Principles for the selection of low-voltage electrical appliances

issuing time:2021-05-25
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Low-voltage electrical equipment refers to the equipment in the 380/220V circuit. The principle of choosing low-voltage electrical equipment is to meet the requirements of safe electricity use, ensure its reliable operation, and not be damaged when passing the maximum possible short-circuit current. Sometimes it is necessary to calibrate the electrical equipment according to the electric power, that is, the heating effect generated by the short-circuit current. Test.
 
1. Selection principle
(1) The rated voltage of the electrical appliance should be compatible with the nominal voltage of the circuit where it is located. The line voltage loss should meet the requirements of the terminal voltage during the normal operation of the electrical equipment and the start-up.
 
 
(2) The rated current of the electrical appliance should not be less than the calculated current of the circuit in which it is located, and the rated power of the electrical appliance should be compatible with the frequency of the circuit in which it is located. The rated power of the electrical appliance should be compatible with the frequency of the loop in which it is located.
 
(3) Electrical appliances should meet the requirements of dynamic stability and thermal stability under short-circuit conditions. Electrical appliances used to disconnect short-circuit current should meet the short-circuit conditions of making and breaking capacity. To check the making and breaking capacity of electrical appliances under short-circuit conditions, the effective value of the expected short-circuit current periodic component at the installation location should be adopted. 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 motor feedback current. influences.
 
2. Electrical appliances should adapt to environmental conditions
. Isolating electrical appliances should be installed when the equipment for maintenance, testing and overhaul needs to be disconnected from the power supply. Isolating electrical appliances should isolate the circuit where they are located from the live parts. When the misoperation of the isolated electrical appliances will cause serious accidents, measures should be taken to prevent misoperation. Isolating electrical appliances should use switches that disconnect all poles of the power supply at the same time.
 
The types of similar equipment should be reduced as much as possible, and the construction standards of the entire project should be harmonized. All new products selected should have reliable test data and be formally qualified. The selection of new products that have not been formally authenticated should be approved by the supervisor.
 
Low-voltage power distribution design implements national technical and economic policies to ensure personal safety, reliable power distribution, qualified power quality, energy saving, advanced technology, reasonable economics, and convenient installation and maintenance. It is used for the design of AC and low-voltage power distribution below 500V in new construction and expansion projects. Low-voltage power distribution design should save non-ferrous metals, and rationally use copper and aluminum conductors.
 
The main points of choosing low-voltage electrical appliances
For the secondary and below power load, when it is used for ring network and terminal power supply, the ring network load switch should be preferred when the technical conditions of the high-voltage 10kV power system are met. Substations in residential quarters should give preference to complete sets of outdoor substation equipment. If a box-type substation is used, the continuous working current of the box-type substation equipment will be reduced by 1% when the ambient temperature is 1°C higher than the average temperature (35°C).
 
The low-voltage circuit breaker and the low-voltage side of the transformer should be connected to the main bus through an isolating switch or plug. The two power lines supplying the primary load should not be laid in the same cable trench. When it is impossible to separate, the two power lines should use cables whose insulation and sheath are non-flammable materials, and they should be installed on the brackets on both sides of the cable trench.
 
1. Selection content The
allowable maximum operating voltage of the electrical appliances selected in the design shall not be lower than the maximum operating voltage of the circuit. The long-term allowable current of the conductors and electrical appliances selected in the design shall not be less than the maximum continuous working current of the circuit; the influence of sunlight on their current carrying capacity should be taken into account for the conductors and electrical appliances outside the house. The short-circuit current used for checking the dynamic stability and thermal stability of conductors and electrical appliances, as well as the breaking current of electrical appliances, should be calculated according to the design and planning capacity, and the long-term development plan of the power system should be considered.
 
2. Determining the short-circuit current When
determining the short-circuit current, it should be calculated according to the normal wiring method that may cause the maximum short-circuit current.
 
Checking the short-circuit current for conductors and electrical appliances, except for calculating the decay time constant of the short-circuit current, the resistance of the component can be omitted. In the electrically connected network, the influence of the asynchronous motor with feedback function and the influence of the discharge current of the capacitor compensation device should be taken into account. The dynamic and thermal stability of conductors and electrical appliances, as well as the short-circuit breaking current of electrical appliances, can be checked according to three-phase short-circuits. When single-phase and two-phase grounding short circuits are more serious than three-phase short circuits, they should be checked according to serious conditions. To check the calculation time of the conductor short-circuit thermal effect, the main protection action time plus the corresponding full opening time of the circuit breaker should be adopted. When the main protection has a dead zone, the backup protection action time that works on the dead zone should be used, and the corresponding short-circuit current value should be adopted.
 
