I. Introduction
In the lightning protection design of buildings, the designers attach more importance to the lightning protection design of the first and second-level lightning protection buildings, and there are few omission errors, but the lightning protection design of a large number of three-level lightning protection buildings is often overlooked. Due to the design quality management regulations: For the electrical design of general engineering, it is not allowed to calculate the book. Therefore, many designers do not design and calculate the lightning protection design of the three-level lightning protection structure and design it based on experience alone. For the installation of lightning protection facilities and various safety clearances for lightning protection facilities, no calculations or verifications have been carried out. As a result, the lightning protection design and construction of a large number of three-level lightning protection buildings have a large degree of blindness. The level of lightning protection was increased, and the construction cost was increased. However, some projects were not designed and constructed in accordance with the specifications, causing mistakes to be missed and causing great hidden troubles and undue losses.
II. Overview and comparison of lightning protection specifications for buildings
The lightning protection standards of buildings today have the recommended industrial standards of “Civil Building Electrical Design Specification†(JGJ/T16-92) implemented from August 1, 1993 and the “Lightning Protection Design Code for Buildings†implemented from November 1, 1994. GB50057-94 mandatory national standards. GB50057-94 gradually makes the lightning protection design and construction of the building meet the International Electrotechnical Commission IEC lightning protection standards, and the design and construction are more standardized and standardized.
GB50057-94 divides civil buildings into two categories, and JCJ/T16-92 divides the lightning protection design of civil buildings into three levels, which are more specific and meticulous in order to avoid causing certain civil buildings to lose their due safety. Some buildings may appear unnecessary waste. In order to better grasp the essence of IEC, GB50057-94, and JCJ/T16-92, the main terms of their choice are listed in Table 1. The following analysis and calculations are cited in JCJ/T16-92.
Third, the estimated number of lightning strikes in each year will be determined by installing lightning protection facilities.
In addition to a small number of first- and second-tier lightning protection buildings, a large number of or three-level lightning protection and lightning protection of buildings other than grades, and most designers of such buildings do not calculate the number of expected lightning strikes N, so many do not Lightning protection measures must be designed to design mine-protected buildings. The design is conservative and people, materials, and objects are wasted. The calculation now illustrates an example:
Example 1: Designing a residential building in a flat residential area: 6-story high-rise buildings without basement, basement height 2.2m, three units, of which: length L = 60m, width W = 13m, height H = 20m, The average annual local thunderstorm day is Td=33.2d/a. Since the residential building is inside the community, the correction factor K=1.
According to JCJ/T16-92 formulae D.2-1, D.2-2, D.2-3, D.2-4, the equivalent area km2 of the same number of lightning strikes as the building interception: Ae=L W+2L+WH200-H+πH200-H×10-6=60×13+2(60+13)20(200-20)+3.14×20(200-20)×10-6=0.02084km2
The average annual density of lightning strikes in the area where the building is located:
Ng=0.024Td1.3=0.024×33.21.3=2.28 times/km2·a
Estimated number of lightning strikes for the year of the building:
N=KNgAe=1×2.28×0.02084=0.0475 times/a
According to JCJ/T16-92 Article 12.3.1, only in N ≥ 0.05GB50057-94: N ≥ 0.06 is set to three levels of lightning protection, and in this case: N = 0.0475 <0.05, and the residential building in a residential building The group is not the highest and is not at the edge of the tower. Therefore, the residential building does not need lightning protection facilities.
According to the above calculation steps, now with L = 60m, W = 13m, respectively, H = 7m, 10m, 15m, 20m four different heights, K values ​​take 1,1.5,1.7,2,Ng=2.28km2·a Calculate the N value. The calculation results are shown in Table 2.
From the data in Table 2, it can be seen that in this area: 1 When K=1, the buildings in the example all have N<0.05, and no lightning protection facilities are required. 2 When K = 1.5, that is, a building or a particularly damp building at the height of the soil resistivity, groundwater outcrop, soil hill top, valley mouth, etc. at the height of the river, the lake, below the hillside or in the mountains, at the height Up to 15m or above, three lightning protection measures must be set. 3 When K = 1.7, that is, a building with a metal brick-wood structure and a height of 7 m or more, three-level lightning protection measures must be provided. 4 When K=2, that is, the building is located in an isolated position in the wilderness and two or more heights of 7m are provided, three-level lightning protection measures are all set.
It can be seen that some buildings do not need lightning protection measures at a height of 20m, and some buildings have a height of 7m, and three-level lightning protection measures must be installed. The key factor is the location, environment, soil quality and lightning activity of the building.
At the same time, in the completed project, we also saw that in the civil building of Example 1, many similar projects should not be equipped with lightning protection but designed and constructed in accordance with the three-level lightning protection. The lightning protection grounding device after construction is shown in the figure. 1 shows.
