What are the methods for calculating the outer sheath of mineral-insulated fire-resistant cables?
Release date:
2022-08-19 10:14
Source:
Mineral-insulated fire-resistant cable Use heat-resistant, non-combustible inorganic minerals as the insulation material. The outer sheath of mineral-insulated cables boasts excellent tensile strength—capable of withstanding pulling forces while also allowing the cable to pass through structural layers. So, what are the calculation methods for determining the specifications of the outer sheath in mineral-insulated fire-resistant cables?
1. Mineral-insulated fire-resistant cables use heat-resistant, non-combustible inorganic minerals as the insulation material, and their outer sheath boasts excellent tensile strength. Not only can the cable itself withstand significant pulling forces, but its structural design and material selection also enable it to endure even greater tension, further enhancing its fire protection performance. Even when exposed to flames, these cables continue to function reliably without interruption, and they are halogen-free. Additionally, they feature a green insulation layer and sheath, ensuring that no corrosive gases are emitted during use—and neither toxic fumes nor harmful gases are produced during manufacturing or combustion.
2. Sometimes, certain issues may arise. The calculated nominal diameter doesn't always match the actual size of the cable produced. In edge cases, even a slight deviation in the calculated diameter—resulting in the sheath thickness not aligning with the actual diameter—can raise concerns. Additionally, different manufacturers employ varying module rail dimensions and calculation methods, leading to discrepancies in nominal diameters. As a result, cables based on the same fundamental design may end up using sheaths of differing thicknesses.
3. To avoid these complications, a hypothetical design algorithm has been adopted, which disregards the wire’s shape and degree of compression. Instead, it calculates an assumed diameter based on the wire’s nominal cross-sectional area, the nominal thickness of the insulation, and the number of cable cores. In this way, the sheath thickness of low-voltage power cables—and the thicknesses of other protective layers—can be correlated with the assumed diameter through formulas or tables. The method for calculating the assumed diameter is clearly defined, ensuring that minor variations in actual manufacturing processes have no impact on the specified protective layer thicknesses. This approach standardizes the design of low-smoke, halogen-free, flame-retardant, and fire-resistant cables, allowing for the pre-calculation of protective layer thicknesses corresponding to each conductor’s cross-sectional area. Notably, the assumed diameter is used solely for determining the dimensions of the cable sheath and outer jacket.
4. Instead of using the precise calculation process required for determining the nominal diameter, the actual nominal diameter should be calculated separately. The following prescribed method for estimating the thickness of various low-voltage cable sheaths should be applied to ensure that any discrepancies arising from the separate calculation—such as the inevitable differences between the assumed conductor size and nominal diameter versus the actual diameter—are accounted for. If the thickness does not exceed 0.3 millimeters, all thickness values and diameters must be rounded to one decimal place according to the guidelines in Appendix B (e.g., cable ties wrapped in a reverse spiral around the armor). Additionally, the assumed diameter of each conductor based on its nominal cross-sectional area should be determined without considering factors like shape or compaction level.
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