Commonly used low-voltage electrical appliance symbols and their functions:
Knife switch (QS): mainly used to clearly isolate the circuit from the power source after the power is cut off, so as to ensure the safety of maintenance personnel.
Combination switch (QS): Used for manually and infrequently connecting and disconnecting circuits, switching power supplies or loads, and can also control small-capacity asynchronous motors.
Automatic air circuit breaker (QF): mainly used for distributing electrical energy in low-voltage power circuits and for infrequent switching on and off of circuits, and has an automatic tripping function in case of fault.
Control button (SB): Used in control circuits to briefly turn on and off small current control circuits.
Limit switches (SQ): These switches operate by the collision of moving mechanical parts, used to disconnect or connect control circuits. They are mainly used to detect the position of moving machinery, control the direction and stroke of moving parts, and provide limit protection.
Proximity switch (SP): When a moving object approaches the sensing head of the proximity switch, it outputs an electrical signal to control the opening and closing of the circuit.
Contactor (KM): It can frequently connect and disconnect AC and DC main circuits and can realize remote control. It is mainly used to control motors, but can also control electrical loads such as capacitors, electric furnaces and lighting fixtures.
Intermediate relay (KA): expands the number of contacts and amplifies the signal.
Current relay (KA): controls the operation of output contacts based on changes in the magnitude of the input current.
Voltage relay (KV): controls the operation of output contacts based on changes in the input voltage.
Time relay (KT): Connects or disconnects the circuit according to a predetermined time.
Thermal relays (FR): These provide overload protection for continuously running motors to prevent them from overheating and burning out. Most thermal relays, in addition to overload protection, also offer phase loss protection, temperature compensation, and automatic and manual reset functions.
Speed relay (KS): It is mostly used for reverse braking control of three-phase AC asynchronous motors. When the reverse braking process of the motor ends and the speed crosses zero, the reverse phase sequence power supply is automatically cut off to ensure reliable stopping of the motor.
Fuse (FU): It mainly serves as a short-circuit protection in low-voltage power distribution circuits.
Indicator light (HL): Used to indicate the working status of the circuit, and can also be used to indicate the working status, warning, fault and other signals.
Low-voltage electrical appliances can automatically or manually change the state and parameters of the circuit according to the requirements of operating signals or external field signals, so as to realize the control, protection, measurement, indication and regulation of the circuit or the controlled object.
The functions of low-voltage electrical appliances include:
(1) Control functions such as the elevator’s up and down movement, automatic switching between fast and slow speeds and automatic stopping at floors.
(2) The protective function can automatically protect the equipment, environment and personnel according to the characteristics of the equipment, such as overheat protection of motor, short circuit protection of power grid, leakage protection, etc.
(3) Measurement function: Using instruments and compatible electrical appliances, to measure equipment, power grid or other non-electrical parameters, such as current, voltage, power, speed, temperature, humidity, etc.
(4) Regulation function: Low-voltage electrical appliances can adjust some electrical and non-electrical quantities to meet the user's requirements, such as adjusting the diesel engine throttle, adjusting the room temperature and humidity, and automatically adjusting the illuminance.
(5) Indication function: Utilizing the control and protection functions of low-voltage electrical appliances, the operating status of equipment and the working status of electrical circuits can be detected, such as insulation monitoring and protection flag indication.
(6) The conversion function is to switch between electrical equipment or to put low-voltage electrical appliances and control circuits into operation in a time-sharing manner to achieve function switching, such as the manual and automatic switching of excitation devices, and the switching of mains power and backup power supply.
Of course, the functions of low-voltage electrical appliances go far beyond these. With the development of science and technology, new functions and new equipment will continue to emerge. The main types and uses of commonly used low-voltage electrical appliances are shown in the table.
Low-voltage power distribution equipment requires strong arc-extinguishing capability, good breaking capacity, good thermal stability, and accurate current limiting. Low-voltage control equipment requires reliable operation, high operating frequency, long lifespan, and a certain load capacity.
Main types and uses of common low-voltage electrical appliances