Among the eight groups, the 412 main ribs of the structural columns in the downhole were used, and the horizontal loop grounding body was buried at a depth of 1m and was 1m away from the external wall of the building. All of the above steels are galvanized. Totally 0.192t of galvanized steel is required. The labor cost is 2,950 yuan. The fixed budget project costs approximately 0.75 million yuan. Similar residential buildings, office buildings and other civil buildings other than such three-level lightning protection have completed approximately 600 to 800 buildings in our region in 1998. The additional direct installation cost for lightning protection facilities is approximately 4.5 to 6 million yuan. By analogy, the figures for increasing the cost of project construction due to raising the level of lightning protection throughout the province and the country are enormous. Therefore, the designer must calculate the predicted number of lightning strikes for the building in the lightning protection design of the civil building. Based on the calculation results and the specific conditions, it is determined whether to install the lightning protection facilities.
Fourth, the safety distance of lightning protection facilities and people, metal pipelines, etc. 1. The relationship between the lightning current counterattack voltage and the lead-off line spacing
When a building is struck by a lightning, the lightning current is passed through the lightning protection net laid on the roof of the building, and is grounded to the grounding device. The raised potential on the grounding device is equal to the product of the current and the resistance. The ground potential at the height h from the ground is Uo=UR+UL=IkRq+L1
Where Ik - lightning current amplitude kA
Rq—grounding resistance of lightning protection device
L—the inductance of the grounding device, μH, from the point where the lightning strikes a point off the ground
Lightning current steepness kA/μH
The first item on the right side of the formula is the resistance component of potential IkRq, and the second term UL is the inductance component of the potential. According to the relevant provisions of GB50057-94, the lightning current Ik=100kA is desirable in the class III lightning protection building. The shape of the wave head is beveled, and the length of the wave head is 10 μs. Then the steepness of the lightning current head ==10 kA/μs, and the inductance per unit of the down wire is Lo = 1.4 μH/m, then Uo=100Rq+1 can be obtained from the formula 1. 4×h×10=100Rq+14hkV2
According to formula 2, when different grounding resistances Rq and heights h, the corresponding Uo values ​​can be obtained. However, if the number of downconductors is different, the Uo values ​​are quite different. The following examples assume that the lead wires in Example 1 are 4 and 8 wires, respectively, assuming that each lead wire has a lightning current of the same magnitude, and the calculated UR/UL value is ignored, regardless of the resistance and inductance drop of the lightning current on the horizontal lightning arrester. Listed in Table 3.
From Table 3, it can be seen that even if the grounding resistance Rq is zero, the potential inductance component generated by the inductance at the grounding down conductors of different heights is also quite high, and a counterattack flashover will also occur.
2. The safety clearance between the deflector and the human body
The lightning strike current flows through the down conductor and the lightning strike voltage generated on the grounding body. The resistance component exists within the lightning wave duration of several tens of μs, and the inductance component only exists within the wave head time of 5 μs, so the two have air insulation effect. Differently, the desired air breakdown strength: inductance UL = 700 kV/m, resistance ER = 500 kV/m. The breakdown strength of the concrete wall is equal to the air breakdown strength, and the breakdown strength of the brick wall is half of the air breakdown strength.
According to the data calculated in Table 3, the safety distance between the underwire and the human body is calculated below. Because each group of deflectors uses the 412 steel in the structural column, it can be considered that the air gap between the deflector and the human body, the metal pipe, and the metal object is the air gap, and the spacer layer between the deflected air and the air is considered as a plaster layer. can be ignored.
(1) When the deflected line is 4 groups, the person stands on the first floor, h1=3m, Rq=30Ω, then URI=750kVUL1=10.5kV The safety distance between the human body and the downleader L Safety 1>
Only counterattack. People stand on the 5th floor, h2=15m, Rq=30Ω, then: UR2=750kVU12=52.5kV then the safety distance L safety 2>1.575m<1.83m. In these two rooms, it is difficult to maintain such a distance, so there is a very dangerous lightning voltage counterattack.
(2) When the downlead is 8 groups, when standing in a room, h1 = 3m, Rq = 30Ω, then UL1 = 5.25kVUR1 = 3.75kV then the safety distance L safety 1> 0.757m. When people stand on the 5th floor, h2=15m then UL2=26.25kVUR2=375kV then the safety distance L safety 2>
It can be seen that the number of deflected wires has doubled and the safety margin has been reduced by half. Therefore, after setting up the lightning protection facilities, the number and spacing of downholes should be set strictly in accordance with the specifications. At the same time, it is suggested that the downlead spacing specified in the specification can be shortened, and a certain number of downleads can be set up to reduce the lightning voltage counterattack. In this way, the construction cost will be minimal.
