"Distribution box", also known as distribution cabinet, is a general term for the motor control center. The distribution box is to assemble the switchgear, measuring instruments, protective appliances and auxiliary equipment in a closed or semi-enclosed metal cabinet or screen according to the requirements of electrical wiring, forming a low-voltage power distribution device.
The purpose of the distribution box
It is easy to manage and facilitates maintenance when a circuit failure occurs. The distribution box and distribution cabinet switchboard distribution certificate are complete sets of devices for centralized installation of switches, instruments and other equipment.
There are two kinds of commonly used distribution boxes: wooden and iron, and now the electricity consumption is quite large, so there are more iron ones.
The purpose of the distribution box: of course, it is convenient to stop and transmit power, and play the role of metering and judging stop and power transmission.
Distribution box composition
It is mainly divided into two parts:
The first is a complete set of components, that is, the distribution box shell and its related accessories.
The second is electrical components and related accessories, that is, air switches and the accessories they need.
1. Circuit breaker
Circuit breaker: the switch, is the main component of the distribution cabinet, commonly used air switch, leakage switch, dual power automatic transfer switch.
1. Air switch:
A. The concept of air switch:
The air switch is also an air circuit breaker, which is used to turn on, break and carry the rated working current and fault currents such as short circuit and overload in the circuit, and can quickly break the circuit in the case of overload, short circuit, undervoltage, etc.
There are various types of dynamic and static contacts and contact rods of circuit breakers, but the main purpose is to improve the breaking capacity of circuit breakers. At present, the current limiting principle of limiting the peak value of short-circuit current during breaking by using a certain contact structure has an obvious effect on improving the breaking capacity of the circuit breaker, and is widely used.
B. The working principle of the air switch:
Automatic air switches, also known as low-voltage circuit breakers, can be used to turn load circuits on and off, as well as to control infrequently started motors. Its function is equivalent to the sum of some or all the functions of electrical appliances such as knife switch, overcurrent relay, loss of voltage relay, thermal relay and leakage protector, and is an important protection appliance in the low-voltage distribution network.
Automatic air switch has a variety of protection functions (overload, short circuit, undervoltage protection, etc.), adjustable action value, high breaking capacity, easy operation, safety and other advantages, so it is widely used at present.
2. Leakage protection switch:
A. Concept of leakage protection switch:
It has the function of leakage protection, and the person will trip when he touches the live energy to ensure personal safety, which is the main function of the leakage protector; if the electrical equipment is not well insulated and leaks to the shell, the leakage protector will also trip to avoid electric shock when the human body touches. At the same time, it has the function of current breaking, overload protection and short circuit protection.
B. The working principle of leakage protection switch:
Diagram of the working principle of the leakage protector. LH is a zero-sequence current transformer, which consists of a permalloy core and a secondary coil wound on a toroidal core. The power phase and neutral wires pass through the round hole to become the primary coil of the zero-sequence transformer. The rear outlet of the transformer is the protection range.
C. The function of leakage protection switch:
(1) When the electrical equipment or line leakage or grounding fault occurs, the power supply can be cut off before the person touches it.
(2) When the human body touches a live object, the power supply can be cut off within 0.1s, so as to reduce the degree of damage to the human body caused by the current.
(3) It can prevent fire accidents caused by leakage.
3. Dual-power automatic transfer switch:
Concept: Dual-power automatic transfer switch is an automatic switching system for two power supplies, after the first fault of the dual power supply automatic transfer switch will automatically switch to the second way to supply power to the load, and the second power supply automatic transfer switch will automatically switch to the first way to supply power to the load.
It is suitable for uninterruptible power conversion of any two power sources, such as UPS-UPS, UPS-generator, UPS-mains, mains-mains, etc.
2. Surge protector
A. The concept of surge protector:
Surge protector, also known as lightning protector, is an electronic device that provides safety protection for various electronic equipment, instruments and meters, and communication lines. When the electrical circuit or communication line suddenly generates a peak current or voltage due to external interference, the surge protector can turn on and shunt in a very short time, so as to avoid the damage of the surge to other equipment in the circuit.