3. Checking electrical appliances
It is advisable to use the backup protection action time plus the corresponding full opening time of the circuit breaker. The voltage transformer circuit protected by the fuse can not be checked for dynamic stability and thermal stability. To check the breaking capacity of the circuit breaker, the actual breaking short-circuit current of the circuit breaker should be taken as the check condition. Checking the making and breaking capacity under short-circuit conditions should use the effective value of the expected short-circuit current periodic component at the installation location. When the sum of the rated currents of the motors connected near the short-circuit point exceeds 1% of the short-circuit current, the influence of the motor feedback current should be included. When the maintenance, testing and repairing equipment needs to be disconnected from the power supply, isolation electrical appliances should be installed. For circuit breakers equipped with automatic reclosing devices, the influence of reclosing on the rated breaking current shall be taken into account.
 
4. The circuit breaker
used to cut off the capacitor bank The circuit breaker used to cut the combined compensation capacitor bank should be a circuit breaker with excellent breaking performance. The normal maximum operating temperature of bare conductors should not be greater than +70°C. When taking into account the effects of sunlight, steel core aluminum wires and tubular conductors should not be greater than +80°C. When the bare conductor contact surface is covered with a reliable tin-plated (lined) tin, its maximum operating temperature can be increased to +85°C. When the short-circuit thermal stability is checked, the maximum allowable temperature of the bare conductor is +200℃ for duralumin and aluminum-manganese alloy, and +300℃ for hard copper. The conductor temperature before short-circuit should be the working temperature under rated load.
 
When selecting the section of bare conductor according to the normal working current of the loop, the long-term allowable current carrying capacity of the conductor should be corrected according to the altitude and ambient temperature of the area. When the conductor adopts a multi-conductor structure, the influence of proximity effect and thermal shielding on the current carrying capacity should be considered.
 
5. Isolating electrical appliances.
Isolating electrical appliances should isolate the loop from the live part. When the misoperation of isolated electrical equipment will cause serious accidents, measures should be taken to prevent misoperation. Isolating electrical appliances should use switches that disconnect all poles of the power supply at the same time or single switches close to each other. Electrical equipment can be used as the disconnecting device; single-pole or multi-pole disconnecting switch, isolation plug; plug and socket; connecting piece; special terminal without removing the wire; fuse. Semiconductor electrical appliances are strictly prohibited to be used as isolation electrical appliances.
 
The operating power supply for on-off current can use the following electrical appliances; load switches and circuit breakers; relays and contactors; semiconductor electrical appliances; plugs and sockets of 10A and below.
 
6. Insulators and bushings
The 3-20kV outdoor pillar insulators and wall bushings of power plants can be used with higher voltage products. 3~6kV outdoor pillar insulators and wall bushings can be used to increase two-level voltage products. During normal operation and short circuit, the maximum force of the electrical lead should not be greater than the allowable load of the electrical terminal. The conductors, bushings, insulators and fittings of outdoor power distribution devices shall be calculated mechanically according to local weather conditions and different stress states. The safety factor should not be less than the regulations in Table 5-38.
 
The safety factor of the suspension insulator should correspond to the failure load. If it corresponds to the 1h electromechanical test load, the safety factor should be 4 and 2.5 respectively. The safety factor of the hard conductor corresponds to the failure stress. If it corresponds to the yield point stress, the safety factor should be 1.6 and 1.4 respectively. When checking the short-circuit dynamic stability, the maximum allowable stress of the hard conductor should meet the requirements of Table 5-39.
 
The hard conductor stress calculation of important circuits should take into account the influence of dynamic effects. There should be reliable connection joints between the conductor and the conductor, the conductor and the electrical appliance. The connection between hard conductors should be soldered. The joints that need to be disconnected and the connections between conductors and electrical terminals shall be connected by bolts. When connecting conductors of different metals, according to environmental conditions, measures such as installing transition joints should be taken. When using hard conductors, expansion joints should be installed at appropriate locations or shock-proof measures should be taken according to temperature changes, uneven settlement and vibration.
 
7. Cable interlayer The
arrangement of low-voltage power distribution cabinets should be parallel to the beams of the cable interlayer. When the power distribution cabinet is arranged vertically with the beam, it should meet the condition that two cables (three-core 240mm2) can enter under each screen.
 
When the cable trench is used under the high and low voltage power distribution cabinet, it should not be less than the following values; the
 
trench depth of the high voltage power distribution cabinet is ≥1.5m wide and 1m,
 
and the line trench depth of the low voltage power distribution cabinet is ≥1.2m wide and 1.5m (including the under and rear parts of the cabinet) ) The
 
cable nozzle in the trench should meet the requirements of the cable bending radius; the clear height of the cable interlayer should not be less than 1.8m. The fronts of power distribution cabinets and boxes used for emergency lighting and fire-fighting electrical equipment shall be painted with a red frame as a mark.