Electrical control systems are generally referred to as secondary control circuits for electrical equipment. Different devices have different control circuits, and the control methods for high-voltage and low-voltage electrical equipment are also different. The main function is to ensure the reliability and safety of equipment operation, which requires many auxiliary electrical equipment to serve it. The combination of several electrical components that can achieve a certain control function is called a control circuit or secondary circuit. These devices should have the following functions:

1. Automatic control function: High voltage and high current switchgear have a large volume and are generally controlled by operating systems for opening and closing. Especially when the equipment malfunctions, the switch needs to automatically cut off the circuit. A set of automatic control electrical operating equipment is required to automatically control the power supply equipment;

2. Protection function: Electrical equipment and circuits may malfunction during operation, and the current (or voltage) may exceed the allowable working range and limit of the equipment and circuits. This requires a set of protection equipment that detects these fault signals and automatically adjusts (disconnects, switches, etc.) the equipment and circuits;

3. Monitoring function: Electricity is invisible to the eyes, and it is impossible to distinguish whether a device is powered on or off from the outside. This requires setting up various audio-visual signals, such as lights and sound systems, to monitor the electrical performance of the device;

4. Measurement function: Light and sound signals can only qualitatively indicate the working status of the equipment (with or without power). If you want to quantitatively know the working condition of electrical equipment, various instrument measurement devices are needed to measure various parameters of the circuit, such as voltage, current, frequency, and power.

In equipment operation and monitoring, traditional operating components, control appliances, instruments, and signal devices can mostly be replaced by computer control systems and electronic components, but there is still a certain range of applications in small equipment and local control circuits. This is also the foundation for implementing microcomputer automation control in circuits.