3. The distance between the down conductor and the indoor metal pipes and metal objects
1 When the lightning protection and grounding device is not connected to the buried part of the metal pipe, according to the data in Example 1: The height of the downline of the roof, h=Lx=20m, Rq=30Ω, according to JCJ/T16-92, 12.5. Clause 7 states that if Lx<5Rq=5×30=150m, then Sal≥0.2KcRi+0.1Lx
In the formula Kc-shunt coefficient, due to multiple deflectors, take 0.44
Ri—impact resistance of lightning protection grounding device, because of the loop grounding body, Ri=Rq=30Ω
Sal—The safety distance between the deflector and the metal object/m
Then Sal ≥ 0.2 × 0.44 × 30 + 0.1 × 20 = 2.816m.
2 When the lightning protection grounding body is connected to the buried part of the metal pipe, according to Equation 12.3.6-3, Sa2 ≥ 0.075 KcLx = 0.075 × 0.44 × 20 = 0.66
From the above-calculated Sal ≥ 2.816m, Sa2 ≥ 0.66m, it is difficult to guarantee the above distance in actual construction, because the metal pipe is installed about 0.1m from the wall, and because Sa2 ≤ Sal, the lightning protection grounding device can be The buried part of the metal pipe is connected. At the same time, the down conductor should be connected with the metal pipe object in the floor to prevent lightning strike back.
4. The distance between the down conductor grounding device and underground metal pipes and other grounding devices.
According to JCJ/T16-92 Article 12.5.7 formula 12.3.6-4: Sed ≥ 0.3KcRi = 0.3 × 0.4 × 30 = 3.96m, and in actual construction, the groundwater heating pipelines are interlaced, before lightning protection and electrical When the grounding device is used for construction, it is difficult to guarantee Sed ≥ 3.96m, and it is difficult to ensure that the requirement is not less than 2m. Therefore, the lightning protection grounding device can be shared with various grounding devices, ie, one building can be implemented. Grounding body. Connect the grounding device to various metal pipes that enter and exit the building underground and implement a total equipotential bonding.
To sum up, after carrying out a construction with a total potential connection and a common grounding body, the lightning protection pin should be connected with metal objects protruding from the roof metal pipe at the top of the building. The auxiliary equipotential bonding shall be implemented within the object, that is, when the down conductor passes through each floor, all the steel bars and metal structures in the floor shall be connected together, so no matter how high the potential of the down conductor rises, the same floor building All metal objects including the steel bars in the ground, metal pipes, and safety grounds of electrical equipment are raised to the same potential at the same time to eliminate counterattack by lightning voltage.
Fifth, step voltage and grounding device buried depth
The step voltage refers to the potential difference between two feet of a person's feet touching the ground, and generally takes the potential difference within 0.8 m from the person's span. There are many factors such as the size of the step voltage and the buried body depth, soil resistivity, and lightning potential amplitude. When the grounding body is a horizontal grounding strip,
3 type Ï—soil resistivity/Ω.m
L - horizontal grounding body length m
Ik - lightning current amplitude kA
K—depth coefficient of grounding device, see Table 4
Ukmax—step voltage maximum kV
According to the calculation of the grounding device in Example 1, the length of the grounding body L = 146m, take Ik = 150k, the soil is sand clay, Ï = 300Ω.m, according to the buried depth of 0.3m, 0.5m, 0.8m, 1m corresponding The K value takes 2.2, 1.46, 0.97.0.78. According to the formula 3:
Its Ukmax values ​​were 107.97, 71.66, 47.61, 38.28/kV, respectively.
According to the analysis of personal shocks and electric shocks in various countries in the world, it is considered that the human body can withstand a step voltage of 90 to 110 kV for the duration of a lightning current. From the calculation results, it can be seen that when the lightning protection grounding body of this project has a buried depth of 0.8m, the step voltage is within the safe range. JCJ/T16-92 12.9.4 stipulates that the buried depth of the grounding body should not be less than 0.6m. Article 12.9.7 stipulates: The artificial grounding body for lightning protection shall not be less than 3m away from the entrance of the building and the sidewalk, when it is less than 3m. The local buried depth of the grounding body should not be less than 1m, or the horizontal grounding body is partially covered with insulators. It is easy to increase the grounding resistance by covering with insulators, so it is better to use a buried depth greater than 1m. In this way, only a small amount of project cost will be increased, but the grounding device will be handled more safely and reliably, which will have a multiplier effect.
If the basic and ring beams are used as the ring grounding body, but because the three-level lightning protection buildings are mostly based on a rubble foundation, the ring beams buried on the rubble foundation are generally about 0.3m, and the shallower layers cannot be prevented at all. The dangerous step voltage needs to bury the grounding device in a depth of 1m. Therefore, it is not appropriate to use a ring beam as a ring grounding body to refer to a three-level lightning protection structure.
Six, distinguish the frequency, impact grounding resistance
The difference and relationship between the power frequency and the impact grounding resistance can not be distinguished and clarified by many construction technicians. Therefore, the grounding resistance of some lightning protection devices of the project has reached the design value, and blind resistance measures are still adopted, which increases the project cost.
