Relay, also known as electrical relay, is an electronic control device. It has a control system (also known as an input circuit) and a controlled system (also known as an output circuit). It is usually in an automatic control circuit. In the circuit, it plays the role of automatic adjustment, safety protection, and conversion circuit. The relay coil is represented by an oblong symbol in the circuit. If the relay has two coils, draw two parallel long wires. At the same time, mark the text symbol “J” of the relay in or beside the long box. There are two ways to represent the wiring of relays: one is to draw them directly on a long page, which is more intuitive.

     The relay box is a secondary circuit control box. It is a low-voltage electrical equipment used to monitor, control, measure, regulate and protect the working conditions of the power system and primary equipment, including measuring instruments, primary equipment control, operation monitoring signals, and Automatic monitoring system, relay protection and safety automatic device, communication equipment, etc. The electrical circuit formed by connecting the secondary equipment according to certain functional requirements is collectively called the secondary wiring or the secondary circuit, which is an important part of ensuring the safe production, economic operation and reliable power supply of the power system.