The Composition of Low-voltage Electrical Appliances - Sensing Part, Execution Part

A low-voltage electrical appliance is a component or device that can switch, control, protect, detect, transform and adjust circuits or non-electrical objects by manually or automatically switching on and off circuits according to external signals and requirements. Control electrical appliances can be divided into high-voltage control electrical appliances and low-voltage control electrical appliances according to their working voltage, with AC 1200V and DC 1500V as the boundary. Generally speaking, low-voltage electrical appliances can be divided into two categories: power distribution appliances and control appliances, which are the basic components of complete sets of electrical equipment. In industry, agriculture, transportation, national defense and people's power consumption departments, most of them use low-voltage power supply, so the quality of electrical components will directly affect the reliability of low-voltage power supply system.


Low-voltage Electrical Appliances


1. Definition of low-voltage electrical appliances


Low-voltage electrical appliances generally refer to electrical appliances that work below AC voltage 1200V or DC voltage 1500V. Common low-voltage devices include switches, fuses, contactors, leakage protectors, and relays. When installing electrical circuits, the power supply and the load (such as a motor) are connected with low-voltage electrical appliances through wires, so that control functions such as on, off, and protection of the load can be realized.


2. Basic structure of low-voltage electrical appliances


Low-voltage electrical appliances generally have two basic parts: one is the sensing part, which senses external signals and responds regularly. In self-controlled electrical appliances, the sensing part is mostly composed of electromagnetic mechanisms. In controlled electrical appliances, the sensor The measuring part is usually the operating handle, etc.; the other is the execution part, such as the contact is connected or cut off according to the instruction.


3. Equipment introduction of low-voltage electrical appliances


Control electrical appliances can be divided into high-voltage control electrical appliances and low-voltage control electrical appliances according to their working voltage, with AC 1200V and DC 1500V as the boundary. AC 1200V and below, DC 1500V and below are called low-voltage electrical appliances.


The development of low-voltage electrical appliances depends on the development of the national economy and the needs of the development of modern industrial automation, as well as the research and application of new technologies, new processes, and new materials. At present, it is developing in the direction of high performance, high reliability, miniaturization, digital modeling, modularization, combination and generalization of parts and components.


4. The role of low-voltage electrical appliances


Low-voltage electrical appliances can automatically or manually change the state and parameters of the circuit according to the requirements of the operation signal or external field signal, so as to realize the control, protection, measurement, indication and adjustment of the circuit or the controlled object.


The functions of low-voltage electrical appliances are:

Control functions, such as the up and down movement of the elevator, automatic switching between fast and slow speeds, and automatic floor stop.


Regulating function, low-voltage electrical appliances can adjust some electric and non-electrical quantities to meet user requirements, such as adjustment of diesel engine throttle, adjustment of room temperature and humidity, automatic adjustment of illumination, etc.


The protection function can automatically protect the equipment, the environment, and the person according to the characteristics of the equipment, such as the overheating protection of the motor, the short circuit protection of the power grid, and the leakage protection.


Indicating function, using the control and protection functions of low-voltage electrical appliances to detect the operation status of equipment and the operation status of electrical circuits, such as insulation monitoring, protection card drop indication, etc.


5. Classification of low-voltage electrical appliances


There are many types of low-voltage electrical appliances, and there are many classification methods.


5.1 According to the action mode, it can be divided into:

5.1.1 Manual electrical appliances--electrical appliances that are switched by direct operation by external force, such as knife switch, push button switch, etc.

5.1.2 Automatic electrical appliances - electrical appliances that operate automatically depending on instructions or changes in physical quantities, such as contactors, relays, etc.


5.2 According to the application, it can be divided into:

5.2.1 Low-voltage control appliances—mainly play a control role in low-voltage power distribution systems and power equipment, such as knife switches, low-voltage circuit breakers, etc.

5.2.2 Low-voltage protection appliances—mainly play a protective role in low-voltage power distribution systems and power equipment, such as fuses and thermal relays.


5.3 According to the type, it can be divided into:

Knife switch, knife switch, fuse, low-voltage circuit breaker, contactor, relay, master electrical appliance and automatic switch, etc.


6. Equipment selection for low-voltage electrical appliances


6.1 The electrical appliances selected for the design of low-voltage power distribution shall meet the relevant current national standards and shall meet the following requirements.


6.1.1 The rated voltage of the electrical appliance should be compatible with the nominal voltage of the circuit where it is located

6.1.2 The rated current of the electrical appliance should not be less than the calculated current of the circuit where it is located

6.1.3 The rated frequency of the electrical appliance should be compatible with the frequency of the circuit where it is located

6.1.4 Electrical appliances should adapt to the environmental conditions of the place

6.1.5 Electrical appliances should meet the requirements of dynamic stability and thermal stability under short-circuit conditions. The electrical appliances used to break the short-circuit current shall meet the on-off capacity under short-circuit conditions.


