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Some problems that should be paid attention to in the electrical design of buildings

There are many specifications involved in electrical design of buildings. In recent years, the specification quality has been deteriorating, and the wording has been inaccurate. Each word is ambiguous. Some specifications are obviously unreasonable, and the specifications are self-contradictory. There are many contradictions between different specifications. Bring a lot of confusion and trouble to the electrical designers. In the process of participating in the construction plan review, some inappropriate or even wrong practices were found, which resulted in waste or security risks. The author is not very shy, pointing out that there are many problems in electrical design, pointing out its inadequacies and proposing correct practices, please refer to colleagues.
1. The provisions of the building's automatic fire alarm system are unreasonable

The "Civil Building Electrical Design Code" JGJ16-2008 Section 13.1.3-2-2) stipulates: "The building height does not exceed 24m single-storey and multi-storey public buildings" should be set up automatic fire alarm system. According to this rule, as long as the building height does not exceed 24m public buildings, regardless of their size and purpose, fire alarm systems must be set up. Obviously, there are many problems with this provision. It is recommended that this chapter stipulates invalidity. Whether or not a fire automatic alarm system is set up shall comply with the relevant provisions of the “Code for the Prevention of Fire Protection of Building Design” GB50016-2014. How to set up an automatic fire alarm system shall comply with the relevant provisions in the “Design Specification for Automatic Fire Alarm System” GB50116-2013.
2. The regulations on the classification of lightning protection for buildings are unreasonable

The “Civil Building Electrical Design Specification” JGJ16-2008 No. 11.2.4-6 stipulates that “the tallest building in a building group or a building located at the edge of a building group more than 20m in height” shall be classified as a Category III lightning protection building. Whether or not a building needs lightning protection shall be calculated and determined based on the purpose, mass, location, meteorological conditions and geological characteristics of the building. For areas where the thunderstorms are small, the tallest building in the complex may not need lightning protection. In an area where the thunderstorms are large, a building not exceeding 20m in height at the edge of the building may also need lightning protection. In summary, the provisions of No. 11.2.4-6 of JGJ16-2008 Electrical Design Code for Civil Buildings are also inappropriate. It is recommended that the lightning protection design does not take into account the requirements of Chapter 11 of the “Guidelines for Electrical Design of Civil Buildings” JGJ16-2008. It is only required to comply with the relevant regulations of GB50057, GB50343 and the construction profession.

3. The fire protection requirements for transmission lines of fire automatic alarm systems are not perfect

Article 11.2.3 of the “Automatic Fire Alarm System Design Specification” GB50116-2013 only proposes a fire prevention method in which the FAS circuit is concealed and is laid in an incombustible body and the protective layer thickness is not less than 30mm. Any fire protection requirement is obviously wrong. FAS trunks are often laid in tanks with organic refractory wires for long distances. Due to the low fire resistance of organic fire-resistant wires, fires may occur at any part of the FAS Mingfa main-line path, and FAS trunks may be damaged, resulting in failure of FAS. Therefore, fire protection measures shall be taken when the FAS line is exposed, and its fire-resistance limit shall not be lower than the continuous working time of the fire-fighting equipment being controlled.
4, low voltage distribution conductor end branch line is too long

"Civil Building Electrical Design Code" JGJ16-2008 Section 7.4.2 stipulates that the low-voltage distribution conductor cross-section should meet: 1) conductor carrying capacity to meet the requirements; 2) voltage loss to meet the requirements: 3) thermal stability should meet the requirements; 4) The mechanical strength should meet the requirements. In the actual design, the designer does not pay enough attention to paragraphs 2) and 3) of this article, especially when the terminal branch line of the low-voltage distribution conductor is long, such as the single-phase branch circuit of the lighting outlet led by the lighting distribution box. The voltage loss often exceeds the allowable value, and more seriously, the short circuit current is too small due to the long line and thin wire, and the short circuit protection device cannot act reliably, resulting in the entire fault distribution circuit wire and the The insulation of the distribution circuit lead that the circuit is in close contact with will be scrapped due to high temperature melting, and there are also fire hazards.

