Views: 0 Author: Site Editor Publish Time: 2024-11-20 Origin: Site
1. Test method
The test method and performance criteria are based on the methods and criteria corresponding to the B1 grade combustion performance level in the GB 31247-2014 "Classification of Combustion Performance of Cables and Optical Cables" standard.
2. Test device
According to the GB/T 31248-2014 "Test method for flame spread, heat release and smoke production characteristics of cables or optical cables under fire conditions", the B1 grade cable test device consists of a combustion chamber, an air supply system, a standard ladder, an ignition source, etc. The test device refers to the corresponding content in the standard. ✎Sheath material selection, testing and verification 1
Sheath material selection, testing and verification
1. B1 grade cable model specifications and voltage levels
Among B1 grade cables, 0.6/1kV power cables and 450/750V control cables are commonly used products with a wide range of uses. Compared with 0.6/1kV power cables, 450/750V control cables have a higher proportion of non-metallic materials. Therefore, the difficulty of the B1-level combustion test of 450/750V control cables is generally higher than that of 0.6/1kV power cables. Based on this, this work takes the commonly used specifications of control cables WDZB1-KYJY-450/750 4x1.5 as an example for verification, and its structural schematic diagram is shown in Figure 1.
2. Sheath material test
According to the B1-level combustion performance criterion and the characteristics of the cable burning from the outside to the inside and upward when burning vertically, the outermost sheath material in the cable structure plays a vital role in the combustion performance of the B1-level cable. In order to make the comparison meaningful, this test conducted a B1-level combustion performance test on two groups of WDZB1-KYJY-450/750 4×1.5 cables with the same structure, model and specification. The first group of cables are copper conductor, cross-linked polyethylene insulation, two layers of high flame retardant tape and conventional low-smoke halogen-free flame retardant polyolefin sheath from inside to outside. The only difference between the second group of cables is that the sheath is a low-smoke halogen-free flame retardant polyolefin sheath with burning crust. The test data is shown in Table 1.
The results show that when the cable uses a low-smoke halogen-free flame retardant polyolefin sheath material with burning crust, its flame spread length, peak heat release rate, total heat release within 1200s of fire, combustion growth rate index, peak smoke production rate, total smoke production within 1200s of fire, transmittance and vertical flame spread are all better than conventional low-smoke halogen-free flame retardant polyolefin sheath materials.
3. Selection and control of B1-level sheath materials
Through the experimental analysis in the previous section, when the sheath material has the characteristics of combustion crusting, the B1-level combustion performance index of the cable can be significantly improved. However, since the technical requirements of this index are not clearly defined in the standards GB 31247-2014 and GB/T 32129-2015, there is a lack of means to control the B1-level combustion performance from the source.
To solve this problem, this work, based on the premise that the basic performance of the raw materials meets the national standard GB/T 32129-2015, draws on the principle of single vertical combustion of cables, designs a "combustion crusting test device" for B1-level sheath materials, conducts multiple combustion tests on the two different sheath materials in the previous section, and obtains a B1-level sheath material selection and control method, as follows:
1) Sample preparation. According to the 5.2 molding method in the national standard GB/T 32129-2015, the cable material particles are plasticized and sliced on the plasticizer, placed in the hydraulic press, preheated for 10 minutes without pressure, and then pressurized and maintained for 5 minutes. The processing temperature of the plasticizer and hydraulic press is set according to the plasticization characteristics of the material to ensure that the material can be fully plasticized and not decomposed during the processing. The pressurization pressure of the hydraulic press should not be less than 15MPa, and then pressurized and cooled to room temperature. The reference length × width × thickness of the sample is 160mm × 160mm × 3mm, and then a molded knife is used to press out a rectangular specimen with a length × width × thickness of 140mm × 6mm × 3mm.
2) Test conditions. The test should be carried out in an environment of (23±2)℃.
3) Test device. The test device consists of a BDP-220-A card-type pure butane blowtorch and a fixed bracket, see Figure 2.
