Erection of external power protection frame with t

2022-08-22
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In recent years, with the continuous development of urban construction, new projects continue to appear, some of which are located in the narrow old urban area. The narrow construction site and the small safety distance between the working face and the original urban electricity are the prominent characteristics of this kind of project. Nearby substation facilities and lines also bring certain dangers to the construction. In order to solve this problem, it is necessary to set up an external power protection frame to ensure safe production. JGJ, technical code for safety of temporary electricity on construction site, has no specific requirements for the material of protective frame. In the "guidelines for the implementation of standards for safety inspection of building construction" prepared by the Ministry of construction, the construction power parts are described as follows: "protective frames are generally erected on the construction site, and their materials should be wood and other insulating materials. When steel pipes and other metal materials are used, they should be well grounded." According to the general practice, the protective frame is erected with wooden poles. However, due to the current lack of use of wooden scaffolds, material procurement, turnover and other reasons, construction enterprises have certain difficulties in erecting a large area of protective frame. Adopting steel pipe fasteners to set up external electrical protection provides another solution for construction. This year, the Tianjin book building project undertaken by our company carried out a pilot project of setting up external electrical protective shelves with steel pipe fasteners, which not only ensured safety, but also achieved good economic benefits. Taking Tianjin book building as an example, this paper expounds the instigation of external electrical protective shelves. The electrical interference of instruments is often a random and symmetrical design and construction feature, and I hope you can correct it if there is anything inappropriate

1. project overview:

Tianjin book building project is located in the downtown business district, covering an area of 7116m2, with one floor underground and 20 floors above the ground, and the building height is 88.8m. There is a 10kv High-voltage Overhead Line on one side of the traffic trunk road adjacent to the building, with a length of nearly 100m, between which there are two 315KVA and 420kva transformers. The high-voltage 10kV line is only 3M from the nearest building

2. scheme demonstration:

2.1 economic demonstration: wooden poles are used for erection, and the one-time material investment is large; The use of steel pipe fasteners can make use of existing materials, which can be recycled and used to save material costs

2.2 safety demonstration: wooden poles are used for erection, and the frame has good insulation; After adopting certain safety technical measures for the erection of steel pipe fasteners, the safety requirements can also be met

2.3 quality demonstration: it is difficult to use wooden poles for erection, and the erection quality should not be controlled; Steel pipe fasteners are used for erection, the erection quality is easy to control, and the protective frame is firm and reliable

2.4 other Argumentation: the project is located in a prosperous commercial area and is also a key project of the city, which requires the overall image of the construction site. The protective frame erected with steel pipe fasteners is civilized and beautiful, and the materials are consistent with the steel pipe scaffold erected on the site, which is conducive to civilized construction, and the site is clean and beautiful

through the above demonstration, it is decided to use steel pipe fasteners to set up the external electrical protection frame

3. Discussion on relevant technical problems:

3.1 analyze the causes of electrification of steel pipe fastener protection frame:

3.1.1 electric and magnetic induction of high-voltage line: due to the strong electric field around the high-voltage line, electromagnetic induction is generated on the nearby conductors

3.1.2 electromagnetic induction of lightning current: in thunderstorm weather, lightning produces a strong changing magnetic field, which makes the frame produce induced current

3.1.3 lightning strike: lightning strike is caused by the direct discharge of thunderclouds on protrusions. At this time, the amplitude of lightning current can reach tens to hundreds of kiloamps

3.1.4 the frame is electrified due to other reasons

the above reasons may cause the frame to be electrified, which may cause electric shock accidents. When the induced current, especially the large current caused by lightning, passes through the frame, if the local resistance in the conductor is large or the contact is poor at this time, local overheating will occur, causing metal melting or igniting combustibles, causing secondary damage

3.2 corresponding safety technical measures:

3.2.1 in order to reduce high-voltage induced electricity, the erection of steel pipe fastener frame must ensure a certain safety distance (generally, the minimum electrical safety distance of 10kv High-voltage Transmission line is 1m, the minimum safe operation distance is 6m, and the minimum electrical safety distance and the minimum safe operation distance are different with the voltage level of high-voltage transmission line)

3.2.2 in view of the possible electrification of the frame, reliable grounding measures must be taken for the frame

3.2.3 all parts of the frame must have good electrical connection performance to prevent local resistance or poor contact