The specification does not specify the length of the terminal branch line of the low-voltage distribution conductor. The technical measures should not exceed 50m. The instantaneous operating current value of the C-type miniature circuit breaker is 5 to 10 times longer than the long-time setting current. It is calculated that the C-type miniature circuit breaker setting current is 16A to protect the 2.5mm2 copper conductor and the line length does not exceed 50m. Voltage loss and short circuit protection sensitivity can meet the specification requirements. In order to reduce the workload of the electrical designer, voltage loss and short-circuit protection sensitivity may not be verified when the above conditions are satisfied. When the line is longer, voltage loss and short circuit protection sensitivity must be verified. If the short-circuit protection sensitivity does not meet the requirements, the long-delay setting current setting value of the miniature circuit-breaker should be reduced to reduce the short-circuit protection instantaneous operating current value. When the current setting of the miniature circuit breaker can not be reduced by the load current limit, the current setting value can be reduced. Switching to a miniature circuit breaker with type B tripping characteristics, the instantaneous operating current value is only 3 to 5 times longer than the long-time setting current.
5. The cross section of the outlet cable of the low-voltage distribution cabinet in the substation is too small

Many designers only select the low-voltage distribution conductor cross-section according to the voltage loss and the allowable current-carrying capacity, and use a small conductor cross section for a small-capacity distribution circuit, but do not consider that the low-voltage distribution conductor cross-section must meet the requirements of thermal stability. Especially in the case where the high-side system has a large short-circuit capacity and the transformer capacity is also large, the short-circuit current at the low-voltage side of the transformer is large. In the event of a short-circuit fault, cables with small cross-sections will be burned because they do not meet the thermal stability requirements. This is very wrong.
Therefore, in addition to the requirements for overload protection and voltage loss, the cross-sectional area of ​​the outgoing line of the low-voltage distribution cabinet in the power distribution and distribution section must verify the thermal stability of the cable when the cross-sectional area of ​​the cable is small.
6, the bridge selection is not reasonable, the bridge type, material is not clear

When many designers choose bridges, they do not specify the type and material of the bridge. There are also some designers who do not consider the use of wires and cables and their use. As long as they are laid in the bridge, they all use cable trays. Even mineral insulated cables are laid inside the trays. These practices are very wrong.

Bridges are collectively referred to as ladder frames, trays, and slot boxes according to different structural forms. They are divided into metal bridges and non-metal bridges according to different materials. Metal bridges can be divided into steel bridges and aluminum alloy bridges. Different types of bridges have different heat dissipation effects, and the impact on the allowable ampacity of cables is naturally different. The minimum allowable ampacity correction coefficient is 0.78 when the cables are laid in the ladder frame, and the cable allows the ampacity when laying in the slot box. The minimum value of the correction coefficient is 0.38. It can be seen that different types of bridges have a great influence on the allowable ampacity of cables. In addition, different types of bridges have different prices and installation requirements. For example, when non-metallic bridges are used, special grounding wires should be laid along the entire length of the bridges; when using aluminum alloy bridges fixed on steel supports and hangers, they should be prevented. Electrochemical corrosion measures; when the surface of the metal bridge has a non-conductive coating, the joint should be connected to the grounding wire. Therefore, the outline size, type, and material of the bridge must be specified in the design drawings. Otherwise, it is not only impossible to do the project budget, but also unable to order and construct. What's more, it is impossible to determine the cable cross-section.


