A cable ampacity correction design method based on loop segment isolation

Abstract

The application discloses a cable ampacity correction design method based on loop segmentation isolation and belongs to the technical field of building electrical power supply and distribution. In view of the problem that 0.7 ampacity correction coefficient must be used when the existing residential community distribution room outlet bridge is densely laid, leading to resource waste, the application divides the cable laying path into a front section and a rear section; the front section adopts a bridge or a cable trench to lay single-layer or multi-layer cables in a close arrangement; the rear section adopts a bridge to lay single-layer cables with a cable spacing of 3d, wherein d is the outer diameter of a single cable; a circuit breaker is arranged on the wall side of the distribution room, the front section cable is connected to the input end of the circuit breaker, and the rear section cable is connected to the output end of the circuit breaker; the rear section cable is selected according to the rated ampacity, and the ampacity correction coefficient is increased from 0.7 to 1.0. The application makes the rear section cable operate in a good heat dissipation condition through segmented and differentiated laying and loop isolation, exempts from the 0.7 coefficient, reduces the cable section by 1-2 grades, reduces the engineering cost by 20%-30%, and has strong engineering adaptability.

Description

Technical Field

[0001] This invention belongs to the field of building electrical power supply and distribution technology, specifically relating to a cable current carrying capacity correction design method based on circuit segmentation and isolation. Background Technology

[0002] According to the national standard GB50217-2018 "Design Standard for Cables in Power Engineering", when multiple parallel cables are installed on cable trays without spacing, the current carrying capacity correction factor is 0.7. Based on this, the Shandong Provincial Engineering Construction Standard DB37 / T5061-2016 "Construction Standard for Power Supply and Distribution Facilities in Residential Communities" further clarifies the requirement that when cables are densely laid side-by-side from the distribution room outlet to the external cable tray, the current carrying capacity of low-voltage cables should be uniformly reduced by a correction factor of 0.7.

[0003] In current engineering practice, multiple cables are closely arranged and in contact with each other within the cable tray, resulting in densely packed cables, high tray fill rate, and poor heat dissipation. The design forces a current-carrying capacity correction factor of 0.7, which in turn leads to the selection of larger cable cross-sections, increased copper usage, significantly higher engineering costs, and obvious waste of non-ferrous metal resources.

[0004] At the same time, the actual load of residential areas is intermittent and dispersed, and the long-term load rate of cables is low. The conservative correction factor of 0.7 is seriously inconsistent with the actual operating conditions.

[0005] In summary, existing technologies suffer from problems such as oversized cable selection, high engineering costs, resource waste, and low utilization of cable current carrying capacity. There is an urgent need for an optimized design scheme that can meet safety standards while reasonably eliminating or reducing correction coefficients. Summary of the Invention

[0006] To address the problem of excessively large cable cross-sections, high costs, and resource waste caused by the dense laying of cable trays in power distribution rooms in residential communities, which necessitates the use of a fixed correction coefficient of 0.7, this invention proposes a cable current-carrying capacity correction design method based on circuit segmentation and isolation. Without reducing the safety margin, this method eliminates the 0.7 current-carrying capacity correction coefficient for low-voltage cables, thereby reducing cable cross-sections, saving copper materials, lowering costs, and improving cable utilization.

[0007] A cable current-carrying capacity correction design method based on loop segment isolation includes the following steps: S1: Divide the cable laying path from the low-voltage outgoing cabinet in the power distribution room to the load end into a front section and a rear section. The front section is from the low-voltage outgoing cabinet to the wall side of the power distribution room, and the rear section is from the wall side of the power distribution room to the load end. S2: Cable trays or cable trenches are used in the front section for single-layer or multi-layer dense cable laying, with cables closely arranged and in contact with each other in the front section of the cable trays or cable trenches; S3: Cable trays are used for single-layer cable laying in the latter section. The cable spacing in the cable tray in the latter section is 3d, where d is the outer diameter of a single cable. S4: Install a circuit breaker independently for each power supply circuit on the side of the distribution room wall, connect the front cable to the input terminal of the circuit breaker and the rear cable to the output terminal of the circuit breaker to achieve independent electrical segmentation isolation of a single circuit; S5: The cross-section of the downstream cable is selected according to the rated current carrying capacity, and the current carrying capacity correction factor of the downstream cable is increased from 0.7 to 1.0.

