In recent years, in the global power system fire accidents, the proportion of power system power supply interruptions caused by power cable fires has increased. Therefore, in power plants, substations and other places where there are a large number of electrical equipment and a large number of cables are intensively laid, flame-retardant cables are used to prevent the spread of fire, and it is increasingly important to use flame-resistant cables to ensure uninterrupted power supply for important circuits. However, some electrical designers are not clear about the concept of flame-retardant cables and fire-resistant cables, and do not know much about the structure and characteristics of the two, which leads to the failure to correctly design and select these two cables according to the power supply requirements, and conduct design agency or supervision on site The laying workers of these two types of cables cannot be properly guided during work. Therefore, a comprehensive and correct understanding of flame-retardant and fire-resistant cables is the first condition for the correct selection of them. Baotuquan Cable will give you the answer.

1 Flame retardant cable

Flame-retardant cable refers to a cable that is burned under specified test conditions, and after the test fire source is removed, the flame spreads only within a limited range, and the residual flame or residual burn can be extinguished by itself within a limited time. Its fundamental feature is: it may be burnt out and cannot operate in a fire situation, but it can prevent the spread of fire. In layman's terms, if a cable catches a fire, it can limit the combustion to a local area without spreading, so as to protect other equipment and avoid greater 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. The difference is that all or part of its insulating layer, sheath, outer sheath and auxiliary materials (tape and filling) are flame-retardant materials.  

1.2 Classification of flame-retardant cables

 According to different cable flame-retardant materials, flame-retardant cables are divided into two categories: halogen-containing flame-retardant cables and halogen-free low-smoke flame-retardant cables.

Among them, the insulating layer, sheath, outer sheath and auxiliary materials (tape and filling) of halogen-containing flame-retardant cables are all or part of halogen-containing polyethylene (PVC) flame-retardant materials, which have good flame-retardant properties. However, when the cable burns, a large amount of dense smoke and halogen acid gas will be released. The halogen acid gas is corrosive to the surrounding electrical equipment. Rescuers need to wear gas masks to approach the scene to extinguish the fire. When the cable burns, it will cause harm to the surrounding electrical equipment and rescuers, which is not conducive to the fire fighting and rescue work, resulting in serious "secondary hazards".

The insulation layer, sheath, outer sheath and auxiliary materials (tape and filling) of halogen-free low-smoke flame-retardant cables are all or partly made of halogen-free cross-linked polyethylene (XLPE) flame-retardant materials, which not only have Better flame-retardant properties, and no halogen acid gas is emitted when the cable is burned, and the amount of smoke generated by the cable is also small. The corrosive gas generated by the cable burning is also a compromise between the cable flame retardancy and reducing the amount of halogen acid gas. Method developed a low-halogen, low-smoke flame-retardant cable Its halogen content is about 1/3 of that of halogen-containing flame-retardant cables. The amount of smoke is also close to the recognized "low smoke" level.

According to the "Wire and Cable Combustion Test Method", flame retardant cables assessed by the bundled combustion test method are classified into categories A, B, and C. Among them, category A cables have the most demanding test conditions and have better performance than categories B and C. The price Also the most expensive.

In order to achieve the flame retardant of the cable or to improve the flame retardant level of the cable, people have developed a new type of extruded so-called "oxygen barrier" or "fire barrier" as filling or wrapping fire barrier tape (fire barrier). Structure flame-retardant cable, and call this cable "oxygen (fire) layer" cable. The main principle is that metal hydrate is added to the material, which can also realize low-halogen and low-smoke, halogen-free and low-smoke, and flame retardancy of cables. However, the so-called "oxygen barrier" and "fire barrier" are only a means to achieve cable flame retardancy. In the final analysis, it is a flame retardant cable, and the highest flame retardant level it can achieve is also Class A. Therefore, flame-retardant cables cannot be called "oxygen barrier" cables, but only a kind of flame-retardant cables.

1.3 Problems to pay attention to when choosing

 The designer should design, select and install the flame-retardant cable according to the specific characteristics of the flame-retardant cable, and pay attention to the following issues:

(1) Since halogen-containing flame-retardant cables (including flame-retardant cables, low-halogen and low-smoke flame-retardant cables) will release corrosive halogen acid gas when burned, it will greatly hinder firefighting work, delay firefighting time and increase the spread of fire. In public places with high population density, halogen-free low-smoke flame-retardant cables should be designed as far as possible. Any kind of flame-retardant cable can be used in work areas with a small population density.

