A "distribution box," also called a distribution cabinet, is a general term for the motor control center. A distribution box is a low-voltage power distribution device that assembles switching equipment, measuring instruments, protective devices, and auxiliary equipment in a closed or semi-closed metal cabinet or panel according to electrical wiring requirements. During normal operation, circuits can be connected or disconnected manually or automatically. In case of faults or abnormal operation, protective devices will disconnect the circuit or trigger an alarm. Measuring instruments can display various operating parameters, allow adjustment of certain electrical parameters, and provide alerts or signals for deviations from normal operating conditions.
Purpose of a distribution box
For ease of management and troubleshooting in case of circuit faults, distribution boxes, distribution cabinets, distribution panels, and other similar equipment are complete sets of devices that centrally house switches, meters, and other equipment. Commonly used distribution boxes are made of wood or iron; given the high electricity consumption everywhere, iron boxes are more commonly used. The purpose of a distribution box is, of course, to facilitate power outages and restorations, and to measure and determine power outages and restorations.
The distribution box is mainly composed of two parts.
The first is the complete set of components, namely the distribution box casing and its related accessories.
Second, electrical components and related accessories, namely air switches and their required accessories.
The cabinet consists of the following parts.
I. Circuit Breaker
Circuit breakers: also known as switches, are the main components of distribution cabinets. Common types include air switches, residual current circuit breakers, and automatic transfer switches.
1. Air switch:
A. The concept of an air switch:
An air switch, also known as an air circuit breaker, is used in circuits to connect, disconnect, and carry rated operating current and fault currents such as short-circuit and overload. It can quickly disconnect the circuit and provide reliable protection in the event of overload, short circuit, or undervoltage conditions. While the design of the moving and stationary contacts and contact rods of circuit breakers varies, maximizing the breaking capacity is the primary objective. Currently, the current-limiting principle, which uses specific contact structures to limit the peak short-circuit current during disconnection, is widely used and has a significant effect on improving the breaking capacity of circuit breakers.
B. Working principle of air switch:
Automatic air circuit breakers, also known as low-voltage circuit breakers, are used to connect and disconnect load circuits and to control infrequently started motors. Their function is equivalent to some or all of the functions of a knife switch, overcurrent relay, undervoltage relay, thermal relay, and residual current device (RCD), making them an important protective electrical appliance in low-voltage power distribution networks.
Automatic air circuit breakers have multiple protection functions (overload, short circuit, undervoltage protection, etc.), adjustable operating values, high breaking capacity, convenient operation, and safety, so they are widely used at present.
2. Residual current circuit breaker:
A. Concept of residual current circuit breaker:
The primary function of a residual current device (RCD) is to provide leakage current protection, tripping the circuit breaker when a person touches a live conductor to ensure personal safety. If the insulation of electrical equipment is poor and leakage occurs to the casing, the RCD will also trip, preventing electric shock. It also features current interruption, overload protection, and short-circuit protection.
B. Working principle of residual current circuit breaker:
The working principle diagram of a residual current device (RCD). LH is a zero-sequence current transformer, which consists of a permalloy core and a secondary coil wound around the toroidal core, forming the detection element. The power phase line and neutral line pass through the circular hole to become the primary coil of the zero-sequence current transformer. The output wires at the rear of the transformer define the protection range.
C. The function of a residual current device (RCD):
1. When electrical equipment or lines experience leakage or grounding faults, the power supply can be cut off before a person can touch it.
2. When a person touches a charged object, the power supply can be cut off within 0.11 seconds, thereby reducing the degree of harm to the human body from the current.
3. It can prevent fire accidents caused by electrical leakage.
3. Dual power automatic transfer switch:
The concept of a dual power automatic transfer switch:
The dual power automatic transfer switch is a two-way power selection automatic switching system. If the first power supply fails, the dual power automatic transfer switch will automatically switch to the second power supply to power the load. If the second power supply fails, the dual power automatic transfer switch will automatically switch back to the first power supply to power the load.
Suitable for uninterrupted power switching between any two power sources, such as UPS-UPS, UPS-generator, UPS-mains, and mains-mains.
II. Surge protectors:
A. Concept of surge protector:
A surge protector, also known as a lightning arrester, is an electronic device that provides safety protection for various electronic devices, instruments, and communication lines. When a surge current or voltage spike suddenly occurs in an electrical circuit or communication line due to external interference, the surge protector can conduct and divert the current in a very short time, thereby preventing damage to other equipment in the circuit.