Figure 2 Combustion crust performance test device
4) Test steps. Fix the sample vertically on the bracket, adjust the sample height so that the bottom of the sample and the lower edge of the blowtorch nozzle are basically on the same horizontal line, then adjust the blowtorch distance so that the horizontal distance between the blowtorch nozzle and the sample is 120mm, light the blowtorch, adjust the blowtorch valve to the blue flame length of 100mm, start timing, and keep it for 3 minutes.
5) Result determination. There should be no burning dripping during the test, and the flame should automatically extinguish within 5s after the fire supply is stopped. The result of combustion crusting is shown in Figure 3.
4. B1-level sheath material selection verification
In order to verify whether the selection and control method of B1-level sheath material is effective, this work selected B1-level sheath material and conventional sheath material from a certain supplier, and compared the raw materials and finished products respectively.
The basic performance test of raw materials is carried out in accordance with the national standard GB/T 32129-2015. The performance comparison of B1-level sheath material and conventional sheath material from a certain supplier is shown in Table 2.
The comparison results of the B1-level combustion test of the finished cable WDZB1-KYJY-450/750 4×1.5 using conventional sheath materials and a supplier's B1-level sheath material are shown in Table 4.
The following conclusions can be drawn from the results of Tables 2, 3 and 4:
1) The B1-level sheath material with combustion crusting function can meet the requirements of the national standard GB/T 32129-2015 in terms of tensile strength, elongation at break, cracking resistance, low temperature performance, high temperature performance and oxygen index as conventional sheath materials.
2) The finished cable corresponding to the sheath material selected according to the B1-level sheath material selection and control method designed in this work passed the B1-level combustion test specified in the national standard GB 31247-2014.
The influence of wrapping structure on combustion performance
After a large number of burning tests, it was found that WDZB1-KYJY23-450/750 4×1.5 armored cable was prone to burning dripping in the B1 level combustion test. Observation of the B1 level combustion test process found that the cable sheath would expand and form a fire protection layer when burning, but because this type of cable is equipped with a metal armor layer, the surface is relatively smooth, which makes it difficult for the sheath material to adhere to the cable core and fall off during combustion.
In order to improve this problem, under the premise of meeting the product standards, two layers of high flame retardant tape are added outside the metal armor layer to verify the influence of the wrapping structure on the B1 level combustion performance of the armored cable.
1. B1-level cable samples
This verification takes WDZB1-KYJY23-450/750 4×1.5 armored control cable as an example, and sets two groups of cable samples. The first group of cable samples has no tape structure outside the armor layer; the second group of cable samples has two layers of high flame retardant tape structure outside the armor layer. There is no difference between the two groups of samples.
2. Burning test with different wrapping structures
The burning test results of WDZB1-KYJY23-450/750 4×1.5 cable are shown in Figure 4.
Figure 4 Burning test results of cables with different wrapping structures
As shown in Figure 4, the sheath of the cable without tape structure outside the armor layer falls off obviously after the B1-level burning test, exposing the metal armor layer, and there are burning drips at the bottom of the test box, while the cable with tape structure outside the armor layer has no burning drips after the B1-level burning test. It can be seen that setting a high flame retardant tape outside the armor layer can significantly improve the problem of dripping from the burning of B1-class armored cables.
This work studies the influence of the burning crusting sheath material of WDZB1-KYJY-450/750 4×1.5 cable and the outer wrapping structure of WDZB1-KYJY23-450/750 4×1.5 cable armor on the B1-class combustion performance. It is found that compared with conventional sheath materials, the use of sheath materials with burning crusting function can significantly improve the B1-class combustion performance of cables; compared with cables without flame retardant tape outside the metal armor, adding a high flame retardant tape outside the metal armor can significantly reduce the sheath dripping problem during the B1-class combustion test of cables. In this work, a "combustion encrustation test device" for B1-class sheath materials was designed, and the B1-class combustion performance was verified through the sheath material combustion encrustation test. The results show that the sheath material combustion encrustation has a certain influence on the B1-class combustion performance of the cable, which provides a reference for the selection of B1-class sheath materials and improves the current situation where there is no standard for B1-class sheath materials.