B. Basic knowledge of surge:
The main function of a surge protector system is to protect electronic equipment from "surges". So, if you're wondering what surge protectors do, you need to figure out two questions: What are surges? Why do electronic devices need their protection?
A surge, also known as a surge, is an instantaneous overvoltage that exceeds the normal operating voltage. Essentially, a surge is a violent pulse that occurs in just a few millionths of a second. Surges can be caused by heavy equipment, short circuits, power switching, or large engines.
A surge or transient voltage is when a voltage substantially exceeds its rated level as electrical energy flows.
In the United States, the standard voltage for wiring in the general home and office environment is 120 volts. If the voltage exceeds 120 volts, it can create a problem, and a surge protector helps prevent that problem from damaging the computer.
C. The function of surge protector:
The first line of defense should be a large-capacity power surge protector connected between the phases of the inlet line of the user's power supply system and the ground. Generally, this level of power protector is required to have a maximum impact capacity of more than 100KA/phase, and the required limiting voltage should be less than 2800V. We call this a Class I Power Surge Protector (SPD).
These power surge protectors are designed to withstand high current and high energy surge energy absorption from lightning and induced lightning strikes, shunting large amounts of surge current to earth. THEY ONLY PROVIDE A MEDIUM LEVEL OF PROTECTION AGAINST THE LIMITING VOLTAGE (THE MAXIMUM VOLTAGE THAT OCCURS ON THE LINE WHEN THE INRUSH CURRENT FLOWS THROUGH THE SPD) BECAUSE CLASS I PROTECTORS ARE PRIMARILY USED TO ABSORB LARGE INRUSH CURRENTS. They alone cannot completely protect the sensitive electrical equipment inside the power supply system.
The second line of defense should be a power surge protector installed at the shunt distribution equipment that supplies power to important or sensitive electrical equipment.
These SPDs provide a better absorption of the remaining surge energy through the inlet surge arrester and provide excellent suppression of transient overvoltages. The maximum impact capacity of the power surge protector used here is more than 40 KA/phase, and the required limiting voltage should be less than 2000V. WE CALL THIS A CLASS II POWER SURGE PROTECTOR. The general user power supply system can meet the requirements of the operation of electrical equipment by providing the second level of protection.
The last line of defense is to use a built-in power surge protector in the power supply section of the consumer to completely eliminate the transient overvoltage of small transients.
The power surge protector used here requires a maximum impact capacity of 20 KA/phase or less, and the required limiting voltage should be less than 1800V. For some particularly important or sensitive electronic devices, a third level of protection is necessary. At the same time, it also protects the electrical equipment from transient overvoltages generated within the system.
3. Watt-hour meter
A. The concept of watt-hour meter:
The electric energy meter commonly used by electrical workers is a meter used to measure electrical energy, commonly known as an energy meter.
B. The working principle of the watt-hour meter:
(1) The working principle of mechanical watt-hour meter:
When an energy meter is connected to a circuit, the magnetic fluxes generated by the voltage coil and the current coil pass through the disk, and these fluxes are out of phase in time and space, inducing eddy currents on the disk.
Due to the interaction between magnetic flux and eddy current, the rotating torque is generated to rotate the disk, and the speed of the disk reaches a uniform motion due to the braking effect of the magnet.
Since the magnetic flux is proportional to the voltage and current in the circuit, the disc moves at a speed proportional to the load current under its action, and the rotation of the disc is transmitted to the gauge through the worm, and the indicator of the gauge is the actual electrical energy used in the circuit.
(2) The basic principle of electronic watt-hour meter:
The electronic watt-hour meter is to use electronic circuits/chips to measure electric energy, use voltage divider resistor or voltage transformer to turn the voltage signal into a small signal that can be used for electronic measurement, use a shunt or current transformer to turn the current signal into a small signal that can be used for electronic measurement, use a special electric energy measurement chip to convert the converted voltage and current signals for analog or digital multiplication, and accumulate the electric energy, and then output a pulse signal whose frequency is proportional to the electric energy;
The pulse signal drives the stepper motor to drive the mechanical meter display, or sends it to the microcomputer for digital display after processing.