B. Basic knowledge of surges:
The primary function of a surge protector system is to protect electronic equipment from damage caused by surges. Therefore, to understand the role of surge protectors, you need to clarify two questions: What is a surge? And why do electronic devices need their protection?
A surge, also called a voltage spike, is, as the name suggests, a momentary overvoltage exceeding the normal operating voltage. Essentially, a surge is a violent pulse that occurs within a fraction of a second. Possible causes of surges include: heavy equipment, short circuits, power switching, or large motors.
A surge, or transient voltage, refers to a voltage that significantly exceeds the rated level during the flow of electrical energy. In the United States, the standard voltage for wiring in typical home and office environments is 120 volts. If the voltage exceeds 120 volts, problems can occur, and surge protectors help prevent these problems from damaging computers.
C. Function of surge protectors:
The first line of defense should be a high-capacity surge protector connected between each phase of the incoming power supply line to the ground. Generally, this type of surge protector is required to have a maximum impulse capacity of 100KA/phase or higher, and the limiting voltage should be less than 2800V. We call this a Class CLASSI surge protector (SPD).
These surge protectors are specifically designed to absorb the high current and energy surge energy of lightning and induced lightning strikes, diverting large amounts of surge current to the ground. They only provide a medium level of protection at limiting voltage (the maximum voltage that appears on the line when the surge current flows through the SPD is called the limiting voltage), because CLASSI-class protectors are primarily designed to absorb large surge currents. They alone cannot fully protect sensitive electrical equipment within the power supply system.
The second line of defense should be surge protectors installed at the branch distribution equipment supplying power to important or sensitive electrical equipment. These surge protectors (SPDs) more effectively absorb the residual surge energy that has passed through the surge arrester at the user's power supply inlet, and have excellent suppression of transient overvoltages. The surge protectors used here are required to have a maximum impulse capacity of 40KA/phase or higher, and the limiting voltage should be less than 2000V. We call these CLASS II surge protectors. For general user power supply systems, this second level of protection is sufficient to meet the operational requirements of the electrical equipment.
The last line of defense can be a built-in surge protector in the power supply section of the electrical equipment to completely eliminate minor transient overvoltages. The surge protector used here should have a maximum impulse capacity of 20kA/phase or lower, and the limiting voltage should be less than 1800V. For some particularly important or sensitive electronic equipment, a third level of protection is necessary. This can also protect the equipment from transient overvoltages generated within the system.
III. Electricity Meter:
A. The concept of an electricity meter:
Electricity meters, commonly used by electricians, are instruments used to measure electrical energy and are commonly known as electricity meters.
B. Working principle of an electricity meter:
① Working principle of mechanical electricity meter: When the electricity meter is connected to the circuit, the magnetic flux generated by the voltage coil and the current coil passes through the disk. These magnetic fluxes are out of phase in time and space, inducing eddy currents on the disk. Due to the interaction between the magnetic flux and the eddy currents, a torque is generated, causing the disk to rotate. Due to the braking effect of the magnet, the rotation speed of the disk reaches a uniform speed. Since the magnetic flux is directly proportional to the voltage and current in the circuit, the disk moves at a speed proportional to the load current under its action. The rotation of the disk is transmitted to the counter via a worm gear. The reading of the counter is the actual electrical energy used in the circuit.
②Basic principle of electronic energy meters: Electronic energy meters use electronic circuits/chips to measure electrical energy; voltage signals are converted into small signals that can be used for electronic measurement using voltage divider resistors or voltage transformers, and current signals are converted into small signals that can be used for electronic measurement using shunts or current transformers. A dedicated energy measurement chip performs analog or digital multiplication on the converted voltage and current signals and accumulates the electrical energy, and then outputs a pulse signal with a frequency proportional to the electrical energy; the pulse signal drives a stepper motor to drive a mechanical counter for display, or is sent to a microcomputer for processing and digital display.
IV. Ammeter:
A. Working principle of an ammeter:
A galvanometer is made based on the principle of a current-carrying conductor experiencing a magnetic force in a magnetic field. When current flows through it, it travels along a spring and a rotating shaft through the magnetic field. The current cuts magnetic field lines, thus experiencing a magnetic force that causes the coil to deflect, leading to the deflection of the shaft and pointer. Since the magnitude of the magnetic force increases with the current, the degree of pointer deflection can be used to observe the magnitude of the current. This is called a magnetoelectric galvanometer.
B. Rules for using an ammeter:
① The ammeter must be connected in series in the circuit (otherwise it will short-circuit).
