Common problems and Countermeasures in the electri

Common problems and Countermeasures in building electrical design

1. Lightning protection calculation problems

lightning protection is divided into three levels in the code. In practice, designers have limited access to a large number of three-level lightning protection buildings. There are two standards for level III lightning protection, article 12.2.3.1: when the number of lightning strikes in that year is greater than or equal to 0.05, or the buildings that need lightning protection are confirmed through investigation. Article 12.2.3.2: the highest building in the building group or the building located at the edge of the building group with a height of more than 20m

it can be seen that to determine whether a building should be protected against grade III lightning, it should first be calculated according to Article 1. However, in the actual design, the designers neglect the calculation because they are afraid of trouble, and simply choose according to Article 2, so it is easy to make some buildings that should have been designed for lightning protection do not have lightning protection design. The empirical formula for the number of annual lightning strikes on buildings is

n = 0.024k 1.3dtae

when the height of the building is h <100m

ae = [lw+2 (l+w) H (200-h) + H (200-h)]

when the height of the building is h 100m

ae = [lw+2h (l+w) + H2]

in the formula, K -- correction coefficient, generally taken as 1

td-- annual average thunderstorm days

ae-- equivalent area of building

l, W, h -- are the length, width and height of the building respectively

therefore, the number of annual lightning strikes of buildings is related not only to the volume of buildings, but also to the number of local thunderstorm days. For example, if a building with a bottom area of 40m and 15m is located in Jinghong County, Yunnan Province, which has the highest thunderstorm days (120.8) and Golmud City, Qinghai Province, which has the lowest thunderstorm days (2.3), the above formula is used for checking calculation. The results show that the building in Jinghong county should also be protected against lightning (n> 0.05) when it is as low as 5m, while in Golmud City, when the building is as high as 475m, there is n 0.05. Therefore, when determining whether a building should be designed for three-level lightning protection, the empirical formula should be used for calculation, so that the design has a basis

2 ﹐ the laying of fire lines

it is found in many electrical design drawings that the fire lines are protected by plastic pipes (PVC) and routed from the ceiling. However, article 24.8.5 of the Code stipulates that fire linkage control, automatic fire extinguishing control, communication, emergency lighting, emergency broadcasting and other lines should be protected by metal pipes and concealed in non combustible structures, and the thickness of the protective layer should not be less than 3cm. When exposed laying is necessary, waterproof measures shall be taken on the metal pipe. Obviously, many designers are negligent about this. There are obstacles in the lifting process of the pendulum

from the study of the description of the provisions of the code, the author understands that the reason why this article does not include the fire detector line is that the detector line only plays a role in the initial smoke stage of the fire, while the fire linkage control, automatic fire extinguishing control, communication, emergency lighting and emergency broadcasting lines specified in the provisions also play a role in a period of time after the fire, so during this period of time, These lines should be absolutely safe

because the lines laid in the ceiling are not safe in case of fire, and sometimes the ceiling is also a fire prone area. Therefore, designers should pay enough attention to this article of the code. In practice, all newly designed buildings should wear metal pipes to protect the lines specified in this article and lay concealed wiring in the cast-in-place slab and wall. In the reconstruction project, when the concealed laying cannot be carried out due to conditions, fire prevention measures should be taken to protect the steel pipe, such as painting fire-proof paint

3. The end protection of dry-type power supply trunk line

trunk power supply is one of the most commonly used ways in power supply system, which refers to connecting several loads on the same circuit power supply trunk line in parallel. The author found in the construction drawings that, for the purpose of saving money, the designers reduced the conductor section of the end or several sections of the trunk line without checking calculation, which not only caused hidden dangers in the power supply system, but also did not meet the requirements of the specification. According to the specification, only when the core section is reduced or the branch meets the following two points can the data show that short-circuit protection is not allowed: 1) the protective appliance of the upper level line can effectively protect the line. 2) The power supply side is equipped with the circuit protected by protective appliances with a rated current of not more than 20A (see article 8.6.2.4 of the specification). The author believes that in order to ensure the safety and reliability of the power supply system, designers should not blindly change the section of the trunk power supply trunk

4. Common grounding problem

there are many grounding systems in modern buildings: lightning protection grounding; Grounding of power system, such as repeated grounding; Electronic equipment grounding, including automatic fire protection system grounding, television system grounding, system grounding, building automation system grounding, etc. In modern buildings, especially in frame structure buildings and high-rise buildings, it is difficult to truly separate these grounding systems. In view of this, article 14.7.4.3 of the specification stipulates that the grounding of electronic equipment should share the grounding with the lightning protection grounding system. But at this time, the grounding resistance should not be greater than 1. If it is separated from the lightning protection grounding system, the distance between the two grounding systems should not be less than 20m. Article 24.14.2: when common grounding is adopted, the special grounding trunk line shall be introduced from the grounding plate of the fire control room to the grounding body (the same is true for other electronic equipment grounding systems)

therefore, the learning and understanding of the regulations on grounding is that the grounding of each system should share the grounding body, but not the grounding wire, and the foundation reinforcement of the building should be used first. The correct approach is that when each grounding system uses the building foundation reinforcement to share the grounding, each grounding system should use a bv-25mm2 copper core wire to pass through pvc20 and directly lead to the foundation, and reliably connect with the main reinforcement in the foundation

in ordinary design, designers can make fewer mistakes and improve the design level as long as they carefully study, understand and abide by the specifications