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The \"interlock\" in high-voltage switchgear is an important measure to ensure the safe operation of power grids, protect equipment and personnel, and prevent misoperations. GB3906-1991 \"AC Metal-Enclosed Switchgear for 3–35 kV\" specifies this clearly. \"Interlocking\" is generally described as a mechanism that prevents the accidental opening or closing of circuit breakers; prevents the opening or closing of isolators while they are under load; prevents the installation (closing) of grounding wires (grounding switches) while there is electricity present; prevents closing switches while they are in a grounded state; and prevents entry into areas where electricity is present. The above five measures to prevent electrical misoperations are collectively referred to as \"five protections\". "\"Five-prevention\" devices can generally be divided into three categories: mechanical, electrical, and integrated. Currently, there are many types of high-voltage switches available on the market, and most of them come with fairly comprehensive interlock mechanisms. However, the interlocks of many high-voltage switchgear units, especially the mechanical interlocks, are not yet perfect, and they fail to fully meet the “five protections” requirements. To this end, this paper focuses on presenting some views regarding mechanical interlocks for discussion with colleagues. Interlock implementation method: In fixed cabinets and trolley cabinets. As is well known, isolating switches do not have a dedicated arc-quenching device, and generally cannot be used to connect or disconnect load currents. In fixed cabinets, the interlocking relationship between circuit breakers and isolators is clear: the isolator can only be operated when the circuit breaker is in the open position. The mechanical interlocking between the isolator and the circuit breaker is relatively easy to implement. As shown in CC-1A(F), to prevent the disconnection or connection of the isolator while it is under load, a fan-shaped striker and a disc structure are commonly used. These work in conjunction with the elastic positioning lock on the CS6-1 mechanism used to operate the isolator; when the circuit breaker is closed, the disc prevents the positioning lock from being pulled out, thus preventing disconnection while under load. However, the situation with trolley cabinets is different; the insertion of a trolley is essentially equivalent to the opening and closing operation of an isolator in a fixed cabinet. Therefore, the interlock requirements for isolators also apply to the insertion and removal of trolleys. When the trolley moves between the test position and the operating position, it must be ensured that the circuit breaker remains in the open position and cannot be closed; in other words, there should be a so-called \"closing interlock\". In various types of drawer cabinets in service, the electrical interlock for \"closing\" is generally achieved by inserting the contacts of a travel switch that indicates the position of the drawer into the circuit for closing the circuit breaker. Operation sequence of the isolators on both sides of the circuit breaker. The operation sequence for the isolators on both sides of the circuit breaker as specified in the GG-1A(F)–07 plan is as follows: during power outage, first disconnect the isolator on the line side ； When supplying power, the busbar-side isolating switch must be closed first. Its purpose is to ensure that in the event of an incorrect operation, the protective function of the circuit breaker can be utilized to minimize the scope of the incident and prevent human intervention from exacerbating it. The reason for disconnecting the circuit-side isolator first in case of a power outage is that if an error occurs during the outage – such as the circuit breaker not having yet cut off the power supply while the isolator is disconnected first, resulting in switching under load; or if, while disconnecting the isolator, the wrong isolator of a circuit that should remain connected is disconnected – this can lead to an arc flash short circuit. Under the above conditions, if the circuit-side isolator is opened first, since the arc flash short-circuit point is located outside the circuit breaker, the protective device of the switch will activate to trip and isolate the fault, thereby reducing the scope of the incident. Conversely, during power transmission, if the circuit breaker is accidentally in the closed position and the busbar-side isolator is closed afterward, it is equivalent to supplying power to the busbar side under load. An arc short circuit will inevitably occur, thereby exacerbating the fault. In this situation, if the busbar-side isolator is closed first and then the line-side isolator, it is equivalent to closing the line-side isolator under load. In the event of an arc flash short circuit, since the short-circuit point is located outside the circuit breaker, the circuit breaker’s protection mechanism will activate to trip the circuit breaker, thereby eliminating the fault and limiting the scope of the incident. Therefore, the busbar-side isolating switch must be closed first when supplying power. It is certainly good to be able to open and close the isolators in the aforementioned sequence, but in practice it is difficult to ensure that the isolators on both sides of the circuit breaker are operated strictly in that order. For ring main switches and bus coupler switches, it is relatively difficult to distinguish between the busbar side and the line side. In the case of counterbalance cabinets, the contacts on both sides of the circuit breaker enter and exit simultaneously. The situation will also be different if the current transformer is installed outside the isolating switch. Therefore, only by ensuring the reliability of the interlock between the circuit breaker and the isolator can the problem of accidental