[0008] Furthermore, in the densely laid section at the front end of the power distribution room, the cable selection and current carrying capacity verification are still carried out in accordance with the specifications using a current carrying capacity correction factor of 0.7 to ensure compliance and safety of the densely laid indoor working conditions.

[0009] Furthermore, the fill rate of the rear section of the cable tray is ≤40%.

[0010] Furthermore, the circuit breaker is installed on the outside or inside of the distribution room wall, the rated current of the circuit breaker matches the maximum load current of the downstream circuit, and the ultimate breaking capacity of the circuit breaker meets the short-circuit current parameter requirements of the regional power distribution system.

[0011] Furthermore, the front section is the main inlet section from the power distribution room to the circuit breaker, and the rear section is the branch power supply section from the circuit breaker to the terminal load.

[0012] Furthermore, the method is applicable to low-voltage power supply and distribution projects in residential communities.

[0013] The beneficial technical effects of this invention are as follows: The 0.7 correction factor is compliantly eliminated: Through a segmented, differentiated design of narrow cable trays (with single or multiple layers of tightly packed, interconnected cables) and wide cable trays (cable spacing ≥ 3d), coupled with circuit breakers installed on the wall side of the distribution room, the downstream cables operate under conditions of wide spacing, low fill rate, and independent isolation, ensuring good heat dissipation and thus achieving the compliant elimination of the 0.7 correction factor. This fully complies with the core principle of GB50217-2018—adjusting the correction factor based on laying density and spacing—without reducing the system's safety margin.

[0014] Significant savings in engineering costs and copper resources: After eliminating the 0.7 coefficient in the later stage, the cable can be directly selected according to the rated current carrying capacity. Under the same load conditions, the cross-section can be reduced by 1 to 2 specifications. Under the premise of meeting power supply safety, current carrying capacity, and voltage drop specifications, the overall cable project cost is reduced by 20% to 30%.

[0015] Improve the utilization rate of cable current carrying capacity: Break away from the traditional design inertia of dense laying and forced capacity reduction throughout the entire circuit, so that the actual current carrying capacity of the downstream cable can be fully utilized, avoiding resource waste caused by conservative coefficients.

[0016] Improved system safety margin: The wide cable tray at the rear section ensures good heat dissipation conditions, the circuit breaker has overload, short circuit and leakage protection functions, and can be independently disconnected without affecting other circuits when a single circuit fails. The overall power supply reliability and safety margin are no less than or even higher than the traditional 0.7 coefficient scheme.

[0017] This invention is applicable to conventional enclosed cable trays, trough-type cable trays, and tray-type cable trays, without requiring modification of the cable tray structure, alteration of the cable tray opening form, or the addition of cable equidistant fixing clamps. Attached Figure Description

[0018] Figure 1 A flowchart of a cable current-carrying capacity correction design method based on loop segmentation isolation; Detailed Implementation

[0019] The specific embodiments of the present invention will be further described below with reference to specific examples: Example: Outgoing line project of power distribution room in a residential community in Shandong Province; This embodiment takes a newly built residential community as an example. The power distribution room is equipped with a low-voltage outgoing cabinet, from which 12 power supply circuits are led out and laid along the same enclosed cable tray to the power distribution points of each building. The space inside the power distribution room is limited, and the width of the outgoing cable tray can only accommodate a single or multiple layers of tightly arranged cables. After exiting the wall and entering the underground garage, the space for laying the cable tray is ample, and the width of the cable tray can be greatly expanded.

[0020] like Figure 1 As shown, the specific implementation steps are as follows: Step 1: Laying the first section of narrow cable trays (inside the power distribution room); Inside the distribution room, from the low-voltage outgoing cabinet to the wall side of the distribution room, 12 YJV-0.6 / 1kV cables are laid. The cables in the upstream cable tray are tightly packed without spacing, with the 12 cables laid in single or multiple layers side-by-side in a dense arrangement, resulting in a high fill rate and generally poor heat dissipation. This section still retains a 0.7 current-carrying capacity correction factor for safety verification according to current specifications to ensure that the upstream section through the wall meets the standard requirements.

[0021] Step 2: Install circuit breakers at the boundary nodes; An independent circuit breaker is installed for each cable, either inside or outside the wall of the distribution room. The upstream cable is connected to the input terminal of the circuit breaker, and the downstream cable is connected to the output terminal. The rated current of the circuit breaker matches the maximum load current of the downstream circuit, and the ultimate breaking capacity of the circuit breaker meets the short-circuit current parameter requirements of the area power distribution system.