Power frequency grounding resistance is the resistance obtained by the power frequency current flowing into the ground through the grounding body. It can be considered that the grounding body is less than 20m away from the earth, and ground is not an electrical zero point. The resistance measured by the ground resistance meter is the power frequency ground resistance.
It can be seen from Table 4 that when the grounding body is a loop grounding body surrounding the building and a grounding body laid in a soil with a resistivity Ï â‰¤ 100Ω such as clay clay, marsh, black soil, and sandy clay, the ground frequency resistance of the power frequency is Equal to impact resistance. However, when laid in the environment of sand, gravel, gravel, gravel, rocky mountains, the frequency-frequency grounding resistance is 2 to 3 times of the impact grounding resistance. Therefore, if a grounding body is laid in the ground as described above, if the grounding resistance measured by the grounding resistance meter does not exceed 2 to 3 times the impact resistance value of the design requirements, the design requirements can be met. Need to take measures to reduce resistance. If the soil and grounding conditions of the grounding device installation site are not analyzed, it is found that the grounding resistance meter's measured value is greater than the design requirement value, blindly adding artificial grounding body or adopting a drag reducing agent to pursue the design value must result in human and material waste. The project cost has been raised, but this phenomenon is universal.
In the lightning protection design of buildings, the designers attach more importance to the lightning protection design of the first and second-level lightning protection buildings, and there are few omission errors, but the lightning protection design of a large number of three-level lightning protection buildings is often overlooked. Due to the design quality management regulations: For the electrical design of general engineering, it is not allowed to calculate the book. Therefore, many designers do not design and calculate the lightning protection design of the three-level lightning protection structure and design it based on experience alone. For the installation of lightning protection facilities and various safety clearances for lightning protection facilities, no calculations or verifications have been carried out. As a result, the lightning protection design and construction of a large number of three-level lightning protection buildings have a large degree of blindness. The level of lightning protection was increased, and the construction cost was increased. However, some projects were not designed and constructed in accordance with the specifications, causing mistakes to be missed and causing great hidden troubles and undue losses.
II. Overview and comparison of lightning protection specifications for buildings
The lightning protection standards of buildings today have the recommended industrial standards of “Civil Building Electrical Design Specification†(JGJ/T16-92) implemented from August 1, 1993 and the “Lightning Protection Design Code for Buildings†implemented from November 1, 1994. GB50057-94 mandatory national standards. GB50057-94 gradually makes the lightning protection design and construction of the building meet the International Electrotechnical Commission IEC lightning protection standards, and the design and construction are more standardized and standardized.
GB50057-94 divides civil buildings into two categories, and JCJ/T16-92 divides the lightning protection design of civil buildings into three levels, which are more specific and meticulous in order to avoid causing certain civil buildings to lose their due safety. Some buildings may appear unnecessary waste. In order to better grasp the essence of IEC, GB50057-94, and JCJ/T16-92, the main terms of their choice are listed in Table 1. The following analysis and calculations are cited in JCJ/T16-92.
Third, the estimated number of lightning strikes in each year will be determined by installing lightning protection facilities.
In addition to a small number of first- and second-tier lightning protection buildings, a large number of or three-level lightning protection and lightning protection of buildings other than grades, and most designers of such buildings do not calculate the number of expected lightning strikes N, so many do not Lightning protection measures must be designed to design mine-protected buildings. The design is conservative and people, materials, and objects are wasted. The calculation now illustrates an example:
Example 1: Designing a residential building in a flat residential area: 6-story high-rise buildings without basement, basement height 2.2m, three units, of which: length L = 60m, width W = 13m, height H = 20m, The average annual local thunderstorm day is Td=33.2d/a. Since the residential building is inside the community, the correction factor K=1.
According to JCJ/T16-92 formulae D.2-1, D.2-2, D.2-3, D.2-4, the equivalent area km2 of the same number of lightning strikes as the building interception: Ae=L W+2L+WH200-H+πH200-H×10-6=60×13+2(60+13)20(200-20)+3.14×20(200-20)×10-6=0.02084km2
The average annual density of lightning strikes in the area where the building is located:
Ng=0.024Td1.3=0.024×33.21.3=2.28 times/km2·a
Estimated number of lightning strikes for the year of the building:
N=KNgAe=1×2.28×0.02084=0.0475 times/a
According to JCJ/T16-92 Article 12.3.1, only in N ≥ 0.05GB50057-94: N ≥ 0.06 is set to three levels of lightning protection, and in this case: N = 0.0475 <0.05, and the residential building in a residential building The group is not the highest and is not at the edge of the tower. Therefore, the residential building does not need lightning protection facilities.
According to the above calculation steps, now with L = 60m, W = 13m, respectively, H = 7m, 10m, 15m, 20m four different heights, K values ​​take 1,1.5,1.7,2,Ng=2.28km2·a Calculate the N value. The calculation results are shown in Table 2.