3.2.4 use flame-retardant dense mesh to close the frame, and prohibit other temporary power lines from crossing and laying with the help of the frame, so as to prevent other accidents from electrifying the frame

3.2.5 strengthen the on-site dynamic management and control requirements, and regularly inspect and measure the resistance of the frame to ensure safe use

4. erection of scaffold:

4.1 material selection: adopt φ 48mm, wall thickness ≮ 3.5mm steel pipe, fastener connection, flame retardant dense mesh. The materials used shall be free of deformation, corrosion, damage, etc

4.2 Foundation: after the foundation is leveled and compacted, a 5cm thick scaffold board is placed at the bottom of the frame

4.3 erection: (see Figure 1)

Figure 1 profile of protective wire frame erection

4.3.1 the spacing between vertical poles of protective frame is 1.5m, and the spacing between horizontal poles is 1.5m~1.8m. The diagonal bracing is continuously set along the outside of the frame body, with a width of 5~7 vertical rods, and the included angle between the diagonal bracing and the ground is 45~60 degrees

4.3.2 set a longitudinal diagonal brace every 4.5m along the longitudinal direction of the frame to increase the overall stability

4.3.3 the erection of the frame is 1.5m higher than the external power, and the left and right sides are not less than 1m

4.3.4 the top of the protective frame is covered with 5cm thick wooden scaffold board

4.3.5 the perimeter of the external electrical protection frame is closed by fire-retardant dense eyes, and the highest part of the frame is set with red flags and red warning lights (with an interval of 3M)

5. grounding protection measures of the frame:

in order to prevent lightning or high-voltage induction in wet weather from electrifying and trying to change the frame of all 3D printing markets, effective lightning protection grounding and protective grounding must be taken to discharge the current on the frame into the earth in time

5.1 specific measures (see Figure 2):

Figure 2 electrical connection diagram

5.1.1 set 40 points horizontally along the frame at the bottom of the protective frame 30cm from the ground × 4. Galvanized flat iron is made into a ring and welded inside the upright of the protective frame φ 10 galvanized bolts shall be used as support, and at the same time, all upright pipe connections shall be φ The steel pipe is welded. The ring is welded with the grounding body and lightning conductor, and its lap length is 8 times the width of flat iron, which is welded on both sides

5.1.2 adopt 20 at the top cross bar of the frame × 2 galvanized flat iron is welded as a support, with a height of 15cm and an interval of 1.2m, which is used to support the top of the frame φ The annular lightning strip made of 10 steel bars is adopted at the four corners and middle of the frame at the cross bar on the top of the frame φ 20. The reinforcement is welded into a lightning rod and welded with the top lightning strip. The lightning downlead is directly along the pole along the lightning rod with -20 × 2. The galvanized flat iron is used as the support, and the support is 12cm with an interval of 1.5m, which is led to the annular flat iron horizontally set at the bottom of the frame, and overlapped and welded on both sides. Install PV on the downlead from the ground to 2m to strengthen the tracking and management of 225 projects in the early stage. C tube is painted with red and white paint to prevent touching and play an eye-catching role. Such measures can transmit the current on the frame to the earth in time

5.1.3 the grounding body is welded with the main reinforcement of the pile column of deep foundation engineering, and is repeatedly grounded and welded with the tower crane. The measured resistance values are 0.6 and 0.7 Ω, which is well grounded and can meet the requirements of Safety specifications

5.2 key problems:

5.2.1 after testing the electrical connection performance of each point of the frame, it is found that the resistance value is concentrated at the connection of the frame fastener, and the φ 8. After the galvanized round steel is connected, the electrical connection performance of each point is good

5.2.2 the setting of grounding body should not use the original natural grounding body for grounding. Here, it is connected with the reinforcement of pile cap. The test results are 0.6 Ω and 0.7 Ω respectively, and the grounding is good

6. conclusions and suggestions:

external electrical protection is actually a kind of screen protection. The material of screen protection is not specified. Generally, mechanical strength and fire resistance are required, and metal materials should be well grounded. As the distribution box is made of wood material, it is not fireproof, low mechanical strength, conductive after wood is wet, etc., and the effect of using metal material as protective grounding is better. The steel pipe fastener type external electrical protection frame provides a solution to the problem of external electrical protection at the construction site. Through practice, it saves materials and ensures safe use. Due to the different conditions of the construction site, corresponding measures should be taken according to the specific conditions of the local project in the actual use process. (Cao Xiangzhe, Xing Wenchang)

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