulled out in the previous step into the mechanical lock of bypass K3 knife gate A3, unlock K3 knife gate, then close K3 knife gate and pull out key A1, K3 knife gate is locked in the closed state → finally insert key A1 into the mechanical lock of high-pressure side switch of primary fan, and carry out power transmission operation for the opening and closing of primary fan.