Abstract: This paper discusses the development trend of dual-power automatic transfer switches (ATSEs). From the development process and trends of ATSEs, it can be seen that PC-grade ATSEs will become mainstream in engineering applications. I. Dual-Power Automatic Transfer Switches (ATSEs) ATSEs consist of one (or more) transfer switches and other necessary electrical components (transfer controllers). They are used to monitor power circuits and transfer one or more load circuits from one power source to another. As terminal switching devices for fire-fighting loads and other important loads, ATSEs are widely used in engineering. Correct and reasonable selection of ATSEs can ensure reliable power supply to important loads. ATSEs are an indispensable and important component in the power supply system for important loads. Currently, ATSEs in China have gone through four development stages: two-contactor type, two-circuit breaker type, excitation-type dedicated transfer switch, and motorized dedicated transfer switch. The two-contactor type transfer switch is the first generation and the earliest dual-power transfer switch produced in China. It is a simple power supply formed by connecting two contactors. Due to its unreliable mechanical interlocking and high power consumption, this device is increasingly rarely used in engineering. The second generation of two-circuit breaker type transfer switches, also known as CB-class ATSE in Chinese national standards and IEC standards, is a modification of two circuit breakers with an additional mechanical interlocking device. It provides short-circuit or overcurrent protection, but the mechanical interlocking is unreliable. The third generation is the excitation-type dedicated transfer switch, an integrated device consisting of an excitation contactor and a controller. It features reliable mechanical interlocking, and the switching is driven by the attraction force generated by an electromagnetic coil, resulting in fast switching speed. The fourth generation is the motorized dedicated transfer switch, a PC-class ATSE. Its main body is a compliant isolating switch, an electromechanical integrated switching device. The switching is driven by a motor, providing smooth and fast switching, and it also has a zero-position crossing function. II. Development Trends of Automatic Transfer Switches (ATSEs) The development trends of ATSEs mainly include two aspects. Firstly, the switch body itself should possess high resistance to inrush current and be capable of frequent switching; it should have reliable mechanical interlocking to ensure that the two power supplies cannot operate in parallel under any circumstances; it should not have fuses or tripping devices to prevent the dual power supply switch from losing power at the output due to overload; it should have a zero-position function and a large isolation distance to withstand higher inrush voltages (above 8KV); the four-stage switch should have a neutral-position (N-stage) pre-closing function to prevent potential drift on the neutral line in different systems during ATSE switching, which could cause inconsistent current flow or current shunting and lead to malfunction of the residual current protection device. Secondly, the controller should adopt a microprocessor-based intelligent product. The detection module should have high detection accuracy and a wide parameter setting range, including voltage, frequency, and delay time; it should have good electromagnetic compatibility and be able to withstand voltage fluctuations, surge protection, harmonic interference, and electromagnetic interference in the main circuit; the switching time should be fast and the delay adjustable; and it should provide users with various signal and fire linkage interfaces, as well as communication interfaces. From the development process and trends of ATSEs, it can be seen that PC-class ATSEs will become the mainstream in engineering applications. It is worth mentioning that the "Stationary Fire Pump Drivers - Controllers" (IEC standard revision draft) stipulates that ATSEs should not have short-circuit and overcurrent protection functions. CB-class ATSEs cannot meet this requirement; in the event of a short circuit or overcurrent, the trip unit will trip, resulting in power supply to the power source while the load is de-energized, failing to meet the power supply requirements of primary and secondary loads. The trend of IEC standard revision also proves that the promotion of PC-class ATSEs in engineering is inevitable. This is why we are discussing PC-class ATSEs separately. III. Selection of PC-class ATSEs Before discussing how to select a PC-class ATSE, let's analyze the ATSE switching procedure. 1. If a deviation is detected in the primary power supply, the load is automatically switched from the primary power supply to the backup power supply; 2. If the primary power supply returns to normal, the load is automatically switched back to the primary power supply. Dual power automatic transfer switches are used for switching between primary and backup power supplies. The operating mechanism of the transfer switch should not permanently disconnect the load circuit from either the primary or backup power supply. The transfer switch should provide auxiliary contacts indicating the position of the connected (primary or backup) power supply. Therefore, when selecting a PC-class ATSE, in addition to following the normal parameters (similar to other low-voltage distribution equipment, which will not be elaborated here), the following aspects should be considered: 1. Electrical isolation, 0-position, and padlock function: To ensure the long-term stability, safety, and remote