(2) Flame-retardant cables are divided into categories A, B, and C3, of which category A flame-retardant cables have better performance than category B and C, and are the most expensive. The designer should indicate the type of flame-retardant cable when providing the order list.

(3) The A, B, and C categories of flame-retardant cables cannot be distinguished from the appearance. Only the manufacturer can guarantee 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 and mechanical properties are significantly reduced. Therefore, when laying cables, halogen-free low-smoke flame-retardant cables The cable should have a larger bending radius than the halogen-containing flame-retardant cable.

(5) When designing cable laying, it is not advisable to lay non-flame-retardant cables and flame-retardant cables in parallel, or to lay flame-retardant cables of different flame-retardant categories in parallel.  

2 Fire-resistant cable series

Fire-resistant cable refers to: under the specified test conditions, the sample is burned in the flame and can still maintain the performance of normal operation 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 case of fire, the cable will not burn at once, and the circuit is safer. Therefore, the main difference between fire-resistant cables and flame-retardant cables is: fire-resistant cables can maintain normal power supply for a period of time when a fire occurs, while flame-retardant cables do not have this feature.

This feature determines that fire-resistant cables play an important role in modern cities and industrial buildings, because once a fire occurs, the power supply loops of the control, monitoring, guidance and alarm systems must maintain normal operation. Therefore, Baotuquan cables are mainly used in power supply circuits from emergency power supply to user fire-fighting equipment, fire alarm equipment, ventilation and smoke exhaust equipment, guide lights, emergency power sockets, emergency elevators, etc.

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. The difference is that the conductor of the fire-resistant cable uses a copper conductor with good fire resistance (the melting point of copper is 1083°C), and a fire-resistant layer is added between the conductor and the insulating layer. The refractory layer is wrapped by multiple layers of mica tape. Because the allowable working temperature of different mica tapes varies greatly, the key to the fire resistance of the cable is the mica tape.

2.2 Classification of fire-resistant cables

In China, ordinary fire-resistant cables are divided into Type A and Type B: Type B cables can withstand combustion for at least 90 minutes in a flame at 750°C to 800°C and at a rated voltage without the cable being broken down (that is, the 3A fuse does not blow). On the basis of improving the manufacturing process of the refractory layer and adding the refractory layer, a Class A refractory cable has been developed. It can withstand combustion for at least 90 minutes in a flame at 950℃～1000℃ and at a rated voltage without the cable being broken down ( That is, the 3A fuse does not blow). The fire resistance of Class A fire-resistant cable is better than that of Class B.

In addition, mineral insulated cables are one of the best fire-resistant cables. They are made of copper core, copper sheath, and magnesium oxide insulation materials, referred to as MI (minerlinsulated cables) cables. The cable is completely composed of inorganic materials as the fire-resistant layer, while the fire-resistant layer of the ordinary fire-resistant cable is composed of inorganic and general organic materials. Therefore, the MI cable has better fire resistance than ordinary fire-resistant cables and will not decompose due to combustion and cause corrosion. gas. MI cable has good fire resistance characteristics and can work under high temperature of 250 ℃ for a long time, and also has explosion-proof, strong corrosion resistance, large current carrying capacity, radiation resistance, high mechanical strength, small size, light weight, long life, and smokeless specialty. However, the price is expensive, the process is complicated, and the construction is difficult. In oil-irrigated areas, important wooden public buildings, high-temperature places and other occasions with high fire resistance requirements and acceptable economic efficiency, this kind of cable with good fire resistance can be used.

2.3 Problems to pay attention to when choosing

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 tunnels, cable interlayers with dense cables, or in flammable places such as oil pipes and oil depots, Class A fire-resistant cables should be selected first. In addition to the above cases and when the number of cables 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 and require normal operation in case of fire. As the ambient temperature rises sharply during a fire, in order to ensure the transmission capacity of the line and reduce the voltage drop, for circuits with long power supply lines and strictly limited allowable voltage drops, the cross-section of the fire-resistant cable should be enlarged by at least one level.

(3) Fire-resistant cables cannot be used as high-temperature cables.

(4) In order to reduce the failure probability of cable joints in fire accidents, the number of joints should be minimized during installation to ensure that the lines can work normally in a fire. If branch wiring is needed, fire-proof the joints should be done.