From the data in Table 2, it can be seen that in this area: 1 When K=1, the buildings in the example all have N<0.05, and no lightning protection facilities are required. 2 When K = 1.5, that is, a building or a particularly damp building at the height of the soil resistivity, groundwater outcrop, soil hill top, valley mouth, etc. at the height of the river, the lake, below the hillside or in the mountains, at the height Up to 15m or above, three lightning protection measures must be set. 3 When K = 1.7, that is, a building with a metal brick-wood structure and a height of 7 m or more, three-level lightning protection measures must be provided. 4 When K=2, that is, the building is located in an isolated position in the wilderness and two or more heights of 7m are provided, three-level lightning protection measures are all set.
It can be seen that some buildings do not need lightning protection measures at a height of 20m, and some buildings have a height of 7m, and three-level lightning protection measures must be installed. The key factor is the location, environment, soil quality and lightning activity of the building.
At the same time, in the completed project, we also saw that in the civil building of Example 1, many similar projects should not be equipped with lightning protection but designed and constructed in accordance with the three-level lightning protection. The lightning protection grounding device after construction is shown in the figure. 1 shows.
Among the eight groups, the 412 main ribs of the structural columns in the downhole were used, and the horizontal loop grounding body was buried at a depth of 1m and was 1m away from the external wall of the building. All of the above steels are galvanized. Totally 0.192t of galvanized steel is required. The labor cost is 2,950 yuan. The fixed budget project costs approximately 0.75 million yuan. Similar residential buildings, office buildings and other civil buildings other than such three-level lightning protection have completed approximately 600 to 800 buildings in our region in 1998. The additional direct installation cost for lightning protection facilities is approximately 4.5 to 6 million yuan. By analogy, the figures for increasing the cost of project construction due to raising the level of lightning protection throughout the province and the country are enormous. Therefore, the designer must calculate the predicted number of lightning strikes for the building in the lightning protection design of the civil building. Based on the calculation results and the specific conditions, it is determined whether to install the lightning protection facilities.
Fourth, the safety distance of lightning protection facilities and people, metal pipelines, etc. 1. The relationship between the lightning current counterattack voltage and the lead-off line spacing
When a building is struck by a lightning, the lightning current is passed through the lightning protection net laid on the roof of the building, and is grounded to the grounding device. The raised potential on the grounding device is equal to the product of the current and the resistance. The ground potential at the height h from the ground is Uo=UR+UL=IkRq+L1
Where Ik - lightning current amplitude kA
Rq—grounding resistance of lightning protection device
L—the inductance of the grounding device, μH, from the point where the lightning strikes a point off the ground
Lightning current steepness kA/μH
The first item on the right side of the formula is the resistance component of potential IkRq, and the second term UL is the inductance component of the potential. According to the relevant provisions of GB50057-94, the lightning current Ik=100kA is desirable in the class III lightning protection building. The shape of the wave head is beveled, and the length of the wave head is 10 μs. Then the steepness of the lightning current head ==10 kA/μs, and the inductance per unit of the down wire is Lo = 1.4 μH/m, then Uo=100Rq+1 can be obtained from the formula 1. 4×h×10=100Rq+14hkV2
According to formula 2, when different grounding resistances Rq and heights h, the corresponding Uo values ​​can be obtained. However, if the number of downconductors is different, the Uo values ​​are quite different. The following examples assume that the lead wires in Example 1 are 4 and 8 wires, respectively, assuming that each lead wire has a lightning current of the same magnitude, and the calculated UR/UL value is ignored, regardless of the resistance and inductance drop of the lightning current on the horizontal lightning arrester. Listed in Table 3.
From Table 3, it can be seen that even if the grounding resistance Rq is zero, the potential inductance component generated by the inductance at the grounding down conductors of different heights is also quite high, and a counterattack flashover will also occur.
2. The safety clearance between the deflector and the human body
The lightning strike current flows through the down conductor and the lightning strike voltage generated on the grounding body. The resistance component exists within the lightning wave duration of several tens of μs, and the inductance component only exists within the wave head time of 5 μs, so the two have air insulation effect. Differently, the desired air breakdown strength: inductance UL = 700 kV/m, resistance ER = 500 kV/m. The breakdown strength of the concrete wall is equal to the air breakdown strength, and the breakdown strength of the brick wall is half of the air breakdown strength.
According to the data calculated in Table 3, the safety distance between the underwire and the human body is calculated below. Because each group of deflectors uses the 412 steel in the structural column, it can be considered that the air gap between the deflector and the human body, the metal pipe, and the metal object is the air gap, and the spacer layer between the deflected air and the air is considered as a plaster layer. can be ignored.
