Low-voltage electrical appliances are conventionally divided into the following two categories according to the relationship between the systems used:
1. Low-voltage distribution appliances Low-voltage distribution appliances are mainly used in low-voltage power supply systems. When the circuit has abnormal conditions such as overload, short circuit, undervoltage, loss of voltage, phase failure or leakage, the low-voltage power distribution equipment should play a protective role and automatically disconnect the faulty circuit. Therefore, the main technical requirements for low-voltage distribution appliances are: reliable operation under fault conditions, sufficient dynamic stability and thermal stability. The dynamic stability of the electrical appliance refers to the ability of the electrical appliance to be damaged by the electromotive force of the short-circuit (impact) current; the thermal stability of the electrical appliance refers to the ability of the electrical appliance to withstand the thermal effect of the short-circuit current within a specified time without damage. Such low-voltage electrical appliances include low-voltage circuit breakers, fuses, knife switches and transfer switches.
2. Low-voltage control appliances Low-voltage control appliances are mainly used in electric drive control systems. Such low-voltage electrical appliances include contactors, relays, controllers and main electrical appliances. The main technical requirements for this type of electrical appliances are a certain on-off capability, high operating frequency, and long electrical and mechanical life. Low-voltage control appliances should be able to connect and disconnect the overload current, but not the short-circuit current. Low-voltage electrical appliances can be divided into general industrial electrical appliances, special industrial and mining electrical appliances, safety electrical appliances, marine electrical appliances and traction electrical appliances according to the occasion of use; according to the operation mode, they are divided into automatic electrical appliances and manual electrical appliances; according to the working principle, they are divided into electromagnetic electrical appliances and non-electrical appliances Control electrical appliances, etc. The characteristic of electromagnetic low-voltage electrical appliances is to use electromagnetic phenomena to complete signal detection and working state conversion. Electromagnetic low-voltage electrical appliances are the most typical and widely used in traditional low-voltage electrical appliances.
The working principle and structure of various electromagnetic appliances are basically the same. From the structural point of view, it consists of two parts: the sensing part and the execution part. The sensing part accepts the signal input from the outside, and through the conversion, amplification, and judgment, make a regular response, make the execution part act, output the corresponding command, and achieve the purpose of control. For contact-type electromagnetic low-voltage electrical appliances, the sensing part is the electromagnetic mechanism, and the implementing part is the contact system. For low-voltage electrical appliances such as low-voltage circuit breakers, there is also a middle part that connects the sensing part and the execution part, so that the two parts work in concert and act according to certain rules. For non-electromagnetic automatic appliances, the working principle of the sensing part is different, but the execution part is still a contact system.
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