6.2 To check the on-off capacity of electrical appliances under short-circuit conditions, the effective value of the periodic component of the expected short-circuit current at the installation site should be used. When the sum of the rated current of the motor connected near the short-circuit point exceeds 1% of the short-circuit current, it should be included in the feedback current of the motor Impact.


7. Precautions for the use of low-voltage electrical appliances


Low-voltage electrical appliances choose circuit breakers with different types of short-circuit breaking capacity to meet the needs of different line expected short-circuit currents (when I is in the same situation).


7.1 Different types of circuit breakers should be selected for different load low-voltage electrical appliances

The most common loads of low-voltage electrical appliances are three categories: distribution lines, motors, and household and similar households (lighting, household appliances, etc.). Most of DW15 series, DW17(ME) series, AH series and DW40, DW45 series of universal (also known as frame type) circuit breakers are type B, while DZ5, DZ15, DZ20, TO, TG, CM1, TM30 and HSM1 and other series and some specifications of universal DW15.DW17 are non-selective class A circuit breakers because they only have the second-stage protection of overload long-time delay and short-circuit instantaneous. Selective protection.


When the point F is short-circuited, only the QF2 circuit breaker close to the F point operates, and the QF1 circuit breaker at the upper position does not operate, which is selective protection (because QF1 does not operate, the QF3 and QF4 branches that have not failed to maintain power supply ). If both QF2 and QF1 are Class A circuit breakers, a short circuit occurs at point F, and when the short circuit current reaches a certain value, QF1 and QF2 act at the same time, and all the circuits of the circuit breaker of QF1 and its lower branches are powered off, which is not selective protection. . The reason why selective protection can be realized is that QF1 is a class B circuit breaker, which has short-circuit and short-delay performance. When point F is short-circuited, the short-circuit current flows through the QF2 branch and also flows through the QF1 circuit, and the instantaneous action of QF2 trips The device acts (usually its full breaking time is not greater than 0.02s), because of the short delay of QF1, QF1 will not act within 0.02s (its short delay ≥ 0.1s or When the QF2 action cuts off the fault line, the whole system returns to normal. It can be seen that it can choose Class A circuit breakers (including molded case and universal).


8. Selection of conductors for low-voltage electrical appliances


8.1 The type of conductor should be selected according to the laying method and environmental conditions. Insulated conductors should meet the requirements of working voltage in addition to meeting the above conditions.


8.2 Select the cross section of the conductor, which should meet the following requirements:


8.2.1 Line voltage loss should meet the requirements of terminal voltage for normal operation and start-up of electrical equipment

8.2.2 The current carrying capacity of the conductor determined according to the laying method and environmental conditions shall not be less than the calculated current

8.2.3 The conductor should meet the requirements of dynamic stability and thermal stability

8.2.4 The minimum cross-section of conductors should meet the requirements of mechanical strength, and the minimum core wire cross-section of fixedly laid conductors should meet the requirements.


8.3 When laying insulated wires and cables along paths with different cooling conditions, when the length of the section with the worst cooling condition exceeds 5m, the section of the insulated wire and cable should be selected according to the conditions of this section, or only the insulated wire and cable with a large section should be used for this section. cable.


9. Types of low-voltage electrical appliances


9.1 Development direction of power circuit breakers

As one of the most important products in low-voltage electrical appliances, power circuit breakers have been favored by many people for their development. Judging from the new products launched, it can be said that the competition is fierce. Frame circuit breaker (ACB) The new generation frame circuit breaker not only has its overall performance and main technical indicators greatly improved, but also the improved technical performance indicators highlight the practicality. The high-current integral structure has become a mainstream in the development of large-grade specifications for the new generation of frame circuit breakers. The new generation of frame circuit breakers not only further reduces the volume of each specification product, but also reduces the number of specifications of the whole series. Most of the contact arc extinguishing systems in the new generation of frame circuit breakers adopt a single break point structure. The internal and external accessories of many new-generation frame circuit breakers adopt modular design, which is not only convenient for standardized assembly production, but also convenient for disassembly, replacement and maintenance. The electronic controllers in the new generation of frame circuit breakers have powerful functions. All new-generation products are equipped with communication interfaces, which can use internal buses with some new-generation frame circuit breakers of major industrial fieldbus systems.


9.2 Molded case circuit breaker (MCCB)

Compared with the older generation of molded case circuit breakers, the new generation of molded case circuit breakers has made great breakthroughs in product structure, operating mechanism, contact arc extinguishing system, adjustable thermal magnetic release and electronic release. The comprehensive technical and economic indicators of the product have been greatly improved. The development of the new generation of molded case circuit breakers pays great attention to improving the technical performance indicators while reducing the volume, so the new double break point breaking technology is getting more and more attention.


