1. Introduction With the development of modern science and technology, power users have increasingly higher requirements for power quality, prompting substations and other power distribution departments to place higher demands on the continuity of power supply. Therefore, improving the reliability of the low-voltage power distribution system for the safe operation of substations is becoming increasingly important. Typically, the low-voltage power distribution system of a substation is designed with dual power supply lines. Substations of 35kV and below often adopt a one-main-one-standby operation mode; while substations of 110kV and above are often designed with a single busbar segmented dual power supply standby automatic transfer mode. Meanwhile, given the large low-voltage load used in 110kV substations, generally above 400A, high-breaking-capacity frame-type switches such as DW15 and ME are commonly used in conventional designs. However, this type of switch often requires dedicated low-voltage distribution bays. Therefore, the low-voltage power distribution system of 110kV and above substations using frame-type switch designs has a large footprint and high equipment investment costs. With the advent of ABB's S-series high-capacity, high-breaking-capacity low-voltage molded case switches and the application of their matching electric operating mechanisms, it has become possible to design low-voltage automatic transfer switch (ATS) distribution systems using ABB S-series molded case switches. This distribution system can be easily integrated with secondary protection equipment, thus saving installation space and significantly reducing equipment investment costs. This article introduces a 400V ATS distribution system implemented using ABB S-series molded case switches. This system employs ABB SACE Isomax S-series low-voltage molded case switches + electric operating mechanisms and static low-voltage relays, resulting in a small footprint, easy panelization, and superior electrical performance. Trial testing has shown good operational performance. 2. Primary Circuit of the Low-Voltage Automatic Transfer Switch Distribution System The primary circuit of this ATS distribution system uses dual power supply lines: one from a 35KV substation and the other from another substation. A schematic diagram is shown in Figure 1. 2.1 Primary Circuit Overview: The primary circuit is connected via two cable-inbound lines, connecting to the upper ends of S-series molded case switches 1JK and 2JK respectively. These switches connect to busbars I and II. Busbars I and II are also connected via an S-series molded case switch LK equipped with an electrically operated mechanism. The primary circuit operates normally with both 1JK and 2JK in the closed position, the two busbars operating separately, and LK cannot be closed. When either busbar loses voltage, its corresponding incoming S-series molded case switch automatically disconnects (to prevent backfeeding), and then the connecting S-series switch LK automatically closes. When the faulty line returns to normal, the incoming switch can be manually closed (or designed to close automatically). In this case, the connecting switch LK should automatically trip before the incoming switch to prevent the two incoming power supplies from operating in parallel. 2.2 Selection of Primary Components Given that the purpose of this design is to simplify the primary system, reduce equipment costs, enhance equipment reliability, and facilitate maintenance, the selection of primary components is crucial. Considering system reliability and future maintenance, this scheme adopts ABB's S-series molded case switches and their electric operating mechanisms. Specifically, 1JK and 2JK use manual operating mechanisms (which can also be designed as electric operating mechanisms), while LK uses an electric operating mechanism. This design scheme has a primary current range of 225A-1600A and a short-circuit breaking capacity range of 35KA-100KA. 3 Secondary Circuit of Low-Voltage Automatic Transfer Switching System 3.1 Secondary Circuit Schematic Diagram For convenience, the secondary circuit power supply in this scheme uses an AC 220V uninterruptible power supply. 3.1.1 Design of the Schematic Diagram for the 1# Incoming Line Control The schematic diagram for the 1# incoming line control is shown in Figure 2. During normal operation: Incoming line switches 1JK and 2JK are in the closed position, and 1ZJ is de-energized. When the 1# incoming line loses voltage, the low-voltage relay 2YJ operates, and the normally closed contact of 2YJ closes, activating the intermediate relay 1ZJ, which first trips the 1JK incoming line switch. Simultaneously, a signal is sent to close the tie switch LK (see the tie switch control schematic diagram). 3.1.2 Design of the Schematic Diagram for the Tie Switch Control The tie switch control schematic diagram shows that KK is a manual/automatic switch. When in the manual position, a manual button controls the opening and closing of the tie switch. Normally, it is in the automatic position. When the incoming line switch 1JK trips, its auxiliary normally closed contact 1JK closes, activating the tie switch closing coil HQ, and the tie switch automatically closes. The delayed closing of the normally open contact of 1ZJ ensures that 1JK is disconnected before the LK switch is closed. When 1JK and 2JK are both in the closed position, the normally closed contacts of 1JK and 2JK ensure that the tie switch LK cannot be closed. When any incoming switch is in the closed position, if the other incoming switch is to be closed, the normally open contacts of 1JK and 2XJ ensure that the tie switch trips first. The delayed closing contacts of 1XJ and 2XJ are used to ensure the incoming switch is protected against tripping. 3.1.3 Design of the Control Principle Diagram for Incoming Switch #2 The control principle diagram for incoming switch #2 is shown in Figure 4. Its control process is as follows: Incoming switch 1JK is not described further here. 3.2 Selection of Secondary Components Given the reliability of the secondary control circuit, which is crucial for ensuring the safe operation of the system, the selection of secondary components should be particularly careful. This scheme uses static low-voltage relays for key components 1YJ and 2YJ, and a combination of contactor-time relays for 1ZJ, 2ZJ, 1XJ, and 2XJ. 4. Conclusion Based on ABB's SACE Isomax S series low-voltage molded case switch and electric operating mechanism, the 400V low-voltage automatic transfer switch (ATS) power distribution system has demonstrated stability and reliable automatic switching since its trial operation at a 220kV substation in June 2004. Furthermore, compared to low-voltage ATS distribution systems implemented with microprocessor-based ATS devices, it significantly reduces equipment investment costs. The system demonstrates excellent practical performance and broad application prospects.