Fourth, the ammeter
A. The working principle of the ammeter:
The flow meter is made by following the action of an energized conductor in a magnetic field by the force of a magnetic field. When there is a current passing through, the current passes through the magnetic field along the spring and the rotating shaft, and the current cuts the magnetic inductance line, so it is deflected by the magnetic field force, and the rotating shaft and pointer are deflected.
Since the magnitude of the magnetic field increases with the increase of the current, the magnitude of the current can be observed by the degree of deflection of the pointer. This is called a magnetoelectric ammeter.
B. Rules for the use of ammeter:
(1) The ammeter should be connected in series in the circuit (otherwise it will be short-circuited.)
(2) The measured current should not exceed the range of the ammeter (you can use the method of touch test to see if the range is exceeded.)
(3) It is absolutely forbidden to connect the ammeter to the two poles of the power supply without passing through the electrical appliances (the internal resistance of the ammeter is very small, equivalent to a wire.) If the ammeter is connected to the two poles of the power supply, the pointer will be crooked, and the ammeter, power supply, and wire will be burned at worst.
(4) See the position of the watch hand (be sure to observe it from the front)
5. Voltmeter
A. The concept of voltmeter:
Voltmeter is an instrument for measuring voltage, commonly used voltmeter - voltmeter symbol: V, there is a permanent magnet in the sensitive galvanometer, a coil composed of wires is connected in series between the two binding posts of the galvanometer, the coil is placed in the magnetic field of the permanent magnet, and is connected with the pointer of the meter through the transmission device. The voltmeter is a fairly large resistor, ideally thought of as an open circuit.
B. Working principle of voltmeter:
The voltmeter is assembled with an ammeter, the internal resistance of the ammeter is very small, so a large resistance can be directly connected to the two points that need to measure the voltage, according to the relationship between Ohm's law, it can be known that the current displayed by the ammeter is proportional to the external voltage, so the voltage can be measured.
C. The use of voltmeter:
The voltmeter can directly measure the power supply voltage, and the voltmeter should be connected in parallel in the circuit when used. The following points should be noted when using a voltmeter:
(1) When measuring voltage, the voltmeter must be connected in parallel at both ends of the circuit to be tested;
(2) Choose the range correctly, and the measured voltage should not exceed the range of the voltmeter. When used in parallel in a circuit, if connected in series, the power supply EMF is measured.
However, the above mentioned components are the most basic components in the distribution box, in the actual production process will be according to the different uses of the distribution box, as well as the use of the distribution box requirements to add other components, such as: AC contactor, intermediate relay, time relay, button, signal indicator, KNX intelligent switch module (with capacitive load) and background monitoring system, intelligent fire evacuation lighting and background monitoring system, electrical fire/leakage monitoring detector and background monitoring system, EPS power battery and so on.
Installation requirements for distribution boxes
1. The distribution box should be made of non-combustible materials;
2. Production sites and offices with low risk of electric shock can be installed with open distribution boards;
3. Closed cabinets should be installed in processing workshops, casting, forging, heat treatment, boiler rooms, woodworking rooms and other places with high risk of electric shock or poor working environment;
4. In dangerous workplaces with conductive dust or flammable and explosive gases, closed or explosion-proof electrical facilities must be installed;
5. The electrical components, instruments, switches and circuits of the distribution box should be arranged neatly, installed firmly, and easy to operate;
6. The bottom surface of the board (box) installed on the ground should be 5~10 mm higher than the ground;
7. The center height of the operating handle is generally 1.2~1.5m;
8. There are no obstacles in the range of 0.8~1.2m in front of the box;
9. The protective wire is connected reliably;
10. There shall be no bare charged body exposed outside the box;
11. The electrical components that must be installed on the outer surface of the box or on the distribution board must have reliable screen protection.
