Outline 
 Electrical control systems are generally referred to as the secondary control loop electrical equipment, different devices have different control circuits, and high-voltage and low voltage electrical equipment, electrical equipment, control is not the same. 
 Main function 
 In order to ensure a reliable and safe operation of equipment, there are many auxiliary electrical equipment needed to serve, to achieve a control of a number of electrical components of the combination, known as the control circuit or secondary circuit. These devices have the following functions: 
 (1) automatic control. High voltage and high current switching devices is a great size, the operating system is generally used to control the points, closing, especially when equipment broke down, you need to switch automatically cut off the circuit, we should have automatic control of electrical operating equipment, automatic control of power supply equipment. 
 (2) protection. Electrical equipment and lines will be in the running process fails, the current (or voltage) in excess of equipment and lines to allow the scope and limits of work, which requires a detection of these fault lines and equipment, and automatic adjustment (off, switch, etc.) to protect equipment. 
 (3) monitoring. Electricity is invisible to the eye, a device is charged or off, can not tell from the appearance, which need to set up audio-visual signals, such as lighting and sound, etc., the electrical equipment on a monitor. 
 (4) measurement. Lighting and sound signals that can only be qualitatively the working status of the device (an electric or power), if you want to know quantitatively the work of electrical equipment, but also a variety of instruments measuring equipment, measuring line parameters such as voltage , current, frequency and power of size. 
 Among the equipment operation and monitoring, the traditional components of the operation, control electronics, instrumentation and signal equipment are mostly computer-controlled system and can be replaced by electronic components, but the equipment and place in small local control circuit is still some applications. This circuit are also the basis for computer automation. 
 Edit this paragraph system components 
 Common control lines back to the basic routing of the following components. 
 (1) power supply circuits. Power supply circuit of AC380V and 220V power supply has a variety. 
 (2) protection circuit. Protection (secondary) circuit work with single-phase 220,36 V power supply or DC 220,24 V variety of electrical equipment and wiring for short circuit, overload and loss of pressure and other protection, the fuses, thermal relays, loss of pressure coil, rectifier and voltage regulator components and other components to protect components. 
 (3) signal circuit. Timely to reflect or display device and line work normal and abnormal state information loop, such as lights of different colors, different sounds of the audio equipment. (4) Automatic and manual ask. In order to improve the efficiency of electrical equipment, are generally equipped with automatic link, but in the installation, commissioning and emergency treatment, the control circuit also need to set the link manually, through a combination of switches or switch and other automatic with manual mode conversion. 
 (5) parking brake circuit. Cut off the power supply circuit, the brake and take some measures to quickly stop the motor control areas, such as braking, power reverse braking, regenerative braking and reverse pull down power braking. 
 (6) self-locking and locking the same way. After the start button is released, to maintain power lines, electrical equipment can continue to work part of the electrical part called the self-locking, such as the contactor contacts move together in series in the coil circuit. Two or more electrical devices and components, in order to ensure the safe and reliable equipment operation, only one power-up, another can not power-up protection areas, called the lock aspect. If two contactor contacts were moving off in the other coil in series circuit.