In order to popularize popular science knowledge among all employees and readers, and strive to create a good atmosphere to understand science, love science, increase knowledge and carry out technological innovation, this newspaper and the Association for Science and Technology jointly open the "science garden" column, and hope to get everyone's support. Enthusiastic submission.In China's economic construction and people's lives, the application of electricity more and more widely. To achieve the modernization of industry, agriculture, national defense and science and technology, it is even more inseparable from electrification. For the safe and reliable use of electrical energy, the circuit must be equipped with a variety of electrical equipment. These electrical devices are collectively referred to as electrical appliances. It can be divided into high-voltage electrical appliances and low-voltage electrical appliances according to the working voltage level. Accustomed to more than 3KV electrical circuits collectively referred to as high-voltage electrical appliances. Low-voltage electrical appliances are used in AC 50Hz or 60Hz, rated voltage of 1200V and below, or DC 1500V and below the electrical circuit. It plays a role in protection, control, regulation, conversion and on-off in the power system. Low-voltage electrical appliances according to the purpose and control of different objects can be broadly divided into:Distribution electrical appliances. Mainly used for power network system. Such a low-voltage circuit breakers, fuses, knife switches, switches and so on. The main technical requirements of the electrical distribution is breaking capacity, limiting the effect of good, dynamic stability and high thermal stability, low operating voltage.Control of electrical appliances. Mainly used for electric drag and automatic control system. There are contactors, starters, the main electrical appliances, all kinds of control relays, and so on. The main technical requirements for the control of electrical appliances is the appropriate conversion cap