Operating procedures for distribution cabinets
(1) The power distribution cabinet is the normal operation of the ship's power distribution center and equipment, and no irrelevant person shall flip the switch on the board.
(2) After the generator set is started, it should be slowly accelerated manually by using the speed up switch of the power screen until the generator enters the normal working state and the voltage and frequency reach the specified value, and then the power can be closed.
(3) After the distribution board enters the power distribution state, the speed switch of the power screen shall not be pulled and pulled at will, and the locking switch of the air circuit breaker shall not be used in a non-emergency situation.
(4) The parallel operation of the generator should be operated in strict accordance with the requirements and regulations of the parallel conditions, and it is necessary to pay attention to the phenomena such as reverse power (reverse current) and failure of parallel operation.
(5) When shutting down, the load of the generator should be cut off first, and then the no-load parking should not be stopped.
(6) When interplugging in shore power, the power switches of the shore power panel should be cut off first, and then the correctness of the wiring and phase sequence should be checked, and the conversion of ship shore power can be carried out only after confirming the correctness, and it is strictly forbidden to operate with load.
(7) The distribution cabinet should be cleaned and maintained regularly so that the equipment is always in good working condition.
(8) When the generator is working and the turbine personnel are operating the distribution board, they should concentrate their thoughts and operate cautiously to prevent accidents, otherwise they will be held accountable for personal accidents.
(9) The charging and discharging board is the ship's emergency distribution board, and the engineer on duty should check its working condition frequently, ensure that the low-voltage power is sufficient at any time, and grasp the working status of the magnetic saturation regulator through the instrument on the board.
(10) During normal navigation, the switches on the distribution board should be turned on to ensure that the generator can be started at any time and can be put into use at any time.
Secondary wiring process
1. According to the schematic diagram, if it is not in the same position, it is necessary to have the upper terminal, and do not connect 3 wires with one terminal. Checking the error is not so checking, you can only follow the schematic diagram to get it right one by one.
(1) The selection of the cross-section of the wire
The line of the mains (AC 220V) voltage circuit is 1.5 square millimeters, and the current circuit is 2.5 square millimeters. The battery is generally 1.5 square millimeters.
(2) When wiring, check whether the signals at both ends of the wire correspond to each other, so as not to cause unnecessary errors.
(3) The most important thing is to understand the schematic diagram and wiring diagram.
2. If you are a novice, you should review the drawings first, sort out your own ideas, and you can also check whether there are any problems with the drawings, and you can figure it out first if you don't understand, which is conducive to making lines. And only then do the wiring begin.
Requirements for secondary wiring construction: construction according to the drawings, correct wiring; wire and electrical components should be bolted, plugged, welded or crimped, etc., should be firm and reliable, good wiring; wiring neat, clear and beautiful; wire insulation is good, no damage; there should be no connector in the wires in the cabinet; the circuit number is correct, and the handwriting is clear.
The selection of the cable core section should also meet the following requirements:
(1) Current circuit: the working accuracy of the current transformer should be made, at this time, if there is no reliable basis, the maximum short-circuit current can be determined according to the current capacity of the circuit breaker.
(2) Voltage circuit: When all protection devices and safety automatic devices are in operation (considering the development, when the load of the voltage transformer is the largest), the voltage drop of the cable from the voltage transformer to the protection and automatic device screen should not exceed 3% of the rated voltage.
(3) Operation loop: Under the maximum load, the voltage drop from the operating bus to the equipment should not exceed 10% of the rated voltage.
The secondary winding of the current transformer is not allowed to open the circuit, and the secondary side of the voltage transformer is not allowed to be short-circuited before the secondary wiring, you should be familiar with the drawings:
(1) Schematic diagram (representing the working principle and interaction of each loop.) The drawings soon show how the components in the secondary circuit are connected, and also the connections related to the primary circuit)
(2) Expand the diagram
(3) Terminal Diagram