1. The number of wrapping layers should generally be two with a 50% overlap, or four with a 5% overlap.
2. Choose the right fire-resistant mica tape. Based on combustion performance analysis, the following types are available: Double-sided synthetic mica tape: Best fire resistance (1000°C), recommended for key projects. Single-sided synthetic mica tape: Fair fire resistance (1000°C), recommended for key projects. Double-sided phlogopite mica tape: Slightly better fire resistance (800°C), recommended for general projects. Single-sided phlogopite mica tape: Slightly better fire resistance (800°C), recommended for general projects. Single-sided phlogopite mica tape: Slightly better fire resistance (800°C), recommended for general projects. Single-sided phlogopite mica tape: Slightly better fire resistance (800°C), recommended for general projects. Since single-film mica tapes on the market are for motors, not cables, we recommend against their use.
3. The wrapping process is also crucial; extreme care and caution are required. The production of fire-resistant wire and cable itself is not complex, but achieving fully qualified results is not. In addition to selecting the right mica tape, cable manufacturers often also need to carefully control their own tape wrapping process.
Here's some information for your reference, which I hope will be helpful:
c) Taping of Fire-Resistant Mica Tape
The tape wrapping process is crucial in the production of fire-resistant cables. Taping machines can be categorized as vertical or horizontal by structure, and high-speed or low-speed by speed. However, cable manufacturers rarely have dedicated mica tape wrapping machines. Most manufacturers utilize existing paper wrapping machines and cable twisting machines with modified wrapping heads. Regardless of the equipment used, operators must be meticulous and responsible. Under the guidance of technicians, operators must adjust and control the tape wrapping tension and angle based on test results, ensuring that tape breakage and tape leaks are avoided. The surface of the insulated core remains smooth and flat after wrapping, with no mica shedding. Joints must also be carefully prepared for the next step. It should be noted that when winding small-section conductors, the mica tape can easily fall off due to the small curvature radius of the wrap. Even if the synthetic mica tape does not fall off, it is more brittle than phlogopite and can easily break, which can easily result in substandard fire-resistant cable performance. Therefore, cable manufacturers should adjust and improve their processes based on the characteristics of their own tooling and equipment, as well as on test results and continuous experience. Strict control of this critical wrapping process will significantly improve the yield of finished products. Once wrapping is properly addressed, the impact of the other steps on fire-resistant performance will be relatively minimal. The mica-taped conductor should be evenly wound on the spool, taking care to avoid scratching the mica layer. This is especially important for fire-resistant cables with larger cross-sections, as scratches from the two conductors rubbing against each other are particularly important.
d) Number of Mica Tape Wraps and Lap Ratio
It's difficult to pinpoint the exact number of mica tape layers and the desired lap ratio. Mica tape manufacturers generally recommend two layers, with a lap ratio equal to or slightly greater than 50%. However, due to the varying material properties, model specifications, and other factors affecting cable products, the precise number of wraps or lap ratios required to pass fire resistance tests can only be determined by product designers based on multiple test results. While more wraps increase the product's pass rate, and vice versa, this approach not only wastes mica tape but also increases the outer diameter of the cable's insulation and sheath, increasing the plastic usage, raising manufacturing costs, and reducing product competitiveness. Therefore, the number of wraps should be determined based on user requirements, cable structure, process complexity, voltage rating, and test results. Furthermore, given a fixed number of wraps, the lap ratio is a crucial indicator, determining the actual number of wraps. If the overlap ratio for one wrapping layer is 50%, the actual number of wrapping layers is 2. If the overlap ratio is less than 50%, assuming a 25% overlap ratio, the actual number of wrapping layers is 1.5, meaning there are two layers in some areas and only one in others. Obviously, the areas with only one layer are more susceptible to breakdown. If two layers of mica tape are wrapped, and the overlap ratio is both less than 50%, the single layers of the inner and outer wrapping layers may overlap, resulting in only two layers of mica tape at that location, making breakdown more likely than four layers. Ensuring an exact overlap ratio of 50% is technically difficult to achieve. In actual use, bending and other factors can also cause the overlap ratio to fall below 50%. Therefore, in actual production, the optimal overlap ratio should be controlled at slightly above 50%.
e) Precautions for Mica Tape Wrapping
Since the conductor diameter of small-section power cables is small, the width of the mica tape used is relatively narrow. Under high temperature conditions, the insulation resistance of the mica tape drops sharply, making it prone to flashover (electrical flashover) in the wrapping gap. Excessive flashovers can cause a fast-acting fuse to blow, but the cause cannot be identified during analysis. In this case, in addition to increasing the number of wrapping layers, wider mica tape should be used, if production conditions permit, and the wrapping gaps should be staggered. For fire-resistant cables with larger cross-sections, this problem is not a significant issue; as long as care is taken during the manufacturing process to avoid damaging the mica tape, the qualified rate of the finished cable will be high. Because fire-resistant mica tape is easily damaged, proper storage is as important as its use. During the wrapping process, transfer and storage of fire-resistant mica tape must be protected from collisions, scratches, and abrasions that could damage the tape. Extrusion of the finished insulation should be performed as soon as possible to prevent moisture absorption. During extrusion, the guide roller radius should be minimized, and the extrusion die should be careful not to scratch the mica layer.
4. General Requirements for Fire-Resistant Mica Tape
Fire-resistant mica tape is an electrical insulating material made from glass fabric or plastic film, impregnated and bonded with powdered mica or synthetic mica, and has a melting point of 1200-1300°C. It is used as a fire-resistant insulation layer for cables to ensure continued operation for a certain period after a fire. The selection of fire-resistant mica tape should be based on actual requirements, and the selected fire-resistant mica tape should meet the fire resistance requirements of fire-resistant cables.
a) The production process for fire-resistant mica tape should comply with the International Electrotechnical Commission (IEC) 371-3-8:1995.
b) The nominal thickness of fire-resistant mica tape ranges from 0.05mm to 0.15mm.
c) Each roll of fire-resistant mica tape should be of consistent quality from beginning to end and meet standard requirements.
d) The surface of the fire-resistant mica tape should be free of defects such as bubbles, pinholes, and creases.
e) Fire-resistant mica tape is supplied to users in a flexible roll. The roll should be able to be unwound continuously and easily separated without sticking. f) Fire-resistant properties of fire-resistant mica tape: Currently, there is no fire-resistant test method for fire-resistant mica tape. However, experience has shown that fire-resistant mica tape can be used in properly designed fire-resistant cables. Fire-resistant mica tape used in properly designed cables should meet the fire resistance test requirements of GB19216-2003/IEC 60331-21:1999, "Tests for line integrity of electrical or optical fiber cables under fire conditions."
J) Storage of fire-resistant mica tape:
Fire-resistant mica tape should be sealed in plastic bags and placed in wooden or cardboard boxes.
Fire-resistant mica tape should be stored in a moisture-proof, ventilated, and dry warehouse.
Fire-resistant mica tape has a shelf life of six months from the date of manufacture. It may still be used after passing inspection and exceeding the shelf life.