[0022] Step 3: Laying the rear wide cable tray (outside the wall); From the wall side of the distribution room to the load branches of each building, the cable spacing in the latter section of the cable tray is 3d, the cable tray fill rate is ≤40%, and the heat dissipation conditions are close to those of a single exposed cable. Due to the large cable spacing, low fill rate, independent switch isolation for each cable, and no heat accumulation from group cables, the heat dissipation conditions of this section of cable are close to those of a single exposed cable. Therefore, the 0.7 correction factor for the current carrying capacity is cancelled, and the rated current carrying capacity is calculated directly.

[0023] Step 4: Remove the 0.7 coefficient and select the model based on the rated current carrying capacity. Because the rear wide cable tray has multiple good heat dissipation and operating conditions such as cable spacing of 3d, filling rate of ≤40%, and electrical isolation of each circuit by circuit breaker, according to the provisions of Appendix D of GB50217-2018 that "the correction coefficient for parallel laying of multiple circuits is directly related to the spacing and arrangement density", the current carrying capacity correction coefficient of this section of cable is increased from 0.7 to 1.0, and the cross-section is directly selected according to the standard rated current carrying capacity.

[0024] Comparison of data between the old and new schemes (example of a 300A load circuit): Traditional conservative approach: Calculated current carrying capacity = 300 ÷ 0.7 ≈ 428.6A, select YJV-4×240; Optimization scheme of the present invention: Select the YJV-4×120 model directly based on the 300A rated current carrying capacity. The cross-section is significantly downgraded, the current carrying capacity is compliant, and the voltage drop and temperature rise all meet the specifications. The cost of copper materials in a single loop is greatly reduced, resulting in huge savings for batch projects.

[0025] Step 5: Safety verification and operational validation; The circuit breaker reliably disconnects during overload tests, and can be independently de-energized during maintenance of a single circuit without affecting the normal operation of the other 11 circuits. The overall system meets all safety requirements of GB50217-2018 and DB37 / T 5061-2016, with a safety margin no less than that of the traditional 0.7 coefficient scheme.

[0026] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A cable current-carrying capacity correction design method based on loop segment isolation, characterized in that, Includes the following steps: S1: Divide the cable laying path from the low-voltage outgoing cabinet in the power distribution room to the load end into a front section and a rear section. The front section is from the low-voltage outgoing cabinet to the wall side of the power distribution room, and the rear section is from the wall side of the power distribution room to the load end. S2: Cable trays or cable trenches are used in the front section for single-layer or multi-layer dense cable laying, with cables closely arranged and in contact with each other in the front section of the cable trays or cable trenches; S3: Cable trays are used for single-layer cable laying in the latter section. The cable spacing in the cable tray in the latter section is 3d, where d is the outer diameter of a single cable. S4: Install a circuit breaker independently for each power supply circuit on the side of the distribution room wall, connect the front cable to the input terminal of the circuit breaker and the rear cable to the output terminal of the circuit breaker to achieve independent electrical segmentation isolation of a single circuit; S5: The cross-section of the downstream cable is selected according to the rated current carrying capacity, and the current carrying capacity correction factor of the downstream cable is increased from 0.7 to 1.

0.

2. The cable current-carrying capacity correction design method based on loop segmentation isolation according to claim 1, characterized in that, In the densely laid section at the front end of the power distribution room, the cable selection and current carrying capacity verification shall still be carried out in accordance with the specifications using a current carrying capacity correction factor of 0.7 to ensure compliance and safety of the densely laid indoor working conditions.

3. The cable current-carrying capacity correction design method based on loop segmentation isolation according to claim 1, characterized in that, The fill rate of the rear section of the cable tray is ≤40%.

4. The cable current-carrying capacity correction design method based on loop segmentation isolation according to claim 1, characterized in that, The circuit breaker is installed on the outside or inside of the distribution room wall. The rated current of the circuit breaker matches the maximum load current of the downstream circuit, and the ultimate breaking capacity of the circuit breaker meets the short-circuit current parameter requirements of the regional power distribution system.

5. The cable current-carrying capacity correction design method based on loop segmentation isolation according to claim 1, characterized in that, The front section is the main inlet section from the power distribution room to the circuit breaker, and the rear section is the branch power supply section from the circuit breaker to the terminal load.

6. The cable current-carrying capacity correction design method based on loop segmentation isolation according to claim 1, characterized in that, The method is applicable to low-voltage power supply and distribution projects in residential communities.

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