(1) When the deflected line is 4 groups, the person stands on the first floor, h1=3m, Rq=30Ω, then URI=750kVUL1=10.5kV The safety distance between the human body and the downleader L Safety 1>
Only counterattack. People stand on the 5th floor, h2=15m, Rq=30Ω, then: UR2=750kVU12=52.5kV then the safety distance L safety 2>1.575m<1.83m. In these two rooms, it is difficult to maintain such a distance, so there is a very dangerous lightning voltage counterattack.
(2) When the downlead is 8 groups, when standing in a room, h1 = 3m, Rq = 30Ω, then UL1 = 5.25kVUR1 = 3.75kV then the safety distance L safety 1> 0.757m. When people stand on the 5th floor, h2=15m then UL2=26.25kVUR2=375kV then the safety distance L safety 2>
It can be seen that the number of deflected wires has doubled and the safety margin has been reduced by half. Therefore, after setting up the lightning protection facilities, the number and spacing of downholes should be set strictly in accordance with the specifications. At the same time, it is suggested that the downlead spacing specified in the specification can be shortened, and a certain number of downleads can be set up to reduce the lightning voltage counterattack. In this way, the construction cost will be minimal.
3. The distance between the down conductor and the indoor metal pipes and metal objects
1 When the lightning protection and grounding device is not connected to the buried part of the metal pipe, according to the data in Example 1: The height of the downline of the roof, h=Lx=20m, Rq=30Ω, according to JCJ/T16-92, 12.5. Clause 7 states that if Lx<5Rq=5×30=150m, then Sal≥0.2KcRi+0.1Lx
In the formula Kc-shunt coefficient, due to multiple deflectors, take 0.44
Ri—impact resistance of lightning protection grounding device, because of the loop grounding body, Ri=Rq=30Ω
Sal—The safety distance between the deflector and the metal object/m
Then Sal ≥ 0.2 × 0.44 × 30 + 0.1 × 20 = 2.816m.
2 When the lightning protection grounding body is connected to the buried part of the metal pipe, according to Equation 12.3.6-3, Sa2 ≥ 0.075 KcLx = 0.075 × 0.44 × 20 = 0.66
From the above-calculated Sal ≥ 2.816m, Sa2 ≥ 0.66m, it is difficult to guarantee the above distance in actual construction, because the metal pipe is installed about 0.1m from the wall, and because Sa2 ≤ Sal, the lightning protection grounding device can be The buried part of the metal pipe is connected. At the same time, the down conductor should be connected with the metal pipe object in the floor to prevent lightning strike back.
4. The distance between the down conductor grounding device and underground metal pipes and other grounding devices.
According to JCJ/T16-92 Article 12.5.7 formula 12.3.6-4: Sed ≥ 0.3KcRi = 0.3 × 0.4 × 30 = 3.96m, and in actual construction, the groundwater heating pipelines are interlaced, before lightning protection and electrical When the grounding device is used for construction, it is difficult to guarantee Sed ≥ 3.96m, and it is difficult to ensure that the requirement is not less than 2m. Therefore, the lightning protection grounding device can be shared with various grounding devices, ie, one building can be implemented. Grounding body. Connect the grounding device to various metal pipes that enter and exit the building underground and implement a total equipotential bonding.
To sum up, after carrying out a construction with a total potential connection and a common grounding body, the lightning protection pin should be connected with metal objects protruding from the roof metal pipe at the top of the building. The auxiliary equipotential bonding shall be implemented within the object, that is, when the down conductor passes through each floor, all the steel bars and metal structures in the floor shall be connected together, so no matter how high the potential of the down conductor rises, the same floor building All metal objects including the steel bars in the ground, metal pipes, and safety grounds of electrical equipment are raised to the same potential at the same time to eliminate counterattack by lightning voltage.
Fifth, step voltage and grounding device buried depth
The step voltage refers to the potential difference between two feet of a person's feet touching the ground, and generally takes the potential difference within 0.8 m from the person's span. There are many factors such as the size of the step voltage and the buried body depth, soil resistivity, and lightning potential amplitude. When the grounding body is a horizontal grounding strip,
3 type Ï—soil resistivity/Ω.m
L - horizontal grounding body length m
Ik - lightning current amplitude kA
K—depth coefficient of grounding device, see Table 4
Ukmax—step voltage maximum kV
According to the calculation of the grounding device in Example 1, the length of the grounding body L = 146m, take Ik = 150k, the soil is sand clay, Ï = 300Ω.m, according to the buried depth of 0.3m, 0.5m, 0.8m, 1m corresponding The K value takes 2.2, 1.46, 0.97.0.78. According to the formula 3:
Its Ukmax values ​​were 107.97, 71.66, 47.61, 38.28/kV, respectively.