operation of the isolator be resolved fundamentally. Conflict between electrical safety regulations and interlock requirements Over the years, statistics on electrical accidents have shown that injuries caused by electric shock and failures of electrical equipment are often directly related to the technical skills of the electrical workers involved. Operating strictly in accordance with electrical safety procedures can effectively reduce and prevent the occurrence of misoperation accidents. However, in practical work, conflicts between procedures and interlocks often occur. In switchgear models such as KYN28A-12 and JYN6-12, which are equipped with a grounding switch on the output side, the interlock function ensures that the door (or cover) of the cable compartment cannot be opened until the grounding switch is closed, thereby preventing accidental entry into live compartments. The regulations state that the ground switch can only be closed after confirming that the circuit is de-energized; this means that the cabinet door must be opened to verify that the circuit is indeed de-energized before the ground switch can be operated, resulting in a conflict between the regulations and the interlock requirements. To address such issues, the following methods can be employed: first, install a live display on the line side; after confirming that there is no voltage, close the grounding switch. Alternatively, an electromagnetic lock can be installed on the operating lever of the grounding switch to ensure that it cannot be closed when the line is under voltage. Second, making notches in the cabinet door allows for careful voltage testing. Third, an emergency unlocking device should be installed on the cable room door (or cover), to be unlocked by professionals using special tools. Active interlocking and passive interlocking: For each interlocking requirement within the \"five protections\" concept, whether it involves fixed cabinets or pull-out cabinets, various designs can be developed to achieve the interlocking function, despite the diversity in cabinet types and primary circuit configurations. In summary, mechanical interlocks mainly come in two forms: active and passive. The so-called \"active interlock\" refers to the situation where, under this type of interlock, it will not be unlocked in the absence of proper unlocking conditions, thereby ensuring that accidental operations cannot take place at all. For example, when the circuit breaker is in the closed position, mechanically it should be ensured that the isolator or isolation plug is locked and cannot be operated, so that no adverse effects occur even if an incorrect operation is attempted. "\"Passive interlocking\" refers to a situation where, under such interlocking conditions, unlocking conditions may arise due to abnormal reasons, resulting in the release of the lock. If the isolating switch or pull-cord is operated in the locked state, although the isolating switch has not yet moved or the isolation plug has not shifted, the circuit breaker trips due to the interlock mechanism, thereby releasing the interlock between the isolating switch and the isolation plug. Obviously, this form of passive interlocking can cause the circuit breaker to trip accidentally. In the design of high-voltage switchgear, \"passive\" interlocks should be avoided, and \"active\" interlocks should be used as much as possible. For example, in switchgear that uses levers to move the trolley in and out, when the circuit breaker is in the closed position, the holes through which the lever used to move the trolley are mechanically blocked, preventing the lever from being inserted; as a result, the trolley cannot move at all. Furthermore, the insertion hole for operating the grounding switch is designed to open only under permitted conditions; otherwise, it remains sealed, preventing any incorrect operation from taking place. In the various switchgear units in use today, many mechanical interlocks employ a \"passive\" type of interlocking; as the design standards for switchgear improve, it is necessary to avoid this approach as much as possible. “Integrity of the “five-prevention” interlocks **The standards require that, regarding the “five-prevention” measures, aside from allowing advisory methods to prevent accidental opening or closing of circuit breakers, all other interlock relationships must be fully implemented within the structure of the switchgear. From the perspective of switchgear operation, each interlock requirement is reflected in a specific set of operating procedures; therefore, both the closing procedure and the opening procedure must meet the interlock requirements. Prevent opening and closing of isolators under load ； Prevent connecting (closing) grounding wires (switches) while they are energized, and closing switches while they are connected to grounding wires. However, these two requirements sometimes need to be met not within the same container but between containers, and in such cases mechanical interlocks often fail to meet the requirements. Reliability of mechanical interlocks Regardless of the frequency of use, the mechanical interlock devices in high-voltage switchgear should remain flexible and reliable throughout their service life. They must also be effective at preventing moisture, mold, and rust, and should not get stuck; their design should be simple and straightforward to ensure easy operation and maintenance. During use, high-voltage switchgear may also experience \"abnormal operation\" due to the operator’s negligence or abnormal operating forces. Under such circumstances, a highly reliable interlock system should have the capability to fully prevent or stop any \"abnormal operations\" that the operator might carry out. Even if errors occur under certain circumstances, it should be possible to correct those errors and easily return to normal operation, so as to avoid serious equipment or personal accidents.