According to Xingke Hardware Network, the structure, characteristics and choice of flame-retardant cable and fire-resistant cable use ZR cable? Under what circumstances do you use an NH cable? What is the difference between applications? In recent years, in the global power system fire accidents, the proportion of power system power supply interruption due to the ignition of power cables has increased. Therefore, in power plants, substations and other places where there are a large number of electrical equipment and densely laid a large number of cables, flame-retardant cables are used to prevent the spread of fire. It is increasingly important to use fire-resistant cables to ensure uninterrupted power supply of important circuits. However, some electrical designers have unclear the concept of flame-retardant cables and fire-resistant cables, and do not understand the structure and characteristics of the two, resulting in the inability to properly design and select these two cables according to the power supply requirements, and design agents or supervision on site. The two cable installations cannot be properly guided during work. Therefore, a comprehensive and correct understanding of flame retardant and fire resistant cables is the primary condition for the correct selection of them.
1, flame retardant cable
The flame-retardant cable refers to the cable that is burned under the specified test conditions. After the test fire source is removed, the flame spreads only within the limited range, and the residual flame or residual burn can be self-extinguished within a limited time. Its fundamental characteristic is that it may be burned out in the event of a fire and cannot operate, but it can prevent the spread of fire. In layman's terms, if the cable is in a fire, it can limit the combustion to a local extent, without spreading, and keep other equipment to avoid further losses.
1.1 Structural characteristics of flame retardant cables
The structure of the flame-retardant cable is basically the same as that of the ordinary cable, except that its insulating layer, sheath, outer sheath and auxiliary materials (packing and filling) are all or partially made of flame retardant material.
1.2 Classification of flame retardant cables
According to the different flame retardant materials of cables, flame retardant cables are divided into two categories: halogen-containing flame-retardant cables and halogen-free low-smoke flame-retardant cables.
The insulating layer, the sheath, the outer sheath and the auxiliary materials (including and filling) of the halogen-containing flame-retardant cable are all or partially made of halogen-containing polyethylene (PVC) flame retardant material, and thus have good flame retardant properties. However, when the cable is burned, a large amount of smoke and halogen acid gas will be released. The halogen acid gas is corrosive to the surrounding electrical equipment, and rescuers need to wear a gas mask to get close to the scene to extinguish the fire. When the cable burns, it will cause harm to surrounding electrical equipment and rescue personnel, which is not conducive to fire fighting and rescue work, resulting in serious "secondary hazard". The insulating layer, sheath, outer sheath and auxiliary materials (band and filling) of the halogen-free low-smoke flame-retardant cable are all or partially made of halogen-free cross-linked polyethylene (XLPE) flame retardant material, which not only has Better flame retardant properties, and no halogen acid gas is released when the cable is burned, the amount of smoke generated by the cable is small, and the corrosive gas generated by the cable combustion is also compromised between the flame retardancy of the cable and the reduction of the amount of halogen acid gas. The method developed a low-halogen and low-smoke flame-retardant cable. Its halogen content is about 1/3 of that of halogen-containing flame retardant cable. The amount of smoke is also close to the recognized "low smoke" level.
According to the provisions of the "Test Method for Wire and Cable Combustion", the flame-retardant cables evaluated by the bundled burning test method are classified into categories A, B, and C3. Among them, the test conditions of the Class A cable are the most demanding, and the performance is better than that of the B and C categories. Also the most expensive. In order to achieve the flame retardancy of the cable or improve the flame retardant level of the cable, a new type of so-called "oxygen barrier layer" or "fire barrier layer" has been developed as a new type of fire-retardant tape (fire barrier) for filling or wrapping fire. The structure of the flame-retardant cable, and this cable is called "oxygen (fire) layer" cable. The main principle is to add metal hydrate components to the materials, and also realize low-halogen low-smoke, halogen-free low-smoke and flame-retardant of cables. However, the so-called "oxygen barrier" and "fire barrier" are only a means to achieve flame retardant cable. In the final analysis, it is also a flame retardant cable, and the highest flame retardant level that can be achieved is also Class A. Therefore, the flame retardant cable cannot be called an "oxygen barrier" cable, but only the "oxygen barrier" cable is a flame retardant cable.
1.3 Problems in selection
The designer should design and install the flame-retardant cable according to the specific characteristics of the flame-retardant cable, and pay attention to the following problems:
(1) Because halogen-containing flame-retardant cables (including flame-retardant cables, low-halogen and low-smoke flame-retardant cables) release corrosive halogen acid gas during combustion, which greatly hinders fire fighting work and delays fire fighting time and exacerbates fire spread. In public places with high population density, we should try to design halogen-free low-smoke flame-retardant cables. Any type of flame retardant cable can be used in the work area where the population density is small.
