BTTZ is a cable that uses mineral (magnesium oxide) as an insulating material. It is usually composed of copper conductor, mineral insulation, copper sheath, and contains no organic materials. It is non-combustible, smoke-free, non-toxic and fire-resistant.
Copper core copper sheathed magnesium oxide insulated heavy-duty fire-resistant cable (also called heavy-duty mineral insulated cable) is a kind of insulating material with a seamless copper tube sheath in the outer layer and magnesium oxide crystal powder filled in the middle. The conductor is a single strand A new type of cable consisting of copper rods.
It has high temperature resistance, fire resistance, explosion protection, non-combustion (continuous long-time operation at 250 ° C, and short-term operation at 30 ° C under extreme conditions), large current carrying capacity, small outer diameter, high mechanical strength, and long service life. Long, generally does not require the characteristics of a separate ground wire.
Better fire resistance is one of the superior characteristics of mineral insulated cables. Therefore, when choosing mineral insulated cables, people associate them with fire resistance requirements, and they are also accustomed to calling them "fire-resistant cables". In fact, it has a wide range of applications, which can be applied at sea, land, indoor and outdoor, above ground and underground; especially in historic buildings, super high-rises, hotels, shopping malls, hospitals, airports, television stations, communications hub projects, ships , Theaters, ancient buildings, subways, civil air defense projects, crowded public places, and fire-prone dangerous places (such as natural gas plants, chemical plants, refineries, offshore oil platforms) are widely used. It can also be applied to places with high ambient temperature, such as power plants and steel plants. For special environments, such as anti-electromagnetic interference, animal bite resistance, waterproofing, and nuclear power plants, food processing plants with high sanitary requirements are also used.
Mineral insulated cables can be used in the above places for trunks, branch and distribution systems, general lighting, internal wiring of machinery and equipment, intrinsically safe circuits, emergency lighting, emergency backup power supplies, information and data transmission and control lines for
fire-related equipment, and potential explosions Dangerous area, etc.
Due to the special structure of the BTTZ cable, it has some characteristics different from traditional cables. These advantages and disadvantages should attract attention in selection and construction.
1.The main advantages of BTTZ cable
1) Completely fireproof BTTZ cable itself does not burn at all, and it will not cause a fire. Even in the case of flame grilling, as long as the flame temperature is lower than the melting point of copper, the cable can continue to be used without replacement after the flame is eliminated. Does not produce toxic fumes and gases when grilled by flames.
2) Strong overload protection When the line is overloaded, the cable will not be damaged as long as the heat does not reach the melting point of copper. Even if the breakdown occurs instantaneously, the magnesium oxide crystals at the breakdown place will not form carbides. After the overload is eliminated, the cable performance will not change, and normal use can still be continued.
3) High working temperature Because the melting temperature of the insulating magnesium oxide crystal is much higher than the melting temperature of copper, the normal working temperature of the cable can reach 250 ℃, and it can continue to operate at a temperature close to the melting point of copper at 1083 ℃ in the short term.
4) Good anti-corrosion and explosion-proof performance. Because the seamless copper tube is used as the sheath, the BTTZ cable has the performance of waterproof, moisture, oil and some chemical substances. Copper tube has considerable mechanical strength and therefore has better explosion-proof performance.
5) Long service life All BTTZ cables are made of inorganic materials, so there is no insulation aging, and the service life can reach more than three times that of ordinary cables.
6) Greater flexibility in laying BTTZ cables can be laid on the bridge with ordinary cables, and can also be exposed with a special bracket, which is more flexible than ordinary cables, saves the investment in cable trays, and can reduce the overall cost of the project.
2.The main disadvantages of BTTZ cable
1) High investment cost Because the outer sheath of BTTZ cable is made of seamless copper pipe, the overall copper content is much more than that of ordinary cable. At the same time, the construction process of BTTZ cable requires higher equipment than ordinary cable. That's about 30% higher.
2) The joint is susceptible to moisture at the joint. The insulating layer is composed of mineral magnesium oxide, which easily reacts with moisture in the air to form conductive magnesium hydroxide. In the construction of cable heads, when the cable ends are stripped of bare conductors, the insulation resistance of the cable is generally above 10MΩ, but if the cable head is not completed within 1 hour, the insulation resistance can drop below 10MΩ, or even drop to 0.5. In the case of below MΩ, if the outer layer is not noticed and the seal and moistureproof treatment are not found in time, the insulation value will drop quickly and gradually drop to 0, which will make the cable unusable.
3) The construction is difficult. The hardness of BTTZ cables is higher than that of ordinary cables, and the weight is about twice that of ordinary cables. It is not easy to achieve a parallel and neat look and feel when laying, and the length of the line and the number of joints make it difficult to find fault points. Large, difficult to bend in and out of the wiring box and the bridge.