② The current being measured should not exceed the range of the ammeter (a trial contact method can be used to check if the range is exceeded);
③ Never connect an ammeter directly to the power source without an appliance in between (the ammeter's internal resistance is very small, essentially equivalent to a wire. Connecting the ammeter directly to the power source will at least cause the pointer to deflect incorrectly, and at worst, burn out the ammeter, the power source, and the connecting wires).
④. Observe the position where the hands stop (be sure to observe from the front).
V. Voltmeter:
A. The concept of a voltmeter:
A voltmeter is an instrument used to measure voltage. The common symbol for a voltmeter is V. Inside a sensitive galvanometer is a permanent magnet. A coil made of wire is connected in series between the two terminals of the galvanometer. The coil is placed in the magnetic field of the permanent magnet and is connected to the pointer of the meter through a transmission mechanism. A voltmeter is a fairly large resistor and is ideally considered as an open circuit.
B. Working principle of a voltmeter:
A voltmeter is assembled from an ammeter. Since the ammeter has a very small internal resistance, by connecting a large resistor in series, it can be directly connected in parallel to the two points where the voltage needs to be measured. According to Ohm's law, the current displayed by the ammeter is proportional to the external voltage, thus allowing the voltage to be measured.
C. Use of a voltmeter:
A voltmeter can directly measure the power supply voltage. When using a voltmeter, it must be connected in parallel in the circuit. The following points should be noted when using a voltmeter: (1) When measuring voltage, the voltmeter must be connected in parallel across the two ends of the circuit being measured;
(2) Select the correct range; the voltage being measured should not exceed the range of the voltmeter. When using it, connect it in parallel in the circuit; if connected in series, the measured value is the electromotive force of the power source.
However, the components mentioned above are the most basic components in a distribution box. In actual production, other components will be added according to the different uses and requirements of the distribution box, such as: AC contactors, intermediate relays, time relays, buttons, signal indicator lights, KNX intelligent switch modules (with capacitive loads) and background monitoring systems, intelligent fire evacuation lighting and background monitoring systems, electrical fire/leakage monitoring detectors and background monitoring systems, EPS power batteries, etc.
Classification by structural features and uses
(1) Fixed panel switchgear, commonly known as switchboard or distribution panel. It is an open switchgear with a panel covering. The front is protected, but the back and sides can still be accessed for live parts. It has a low protection level and can only be used in industrial and mining enterprises with low requirements for power supply continuity and reliability for centralized power supply in substations.
(2) Protective (i.e., enclosed) switchgear refers to a type of low-voltage switchgear where all sides except the mounting surface are enclosed. The electrical components, such as switches, protection devices, and monitoring and control systems, are all installed within a closed enclosure made of steel or insulating material, which can be mounted against a wall or away from a wall. Isolation measures may not be used between circuits within the cabinet, or grounded metal plates or insulating plates may be used for isolation. Typically, the door is mechanically interlocked with the main switch operation. There are also protective tabletop switchgear (i.e., control consoles), with control, measurement, and signal electrical components mounted on the panel. Protective switchgear is mainly used as a power distribution device in process sites.
(3) Drawer-type switchgear: This type of switchgear uses a steel plate enclosed shell. The electrical components of the incoming and outgoing circuits are installed in removable drawers, forming functional units that can complete a certain type of power supply task. The functional units are separated from the busbars or cables by grounded metal plates or plastic functional plates, forming three areas: busbars, functional units, and cables. There are also isolation measures between each functional unit. Drawer-type switchgear has high reliability, safety, and interchangeability, and is a relatively advanced type of switchgear. Most of the switchgear currently produced is drawer-type. They are suitable for industrial and mining enterprises and high-rise buildings that require high power supply reliability, serving as centralized control power distribution centers.
(4) Power and lighting distribution control boxes are mostly enclosed and vertically installed. The enclosure protection level varies depending on the application. They are mainly used as power distribution devices in the production sites of industrial and mining enterprises.
The installation requirements for distribution boxes are:
Distribution boxes should be made of non-combustible materials. Open-type distribution boards can be installed in production areas and offices with low risk of electric shock. Enclosed cabinets should be installed in processing workshops, foundries, forging plants, heat treatment plants, boiler rooms, and woodworking shops where the risk of electric shock is high or the working environment is poor. Enclosed or explosion-proof electrical facilities must be installed in hazardous work areas with conductive dust or flammable/explosive gases. All electrical components, instruments, switches, and wiring in the distribution box should be neatly arranged, securely installed, and easy to operate. The bottom of floor-mounted panels (boxes) should be 5-10mm above the ground. The center height of the operating handle is generally 1.2-1.5m. There should be no obstructions within 0.8-1.2m in front of the box. Protective wiring connections should be reliable. No bare live parts should be exposed outside the box. Electrical components that must be installed on the outer surface of the box or on the distribution board must have reliable shielding.