Since the cable tray box has a great influence on the cable current carrying capacity, except that the cable should be laid in the ceiling, the slot box should be used. The fire-resisting distribution circuit adopts the organic fire-resistant cable. When laying in the bridge frame, the fire box should be used. The ordinary distribution circuit cable Cable ladders or cable trays can be used when laying in the bridge.
7. Ordinary distribution lines and fire protection distribution lines are laid in the same cable tray box

There are fire protection requirements for fire protection distribution lines. When the fire protection distribution trunks are made of organic-insulated fire-resisting wires and cables, their laying methods should have fire prevention measures. If cable bridges are used, fireproof cable trays shall be used, or ordinary metal cable trays shall be used to coat the outer wall with a fireproof protective layer. Because the cable tray box has a great influence on the allowable ampacity of the wire and cable, when the fire extinguishment distribution trunk adopts the organic insulating fire-resistant wire and cable, when the ordinary distribution line and the fire extinguishing line are laid in the same cable box, the conductor is cut. The area will inevitably increase a lot and increase investment. In addition, ordinary distribution lines shall be separated from the fire distribution lines and the 2 lines of the same load by means of fire-proof partitions. There shall be at least 2 fire barriers in the cable trays. This practice will not only increase investment, but also make construction inconvenient. This is very inappropriate.

It is reasonable that ordinary distribution lines and fire distribution lines are laid in their respective bridges. Ordinary distribution lines can use cable ladders; fire protection distribution trunks are laid in fireproof cable trays with fire-proof partitions in the middle or ordinary metal cable trays coated with fire-protective layers on the outer wall, and 2 wires and cables of the same load. Laid on both sides of the partition. If the fire protection distribution trunk is made of mineral insulated cable, it can be applied along the wall, or it can be laid on the cable ladder or on the bracket.
8, use non-existent wire and cable models

During the review of construction drawings, it was often found that some designers used WDZ-BV wire, WDZ-VV cable, WDZ-YJV cable or WDZN-BV wire, WDZN-VV cable, and WDZN-YJV cable. As we all know, "W" in wire and cable models indicates halogen-free, "D" indicates low smoke, "V" indicates polyvinyl chloride, and BV indicates copper-core PVC insulated wire, VV indicates copper-core PVC insulated and polychlorinated. Ethylene sheathed power cable, YJV said copper core cross-linked polyethylene insulated PVC sheathed power cable. The insulation or/and sheathing of the above wire and cable is made of polyvinyl chloride, and the chlorine contained in polyvinyl chloride is a halogen element. That is, as long as the wire and cable model has the letter “V”, it must contain halogen. Therefore, there is no WDZ-BV wire, WDZ-VV cable, WDZ-YJV cable or WDZN-BV wire, WDZN-VV cable, WDZN-YJV cable.
9, abuse 4P switch

Some designers were misled by technical measures and designed the 4k switch for the main switch and contact switch on the low voltage side of the 10kV transformer substation. Some designers also designed the low-voltage entry main switch as a 4p leakage circuit breaker. This is very wrong. of.

When the 4P switch is turned on and off, the N pole cannot produce an arc that cleans the contacts. This can easily cause N pole contact failure and cause a so-called “zero fault”. In the TN system, only the 4P switch can be used when the stray current path needs to be cut off.

The regulations stipulate that the multi-power TN system in the 10kV substation should be grounded at one point of the low-voltage distribution cabinet, and the stray current is no longer accessible. The design of the low-side main switch and contact switch of the 10kV transformer substation as a 4P switch not only increases the cost, but also has potential safety hazards of “zero-breaking fault”, and the distribution circuit of TN-C and TN-CS cannot be configured. . Therefore, the main switch and contact switch on the low-voltage side of the transformer should use 3P switches.
When the low-voltage inlet switch is selected to use the leakage circuit breaker for electrical fire protection, the 4P leakage circuit breaker must not be used to avoid “zero-break fault”. Should use 3P4W leakage circuit breaker, this type of leakage circuit breaker factory release code is 4300A, that is, N and L are connected to the leakage circuit breaker, N line does not pass the contact, always connected.
10, wire and cable capacity is not corrected according to regulations