With the improvement of short-circuit breaking capacity, many products have realized Ics=Icu, and some series of products have Icw index in the high-current level specification, which makes it possible to realize selective protection between the upper and lower stages of the molded case circuit breaker. Many new-generation products are not only greatly reduced in size, but also have a tendency to reduce the frame level. With the size reduction and cost reduction of electronic trip units, it is an obvious trend to adopt electronic trip units in the whole series, and the protection performance of molded case circuit breakers using electronic trip units is more perfect.


The new generation of molded case circuit breakers all have the function of ground fault protection and residual current protection. The new generation of molded case circuit breakers also have the function of residual current protection. There are generally two structural methods: twin structure and assembled structure. The twin structure is small in size and easy to maintain and replace; the assembled structure is flexible.


In order to meet the needs of the system, more and more attention is paid to the development of accessories, and a wealth of internal accessories and external accessories are provided. The new generation of molded case circuit breakers all have communication functions, which are conveniently connected to various fieldbus systems (such as Profibus, Devicenet, Modbus, etc.) through external communication adapters to realize remote monitoring.


9.3 Development trend of control appliances

With the rapid development of electronic technology, the development of control appliances is getting faster and faster, especially the development of control and protection switching appliances, electronic motor protectors, soft starters and frequency converters. In the past ten years, some Most of them have developed two generations of products, which shows the rapid development of technology.


9.4 Control and protection switchgear (CPS)

In recent years, major companies have introduced the concept of overall solutions into motor control and protection devices, and directly assembled motor protection circuit breakers, contactors, and protection relays into compact motor starters (that is, combined types such as MOELLER's Xstart series, etc.), And in the design of components, the mutual connection and combination requirements are considered from the aspects of appearance, size, terminal connection and performance coordination. With the development of technology and the increase of demand, integrated CPS has also appeared (such as Schneider's TesysU series), which is more compact and reasonable in structure and more powerful in function. Integral CPS is represented by Schneider's Tesys U series products. High integration not only greatly reduces the volume compared with combined type, but also has perfect protection performance and powerful functions. Therefore, integral CPS is likely to become the mainstream of CPS development in the future.


9.5 Electronic motor protector

Electronic motor protectors are electrical appliances that were born with the development of electronic technology and are professionally used for the protection of motors. They are divided into high, medium and low-end product series according to the level of motor protection requirements. High-end motor protectors are very powerful, such as one of the main products in the motor protector product series under MM2 and GEMultlin, which are mainly used in low-voltage motor control centers. [4]

The function expansion of the electronic motor protector also presents a variety of schemes. The protection scope is not only motor protection, but also extends to other equipment protection, such as substation protection, transformer protection, etc. The mechanical structure and function of low-end and mid-range electronic motor protectors are similar to those of traditional thermal protection relays. They can be directly assembled with contactors of the same level to form compact motor starters, but their functions are more powerful than thermal relays and will gradually replace thermal relays. Relays occupy a place in the field of motor control and protection.


9.6 Soft starter

The soft starter technology has developed rapidly, and has basically replaced the original step-down starter, auto-coupling decompression starter and other products, and has become an important motor starter. Generally, there are two series of soft starters, economical and advanced. Launcher. Economical soft starters generally have simple functions, only have soft start function, and have no other protection functions, but they are low in cost, compact in structure, and small in size. Some economical soft starters can also be used as semiconductor contactors. Advanced soft starter, in addition to having a soft start function. It also has various other protection functions.


Motor frequency conversion speed control device (frequency converter) motor frequency conversion speed control device can adjust the motor speed arbitrarily by changing the output frequency of the power supply, and realize smooth stepless speed regulation. It is widely used in industrial control that requires speed regulation, so each is very Pay attention to the development of frequency conversion speed control device. The general characteristics of the new generation of motor frequency conversion speed control devices are: high performance; strong ease of use, further simplification of installation and initialization settings; wide power range, excellent speed control and torque control characteristics; power structure and control unit Modular, the control unit can support plug-and-play functions; intelligent, support multi-bus communication, provide an open field programmable structure; high power density, compact size; less affected by EMI/RFI harmonics. With the development of frequency converter control technology, its advanced products have high control performance, not only can control AC asynchronous motor, but also can control AC permanent magnet synchronous motor, and its control performance level has reached the level of general servo controller. Motor-integrated frequency converters and component frequency converters have been developed rapidly. This type of frequency converter will become the mainstream of the development of small frequency converters.


9.7 Development Trend of Terminal Appliances

The development of terminal electrical appliances has experienced rapid development in the 1990s, and has shown a steady development trend in recent years. The product series has complete specifications, high technical performance indicators, modular size, and track-based installation.