4) BTTZ cables with a large amount of construction work are single-core cables with specifications exceeding 35mm. For example, a 70mm cable, ordinary cables need only 3 × 70 2 × 35 five conductors in the same outer sheath, and To reach the same specifications of BTTZ, three 70mm and one 35mm single-core cables are assembled. The single-core cable has a short delivery length. For example, a 240mm cable has a delivery length of 69 meters. If the laying distance is long, a large number of intermediate joints will be produced, which will double the construction workload.
In view of the above disadvantages, it is particularly important to control the construction quality of BTTZ cables. According to the construction experience of nearly 15,000 meters of BTTZ cables in the comprehensive building project of the Party School of the Central Committee of the Communist Party of China, the author believes that the quality of construction should be controlled from the following aspects.
3.Quality control during BTTZ cable laying
1) Matters to be paid attention to when laying the cable BTTZ cable has a large hardness, so crossover should be avoided as much as possible during the laying. Before laying, the “cable laying plan” should be drawn according to the design drawing, and the number, specifications, length, direction, intermediate joint position and the distance of crossing with other pipes of the cables should be carefully checked. Laying should be carried out on a special cable pay-off stand. Be careful when removing the packaging. Do not let the knife penetrate the packaging layer to avoid damaging the copper sheath. Allow sufficient operating margin when handling the intermediate joints and terminal heads. At the corners, branches, etc. of steel pipes and bridges, the transition should be smooth and even in accordance with the pre-arranged order to avoid crossing and overlapping.
2) Cable loops should be numbered and pasted at the end and start of each loop, at each intermediate joint, through the wall hole, etc. The number and phase order of each loop should be marked by hanging signboards or sticky signs to avoid multiple loops. 2. There are too many joints to distinguish, and there is a loop and phase sequence connection error.
3) Reduce eddy current loss In practical applications, BTTZ cables are mostly composed of single-core cables, so it is easy to generate induced eddy currents in cable fixing hardware. If the eddy current is too large, it will not only generate a large amount of eddy current loss, but also accelerate the aging speed of the fixing fixtures of the cable. Therefore, in the actual construction process, eddy current should be avoided or minimized. Cables are usually tied at the site with non-metallic fixtures, while reasonable cable phase arrangements are used to minimize eddy current generation.
4) Cable moisture-proof When you straighten the cable, be careful to avoid damaging the copper sheath of the cable during the straightening process. Before laying, the insulation value and the end and copper sheath should be tested carefully for nakedness and scratches. It should be sealed in time after discovery, and paraffin wax is generally available on site as a temporary sealing material. The remaining sawn-off place when putting out the wire should also be sealed immediately. Make sure that moisture in the air does not enter the insulation layer. If the resistance value in the test does not meet the requirements of the specification, it shall not be laid, and it shall be laid after the dehumidification treatment is performed to meet the requirements.
5) Bending of cables: T-bends of bridges, L-bends, through-wall holes, electrical shafts, and access to power distribution cabinets. When laying large, narrow spaces, pay attention to the application of force when laying. When handling bends, To use a special bending tool provided by the manufacturer, perform cold bending according to the bending method and strength of the installation instructions. Do not use ordinary tools or manual forced bending to avoid damaging the copper sheath of the cable during operation.
6) Setting of expansion ring Due to the high hardness of BTTZ cable, in order to avoid damage to the cable caused by temperature changes, expansion rings should be installed on permitted occasions when the cable is laid more than 70 meters in straight line, usually S-type expansion ring is used. In addition, an expansion ring should also be installed at the connection between the cable and the motor, water pump, fan and other equipment with vibration. The Ω-type expansion ring is usually used here. S-type expansion ring Ω-type expansion ring
7) The protection after cable laying is in the same bridge frame. Cables with different phase sequence in the same circuit should be laid at the same time. After the laying, the bridge cover plate should be covered in time for protection to prevent cables from being damaged by tools and building materials during other professional construction processes. The outer sheath of the cable is damaged due to impact or burns caused by impact or welding sparks.
4. Quality control of BTTZ cable head production The production of cable head is an important part of the cable laying project. Because BTTZ cable has its distinct characteristics compared to ordinary cables, the insulation resistance has been shaken after the laying. 80% of the low cases are caused by quality defects in the production of cable heads, so the quality control in the production of cable heads is even more important. There are two types of cable heads for BTTZ cables: cable terminal heads and cable intermediate connectors. The component components of these two types of cables can be provided as a set by the cable manufacturer, as shown in the following figure: Before making, you should also pay attention to the following aspects: The cable insulation resistance shake value should meet the requirements of the specification, otherwise dehumidification operation should be performed; be careful when stripping the copper sheath to avoid damaging the core; use clean after removing the magnesium oxide powder The remaining powder on the core is cleaned with clean cotton yarn. It is strictly forbidden to blow with the mouth to prevent the cable from getting wet. The cable joints must be made with qualified silicone, sealing paste and special construction tools; Start at the place and work in turn towards the load terminal.