(2) Flame-retardant cables are classified into categories A, B, and C3. Among them, the performance of Class A flame-retardant cables is better than those of Class B and C, and the price is also the most expensive. Designers should indicate the type of flame retardant cable when ordering.
(3) The categories A, B, and C of the flame-retardant cable cannot be distinguished from the appearance. Only the manufacturer guarantees the supply. Therefore, the cable manufacturer should be carefully selected when designing and selecting.
(4) Compared with halogen-containing flame-retardant cables, halogen-free low-smoke flame-retardant cables have the advantages of low corrosion and low smoke, but the electrical properties and mechanical properties are significantly reduced. Therefore, halogen-free low-smoke flame retardant is used in cable laying. The cable should have a larger bend radius than the halogen-containing flame retardant cable.
(5) When designing the cable laying, it is not advisable to lay the non-flame retardant cable and the flame-retardant cable side by side. It is not appropriate to lay the flame-retardant cables of different flame retardant categories side by side.
2, fire resistant cable series
Refractory cable means that under the specified test conditions, the sample is burned in the flame and can maintain normal operation performance for a certain period of time. Its fundamental characteristic is that the cable can still maintain the normal operation of the line for a period of time under burning conditions. In layman's terms, in the event of a fire, the cable will not burn at a time, and the circuit is safer. Therefore, the main difference between the fire-resistant cable and the flame-retardant cable is that the fire-resistant cable can maintain a normal power supply for a period of time in the event of a fire, and the flame-retardant cable does not have this characteristic. This property determines that fire-resistant cables play an important role in modern urban and industrial buildings, because once a fire occurs, the system's power supply loops such as control, monitoring, guidance, and alarm must be maintained. Therefore, the cable is mainly used in emergency power supply to the user's fire-fighting equipment, fire alarm equipment, ventilation and smoke exhaust equipment, guide lights, emergency power outlets, emergency elevators and other power supply circuits.
2.1 Structural characteristics of fire resistant cables
The structure of the fire resistant cable is basically the same as that of the ordinary cable, except that the conductor of the fire resistant cable is made of a copper conductor having a good fire resistance (the melting point of copper is 1 083 ° C), and a refractory layer is added between the conductor and the insulating layer. The refractory layer is wrapped by a multi-layer mica tape. Because the allowable operating temperature difference of different mica tapes is large, the key to the fire resistance of the cable is the mica tape.
2.2 Classification of fire resistant cables
Common fire-resistant cables in China are classified into Class A and Class B: Class B cables can withstand combustion for at least 90 minutes in flames and rated voltages of 750 ° C to 800 ° C without cable breakdown (ie, 3 A fuses are not blown). Based on the improvement of the refractory layer manufacturing process and the increase of the refractory layer, a Class A fire-resistant cable was developed, which can withstand combustion for at least 90 min in the flame at 950 ° C to 1 000 ° C and the rated voltage without the cable being broken down. (ie 3A fuse does not blow). Class A fire resistant cables have better fire resistance than Class B. In addition, the mineral insulated cable is one of the better performances in the fire resistant cable. It is made of copper core, copper sheath and magnesium oxide insulating material, referred to as MI (minerl insulated cables). The cable is composed entirely of inorganic materials, and the refractory layer of the ordinary fire-resistant cable is made of inorganic materials and common organic materials. Therefore, the fire resistance of the MI cable is superior to that of the ordinary fire-resistant cable and does not decompose and burn due to combustion. gas. MI cable has good fire resistance and can work under high temperature of 250 °C for a long time. It also has explosion-proof, strong corrosion resistance, large current capacity, radiation resistance, high mechanical strength, small volume, light weight, long life and smokeless. specialty. However, the price is expensive, the process is complicated, and the construction is difficult. In the oil irrigated area, the important wooden structure public building, the high temperature place and the like, the fire resistance is high and the economy is acceptable, and the cable with good fire resistance can be used.
2.3 Problems in selection
According to the specific characteristics of fire-resistant cables, designers should pay attention to the following issues when designing and selecting:
(1) When fire-resistant cables are used in cable-dense cable tunnels, cable interlayers, or in flammable locations such as oil pipes and oil depots, Class A fire-resistant cables should be used first. In addition to the above, and the number of cable configurations is small, Class B fire-resistant cables can be used.
(2) Fire-resistant cables are mostly used as power supply circuits for emergency power supplies, requiring normal operation during fire. Due to the sharp rise of the ambient temperature during the fire, in order to ensure the transmission capacity of the line and reduce the pressure drop, the cross section of the fire resistant cable should be at least one step for the circuit with a long supply line and strict limits on the allowable voltage drop.
(3) Fire resistant cables cannot be used as high temperature resistant cables.
(4) In order to reduce the probability of failure of the cable joint in a fire accident, the number of joints should be minimized during installation to ensure that the line can work normally in a fire. If branch wiring is required, the joint should be fireproofed.
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Label: difference between flame retardant cable and fire resistant cable
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