management of the dual power supply system, the electrical isolation characteristics of the ATSE's main switch are crucial. Its input and output terminals withstand the voltages of both power supplies. Contactors, circuit breakers, and disconnect switches have different functions and should be selected accordingly. Disconnect switches should have a large breaking distance in the open position. National standards stipulate that their line-to-line and disconnect contact-to-line contact must withstand a rated impulse withstand voltage of 8KV. It is recommended to choose an ATSE with a disconnect switch as the main switch. In the event of a fire caused by a non-fire-fighting power supply, and during the inspection and maintenance of electrical equipment downstream of the ATSE, the ATSE should have a 0-position function. Some ATSEs already have a 0-position interface function, allowing connection to the fire control center. Furthermore, during 0-position maintenance, a padlock function should be provided to ensure the safety of maintenance personnel and equipment. 2. Delay Setting and Level Selection: When switching from the mains power supply to the backup power supply, to prevent damage to the backup power supply due to transient fluctuations or voltage loss in the mains power, the ATSE should have a delay detection function. Civil regulations require a delay of no more than 30 seconds. Many products have a switching delay, generally set to 1-8 seconds. The author believes that 3 seconds is more appropriate, as it will not affect the normal use of electrical equipment or lighting. When switching from the backup power supply to the mains power supply, most manufacturers provide a delay of 1-300 seconds to confirm that the mains power supply has returned to normal and is providing stable power. The author believes that 2 minutes is more appropriate. During the delay time, the ATSE continuously supplies power to the load and will not affect the use of electrical equipment. When selecting an ATSE (Automatic Transfer Switch), a four-stage switch should be used, and the neutral (N) line should be completely isolated. This is to prevent potential drift on the N line in different systems during ATSE switching, which could cause current deviation and malfunction of the residual current protection device. 3. Regarding the electromechanical integrated intelligent dual-power automatic transfer switch, Shenyang Siwo Electric Co., Ltd. produces switches with high automation and good safety and reliability, making them a growing trend. The switch consists of a switch body and a drive control section. It utilizes integrated control technology, zero-crossing technology, and unique contact opening and closing technology. Its performance is analyzed below: 1. The drive control section consists of a logic control circuit and a gear motor. The circuit control core uses CPU control, and the power supply uses a switching power supply voltage regulation system, ensuring reliable power supply. The circuit has good electromagnetic compatibility, and the gear motor has strong resistance to damp heat and high temperatures, providing excellent safety protection functions. 2. The mechanical interlocking section features multiple mechanical interlocks to ensure that the two power supplies cannot operate in parallel under any circumstances. 3. The switch protection functions include three-phase loss protection, over/under voltage protection, motor protection, and frequency detection. 4. GLD Control Board Performance: Utilizing an integrated switching power supply, the circuit features overload and short-circuit protection, providing 5V, 8V, and 12V respectively. 5V powers the CPU chip, 8V powers the comparison and detection circuit, and 12V powers the power supply, execution relays, and external input signals. The sampling and comparison circuit employs four voltage comparators to ensure detection of over-voltage, under-voltage, phase loss, and short-circuit. The program control chip (CPU) uses a PIC16C71 microcontroller, featuring power-on reset, program interrupt, and dual-phase input/output functions. IV. Applications of PC-Level ATSEs in Engineering: ATSEs are widely used in engineering, with various wiring methods including bridging, three-point, four-point, and five-point connections, which will not be elaborated upon here. PC-level ATSEs are primarily used at the end of power supply lines for dual-power switching, using a three-point connection. Multiple ATSEs can be used in conjunction to increase the reliability of the power supply system. V. Related Products : Currently, ATSEs on the market are mainly divided into three series: excitation type, double-circuit breaker type, and electromechanical integrated intelligent type. Among them, the mechatronic intelligent (PC-level) type includes the GLD series products manufactured by Shenyang Siwo Electric Co., Ltd. in China, and the Schneider series products manufactured abroad. From a performance-price ratio perspective, GLD has a greater advantage. Of course, this article does not intend to offer excessive product evaluation; each owner can choose the appropriate product based on their own actual situation. However, the application prospects of PC-level ATSEs are extremely broad. References: 1. National Standard "Low-voltage Switchgear and Controlgear" Part 6: Multifunctional Electrical Appliances, Chapter 1: Automatic Transfer Switchgear 2. International Electrotechnical Commission: IEC60947-6-1:1998, IDT 3. Ji Huiyu et al., Shanghai Electrical Apparatus Research Institute, "Introduction to the Draft Content of IEC Standard for Fixed Fire Pump Drivers and Controllers", "Low-voltage Electrical Apparatus", 2001, No. 3