Operating Procedures
(1) The power distribution cabinet is the central hub for the ship's power distribution and the normal operation of the equipment. No unauthorized personnel may operate the switches on the panel.
(2) After the generator set is started, the speed should be manually accelerated slowly using the power panel speed-up switch until the generator enters normal working state and the voltage and frequency reach the specified values before the circuit breaker can be closed to supply power.
(3) After the power distribution board enters the power distribution state, the power panel speed-up switch shall not be turned on at will, and the interlocking switch of the air circuit breaker shall not be used except in an emergency.
(4) When operating generators in parallel, the operation must be carried out in strict accordance with the requirements and regulations for parallel operation. Attention should be paid to phenomena such as reverse power (reverse current) and parallel operation failure.
(5) When shutting down, the generator load should be disconnected first, and then the machine should be stopped under no-load conditions. It is not allowed to shut down the machine directly under load.
(6) When switching to shore power, the power switches of the shore power panel should be turned off first, and then the wiring and phase sequence should be checked for correctness. Only after confirming that it is correct can the ship-to-shore power conversion be carried out. It is strictly forbidden to operate under load.
(7) The distribution cabinet should be cleaned and maintained regularly to ensure that the equipment is always in good working condition.
(8) When the generator is working, the engine room personnel should concentrate and operate the switchboard with care to prevent accidents. Otherwise, they will be held personally responsible for the accident.
(9) The charging and discharging board is the ship's emergency power distribution board. The on-duty engine personnel should frequently check its working status to ensure that the low-voltage power is sufficient at all times, and monitor the working status of the magnetic saturation voltage regulator through the instruments on the board.
(10) During normal navigation, all switches on the power distribution board should be turned on to ensure that the generator can be started at any time and put into use at any time when needed.
Secondary wiring process
1. Follow the schematic diagram. Wires not in the same position need to be connected to terminals. Never connect three wires to one terminal. Troubleshooting is not so straightforward; you have to check each wire against the schematic diagram one by one.
(1) Selection of conductor cross-section
For mains power (220V AC) circuits, use 1.5 square millimeter wires; for current circuits, use 2.5 square millimeter wires. For storage batteries, 1.5 square millimeter wires are generally sufficient.
(2) When wiring, check whether the signals at both ends of the wire correspond to each other to avoid unnecessary errors.
(3) The most important thing is to understand the schematic diagram and wiring diagram.
2. If you're a beginner, you should first review the drawings, organize your thoughts, and check for any problems. Clarify any unclear points first, as this will help with wiring. Only then should you begin connecting the wires. The entire wiring process requires meticulous attention. Of course, if you're an experienced wirer, you don't need to go into so much detail.
Construction personnel should carefully read and familiarize themselves with the secondary wiring symbols, and verify the secondary wiring diagram against the schematic diagram to ensure that the wiring diagram is correct.
Requirements for secondary wiring construction: Construction should be carried out according to the drawings, and the wiring should be correct; the connection between wires and electrical components by bolts, plugs, welding or crimping should be firm and reliable, and the wiring should be good; the wiring should be neat, clear and aesthetically pleasing; the wire insulation should be good and undamaged; there should be no joints in the wires inside the cabinet; the circuit numbering should be correct and the writing should be clear.
The selection of cable core cross-section should also meet the following requirements:
(1) Current loop: The current transformer should be made to operate with an accuracy rating. 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 activated (considering development, when the load of the voltage transformer is at its maximum), the voltage drop of the cable from the voltage transformer to the protection and automatic device panel should not exceed 3% of the rated voltage.
(3) Operating circuit: Under maximum load, the voltage drop from the operating bus to the equipment should not exceed 10% of the rated voltage.
2. The secondary winding of a current transformer must not be open-circuited, and the secondary side of a voltage transformer must not be short-circuited. Before connecting the secondary windings, familiarize yourself with the drawings:
(1) Schematic diagram (showing the working principle and interaction of each circuit. The diagram not only shows the connection method of each component in the secondary circuit, but also the connection with the primary circuit)
(2) Unfolded diagram
(3) Terminal block diagram
(4) Installation wiring diagram
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