According to the analysis of personal shocks and electric shocks in various countries in the world, it is considered that the human body can withstand a step voltage of 90 to 110 kV for the duration of a lightning current. From the calculation results, it can be seen that when the lightning protection grounding body of this project has a buried depth of 0.8m, the step voltage is within the safe range. JCJ/T16-92 12.9.4 stipulates that the buried depth of the grounding body should not be less than 0.6m. Article 12.9.7 stipulates: The artificial grounding body for lightning protection shall not be less than 3m away from the entrance of the building and the sidewalk, when it is less than 3m. The local buried depth of the grounding body should not be less than 1m, or the horizontal grounding body is partially covered with insulators. It is easy to increase the grounding resistance by covering with insulators, so it is better to use a buried depth greater than 1m. In this way, only a small amount of project cost will be increased, but the grounding device will be handled more safely and reliably, which will have a multiplier effect.
If the basic and ring beams are used as the ring grounding body, but because the three-level lightning protection buildings are mostly based on a rubble foundation, the ring beams buried on the rubble foundation are generally about 0.3m, and the shallower layers cannot be prevented at all. The dangerous step voltage needs to bury the grounding device in a depth of 1m. Therefore, it is not appropriate to use a ring beam as a ring grounding body to refer to a three-level lightning protection structure.
Six, distinguish the frequency, impact grounding resistance
The difference and relationship between the power frequency and the impact grounding resistance can not be distinguished and clarified by many construction technicians. Therefore, the grounding resistance of some lightning protection devices of the project has reached the design value, and blind resistance measures are still adopted, which increases the project cost.
Power frequency grounding resistance is the resistance obtained by the power frequency current flowing into the ground through the grounding body. It can be considered that the grounding body is less than 20m away from the earth, and ground is not an electrical zero point. The resistance measured by the ground resistance meter is the power frequency ground resistance.
It can be seen from Table 4 that when the grounding body is a loop grounding body surrounding the building and a grounding body laid in a soil with a resistivity Ï â‰¤ 100Ω such as clay clay, marsh, black soil, and sandy clay, the ground frequency resistance of the power frequency is Equal to impact resistance. However, when laid in the environment of sand, gravel, gravel, gravel, rocky mountains, the frequency-frequency grounding resistance is 2 to 3 times of the impact grounding resistance. Therefore, if a grounding body is laid in the ground as described above, if the grounding resistance measured by the grounding resistance meter does not exceed 2 to 3 times the impact resistance value of the design requirements, the design requirements can be met. Need to take measures to reduce resistance. If the soil and grounding conditions of the grounding device installation site are not analyzed, it is found that the grounding resistance meter's measured value is greater than the design requirement value, blindly adding artificial grounding body or adopting a drag reducing agent to pursue the design value must result in human and material waste. The project cost has been raised, but this phenomenon is universal.
Stainless Steel Pillow Bearing
Stainless Steel Pillow Blocks mounted units are made of corrosion resistant stainless steel and are ideal for the food handling industry, water treatment plants, and salt water desalinization plants. Balls & Rings are made out of 440 Stainless Steel, Set Screws 410 Stainless Steel, slinger & Seal Frame 302 Stainless Steel, Retainer 302 Stainless Steel, Set Screw locking Collar (SNA only) 304 Stainless Steel, Seal Silicone Rubber Lubrication H-1 Food Grade Grease Operating Temp. Range -10° F to 250° F. Housings are 304 Stainless Steel Stainless steel. They are generally the most effective solution for highly corrosive applications. They do not experience the same flaking, cracking, deformation or discoloration issues associated with other corrosion resistant housing materials. They are virtually unaffected by scrapes and can easily handle significant radial loads.
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Product Name
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Pillow Block Ball Bearing
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Material
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Stainless Steel
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Type
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UC,UK,SA,SB,UEL Series Insert bearing, UCF,UCP,UCT,UCFL,UCPA,UCFC,UCFA,UCFB,UCPH Series Insert Bearing With Pillow Block
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Size
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d:3-110mm,D:9-240mm
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Grade
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PO,P6,P5,P4,P2
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Shield
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ZZ ,2RS, OPEN
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Used For
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Mining, metallurgy, agriculture, chemical industry, textile, printing and dyeing, conveying machinery, etc.
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Warranty
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1 year
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Brand
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OEM or DFN
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Packing
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According to the buyer requests for packaging
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Specifition
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Bearing No.