1) Quality control of BTTZ cable intermediate connector in the manufacture of intermediate joints includes 1 brass tube, 2 brass bunch heads and 2 brass sealing cups. Main points for quality control: When making the wire, thoroughly remove the powder and debris to avoid affecting the sealing effect. When using sealing cream or silicone, there should be no contamination and contamination. When twisting the sealing cup, the wire should be accurate and the force should be uniform, and it should not be dry. Special attention should be paid in the production of the multi-core cable middle head: carefully check the corresponding phases of the cores before the connector to avoid phase connection errors, the connection parts of the cores of different phases in the connector should be staggered, and they should be kept secret when filling the insulation seal In order to ensure the insulation between the phases, as shown in the following figure: cross-section view of the middle connector of the four-core cable
2) Manufacture of the terminal head The BTTZ cable terminal connector includes a brass sealing cup, a brass bundle head, and a ground terminal. The manufacturing process is basically the same as that of the intermediate connector. The main points of quality control are: before sealing, the cable must be measured with a 500V shake meter, and its resistance value must be greater than 100MΩ before sealing. The sealing process should ensure that the sealing packing is clean and free of debris, and tightly sealed. For multi-core BTTZ cable terminals, special attention should be paid to the spacing and insulation between the cores, between the cores and the copper sheath during installation. After the BTTZ cable is installed in the terminal head and the middle head, an insulation shake test should be performed again. In the actual measurement, the pointer of the megohmmeter should point to ∞, which indicates that the cable line performance is good. If the resistance value decreases during the measurement, the fault point should be found. Usually the fault point is at the joint. At this time, the joint should be removed and dehumidified until its insulation resistance is restored, and then the joint is remade.
5. Quality Control of Moisture and Dehumidification of BTTZ Cables As the magnesium oxide crystal powder as an insulator in BTTZ cables is a material with a very high moisture absorption rate, once the outer sheath is slightly damaged during the cable transportation and laying process, it will cause oxidation. The magnesium crystal powder is damp, which causes the resistance to be greatly reduced. In addition, the cable must be cut in the middle of the cable and the terminal connector is made. At this time, the magnesium oxide crystal powder will also be wet. Corresponding measures should be taken at the scene for these situations. There are usually two situations:
1) The dehumidification method of the cable wet end is at the cable end. Generally, the moisture intrusion range is 300-400mm. However, if the cable end is not exposed to air for a long time, the moisture can invade about 1m. At this time, the tail end of the cable can be tilted upwards with a gasoline torch to dehumidify it from about 1m away from the end of the cable, so that the moisture in the cable's magnesium oxide gradually radiates from the inside out, and the flame should move slowly during operation To achieve uniform heating, pay attention to control the surface temperature of the copper sheath within 200 ℃, to eliminate moisture. If the resistance of the measured insulation resistance does not increase much after baking at one end, apply the same method to continue baking the other end of the cable until the insulation resistance is acceptable.
2) Finding and repairing the damage of the copper sheath of the cable During the construction, the copper sheath of the middle part of the cable is broken. The method of finding and repairing it is as follows: first, if the insulation layer at both ends of the cable is dehumidified by the flame, the insulation resistance does not increase or Only when the change is small, it can be determined as the middle section failure. You can bake the full length of the cable with gentle flames and use a multimeter high resistance file to find it. The specific method is: adjust the multimeter to the high resistance range, one end of the meter head is connected to the cable core, and the other end is connected to the outer sheath. Use a torch to bake and heat the cable along the first end of the cable, and observe the change in the pointer reading. When the reading varies greatly in a certain part, the fault point can be determined here. Then, cut the cable and bake it with gentle heat to dehumidify. After dehumidification, check whether the insulation value is qualified. Use the intermediate connector to connect the cables after the resistance value is qualified. When dehumidifying, pay attention to changes in heating temperature and resistance value. At the same time, the resistance change curve of the damped cable insulation layer at 200 ° C. At the same time, in the selection and use of BTTZ cables, some related issues should be noted: First, BTTZ cables should be used in important buildings, such as high-rise buildings, airports, and docks. The key parts of railway stations, such as electrical control circuits for fire control centers, emergency lighting, fire pumps, standby power supplies, smoke exhaust fans, and fire alarm devices, are conducive to reducing project costs, shortening construction periods, and facilitating construction and maintenance.
Secondly, the terminal end of the BTTZ cable is longer and more difficult to bend than ordinary cables, so full consideration should be given to allowing sufficient operating space when customizing the distribution box. When the cable is laid, special tools should be used for cold work to prevent damage to the copper jacket. The production of terminal heads and intermediate joints shall strictly follow the operating procedures, keep the operation site tidy and free of contamination from sealing glue, sealing paste, and silicone, and ensure that its insulation resistance meets the requirements of the specification.
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