9.8 Development of Miniature Circuit Breakers (MCBs)

As the main product in terminal electrical appliances, the development of miniature circuit breakers has always dominated the development of terminal electrical appliances. The selective protection of the terminal power distribution system has not been realized in the past. The birth of miniature circuit breakers with selective protection, such as ABB’s S700, has completely solved the problem of selective protection of the upper and lower levels of the terminal power distribution system. The safety of electricity use, Reliability is greatly improved. P+N structure miniature circuit breakers have developed rapidly in recent years. At present, P+N structure miniature circuit breakers are developing in the direction of large capacity and high breaking capacity. With the breakthrough of P+N structure and miniature circuit breaker technology, smaller volume (Estimated single-pole 12mm wide) miniature circuit breakers will also be available in the near future, which will usher in another development peak of miniature circuit breakers. The same miniature circuit breaker has various tripping characteristics, which can meet various standards, meet the requirements of different power distribution systems, and meet the needs of global trade. In addition to its own technological development, miniature circuit breakers have more complete accessories. There are new breakthroughs in the installation method of miniature circuit breakers, from screw installation to rail installation, which makes the installation of miniature circuit breakers more convenient. At present, the integrated technology of busbar connection and installation is adopted by more and more miniature circuit breakers to improve production efficiency .


9.9 Development of residual current circuit breakers RCCB and RCBO

In recent years, the development of residual current circuit breakers RCCB and RCBO has mainly focused on B-type residual current circuit breakers and residual current circuit breakers with self-test function. The introduction and continuous breakthrough of new technologies not only make the protection performance of residual current circuit breakers more Perfect, more functional, and shrinking in size. The emergence of small-volume B-type RCCB improves the performance of residual current protection on the one hand, and lays the foundation for wider application in the future on the other hand. The birth of the residual current circuit breaker with self-test function is another highlight in the development of the residual current circuit breaker family. The application of this technology will greatly improve the safety and reliability of residual current circuit breaker terminals.


9.10 Development of Terminal Combined Appliances

After nearly two decades of development, most of the protective shells of terminal combined electrical appliances are made of high-strength engineering plastics, with novel structure, beautiful appearance, bright colors, and the protection level of shells has been continuously improved, from IP30, IP40 to IP55, IP65. Modular dimensions allow them to be used alone or in combination. The integrated technology of busbar connection and installation is increasingly used in the installation of terminal combined electrical appliances and busbar connection, which will become the mainstream structure of protective enclosures.


9.11 Development trend of low-voltage surge protective device (SPD)

With the development of electronic technology, more and more countries pay attention to lightning protection technology and lightning protection products, and all major countries have launched a complete series of products. In addition to graded products, in order to facilitate the use of supporters, Haoduo has introduced SPDs with a combined structure of I+II, which can be directly used for the protection of LPZ0 to LPZ2, which simplifies installation and wiring, and can reduce costs.


10. Principles of installation and use of low-voltage electrical appliances


10.1 Low-voltage electrical appliances should be installed vertically. Especially for oil-immersed decompression starters. In order to prevent the insulating oil from overflowing, the tank should not be tilted more than 5. It should be fixed on the support with bolts instead of welding: the installation position should be easy to operate, and a certain distance should be kept between the handle and the surrounding buildings. Not easy to be damaged.


10.2 Low-voltage electrical appliances should be installed in a place without severe vibration and have an appropriate height from the ground. Power lines such as knife switches and load switches must be connected to fixed contacts. It is strictly forbidden to hook the power cord on the knife switch.


10.3 The metal casing or metal support of low-voltage electrical appliances must be grounded (or connected to zero). The exposed part of the electrical appliance should be covered with a protective cover. The opening position of the double-throwing knife switch should have a device to prevent self-closing.


10.4 In the factory building with flammable and explosive gas or dust, the electrical appliances should be installed outdoors in a sealed manner with rainproof measures. Explosion-proof electrical appliances must be used in places with explosion hazards.


10.5 When in use, keep the surface of electrical contacts clean, smooth, and in good contact. The contacts should have sufficient pressure, the movements of the contacts of each phase should be consistent, and the arc extinguishing device should be kept intact.


10.6 Before use, the protective oil layer on each contact surface should be removed. Before putting into operation, it should be operated several times to check the action. The static contacts of low-voltage electrical appliances should be connected to the power supply, and the moving contacts should be connected to the load.


10.7 The single pole switch must be connected to the phase line. Floor-mounted low-voltage electrical appliances. Its bottom should be 100mm above the ground, and the name of the installed equipment and the circuit number or route should be marked on the panel where the low-voltage electrical appliance is installed.

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