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Weight
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UCP
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UCF
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UCT
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UCFL
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UCPA
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UCFA
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UCPH
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KG
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UC201
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0.16
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UCP201
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UCF201
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UCT201
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UCFL201
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UCPA201
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UCFA201
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UCPH201
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UC202
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0.18
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UCP202
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UCF202
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UCT202
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UCFL202
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UCPA202
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UCFA202
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UCPH202
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UC203
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0.19
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UCP203
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UCF203
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UCT203
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UCFL203
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UCPA203
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UCFA203
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UCPH203
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UC204
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0.21
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UCP204
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UCF204
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UCT204
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UCFL204
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UCPA204
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UCFA204
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UCPH204
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UC205
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0.29
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UCP205
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UCF205
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UCT205
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UCFL205
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UCPA205
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UCFA205
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UCPH205
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UC206
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0.31
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UCP206
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UCF206
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UCT206
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UCFL206
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UCPA206
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UCFA206
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UCPH206
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UC207
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0.49
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UCP207
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UCF207
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UCT207
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UCFL207
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UCPA207
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UCFA207
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UCPH207
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UC208
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0.63
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UCP208
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UCF208
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UCT208
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UCFL208
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UCPA208
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UCFA208
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UCPH208
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UC209
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0.68
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UCP209
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UCF209
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UCT209
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UCFL209
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UCPA209
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UCFA209
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UCPH209
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UC210
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0.79
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UCP210
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UCF210
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UCT210
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UCFL210
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UCPA210
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UCFA210
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UCPH210
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UC211
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1.04
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UCP211
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UCF211
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UCT211
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UCFL211
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UCPA211
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UCFA211
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UCPH211
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UC212
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1.47
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UCP212
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UCF212
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UCT212
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UCFL212
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UCPA212
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UCFA212
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UCPH212
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UC213
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1.73
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UCP213
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UCF213
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UCT213
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UCFL213
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UCPA213
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UCFA213
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UCPH213
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UC214
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2.08
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UCP214
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UCF214
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UCT214
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UCFL214
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UCPA214
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UCFA214
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UCPH214
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UC215
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2.25
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UCP215
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UCF215
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UCT215
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UCFL215
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UCPA215
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UCFA215
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UCPH215
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UC216
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2.86
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UCP216
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UCF216
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UCT216
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UCFL216
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UCPA216
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UCFA216
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UCPH216
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UC217
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3.42
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UCP217
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UCF217
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UCT217
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UCFL217
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UCPA217
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UCFA217
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UCPH217
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UC218
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4.4
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UCP218
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UCF218
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UCT218
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UCFL218
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UCPA218
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UCFA218
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UCPH218
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Bearing No.
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Weight
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UCP
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UCF
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UCT
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UCFL
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UCPA
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UCFC
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UCPH
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kg
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UC 305
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0.45
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UCP305
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UCF305
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UCT305
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UCFL305
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UCPA305
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UCFC305
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UCPH305
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UC 306
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0.57
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UCP306
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UCF306
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UCT306
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UCFL306
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UCPA306
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UCFC306
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UCPH306
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UC 307
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0.72
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UCP307
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UCF307
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UCT307
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UCFL307
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UCPA307
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UCFC307
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UCPH307
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UC 308
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1.00
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UCP308
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UCF308
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UCT308
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UCFL308
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UCPA308
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UCFC308
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UCPH308
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UC 309
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1.30
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UCP309
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UCF309
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UCT309
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UCFL309
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UCPA309
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UCFC309
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UCPH309
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UC 310
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1.67
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UCP310
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UCF310
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UCT310
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UCFL310
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UCPA310
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UCFC310
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UCPH310
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UC 311
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2.10
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UCP311
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UCF311
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UCT311
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UCFL311
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UCPA311
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UCFC311
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UCPH311
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UC 312
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2.62
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UCP312
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UCF312
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UCT312
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UCFL312
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UCPA312
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UCFC312
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UCPH312
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UC 313
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3.19
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UCP313
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UCF313
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UCT313
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UCFL313
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UCPA313
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UCFC313
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UCPH313
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UC 314
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3.88
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UCP314
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UCF314
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UCT314
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UCFL314
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UCPA314
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UCFC314
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UCPH314
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UC 315
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4.68
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UCP315
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UCF315
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UCT315
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UCFL315
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UCPA315
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UCFC315
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UCPH315
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UC 316
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5.50
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UCP316
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UCF316
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UCT316
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UCFL316
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UCPA316
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UCFC316
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UCPH316
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UC 317
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6.67
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UCP317
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UCF317
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UCT317
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UCFL317
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UCPA317
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UCFC317
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UCPH317
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UC 318
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7.50
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UCP318
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UCF318
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UCT318
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UCFL318
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UCPA318
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UCFC318
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UCPH318
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UC 319
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8.80
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UCP319
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UCF319
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UCT319
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UCFL319
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UCPA319
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UCFC319
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UCPH319
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UC 320
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10.94
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UCP320
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UCF320
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UCT320
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UCFL320
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UCPA320
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UCFC320
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UCPH320
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UC 322
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14.50
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UCP322
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UCF322
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UCT322
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UCFL322
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UCPA322
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UCFC322
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UCPH322
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UC 324
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18.75
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UCP324
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UCF324
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UCT324
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UCFL324
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UCPA324
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UCFC324
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UCPH324
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Pillow Block Bearing,Stainless Steel Bearing,Stainless Steel Pillow Block Bearing,Steel Pillow Block Bearing
Shijiazhuang Longshu Mechanical & Electrical Equipment Trading Co., Ltd. , https://www.longsbearings.com