Discussion on several problems of construction ele

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Discussion on several problems of on-site construction power consumption I. The article on "design ideas for construction power consumption of small and medium-sized construction projects" published in the ninth issue of building safety in 2002, combined with the fact that Shanghai Standardization Committee has already cancelled the disconnecting switch, has presented its own views and arguments on this basis, It also puts forward the practical principle that "more disconnectors are bound to have more contacts, more contacts are bound to have more faults, more faults are bound to have more maintenance time, and the inspection should establish and improve the comprehensive evaluation criteria for plastic granulator equipment, and more industry standard maintenance time is bound to have more possibility of electric shock". As we all know, no matter how good the contact is, it is not as good as a wire connection. If there is a poor contact, resistance heat will be generated. Over time, it will cause frequent accidents such as phase loss, motor burning or electrical appliance heating and fire. Therefore, it is reasonable to say that canceling the isolating switch is equivalent to canceling the potential safety hazard. The author believes that under the premise of ensuring safety, the more simplified the control of temporary electricity use on the construction site, the better. For example, a simple bridge rectifier circuit is composed of four rectifier diodes, while the electronic manufacturer produces a full bridge rectifier circuit, which is small in size and high in power. The front end inputs AC and the back end can output DC. Another example is that an ordinary integrated circuit can top hundreds of components, which occupies a small area, has a large working range, and has stable performance. Why doesn't the safe use of electricity at the construction site develop in this direction? If all leakage switches with overload, short circuit and leakage protection functions are selected, and the temporary disordered connection at the disconnector can be effectively controlled. The author believes that as long as the safety can be more effectively guaranteed, the reliability and usability can be improved, and the purpose of saving can be achieved, the cancellation of the disconnector does have certain benefits, and also meets the requirements of safe power consumption, which is worth popularizing. However, before the promotion, the inspection did not meet the requirements of the scoring table for construction power utilization inspection. The implementation of this practice must be approved or approved by the superior competent department or issued with hard documents, otherwise it is difficult to implement on site

II. Opinions on on-site trunk type power distribution

many on-site trunk type power distribution were adopted before the 1980s. With the development of the construction industry and the standardized and standardized safety management on site, the trunk type power supply and distribution mode is no longer suitable for modern construction sites. Since the density of steel is 7.8g/cm3, overhead lines are mostly used for trunk type power distribution, which brings a lot of inconvenience to the on-site road transportation and material hoisting, as well as potential safety hazards. At present, rubber insulated cables are mostly buried on site, and the on-site power supply is radial, from the on-site general distribution box to the distribution box to the switch box (see the schematic diagram of parallel connection and socket connection). Therefore, there is no overhead line or trunk type power supply on site. This method is not suitable for the safety management of civilized construction site. The trunk type power supply is used upstairs. The author thinks that there are too many connectors, and the interior and exterior decoration makes it difficult to set up the main line, fix it firmly, and its security is also poor. Therefore, it is more practical to use the insulated cable chain power supply for a beautiful scenery line of the exhibition to achieve maximum environmental protection (see Figure 1)

Figure 1 Schematic diagram of chain power supply

III. repeated grounding is not necessary according to the working grounding requirements

about repeated grounding, it is regarded as working grounding, and the grounding resistance is required to be less than 4 Ω, which the author thinks is not necessary. Specifications; "The working grounding resistance shall not be greater than 4 Ω; the grounding resistance of each repeated grounding device shall not be greater than 10 Ω; the impact grounding resistance of all lightning protection devices in the construction site shall not be greater than 30 Ω". The specification also stipulates that the same grounding body can be used for repeated grounding and lightning protection grounding of the same electrical equipment, and the grounding resistance value shall meet the requirements of repeated grounding resistance value. Most of the on-site working grounding wires are introduced from the transformation and distribution room of Party A. the transformation and distribution room is specially inspected and accepted by the power industry department, and the grounding resistance also meets the requirements. The on-site protective neutral connection is led out from the zero line at the power side of the first level leakage protector, and repeated grounding is set at the ends of the main box, sub box and remote closing box on the site to make electrical connection with the protective zero line and the electrical equipment shell. The Ministry's standard technical code for safety of temporary power utilization on construction site has been proved by years of on-site power utilization practice that the requirements have been fully met as long as the grounding resistance value of each repeated grounding device is not greater than 10 Ω. Of course, the smaller the grounding resistance, the better. At present, the foundation reinforcement is mostly used as the grounding body on the site, and the measured grounding resistance of the grounding body from some large foundations reaches about 1 Ω, which further enhances the safety guarantee of repeated grounding and lightning protection grounding. In addition, the protective neutral line can only have a small current when there is leakage, and usually there is no current. Therefore, repeated grounding is not necessary to be set according to the requirements of working grounding

IV. where is the best place to set the protective zero line

the specification stipulates that the metal shell of the electrical equipment must be connected with the special protective zero line, and the protective zero line must be connected to the grounding point with grounding symbol in the motor junction box on site. It is considered unqualified during the inspection, and it is considered that the metal outer shell is the place where the motor can be seen (such as the fixing screw of the junction box cover or the fixing screw of the cooling fan cover), which is inconvenient to be connected inside for inspection. However, the author believes that it is better to connect the inner side than the outer side. The inner side is a special protective grounding point, which can be well and properly contacted. The outer side needs to be attacked by corrosive substances such as wind and construction mud for a long time, and the screws are seriously rusted, which will also cause artificial disconnection and unreliable electrical connection. It is better to connect the protective zero line to the box

v. understanding of sub box socket connection

due to the insufficient number of output terminals of the main distribution box, the phenomenon of sub box socket connection often occurs on site. During the inspection, it is considered that one switch is used for multiple purposes and points will be deducted. The output of a 600A general distribution box also has three routes. Generally, the site is divided into three boxes based on mechanical settings: reinforcement machinery, woodworking machinery, electric welding workshop, on-site vertical transportation machinery and mixer, upstairs power supply, kitchen, living area and so on. At least one sub box is required. A 600A general distribution box with three-way output cannot meet the requirements of a small project. Usually, the output end of the main box is connected in parallel with the sub box or the input end of the distribution box is sleeved with the sub box. As shown in figure 2:

Figure 2 Schematic diagram of parallel and socket power distribution

from the analysis of the figure, the main box is connected in parallel by one output to control two distribution boxes; The socket connection of the distribution box is only the chain power supply of the local distribution box on the site. It is also a one-way output control two-way distribution box. If the distribution box is connected to the output end of the distribution box, the capacity is not enough (the 250A distribution box only outputs 4 circuits of 60a). Therefore, from the perspective of distribution principle, it meets the requirements of three-level distribution and two-level leakage protection. In terms of system, they all have separate main box, sub box, switch box and separate leakage protection system, which cannot be considered as one switch for multiple purposes. The author thinks that only when one switch box controls two devices, it is really a multi-purpose switch. Is this the case with your peers

(Zhao Jing, Tan Ding, Jinhui)

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