Some people think that many distribution circuits are not fully loaded, and the actual current of some distribution circuits is much smaller than the allowable ampacity of the circuit cables. It is not necessary to verify the ampacity of the cables and not to allow the ampacity of the cables. Nothing happened when checking. As we all know, some people replace the fuses of the protection circuit with copper wires, or increase the setting current of the protection switch without increasing the cross section of the conductor. Long-term operation may not necessarily lead to failure. Even so, no electrical designer will think that the distribution line does not need protection. The regulations stipulate that overload, short-circuit, and other protections must be provided for the distribution line. It is not possible to operate normally without the above-mentioned protection of the distribution line, but only to ensure that the distribution line is not damaged in the event of a fault. Although "a lot of distribution circuits are not under full-load operation, the actual current of some distribution circuits is much smaller than the allowable current-carrying capacity of the circuit cables", there is no measure to prevent the above-mentioned distribution lines from running at full capacity. . Therefore, it is very wrong to check the cable's current-carrying capacity and it won't happen.
Ambient temperature and laying methods affect the current carrying capacity of wires and cables. In the wire or cable current-carrying capacity table, only the current-carrying capacity at a certain number of ambient temperatures is given. When the actual condition of the wire or cable laying is inconsistent with the conditions given in the ampacity table, the current-carrying capacity of the wire and cable must be corrected according to the relevant provisions of Section 7.4 of the “Civil Building Electrical Design Specification” JGJ16-2008.

The cable tray box is a closed structure, which makes it difficult to dissipate heat. When the multi-circuit distribution cable is laid in the cable tray box, the current carrying capacity thereof is greatly reduced. The correction factor is 0.5 for 9 loops and 0.38 for 20 loops or more. This must cause the electrical designer's attention.

When wires and cables are laid in indoor cable trays, the ambient temperature is generally set at 35°C. The core allows long-term working temperature. PVC insulation is 70°C. XLPE or EPR insulation is 90°C. Backup circuits, circuits that are not normally operated, and circuits where the actual load current is not greater than 30% of the allowable ampacity are not counted in the total number of circuits.
11. The emergency light comes with a battery for 180 minutes

Some designers require that emergency lights come with batteries that last 180 minutes. This is very unreasonable. Especially when the lamp capacity is large, such as two 36W straight fluorescent lamps, the battery needs about 10kg of weight for 180 minutes of continuous power supply. In addition, auxiliary devices such as rectification and inverter are very large and cannot be installed in the lamps. Inside. Where such a large weight and volume are installed and how to fix it are all problems that are difficult to handle.

In addition to the three-tier load-bearing buildings, the duration of emergency lighting should be guaranteed by the distribution system. The emergency light comes with the battery only in the transition period of the power conversion, especially when the mains and self-powered generators are converted, so that they can meet the emergency lighting conversion time of the auxiliary measures.
12. Emergency lighting adopts the forced lighting mode to cut off the power

When some designers design emergency lighting systems, the emergency lighting distribution box uses dual power supplies, and the emergency lights have their own batteries. In the event of a fire, the power supply circuit of the emergency lighting is cut off in linkage, and the emergency light is turned on due to the power cut and is powered by the battery. This practice is very wrong.

The regulations stipulate that in the event of a fire, the non-fire-fighting power supply at the relevant site should be removed. At any time, especially in the event of a fire, the reliability of the power supply to the fire distribution circuit must be ensured.

The emergency lighting is the fire load. In the event of a fire, the switch-off of the distribution circuit is obviously in violation of the regulations. In addition, the battery capacity of the emergency lamp gradually decreases with the increase in the number of charge and discharge times. In the event of a fire, the power supply circuit that cuts off the emergency lighting is powered by its own battery and cannot guarantee the continuous supply time required by the regulations.

Article 3.4.2 of the “Fire Automatic Alarm System Design Specification” GB50116-2013 stipulates that the fire control room graphic display device installed in the fire control room shall be able to display the fault status and emergency working status of the fire emergency lighting and evacuation indication system. Emergency lighting adopts the forced lighting mode that cuts off the power supply and cannot meet the above specification requirements.
13. Three-level fire-fighting load distribution adopts two-circuit end mutual investment

When the fire protection equipment in the building is set to three-level load, the building only needs

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