In the electrical control equipment of industrial and mining enterprises, the components utilized are predominantly low-voltage electrical devices. Consequently, low-voltage electrical devices serve as the fundamental building blocks of electrical control systems; the overall quality and performance of a control system are directly contingent upon the performance of these low-voltage components. Driven by economic expansion and the ever-increasing demand for electrical energy, the market for low-voltage electrical devices has grown to a massive scale. Taking the development of my country's power industry as an example-based on an estimated annual increase of 20 million kW in installed capacity-the sector requires approximately 200,000 high-voltage circuit breakers and around 200 million low-voltage electrical components annually. Specifically, this demand includes approximately 460,000 air circuit breakers, 4.4 million molded-case circuit breakers, and 32 million AC contactors per year [2]. As electrical engineering professionals, it is essential to possess a thorough understanding of the structure, operating principles, and application methods of low-voltage electrical devices. In electrical control systems, Programmable Logic Controllers (PLCs) require the integration of numerous low-voltage control components to constitute a complete and functional control system; therefore, mastering the fundamentals of low-voltage electrical devices serves as a prerequisite for learning how to utilize PLCs. Within industrial control systems, relays-as quintessential control devices-play pivotal roles such as signal amplification, control switching, electrical isolation between main power circuits and control circuits, expansion of control capabilities, and circuit protection. Optocoupler relays, characterized by their superior electrical isolation properties and rapid response times, find extensive application in fields such as automation control and power electronics.
Low-voltage electrical devices are defined as electrical apparatus with a rated voltage level of 1200 V AC or less, or 1500 V DC or less. In industrial control circuits within my country, the most commonly utilized three-phase AC voltage level is 380 V; other voltage levels are typically employed only in specific industrial environments-for instance, 127 V for underground electric drills in coal mines, 660 V for conveyor systems, and 1140 V for coal mining machinery. For single-phase AC applications, the most prevalent voltage level is 220 V; however, a 127 V level is frequently adopted for machine tools, thermal instrumentation, and mine lighting systems. Additionally, other voltage levels-such as 6 V, 12 V, 24 V, 36 V, and 42 V-are generally reserved for lighting and signaling applications in safety-critical environments, as well as for serving as control voltages within electrical systems. Common DC voltage levels include 110V, 220V, and 440V, which are primarily used for power applications; 6V, 12V, 24V, and 36V are mainly used for control systems; and in electronic circuits, voltage levels such as 5V, 9V, and 15V are also utilized.
